ASHRAE REFRIGERATION SI CH 7-2010 CONTROL OF MOISTURE AND OTHER CONTAMINANTS IN REFRIGERANT SYSTEMS《制冷系统中水分控制和其它污染物》.pdf

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1、7.1CHAPTER 7CONTROL OF MOISTURE AND OTHER CONTAMINANTS IN REFRIGERANT SYSTEMSMoisture 7.1Other Contaminants 7.6System Cleanup Procedure After Hermetic Motor Burnout . 7.8Contaminant Control During Retrofit. 7.9Chiller Decontamination. 7.10MOISTUREOISTURE (water) is an important and universal contami

2、nantM in refrigeration systems. The amount of moisture in a refrig-erant system must be kept below an allowable maximum for satis-factory operation, efficiency, and longevity. Moisture must beremoved from components during manufacture, assembly, and ser-vice to minimize the amount of moisture in the

3、 completed system.Any moisture that enters during installation or servicing should beremoved promptly.Sources of MoistureMoisture in a refrigerant system results from Inadequate equipment drying in factories and service operationsIntroduction during installation or service operations in the fieldLea

4、ks, resulting in entrance of moisture-laden airLeakage of water-cooled heat exchangersOxidation of some hydrocarbon lubricants that produce moistureWet lubricant, refrigerant, or desiccantMoisture entering a nonhermetic refrigerant system throughhoses and sealsDrying equipment in the factory is disc

5、ussed in Chapter 5. Properinstallation and service procedures as given in ASHRAE Standard147 minimize the second, third, and fourth sources. Lubricants arediscussed in Chapter 12. If purchased refrigerants and lubricantsmeet specifications and are properly handled, the moisture contentgenerally rema

6、ins satisfactory. See the section on Electrical Insula-tion under Compatibility of Materials in Chapter 6 and the sectionon Motor Burnouts in this chapter.Effects of MoistureExcess moisture in a refrigerating system can cause one or all ofthe following undesirable effects:Ice formation in expansion

7、valves, capillary tubes, or evaporatorsCorrosion of metalsCopper platingChemical damage to motor insulation in hermetic compressors orother system materialsHydrolysis of lubricants and other materialsSludge formationIce or solid hydrate separates from refrigerants if the water con-centration is high

8、 enough and the temperature low enough. Solidhydrate, a complex molecule of refrigerant and water, can form attemperatures higher than those required to separate ice. Liquid waterforms at temperatures above those required to separate ice or solidhydrate. Ice forms during refrigerant evaporation when

9、 the relativesaturation of vapor reaches 100% at temperatures of 0C or below.The separation of water as ice or liquid also is related to the sol-ubility of water in a refrigerant. This solubility varies for differentrefrigerants and with temperature (Table 1). Various investigatorshave obtained diff

10、erent results on water solubility in R-134a andR-123. The data presented here are the best available. The greaterthe solubility of water in a refrigerant, the less the possibility that iceor liquid water will separate in a refrigerating system. The solubilityof water in ammonia, carbon dioxide, and

11、sulfur dioxide is so highthat ice or liquid water separation does not occur.The concentration of water by mass at equilibrium is greater inthe gas phase than in the liquid phase of R-12 (Elsey and Flowers1949). The opposite is true for R-22 and R-502. The ratio of massconcentrations differs for each

12、 refrigerant; it also varies with tem-perature. Table 2 shows the distribution ratios of water in the vaporphase to water in the liquid phase for common refrigerants. It canbe used to calculate the equilibrium water concentration of the liq-uid-phase refrigerant if the gas phase concentration is kno

13、wn, andvice versa.Freezing at expansion valves or capillary tubes can occur whenexcessive moisture is present in a refrigerating system. Formationof ice or hydrate in evaporators can partially insulate the evaporatorand reduce efficiency or cause system failure. Excess moisture cancause corrosion an

14、d enhance copper plating (Walker et al. 1962).Other factors affecting copper plating are discussed in Chapter 6.The preparation of this chapter is assigned to TC 3.3, Refrigerant Contami-nant Control.Table 1 Solubility of Water in Liquid Phase ofCertain Refrigerants, ppm (by mass)Temp., CR-11 R-12 R

15、-13 R-22 R-113 R-114 R-123R-134aR-410A R-50270 470 620 3900 460 480 2500 4100 180060 350 430 3100 340 340 2000 3200 7200 140050 250 290 2500 250 230 1600 2500 4800 110040 180 190 1900 180 158 1300 1900 3100 84030 120 120 1500 120 104 1000 1400 2000 62020 83 72 1100 83 67 740 1010 1200 46010 55 43 35

16、 810 55 42 550 720 700 3300 35 24 20 581 35 25 400 500 400 23010 21 13 10 407 22 14 290 340 220 15020 13 7.0 5 277 13 8 200 230 110 10130 7 3.5 2 183 8 4 135 143 54 6440 4 1.6 1 116 2 88 87 25 3950 2 0.7 71 1 55 51 2360 1 0.3 42 0.4 33 28 1370 0.4 23 0.2 19 15 7Data on R-134a adapted from Thrasher e

17、t al. (1993) and Allied-Signal Corporation.Data on R-123 adapted from Thrasher et al. (1993) and E.I. DuPont de Nemours the color changesat a low enough level to be safe. Manufacturers instructions must befollowed because the color change point is also affected by liquid-line temperature and the ref

18、rigerant used.Moisture MeasurementTechniques for measuring the amount of moisture in a compres-sor, or in an entire system, are discussed in Chapter 8. The followingmethods are used to measure the moisture content of various halo-carbon refrigerants. The moisture content to be measured is gener-ally

19、 in the milligram-per-kilogram range, and the proceduresrequire special laboratory equipment and techniques.The Karl Fischer method is suitable for measuring the moisturecontent of a refrigerant, even if it contains mineral oil. Although dif-ferent firms have slightly different ways of performing th

20、is test andget somewhat varying results, the method remains the commonindustry practice for determining moisture content in refrigerants.The refrigerant sample is bubbled through predried methyl alcoholin a special sealed glass flask; any water present remains with thealcohol. In volumetric titratio

21、n, Karl Fischer reagent is added, andthe solution is immediately titrated to a “dead stop” electrometricend point. The reagent reacts with any moisture present so that theamount of water in the sample can be calculated from a previous cal-ibration of the Karl Fischer reagent. In coulometric titratio

22、n (AHRI Standard 700C), water is titratedwith iodine that is generated electrochemically. The instrument mea-sures the quantity of electric charge used to produce the iodine andtitrate the water and calculates the amount of water present.These titration methods, considered among the most accurate,ar

23、e also suitable for measuring the moisture content of unused lubri-cant or other liquids. Special instruments designed for this particu-lar analysis are available from laboratory supply companies.Haagen-Smit et al. (1970) describe improvements in the equipmentand technique that significantly reduce

24、analysis time.The gravimetric method for measuring moisture content ofrefrigerants is described in ASHRAE Standards 35 and 63.1. It isnot widely used in the industry. In this method, a measuredamount of refrigerant vapor is passed through two tubes in series,each containing phosphorous pentoxide (P2

25、O5). Moisture presentin the refrigerant reacts chemically with the P2O5and appears asan increase in mass in the first tube. The second tube is used as atare. This method is satisfactory when the refrigerant is pure, butthe presence of lubricant produces inaccurate results, because thelubricant is we

26、ighed as moisture. Approximately 200 g of refriger-ant is required for accurate results. Because the refrigerant mustpass slowly through the tube, analysis requires many hours.DeGeiso and Stalzer (1969) discuss the electrolytic moistureanalyzer, which is suitable for high-purity refrigerants. Other

27、elec-tronic hygrometers are available that sense moisture by the adsorp-tion of water on an anodized aluminum strip with a gold foil overlay(Dunne and Clancy 1984). Calibration is critical to obtain maximumaccuracy. These hygrometers give a continuous moisture readingand respond rapidly enough to mo

28、nitor changes. Data showing dry-down rates can be gathered with these instruments (Cohen 1994).Brisken (1955) used this method in a study of moisture migration inhermetic equipment.Thrasher et al. (1993) used nuclear magnetic resonance spectros-copy to determine the moisture solubilities in R-134a a

29、nd R-123.Another method, infrared spectroscopy, is used for moisture analy-sis, but requires a large sample for precise results and is subject tointerference if lubricant is present in the refrigerant.DesiccantsDesiccants used in refrigeration systems adsorb or react chemi-cally with the moisture co

30、ntained in a liquid or gaseous refrigerant/lubricant mixture. Solid desiccants, used widely as dehydratingagents in refrigerant systems, remove moisture from both new andfield-installed equipment. The desiccant is contained in a devicecalled a drier (also spelled dryer) or filter-drier and can be in

31、stalledin either the liquid or the suction line of a refrigeration system.Desiccants must remove most of the moisture and not react unfa-vorably with any other materials in the system. Activated alumina,silica gel, and molecular sieves are the most widely used desiccantsacceptable for refrigerant dr

32、ying. Water is physically adsorbed onthe internal surfaces of these highly porous desiccant materials.Activated alumina and silica gel have a wide range of pore sizes,which are large enough to adsorb refrigerant, lubricant, additives,and water molecules. Pore sizes of molecular sieves, however, areu

33、niform, with an aperture of approximately 0.3 nm for a type 3Amolecular sieve or 0.4 nm for a type 4A molecular sieve. The uni-form openings exclude lubricant molecules from the adsorptionsurfaces. Molecular sieves can be selected to exclude refrigerantmolecules, as well. This property gives the mol

34、ecular sieve the ad-vantage of increasing water capacity and improving chemical com-patibility between refrigerant and desiccant (Cohen 1993, 1994;Cohen and Blackwell 1995). The drier or desiccant manufacturercan provide information about which desiccant adsorbs or excludesa particular refrigerant.D

35、rier manufacturers offer combinations of desiccants that can beused in a single drier and may have advantages over a single desic-cant because they can adsorb a greater variety of refrigeration con-taminants. Two combinations are activated alumina with molecularsieves and silica gel with molecular s

36、ieves. Activated carbon is alsoused in some combinations.Desiccants are available in granular, bead, and block forms. Solidcore desiccants, or block forms, consist of desiccant beads, granules,or both held together by a binder (Walker 1963). The binder isusually a nondesiccant material. Suitable fil

37、tration, adequate contactbetween desiccant and refrigerant, and low pressure drop are7.4 2010 ASHRAE HandbookRefrigeration (SI)obtained by properly sizing the desiccant particles used to make upthe core, and by the proper geometry of the core with respect to theflowing refrigerant. Beaded molecular

38、sieve desiccants have higherwater capacity per unit mass than solid-core desiccants. The compo-sition and form of the desiccant are varied by drier manufacturers toachieve the desired properties.Desiccants that take up water by chemical reaction are not recom-mended. Calcium chloride reacts with wat

39、er to form a corrosive liquid.Barium oxide is known to cause explosions. Magnesium perchlorateand barium perchlorate are powerful oxidizing agents, which arepotential explosion hazards in the presence of lubricant. Phosphorouspentoxide is an excellent desiccant, but its fine powdery form makes itdif

40、ficult to handle and produces a high resistance to gas and liquidflow. A mixture of calcium oxide and sodium hydroxide, which haslimited use as an acid scavenger, should not be used as a desiccant.Desiccants readily adsorb moisture and must be protectedagainst it until ready for use. If a desiccant

41、has picked up moisture,it can be reactivated under laboratory conditions by heating forabout 4 h at a suitable temperature, preferably with a dry-air purgeor in a vacuum oven (Table 3). Only adsorbed water is driven off atthe temperatures listed, and the desiccant is returned to its initialactivated

42、 state. Avoid repeated reactivation and excessive tempera-tures during reactivation, which may damage the desiccant. Desic-cant in a refrigerating equipment drier should not be reactivated forreuse, because of lubricant and other contaminants in the drier aswell as possible damage caused by overheat

43、ing the drier shell.Equilibrium Conditions of Desiccants. Desiccants in refrigera-tion and air-conditioning systems function on the equilibrium prin-ciple. If an activated desiccant contacts a moisture-laden refrigerant,the water is adsorbed from the refrigerant/water mixture onto the des-iccant sur

44、face until the vapor pressures of the adsorbed water (i.e., atthe desiccant surface) and the water remaining in the refrigerant areequal. Conversely, if the vapor pressure of water on the desiccant sur-face is higher than that in the refrigerant, water is released into therefrigerant/water mixture,

45、and equilibrium is reestablished.Adsorbent desiccants function by holding (adsorbing) moistureon their internal surfaces. The amount of water adsorbed from a re-frigerant by an adsorbent at equilibrium is influenced by (1) porevolume, pore size, and surface characteristics of the adsorbent;(2) tempe

46、rature and moisture content of the refrigerant; and (3) sol-ubility of water in the refrigerant.Figures 1 to 3 are equilibrium curves (known as adsorption iso-therms) for various adsorbent desiccants with R-12 and R-22.These curves are representative of commercially available materi-als. The adsorpt

47、ion isotherms are based on the technique developedby Gully et al. (1954), as modified by ASHRAE Standard 35.ASHRAE Standards 35 and 63.1 define the moisture content of therefrigerant as equilibrium point dryness (EPD), and the moistureheld by the desiccant as water capacity. The curves show that for

48、 anyspecified amount of water in a particular refrigerant, the desiccantholds a corresponding specific quantity of water.Figures 1 and 2 show moisture equilibrium curves for threecommon adsorbent desiccants in drying R-12 and R-22 at 24C. Asshown, desiccant capacity can vary widely for different ref

49、rigerantswhen the same EPD is required. Generally, a refrigerant in whichmoisture is more soluble requires more desiccant for adequate dry-ing than one that has less solubility.Figure 3 shows the effect of temperature on moisture equilib-rium capacities of activated alumina and R-12. Much higher watercapacities are obtained at lower temperatures, demonstrating theadvantage of locating alumina driers at relatively cool spots in thesystem. The effect of temperature on molecular sieves watercapacity is much smaller. AHRI Standard 711 requires de