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 mg/kg of moisture.Synthetic POE lubricants have 50 to 85 mg/kg; they are highlyhygroscopic, so they must be handled appropriately to preventmoisture contamination. Refrigerants have an accepted commercialtolerance of 10 to 15 mg/kg on bulk shipme
11、nts. Controls at the fac-tory are needed to ensure these moisture levels in the oils and refrig-erant are maintained.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 tha
12、tthe stator, with 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 80 to 170C are usu-ally maintained. The oven temperature should be selec
13、ted carefullyto 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
14、economically 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 tem
15、perature and pressure for water at saturation.Vacuum is classified according to the following absolute pres-sure ranges:Low Vacuum 101.325 to 3.5 kPaMedium Vacuum 3500 to 0.130 PaHigh Vacuum 130 to 0.13 mPaVery High Vacuum 130 to 0.13 PaUltrahigh Vacuum 0.13 Pa and belowThe degree of vacuum achieved
16、 and 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 andfi
17、tting 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
18、 long 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, P
19、ositive Displace-ment Compressors.8.2 2010 ASHRAE HandbookRefrigeration (SI)equipment 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 becomin
20、g totally 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 MethodsEachofthefollowingmethodscanbeeff
21、ectiveifcontrolledcare-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, m
22、aking the 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 d
23、rives moisture 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 73C. Heat sources are the same as those men-tioned previously. Heat can be combined with a vacuum to acceler-ate the process. The he
24、at and dry-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 sy
25、stem or component design and the resultsTable 1 Typical Factory Dehydration and Moisture-Measuring Methods for Refrigeration SystemsComponent Dehydration Method Moisture Audit Moisture LimitCoils and tubing 121Coven, 57Cdry-air sweep Dew point recorder 10 mgEvaporator coilsSmall 59Cdp dry-air sweep,
26、 240 s P2O525 mgLarge 59Cdp dry-air sweep, 240 s P2O565 mgEvaporators/condensers 149Coven, 1 h, dry-air sweep Cold trap 200 mgDry-air sweep Nesbitt tube 90 mg/m2surf. areaCondensing unit (1 to 25 kW) Purchase dry P2O525 to 85 mgDry-air sweep Nesbitt tube 90 mg/m2surf. areaAir-conditioning unit Evacu
27、ate to 32 Pa P2O535 mg/kg3 h winding heat, 0.5 h vacuum Refrigerant moisture check 25 mg/kgRefrigerator 121Coven, dc winding heat, vacuum Cold trap 200 mgFreezer 59Cdp dry-air ambient, 40C dp air sweep P2O510 mg/kgCompressorsdc Winding Heat0.5 h dc winding heat 177C, 0.25 h vacuum/repeat Cold trap 2
28、00 mgdc winding heat 88C, 0.5 h vacuum Cold trap 1.200 mg7 to 210 kW semihermetic dc winding heat, 30 min, evacuation, N2charge Cold trap 1.000 to 3.500 mgOven Heat121Coven,4hvacuum Cold trap 180 mg121Coven, 5.5 h at 51Cdp air Cold trap 200 mg2 to 40 kW hermetic 149Coven 4 h, 59Cdp air 3.5 min Cold
29、trap 150 to 400 mg175 to 350 kW Oven at 132C,4hevacuate to 133 Pa Cold trap 1000 mg5 to 20 kW hermetic 171Coven, 73Cdp dry air, 1.5 h Cold trap 100 to 500 mg7 to 140 kW semihermetic 121Coven, 73Cdp dry air, 3.5 h Cold trap 0.100 to 1.100 mg20 to 525 kW open 79Coven, evacuate to 133 Pa Cold trap 0.40
30、0 to 2.700 mgScroll 7 to 35 kW hermetic 149Coven 4 h, 50 s evacuation and 10 s 59Cdp air charge/repeat 7 timesCold trap 300 to 475 mgHot Dry Air, N210 to 20 kW Dry air at 135C, 3 h Cold trap 250 mg25 to 55 kW Dry air at 135C, 0.5 h vacuum Cold trap 750 mg70 to 140 kW Dry N2sweep at 135C, 3.5 h evacu
31、ate to 27 Pa Cold trap 750 mgDry N2FlushReciprocating, semihermetic N2run, dry N2flush, N2charge Screw, hermetic/semihermetic R-22 run, dry N2flush, N2charge Screw, open N2run, dry N2flush, N2charge Evacuation OnlyScrew, open, 175 to 5300 kW Evacuate 200 Pa, N2charge Refrigerants As purchased Electr
32、onic analyzer Typically 10 mg/kgLubricantsMineral oil As purchased Karl Fischer method 25 to 35 mg/kgAs purchased and evacuation Hygrometer 10 mg/kgSynthetic polyol ester As purchased Karl Fischer method 50 to 85 mg/kgdp = dew pointEquipment and System Dehydrating, Charging, and Testing 8.3expected.
33、 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 method andmuch less expensive, although it incorporates a 1.5 h e
34、vacuationafter the hot-air purge. Tests by manufacturers show that a 138Coven bake for 1.5 h, followed by a 20 min evacuation, effectivelydehydrates compressors that use newer insulating materials.MOISTURE MEASUREMENTMeasuring the correct moisture level in a dehydrated system orpart is important but
35、 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. This common method of determiningresidual moisture monitors the prod
36、uction 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 56 to135C (depending on the limitations of the sample) for 4 to 6 h.During this t
37、ime, 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 73C. Vacuum levels are between1.3 and 13 Pa, with lower levels preferred. Important factors areleaktightness of the vacuum system and cleanliness and dryne
38、ss ofthe 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 unit or system evacuation that removes the noncondensablesbefore final chargi
39、ng. 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) show that a smallsurface area in connection with a relatively large volume of
40、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 air as it leaves the partbeing dried. If airflow is relatively slow, there sh
41、ould 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 allsystems and methods described in this chapter, acceptable valuesdepend on the
42、 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 flasks containing phosphorous pentoxide (P2O5), and themass increase of the chemic
43、al (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 under carefullycontrolled conditions. Also, it is time-consuming and cannot beuse
44、d when production quantities are high. Furthermore, the methodis not effective in systems containing only small charges of refrig-erant because it requires 200 to 300 g of refrigerant for accurateresults. If it is used on systems where withdrawal of any amount ofrefrigerant changes the performance,
45、recharging is required.Aluminum Oxide Hygrometer. This sensor consists of an alu-minumstripthatisanodizedbyaspecialprocesstoprovideaporousoxidelayer.Averythincoatingofgoldisevaporatedoverthisstruc-ture. The aluminum base and gold layer form two electrodes thatessentially form an aluminum oxide capac
46、itor.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 water molecules absorbed in the oxidestructure determines the sensors electrical i
47、mpedance, 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 70 to 110C and a pressure rangeof about 1 Pa to 34.5 MPa. The Henrys Law constant (saturationparts per mil
48、lion by mass of water for the fluid divided by the satu-rated 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 temperature at the sensor.Christensen Moisture Detector. The Christensen moisture de-
49、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 detector are 2 to 60 mg.Corrections must be made for variations in desiccant grain size, thequanti
