ASHRAE HVAC SYSTEMS AND EQUIPMENT SI 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 about1200 mm). 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 ex

4、hausting 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 m

5、ay 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 abov

7、e 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

8、refrigerantthen 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

9、Hz 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-outle

10、t, 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 1.5 to2.3 kW, and are typically used in single-room ap

11、plications. Larger-capacity 115 V units are in the 3.5 to 4.4 kW range. Capacities for230, 208, or 230/208 V units range from 2.3 to 10.6 kW. Thesehigher-voltage units are typically used in multiple-room installa-tions.Heat pump models are also available, usually for 208 or 230 Vapplications. These

12、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.Another type of h

13、eating 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 theoverall system desi

14、gn: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 TC 8.11, Unitary

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

16、ough 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 on the variou

17、s 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 performance and passes

18、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 may not be ac

19、ceptable. 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 pressure, which r

20、esults in ahigh COP.CompressorsRoom air conditioner compressors range in capacity from about1.2 to 10 kW. Design data are available from compressor manufac-turers at the following standard rating conditions:Compressor manufacturers offer complete performance curvesat various evaporating and condensi

21、ng 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. Information on coilperform

22、ance 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 (W/m2), (2) dry-bulb temperature and mois-ture content of entering air, (3) air-side fri

23、ction 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 main types of restrictor

24、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 (capillary). Thecapillary i

25、s 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, and restrictor sizing

26、 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 itfrom the unit, and measu

27、re 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.Fan Motor and Air Imp

28、eller 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 blower wheel and (2)

29、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-els than fan blades.Th

30、e 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 in practice because o

31、f 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; modulated fan speeds;

32、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 and certifiesthe cooling

33、 and heating capacities, COP, 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)Enclosure sweatFreeze-

34、upRecirculated air quantityMoisture removalVentilating air quantity and exhaust air quantityElectrical input (heating and cooling)Power factorCondensate disposalApplication heating capacityOutdoor coil deicingEfficiencyEfficiency is capacity in watts divided by input in watts. Forroom air conditione

35、rs, it may be called energy efficiency ratio(EER) or coefficient of performance (COP). To convert EER toCOP, multiply EER 0.2931.Evaporating temperature 7.2CCompressor suction temperature 18.3CCondensing temperature 54.4CLiquid temperature 46.1CAmbient temperature 35.0CRoom Air Conditioners and Pack

36、aged 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 capacity, and hot, humidareas like New Orleans and arid locales like

37、 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 sensible heat ratio or the volume per hour of con-densate removed as an a

38、id 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 Conservation Act of 2005 (Public Law 109-58)provides a commercial building dedu

39、ction 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 of minimum efficiency standards for majorappliances, including room

40、 air conditioners. The room air condi-tioner portion of NAECA originally specified minimum efficienciesfor 12 classes, based on physical conformation, with minimumsranging from a COP of 2.3 to 2.6 and applying to all units manufac-tured on or after January 1, 1990.The U.S. Department of Energy (DOE)

41、 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 range from a COP of 2.3 to 2.9. For themost popular classes (

42、cooling-only units with louvered sides rang-ing in capacity from less than 1800 to 5900 W) the minimum stan-dards are either a COP of 2.8 or 2.9. Table 2 shows the U.S.Environmental Protection Agency (EPA) and DOEs ENERGYSTARrequirements for room air conditioners. ENERGY STAR-qualified room air cond

43、itioners use at least 10% less energy thanminimum-efficiency models. The EPAs ENERGY STAR Web siteprovides additional information on qualifying products (EPA 2012).All state and local minimum efficiency standards in the UnitedStates are automatically superseded by federal standards. Many othercountr

44、ies 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 an-nual hours H of operation of a room air conditione

45、r 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 maps have beendeveloped by DOE and AHAM that can help

46、 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 estimated cost of operation is as follows:C = RHW/1000

47、whereC = annual cost of operation, $/yearR = average cost, $/kWhH = annual hours of operationW = input, WHigh-Efficiency DesignThe COP can be affected by three design parameters. The first iselectrical efficiency. Fan motor efficiency ranges from 25 to 65%;compressor motors range from 60 to 85%. The

48、 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 smaller displacement and a high-efficiencymotor. The thir

49、d 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 COPs 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 conditioners are designed to minimize theirextension beyond the b