ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 2-2012 DECENTRALIZED COOLING AND HEATING.pdf

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1、2.1CHAPTER 2DECENTRALIZED COOLING AND HEATINGSystem Characteristics. 2.1Design Considerations. 2.2Window-Mounted and Through-the-Wall Room HVAC Units 2.3Water-Source Heat Pump Systems . 2.4Multiple-Unit Systems 2.5Residential and Light Commercial Split Systems. 2.6Commercial Self-Contained (Floor-by

2、-Floor) Systems . 2.7Commercial Outdoor Packaged Systems 2.8Automatic Controls and Building Management Systems . 2.10Maintenance Management . 2.10Building System Commissioning 2.10OR MOST small to mid-size installations, decentralized cool-Fing and heating is usually preferable to a centralized syst

3、em(see Chapter 3). Frequently classified as packaged unit systems (al-though many are far from being a single packaged unit), decentral-ized systems can be found in almost all classes of buildings. Theyare especially suitable for smaller projects with no central plant,where low initial cost and simp

4、lified installation are important.These systems are installed in office buildings, shopping centers,manufacturing plants, schools, health care facilities, hotels, motels,apartments, nursing homes, and other multiple-occupancy dwell-ings. They are also suited to air conditioning existing buildings wi

5、thlimited life or income potential. Applications also include facilitiesrequiring specialized high performance levels, such as computerrooms and research laboratories.Although some of the equipment addressed here can be ap-plied as a single unit, this chapter covers applying multiple unitsto form a

6、complete heating and air-conditioning system for abuilding and the distribution associated with some of these sys-tems. For guidance on HVAC system selection, see Chapter 1.SYSTEM CHARACTERISTICSDecentralized systems can be one or more individual HVACunits, each with an integral refrigeration cycle,

7、 heating source, anddirect or indirect outdoor air ventilation. Components are factory-designed and assembled into a package that includes fans, filters,heating source, cooling coil, refrigerant compressor(s), controls,and condenser. Equipment is manufactured in various configura-tions to meet a wid

8、e range of applications. Examples of decentral-ized HVAC equipment include the following: Window air conditionersThrough-the-wall room HVAC unitsAir-cooled heat pump systemsWater-cooled heat pump systemsMultiple-unit variable-refrigerant-flow (VRF) systemsResidential and light commercial split syste

9、msSelf-contained (floor-by-floor) systemsOutdoor package systemsPackaged, special-procedure units (e.g., for computer rooms)For details on window air conditioners and through-the-wallunits, see Chapter 50; the other examples listed here are discussedfurther in Chapter 49. (Multiple-unit systems are

10、also covered inChapter 4.)Commercial-grade unitary equipment packages are availableonly in preestablished increments of capacity with set performanceparameters, such as the sensible heat ratio at a given room conditionor the airflow per kilowatt of refrigeration capacity. Components arematched and a

11、ssembled to achieve specific performance objectives.These limitations make manufacture of low-cost, quality-controlled,factory-tested products practical. For a particular kind and capacityof unit, performance characteristics vary among manufacturers. Allcharacteristics should be carefully assessed t

12、o ensure that the equip-ment performs as needed for the application. Several trade associa-tions have developed standards by which manufacturers may test andrate their equipment. See Chapters 49 and 50 for more specific infor-mation on pertinent industry standards and on decentralized coolingand hea

13、ting equipment used in multiple-packaged unitary systems.Large commercial/industrial-grade packaged equipment can becustom-designed by the factory to meet specific design conditionsand job requirements. This equipment carries a higher first cost andis not readily available in smaller sizes.Self-cont

14、ained units, often called rooftop units, can use multiplecompressors to control refrigeration capacity. For variable-air-volume (VAV) systems, compressors are turned on or off or unloadedto maintain discharge air temperature. Variable-speed compressorscan be factory integrated for close control. As

15、zone demand de-creases, the temperature of air leaving the unit can often be reset up-ward so that a minimum ventilation rate is maintained.Multiple packaged-unit systems for perimeter spaces are fre-quently combined with a central all-air or floor-by-floor system.These combinations can provide bett

16、er humidity control, air purity,and ventilation than packaged units alone. Air-handling systemsmay also serve interior building spaces that cannot be conditionedby wall or window-mounted units. Water-source heat pump systems often combine packaged units(heat pumps) with a central piping system for h

17、eat rejection and heatgain. These systems require heat rejection equipment (groundsource or cooling tower) and heat source (ground source or boiler)provided separately from the packaged heat pump.Heating can be included in a packaged unit. Gas heat exchangersor electric heat coils can be provided. H

18、eat can be turned on or offin stages to meet zone demands. In some applications, heat from acentralized source, like a boiler system, are combined with decen-tralized packaged units, and steam or hot-water coils can be factorymounted in packaged units for connection to local piping systems.For suppl

19、ementary data on air-side design of decentralized sys-tems, see Chapter 4.AdvantagesHeating and cooling can be provided at all times, independent ofthe mode of operation of other building spaces.Manufacturer-matched components have certified ratings andperformance data.Assembly by a manufacturer hel

20、ps ensure better quality controland reliability.Manufacturer instructions and multiple-unit arrangements sim-plify installation through repetition of tasks.Only one zone of temperature control is affected if equipmentmalfunctions.The equipment is readily available.The preparation of this chapter is

21、assigned to TC 9.1, Large Building Air-Conditioning Systems.2.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)Multiple vendors manufacture similar equipment, providing mul-tiple sources.One manufacturer is responsible for the final equipment package.For improved energy control, equipment servin

22、g vacant spacescan be turned off locally or from a central point, without affectingoccupied spaces.System operation is simple. Trained operators are not usuallyrequired.Less mechanical and electrical room space is required than withcentral systems.Initial cost is usually low.Equipment can be install

23、ed to condition one space at a time as abuilding is completed, remodeled, or as individual areas are occu-pied, with favorable initial investment.Energy can be metered directly to each tenant.Air- or water-side economizers may be applicable, depending ontype of decentralized system used.Disadvantage

24、sPerformance options may be limited because airflow, cooling coilsize, and condenser size are fixed.Larger total building installed cooling capacity is usually requiredbecause diversity factors used for moving cooling needs do notapply to multiple dedicated packages.Temperature and humidity control

25、may be less stable, especiallywith mechanical cooling at very low loads.Standard commercial units are not generally suited for large per-centages of outdoor air or for close humidity control. Custom orspecial-purpose equipment, such as packaged units for computerrooms, or large custom units, may be

26、required.Energy use is usually greater than for central systems if efficiencyof the unitary equipment is less than that of the combined centralsystem components.Low-cost cooling by economizers is not always available or prac-tical.Air distribution design may be limited by fan capacity available.Oper

27、ating sound levels can be high, and noise-producing ma-chinery is often closer to building occupants than with centralsystems.Ventilation capabilities may be limited by equipment selectionand design.Equipments effect on building appearance can be unappealing.Air filtration options may be limited.Dis

28、charge temperature varies because of on/off or step control.Condensate drain is required with each air-conditioning unit.Maintenance may be difficult or costly because of multiple piecesof equipment and their location.DESIGN CONSIDERATIONSRating classifications and typical sizes for equipment addres

29、sedin this chapter can be found in Chapters 49 and 50, which alsoaddress available components, equipment selection, distributionpiping, and ductwork.Selection of a decentralized system should follow guidance pro-vided in Chapter 1. The design engineer can use the HVAC systemanalysis selection matrix

30、 to analytically assess and select the opti-mum decentralized system for the project. Combined with thedesign criteria in Chapters 49 and 50, the basis of design can be doc-umented.Unlike centralized cooling and heating equipment, capacitydiversity is limited with decentralized equipment, because ea

31、chpiece of equipment must be sized for peak capacity.Noise from this type of equipment may be objectionable andshould be checked to ensure it meets sound level requirements.Chapter 48 of the 2011 ASHRAE HandbookHVAC Applicationshas more information on HVAC-related sound and vibration con-cerns.Decen

32、tralized packaged equipment requires design considerationbecause the building structure must support the units: a typical fullcompressorized package unit weighs more than a similarly sizedair-handling-only system.When packaged units are located outdoors, designers must con-sider weather conditions a

33、nd accommodate rain, snow, or highwinds. Outdoor packaged units can conveniently be provided withintegrated outdoor air intakes or air economizers. Gas-fired furnaceson outdoor package units can be vented directly from the unit.When packaged units are located indoors, the designer must con-sider a s

34、eparate source for ventilation air and/or economizer. Gasheat might be limited by combustion air source and available fluepathways. When split-system packaged units are used, design limi-tations for refrigerant piping and distances must be considered.Design must consider noise generated by packaged

35、units com-pressors, fans, or both. Units mounted on roofs should not be locatedabove sound-sensitive spaces, such as conference rooms or sleepingareas. Units inside the building might require enclosure in a mechan-ical room or closets with sound-absorbing wall construction.Decentralized units requir

36、e electric and/or gas to each location.Designers must consider building type and installation costs for theutilities in addition to the HVAC system cost.Air-Side EconomizerWith some decentralized systems, an air-side economizer is an op-tion, if not an energy code requirement (check state code for c

37、riteria).The air-side economizer uses cool outdoor air to either assist me-chanical cooling or, if the outdoor air is cool enough, provide totalcooling. It requires a mixing box designed to allow 100% of the sup-ply air to be drawn from the outdoor. It can be a field-installed acces-sory that includ

38、es an outdoor air damper, relief damper, return airdamper, filter, actuator, and linkage. Controls are usually a factory-installed option.Self-contained units usually do not include return air fans. Abarometric relief, fan-powered relief fan, or return/exhaust fan maybe provided as an air-side econo

39、mizer. The relief fan is off and dis-charge/exhaust dampers are closed when the air-side economizer isinactive.AdvantagesSubstantially reduces compressor, cooling tower, and condenserwater pump energy requirements, generally saving more energythan a water-side economizer.Has a lower air-side pressur

40、e drop than a water-side economizer.Reduces tower makeup water and related water treatment.May improve indoor air quality by providing large amounts ofoutdoor air during mild weather.DisadvantagesIn systems with larger return air static pressure requirements,return or exhaust fans are needed to prop

41、erly exhaust building airand take in outdoor air.If the units leaving air temperature is also reset up during the air-side economizer cycle, humidity control problems may occur andthe fan may use more energy. Humidification may be required during winter.More and/or larger air intake louvers, ducts,

42、or shafts may berequired for indoor package units.Water-Side EconomizerThe water-side economizer is another option for reducing energyuse. ASHRAE Standard 90.1 addresses its application, as do somestate energy codes. The water-side economizer consists of a watercoil in a self-contained unit upstream

43、 of the direct-expansion cool-ing coil. All economizer control valves, piping between economizerDecentralized Cooling and Heating 2.3coil and condenser, and economizer control wiring can be factoryinstalled. This applies typically for indoor packaged compressorunits using a water loop for heat rejec

44、tion.The water-side economizer uses the low cooling tower or evapo-rative condenser water temperature to either (1) precool entering air,(2) assist mechanical cooling, or (3) provide total system cooling ifthe cooling water is cold enough. If the economizer is unable to main-tain the air-handling un

45、its supply air or zone set point, factory-mounted controls integrate economizer and compressor operation tomeet cooling requirements. For constant condenser water flow con-trol using a economizer energy recovery coil and the unit condenser,two control valves are factory-wired for complementary contr

46、ol, withone valve driven open while the other is driven closed. This keepswater flow through the condenser relatively constant. In variable-flow control, condenser water flow varies during unit operation. Thevalve in bypass/energy recovery loop is an on/off valve and is closedwhen the economizer is

47、enabled. Water flow through the economizercoil is modulated by its automatic control valve, allowing variablecooling water flow as cooling load increases (valve opens) andreduced flow on a decrease in cooling demand. If the economizer isunable to satisfy the cooling requirements, factory-mounted con

48、trolsintegrate economizer and compressor operation. In this operatingmode, the economizer valve is fully open. When the self-containedunit is not in cooling mode, both valves are closed. Reducing or elim-inating cooling water flow reduces pumping energy.AdvantagesCompressor energy is reduced by prec

49、ooling entering air. Often,building load can be completely satisfied with an entering con-denser water temperature of less than 13C. Because the wet-bulbtemperature is always less than or equal to the dry-bulb tempera-ture, a lower discharge air temperature is often available.Building humidification does not affect indoor humidity by intro-ducing outdoor air.No external wall penetration is required for exhaust or outdoor airducts.Controls are less complex than for air-side economizers, becausethey are often inside the packaged unit.The coil can be mechanically cleaned.More net usable