ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 3-2012 CENTRAL COOLING AND HEATING.pdf

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1、3.1CHAPTER 3CENTRAL COOLING AND HEATINGSystem Characteristics. 3.1Design Considerations. 3.2Equipment 3.3Distribution Systems 3.6Acoustic, Vibration, Wind, and Seismic Considerations 3.7Space Considerations. 3.7Automatic Controls and Building Management Systems . 3.8Maintenance Management Systems .

2、3.9Building System Commissioning 3.9ENTRAL cooling and/or heating plants generate coolingCand/or heating in one location for distribution to multiple loca-tions in one building or an entire campus or neighborhood, andrepresent approximately 25% of HVAC system applications. Cen-tral cooling and heati

3、ng systems are used in almost all classes ofbuildings, but particularly in very large buildings and complexes orwhere there is a high density of energy use. They are especiallysuited to applications where maximizing equipment service life andusing energy and operational workforce efficiently are imp

4、ortant.The following facility types are good candidates for central cool-ing and/or heating systems: Campus environments with distribution to several buildingsHigh-rise facilitiesLarge office buildingsLarge public assembly facilities, entertainment complexes, stadi-ums, arenas, and convention and ex

5、hibition centersUrban centers (e.g., city centers/districts) Shopping mallsLarge condominiums, hotels, and apartment complexesEducational facilitiesHospitals and other health care facilitiesIndustrial facilities (e.g., pharmaceutical, manufacturing)Large museums and similar institutionsLocations whe

6、re waste heat is readily available (result of powergeneration or industrial processes)This chapter addresses design alternatives that should be consid-ered when centralizing a facilitys cooling and heating sources. Dis-tribution system options and equipment are discussed when theyrelate to the centr

7、al equipment, but more information on distributionsystems can be found in Chapters 11 to 15.SYSTEM CHARACTERISTICSCentral systems are characterized by large chilling and/or heatingequipment located in one facility or multiple smaller installationsinterconnected to operate as one. Equipment configura

8、tion and ancil-lary equipment vary significantly, depending on the facilitys use.See Chapter 1 for information on selecting a central cooling or heat-ing plant.Equipment can be located adjacent to the facility, or in remotestand-alone plants. Also, different combinations of centralized anddecentrali

9、zed systems (e.g., a central cooling plant and decentral-ized heating and ventilating systems) can be used.Primary equipment (i.e., chillers and boilers) is available in dif-ferent sizes, capacities, and configurations to serve a variety ofbuilding applications. Operating a few pieces of primary equ

10、ipment(often with back-up equipment) gives central plants different bene-fits from decentralized systems (see Chapter 2).Multiple types of equipment and fuel sources may be combinedin one plant. The heating and cooling energy may be a combinationof electricity, natural gas, oil, coal, solar, geother

11、mal, waste heat,etc. This energy is converted into chilled water, hot water, or steamthat is distributed through the facility for air conditioning, heating,and processes. The operating, maintenance, and first costs of allthese options should be discussed with the owner before final selec-tion. When

12、combining heating generation systems, it is important tonote the presence of direct-firing combustion systems and chilled-water production systems using refrigerants, because the safetyrequirements in ASHRAE Standard 15 must be met.A central plant can be customized without sacrificing the stan-dardi

13、zation, flexibility, and performance required to support the pri-mary cooling and heating equipment through careful selection ofancillary equipment, automatic control, and facility management.Plant design varies widely based on building use, life-cycle costs,operating economies, and the need to main

14、tain reliable buildingHVAC, process, and electrical systems. These systems can requiremore extensive engineering, equipment, and financial analysis thandecentralized systems do.In large buildings with interior areas that require cooling whileperimeter areas require heating, one of several types of c

15、entralizedheat reclaim units can meet both these requirements efficiently.Chapter 9 describes these combinations, and Chapters 13 to 15 givedesign details for central plants.Central plants can be designed to accommodate both occupied/unoccupied and constant, year-round operation. Maintenance canbe p

16、erformed with traditional one-shift operating crews, but mayrequire 24 h coverage. Higher-pressure steam boiler plants usuallygreater than 100 kPa (gage) or combined cogeneration and steamheating plants require multiple-operator, 24 h shift coverage.AdvantagesPrimary cooling and heating can be provi

17、ded at all times, inde-pendent of the operation mode of equipment and systems outsidethe central plant.Using larger but fewer pieces of equipment generally reduces thefacilitys overall operation and maintenance cost. It also allowswider operating ranges and more flexible operating sequences.A centra

18、lized location minimizes restrictions on servicing acces-sibility.Energy-efficient design strategies, energy recovery, thermalstorage, and energy management can be simpler and more cost-effective to implement.Multiple energy sources can be applied to the central plant, pro-viding flexibility and lev

19、erage when purchasing fuel.Standardizing equipment can be beneficial for redundancy andstocking replacement parts. However, strategically selectingdifferent-sized equipment for a central plant can provide betterpart-load capability and efficiency.Standby capabilities (for firm capacity/redundancy) a

20、nd back-upfuel sources can easily be added to equipment and plant whenplanned in advance.The preparation of this chapter is assigned to TC 9.1, Large Building Air-Conditioning Systems.3.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)Equipment operation can be staged to match load profile andta

21、ken offline for maintenance.A central plant and its distribution can be economically expandedto accommodate future growth (e.g., adding new buildings to theservice group).Load diversity can substantially reduce the total equipment capac-ity requirement.Submetering secondary distribution can allow in

22、dividual billingof cooling and heating users outside the central plant.Major vibration and noise-producing equipment can be groupedaway from occupied spaces, making acoustic and vibration con-trols simpler. Acoustical treatment can be applied in a single loca-tion instead of many separate locations.

23、Issues such as cooling tower plume and plant emissions are cen-tralized, allowing a more economic solution.DisadvantagesEquipment may not be readily available, resulting in long leadtime for production and delivery.Equipment may be more complicated than decentralizedequipment, and thus require a mor

24、e knowledgeable equipmentoperator.A central location within or adjacent to the building is needed.Additional equipment room height may be needed.Depending on the fuel source, large underground or surface stor-age tanks may be required on site. If coal is used, space for storagebunker(s) will be need

25、ed.Access may be needed for large deliveries of fuel (oil or coal).Fossil-fuel heating plants require a chimney or flue and possiblyemission permits, monitoring, and treatments.Multiple equipment manufacturers are required when combiningprimary and ancillary equipment.System control logic may be com

26、plex.First costs can be higher.Special permitting may be required.Safety requirements are increased.A large pipe distribution system may be necessary (which mayactually be an advantage for some applications).DESIGN CONSIDERATIONSCooling and Heating LoadsDesign cooling and heating loads are determine

27、d by consideringindividual and simultaneous loads. The simultaneous peak or instan-taneous load for the entire portion or block of the building served bythe HVAC and/or process load is less than the sum of the individualcooling and heating loads (e.g., buildings do not receive peak solarload on the

28、east and west exposures at the same time). The differencebetween the sum of the space design loads and system peak load,called the diversity factor, can be as little as 5% less than the sum ofindividual loads (e.g., 95% diversity factor) or represent a more sig-nificant portion of the load (e.g., 45

29、% diversity factor), as is possiblein multiple-building applications. The peak central plant load can bebased on this diversity factor, reducing the total installed equipmentcapacity needed to serve larger building cooling and heating loads. Itis important for the design engineer to evaluate the ful

30、l point-of-useload requirements of each facility served by the central system.Opportunities for improving energy efficiency includeStaging multiple chillers or boilers for part-load operation. Usingcorrectly sized equipment is imperative to accurately provide themost flexible and economical sequenci

31、ng of equipment.For central chiller plants, consider incorporating variable-frequency drives (VFDs) onto one base-loaded primary chiller.Multiple VFD installations on chillers allow more flexibility inenergy control of chiller plant operation. Note that increased part-load efficiency can only be ach

32、ieved if the required lift changes.That occurs when the condensing temperature is lowered (coolercondensing fluid) or the discharge chilled-water temperature israised.Discrete loads (e.g., server rooms) may be best served by anindependent system, depending on the overall load profile of sys-tems ser

33、ved by the plant; a small, independent system designed forthe discrete load may be the most cost-effective approach. Centralplants sized for minimum part-load operation may not be able toreliably serve a small, discrete load. For example, an 8 MW chillerplant serving multiple facilities could be sel

34、ected with four 2 MWchillers. In a remote facility connected to the central distributionsystem, an independent computer room server has a year-round20 kW load. Operating one 2 MW chiller at less than 500 kW reli-ably may be extremely inefficient and possibly detrimental to thechiller plant operation

35、. Serving the constant remote load indepen-dently allows the designer the flexibility to evaluate the chiller plantas a whole and individual subsystems independently, so as to notadversely affect both systems.Peak cooling load time is affected by outdoor ventilation, out-door dry- and wet-bulb tempe

36、ratures, period of occupancy, interiorequipment heat gain, and relative amounts of north, east, south, andwest exposures. For typical office and/or classroom buildings witha balanced distribution of solar exposures, the peak usually occurson a midsummer afternoon when the west solar load and outdoor

37、dry-bulb temperature are at or near concurrent maximums. How-ever, the peak cooling period can shift, depending on required ven-tilation rates and internal load profiles.Diversity of building occupancies served can significantly affectthe diversity factor. For example, in a system serving an entire

38、col-lege campus, the peak cooling period for a classroom is differentfrom that for a dormitory or an administration building. Planningload profiles at academic facilities requires special consideration.Unlike office and residential applications, universities and collegestypically have peak cooling l

39、oads during late summer and fall.Peak heating load has less opportunity to accommodate a diver-sity factor, so equipment is most likely to be selected on the sum ofindividual heating loads. This load may occur when the buildingmust be warmed back up to a higher occupied-space temperatureafter an uno

40、ccupied weekend setback period. Peak demand mayalso occur during unoccupied periods when the ambient environ-ment is harshest and there is little internal heat gain to assist theheating system, or during occupied times if significant outdoor airmust be preconditioned or some other process (e.g., pro

41、cess heat-ing) requires significant heat. To accommodate part-load conditionsand energy efficiency, variable-flow may be the best economicalchoice. It is important for the designer to evaluate plant operationand system use.System Flow DesignThe configuration of a central system is based on use and a

42、ppli-cation. Two types of energy-efficient designs used today are primaryvariable flow and primary/secondary variable flow.Primary variable flow uses variable flow through the pro-duction energy equipment (chiller or heating-water generator) anddirectly pumps the medium, usually water, to the point

43、of use. Vari-able flow can be achieved using two-way automatic control valvesat terminal equipment and either variable-frequency drive (VFD)pumping (Figure 1) or distribution pressure control with bypassvalve (Figure 2). Both concepts function based on maintaining sys-tem pressure, usually at the hy

44、draulically most remote point (lastcontrol valve and terminal unit) in the water system.Primary/secondary variable flow hydraulically decouples theprimary production system (chilled- or heating-water source), whichis commonly constant flow. A variable-flow secondary piping sys-tem distributes the ch

45、illed or heating medium to the point of use(Figures 3 and 4).Central Cooling and Heating 3.3Another design is a straight constant-volume primary system.Hydronic pumps distribute water through the energy equipmentstraight to the point of use. Pumping energy is constant, as is the dis-tribution flow,

46、which generally requires a means to maintain thedesign flow rate through the system while in operation. Thus, thesesystems are generally more expensive to operate and less attractivein central system designs. Particular care should be taken with thistype of system design, because many current energy

47、 codes do notallow use of pump motors without implementing variable-frequency drives. When using either primary/secondary or primary variable-flowdesigns, the engineer should understand the design differencesbetween two- and three-way modulating valves. Variable-flowdesigns vary the flow of chilled

48、or heating water through the distri-bution loop. As terminal units satisfy demand, the valve modulatestoward the closed position. In the distribution system, the pump, ifat design operating conditions, increases system pressure above thedesign point. To compensate, a VFD is typically installed on th

49、e sec-ondary pump. Pump speed is usually controlled by a pressure dif-ferential sensor or sensing control valve position. As the pressuredifferential increases or the valve closes, the sensor sends a signal tothe VFD to reduce speed. The affinity laws then allow flow toreduce in direct proportion to the change in flow. As system demandrequires increased flow, the control valve modulates open, reducingsystem differential pressure. The reduction in pressure differencemeasured causes a corresponding increase in pump speed (andtherefore flow) to meet plant demand. With a primary/secondary