ASHRAE REFRIGERATION IP CH 11-2010 REFRIGERANT-CONTROL DEVICES《制冷控制设备》.pdf

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1、11.1CHAPTER 11REFRIGERANT-CONTROL DEVICESCONTROL SWITCHES 11.1Pressure Switches. 11.1Temperature Switches (Thermostats) . 11.2Differential Switches 11.2Float Switches 11.3CONTROL SENSORS 11.4Pressure Transducers. 11.4Thermistors 11.4Resistance Temperature Detectors. 11.4Thermocouples. 11.4Liquid Lev

2、el Sensors 11.4CONTROL VALVES . 11.4Thermostatic Expansion Valves . 11.5Electric Expansion Valves 11.10REGULATING AND THROTTLING VALVES. 11.11Evaporator-Pressure-Regulating Valves 11.12Constant-Pressure Expansion Valves. 11.14Suction-Pressure-Regulating Valves 11.14Condenser-Pressure-Regulating Valv

3、es . 11.15Discharge Bypass Valves 11.16High-Side Float Valves. 11.17Low-Side Float Valves 11.17Solenoid Valves 11.17Condensing Water Regulators 11.20Check Valves. 11.21Relief Devices. 11.22DISCHARGE-LINE LUBRICANT SEPARATORS 11.23CAPILLARY TUBES 11.24Adiabatic Capillary Tube Selection Procedure 11.2

4、6Capillary-Tube/Suction-Line Heat Exchanger Selection Procedure 11.29SHORT-TUBE RESTRICTORS 11.31ONTROL of refrigerant flow, temperatures, pressures, and liq-Cuid levels is essential in any refrigeration system. This chapterdescribes a variety of devices and their application to accomplishthese impo

5、rtant control functions.Most examples, references, and capacity data in this chapter referto the more common refrigerants. For further information on controlfundamentals, see Chapter 7 of the 2009 ASHRAE HandbookFun-damentals and Chapter 46 of the 2007 ASHRAE HandbookHVACApplications.CONTROL SWITCHE

6、SA control switch includes both a sensor and a switch mechanismcapable of opening and/or closing an electrical circuit in response tochanges in the monitored parameter. The control switch operatesone or more sets of electrical contacts, which are used to open orclose water or refrigerant solenoid va

7、lves; engage and disengageautomotive compressor clutches; activate and deactivate relays,contactors, magnetic starters, and timers; etc. Control switchesrespond to a variety of physical changes, such as pressure, temper-ature, and liquid level.Liquid-level-responsive controls use floats or electroni

8、c probesto operate (directly or indirectly) one or more sets of electrical con-tacts.Refrigeration control switches may be categorized into threebasic groups: Operating controls (e.g., thermostats) turn systems on and off.Primary controls provide safe continuous operation (e.g., com-pressor or conde

9、nser fan cycling).Limit controls (e.g., high-pressure cutout switch) protect a sys-tem from unsafe operation.PRESSURE SWITCHESPressure-responsive switches have one or more power elements(e.g., bellows, diaphragms, bourdon tubes) to produce the forceneeded to operate the mechanism. Typically, pressur

10、e-switch powerelements are all metal, although some miniaturized devices for spe-cific applications, such as automotive air conditioning, may use syn-thetic diaphragms. Refrigerant pressure is applied directly to theelement, which moves against a spring that can be adjusted to con-trol an operation

11、at the desired pressure (Figure 1). If the control isto operate in the subatmospheric (or vacuum) range, the bellows ordiaphragm force is sometimes reversed to act in the same directionas the adjusting spring.The force available for doing work (i.e., operating the switchmechanism) in this control de

12、pends on the pressure in the systemand on the area of the bellows or diaphragm. With proper area,enough force can be produced to operate heavy-duty switches. Inswitches for high-pressure service, the minimum differential is rel-atively large because of the high-gradient-range spring required.Miniatu

13、rized pressure switches may incorporate one or moresnap disks, which provide positive snap action of the electrical con-tacts. Snap-disk construction ensures consistent differential pres-sure between on and off settings (Figure 2); it also substantiallyreduces electrical contact bounce or flutter, w

14、hich can damage com-pressor clutch assemblies, relays, and electronic control modules.Some snap-disk switches are built to provide multiple functions inThe preparation of this chapter is assigned to TC 8.8, Refrigerant SystemControls and Accessories.Fig. 1 Typical Pressure SwitchFig. 1 Typical Press

15、ure Switch11.2 2010 ASHRAE HandbookRefrigerationa single unit, such as high-pressure cutout (HPCO), high-side low-pressure (HSLP), and high-side fan-cycling (HSFC) switches.Pressure switches in most refrigeration systems are used primar-ily to start and stop the compressor, cycle condenser fans, and

16、 initi-ate and terminate defrost cycles. Table 1 shows various types ofpressure switches with their corresponding functions.TEMPERATURE SWITCHES (THERMOSTATS)Temperature-responsive switches have one or more metal powerelements (e.g., bellows, diaphragms, bourdon tubes, bimetallic snapdisks, bimetall

17、ic strips) that produce the force needed to operate theswitch.An indirect temperature switch is a pressure switch with thepressure-responsive element replaced by a temperature-responsiveelement. The temperature-responsive element is a hermeticallysealed system comprised of a flexible member (diaphra

18、gm or bel-lows) and a temperature-sensing element (bulb or tube) that are inpressure communication with each other (Figure 3). The closed sys-tem contains a temperature-responsive fluid.The exact temperature/pressure or temperature/volume relation-ship of the fluid used in the element allows the bul

19、b temperature tocontrol the switch accurately. The switch is operated by changes inpressure or volume that are proportional to changes in sensor tem-perature.A direct temperature switch typically contains a bimetallicdisk or strip that activates electrical contacts when the temperatureincreases or d

20、ecreases. As its temperature increases or decreases,the bimetallic element bends or strains because of the two metalsdifferent coefficients of thermal expansion, and the linked electricalcontacts engage or disengage. The disk bimetallic element providessnap action, which results in rapid and positiv

21、e opening or closing ofthe electrical contacts, minimizing arcing and bounce. A bimetallicstrip (Figure 4) produces very slow contact action and is only suit-able for use in very-low-energy electrical circuits. This type ofswitch is typically used for thermal limit control because the switchdifferen

22、tials and precision may be inadequate for many primaryrefrigerant control requirements.DIFFERENTIAL SWITCHESDifferential control switches typically maintain a given differ-ence in pressure or temperature between two pipelines, spaces, orloads. An example is the lubricant pressure differential failur

23、eswitch used with reciprocating compressors that use forced-feedlubrication.Figure 5 is a schematic of a differential switch that uses bellowsas power elements. Figure 6 shows a differential pressure switchused to protect compressors against low oil pressure. These controlshave two elements (either

24、pressure- or temperature-sensitive) simul-taneously sensing conditions at two locations. As shown, the twoelements are rigidly connected by a rod, so that motion of onecauses motion of the other. The connecting rod operates contacts (asshown). The scale spring is used to set the differential pressur

25、e atwhich the device operates. At the control point, the sum of forcesdeveloped by the low-pressure bellows and spring balances theforce developed by the high-pressure bellows.Instrument differential is the difference in pressure or temper-ature between the low- and the high-pressure elements for wh

26、ich theinstrument is adjusted. Operating differential is the change inTable 1 Various Types of Pressure SwitchesType FunctionHigh-pressure cutout (HPCO) Stops compressor when excessive pressure occursHigh-side low-pressure (HSLP) Prevents compressor operation under low ambient or loss of refrigerant

27、 conditionsHigh-side fan-cycling (HSFC) Cycles condenser fan on and off to provide proper condenser pressureLow-side low-pressure (LSLP) Initiates defrost cycle; stops compressor when low charge or system blockage occursLow-side compressor cycling (LSCC)Cycles compressor on and off to provide proper

28、 evaporator pressure and load temperatureLubricant pressure differential failure (LPDF)Stops compressor when difference between oil pressure and crankcase pressure is too low for adequate lubricationFig. 2 Miniaturized Pressure SwitchFig. 2 Miniaturized Pressure SwitchFig. 3 Indirect Temperature Swi

29、tchFig. 3 Indirect Temperature SwitchFig. 4 Direct Temperature SwitchFig. 4 Direct Temperature SwitchRefrigerant-Control Devices 11.3differential pressure or temperature required to open or close theswitch contacts. It is actually the change in instrument differentialfrom cut-in to cutout for any se

30、tting. Operating differential can bevaried by a second spring that acts in the same direction as the firstand takes effect only at the cut-in or cutout point without affectingthe other spring. A second method is adjusting the distance betweencollars Z-Z on the connecting rod. The greater the distanc

31、e betweenthem, the greater the operating differential.If a constant instrument differential is required on a temperature-sensitive differential control switch throughout a large temperaturerange, one element may contain a different temperature-responsivefluid than the other.A second type of differen

32、tial-temperature control uses two sens-ing bulbs and capillaries connected to one bellows with a liquid fill.This is known as a constant-volume fill, because the operating pointdepends on a constant volume of the two bulbs, capillaries, and bel-lows. If the two bulbs have equal volume, a temperature

33、 rise in onebulb requires an equivalent fall in the others temperature to main-tain the operating point.FLOAT SWITCHESA float switch has a float ball, the movement of which operatesone or more sets of electrical contacts as the level of a liquidchanges. Float switches are connected by equalizing lin

34、es to thevessel or an external column in which the liquid level is to bemaintained or monitored. The switch mechanism is generally her-metically sealed. Small heaters can be incorporated to prevent mois-ture from permeating the polycarbonate housing in cold operatingconditions. Other nonmechanical d

35、evices, such as capacitanceprobes, use other methods to monitor the change in liquid level.Operation and SelectionSome float switches (Figure 7) operate from movement of a mag-netic armature located in the field of a permanent magnet. Othersuse solid-state circuits in which a variable signal is gene

36、rated by liq-uid contact with a probe that replaces the float; this method isadapted to remote-controlled applications and is preferred forultralow-temperature applications.ApplicationThe float switch can maintain or indicate the level of a liquid,operate an alarm, control pump operation, or perform

37、 other func-tions. A float switch, solenoid liquid valve, and hand expansionvalve combination can control refrigerant level on the high- or low-pressure side of the refrigeration system in the same way that high-or low-side float valves are used. The hand expansion valve, locatedin the refrigerant l

38、iquid line immediately downstream of the sole-noid valve, is initially adjusted to provide a refrigerant flow rate atmaximum load to keep the solenoid liquid valve in the open position80 to 90% of the time; it need not be adjusted thereafter. From theoutlet side of the hand expansion valve, refriger

39、ant passes through aline and enters either the evaporator or the surge drum.When the float switch is used for low-pressure level control,precautions must be taken to provide a calm liquid level that falls inresponse to increased evaporator load and rises with decreasedevaporator load. The same recom

40、mendations for insulation of thebody and liquid leg of the low-pressure float valve apply to the floatswitch when it is used for refrigerant-level control on the low-pressure side of the refrigeration system. To avoid floodback, con-trols should be wired to prevent the solenoid liquid valve fromopen

41、ing when the solenoid suction valve closes or the compressorstops.Fig. 5 Differential Switch SchematicFig. 5 Differential Switch SchematicFig. 6 Differential Pressure SwitchFig. 6 Differential Pressure SwitchFig. 7 Magnetic Float SwitchFig. 7 Magnetic Float Switch11.4 2010 ASHRAE HandbookRefrigerati

42、onCONTROL SENSORSThe control sensor is the component in a control system thatmeasures and signals the value of a parameter but has no direct func-tion control. Control sensors typically require an auxiliary source ofenergy for proper operation. They may be integrated into electroniccircuits that pro

43、vide the required energy and condition the sensorssignal to accomplish the desired function control.PRESSURE TRANSDUCERSPressure transducers sense refrigerant pressure through a flexibleelement (diaphragm, bourdon tube, or bellows) that is exposed tothe system refrigerant pressure. The pressure acts

44、 across the flexibleelements effective area, producing a force that causes the flexibleelement to strain against an opposing spring within the transducer.The transducer uses a potentiometer, variable capacitor, strain gage,or piezo element to translate the flexible elements movement to aproportional

45、 electrical output.Transducers typically include additional electronic signal pro-cessing circuitry to temperature-compensate, modify, amplify, andlinearize the final analog electrical output. Typically, the outside ofthe pressure-sensing flexible element is exposed to atmosphericpressure and the tr

46、ansducers electrical output is proportional to therefrigerants gage pressure. Transducers capable of measuring abso-lute pressure are also available.Transducers are usually used as control sensors in electronic con-trol systems, where the continuous analog pressure signal providesdata to comprehensi

47、ve algorithm-based control strategies. Forexample, in automotive air-conditioning systems, engine load man-agement can be significantly enhanced. Based on a correlationbetween refrigerant pressure and compressor torque requirements,the electronic engine controller uses the transducer signal to regu-

48、late engine air and fuel flow, compensating for compressor loadvariations. This improves fuel economy and eliminates the powerdrain experienced when the compressor starts. Transducers alsoprovide a signal to electronically controlled variable-displacementcompressors to adjust refrigerant flow from t

49、he evaporator, prevent-ing excessive cooling of the air and further improving fuel economy.THERMISTORSThermistors are cost-effective and reliable temperature sensors.They are typically small and are available in a variety of configura-tions and sheath materials. Thermistors are beads of semiconductormaterials with electrical resistances that change with temperature.Materials with negative temperature coefficients (NTC) (i.e., resis-tance decreases as temperature increases) are frequently used. NTCthermistors typically produce large changes in resistance with rela-tively s