ASHRAE HVAC SYSTEMS AND EQUIPMENT IP CH 50-2012 ROOM AIR CONDITIONERS AND PACKAGED TERMINAL AIR CONDITIONERS.pdf

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1、50.1CHAPTER 50ROOM AIR CONDITIONERS AND PACKAGED TERMINAL AIR CONDITIONERSROOM AIR CONDITIONERS . 50.1Sizes and Classifications 50.1Design 50.1Performance Data 50.2Special Features. 50.3Safety Codes and Standards. 50.4Installation and Service . 50.4PACKAGED TERMINAL AIR CONDITIONERS. 50.5Sizes and C

2、lassifications 50.5General Design Considerations. 50.6Design of PTAC/PTHP Components 50.6Heat Pump Operation 50.7Performance and Safety Testing. 50.7ROOM AIR CONDITIONERSOOM air conditioners are encased assemblies designed prima-R rily for mounting in a window or through a wall. They aredesigned to

3、deliver cool or warm conditioned air to the room, eitherwithout ducts or with very short ducts (up to a maximum of about48 in.). Each unit includes a prime source of refrigeration anddehumidification and a means for circulating and filtering air; itmay also include a means for ventilating and/or exh

4、austing andheating.The basic function of a room air conditioner is to provide com-fort by cooling, dehumidifying, filtering or cleaning, and circulatingthe room air. It may also provide ventilation by introducing outdoorair into the room and/or exhausting room air to the outdoors. Roomtemperature ma

5、y be controlled by an integral thermostat. The con-ditioner may provide heating by heat pump operation, electric resis-tance elements, or a combination of the two.Figure 1 shows a typical room air conditioner in cooling mode.Warm room air passes over the cooling coil and gives up sensibleand latent

6、heat. The conditioned air is then recirculated in the roomby a fan or blower.Heat from the warm room air vaporizes the cold (low-pressure)liquid refrigerant flowing through the evaporator. The vapor thencarries the heat to the compressor, which compresses the vapor andincreases its temperature above

7、 that of the outdoor air. In the con-denser, the hot (high-pressure) refrigerant vapor liquefies, giving upthe heat from the room air to outdoor air. Next, the high-pressure liq-uid refrigerant passes through a restrictor, which reduces its pres-sure and temperature. The cold (low-pressure) liquid r

8、efrigerantthen enters the evaporator to repeat the refrigeration cycle.SIZES AND CLASSIFICATIONSRoom air conditioners have line cords, which may be pluggedinto standard or special electric circuits. Most units in the UnitedStates are designed to operate at 115, 208, or 230 V; single-phase;50 or 60 H

9、z power. Some units are rated at 265 V or 277 V, for whichthe chassis or chassis assembly must provide permanent electricalconnection. The maximum amperage of 115 V units is generally12 A, which is the maximum current permitted by NFPA Standard70 the National Electrical Code(NEC) for a single-outlet

10、, 15 Acircuit. Models designed for countries other than the United Statesare generally for 50 or 60 Hz systems, with typical design voltageranges of 100 to 120 and 200 to 240 V, single-phase. Popular 115 V models have capacities in the range of 5000 to8000 Btu/h, and are typically used in single-roo

11、m applications.Larger-capacity 115 V units are in the 12,000 to 15,000 Btu/h range.Capacities for 230, 208, or 230/208 V units range from 8000 to36,000 Btu/h. These higher-voltage units are typically used inmultiple-room installations.Heat pump models are also available, usually for 208 or 230 Vappl

12、ications. These units are generally designed for reversed-refrigerant-cycle operation as the normal means of supplying heat,but may incorporate electrical-resistance heat either to supplementheat pump capacity or to provide the total heating capacity whenoutdoor temperatures drop below a set value.A

13、nother type of heating model incorporates electrical heatingelements in regular cooling units so that heating is providedentirely by electrical resistance heat.DESIGNRoom air conditioner design is usually based on one or more ofthe following criteria, any one of which automatically constrains theove

14、rall system design:Lowest initial costLowest operating cost (highest efficiency)Energy-efficiency ratio (EER) or coefficient of performance(COP), as legislated by governmentLow sound levelChassis sizeUnusual chassis shape (e.g., minimal depth or height)The preparation of this chapter is assigned to

15、TC 8.11, Unitary and RoomAir Conditioners and Heat Pumps.Fig. 1 Schematic View of Typical Room Air Conditioner50.2 2012 ASHRAE HandbookHVAC Systems and Equipment Amperage limitation (e.g., 7.5 A, 12 A)WeightNote that one of the integral characteristics of an air conditioneris its dehumidification ca

16、pability. Although this is not usually adesign or selection criterion for the unit, maintaining proper humid-ity levels while drawing less power is a desirable feature, especiallyin hot and humid areas (e.g., New Orleans).The following combinations illustrate the effect of an initialdesign parameter

17、 on the various components:Low Initial Cost. High airflow with minimum heat exchangersurface keeps the initial cost of a unit low. These units have a low-cost compressor, which is selected by analyzing various compres-sor and coil combinations and choosing the one that both achievesoptimum performan

18、ce and passes all tests required by Underwrit-ers Laboratories (UL), the Association of Home Appliance Man-ufacturers (AHAM), and others. For example, a high-capacitycompressor might be selected to meet the capacity requirementwith a minimum heat transfer surface, but frost tests under maxi-mum load

19、 may not be acceptable. These tests set the upper andlower limits of acceptability when low initial cost is the prime con-sideration.Low Operating Cost. Large heat exchanger surfaces keep oper-ating cost low. A compressor with a low compression ratio operatesat low head pressure and high suction pre

20、ssure, which results in ahigh EER.CompressorsRoom air conditioner compressors range in capacity from about4000 to 34,000 Btu/h. Design data are available from compressormanufacturers at the following standard rating conditions:Compressor manufacturers offer complete performance curvesat various evap

21、orating and condensing temperatures to aid in selec-tion for a given design specification.Evaporator and Condenser CoilsThese coils are generally tube-and-plate-fin, tube-and-louvered-fin, tube-and-spine-fin, microchannel-tube-and-flat-folded-fins, ormicrochannel-tube-and-louvered-folded-fin. Inform

22、ation on coilperformance is available from suppliers, and original equipmentmanufacturers usually develop data for their own coils. Designparameters to consider when selecting coils are (1) cooling rate perunit area of coil surface (Btu/hft2), (2) dry-bulb temperature andmoisture content of entering

23、 air, (3) air-side friction loss, (4) internalrefrigerant pressure drop, (5) coil surface temperature, (6) airflow,(7) air velocity, and (8) sensible-to-latent heat factor of the coil. SeeChapter 23 for more information on air-cooling and dehumidifyingcoils.Restrictor Application and SizingThree mai

24、n types of restrictor devices are available to thedesigner: (1) a thermostatic expansion valve, which maintains aconstant amount of superheat at a point near the outlet of the evap-orator; (2) an automatic expansion valve, which maintains a con-stant suction pressure; and (3) a restrictor tube (capi

25、llary). Thecapillary is the most popular device for room air conditioner appli-cations because of its low cost and high reliability, even though itsrefrigerant control over a wide range of ambient temperatures is notoptimal. A recommended procedure for optimizing charge balance,condenser subcooling,

26、 and restrictor sizing is as follows:1. Use an adjustable restrictor (e.g., a needle valve), so that testsmay be run with a flooded evaporator coil and various refrigerantcharges to determine the optimum point of system operation.2. Reset the adjustable restrictor to the optimum setting, remove itfr

27、om the unit, and measure flow pressure with a flow comparatorsimilar to that described in ASHRAE Standard 28.3. Install a restrictor tube with the same flow rate as the adjustablerestrictor. Usually, restrictor tubes are selected on the basis ofcost, with shorter tubes generally being less expensive

28、.Fan Motor and Air Impeller SelectionThe two types of motors generally used on room air condition-ers are the (1) low-efficiency, shaded-pole type; and (2) more effi-cient, permanent split-capacitor type, which requires using a runcapacitor. Air impellers are usually of two types: (1) forward-curved

29、 blower wheel and (2) axial- or radial-flow fan blade. Ingeneral, blower wheels are used to move small to moderateamounts of air in high-resistance systems, and fan blades movemoderate to high air volumes in low-resistance applications.Blower wheels and cross-flow fans also generate lower noise lev-

30、els than fan blades.The combination of fan motor and air impellers is so importantto the overall design that the designer should work closely withthe manufacturers of both components. Performance curves areavailable for motors, blower wheels, and fans, but data are forideal systems not usually found

31、 in practice because of physicalsize, motor speed, and component placement limitations.ElectronicsMicroprocessors monitor and control numerous functions forroom air conditioners. These microelectronic controls offer digitaldisplays and touch panels for programming desired temperature;on/off timing;

32、modulated fan speeds; bypass capabilities; and sens-ing for humidity, temperature, and airflow control.PERFORMANCE DATAIn the United States, an industry certification program under thesponsorship of the Association of Home Appliance Manufacturers(AHAM) covers the majority of room air conditioners an

33、d certifiesthe cooling and heating capacities, EER, and electrical input (inamperes) of each for adherence to nameplate rating see AHAM2011) for product ratings. The following tests are specified byAHAM Standard RAC-1:Cooling capacityHeating capacityMaximum operating conditions (heating and cooling)

34、Enclosure sweatFreeze-upRecirculated air quantityMoisture removalVentilating air quantity and exhaust air quantityElectrical input (heating and cooling)Power factorCondensate disposalApplication heating capacityOutdoor coil deicingEfficiencyEfficiency for room air conditioners may be shown in either

35、 oftwo forms:1. Energy efficiency ratio (EER: generally for cooling)(Capacity in Btu/h)/(Input in watts)2. Coefficient of performance (COP: generally for heating)(Capacity in Btu/h)/(Input in watts 3.412)Evaporating temperature 45FCompressor suction temperature 65FCondensing temperature 130FLiquid t

36、emperature 115FAmbient temperature 95FRoom Air Conditioners and Packaged Terminal Air Conditioners 50.3Sensible Heat RatioThe ratio of sensible heat to total heat removal is a useful perfor-mance characteristic for evaluating units for specific conditions. Alow ratio indicates more dehumidification

37、capacity, and hot, humidareas like New Orleans and arid locales like Phoenix might best beserved with units having lower and higher ratios, respectively.Although capacity and efficiency are certified data items withAHAM, dehumidification performance is not. Some manufacturerspublish either the sensi

38、ble heat ratio or the volume per hour of con-densate removed as an aid for consumer selection of equipment.Energy Conservation and EfficiencyIn the United States, two federal energy programs have increasedthe demand for higher-efficiency room air conditioners. First, theEnergy Policy and Conservatio

39、n Act of 2005 (Public Law 109-58)provides a commercial building deduction for energy-efficientbuilding improvements, and provides tax breaks for those makingenergy conservation improvements to their homes. Second, the Na-tional Appliance Energy Conservation Act of 1987 (NAECA) pro-vides a single set

40、 of minimum efficiency standards for majorappliances, including room air conditioners. The room air condi-tioner portion of NAECA originally specified minimum efficienciesfor 12 classes, based on physical conformation, with minimumsranging from 8 to 9 EER and applying to all units manufactured onor

41、after January 1, 1990.The U.S. Department of Energy (DOE) issued increased mini-mum efficiency standards that became effective October 1, 2000(Federal Register, September 24, 1997). (See Table 1) Four addi-tional classes were created, two of which cover casement-type units.The minimum standards rang

42、e from 8 to 9.8 EER. For the most pop-ular classes (cooling-only units with louvered sides ranging incapacity from less than 6000 to 20,000 Btu/h) the minimum stan-dards are either 9.7 or 9.8 EER. Table 2 shows the U.S. Environ-mental Protection Agency (EPA) and DOEs ENERGY STARrequirements for room

43、 air conditioners. ENERGY STAR-qualifiedroom air conditioners use at least 10% less energy than minimum-efficiency models. The EPAs ENERGY STAR Web site providesadditional information on qualifying products (EPA 2012).All state and local minimum efficiency standards in the UnitedStates are automatic

44、ally superseded by federal standards. Many othercountries have or are considering minimum efficiency standards, sosuch standards should be sought as part of the design process.Whether estimating potential energy savings associated with ap-pliance standards or estimating consumer operating costs, the

45、 an-nual hours H of operation of a room air conditioner are important.These figures have been compiled from various studies commis-sioned by DOE and AHAM for every major city and region in theUnited States. The national average is estimated at 750 h/year(AHAM 2010; Federal Register 2011a, 2011b) and

46、 maps have beendeveloped by DOE and AHAM that can help estimating the usagefor a specific location. The California Utilities/National ResourcesDefense Council (NRDC) also supported DOEs allocation of750 h/year to active cooling as reasonable, given available data(California Utilities/NRDC 2010).The

47、estimated cost of operation is as follows:C = RHW/1000whereC = annual cost of operation, $/yearR = average cost, $/kWhH = annual hours of operationW = input, WHigh-Efficiency DesignThe EER can be affected by three design parameters. The first iselectrical efficiency. Fan motor efficiency ranges from

48、 25 to 65%;compressor motors range from 60 to 85%. The second parameter,refrigerant cycle efficiency, is increased by enhancing or enlargingthe heat transfer surface to minimize the difference between therefrigerant saturation temperature and air temperature. This allowsusing a compressor with a sma

49、ller displacement and a high-efficiencymotor. The third parameter is air circuit efficiency, which can beincreased by minimizing pressure drop across the heat transfer sur-face, which reduces the load on the fan motor.Higher EERs are not the complete answer to reducing energycosts. Additional energy savings can be achieved by properly sizingthe unit, keeping infiltration and leakage losses to a minimum,increasing building insulation, reducing unnecessary internal load-ing, providing effective maintenance, and using thermostat setback.SPECIAL FEATURESSome room air conditione