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776 CHAPTER 49 1965 Guide And Data Book DRYING METHODS The final choice of the method used in drying refrigerant systems depends on the desired moisture level, on size, avail able time and cost. (See Chapter 47.) It may be useful to give some details concerning some de hydration methods used in various plants. In the compressor plant described by Larsen and Elliot,u the parts supplied to the assembly line are dehydrated prior to assembly and then the assembled unit is also dehydrated. The evaporator tubing is baked 8 to 10 hr at 280 F, while dry air is passed through it. After assembling the whole unit, it is leak-tested at 300 psig while immersed in a tank of water. Units passing ... this test for relatively large leaks are then evacuated to less ' than 200 microns and heated in an oven at approximately fig. 1 .... Curves Showing Solubility of Water in Certain 250 F. Units which cannot hold this high vacuum, have either Refrigerants* - a leak or contain moisture. .While still being evacuated, the l / unit is placed in an atmosphere of helium. Any leak* will draw helium gas into the system, and these units are rejected at the . next station where, the gas inside the unit is analyzed with a origin as being due to water. Corrosion in a butane or isb- ' mas* spectrometer. Normally, the unit is then charged, test butane system is due to the direct action of water on metals.' run, electrically tested, and leak tested, with an electronic For further information, see Chapter 48. leak detector. Walker and Rinelli* have 6hown that approximately 90 Another drying procedure1* is designed to dehydrate refriger percent of the sludges analyzed by them were due to moisture; ation units to a maximum moisture limit of 0.03 g residual the others examined were due to oil. Prevalence of the burnout moisture per refrigerating machine. Dehydration is ac of hermetic' motors has shifted this percentage in favor'of complished by sending the refrigeration units through an sludges formed by motor insulation breakdown. It is prob oven heated to 300 F for at least 2 hr. While in the oven, the able that many salt type sludges, especially those containing unit is evacuated to below 1000 microns within the first few oil decomposition products, were due to thermal reactions of minutes. It leaves the oven at-a maximum pressure of 50 the oil and refrigerant. This reaction was not known until a microns. In the oven, electrical power is applied to the motor later date. In some systems, sludges are primarily formed by winding to bring the winding temperature to 300 F as quickly overheated motors and bearings and by insulation breakdown. as possible. s':-. ' . .. ,-Elsey7 and Steinle* have separately shown that a reaction A drying procedure for sulfur dioxide machines described may take plaoe at high temperatures between oil and the by McGovern,* includes heating in an oven at 250-275 F under halogenated refrigerants with resulting formation of hydro-, vacuum, greater than 29.5 in.,, for 4 hr. This is intended to chloric and hydrofluoric acids. In addition, the oil discolors reduce the moisture content of the machines to approximately and thickens and, if the reaction goes far enough, carbon is 0.0095 os of water per cu ft of internal machme volume. As a formed. The viscous or carbonized.oil may result in-severe means of removing the water vapor remaining in the machine, damage to bearings and valves. The acids cause severe cor? flushing with dry'air is recommended. A discussion of drying rorion. by vacuum treatment at room temperatore and flushing with Waring1* has shown that a relatively large quantity of mois^ dry air is also included in Reference 5. ture in a reciprocating system using Refrigerant 12 results in - Anderson14 has; shown a very desirable feature asyMurt'** accelerated flapper valve breakage. It was .found that the with the use of dry air, namely, that when dry air is drawn same system, overhauled completely and equipped with an through the equipment, restrictions and stoppages are re efficient drier, operated indefinitely without flapper:valve vealed; whereas neither is indicated when a vacuum is em breakage. Incipient corrosion, probably of an intercrystaUine ployed. nature, was advanced as the, cause, of fatigue failure of the It should be noted that, although winding heat is ^n effec flapper valves. tive method of removing moisture from a'stator, it must be Copper, plating may be the result of the action of, or related carefully controlled to. .prevent overheating, particularly to, moisture.11 (See Chapter 48.) . , when units of various sizes, are dehydrated-at the same time. Consideration of the foregoing shows that the amount of Application of the wrong voltage oh either a 440-volt stator water present in a refrigerant system must be small enough to or a 220-volt stator, for example, may. result in poor moisture avoid ice aeparationand corrosion. ' ... removal or.in overheating, respectively. .. Table. 1 shows the results of a survey of 20 equipment manu The use of either direst or alternating current for winding facturers, conducted by ASHRAE Technical Committee 1.11, heat appears to be optional The application of the latter to determine the maximum tolerable moisture content. These keeps the windings hotter, due to the eddy current heating of data show that the maximum, as well as the actual, moisture the stator laminations. Since, in a sealed system,' the cellulose content of machines is generally higher with Refrigerant 22 of the stator contains much water which evaporates slowly, than with Refrigerant 12. Since the quantity of moisture re prebaking may-be used to reduoe dehydration time. Either quired to produce corrosion is generally mbiiW to be above straight oven heat at 250 F, or storage in a low humidity that causing - freezing, elimination of .freezing frrnlrf nJnr> room is possible. One manufacturer reports the removal of exclude corrosion of the type due to water. .This obviously is 30 cc of'moisture from a hp, 4-pole stator by prebaking. not true-for systems in which expansion valves or capillary Recent developments in motor manufacture indicate that slot tubes operate above the freezing point of-water. ,...if insulation containing little or no moisture may be used in Moisture in Refrigerant Systems 777 Table 1 ... .Results'of Survey on Moisture in-Refrigeration Systems Applkaiho Domestic Refrigerators end Freezers Food Display Cases Frozen Food Cases Room Air Conditioners Residential and Package Air Conditioners Field-Built Systems Max Tolerable Moldwv, PPM Moisfwe in Urah, PPM , Refrigerant 12 Refrigerant 22 Refrigerant 12 Refrigerant 22 10.10.10.15 10,15,15,20 ... 10.10.10.10.15 15,20 10,15,15 5.8.10 5, 10 10 5 15,25' 10,15,15,25,50 15,20,25. 25,25,40,55,60,10, 100 10,15,25,25,35,40, 40, 40,60,100,T80 15,40,40,100 10.10 7,10,15,40 5,15,15,15,20, 60 7, ib; 15, IS, 15,20,; 25.40.50.55.60 * 10.15.60 8--Ne|Si*ble : .' : 1-^ percent . 1-40 percent .i , Indeterminate. beceuee Eeilnrc dne to maietur* bee the mm appearance u taitarc doe to other ceueee. 8--Negligible' 1--3 percent 3--10 percent l--30 percent 1--40 percent hermetic applications. This should materially reduce the stator dryout problem. Wile1* describes, in'very general terms,' a drying method for ice cream cabinet units. Better than ordinary drying', necessitated by the low temperatures involved, is attained1 by dehydrating each piece of equipment at high temperature and a very fine vacuum. This procedure apparently results in sys tems sufficiently dry, .but silica gel driers are placed on each machine as an additional precaution. . Carter1* has also dealt with the field removal of moisture! The heat and vacuum, hot air, carbon dioxide and alcohol,' and double evacuation methods of dehydration are HisraiM*H and illustrated. Methods of heating the equipment include the use of'infrared lamps.'-The double evacuation method is considered most satisfactory, and it is fully discussed audits" efficiency calculated. A moisture indicator in determin ing when dehydration is finished. Fenwick17 describes a system'and apparatus for producing dry air and drying units in the field as well as in the factory. Units of this type are commercially available, *d are usually used in pairs for continuous operation. : - When conditions in the field do not permit heating of the equipment, anhydrous methanol (methyl alcohol) has been used to flush out moisture. Since the alcohol has.a lower boiling point than water, it may be removed more readily.than water by means of a stream of dry air, carbon dioxide, or oil- pumped nitrogen, or by.the application of a vacuum. Under no circumstances should the alcbhol in quantity be allowed to remain in the system since it may produce corrosion.M Anhy drous methanol, in quantities somewhat !** than one percent, does not appear to induce rapid corrosion and, moreover, exerts an antifreeze action. MOISTURE MEASUREMENT . Various methods have been developed for checking the moisture level in the refrigerant system. They are described in detail in Chapter 47 and listed in Table 1 of the chapter. Obviously, the measuring method varies with-the type of drying process employed, but unfortunately the moisture limits accepted by the different manufacturers do not always correspond. One ofJthe most common methods of moisture checking is the cold trap method, and when dry air is used, the dew point of the outgoing air is frequently used-as a check on the effi- . ciency of the process. Fenwick17 advocates the use of. a dew point of -- 40 F (equal to 0.046 gr water per cu ft) for the air leaving the equipment, as proof that it is dry. Anderson.1? determines the residual moisture in.the last machine of a series, which has. been.dried with air, by drawing off the moisture into a liquid air trap and calculating therefrom,the quantity in the machine. McGovern* recommends drawing a vacuum on the equipment being tested, closing off the system and observing the pressure rise. If no pressure rise occurs after a time, tiie machine may be considered dry, since mois ture remaining in the machine will create its own gas pressure. Moisture may also be determined by removing a portion of the liquid refrigerant from a machine which has been operated for several hours until the moisture is in equilibrium through out. The quantity of moisture in .the removed sample is de termined by the phosphorous pentoxide method.1? Walker and Rinelli** developed a method -applicable to methyl chloride, methylene chloride and Refrigerant 12 which makwa possible routine analysis of a very large number of machines. Diniak et al*1 have described a simple rapid method for determining the water content of a circulating refrigerant of the fluoro carbon group. Duncan** determined moisture, in refrigerator units by collecting it in a cold' silica gel drier. The moisture was then expelled from the drier with Refrigerant 12 gas at 370 F and collected in Nesbitt tubes. Elsey2* used a dew point method for determining the moisture contents of gas samples taken from refrigerating units in the plant and in the field. Stork** compared various desiccants by using th*m in operating units. The moisture indicator is a useful device for showing whether a system is wet or dry. See Chapter '48. DESICCANTS Desiccants are used to remove excessive moisture from in stalled equipment, particularly after it has been overhauled and recharged. A desiccant is a chemical compound capable of adsorbing, or reacting chemically with the moisture contained in the liquid or gaseous refrigerant-oil mixture. Considerable information'about desiccants has been-'published in the chemical literature,u-n but not all data given are'directly applicable to the problem of refrigerant drying. Therefore articles dealing specifically with the use of desic cants in refrigerant drying should be consulted.*'**""*4-' In order that a r'W<>mit> substance be acceptable for use as a.desiccant,:it,must posses, m-;addition.to.the property.of