ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 18-2012 VARIABLE REFRIGERANT FLOW.pdf

1、18.1CHAPTER 18VARIABLE REFRIGERANT FLOWStandards 18.3Equipment 18.4VRF Heat Pump System Operation. 18.4Design Considerations . 18.7VRF System Design Example . 18.8ARIABLE-refrigerant-flow (VRF) HVAC systems are aVdirect-expansion (DX) heat pump technology platform builton the standard reverse Rankin

2、e vapor compression cycle. Thesesystems are thermodynamically similar to unitary and other com-mon DX systems, and share many of the same components (i.e.,compressor, expansion device, and heat exchangers). VRF sys-tems transport heat between an outdoor condensing unit and a net-work of indoor units

3、 located near or within the conditioned spacethrough refrigerant piping installed in the building. Attributes thatdistinguish VRF from other DX system types are multiple indoorunits connected to a common outdoor unit (single or combinedmodules), scalability, variable capacity, distributed control, a

4、ndsimultaneous heating and cooling.VRF systems are highly engineered, with single or multiple com-pressors, multiple indoor units (ducted and nonducted types), andoil and refrigerant management and control components. VRF pro-vides flexibility by allowing for many different indoor units (withdiffere

5、nt capacities and configurations), individual zone control,and the unique ability to offer simultaneous heating and cooling inseparate zones on a common refrigerant circuit, and heat recoveryfrom one zone to another. Typical capacities range from 5.3 to223 kW for outdoor units and from 1.5 to 35 kW

6、for indoor units.Many VRF systems are equipped with at least one variable-speedand/or variable-capacity compressor. Figure 1 illustrates capacitycontrol of a single variable-speed compressor; the compressor var-ies its speed to operate only at the levels necessary to maintainindoor environments to t

7、he specified requirements.System TypesThere are three basic types of VRF systems: cooling only (Figure2), heat pump (Figure 2), and heat recovery (see Figures 3 and 4).Heat pumps are air-conditioning systems capable of reversing thedirection of the refrigerant flow to provide heating or cooling to t

8、heindoor space. All indoor units connected to a heat pump system canuse individual control and set points, but they operate in the samemode of either heating or cooling at any given time.Heat recovery units are heat pump systems that can providesimultaneous heating and cooling. All indoor units conn

9、ected to aheat recovery system not only can use individual control and setpoints, but they can also individually operate in heating or coolingmode at any given time. To match the buildings load profiles,energy is transferred from one indoor space to another through therefrigerant line, and only one

10、energy source is necessary to provideboth heating and cooling. VRF systems also operate efficiently atpart load because of the compressors variable capacity control.Note: Figures 2 to 4 represent various methods of VRF systempiping configurations. Please refer to each manufacturers data forspecifics

11、.The following definitions are based on AHRI Standard 1230.A heat pump multisplit system is an encased, factory-made,permanently installed assembly that takes heat from a heat sourceThe preparation of this chapter is assigned to TC 8.7, Variable RefrigerantFlow.Fig. 1 Compressor FrequencyFig. 2 Cool

12、ing-Only Heat Pump VRF SystemFig. 3 Two-Pipe Heat Recovery VRF System18.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)and delivers it to the conditioned space when heating is desired. Itmay remove heat from the conditioned space and discharge it to aheat sink if cooling and dehumidification a

13、re desired from the sameequipment. Normal components include multiple indoor condition-ing coils, compressor(s), and outdoor coil(s). Equipment may be pro-vided in multiple assemblies, intended for use together. Otherfunctions may include cleaning, circulating, and humidifying the air.A VRF multispl

14、it system is a split-system air conditioner or heatpump with a single refrigerant circuit, one or more outdoor units, atleast one variable-speed compressor or other compressor combina-tion that can vary system capacity by three or more steps, and multipleindoor fan-coil units that are individually m

15、etered and individuallycontrolled by an integrated control device and common communica-tions network. A VRF heat recovery multisplit system operates asan air conditioner or as a heat pump, and also can provide simultane-ous heating and cooling operation by transferring recovered energyfrom the indoo

16、r units operating in one mode to other indoor unitsoperating in the other mode. Variable refrigerant flow implies three ormore steps of control on common, interconnecting piping.VRF ApplicationsVRF systems can be used in various applications, such asHigh- or low-rise officesEducational facilities (s

17、chools, universities)Health care facilities, including clinics and long-term-care nurs-ing homesMultiple-tenant residential buildingsRetail storesHospitality centers, restaurants, banquet halls, hotels, and motelsData center cooling-only applicationsCultural facilities, including religious centersZo

18、ned Comfort ControlVRF systems achieve temperature control on a zone-by-zonebasis primarily by using refrigerant-side control. Indoor unitsconstantly react to changes in the zones heating/cooling loads,and maintain conditions by controlling target superheat with anelectronic expansion valve (EEV) or

19、 a linear expansion valve (LEV).The inverter compressors or combination of inverter and constant-speed compressors generally found in VRF systems modulate refrig-erant flow, and work in unison with the refrigerant volume requiredby the indoor units.Maintaining comfort conditions efficiently can be c

20、hallenging forHVAC systems during the “shoulder” seasons, usually periods dur-ing fall and spring when both heating and cooling maybe required.The ideal changeover from predominantly heating operation to pre-dominantly cooling operation, or vice versa, is often determined bya preset outdoor ambient

21、temperature. Many traditional unitaryHVAC systems operate heating and cooling systems simultaneouslyfor extended periods during shoulder seasons, providing heating andcooling through a four-pipe network. With decentralized systemssuch as VRF, buildings can be zoned so that switchover is seamlessand

22、comfort is maintained.Indoor Air QualityIndoor air quality (IAQ) provided by an HVAC system dependson the design zone air change rate, level of ventilation air supplied,and degree of airflow filtration. VRF systems, like traditional unitarysystems, must follow associated filtration levels outlined i

23、n ASH-RAE Standard 62.1 for ventilation, with filtration provided eithercentrally, if using a cooling coil, and/or at zone level, if higher levelsof filtration are required beyond that provided by the central unit.To meet ASHRAE Standard 62.1 requirements, VRF systems arefrequently integrated with a

24、 dedicated outdoor air system (DOAS)where ventilation airflow delivery rates remain consistent, regard-less of the zones peak heating or cooling loads or the minimumdamper (variable-air-volume terminal unit or central air-handlingunit) position.Annual Operating Efficiency CharacteristicsAn HVAC syst

25、ems annual operating efficiency is affected by thebuildings occupancy profile, orientation, design ventilation airrequirements, construction, local outdoor ambient design parame-ters, air-source versus water-source heat rejection strategies, andother factors. VRF systems operate efficiently at part-

26、load condi-tions, particularly in heat recovery mode where the system simulta-neously provides heating and cooling.The variable capacity of both the indoor and outdoor units meansthat the entire system matches the load in any given space. Loadmatching can also provide benefits such as efficiency at

27、part-loadoperations and dehumidification. VRF systems feature decentral-ized individual zoned indoor units, which eliminate the need forlarge, central fans and ductwork.Local and Remote MonitoringVRF systems use a manufacturer-specific controls protocol tocommunicate between outdoor units, indoor un

28、its, and availablesystem-specific accessories. Each VRF manufacturer has an ap-proach to control accessories (e.g., steam or water valves, humidi-fication, dampers, motors), so the designer should consult systemsand component engineering and operation manuals for proper con-trol integration.Controls

29、 must be an integral part of the total system design, andthe buildings application dictates the control package(s) required.VRF systems can function as a stand-alone system using the manu-facturers standard controls, or can integrate with other systemsthrough building automation controls such as BAC

30、net, LonWorks,or the manufacturers method.Life-Cycle Cost ComparisonAn HVAC systems total life-cycle operating costs depend oninstalled capital costs, annual operating energy costs, routine main-tenance costs, equipment life expectancy, and system replacementcosts.Fig. 4 Three-Pipe Heat Recovery VRF

31、 System ExamplesVariable Refrigerant Flow 18.3Installed Capital Costs. In new construction, installed capitalcosts of an air-or water-source VRF system are comparable to thoseof a four-pipe chilled-/hot-water system. A VRF systems equipment-to-labor ratio costs differ from those of a four-pipe chill

32、ed-/hot-watersystem because the primary control components of a VRF system arefactory installed or packaged. In an air-source VRF system, the com-pressor is the main operating component; additional pumps and reg-ulating or control valves are not required. In some climates, air-sourceVRF systems can

33、satisfy the complete building heating load, avoidingthe need for a supplemental or boiler heating system.Life-Cycle Operating Costs. Life-cycle operating costs forVRF systems are divided into four categories:Annual operating costs depend on many factors, includingbuilding load profiles and heat reco

34、very opportunities.Routine maintenance costs include remote monitoring tools,which typically enable ongoing VRF system performance re-views and proactive maintenance strategies. Indoor units requireregular visual inspection and scheduled air filter replacement.Outdoor units require routine inspectio

35、n of key components (e.g.,compressor, heat exchanger).Equipment life expectancy depends, in some instances, on antic-ipated operating hours. Most VRF system manufacturers list 15 to20 years for air-source systems and 20 to 25 years for water-source systems as the average range, if approved routine m

36、ainte-nance is performed.System replacement costs.Life-Cycle Analysis Tools. Industry-recognized building energy-simulation tools (including VRF system performance characteristiccurves) are proving to be the most accurate strategy for capturingVRF systems annual performances. Such simulation softwar

37、e pro-grams can also perform a life-cycle analysis.STANDARDSAHRI has a certification program specifically for VRF multi-split air-conditioning and heat pump equipment. AHRI Standard1230 applies to all VRF multisplit air-conditioning and heat pump/heat recovery equipment with capacities up to 88 kW.

38、It followsAHRI standard rating conditions, and covers all multisplit,matched system air conditioners and heat pumps, regardless ofelectric power source, refrigeration cycle, or secondary fluid (e.g.,air-to-air or water-to-air). In conducting all VRF testing, the follow-ing definition is used to conf

39、igure the systems to be tested. This def-inition has been a key factor is establishing equivalency of testingwith VRF and other unitary systems:3.25 Tested Combination. A sample basic model comprised ofunits that are production units, or are representative of pro-duction units, of the basic model be

40、ing tested. The testedcombination shall have the following features:a. The basic model of a variable refrigerant flow system(“VRF system”) used as a Tested Combination shall con-sist of an outdoor unit (an outdoor unit can include multi-ple outdoor units that have been manifolded into a singlerefrig

41、eration system, with a specific model number) thatis matched with between 2 and 12 indoor units.b. The indoor units shall:b.1 Represent the highest sales model family as deter-mined by type of indoor unit e.g. ceiling cassette,wall-mounted, ceiling concealed, etc.b.2 Together, have a nominal cooling

42、 capacity between95% and 105% of the nominal cooling capacity ofthe outdoor unit.b.3 Not, individually, have a nominal cooling capacitygreater than 50% of the nominal cooling capacity ofthe outdoor unit, unless the nominal cooling capac-ity of the outdoor unit is 7016 W or less.b.4 Have a fan speed

43、that is consistent with the manu-facturers specifications.b.5 All be subject to the same minimum external staticpressure requirement while being configurable toproduce the same static pressure at the exit of eachoutlet plenum when manifolded as per section 2.4.1of 10 CFR Part 430, Subpart B, Appendi

44、x M.Table 1 lists VRF multisplit system classifications.The following certification program ratings (from AHRI Stan-dard 1230-2010) are verified by test:VRF Multisplit Air Conditioners 19 kWStandard rating cooling capacity, kWSeasonal energy efficiency ratio (SEER; total cooling of a systemwith a ca

45、pacity 19 kW during its normal use for cooling, dividedby total electric energy input during the same period), Btu/WhVRF Multisplit Air Conditioners 19 kWStandard rating cooling capacity, kWEnergy efficiency ratio (EER; ratio of cooling capacity to powerinput values at any given set of rating condit

46、ions), Btu/WhIntegrated energy efficiency ratio (IEER; a cooling part-load effi-ciency rating), Btu/WhTable 1 VRF Multisplit System ClassificationsAttribute VRF Multisplit Air Conditioner or Heat Pump VRF Multisplit Heat RecoveryRefrigerant Circuits One shared with all indoor units One shared with a

47、ll indoor unitsCompressors One or more variable speed or alternative method resulting in three or more steps of capacityOne or more variable speed or alternative method resulting in three or more steps of capacityIndoor Units Quantity Greater than one indoor unitOperation Individual zones/temperatur

48、e Individual zones/temperatureOutdoor Unit(s)2Quantity One or multiple-manifolded outdoor units with a specific model numberOne or multiple-manifolded outdoor units with a specific model numberSteps of control Three or more Three or moreMode of operation Air conditioning, heat pump Air conditioning,

49、 heat pump, heat recoveryHeat exchanger One or more circuits of shared refrigerant flow One or more circuits of shared refrigerant flowAHRIClassificationAir conditioner (air-to-air) MSV-A1-CBAir conditioner (water-to-air) MSV-W1-CBHeat pump (air-to-air) HMSV-A1-CB HMSR-A1-CBHeat pump (water-to-air) HMSV-W1-CB HMSR-W1-CBSource: Adapted from AHRI Standard 1230-2010.Notes:1“-A” indicates air-cooled condenser and “-W” indicateswater-cooled condenser.2For purposes of tested combination definition, when twoor more outdoor units are connected, they are consideredas one outdoor u