There are many different types of HVAC Heating, Ventilating and Air Conditioning systems. The purpose of this article is to introduce you to these systems. We will be studying the overall construction and use of these systems. In this article, we will study how each component is used in the system as a whole.
The purpose of HVAC Heating, Ventilating and Air Conditioning is the control of an enclosed environment. An HVAC system provides adequate indoor air quality by: conditioning the air in the occupied space of a building in order to provide for the comfort of its occupants; diluting and removing contaminants from indoor air through ventilation; and providing proper building pressurization. The three main environmental characteristics that are controlled are:
Controlling the temperature of an environment involves the transfer of heat from one area to another. Increasing or decreasing the temperature of the environment can be accomplished by any of several methods. We will talk about some of these methods later in this article.
The amount of water vapor contained in air is measured by relative humidity. Relative humidity is a ratio of how much water vapor is in the air to how much the air can hold at a specific temperature, and it is expressed as a percentage. The lower the relative humidity, the more tendency the air has to draw water from existing sources. When cold air is heated, its ability to hold additional water is increased. For this reason, when providing an HVAC system for personnel, a humidifier is typically provided in conjunction with heaters to control the humidity in the designed range.
The final characteristic that is generally included in HVAC system design consideration is the cleaning of the air. This is accomplished through ventilation and filtration. Dust, gases, and odors are unsatisfactory elements in environmental air, so they must be controlled.
There are many different types of air systems used to deliver the conditioned air to the areas or spaces requiring it. The single and dual duct systems are the two basic types of air duct systems that are used for distribution of conditioned air. The single duct system supplies air to each area at a constant temperature. Temperature control is obtained by adjusting the volume of the supply air furnished. The dual duct system provides warm and cool air in separate ducts. Individual room or area temperature control is obtained by adjusting the amount or ratio of warm and cool air being mixed and introduced to the environment.
The advantages of each system must be considered when selecting basic design. The single duct system is obviously less expensive to install. However, an evaluation of the operating costs should be made.
Efficiency of operation in the dual duct system often provides a cheaper overall cost. The systems that will be discussed are:
The simplest form of a single-zone system is a single conditioner serving a single temperature-controlled zone. A single-zone system responds to only one set of space conditions. Its use is limited to situations where variations occur almost uniformly throughout the zone served or where the load is stable. A single-zone system would be applied to small department stores, small shops in a shopping center, individual classrooms of a small school, computer rooms, etc. Figure 1 shows a schematic of a single-zone central unit.
Figure 1: Single-Zone System
Control of dry-bulb temperature within a space requires that a balance be established between the space load and the air supplied to offset the load. To maintain the balance, you can choose between varying the supply air temperature or varying the volume as the space load changes. Variable air volume systems may be applied to interior or perimeter zones with common or separate fans systems, common or separate air temperature control, and with or without auxiliary heating devices. It is possible to vary zone air volume only, while keeping fan and system volume constant by dumping excess air into a return air ceiling plenum or directly into the return air duct system. Figure 2 shows a schematic of a variable volume system.
Figure 2: Variable Volume System
The reheat system is a modification of the single-zone system. Conditioned air is supplied from a central unit at a fixed cold air temperature designed to offset the maximum cooling load in the space. The control thermostat simply calls for heat as the cooling load in the space drops below maximum. This system is generally applied to hospitals, laboratories, or spaces where wide load variations are expected. Figure 3 shows a schematic of a terminal reheat system.
Figure 3: Terminal Reheat System
Primary air is discharged from nozzles arranged to induce room air into the induction unit approximately four times the volume of the primary air. The induced air is cooled or heated by a secondary water coil. Induction-type units are generally located under the window to offset winter downdrafts. Figure 4 shows a schematic of an induction system.
Figure 4: Induction System
The dual duct system conditions all the air in a central apparatus and distributes it to conditioned spaces through two parallel mains or ducts. One duct carries cold air, and the other duct carries warm air, thus providing air sources for both heating and cooling at all times. In each conditioned space or zone, a mixing valve controlled by a room thermostat mixes the warm and cold air in proper proportions to satisfy the prevailing heat load of the space. Figure 5 shows a schematic of a dual duct system low-velocity.
Figure 5: Low-Velocity Dual Duct System
High-velocity dual duct systems operate in the same manner as the low-velocity systems, except that the supply fan runs at a higher pressure, and each zone requires a mixing box with sound attenuation. Figure 6 shows a schematic of a dual duct system high-velocity.
Figure 6: High-Velocity Dual Duct System
The multi-zone system is applicable for serving a relatively small number of zones, from a single central air handling unit. The requirements of the different zones are met by mixing cold and warm air through zone dampers at the central air handler in response to zone thermostats. The mixed conditioned air is distributed throughout the building by a system of single-one ducts as shown in Figure 7.
Figure 7: Multi-zone System
When the environmental air is to be recirculated, filtration is used to remove undesirable elements. Filtration allows previously conditioned air to be cleaned while maintaining temperature and humidity, thus increasing the systems efficiency.
The placement of filters in HVAC systems obviously produces a differential pressure that system fans must overcome. This differential pressure can drastically reduce airflow and, therefore, system energy efficiency. Several methods of filtration are available for use in environmentally controlled systems. The following types of filters will be discussed in the segments below:
Fibrous media filters, as shown in Figure 8, are composed of a coarse fiber material such as fiberglass, metal mesh, or vegetable fibers. The air to be filtered is forced through the material where dust and other similarly sized particulates become trapped in the matrix. Depending on the type of fibrous material used, it may be washed or replaced when dust impedes the airflow. As dust is trapped in the filter material, the effectiveness of the filter increases the passageways for the air get smaller, but the pressure drop across the filter also increases, creating a higher demand on the systems blower unit.
Figure 8: Typical Fibrous Media Filters
In some circumstances, it is necessary to continuously provide a clean filter. The used filter may be rolled up and discarded or cleaned and reused. Some systems include a mechanism to clean the filter, which is arranged in a continuous belt. This system maintains a constant pressure drop and cleaning efficiency.
By passing air through an electric field typically 12,000 volts, particulates receive a charge. When the charged particulates are subsequently passed through a matrix of oppositely charged plates, the particles are attracted and collected on the plates. Figure 9 diagrams this process. The plates may be removed for cleaning or washed in place periodically. Figure 10 shows a typical electronic air cleaner.
Figure 9: Electronic Air Cleaning Process
Figure 10: Industrial Electronic Air Cleaner
The high-efficiency particulate air HEPA filter is the most efficient air-cleaning system commercially available. Although it was developed for the nuclear industry, it has been found to be extremely useful in the medical and electrical fields.
HEPA filters provide a minimum efficiency of 99.97% on 0.3 micron particulates. A micron is one-millionth of a meter. The filter media is typically a fibrous material with a high surface area to volume ratio. Design velocities are held down to about 5 feet-per-minute. This increases the particulate-holding characteristics of the filter. In the fabrication and installation process of HEPA filters, care must be taken to ensure that all air that passes through the unit goes through the filter material. No cracks or voids may exist that allow unwanted particulates to avoid filtration.
HEPA filters are actually a specialized fibrous material filter. Figure 11 shows a box HEPA filter.
Figure 11: Box HEPA Filter
Activated carbon filters are commonly used to remove gases and vapors from recirculated air. The process involved is adsorption, where the carbon adsorbs the gases and vapors in a sponge-like process. The carbon will also trap some particulate matter.
A hydronic, or all-water, system is one in which hot or chilled water is used to convey heat to or from a conditioned space or process through piping connecting a boiler, water heater, or chiller with suitable terminal heat transfer units located at the space or process.
All water systems may be classified by temperature, generation of flow, pressurization, piping arrangement, and pumping arrangement.
In terms of flow generation, hot water heating systems are of two types: the gravity system, in which circulation of the water is due to the difference in weight between the supply and the return water columns of any circuit or system, and the forced system in which a pump, usually driven by an electric motor, maintains the necessary flow. Water systems can be either once-through or recirculating systems.
Examples of how water systems are classified according to temperature are discussed below.
An LTW is a hot water heating system operating within the pressure and temperature limits of the ASME boiler construction code for low-pressure heating boilers.
The maximum allowable working pressure for low pressure heating boilers is 160 psi with a maximum temperature limitation of 250F. The usual maximum working pressure for boilers for LTW systems is 30 psi, although boilers specifically designed, tested, and stamped for higher pressures may frequently be used with working pressures to 160 psi. Steam-to-water or water-to-water heat exchangers are also often used.
An MTW is a hot water heating system operating at temperatures of 350F or less, with pressures not exceeding 150 psi. The usual design supply temperature is approximately 250 to 325F, with a usual pressure rating for boilers and equipment of 150 psi.
An HTW is a hot water heating system operating at temperatures over 350F and usual pressures of about 300 psi. The maximum design supply water temperature is 400 to 450F, with a pressure rating for boilers and equipment of about 300 psi. It is necessary that the pressure-temperature rating of each component be checked against the design characteristics of the particular system.
A CWS is a chilled water cooling system operating with a usual design supply water temperature of 40 to 55F and normally operating within a pressure range of 125 psi. Antifreeze or brine solutions may be used for systems usually process applications that require temperatures below 40F. Well water systems may use supply temperatures of 60F or higher.
A DTW is a combination hot water heating and chilled water cooling system that circulates hot and/or chilled water to provide heating or cooling using common piping and terminal heat transfer apparatus. They are operated within the pressure and temperature limits of LTW systems, with usual winter design supply of water temperatures about 100F to 150F and summer supply water temperatures 40F to 55F.
Generally, the most economical distribution system layout has mains that are run by the shortest and most convenient route to the terminal equipment having the largest flow rate requirements. Branch or secondary circuits are then connected to these mains.
Water distribution mains are most frequently located in corridor ceilings, above hung ceilings, wall-hung along a perimeter wall, or in pipe trenches, crawl spaces, or basements. Water system piping need not be run at a definite level or pitch but may change up or down as required by architectural or structural needs. Water system piping may be divided into two arbitrary classifications, pipe circuits suitable for complete small systems and terminal or branch circuits for large systems. Examples include:
A series loop is a continuous run of pipe or tube from supply connection to return connection. Terminal units are a part of the loop.
Figure 12 shows a system of two series loops on a supply and return main split series loop. One or many series loops may be used in a complete system. Loops may connect to mains, or all loops may run directly to and from the boilers.
Water temperature drops progressively as each radiator transfers heat to the air; the amount of drop depends on radiator output and water-flow rate. The true system operating water temperature and flow rate must be known to calculate the average water temperature AWT for each unit on the loop. If all terminal units are in series on one loop in one zone of interconnecting air space, the entire set of units can be sized at the AWT of the loop. One floor of a small dwelling with open interior doorways is such an interconnecting space. If individual units on a loop are in separate enclosed spaces, each unit must be sized to actual AWT for that unit.
Figure 12: Series Loop System
A decrease in loop waterflow rate increases temperature drop in each unit and in the entire loop. Average water temperature shifts downward progressively from the first to the last radiator in series. Unit output gradually lowers from first to last on the loop. Consequently, comfort cannot be maintained in separate spaces heated with a single series loop if waterflow rate is varied. Control of output from individual terminal units on a series loop is impractical except by control of heated airflow. Manual dampers can be used on natural convection units; automatic fan or face and bypass damper control can be used on forced air units.
One-pipe circuits use a single loop main, as shown in Figure 13. For each terminal unit, a supply and a return tee are installed on the same main. One of the two tees is a special diverting tee that creates a pressure drop in main flow to divert a portion of main flow to the unit. One return diverting tee is usually sufficient for up-feed units above main systems. Two special fittings supply and return tees are usually required for down-feed units to overcome thermal head. Special tees are proprietary; consult manufacturers literature for flow rates and pressure drop data.
Figure 13: One-Pipe System
One-pipe circuits allow manual or automatic control of flow to individual connected heating units. On-off, rather than flow modulation, control is advisable because of the relatively low pressure and flow diverted. Length and load imposed on a one-pipe circuit are usually small because of the limitations listed.
Two-pipe circuits may be direct-return return main flow direction is opposite supply main flow; return water from each unit takes the shortest path back to the boiler, as shown in Figure 14, or reverse-return return main flow is in the same direction as supply flow; after the last unit is fed, the return main returns all water to the boiler, as shown in Figure 15. The direct-return system is popular because less main pipe length is required; however, circuit-balancing valves usually are required on units or sub-circuits. Since waterflow distance to and from the boiler is virtually the same through any unit on a reverse-return system, balancing valves are seldom adjusted. Operating pumping cost is likely to be higher with direct return because of the added balancing fitting pressure drops at the same flow rate.
Figure 14: Direct-Return Two-Pipe System
Figure 15: Reverse-Return Two-Pipe System
The four basic arrangements exist only to describe function. One type can grade into another; a piping system can contain from one to all four types and, thus, cannot be described as a particular type. Figure 16 illustrates a primary circuit and two secondary pumping circuits. As pipe lengths and number of units vary, and as circuit types are combined, basic names for piping circuits become meaningless; flow, temperature, and head must be determined for each circuit and for the complete system.
Figure 16: Example of Primary and Secondary Pumping Circuits
The three-pipe system satisfies variations in load by providing independent sources of heating and cooling to the room unit in the form of constant temperature primary and secondary chilled and hot water.
The unit contains a single secondary water coil, as shown in Figure 17. A three-way valve at the inlet of the coil admits the water from either the hot or cold water supply, as required. The water leaving the coil is carried in a common pipe to either the secondary cooling or heating equipment. The usual room control for three-pipe systems is a special three-way modulating valve that modulates either the hot or cold water in sequence but does not mix the streams. The three-way valves are a special design in which the hot port gradually moves from open to fully closed and the cold port gradually moves from fully closed to open. The valves are constructed so that, at mid-range, there is an interval in which both ports are completely closed. Room control action is the same during all seasons. The primary air is cold and at the same temperatures year-round.
During the period between seasons, if both hot and cold secondary water is available, any unit can be operated within a wide capacity range from maximum cooling to maximum heating within the limits set by the temperature of the secondary chilled or hot water. Any unit in the system can be operated through its full range of capacity without regard to the operation of any other unit in the system, recognizing the operating cost penalty that will result from simultaneous heating and cooling loads. All units are selected on the basis of their peak capacity requirements.
Figure 17: Return Mix System Room Unit Controls
Four-pipe systems for induction, fan-coil, or radiant panel systems derive their name from the four pipes to each terminal unit. The piping includes a cold water supply, cold water return, warm water supply, and warm water return.
The four-pipe terminal unit is usually provided with two completely separated secondary water coils, one receiving hot water and the second receiving cold water. The coils are operated in sequence by the same thermostat. The coils are never operated simultaneously, and the unit receives either hot water or cold water in varying amounts, or else no flow is present. This is shown in Figure 18. During peak cooling and heating, the four-pipe system performs in a manner similar to the two-pipe system, with essentially the same operating characteristics. During the period between seasons, any unit can be operated at any capacity level from maximum heating to maximum cooling, if both cold water and warm water are being circulated. Any unit can be operated at or between these extremes without regard to the operation of any other unit.
Figure 18: Four-Pipe System Room Unit Control
Since the primary air is supplied at a constant cool temperature at all times, it is sometimes feasible for fan-coil or radiant panel systems to extend the interior system supply to the perimeter spaces, eliminating the need for a separate primary air system.
Figure 18 also shows another unit and control configuration that is sometimes used. A single secondary water coil is provided at the unit, and three-way valves located at the inlet and leaving side of the coil admit the water from either the hot or cold water supply, as required, and divert it to the appropriate return pipe. This arrangement requires a special three-way modulating valve, originally developed for one form of the three-pipe system, which controls the hot or cold water selectively and proportionally but does not mix the streams. The valve at the coil outlet is a two-position valve open to either the hot or cold water return, as required.
When all aspects are considered, the two-coil arrangement provides a superior four-pipe system. The operation of the induction unit controls is the same year-round. Units with secondary air bypass control are not applicable to four-pipe systems.
Because of the nature of water systems, we will discuss some of the components that are associated only with hydronic systems.
If air and other gases are not eliminated from the flow circuit, they may cause air binding in the terminal heat transfer elements and noise in the piping circuit. High points in piping systems and terminal units should be vented with manual or automatic air vents. Because automatic air vents may malfunction, valves should be provided at each vent to permit service without draining the system. The discharge of each vent should be piped to a point where water can be wasted into a drain or container. If a plain expansion tank is used, free air contained in the circulating water should be removed from the piping circuit and trapped in the expansion tank by a boiler dip tube or other air separation devices. If a diaphragm-type tank is used, all air should be vented from the system.
All low points should be equipped with drains. Provisions should be made for separate shutoff and drain of individual equipment and circuits so that the entire system does not have to be drained for service of a particular item.
Balance fittings should be applied as needed to permit balancing of individual terminal and major sub-circuits. Such fittings should be placed at the circuit return when possible.
Piping does not need to pitch but can be run level, providing flow velocities in excess of 1.5 feet per second are maintained.
Strainers should be used where necessary to protect the elements of a system. Strainers placed in the pump suction need to be analyzed carefully to avoid cavitation. Large separating chambers are available that serve as main air venting points and direct strainers ahead of pumps. Automatic control valves or spray nozzles operating with small clearances require protection from pipe scale, gravel, welding slag, etc., which may readily pass through the pump and its protective separator. Individual fine mesh strainers may, therefore, be required ahead of each control valve. Condenser water systems without water regulating valves do not necessarily require a strainer. If a cooling tower is used, the strainer provided in the tower basin will usually be adequate.
Thermometers and/or thermometer wells should be installed to assist the system operator and to use for troubleshooting. Permanent thermometers with correct scale range and separable sockets should be used at all points where temperature readings are regularly needed. Thermometer wells should be installed where readings will be needed only during startup and balancing.
Flexible connectors are sometimes installed at pumps and machinery to reduce pipe vibration and to allow for expansion and contraction of system piping. Vibrations are transmitted through the water column across a flexible connection and reduce the effectiveness of the connector. Flexible connectors, however, prevent damage caused by misalignment of equipment piping flanges.
Gauge cocks should be installed at points where pressure readings will be required. Gauges permanently installed in the system will deteriorate due to vibration and pulsation. They will not be reliable when needed, unless periodic inspection and calibration is performed.
Pump location varies with the size and type of system. A pump in the boiler return is acceptable for small systems when pump head is low 12-foot head or less, the compression tank is on the boiler or a nearby main, and the highest piping and radiation is maintained at a static pressure greater than full pump head. These conditions apply to most residential systems.
When pump head is equal to or greater than the difference between boiler fill and relief valve discharge pressures, or when highest piping or radiation can be at a static pressure less than total pump head, the pump must be located on the supply side of the boiler, with a pressurized head tank also called an expansion or compression tank at the pump inlet, as shown in Figure 19. This ensures that pump cycling will not cause a vacuum at the topmost system points to allow air to be introduced into the system. Pump cavitation is prevented by locating a properly sized pressurized head tank near the pump inlet that supplies a positive pressure to the pump suction.
Figure 19: Boiler Piping for a Multiple-Zone, Multiple-Purpose Heating System
The equipment used in heating, ventilation, and air conditioning comes under three major headings that cover all the equipment in an HVAC system. Those major headings are heating, cooling, and air-handling.
The criteria for the selection of equipment and/or HVAC systems are basically the same. It requires that these eight requirements be looked at:
These include temperature, humidity, ventilation, pressurization, and zoning for better control if needed. In theory, at least, this criterion should have a high priority. In practice, the "comfort" requirement is sometimes subordinated to first cost or the desires of someone in authority. Process requirements are more difficult and require a thorough inquiry by the HVAC designer into the process and its needs. Most often, it will be found that different parts of the process have different temperature, humidity, pressure, and cleanliness requirements; the most extreme of these can penalize the entire HVAC system.
This is usually a code requirement and not an option. State and local building codes almost invariably include requirements limiting the use of new, nonrenewable energy. Nonrenewable refers primarily to fossil fuel sources. Renewable sources include solar, wind, water, processed waste, and reclaimed heat. The strictest codes prohibit any form of reheat except from reclaimed or renewable sources unless humidity control is essential. Most HVAC systems for process environments have opportunities for heat reclaim and other ingenious ways of conserving energy. Off-peak storage systems are becoming popular for energy cost savings although these systems may actually consume more energy than conventional systems.
First cost considers only the initial price once equipment is installed and ready to operate. It ignores such factors as expected life, ease of maintenance and even, to some extent, efficiency, though most energy codes require some minimum efficiency rating. Life-cycle cost includes cost factors like first cost, operation, maintenance, replacement, and estimated energy use, and evaluates the total cost of the system over a period of years. The usual method of comparing the life-cycle costs of two or more systems is to convert all costs to "present worth" values. Typically, first cost governs in buildings being built for speculation or short-term investment. Life-cycle costs are most often used by institutional builders - schools, hospitals, government - and owners who expect to occupy the building for an indefinite period.
Very often, someone in authority lays down guidelines that must be followed by the designer. This is particularly true for institutional owners and major retailers. Here, the designers job is to follow the criteria of the employer or the client unless it is obvious that some requirements are unsuitable in an unusual environment. Examples of such environmental conditions are extremely high or low outside air humidity, high altitude which affects air handling unit and air-cooled condenser capacity, and contaminated outside air which may require special filtration and treatment.
The architect can influence the HVAC system selection by the space he or she makes available in a new building. In retrofit situations, the designer must work with existing space. Sometimes, in existing buildings, it is necessary to take additional space in order to provide a suitable HVAC system. For example, in adding air conditioning to a school, it is often necessary to convert a classroom to an equipment room. Rooftop systems are another alternative where space is limited if the building structure will support such systems. In new buildings, if space is too restricted, it will be desirable to discuss with the architect the implications of the space limitations in terms of equipment efficiency and maintainability. There are ways of providing a functioning HVAC system in very little space, such as individual room units and roof-top units, but these systems often have a high life-cycle cost.
This criterion includes equipment quality mean time between failures is a commonly used term, ease of maintenance are high maintenance items readily accessible in the unit?, and accessibility is the unit readily accessible? Is there adequate space around it for removing and replacing items?. Rooftop units may be readily accessible if there is an inside stair and roof penthouse, but if an outside ladder must be climbed, the adjective "readily" must be deleted. Many equipment rooms are easy to get to but are too small for adequate access or maintenance. This criterion is critical in the life-cycle cost analysis and in the long-term satisfaction of the building owner and occupants.
Central plants may include only a chilled water source, both heating and chilled water, an intermediate temperature water supply for individual room heat pumps, or even a large, central air-handling system. Many buildings have no central plant. This decision, in part, influenced by previously cited criteria and is itself a factor in the life-cycle cost analysis. In general, central plant equipment has a longer life than packaged equipment and can be operated more efficiently. The disadvantages include the cost of pumping and piping, or, for the central Air handling unit AHU, longer duct systems, and more fan horsepower. There is no simple answer to this choice. Each building must be evaluated separately.
Though listed last, this is the most important criterion in terms of how the system will really work. There is an accepted truism that "the operator will soon reduce the HVAC system and controls to his level of understanding." This is not to criticize the operator, who may have had little or no instruction about the system. It is simply a fact of life. The designer who wants or needs to use a complex system must provide for adequate training - and retraining - for the operators. The best rule is to never add an unnecessary complication to the system or its controls.
Heating is the first word in the HVAC acronym.
Proper design of the heating system is even more critical than that of ventilation or cooling. In modern heating system design, the two things of primary concern are proper sizing to achieve comfort and system reliability. Capital and operating costs and pollution control are of secondary consideration. Energy conservation and operating costs go together and have a considerable effect on life-cycle costs.
In a modern heating system, heating can be provided by:
End users are provided heat by:
Boilers can produce low-temperature, medium-temperature, or high-temperature water, low-pressure steam, high-pressure steam including process steam, and thermal liquid.
Low-temperature water boilers Up to 250F are the most widely used type for residential, apartment, and commercial construction. Medium-temperature water boilers 250 to 310F are generally applied to industrial and campus-type facilities. High-temperature water 310 to 400F is used for extended campus-type facilities and industrial process facilities. It is often used where there are significant end-user steam requirements at pressures of 100 psi or more. Thermal liquid heaters are primarily found in industrial applications where both space and process heating are significant loads.
Low-pressure boilers 15 psig are generally found in commercial, apartment house, and single-unit industrial facilities. They are used for space heating and domestic hot water through end-use heat exchangers. High-pressure steam applications 15 to 150 psig are generally found in campus-type facilities, hospitals, and industrial plants where there are significant process requirements. Cogeneration, high-pressure steam boilers are in the range of 600 to 900 psig with some degree of superheat in order to obtain good turbine efficiency. Waste heat from the turbine is used for space heating, domestic hot water, and process requirements.
A unit heater is a package that includes a heating element and a circulating fan. It is designed for installation in or adjacent to the space to be heated. Units are made for horizontal discharge Figure 20 or vertical discharge Figure 21. Most unit heaters have propeller fans. Units with centrifugal fans may be used with duct work to extend the area of coverage.
Figure 20: Horizontal Discharge Unit Heater
Figure 21: Vertical Discharge Unit Heater
The heating element may be a steam or water coil, or it may be direct-fired using fuel gas or electric resistance. Gas heaters require proper venting and safety controls. Unit heaters are normally controlled by means of a room thermostat that starts the fan and energizes the heating element simultaneously.
A duct heater duct furnace is a unit heater without a fan and is installed in a duct or plenum. The duct heater depends on an AHU fan for air circulation. It may be the primary heating element in the main duct or AHU plenum, or it may be used for zone reheat control in branch ducts. Many package air-handling systems use duct heaters.
An outside air heater is a unit heater or duct heater used for preheating outside air, as required for exhaust make-up or combustion. To prevent freeze-up, gas or electric heating is used, with gas preferred on an energy-cost basis. In some installations, codes allow the use of unvented heaters - all the heat and products of combustion are in the air stream but so diluted as to pose no danger. This situation requires that all of the supply air be exhausted.
Radiant unit heaters do not have fans and use radiant heating rather than convective heating. For this purpose, they are installed overhead and equipped with special high-temperature surfaces that radiate primarily in the infrared spectrum. They are used mostly for "spot-heating" at work stations in otherwise unheated or poorly heated buildings. Another use is for heating of outdoor areas is where people need to wait or stand in line, such as under theater marquees or in amusement parks. Radiant heating is a very efficient and economical method of achieving a level of comfort in an area that would be difficult or impossible to heat satisfactorily in any other way.
Terminal heating equipment is equipment installed in or contiguous with the area served. In general, the heating source is remote - water or steam is used - but electric resistance heating is common. Duct heaters and some heat pumps can also be included in this category. In many cases, the terminal equipment is used for both heating and cooling.
A radiator is a heating device that is installed in the space to be heated and transfers heat primarily by radiation. The most common example is the sectional cast-iron column radiator. There are many thousands of these in use throughout the world, although, in new installations, they have been largely supplanted by convector radiators or baseboard radiation. The heat source is hot water or low-pressure steam 5 psig or less. Small "electric radiators" include water and an electric immersion heater.
Radiators are rated in square feet of radiation, or EDR equivalent direct radiation. One square foot EDR is equal to 240 btuh for steam at one psig or 180 btuh for water at 200F. These ratings are no longer readily available but may be obtained from the Hydronics Institute, formerly the Institute of Boiler and Radiator Manufacturers. Some representative data is available in the ASHRAE Handbook.
Radiators are controlled in several ways:
A convector is a heating device that depends primarily on gravity convective heat transfer. The heating element is a finned-tube coil, or coils, mounted in an enclosure designed to increase the convective effect, as shown in Figure 22. The enclosure cabinet is made in many different configurations, including partially or fully recessed into the wall. The usual location is on an exterior wall at or near the floor. Capacity depends on geometry - length, depth, height - and heating element design, as well as hot water temperatures or steam pressure. Ratings are usually based on the test methods specified in "Commercial Standard CS 140-47, Testing and Rating Convectors." Refer to manufacturers catalogs for specific data.
Figure 22: Convector
Baseboard radiation is designed for wall mounting in place of the usual baseboard. It is either a fin-tube system, similar to a convector but much smaller, or a cast-iron section, designed with convective heat channels to augment the radiant effect. Baseboard radiation is usually continuous along exterior walls. Blank covers may be used for appearance if the capacity if not needed.
Finned-tube, or finned-pipe, radiation uses larger tubing or pipe - 1-1/4 inch to 2 inch size - with fins bonded to the pipe. The fins are typically 3-1/2 inch to 4-1/2 inch square. The system is used mostly for perimeter heating, particularly at glass areas. Heat transfer is by convection, and a variety of enclosure types are available. Special enclosures are often made to suit an architectural decor.
All of these heating elements may use either low-pressure steam or hot water as the heating source. Either one-pipe or two-pipe distribution systems are used, although two-pipe is more common in modern practice. Zoning by exposure, using solar compensated sensors, is a frequent practice. Electric baseboard radiation is also available. It is sometimes more economical, for example, in an all-electric situation or where steam or hot water is not available.
A radiant panel is a heating surface designed to transfer heat, primarily by radiation. There may also be a convective component, and in the case of floor panels, convective transfer may be predominant. Panels may be located in the floor, wall, or ceiling, and they may occupy part or all of the available area. Panel surface temperatures are limited by the physiological response of the building occupants, that is, too high a temperature may result in an uncomfortably warm feeling. Typical limitations are 80 to 85F for floor panels, about 100F for wall panels, and 120 to 130F for ceiling panels. The heating source is hot water or electrical resistance heating cable. Hot-water supply temperatures should be consistent with the panel temperature limitations; for floor panels, for example, supply water temperature should be no more than 100F.
Factory-assembled sidewall and ceiling panels and panel systems are available. Most panels are field-fabricated using electrical heating cable, copper tubing, or steel pipe imbedded in the construction. For concrete floor panels, steel pipe is used 3/4 inch or 1 inch size because steel has an expansion coefficient similar to that of concrete. Corrosion at the concrete-pipe interface can be severe. Electric heating cable may be used. Ceiling and wall panels use 1/2 inch to 3/4 inch copper tube or electric cable. Air venting is a serious problem, especially with floor panels.
Control systems are conventional because radiant-heat-sensitive devices are not readily available. Floor panels are very difficult to control because the relatively large mass provides a slow response.
A heat pump is a mechanical refrigeration system arranged and controlled to use the condenser heat for some useful purpose, typically space heating.
Systems may be packaged or built-up, air-to-air, water-to-air, or water-to-water. Earth-coupled systems are also used.
A packaged heat pump is a factory-assembled system, designed to provide either heating or cooling as needed. The standard refrigeration cycle is modified as shown in Figure 23. The key to the operation is the reversing valve. In the cooling position, refrigerant flow is directed first to the outdoor coil, which becomes the condenser. The liquid refrigerant then bypasses metering device no. 1 and flows through metering device no. 2 to the indoor coil. The metering device is a thermal expansion valve, throttling tube, or some other method of reducing the pressure. The indoor coil then becomes the evaporator and cooling is provided. With the reversing valve in the heating position, refrigerant flow is reversed, and the indoor coil becomes the condenser and provides heating; heat is extracted from the outdoor air. Changeover from heating to cooling may be automatic but is usually manual. Most packaged heat pumps are air-to-air. Heating capacity decreases as outdoor air temperature decreases.
Figure 23: Packaged Heat Pump Cycles
While most air-to-air heat pumps will operate satisfactorily down to zero degrees F outdoors, auxiliary heating will be needed except in very mild climates. Figure 24 and Figure 25 illustrate the procedure for determining the auxiliary heat requirements.
Figure 24: Estimating Heating/Cooling Loads versus Outdoor Temperature
Figure 24 shows the method of calculating the net heating load as a function of temperature. For buildings with 24-hour occupancy, solar heat effects should be ignored. Note that the net heat loss is less than the calculated heat loss because of internal heat gains due to people, lights, and other sources.
In Figure 25, this net heating load is plotted against the heat pump capacity from manufacturers data as a function of temperature. The shaded area is the excess of load over capacity, requiring auxiliary heat. Almost any fuel can be used for auxiliary heat, but electric resistance is the most common.
Figure 25: Heating Load versus Heat Pump Capacity
In a water-to-air package heat pump, a water-to-air heat exchanger is substituted for the outdoor coil. A central source for heating or cooling the water can then, in effect, provide the auxiliary heat. Systems of this type are used in apartment houses and hotels to allow maximum control of the room environment by the occupant. The water temperature is controlled at a range of values - perhaps 70oF to 85oF - which is suitable both as a heat source and heat sink for the heat pumps. In mild weather when some units are in heating mode while others are cooling, the central boiler and cooling tower may be idle.
There are many types of cooling systems. This section will give an introduction to the equipment that are used in cooling systems which includes:
Several different methods are used to cool air directly or indirectly. Equipment from the following systems will be discussed along with a brief description of how each system operates:
The steam jet refrigeration system may be used if an abundant supply of high-pressure steam is available. The equipment that is used in the system is a nozzling jet, which works like an air ejector and a tank with an inlet and outlet for water supply to it. Refer to Figure 26. The process creates a partial vacuum in a tank by nozzling a jet of steam over the single opening aspiration. This reduced pressure in the tank permits water to boil at a substantially reduced temperature 40 to 50F. The heat required for evaporation is extracted from the cooling water, which is pumped through the tank.
Figure 26: Steam Jet Refrigeration System
The heat sink method of providing air conditioning requires a large body of cool water, usually subterranean or drawn from deep lakes. The equipment that is used in this system is a pump, piping, and hydronic coils. The cool water is circulated through hydronic coils and subsequently provides cooling to the controlled environment.
The absorption system uses a liquid with a low boiling point, such as ammonia or water, under a low vacuum. The equipment that is used in this system is a chiller, heat exchanger, and three pumps. The chiller is divided into four sections: evaporator, absorber, generator, and condenser. We will discuss the function of each piece of equipment.
The chiller is divided into four sections: evaporator, absorber, generator and condenser.
The function of the heat exchanger is to make the absorption cycle more efficient. It does this by bringing the warm, concentrated absorbent solution coming from the generator in contact with the relatively cool dilute absorbent from the absorber. This lowers the heat input needed for input to the generator and increases the efficiency of the system.
There are three pumps usually associated with an absorption system. These pumps are used to circulate the fluids between the following components and are named for the component that they service.
Because all absorption systems work basically the same, we will describe the operation of the lithium bromide cycle.
Refer to Figure 27 as you go through the cycle. It will help you to understand what is happening in each area of the absorption chiller.
Let us start the cycle by creating a vacuum in the absorber and evaporator and starting these pumps. Water will boil at 40F to 45F with a vacuum of 29.53 inches of mercury Hg. As the refrigerant water is sprayed on the 55F chilled water coil, the refrigerant boils and absorbs the heat from the chilled water. The refrigerant vapor is then absorbed by the lithium bromide and becomes weaker. To have continuous operations, the lithium bromide must be made stronger, and the refrigerant must return to the evaporator. To do this, the generator pump is started and a steam valve is opened. The generator pump forces the weak solution through the heat exchanger where the weak solution is preheated and the strong solution from the generator is cooled, then into the generator. Steam is used to make the refrigerant water go into a vapor again, where it condenses into pure water in the condenser. As the refrigerant level rises in the condenser, the float opens to return the refrigerant into the evaporator for continuous operation.
Figure 27: Lithium Bromide Absorption System
The compressed gas type of refrigeration system is the most widely used in everyday application, the one most people are familiar with, and the one we will discuss the most.
Just as with the other type of refrigeration system, we will look at the major equipment that is used to make the system. The equipment that we will cover is as follows:
The compressor removes the vapor from the evaporator, compresses it, and heats it. This raises the pressure and temperature of the vapor so that it can be condensed at ordinary climatic temperatures. The compressor then discharges the vapor to the condenser. There are four primary types of compressors: reciprocating, rotary, screw, and centrifugal. Regardless of the type of compressors, they all do the same thing, and they are the heart of the compressed gas cycle.
The condenser transfers heat from a place where it is not wanted to a place where it can be discarded. The condenser is a coil of metal tubing that is exposed to a cooling medium, such as water or fan-forced air. The cooling medium absorbs enough heat from the vapor to condense it. There are three types of condensers normally used: water-cooled, air-cooled, and evaporative.
Receivers are installed to collect the liquid refrigerant as it leaves the condenser. In some models, the lower section of the condenser is used as the receiver. A receiver serves as a storage for refrigerant, maintains a liquid seal on the liquid line, and vents any air or non-condensable gases back to the condenser.
Receivers are usually designed to be large enough to hold the complete charge of refrigerant required to operate the unit. They are equipped with stop valves on the inlet and outlet lines to permit the technician to pump the unit down when work is to be performed on another component in the system.
From the receiver, the liquid refrigerant is collected and then directed to the metering device.
The metering device limits, or controls, the flow of refrigerant passing through it on its way back to the evaporator. By controlling the flow, the pressure is reduced so that the liquid will again boil at low temperature in the evaporator.
So that the refrigerating unit may operate automatically, an automatic metering device must be placed in the circuit between the liquid line and the evaporator. This control reduces the high pressure in the liquid line to the low pressure in the evaporator. The six main types of automatic metering devices are:
The evaporator, or cooling coil, is the part of the refrigeration system where heat is removed from the product; air, water, etc., is to be cooled. As the refrigerant enters the passages of the evaporator, it absorbs heat from the product being cooled, and as it absorbs heat from the load, it begins to boil and vaporizes. In this process, the evaporator accomplishes the overall purpose of the system - refrigeration.
Manufacturers develop and produce evaporators in several different designs and shapes to fill the needs of prospective users. The blower coil or forced convection type evaporator is the most common design; it is used both in refrigeration and air-conditioning installations. The six main types of evaporators are:
Figure 28 shows the basic compressed gas cycle. Liquid refrigerant enters the metering device, which separates the high-pressure side of the system from the low-pressure side. This valve regulates the amount of refrigerant that enters the cooling coils of the evaporator. Because of the pressure differential, as the refrigerant passes through the metering device, some of it flashes to a vapor.
Figure 28: Basic Vapor-Compression Refrigeration Cycle
From the metering device, the refrigerant passes into the evaporator. The boiling point of the refrigerant under the low pressure in the evaporator is lower than the temperature of the space in which the cooling coil is installed. This causes the refrigerant to boil and vaporize, picking up latent heat of vaporization from the space being cooled.
The refrigerant leaves the evaporator as low-pressure, superheated vapor. The remainder of the cycle is used to dispose of this heat and convert the refrigerant back into a liquid state so that it can again vaporize in the evaporator and absorb the heat again.
The low-pressure, superheated vapor is drawn out of the evaporator by the compressor, which also keeps the refrigerant circulating through the system. In the compressor, the refrigerant is compressed from a low-pressure, low-temperature vapor to a high-pressure, high-temperature vapor.
The high-pressure vapor is discharged from the compressor into the condenser. Here, the refrigerant condenses, giving up its superheat sensible heat and its latent heat of condensation. The refrigerant, still at high pressure, is now a liquid again. From the condenser, the refrigerant goes to the metering device and the cycle begins again.
Figure 29 shows a graphic illustration of the pressure-temperature relationship for the refrigerant R-22, during each phase of its cycle.
Figure 29: Pressure-Temperature Chart for R-22
Referring to Figure 29, the evaporator state is the point at which the boiling liquid refrigerant enters the evaporator and absorbs sensible heat form the chill water return. As heat is absorbed, the liquid becomes completely vaporized and rises above its saturation temperature. At this point, the vapor is said to be "superheated."
The superheated vapor is then compressed. Compressing the vapor raises the pressure-temperature state of the vapor, which preconditions it for the condensation process.
Condensation is the exact reverse of evaporation and serves to expel the heat absorbed by the refrigerant and to condense the refrigerant vapor back to liquid form for reuse by the evaporator.
The term "chiller" is normally used in connection with a complete chiller package - which includes the compressor, the condenser, the evaporator, the internal piping, and the controls - or for a liquid chiller evaporator only, where the water or brine is cooled.
Liquid chillers are of two general types: flooded and direct expansion. There are several different configurations including shell-and-tube, double tube, shell-and-coil, Baudelot surface, and tank-with-raceway. For HVAC applications, the shell-and-tube configuration is most common.
Figure 30 shows a typical flooded shell-and-tube liquid chiller. Refrigerant flow to the shell is controlled by a high- or low-side float valve or by a restrictor. Waterflow rate through the tubes is defined by the manufacturer but is generally in the range of 6 to 12 fps. Tubes may be plain bare or have a finned surface. The two-pass arrangement shown is most common, although one to four passes are available. The chiller must be arranged with removable water boxes so that the tubes may be cleaned at regular intervals because even a small amount of fouling can cause a significant decrease in heat-exchange capacity. Piping must be arranged to allow easy removal of the water boxes.
Figure 30: Flooded Liquid Chiller
In the DX liquid chiller Figure 31, the refrigerant is usually inside the tubes with the liquid in the shell. Baffles are provided to control the liquid flow. The U-tube configuration shown is typical and less expensive than the straight-through tube arrangement but can lead to problems with oil accumulation in the tubes if refrigerant velocities are too low. Refrigeration flow is controlled by means of a thermal expansion valve.
Figure 31: Direct-Expansion Chiller U-Tube Type
A complete package chiller will include the compressor, the condenser, the evaporator chiller, the internal piping, and the operating and capacity controls. Controls should be in a panel and include all internal wiring with a terminal strip for external wiring connections. In small packages up to 100 tons motor starters may also be included. Some units with air-cooled condensers are designed for outdoor mounting; freeze prevention procedures must be followed. Units with water-cooled condensers require an external source of condensing water.
Chillers with reciprocating compressors are found mostly in the 5 to 100 ton range. Although larger units are made, economics usually favor centrifugal compressor chillers in sizes of 100 tons or more. Screw compressor systems are made in a limited range of sizes, as contrasted with centrifugal compressors. Motor starters are usually separate from the centrifugal or screw packages; they may be turbine-driven, but they more often use electric motors. The typical system is direct-driven at 36,500 rpm. Wye-delta motors are used for reduced voltage starting. In larger units of 1,000 tons or more, it is not unusual to use high-voltage motors; the lower current requirements allow smaller wire sizes and across-the-line starting. An unusual drive system is that used on one of the 8,500-ton chillers at the Dallas-Ft. Worth airport. The utility plant manager replaced the original steam turbine driver with a 5,000 hp, 4,160 volt, variable-speed, variable-frequency electric drive. The chiller capacity was reduced to 5,500 tons, more in line with the actual load.
A cooling tower is a device for cooling water by using the evaporative cooling effect of the water. The cooled water may be used for many purposes, but the principle focus in this is for its use as a heat sink in a refrigerant condenser.
The two main types of cooling towers are open-circuit and closed-circuit. There are also two basic configurations: cross-flow and counter-flow. In either arrangement, the water enters at the top of the tower and flows downward through it. In the counter-flow arrangement, the air enters at the bottom and flows upward. In the cross-flow arrangement, the air enters at one side, flows across the tower, and out the other side.
Towers may be forced- or induced-draft, using fans as shown in Figure 32, or natural draft, using convective chimney effects. Typical of this latter group are the large hyperbolic towers seen at many power plants, as shown in Figure 33. In a forced-draft tower, the air is blown into the tower by the fans; in the induced-draft tower, the air is drawn through the tower.
Figure 32: Forced-Draft Cooling Tower
Figure 33: Natural-Draft Cooling Tower
Towers are spray-filled, with the water distributed through spray nozzles, or splash-filled, where the water flows by gravity and splashes off the tower fill material. In either case, the idea is to maximize the evaporation efficiency. The most important factors in this effort are:
Tower fill material used to be redwood. Now, most fill material is made of PVC or some similar plastic.
The two terms relating to tower efficiency are "range" and "approach". The range is the difference between entering and leaving cooling water temperatures. For HVAC practice, this is usually 10F, although 8F to 15F are used. Approach is the difference between the leaving cooling water temperature and the ambient wet-bulb temperature. This is usually between 6 and 10F, with 8F being typical.
In Figure 34, it can be seen that there is only one water circuit, with a portion of the cooling water being evaporated to cool the remainder. Because the water is exposed to air, with all of its contaminants, and absorbs oxygen, which is corrosive to most piping, the water must be carefully treated. To avoid increasing the concentration of solids as water is evaporated, blowdown must be provided: a portion of the water is wasted to the sewer either continuously or intermittently. A blowdown rate equal to the evaporation rate is considered normal. Ideally, treatment additives and blowdown rate should be controlled automatically by a system that measures water quality and solids concentration.
Figure 34: Cross-Flow Cooling Tower
The closed-circuit tower, shown in Figure 35, is designed to minimize corrosion and fouling in the cooling water circuit by making this a closed circuit. The cooling water flows through a bare tube coil in the tower, and coolant water in a separate circuit is sprayed over the coil and evaporated. This is essentially the same system as the evaporative condenser. The coolant water circuit is open and needs treatment and blowdown. Because of the temperature differential through the tube wall, this system is slightly less efficient than the open circuit, but the lower fouling effect improves the performance of, and decreases maintenance on, the condenser. This tower usually has a higher first cost than the open circuit tower.
Figure 35: Closed-Circuit Cooling Tower
A cooling coil is a finned-tube heat exchanger for use in an air-handling unit. Chilled water, brine, or refrigerant is inside the tubes, and air is blown over the outside, across the fins and tubes. When used with refrigerant, this element is the "evaporator" in the refrigeration cycle and is called a direct expansion DX coil.
Piping systems are the means by which thermal energy fluids are transported from one place to another. The type of fluid and its temperature and pressure influence limit the choice of piping materials. Most systems are closed; that is, the fluid is continually recirculated and no makeup is required except to replace that lost due to leaks. Steam systems are partly to completely open - as when the steam is used for a process or humidification and require continuous makeup. Cooling tower systems are open and need makeup to replace the water evaporated in the tower.
Closed systems require some means of compensating for the changes in volume of the fluid due to temperature changes. Expansion compression tanks are used.
Piping must be properly supported, with compensation for expansion due to temperature changes and anchors to prevent undesired movement.
Centrifugal pumps are used in HVAC for circulation of chilled, hot and condensing water, and brine. They are also used for pumping steam condensate and for boiler feed.
The operating theory of centrifugal pumps is exactly like centrifugal fans. The rotating action of the impeller equivalent to the fan wheel in a scroll housing generates a pressure that forces the fluid through the piping system. The pressure and volume developed are functions of pump size and rotational speed. For higher pressures, multistage pumps are used.
The majority of the centrifugal pumps used in HVAC work have a backward curved blade impeller, as shown in Figure 36. For pumping hot condensate, a turbine-type impeller is used to minimize flashing and cavitation.
Most pumps are direct driven at standard motor speeds such as 3,500 rpm, 1,750 rpm, and 1,150 rpm. Typical arrangements include combinations of alternatives such as end or double-suction, in-line or base-mounted, horizontal or vertical, and close-coupled or base-mounted. Vertical turbine pumps are used in sumps, i.e., in cooling-tower installations.
Figure 36: Backward-Curved Pump
In general, in-line pumps are used in small systems or secondary systems, such as freeze-prevention loops. Base-mounted pumps are used for most applications. Double-suction pumps are preferred for larger water volumes over 300 to 400 gallon per minute gpm because the purpose of the double-suction design is to minimize the end thrust due to water entering the impeller.
A typical pump performance curve, shown in Figure 37, is drawn with coordinates of gpm and feet of head. The curves show the capacity of a specific pump-casing size and design at a specific speed rpm and with varying impeller diameters.
The same impeller is used throughout, but when it is "shaved" machined to reduce its outside diameter, the capacity is reduced. This allows the pump to be matched to the design conditions. The graph includes brake horsepower curves for standard size motors, based on water with specific gravity of 1.0. For brines, or liquids with other specific gravities, the horsepower must be corrected in direct proportion to the specific gravity change. Also shown are efficiency curves.
Figure 37: Pump Performance Curve
The point at which a pump curve intersects the zero flow line is the shutoff head. At this or a higher head, the pump will not generate any flow. If the pump continues to run under no-flow conditions, the work energy input will heat the water. The resulting temperature/pressure rise has been known to break the pump casing.
If the speed of the pump is varied, the result will be a family of curves similar to Figure 38. These data are needed to evaluate a variable-speed pumping design.
Figure 38: Pump Speed versus Capacity and Head
In order to select a pump, it is necessary to calculate the system pressure drop at the design flow rate. Losses include pipe, valves, fittings, control valves, and equipment such as heat exchangers, boilers, or chillers. The design operating point or a complete system curve can then be plotted on a pump performance curve. Usually, several different pump curves will be inspected in order to find the best efficiency and lowest horsepower. In general, for large flows at low heads, lower speed pumps 1,150 rpm or even 850 rpm will be most efficient. For higher heads and lower flow rates, 1,750 rpm or 3,500 rpm will be preferable. Multistage pumps may be needed at very high heads. Always select a motor hp that cannot be exceeded by the selected pump at any operating condition, e.g., the hp curve should be above the pump curve at all points.
When two or more identical pumps are installed in parallel, the performance curve for two pumps has twice the flow of one pump at any given head. When the system curve is superimposed, it can be seen that the curve for one pump will intersect the system curve at about 70% of the design flow rate and about half of the design head. Similar curves can be drawn for three or more pumps in parallel.
Two or more identical pumps in series provide twice the head at any given flow rate. The flow with one pump will be about 75% of design flow. However, unless a bypass is provided around the second pump, the system curve will change somewhat with only one pump running, due to the pressure loss through the second pump. A bypass should be provided around both pumps to allow one to operate while the other is being repaired or replaced.
To completely understand the working of an air conditioning/refrigeration system, a basic understanding of the refrigerant being used and the safety concerns is necessary. A refrigerant is a primary working fluid used for absorbing and transmitting heat in a refrigeration system. All refrigerants absorb heat at a low temperature and low pressure during evaporation and release heat at a high temperature and high pressure during condensation.
The most commonly used refrigerants, most of which are synthetic chemical compounds, can be classified into six groups:
Chloroflourocarbons contain only carbon, chlorine, and fluorine atoms, and they are designated by the prefix CFC. CFCs have a high ozone depletion potential and have been phased out of general use worldwide.
Halons contain bromine, fluorine, and carbon atoms, and they are designated by the prefix BFC. BFCs have a high ozone depletion potential and have been phased out of general use worldwide.
Hydrochloroflourocarbons contain hydrogen, chlorine, fluorine, and carbon atoms, and they are designated by the prefix HCFC. HCFCs cause far less ozone depletion than CFCs or BFCs and have been used in recent years as a transitional refrigerant. HCFCs are scheduled for restricted use starting in the year 2004.
Hydroflourocarbons contain only hydrogen, fluorine, and carbon atoms. They are designated by the prefix HFC. They contain no chlorine atoms and cause no ozone depletion. HFCs may become one of the most widely used refrigerant groups for air conditioning in the next century.
An azeotrope is a blend of two substances that cannot be separated from its constituents by distillation. They are designated by the composite prefixes of their constituents. Azeotropes containing CFCs cause ozone depletion, and they have been phased out of general use.
These compounds include refrigerants used before halocarbons were introduced and include ammonia, water, and air. Many of them are still widely used in refrigeration systems because they do not deplete the ozone layer and have other desirable characteristics. Although ammonia is toxic and flammable, it is still used in refrigeration.
The tropospheric level of the atmosphere ground level to 7 miles above contains ozone as a pungent and bluish pollutant smog, which is a result of air contaminants exposed to the sun. The area of the atmosphere between 7 and 30 miles above the Earth is known as the stratosphere. The stratosphere is made of gases that help form the Earths protective shield. Ozone in the stratosphere above the Earth consists of molecules containing three oxygen atoms. The ozone layer protects the Earth from ultraviolet radiation from the sun.
Man-made CFCs and bromine released into the atmosphere have caused a depletion of ozone in the atmosphere. Air sampling measurements are the strongest evidence that CFCs exist in the stratosphere. The element in refrigerants that causes ozone depletion is chlorine. The chemical found in the upper stratosphere indicating ozone depletion is chlorine monoxide.
Chlorine in the stratosphere has been determined to come primarily from CFCs rather than natural sources because the rise in the amount of chlorine measured in the stratosphere during the past two decades matches the rise in the amount of fluorine. It is also determined that the rise in the amount of chlorine matches the rise in CFC emissions during the same period.
Measurements over the last decade have shown that volcanoes contribute minimal quantities of chlorine to the stratosphere compared to CFCs. Unlike hydrogen chloride from volcanoes, CFCs are not washed out of the atmosphere by rainfall and do not dissolve or break down into components that dissolve in water. The only fluorocarbon refrigerants that cause no harm to the stratosphere ozone are HFCs.
Depletion of the stratospheric ozone layer is a global concern. Estimates indicate that up to 100,000 ozone molecules can be destroyed by a single chlorine atom.
To compare the relative ozone depletion caused by various refrigerants, an index called ozone depletion potential ODP has been developed. The ODP is the ratio of the rate of ozone depletion of one pound of any halocarbon to one pound of CFC-11. The ODP of CFC-11 is assigned a value of 1.
Like the ODP, global warming potential GWP is used to compare the effects of halocarbons on global warming or the greenhouse effect with the effects of CFC-11. The ODPs and GWPs of various refrigerants are shown in Table 1.
Effects on human health levels from ozone depletion and increased levels of ultraviolet radiation range from increased cases of skin cancer and eye cataracts to a weakening of the immune system.
The EPA estimates that over the next century, without controls on CFCs, millions of additional cases of cataracts could result. Cataracts are clouds that develop on the eye lens and limit vision. The EPA also estimates that without controls on CFCs, many millions of additional cases and over 3 million deaths could result from skin cancer.
Reports indicate that people living closer to the equator have increased exposure to the suns ultraviolet rays. This increased exposure weakens the bodys immune system to some diseases.
Global warming, sometimes referred to as the greenhouse effect, presents additional problems. Tropospheric pollutants like CFCs, HCFCs, HFCs, carbon dioxide, and carbon monoxide absorb and reflect the Earths infrared radiation. The redirection of the Earths infrared radiation results in a gradual increase in the earths temperature causing global warming. The warming trend can be reduced by using alternative refrigerants.
Since the Clean Air Act was enacted to prevent depletion of the ozone layer, the intentional venting of CFCs and HCFCs has been prohibited. The first step in avoiding the venting is to use a refrigerant-recovering/recycling unit to remove all of the CFCs and HCFCs refrigerant from a chiller or other refrigeration system.
Figure 39 shows a typical refrigerant recovery unit. It includes a recovery cylinder, a vacuum pump or compressor, a water-cooled condenser, a sight glass, a shutoff float switch, necessary accessories, pipes, and hoses. Recovering refrigerant from a chiller that has been shut down involves two phases: liquid recovery and vapor recovery.
Figure 39: Typical Refrigerant Recovery Unit
Liquid recovery is shown in Figure 39a. The vacuum pump or compressor in the recovery unit creates a low pressure in the recovery cylinder. Liquid refrigerant is then extracted from the bottom of the chiller into the recovery cylinder. If the recovery cylinder is not large enough, the shutoff float switch stops the vacuum pump or compressor when the recovery cylinder is 80% full. Another empty recovery cylinder is used to replace the filled cylinder. If vapor enters the sight glass, which means that the liquid refrigerant is all extracted, the vacuum pump or compressor is stopped and the vapor recovery phase begins.
Vapor recovery is shown in Figure 39b. The vacuum pump, or compressor, extracts the refrigerant vapor from the top of the chiller. Extracted refrigerant vapor is then condensed into a liquid form that flows through the water-cooled condenser and is stored in the recovery cylinder. Non-condensable gases are purged into the atmosphere from the recovery unit. Water at a temperature between 40F and 85F is often used as a condensing/cooling medium. The recovered refrigerant can be recycled or reclaimed as required. Existing refrigeration equipment can be converted to use alternative refrigerants with only minor losses of capacity and efficiency.
Refrigerant testing is primarily concerned with ensuring that the correct refrigerant is being used, it is not contaminated, and there are no leaks in the system.
Equipment is currently available that can be used to test refrigerant systems for the type of refrigerant being used to ensure that no CFCs or HCFCs are in the system. The device can identify R-134a, R-12, R-22, and hydrocarbons, as well as any contaminants such as water or oil that may have gotten into the system.
Refrigerant leakage can easily be detected. If refrigerant leakage is not detected, gradual capacity reduction and eventual failure to provide the required cooling result. In addition, halocarbon leakage causes ozone depletion.
Most CFCs, HCFCs, and HFCs are colorless and odorless. Leakage of refrigerant from a refrigerating system can be detected by three different methods:
This is simple and fast. When air flows over a copper element heated by a methyl alcohol flame, CFC vapor decomposes and changes the color of the flame green for a small leak, bluish with a reddish top for a large leak. This method cannot be used where the ambient air contains explosive or flammable vapors.
This type of detector reveals a variation of electric current due to ionization of decomposed refrigerant between two oppositely charged electrodes. It is sensitive but cannot be used where the ambient air contains explosive or flammable vapors.
A solution of soap or detergent is brushed over the seals and joints where leakage is suspected, producing bubbles that can easily be detected.
Ammonia leakage can be detected by its objectionable odor, even from small leaks. Other inorganic refrigerant leaks can be detected by burning a sulfur candle, which will form a cloud of sulfuric compound in the presence of refrigerants. This method cannot be used when ambient air contains explosive or flammable vapors.
It stands to reason that an air-handling unit of some kind is an essential part of an air-conditioning system.
If the technicians understand the equipment that is used in air systems, it will help them to understand the overall view of HVAC. In this section, we will cover the equipment used in air handling, specifically fans and ductwork.
A fan is "a device used to cause a current of air by movement of a broad surface or a number of such surfaces within a sealed plenum." From this definition, the function of a fan can be stated as "a device that moves air or gas from one place to another." In doing so, it overcomes the resistance to flow by supplying the fluid gas or air with the energy necessary for continued motion. The resistance to flow is caused by duct configuration, the fluid being at rest, etc.
Large central station boilers, regardless of fuel and method of firing, use mechanical draft fans. Forced-draft fans supply large amounts of fresh air for combustion. Induced-draft fans remove combustion products.
A fan moves a quantity of air or gas by adding sufficient energy to the air stream to start motion and overcome resistance to flow. The bladed rotor or impeller does the actual work. The power required depends on the volume of gas moved per unit time, the pressure difference across the fan, and the efficiency of the fan and its drive. The following topics will be covered:
There are two basic types of fans: axial flow and centrifugal. The axial flow fan, shown in Figure 40, moves the gas in a path parallel to the fan rotor. These fans operate most efficiently with a low resistance to flow and provide a high volume of air at low head pressures. Axial fans are normally used as forced-draft fans in a balanced-draft system.
Figure 40: Axial Flow Fan
The centrifugal or radial fan, shown in Figure 41, moves the gas perpendicular to the fan rotor and operates most efficiently in a high head situation. The centrifugal fan is suitable for a forced-draft or pressurized system in which induced draft fans are not.
Figure 41: Centrifugal Radial Fan
The centrifugal radial fan has several advantages over the axial fan. It is cheaper and lighter and, therefore, requires less power. This can be seen on Figure 42. Also, because of its size and weight, it is more easily controlled.
Figure 42: Typical Fan Power Curve
The blades of an axial fan are generally smaller than those of a centrifugal fan and the construction is such that a variable pitch control system can be easily installed. This type of control allows for a rapid change of output and increased efficiency over the centrifugal fan, as shown in Figure 43.
Figure 43: Typical Fan Efficiency Curve
The main disadvantage of an axial fan is that it requires high rotational speeds to generate the required airflow. This can result in noise pollution. However, the primary problem with high speeds is that the fan must be a precision machine, which means that it can fail quickly and catastrophically with little or no warning.
In contrast, the centrifugal radial fan has two primary advantages. It is more durable because of its lower rotational speeds, and it can supply air at high head pressure more efficiently than the radial machine. The axial machine can change the pitch of its rotating or stationary blade, whereas the blades of a centrifugal radial fan are normally fixed. Therefore, a centrifugal radial fans blades establish the fans use and a particular fan is chosen for a specific system.
There are three basic blade shapes used in a fan: a forward curve, a straight blade, and a backward curve. These shapes and the effects on the velocity are shown in Figure 44.
Figure 44: Types of Centrifugal Fan Blades
The straight-bladed fan is generally used for industrial, dust-laden gas flow. This type of fan operates at an efficiency of 50% or less. The forward blade curve is a general-purpose fan best used for medium pressure applications. The fan is fairly quiet during operation because of the low tip speed.
The backward curve blade fan can produce higher discharge pressures than the straight blade fan or the forward curved blade fan. Horsepower requirements are at a maximum at 60% airflow. Above or below 60% airflow, the horsepower requirements are less than maximum.
Very few instances of operations permit fans to operate continuously at the same pressure and volume discharge rates; therefore, to meet the requirements of the system, a convenient means of varying the fan output becomes necessary. Common methods of controlling fan output are damper control, variable speed control, and inlet vane control. In some cases, a combination of controls is used. Damper control provides variable resistance in the system to alter the fan output. However, damper control is inefficient because of the excess pressure energy that must be dissipated by throttling. The advantages to damper control are:
Variable speed control is the most efficient method of controlling fan output since it also reduces power consumption. Speed control results in the same loss in efficiently throughout the entire fan load range. The loss in effectiveness depends on the type of speed variation. Commonly used variable speed systems include magnetic couplings, hydraulic couplings, special mechanical drives, variable speed DC motors, variable speed AC motors, and variable speed steam turbines.
Magnetic couplings consist of two windings in a housing with a variable field. A change in field strength varies the slip and, consequently, the speed of the fan.
A hydraulic coupling varies slip by varying the hydraulic pressure as the speed of the driver changes. The variable pitch V-belt and the variable speed planetary transmission are examples of special mechanical drives.
Two-speed AC motors can be used to supplement damper control. Two-speed AC motors cost less than the variable speed AC drives and improve fan efficiency when coupled with a simple damper control.
Inlet vane control, as shown in Figure 45, regulates airflow entering the fan and requires less horsepower at fractional loads than outlet damper control. The inlet vanes give the air a varying degree of spin in the direction of wheel rotation, enabling the fan to produce the required head at proportionately lower power and, therefore, greater efficiency. Although vane control offers considerable savings in efficiency over damper control at any reduced load, it is most effective for moderate load changes close to full-load operation. Inlet vane control is often used for full load operation and efficiency adjustments.
Inlet vane leakage often makes it difficult to reduce fan airflow at low loads when using a single speed fan drive; therefore, a supplementary damper is used to increase the control range of the vanes. This is especially applicable to forced-draft fans in which a wide load range is required.
Figure 45: Inlet Vanes
Electric motors are normally used for fan drives because they are less expensive and more efficient than any other type of drive. For fans of more than a few horsepower, squirrel-cage induction motors are most common. This type of motor is relatively inexpensive, reliable, and highly efficient over a wide load range. It is frequently used in large sizes with a magnetic or hydraulic coupling for variable speed installations. For some variable speed installations, particularly in the smaller sizes, wound-rotor slip rings induction motors are used. If a DC motor is required, the compound type is usually selected. The steam turbine drive costs more than a squirrel-cage motor but is less expensive than any of the variable speed electric motor arrangements in sizes over 50 horsepower.
Fans are tested by their manufacturers, and the results of the fan operations are presented in characteristic curves. The curves may include the variation in head, capacity, power, and efficiency for a constant speed, or they can be a family of curves for a series of constant speeds. By careful review of the various types of fans and their characteristic curves, the most correct fan for a given system can be selected.
Within a given class or type of fan, there are certain general characteristics that are common to the many different designs. These characteristics are power, pressure, and efficiency. The curves in Figure 46 show the variation in power, pressure, and efficiency for differing capacities at a constant speed for an axial-flow fan. The fairly constant power output over a wide range of capacities is common to most axial-flow fans. Thus, there will be little tendency to overload the driving motor regardless of the change in conditions under which the fan operates. This is called a non-overloading characteristic. The capacity decreases more or less at a constant rate for an increase in resistance or pressure. The efficiency of such a fan is generally somewhat lower than that of centrifugal fans except at low pressure. By varying such things as the pitch diameter and width of the blades, the point of maximum efficiency can be varied to cover a wide range of conditions.
Figure 46: Axial Flow Fan Characteristic Curve
The characteristics of a radial-tip centrifugal fan are shown in Figure 47. The power increases with a decrease in pressure and an increase in capacity, but the increase is not sharp enough to overload the motor if proper selection of the motor is made. Generally, the characteristics of the radial-tip centrifugal fan will be a compromise of the backward-curve-blade and forward-curved-blade fans.
Figure 47: Radial-Tip Blade Fan Characteristic Curve
The backward-curved-blade centrifugal fan will have the characteristics shown in Figure 48. Best efficiencies are obtained with rotors having backward-curved blades, and the power curve for these rotors shows a non-overloading characteristic over the complete range of pressures and capacities. The point of maximum efficiency occurs at the point of maximum power. Above 50% of the maximum capacity, an increase in capacity will decrease the pressure sharply. This fan is excellent for forced-draft service, because, as the fuel bed of a furnace closes and restricts the flow of air from the fan, the fan pressure will rise sharply. This increase in fan pressure will tend to open the fuel bed to admit more air to the furnace.
Figure 48: Backward-Curved-Blade Fan Characteristic Curve
The forward-curved-blade fan has an overloading power characteristic, as shown in Figure 49. If reasonable care is exercised in figuring the conditions under which the fan will operate, a motor can be selected to prevent its overloading. The point of maximum efficiency occurs near the point of maximum pressure.
Figure 49: Forward-Curved-Blade Fan Characteristic Curve
The straight-blade fans discharge pressure rises from full flow to a maximum at no flow, where it falls off, as shown in Figure 50. The maximum efficiency occurs near the fans maximum pressure.
Figure 50: Straight-Bladed Fan Characteristic Curve
Figure 51 represents a modification of a backward-curved-blade fan. The pressure characteristic does not have a steep slope, nor does the horsepower curve have a distinct hump, resulting in maximum efficiency obtainable over a wider operating range.
An air duct is an enclosed conduit through which air is moved from one place to another. In this section, we are going to discuss the equipment that is used, its classification, and duct system accessories.
Air duct design is broken into high and low pressure classifications by the Sheet Metal and Air Conditioning Contractors National Association SMACNA. Most of the low-pressure standards also apply to high-pressure work. High-pressure standards are intended for heavier industrial systems that require additional structural consideration.
Table 2 shows the specific breakdown of the SMACNA pressure-velocity classifications.
The use of the velocity to classify duct construction and design is not common. Velocity classifications are used to describe the system or individual duct run. Traditionally, 2,000 feet per minute is used to separate high and low velocity classifications.
Several accessories are used in the process of distributing the air to make the operation more efficient. These include turning vanes, splitters, dampers, louvers, vents, diffusers, and silencers. Each of these components performs a specified function within HVAC air ducting.
Turbulent airflow increases the amount of friction encountered in the movement of air. By minimizing turbulence, thereby creating laminar flow, the overall efficiency of the HVAC system is increased.
Turbulence occurs when changes in flow direction are encountered. Figure 52 graphically indicates what happens to airflow through a typical elbow. The air entering the elbow is laminar. The portion of the flow that travels along the outside edge of the elbow follows the curve of the outside wall. Air entering on the inside edge continues straight until it runs into the air stream on the outer edge. This collision sets up eddies that increase the friction loss.
Figure 52: Turbulent Airflow in Elbow
To overcome this situation, vanes can be installed to direct the airflow around the elbow. This is shown in Figure 53. The vanes create a series of smaller elbows, which reduce the amount of turbulence.
Figure 53: Reducing Turbulence with Turning Vanes
In some rare instances, splitters can be used to actually establish smaller elbows, as shown in Figure 54. Splitters and vanes are also used to maintain a laminar flow with uniform pressure distribution when a tee or other fitting is near the downstream side of an elbow. This process is shown in Figure 55.
Figure 54: Reducing Turbulence with Splitters
Dampers are used to limit the amount of airflow through a duct or piece of equipment. Three basic types are found in HVAC distribution systems: parallel blade, opposed blade, and pivot blade. Figure 56 shows a schematic representation of each basic type.
Figure 56: Damper Types
Parallel blade dampers normally are used when only full-open or full-shut conditions are required. When parallel blade dampers are in a partially open position, they tend to direct the airflow to one side of the duct, thus causing uneven pressure distribution.
Opposed blade dampers are best suited for situation where the airflow volume is to be regulated. They do not create the uneven airflow that a partially open parallel blade damper does; instead, a mixing or turbulent condition exists.
Pivot or splitter dampers generally are used to direct desired air volume flows at a duct branch, as shown in Figure 57. Pivot dampers are not commonly used due to the force required to move the damper. Parallel and opposed blades have approximately equal forces acting on each side of the rotating axis, as shown in Figure 58.
Figure 57: Pivot Damper at Branch
Figure 58: Forces Acting on Damper Blade
Louvers are similar in appearance to dampers. The blades, however, are fixed. Louvers are installed where intake or exhaust air is vented through the external walls. They may be ducted or simply used for ventilation. Their primary function is to keep rain and snow from entering the building. In addition, louvered openings are often equipped with screens or mesh to prevent insects, birds, animals, and trash from entering the ventilation system.
Figure 59 shows a typical louver configuration.
Figure 59: Typical Louver Configuration
At the terminal ends of an air duct system where the conditioned air is withdrawn from or introduced to the controlled environment, vents are installed to control the distribution and collection of conditioned air. The term "vent" is a general expression for any apparatus that permits transfer of air due to pressure gradient.
In HVAC distribution systems, the term "grille" applies to a flush-mounted grid. A typical grille is shown in Figure 60. When dampers are added to the duct side of a grille, the assembly is known as a register. A typical register is shown in Figure 61. Grilles and registers are used on both supply and return duct systems.
Figure 60: Typical Grille
Figure 61: Typical Register
When the blades of a grille are arranged so the airflow is spread out and distributed into the controlled environment, it is known as a diffuser. Diffusers may be fixed or adjustable blade variety.
Diffusers are only used on the supply side. Figure 62 shows a typical ceiling supply vent with a diffuser and damper.
Figure 62: Ceiling Supply Vent with Diffuser and Damper
Fans and airflow can create unwanted noise that is carried by the HVAC duct system. To eliminate this noise, devices are installed to baffle and adsorb the sound. The most basic of the silencers is an expansion box with the entrance and exit ports skewed, as shown in Figure 63. Other silencing methods include encasing noisy equipment in insulated boxes and lining the inside of the duct with insulation. Noise generators produce sound waves at a reduced pressure variance and, therefore, impede the movement of discernible noise.
Figure 63: Basic Silencer Box
In order for an HVAC system to operate properly, it must be tested and balanced in accordance with proven procedures. This section discusses some of the methods used to properly test and balance HVAC systems and components. The areas that will be covered are:
In this section, we will discuss the various methods of testing the airflow in different types of duct systems and adjustments that can be made to balance the system for proper operation. The topics to be covered include:
In order to balance any HVAC system, the most important thing you need to know is the airflow through that system. The measurement of airflow in ducts and ductwork is a relatively easy procedure as long as you follow a systematic approach. In this section, we will examine airflow measurement in ducts, including:
Airflow measurement in ductwork can be accurately measured with any the following instruments:
As you can see from the list of devices that can be used to measure airflow in a duct, the pilot tube, shown in Figure 64, is commonly used in conjunction with a manometer or similar gauge to provide a simple method of measuring the air velocity in a duct.
The pilot tube is of double tube construction, consisting of an inner tube that is concentrically located inside of the outer tube. The outer "static" tube has eight equally spaced holes around the circumference of the outer tube.
Figure 64: Typical Pilot Tube
Both tubes have a 90-degree radius bend located near the measuring end to allow the open-ended inner "impact" tube to be positioned so that it faces directly into the airstream when the main shaft of the pilot tube is perpendicular to the duct and the side outlet static pressure tube outlet connector is pointed in a parallel direction with airflow facing upstream.
The pilot tube is actually a head-type flow element that measures fluid flow by creating a differential pressure. Figure 65 is an exaggerated view of a pilot tube, showing how it functions inside of the duct. The inner tube is sometimes called the impact tube, or the total pressure tube, while the outer tube is the static tube. As we began to explain, the pilot tube creates a difference of pressure to measure flow or, more directly, fluid velocity. Per Bernoullis equation, the square root of the difference in pressure is proportional to flow. From Figure 65, we can see that the difference between the total pressure Tp and the static pressure Sp is the velocity pressure Vp. The relationship between the pressures is expressed by the equation Tp = Sp + Vp.
Figure 65: Pressure Relationships and the Pilot Tube
The pilot tube is used for the measurement of airstream "total pressure" by connecting the inner tube outlet connector to one side of a manometer, for measurement of airstream "static pressure" by connecting the outer tube-side outlet connector to one side of a manometer, and from measurement of airstream "velocity pressure" by connectors to opposite sides of a manometer or draft gauge. This instrument is commonly used with a draft gauge, manometer, or micromanometer. The pilot tube is a most reliable and rugged instrument and is preferred over any method for the field measurement of air velocity system total air, minimum outdoor air and maximum return air quantities, fan static pressure, fan total pressure, and fan outlet velocity pressures.
Several shapes and sizes of pilot tubes are available for different applications. A reasonably large space is required adjacent to the duct penetration for maneuvering the instrument and care must be taken to avoid pinching the instrument tubing.
The inclined gauge manometer for airflow pressure readings, shown in Figure 66, is usually constructed from a solid transparent block of plastic. It has an inclined scale that expands or lengthens the scale for a given amount of fluid displacement, increasing resolution and allowing more accurate air pressure readings from 0 to 1.0 inch of water gauge in. w.g. and a vertical scale for reading greater pressures.
Figure 66: Inclined-Vertical Manometer
All air pressures are given in "inches of water," which means that the air pressure on one end of a U-shaped tube is enough to force the water higher in the other leg of the tube. Instead of water, this instrument uses colored oil that is lighter than water. Although the scale reads in inches of water, it is longer than a standard rule. Whenever a manometer is used, the oil must be at room temperature, or the reading will not be correct. The manometer must be set level and mounted so it does not vibrate.
The inclined gauge manometer or inclined draft gauge is the standard in the industry. It can be read accurately down to approximately 0.002 in w.g. and contains no mechanical linkage. It is simple to adjust by setting the piston at the bottom until the meniscus of the oil is on the zero line. This instrument is used with a pilot tube or static probe to determine pressure or air velocity in a duct.
The magnehelic gauge, which is shown in Figure 67, is an easy-to-use pressure gauge designed for air system work. Magnehelic pressure gauges come in a wide variety of pressure ranges. Two different ranges 0 to 0.5 in. w.g. and 0 to 1.0 in. w.g. are the most commonly used. Readings should always be made in the mid-range of the scale, and the instrument should be held in the same position as when "zeroed."
Figure 67: Magnehelic Gauge
Like the previously described inclined gauge manometer, the magnehelic gauge is connected to the duct in which the airflow is being measured using a pilot tube. The "high" pressure connection is used relative to the atmosphere for reading positive pressures and the "low" pressure connection is used for negative pressures. By using both, it is possible to measure a pressure drop or rise across components in HVAC systems.
The operation of the thermo-anemometer thermo being "hot wire" depends on the fact that the resistance of a heated wire will change with its temperature. The probe of this instrument is provided with a special type of wire element that is supplied with current from batteries contained in the instrument case. As air flows over the sensing element in the probe, the temperature of the element is changed from that which exists in still air, and the resistance change is indicated as a velocity on the indicating scale of the instrument. Figure 68 shows a picture of a typical thermo-anemometer. One important difference between this type of flow measuring device and the others discussed here is that the thermo-anemometer does not rely on a pilot tube to detect the airflow. This allows the thermo-anemometer to be used in locations where a pilot tube cannot be easily inserted into the ductwork.
Figure 68: Thermo-Anemometer
If the velocity of the airstream under measurement were uniform, one reading at any point would be sufficient. However, the air moving along a duct wall loses speed because of friction; consequently, the velocity in the center of the duct will always be greater assuming no special turbulent motion. Since the velocity pressure is seldom uniform, a series of readings must be taken across the duct section called a duct traverse.
Various industry groups have determined an accurate way to take uniform measurements in round ducts tangential traverse and square ducts. While this methodology is described in detail in the following paragraphs and figures, there are some general points to remember when performing a pilot tube traverse. These general points are:
Velocity pressure readings are taken at equal intervals over a cross section of the duct. Good practice dictates that no less than 16 readings should be taken in any duct. In larger ducts, readings should be taken on not less than 6 inch centers. The velocity pressures are then changed to velocity values, added together, and divided by the total number of readings to get the average velocity. Do not average the velocity pressure readings. It is not unusual to make a negative pressure reading in ducts with considerable turbulence.
The negative readings are added in as a zero value but are counted in the number of readings to obtain the average velocity. For example, assume that while taking pressure readings in a duct, there are 16 positive pressure readings and 4 negative pressure readings. The value of the 16 positive pressure readings would be added together with the zero value of the 4 negative readings and that sum 16 positive values plus 4 zero values would be divided by the total number of readings 20 to find the average pressure in the duct averaged by all 20 readings 16 positive plus 4 zero readings.
For round ducts, the tangential method is the most common traverse. The duct is divided into N zones of equal area by concentric circles of radii, R1, R2, and R3, etc., as shown in Figure 69. A series of ten readings is then taken along the horizontal axis, and ten readings are taken along the vertical axis. One practical aspect to be considered is how do you know where the pilot tube is inside the duct? As Figure 69 shows, reading positions are calculated from the center of the duct, as some position multiplier times the radius of the duct. This is fairly common practice in the industry.
Figure 69: Tangential Pilot Tube Traverse
Table 3 shows the calculated distance from the inside wall to the pilot tube reading point for several duct sizes. Figure 70 shows a pilot tube marked for a 20" diameter duct traverse, as per Table 3. The pilot tube should be marked carefully with a China marking pencil or small strips of duct tape to facilitate accurate placement of the tube for traverse readings.
Figure 70: Marking the Pilot Tube
For square or rectangular ducts, traverse holes are drilled in one wall of the duct in such a manner as to establish equal areas, as shown in Figure 71, for a 48 inch by 36 inch rectangular duct. Note that no reading location is more than six inches from another. The 48" x 36" duct in Figure 71 would be measured at 48 equally spaced stations, and the Pilot tube would be marked accordingly to facilitate those readings. As with circular ducts, tables are available for square/rectangular ducts that eliminate the need to calculate the reading position. All velocity pressure readings would be recorded and transferred into velocity values, added together, and averaged.
Figure 71: Rectangular Duct Traverse
Diffusers are used where ducting delivers conditioned air to spread the flow of air out into the room in a widespread pattern. Because they spread the airflow out over a wide area, it takes different techniques and instruments to measure the flow of air at a diffuser than it does in a duct. In this section, we will examine airflow measurement at diffusers, including:
Air flows from diffusers should be measured with one of the following types of instruments:
Instead of depending on a swinging vane to deflect and indicate a reading, the Alnor 6000P Velometer, which is shown in Figure 72, operates on the pilot tube principle; pressure exerted on a vane that is free to travel in a circular tunnel moves the vane and causes a pointer to indicate the measured value on a scale.
It is not dependent on air density because of the sensing of pressure differential to indicate velocities. Note that the instrument is provided and always used with a dual-hose connection between the meter and the probes, except as noted below.
The Model 6000AP set is an all purpose set that adequately meets the needs of total air balance TAB work. Most major air distribution device manufacturers have set up area factors based on its use. The velometer consists basically of the meter, measuring probes, range selectors, and connecting hoses. The meter is scaled through the following velocity ranges: 0-300, 0-1250, 0-2500, 0-5000, and 0-10,000 fpm.
Figure 72: Velometer Set
Three velocity probes are provided: the low-flow probe, the diffuser probe, and the pilot tube. The low-flow probe is used in conjunction with the 0 to 300 cfm scale for measuring terminal air velocities in rooms or open spaces, and to measure face velocities at ventilating hoods, spray booths, and fume hoods. The low-flow probe is directly mounted to the meter without the use of hoses. The diffuser probe is designed to measure the velocity at diffusers, registers, and grilles. The volume of air being supplied or exhausted can be determined using the following formula:
Air distribution devices, such as diffusers and grilles, cannot be measured without a K factor or flow factor, because the manufacturer must test each outlet along with a particular instrument and designate the precise points on the diffuser where the instrument probe must be placed. The technician must select the K factor for each diffuser type and size from the manufacturers specification sheet. The pilot tube is used to measure velocities in ducts and at return air or exhaust air grilles. The low-flow and diffuser probes are shown in Figure 73.
Figure 73: Velometer Attachments
The FlowRite anemometer, shown in Figure 74, is direct reading; that is, it does not depend on a time interval. It measures velocity pressure and displays velocity fpm on the gauge. The Flo-Rite anemometer may be used in the same manner as the rotating vane anemometer except that discreet velocity points are best, such as 8" x 9" grids over a particular area, reading the velocities at each point. This will be very much like a velocity profile reading obtained by a pilot traverse. To take an approximate reading for a rough balance, a traverse is made in a given time period for the area by moving the anemometer around and visually averaging the velocities present. This is an inaccurate method, but it is fast and it will put you in the "ball park."
Figure 74: FlowRite Anemometer
Each diffuser tested should be marked at locations of readings on the face or vane. The velocity meter inlet jet should be placed in the vena contracta of the face vanes of the diffuser. A minimum of six readings should be made to determine average velocity in feet per minute. All future readings and check readings should be made at the marked locations of each diffuser.
Diffusers are designed to spread the flow of air out into a room or area. Unfortunately, the instruments used to measure this airflow are designed to work with a concentrated airflow, and if they were used to measure the flow of air out of a diffuser, the readings would be quite inaccurate. Fortunately, the flow of air coming out of a diffuser can be easily concentrated for measurement with the help of a simple device known as a "flow measuring hood," or simply a "hood."
The conical or pyramid-shaped hood also called a balancing cone can be used to collect all of the air discharged from an air terminal and guide it over the flow-measuring instrumentation. Hoods generally are constructed so that the outlet tapers down to an area of one square foot. An anemometer velometer tip is installed in the neck to read cfm directly, regardless of the airflow quantity measured.
The balancing cone should be tailored for the particular job. To keep weight to a minimum, aluminum is normally used. The large end of the cone should be sized to fit over the complete diffuser and should have a sponge rubber seal to eliminate leakage and to avoid ceiling marks. When balancing a large number of ceiling diffusers of common size, a hood may permit reading from the floor and eliminate the need for a ladder as does the commercially made hood shown in Figure 75.
Figure 75: Flow Measuring Hood
The disadvantages of using a hood are as follows:
When a hood is applied to an outlet, a certain amount of backpressure is introduced against the flow of air. Since the exact shape of these hoods follows a carefully calculated design, you can determine just exactly how much backpressure can be expected from each of the various sizes so that correction factors can be applied to the results. For example, a 24" x 24" hood funnels down to a 12" x 12" discharge opening at the bottom. Obviously, this creates a 1 square foot free area where the velocity measurements are read. However, the lab tells us that a 1/4" w.g. is created by this particular size hood; therefore, using 1.00 sq ft times the velocity will not give an accurate reading of what the diffuser would be handling without the hoods imposed backpressure. Consequently, the correction factor of 1.25 is applied to the actual free area of the discharge end of the hood, increasing it from 1.00 to 1.25 square feet.
It has been determined that when a hood is applied to any given outlet, the backpressure created by the hood causes the air to back up slightly. This forces more air to be discharged from some other outlet, either in the same zone or at the point of least resistance. This can be proven by testing two outlets individually on the same branch duct and comparing the added total against a traverse reading of the branch duct upstream from both outlets. Without applying the correction factor as outlined above, the reading of the traverse will be greater than the total of the two outlet readings.
Supply grilles and registers are used to deliver concentrated streams of conditioned air into a room. In this section, we will examine airflow measurement at supply grilles and registers, including:
The flow of air from a supply grill or register can easily be measured with one of the following type instruments:
Only the four-inch vane-type anemometer will be described since both the Bacharach Flow-Rite meter and the Alolar Velometer were previously described.
The propeller or rotating vane anemometer consists of a lightweight, wind-driven wheel connected through a gear train to a set of recording dials that read the linear feet of air passing through the wheel in a measured length of time. The instrument is made in various sizes: 3", 4", and 6" sizes being the most common. Figure 76 shows a picture of a rotating vane-type anemometer.
At low velocities, the friction drag of the mechanism is considerable. In order to compensate for this, a gear train that over-speed is commonly used. For this reason, a correction factor or calibration curve must be used and the correction is often additive at the lower range and subtractive at the upper range, with the least correction in the middle of the range. Most of these instruments are not sensitive enough for use below 200 fpm. Their useful range is from 200 to 2,000 fpm.
The instrument reads in feet, so a timing instrument must be used to determine velocity. A stop watch should be used to measure the timed interval, although a wristwatch with a sweep-second hand may give satisfactory results for rough field checks.
Figure 76: Rotating Vane Anemometer
It has been found that a two-minute timed traverse gives better averaging accuracy across the coil face or return air grille than the one-minute pass recommended by some industry groups. It is recommended that two or more traverses be made across the air stream and then averaged.
In the case of coils or filters, an uneven airflow is frequently found because of entrance or exit conditions. The Sheet Metal and Air Conditioning Contractors National Association SMACNA recommends that this variation be taken into account by moving the instrument in a fixed pattern to cover the entire amount of time over all parts of the area being measured so that the varying velocities can be averaged.
In practice it is quite difficult to end the pattern at precisely the proper time. The SMACNA recommends that the area be traversed horizontally, then vertically, and then end with an "x-type" pattern, so that if time runs out and only one bar of the "x" has been completed, it will still be a satisfactory ending point.
The Associated Air Balance Council AABC recommends a different approach to obtain accurate anemometer readings. They recommend that the anemometer be held steady in the air stream for a given period of time. The average anemometer reading should be determined by marking the grille off in sections, taking a reading in front of each section, and averaging the results. A true average reading cannot be obtained by moving the anemometer back and forth across the face because, if the instrument happens to pass over a dead spot or a section where the velocity is low after having passed over one where it is high, the blades are likely to coast over the low section.
The anemometer or Bacharach flowmeter or velometer should be used to determine the flow through a return intake by marking off the face of the intake into sections as was done with supply grilles. The procedure for testing returns and intakes is similar to the testing of supply outlets with the exception that the effective area factor is not considered.
The inclined gauge or calibrated magnehelic gauge should be used in conjunction with a static pressure tip to measure static pressure in ductwork, plenum chambers, across filters, or across coils. Insertion of the tube end or the use of suction cups is not acceptable. Probes should be made in areas considered to have a stabilized pressure. Preferably two or more readings should be taken.
Each system that uses hot and cold mixing should be subject to the test so that a leakage factor can be determined. A temperature sampling should be made in the cold supply duct and in the main hot supply duct. With the room thermostat calling for full cooling, air temperature should be read at the outlet and compared to the temperature of the cold duct air. The same procedure should be followed for testing of the hot supply duct but with the use of full heating. Normal leakage factor should not exceed 5%.
The setting and testing of static pressure dampers should be accomplished in the following manner: during the balancing of the system, the cold static dampers should be held at a maximum open position of 90% with the system calling for full cooling and with the hot dampers in the closed position. For the balancing of the hot side, the same positioning should be made with the hot damper on call for full heating. The final settings of dampers should be made by reading the static pressure required at the sensing tip when the air column at all terminals is as specified on a call for full cooling. This procedure should be reversed for the heating side. Arbitrary settings at gauges should be avoided. This may involve a great deal of checking as the point of least static pressure is generally, but most certainly not always, at the end of the system.
The measurement of coil face velocities should be made using a 4 inch vane anemometer. The test engineer must attach a long handle to the instrument and avoid blocking any airflow motion. Continuous movement across the face of the coil should be avoided. Individual spot readings at set intervals should be made to establish averages. Coil face velocities at best are not very reliable and should not be used as a method of establishing total air except when it is the only available method.
To ensure that you make a completely accurate assessment of any system when testing it, the system should be operated under a standard set of conditions. The conditions under which a system should be operated during a test include the following:
Final balanced conditions must include the setting of outside air quantities and return air quantities. Setting of outside air quantities must be made by adjustment of dampers using direct airflow readings or by temperature methods.
Where possible a duct traverse should be taken to establish total outside supply air, or a 4-inch vane anemometer across the outside air intake may be used. The temperature percentage method of calculation may be used whenever conditions of duct work or installation indicate improper readings or erratic readings at louvre face.
Refer to Figure 77 for the following discussion. When air is delivered into the conditioned area through an acoustical ceiling, balancing the air distribution system cannot end at the duct outlet into the ceiling plenum. It must continue through the testing and adjusting of the plenum and the ceiling tiles themselves.
The ceiling plenum supply system is essentially a sealed area or box into which air is discharged at the required cfm with a predetermined pressure established to force the air from this sealed area through the ceiling, either by means of perforations in the tile itself or slotted runners that hold the tile in place.
Figure 77: Pattern of Air Distribution Through Plenum Ceiling
There are various types of ceilings on the market, but the principle of air supply is essentially the same in all of them. Typical design of such a system consists of a standard air handling unit or supply fan with a supply air duct running to the various sealed plenum areas or boxes over the space to be conditioned. Supply ducts terminate at the plenums, discharging the air into these sealed areas above the ceilings.
There are four critical factors that must be carefully checked in balancing this type of system:
Here is a six-step balancing procedure that, if followed, will enable the technician to ensure the satisfaction of these four major factors and to provide comfort conditions in the areas served:
Making these checks effective requires proper instrumentation and well-trained and experienced engineers.
Information provided by manufacturers of ceiling tile indicate that 30 to 45 fpm is an acceptable velocity at the comfort level, but experience has shown that 60 to 65 fpm is a more realistic figure.
The ceiling plenum system must supply essentially the same type of airflow as a diffuser or grille. As in the case of diffusers, the ceiling plenum outlets must aspirate enough room air to force gentle air motion throughout the occupant zone, otherwise stagnation and stratification will result. When balancing this type of air distribution, it is important to see that the proper airflow velocities are maintained at the proper levels.
If the airflow pattern of the ceiling plenum system is directed horizontally, the throw must be sufficient to produce satisfactory conditions. Under-blowing during cooling can cause the same areas to be too cool and others too warm. Over-blowing can result in objectionable down drafts from any surface the air stream may strike.
Remember that static pressure plays an important part in the general distribution of air throughout a large plenum delivery ceiling. Air flowing through the ceiling system possesses energy that is imparted to it by the fan. When the delivery is confined inside the supply duct, this energy is converted to static pressure. As the static pressure inside the delivery duct becomes higher than that of the plenum, the air is forced from the duct into the plenum and then through the ceiling into the conditioned space.
Sufficient static pressure must be maintained uniformly throughout the ceiling plenum to allow for even distribution. Barriers and seals must be inspected carefully for leakage, for each leakage can account for a considerable loss of static pressure in the air system.
With a ceiling plenum system, as with conventional air delivery, the technician must provide greater air flows across window areas or outside walls. Final balancing of ceiling tiles or slots should provide the air quantities required for this purpose.
A balanced hood and enclosure requires that the airflow through the opening and enclosure itself be such that full protection is maintained without interfering with the experiment or the personnel carrying on the experiment. Minimum airflow conditions that will furnish this protection, yet not waste conditioned or heated air, are also a requirement of good balanced conditions.
Two types of hoods are now generally in use. Type one introduces a source of make-up air and uses a very minimum amount, if any, of the surrounding conditioned air. The second type uses all surrounding air. In either case, the air that flows through the enclosure must use an exhaust fan system to move this contaminated air to the out-of-doors.
To achieve good balance in either type of system, the exhaust fan and supply air must be adjusted to accurate and correct amounts.
In general, the following recommended face velocities should be applied under balanced condition across the hood face:
Four separate tests must be made to properly balance fume hood systems:
These four tests must be performed to provide a satisfactorily balanced condition. The following test standards and test methods can be used to accomplish these conditions:
Figure 78 shows the proper positions of readings to determine exhaust fan readings.
The general use of 100 fpm velocities would apply to most applications and provide sufficient airflow to satisfy laboratory conditions. It should be remembered that when the designer includes exhaust hoods in air-conditioned areas without self-contained makeup air, sufficient outside air should be introduced by the air conditioning system as makeup air. Additional tonnage capacity for cooling conditions must be added, and additional heating capacity must be included for the use of this type of system.
Figure 78: Fume Hood Schematic
When hood enclosures are banked together in a given area, particular attention must be given to the placement of air conditioning outlets. Placing these outlets close to the hoods will cause excessive draft conditions and in some cases, short circuiting of conditioned air. Optimum application is to place air outlets on opposite ends of room areas allowing hoods to draw this air across the room area. When hoods have a self-contained air supply, attention to the above details may not be needed. Because of the potential health hazards of improper hood operations, hood operating tests should be performed at least once a year. Inspection and test results certificates should be placed in a conspicuous location by the testing agency.
To prevent the occurrence of leakage problems, the consulting engineer should include a duct leakage test section in the specification that would include a verification procedure and certification of tightness.
In most cases, it is not practicable for the test and balance agency to perform these tests as it would require having a test technician continuously on the job as the ductwork is installed. From the standpoint of economy, it is more practicable to have the contractor conduct tests in accordance with AABC test standards and have the agency verify the results obtained and issue a certificate. The degree of air tightness in high-velocity ductwork should not be compared with a water distribution system or gas system. Some degree of leakage will exist regardless of all the precautions taken during fabrication and installation; however, this leakage should be minimized to a degree that will not cause excessive problems.
Duct tightness can be determined by the application of proper pressure testing. When not otherwise specified, 1% of the system air volume at one and a half times the duct operating pressure is considered reasonable leakage.
In order to test sections of ductwork as it is installed, a portable means of testing is required. The most practical type of test apparatus that will facilitate field testing should consist of the following:
These items should be assembled into a portable device, as shown in Figure 79.
Figure 79: Portable Test Apparatus
Figure 80: Variation of Airflow Rate with Orifice Differential Pressure
The air conditioning installation or sheet metal contractor should engage the services of a Certified AABC Test Agency to verify results and submit a certification certificate attesting to the results obtained.
Tested sections of the ductwork should be visually marked by the agency with a certification sticker and initials of the field test inspector. Tests should be made before duct sections are concealed.
Balancing means sizing the ducts and adjusting the dampers to ensure that each room receives the correct amount of air.
Conditioned air must be fed in the right amounts to each different room of a multiple-room system, and the correct amount of air must be returned. If the system is not balanced, rooms will maintain different temperatures; some ducts will be noisy, some will have incorrect humidity, and some will have stale air.
To total air balance TAB a system, the air velocity leaving each grille must be measured and the "free" area of the grille or diffuser must be determined. The "free" area is the actual size of the air openings.
To balance a system, complete the following steps:
In some cases, it may be necessary to overcome excess duct resistance by installing an air duct booster. These are fans used to increase airflow when a duct is too small, too long, or has too many elbows.
An effective but simpler technique can be used to balance airflow to the different rooms.
Figure 81: Mounting Thermometer in Folded Cardboard
Remember, if one room is too warm, one or more other rooms should be too cool. Close the damper to the warm room a little, and open dampers a little to the room or rooms that are too cool.
A schematic piping diagram should be made by the technician showing all locations of major components, and flows required should be marked at all locations.
The entire system must be cleaned by the installing contractor prior to the start of balancing.
The following items must be checked before the start of balancing:
The following balancing procedures are basic to all types of hydronic distribution systems:
If the total head is higher than the design total head, the waterflow will be lower than designed. If the total head is less than design, waterflow will be greater; in which case, the pump discharge pressure should be increased by partially closing the balancing cock until the system waterflow is approximately 110% of design. Record the pressures and the waterflow. Check pump motor voltage and amperage, then record. This data should still be within the motor nameplate ratings. Start any secondary system pumps and readjust the balancing cock in the primary circuit pump discharge piping if necessary. Again, record all readings.
NOTE:A heat balance should be run and reported on all main coils. It is well to remember that pump heads, orifice drops, etc., are a means of checking and balancing in an attempt to obtain a given result. The heat balance is the result.
Refer to Figure 82 for the following procedure:
Figure 82: Composite Coil Diagram
Sometimes, an HVAC system will not operate as designed; this may come about due to design problems, component fault, or degradation over a long period of time. It is the responsibility of those who are in charge of maintenance of the HVAC system to see that the maintenance and troubleshooting are done correctly.
When troubleshooting, a careful, systematic approach should be employed to resolve a malfunction. The objective of troubleshooting should be to solve the problem and not just fix the symptom. There is a difference between the two, even though the initial results may seem the same. The act of solving problems involves proper diagnosis of the root cause, correction of the cause, and then to make repairs or replacements of damaged or faulty components. Any fault can be temporarily fixed by replacing the faulty parts with operational ones. This fix is temporary; the part will eventually fail again, resulting in unplanned downtime and increased service cost.
As an example of the difference between fixing and solving a problem, consider a fan motor that faults periodically due to thermal overload devices tripping. The fix is simply to reset the overload or replace it with a higher-rated unit. The solution to the problem involves examination of the motor and its mechanical load to identify and correct the occasional overload condition. The overload devices should first be examined to see whether they were properly selected for the application.
In order to keep the fan in operation, the problem may require temporary fixing, but unless the problem is actually solved it will reoccur. In this section, we will discuss the following topics to get an overview of proper HVAC maintenance and troubleshooting. The topics are:
Troubleshooting techniques were chosen to give basic methods of attacking a problem. The other topics were chosen because they are usually involved with HVAC system problems.
How well a fan performs depends on a few factors such as size, speed, and design of the system. Because the fan plays such a big part in the system, it is necessary for an HVAC technician to understand what common problems occur and how to troubleshoot them. The topics we will discuss are:
The most common fan problem is noise. The experienced technician can usually trace the source of mechanical or vibrational noise quickly.
A stethoscope should be the only instrument needed. If the problem cannot be isolated, the possibility of surge, resonance, or unbalance must then be checked in that sequence.
Surge is a common condition that occurs when a fan is operating somewhere in the unstable area to the left of its fan curve. It is a static pressure problem and must be relieved accordingly by reducing the system static pressure. Duct blockage, jammed fire dampers, plugged filters, or improper fan selection can be the cause. A discharge-to-suction bypass may be the solution. Reducing the fan speed will not eliminate surge.
Resonance is usually caused by transmitted vibration to an apparatus panel. Changing the fan speed by more than plus or minus 10% will eliminate the resonance. A correction or alteration can then be made by damping, absorbing, or stiffening the faulty panel.
If surge or resonance is not found to be the source of noise, the problem may be unbalance. This can be a more serious problem, and the fan must be balanced. The acceptable instrument for checking unbalance is an electronic vibration analyzer.
Another frequent cause of unstable fan operation is fully depressed scroll volume dampers in their shipping position. Check the scroll volume dampers before proceeding with testing.
There are two conditions that may cause poor fan performance: fan inlet restriction and fan inlet spin. As a result of job conditions or poor system engineering, a fan may be located too close to a wall, causing a serious performance reduction. When a fan inlet is closer than one wheel diameter from an adjoining wall, it is suggested that the fan be checked against its performance curve. Fan performance may also be seriously impeded by a drive guard restriction. Belt and drive guards should be kept as far as possible from the inlet opening and, wherever possible, fabricated from expanded metal mesh rather than solid sheet steel. Be alert for fan inlet restrictions.
Inlet spin results from a poor inlet connection and is probably most frequently the cause of poor fan performance. When air is introduced into a fan plenum, it should be directed at the center line of the fan inlet. If the inlet air velocity spin is imparted in the direction of wheel rotation, the fan volume, pressure, and horsepower are lowered. If the air spin is opposite to the wheel rotation, the volume, static pressure, and horsepower will be greater than expected. Either spin situation will cause a reduction in fan efficiency.
It is common to find that the designer has failed to consider all sides of the fan inlet connection. If straight airflow is imparted at the inlet connection, the inlet tangential velocity will be zero; any other angle of velocity at the inlet will adversely affect the fan performance.
By inserting a pilot tube through the flexible connections at the fan inlet, it is easy to demonstrate the condition of spin. With the pilot tube connected to a draft gauge or inclined manometer reading velocity pressure, probe the connection carefully, holding the pilot tube parallel to the fan shaft, as shown in Figure 83.
Figure 83: Probing For Spin
Eccentric flow will be indicated by higher readings on the top, bottom, or side of the connection. The angle of airflow will be indicated by slowly oscillating the pilot tube back and forth. By carefully observing the pressure readings as the pilot tube is slowly twisted back and forth, it is possible to determine the angle of spin. To figure the loss in capacity equivalent to the angle of spin, the technician may consult a reliable inlet damper characteristic curve.
Table 4 is a troubleshooting chart for specific problems.
No system should be tested without first checking the fan rotation. Fan rotation, as defined by the fan manufacturer, is either the clockwise or counter-clockwise spin of the fan impeller. However, the rotation depends upon the position of the viewer relative to the fan. When checking rotation for a centrifugal fan, the fan must be viewed from the drive side. For tubular centrifugals, the fan must be viewed from the outlet side. For axial fans, the fans must be viewed from the inlet side. Figure 84 and Figure 85 show the correct rotation for centrifugal and axial fan impellers.
Figure 84: Centrifugal Fan Impellers
Figure 85: Axial Fan Impellers
Regardless of the type of system, there will be some common operating problems encountered, and the technician must be able to recognize the symptoms, diagnose the cause, and take corrective action. The technician may have to arrive at a diagnosis through the process of elimination of several possible causes of the problem in the refrigeration system.
Technicians must be conscientious while performing their jobs of keeping refrigeration systems in proper operating condition. There have been complaints in this industry that some service provided does not always measure up to the highest standard. For example, adding refrigerant to a system when none is required and then finding that this action did not correct the problem, the refrigerant was not removed. The excess refrigerant becomes the cause of a future service complaint. Some manufacturers of components such as expansion valves have had parts returned that were not defective. The strainers in the valves were merely dirty and clogged. The technician involved had replaced the valve, blaming the system problem on its faulty operation.
In small refrigeration units, major problems that occur are:
Many other electrical problems may occur and should be diagnosed on case-by-case basis.
Three main conditions in operating units that are not cooling satisfactorily are:
It is recommended that, if possible, the technician diagnose this problem without entering the sealed system. Some of the causes of these problems are listed below. They can be diagnosed by using gauges.