ASHRAE REFRIGERATION IP CH 8-2010 EQUIPMENT AND SYSTEM DEHYDRATING CHARGING AND TESTING《脱水设备及系统 充电和测试》.pdf

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1、8.1CHAPTER 8EQUIPMENT AND SYSTEM DEHYDRATING,CHARGING, AND TESTINGDehydration (Moisture Removal) 8.1Moisture Measurement 8.3Charging 8.4Testing for Leaks. 8.5Performance Testing . 8.6ROPER dehydration, charging, and testing of packaged refrig-Peration systems and components (compressors, evaporators

2、,and condensing coils) help ensure proper performance and extendthe life of refrigeration systems. This chapter covers the methodsused to perform these functions. It does not address criteria such asallowable moisture content, refrigerant quantity, and performance,which are specific to each machine.

3、DEHYDRATION (MOISTURE REMOVAL)Factory dehydration may be feasible only for certain sizes ofequipment. On large equipment, which is open to the atmospherewhen connected in the field, factory treatment is usually limited topurge and backfill, with an inert holding charge of nitrogen. In mostinstances,

4、 this equipment is stored for short periods only, so thismethod suffices until total system evacuation and charging can bedone at the time of installation.Excess moisture in refrigeration systems may lead to freeze-upof the capillary tube or expansion valve. It also has a negative effecton thermal s

5、tability of certain refrigeration oils e.g., polyol ester(POE). (Chapter 7 has more information on moisture and othercontaminants in refrigerant systems).Except for freeze-up, these effects are not normally detected bya standard factory test.It is important to use a dehydration technique that yields

6、 a safemoisturelevelwithoutaddingforeignelementsorsolvents,becausecontaminantscancausevalvebreakage,motorburnout,andbearingand seal failure. In conjunction with dehydration, an accuratemethod of moisture measurement must be established. Many fac-tors, such as the size of the unit, its application, a

7、nd type of refriger-ant, determine acceptable moisture content. Table 1 shows moisturelimits recommended by various manufacturers for particular refrig-eration system components.Sources of MoistureMoisture in refrigerant systems can be (1) retained on the sur-faces of metals; (2) produced by combust

8、ion of a gas flame; (3) con-tained in liquid fluxes, oil, and refrigerant; (4) absorbed in thehermetic motor insulating materials; (5) derived from the factoryambient at the point of unit assembly; and (6) provided by freewater. Moisture contained in the refrigerant has no effect on dehy-dration of

9、the component or unit at the factory. However, becausethe refrigerant is added after dehydration, it must be considered indetermining the overall moisture content of the completed unit.Moisture in oil may or may not be removed during dehydration,depending on when the oil is added to the component or

10、 system.Bulk mineral oils, as received, have 20 to 30 ppm of moisture.Synthetic POE lubricants have 50 to 85 ppm; they are highly hygro-scopic, so they must be handled appropriately to prevent moisturecontamination.Refrigerantshaveanacceptedcommercialtoleranceof 10 to 15 ppm on bulk shipments. Contr

11、ols at the factory areneeded to ensure these moisture levels in the oils and refrigerant aremaintained.Newer insulating materials in hermetic motors retain much lessmoisture compared to the old rag paper and cotton-insulatedmotors. However, tests by several manufacturers have shown thatthe stator, w

12、ith its insulation, is still the major source of moisture incompressors.Dehydration by Heat, Vacuum, or Dry AirHeat may be applied by placing components in an oven or byusing infrared heaters. Oven temperatures of 180 to 340F are usu-ally maintained. The oven temperature should be selected carefully

13、to prevent damage to the synthetics used and to avoid breakdown ofany residual run-in oil that may be present in compressors. Air in theoven must be maintained at low humidity. When dehydrating byheat alone, the time and escape area are critical; therefore, the sizeof parts that can be economically

14、dehydrated by this method isrestricted.The vacuum method reduces the boiling point of water belowthe ambient temperature. The moisture then changes to vapor,which is pumped out by the vacuum pump. Table 3 in Chapter 1 ofthe 2009 ASHRAE HandbookFundamentals shows the relation-ship of temperature and

15、pressure for water at saturation.Vacuum is classified according to the following absolute pres-sure ranges:Low Vacuum 29.92 to 1.0 in. HgMedium Vacuum 1.0to4105in. HgHigh Vacuum 4105to4108in. HgVery High Vacuum 4108to 1011in. HgUltrahigh Vacuum 41011in. Hg and belowThe degree of vacuum achieved and

16、the time required to obtainthe specified moisture level are a function of the (1) type and size ofvacuum pump used, (2) internal volume of the component or sys-tem, (3) size and composition of water-holding materials in the sys-tem, (4) initial amount of moisture in the volume, (5) piping andfitting

17、 sizes, (6) shape of the gas passages, and (7) external temper-atures maintained. The pumping rate of the vacuum pump is criticalonly if the unit is not evacuated through a conductance-limiting ori-fice such as a purge valve. Excessive moisture content, such as apocket of puddled water, takes a long

18、 time to remove because of thevolume expansion to vapor.Vacuummeasurementsshouldbetakendirectlyattheequipment(or as close to it as possible) rather than at the vacuum pump. Smalltubing diameters or long tubing runs between the pump and theThe preparation of this chapter is assigned to TC 8.1, Positi

19、ve Displace-ment Compressors.8.2 2010 ASHRAE HandbookRefrigerationequipment should be avoided because line/orifice pressure dropsreduce the actual evacuation level at the equipment.If dry air or nitrogen is drawn or blown through the equipmentfor dehydration, it removes moisture by becoming totally

20、or par-tially saturated. In systems with several passages or blind passages,flow may not be sufficient to dehydrate. The flow rate should obtainoptimum moisture removal, and its success depends on the overallsystem design and temperature.Combination MethodsEachofthefollowingmethodscanbeeffectiveifco

21、ntrolledcare-fully, but a combination of methods is preferred because of theshorter drying time and more uniform dryness of the treated system.HeatandVacuumMethod.Heat drives deeply sorbed moistureto the surfaces of materials and removes it from walls; the vacuumlowers the boiling point, making the

22、pumping rate more effective.The heat source can be an oven, infrared lamps, or an ac or dc cur-rent circulating through the internal motor windings of semiher-metic and hermetic compressors. Combinations of vacuum, heat,and then vacuum again can also be used.Heat and Dry-Air Method. Heat drives mois

23、ture from thematerials. The dry air picks up this moisture and removes it from thesystem or component. The dry air used should have a dew pointbetween 40 and 100F. Heat sources are the same as those men-tioned previously. Heat can be combined with a vacuum to acceler-ate the process. The heat and dr

24、y-air method is effective with open,hermetic, and semihermetic compressors. The heating temperatureshould be selected carefully to prevent damage to compressor partsor breakdown of any residual oil that may be present.Advantages and limitations of the various methods dependgreatly on the system or c

25、omponent design and the resultsTable 1 Typical Factory Dehydration and Moisture-Measuring Methods for Refrigeration SystemsComponent Dehydration Method Moisture Audit Moisture LimitCoils and tubing 250F oven, 70F dp dry-air sweep Dew point recorder 0.154 grEvaporator coilsSmall 70F dp dry-air sweep,

26、 240 s P2O50.386 grLarge 70F dp dry-air sweep, 240 s P2O51.00 grEvaporators/condensers 300F oven, 1 h, dry-air sweep Cold trap 3.09 grDry-air sweep Nesbitt tube 0.131 gr/ft2surf. areaCondensing unit (0.25 to 7.5 ton) Purchase dry P2O50.386 to 1.31 grDry-air sweep Nesbitt tube 0.131 gr/ft2surf. areaA

27、ir-conditioning unit Evacuate to 0.0095 in. Hg P2O535 ppm3 h winding heat, 0.5 h vacuum Refrigerant moisture check 25 ppmRefrigerator 250F oven, dc winding heat, vacuum Cold trap 3.1 grFreezer 70F dp dry-air ambient, 40F dp air sweep P2O510 ppmCompressorsdc Winding Heat0.5 h dc winding heat 350F, 0.

28、25 h vacuum/repeat Cold trap 3.1 grdc winding heat 190F, 0.5 h vacuum Cold trap 18.5 gr2 to 60 ton semihermetic dc winding heat, 30 min, evacuation, N2charge Cold trap 15.5 to 31 grOven Heat250F oven,4hvacuum Cold trap 2.78 gr250F oven, 5.5 h at 60F dp air Cold trap 3.1 gr0.5 to 12 ton hermetic 300F

29、 oven 4 h, 70F dp air 3.5 min Cold trap 2.3 to 6.2 gr50 to 100 ton Oven at 270F,4hevacuate to 0.04 in. Hg Cold trap 11.6 gr1.5 to 5 ton hermetic 340F oven, 100F dp dry air, 1.5 h Cold trap 1.54 to 7.72 gr2 to 40 ton semihermetic 250F oven, 100F dp dry air, 3.5 h Cold trap 1.54 to 17.0 gr5 to 150 ton

30、 open 175F oven, evacuate to 0.04 in. Hg Cold trap 6.17 to 41.7 grScroll 2 to 10 ton hermetic 300F oven 4 h, 50 s evacuation and 10 s 70Fdp air charge/repeat 7 timesCold trap 4.63 to 7.33 grHot Dry Air, N23to5 ton Dry air at 275F, 3 h Cold trap 3.86 gr7.5 to 15 ton Dry air at 275F, 0.5 h vacuum Cold

31、 trap 11.6 gr20 to 40 ton Dry N2sweep at 275F, 3.5 h evacuate to 0.0079 in. Hg Cold trap 11.6 grDry N2FlushReciprocating, semihermetic N2run, dry N2flush, N2charge Screw, hermetic/semihermetic R-22 run, dry N2flush, N2charge Screw, open N2run, dry N2flush, N2charge Evacuation OnlyScrew, open, 50 to

32、1500 ton Evacuate 0.059 in. Hg, N2charge Refrigerants As purchased Electronic analyzer Typically 10 ppmLubricantsMineral oil As purchased Karl Fischer method 25 to 35 ppmAs purchased and evacuation Hygrometer 10 ppmSynthetic polyol ester As purchased Karl Fischer method 50 to 85 ppmdp = dew pointEqu

33、ipment and System Dehydrating, Charging, and Testing 8.3expected. Goddard (1945) considers double evacuation with an airsweep between vacuum applications the most effective method,whereas Larsen and Elliot (1953) believe the dry-air method, ifcontrolled carefully, is just as effective as the vacuum

34、method andmuch less expensive, although it incorporates a 1.5 h evacuationafter the hot-air purge. Tests by manufacturers show that a 280Foven bake for 1.5 h, followed by a 20 min evacuation, effectivelydehydrates compressors that use newer insulating materials.MOISTURE MEASUREMENTMeasuring the corr

35、ect moisture level in a dehydrated system orpart is important but not always easy. Table 1 lists measuring meth-ods used by various manufacturers, and others are described in theliterature. Few standards are available, however, and acceptablemoisture limits vary by manufacturer.Cold-Trap Method. Thi

36、s common method of determiningresidual moisture monitors the production dehydration system toensure that it produces equipment that meets the required moisturespecifications. An equipment sample is selected after completionof the dehydration process, placed in an oven, and heated at 150 to275F (depe

37、nding on the limitations of the sample) for 4 to 6 h.During this time, a vacuum is drawn through a cold-trap bottleimmersed in an acetone and dry-ice solution (or an equivalent),which is generally held at about 100F. Vacuum levels arebetween 0.0004 and 0.004 in. Hg, with lower levels preferred.Impor

38、tant factors are leaktightness of the vacuum system andcleanliness and dryness of the cold-trap bottle.Vacuum Leakback. Measuring the rate of vacuum leakback isanother means of checking components or systems to ensure that nowater vapor is present. This method is used primarily in conjunctionwith a

39、unit or system evacuation that removes the noncondensablesbefore final charging. This test allows a check of each unit, but toorapid a pressure build-up may signify a leak, as well as incompletedehydration.Thetimefactormaybecriticalinthismethodandmustbe examined carefully. Blair and Calhoun (1946) s

40、how that a smallsurface area in connection with a relatively large volume of watermay only build up vapor pressure slowly. This method also does notgive the actual condition of the charged system.DewPoint.When dry air is used, a reasonably satisfactory checkfor dryness is a dew-point reading of the

41、air as it leaves the partbeing dried. If airflow is relatively slow, there should be a markeddifference in dew point between air entering and leaving the part,followed by a decrease in dew point of the leaving air until it even-tually equals the dew point of the entering air. As is the case with all

42、systems and methods described in this chapter, acceptable valuesdepend on the size, usage, and moisture limits desired. Differentmanufacturers use different limits.Gravimetric Method. In this method, described by ASHRAEStandard 35, a controlled amount of refrigerant is passed through atrain of flask

43、s containing phosphorous pentoxide (P2O5), and theweight increase of the chemical (caused by the addition of moisture)is measured. Although this method is satisfactory when the refrig-erant is pure, any oil contamination produces inaccurate results.This method must be used only in a laboratory or un

44、der carefullycontrolled conditions. Also, it is time-consuming and cannot beused when production quantities are high. Furthermore, the methodis not effective in systems containing only small charges of refrig-erant because it requires 0.44 to 0.66 lb of refrigerant for accurateresults. If it is used

45、 on systems where withdrawal of any amount ofrefrigerant changes the performance, recharging is required.Aluminum Oxide Hygrometer. This sensor consists of an alu-minumstripthatisanodizedbyaspecialprocesstoprovideaporousoxidelayer.Averythincoatingofgoldisevaporatedoverthisstruc-ture. The aluminum ba

46、se and gold layer form two electrodes thatessentially form an aluminum oxide capacitor.In the sensor, water vapor passes through the gold layer andcomes to equilibrium on the pore walls of the aluminum oxide indirectrelationtothevaporpressureofwaterintheambientsurround-ing the sensor. The number of

47、water molecules absorbed in the oxidestructure determines the sensors electrical impedance, which mod-ulates an electrical current output that is directly proportional to thewater vapor pressure. This device is suitable for both gases and liq-uids over a temperature range of 158 to 166F and a pressu

48、re rangeof about 2 105to 5000 psia. The Henrys Law constant (satura-tion parts per million by mass of water for the fluid divided by thesaturated vapor pressure of water at a constant temperature) for eachfluid must be determined. For many fluids, this constant must be cor-rected for the operating t

49、emperature at the sensor.Christensen Moisture Detector. The Christensen moisture de-tectorisused foraquick checkof uncharged componentsor unitsonthe production line. In this method, dry air is blown first through thedehydrated part and then over a measured amount of calcium sulfate(CaSO4). The temperature of the CaSO4rises in proportion to thequantity of water it absorbs, and desired limits can be set and mon-itored. One manufacturer reports that coils were checked in 10 swith this method. Moisture limits for this