Document oMv2KwZg02BJZzGXqpjxO9bdw

780 CHAPTER 49 - 1965 GuideiAnd Data Book Adds Hie use of desiccants is an accepted procedure41 for removal of adds and other produets produced during burnouts of hermetic motors. Another method involves flushing of .the equipment, witii a suitable solvent.41 Both methods are widely used. The mechanisms involved in the removal of acids by' desiccants has only recently received research attention. In order to be effective in the cleanup of a burnout, desicr cants and other parts of the drier must remove: (l) inorgahic adds, hydrofluoric, hydrochloric; (2) organic acids due . to oxidation' of oil, 'and a thermal decomposition of organic insulation; and, (3) other decomposition products. Essentially the same requirements may be made, on occasions, of a liquid line drier. \* Recent research -by-Hoffman* and Mays47 shows;that hydrochloric add is adsorbed by silica gel It is first adsorbed and then chemically reacts with activated alumina.and the Zeclytes. Adsorption by the latter is very limited.-The'action of hydrofluoric acid was not investigated, due to the experi mental difficulties involved in handling this add. Hoffman* showed that organic adds are adrorbed with no evidence of a later chemical reaction. Oils taken from machines in the field were shown to contain in some instances both organic and in organic adds; in.others, only..organic add. Adsorption iso-, therms'were determined for an organic add containing oil and various desiccants. Similar adsorption and chemical reaction isotherms were made for an oil of known organic and inorganic add content. This work proves that the add content of an oil can be reduced to a low level by means,of desiccants. Very little research has been devoted to; the problem of what.be:- comesof the water produced by. the.reaction of inorganic adds and desiccants. It may be speculated that .this water is ad sorbed by the portion of the desiccant that has.not reacted, with the adds. If the acid reaction is.excessive, then adsorpt-j tion of water by the suction line drier may no longer be possible. 1 Under' .this. circumstance, /the- wa,teh ;niay_>.pass. through the compressor and be adsorbed by the liquid line drier. In view of this uncertainty, the liquid line drier installed during and after' the'cleanup operation should be larger than' usual.' ' Colors Colors are adsorbed by activated alumina, silica gel mid, to a minor extent, by calcium sulfate. Because of this, leak detectors which act through an oil soluble dye, lose -their efficiency in machines equipped with driers' containing one of these desiccants. The color is completely removed horn the oil-refrigerant mixture and held firmly by the desiccant: Colloids : ' j' There is scant evidence that desiccants remove .colloids from refrigerating systems, but the appearance of numerous gampiflR of silica gel and activated alumina taken from driers makes this a distinct possibility. \ ' Oil Deterioration Products '- (hi in a refrigerating system may deteriorate, producing substances which are adsorbed by activated alumina, silica gel, and calcium sulfate;* this is due to'factore that are not fully understood at the present. The'adsorption of the sub stances makes .the 'desjccant incapabie, of taking up liquid water, but the effectiveness of the desiccant in removing mois ture from the refrigerant is only slightly reduced. : ) Chemicals Desiccants are used industrially for the adsorption of au-' merous chemicals. Hence it is not surprising that chemical' compounds are adsorbed by desiccants in refrigerating sys-' terns. Acrolein, extensively used in the past as a warning agent with methyl chloride, is adsorbed by gel and acti vated ahiidna. Methanol is preferentially adsorbed over mois^ ture by. these same desiccants. Refrigerants are adsorbed' by' these desiccants and a considerable temperature rise of. the drier may result when the refrigerant is first admitted to it.' This temperature'rise is not due to moisture in the refrigerimt. although'it has' been so misinterpreted' many time' Oil additives may be adsorbed by silica' gel and activated alumina. Because of small pore size, the Zeolyte type desic cants do not adsorb additives or the oiL .. DRIERS A drier is a mechanical device containing a desiccant. Its mechanical features are important but are not discussed here. .While .the primary purpose is to collect and hold mois ture,,nevertheless all of tire properties inherent to the desic-' cant, automatically, accrue to the drier.. Since all driers serve as. filters this becomes'an important function. Acid removal Hah assumed greater importance within the last few years principally in connection with the cleanup of burnouts.. Since acids may also be present in a machine which has not. bumed out,. the. liquid line drier must function to remove, these acids..In order to prevent moisture from freezing the1 expansion valve or capillary tube, a drier is ordinarily located in the liquid line close to these devices. Hot locations mustbe avoided. Driers also function on the low side, of the metering devices, but this has never been the preferred loca-, tion, certainly not for fiekl operations.- The oversized drier, which.is used-for the.cleanup of burnouts is installed in the, suction line. At the same time, a larger than usual drier is put in.the liquid line: Liquid line driers are usually installed; permanently and are replaced only when they lose their effectivenes. Suction line driers are removed when .they fail to. function, and always after a cleanup has'been completed. The position of the drier on the machine is important. A liquid feed line should enter the drier in such a manner as to assure uniform contact between refrigerant and desiccant. When the drier is in a vertical position, the liquid inlet should be at the bottom. If a drier is placed vertically in the suction line, thg feed should be at the top and the exhaust at' the bottom so that the oil can be easily blown through the drier. Moisture is reduced to a low level by one pass of the liquid refrigerant through a drier. Since the moisture is usually dis tributed throughout the entire refrigerating system, time is required for the circulating oil-refrigerant mixture to bring it to the drier. Krause, Guise, and Beacham* measured the rate of removal of moisture from laboratory equipment set up to simulate a refrigerating system. They showed that consider able time is required to produce moisture equilibrium in a machine. The actual time varies with the rate of refrigerant flow, internal area of the system, and size of the drier. Brisken" studied the migration of moisture in a hermetic system while Pennington* observed the role of the adsorption type desic cant in refrigerating units. These studies confirm the opinion long held by the industry, that removal of moisture to a safe level of.operation requires a minimum of several hours.'- : Moisture in Refrigerant Systems 781 SELECTING A DRIER Manufacturers of driers normally issue-selection charts which are generally printed on, or inserted in, the drier con tainer. Additional information is contained in their literature. A selection chart should list the amount of desiccants, the refrigerants, - refrigerant flow capacity, filter area, water capacity, and a specific recommendation covering the type and tonnage rating of the machine for which the drier'is designed- . v All of the-following are considered in the design and pro duction of a drier. Their consideration is a must for the manufacturer and their proper.understanding is' of great value to the user. The desiccant is the heart of the drier and its careful selection therefore is of paramount importance. Its properties have been discussed in the section on Desic cants in this chapter. ' .The water capacity of a drier is determined by ARI Stand ard 710.44 It is given in drops of water for Refrigerants 12 and 22, at EPD of 15 and 60 ppm, respectively for the tem peratures 75 and 125 F. These reference points are arbitrary. They were set to prevent confusion arising from determina tions made at other points. A separate EPD is required for each additional refrigerant, it should be noted that the spe cific refrigerant, the amount of desiccant, and the effect of temperature are all considered in the statement of water ca pacity. . Flow capacity is determined in tons per minute, at two pounds pressure drop across the drier, by the official pro cedures of ASHRAE* and ARI.44 Jones* developed a method for determining flow capacities in which Refrigerant 113 is used, and the results converted to Refrigerants 12 and 22. Flow capacity may be quickly reduced when critical quanti ties of solid and semi-solid are filtered out by the drier. There is no way of predicting what this will amount to or when it- may occur. It is obvious that whenever the flow capacity drops below the requirements of the machine, the drier should be replaced. Filter area is very important since a drier acts as a filter for the oil-refrigerant mixture in the liquid line, and for refrigerant gas and oil when installed in the suction line. A large filter area is able to filter out more material than is possible with a small one without cutting down the flow capacity of the drier to the point where the efficiency of the machine is seriously reduced. A solid core drier has a larger filter area than the older conventional loose desiccant type. However, some of the latter have been modified to substan tially increase their filter areas. In addition to quantity removal of solids and semi-solids, size of particles is important. Almost any filter arrangement will take out the larger parti cles. Nothing has been published concerning the lower limit of particle sire, but most manufacturers advertise ability to filter out particles in the low micron range. Obviously all particles capable of causing trouble in the Bystem must be removed. The filter action of a good drier takes care of thi Acid removal is seldom listed in selection charts. The drier manufacturer should be consulted for information. TESTING AND RATING Methods of testing desiccants will be found in Reference 43. Methods of rating driers will be found in Reference 44. Mini, mum standards for listing of refrigerant driers will be found m Standard for Refrigerant Containing Components, Subject 207 (Underwriters' Laboratories). REFERENCES 1W. O. Walker and W. R. Rinelli: The separation of wax from oil-refrigerant mixtures (Refrigerating Engineering, June 194L p. 395). ,- * Private communication from R. J. Thompson. * W. O. Walker. S. Roeen, aad-S. L.Levy: A study of the fac tors influencing the stability of mixtures of Refrigerant 22 and refrigerating oils (ASHRAE Transactions, Vol. 66, 1660, p. 445). - . 4 W. 0. Walker, S. Rosen, and S. L. Levy: Stability of mix tures of refrigerants and refrigerating oils (ASHRAE Journal, August 1962, p. 59). * W. 0. Walker and W.-R. Rinelli (Ansul Newt Nates, VoL 4, No. 2, 1940). ' . .. . .. * E. W. McGovern (Refrigeration Service Engineering, VoL 7, No. 3, I939,p. 23). * H. M. Ebey, Ll C. Flowers, and J. B. Kelley: A method of evaluating refrigerator oils.(Refrigerating Engineering, 'July 1952, p.737). - -J ', * Heins Steinle: Chemical reactions between refrigerants and oils in refrigerating machines (Kilteteehriik, Vol. 2, No. 7, 1950, p. 174). * W. O. Walker (Ansul Newt Notes, VoL 4, No. 2, 1940, p. 10). '* Private communication from C. E. Waring. u E. W. McGovern: Copper pU-tlng in refrigerant compressors (Refrigerating Engineering, July 1939, p. 31). n L- W. Larsen and J. Elliot: Factory methods for dehydrating refrigeration compressors (Refrigerating Engineering, De cember 1953, p. 1325). u Private communication from R. R. Pickett. 14 W. B. Anderson: Domestic refrigerating units--their manu facture and testing (Refrigerating Engineering, May 1941, p. 323). * D. D. Wile: Modern ice cream cabinet design and applica tion (Refrigerating Engineering, April 1941, p. 240). ** F. Y. Carter: Field removal of moisture (Refrigerating Engineering, June 1951, p. 547). 17 A. M. Fenwick: Paper presented at 1940 meeting of Re frigeration Service Engineers Society. u W. O. Walker and W. R. Rinelli (Annd Newt Notes, Vol. 2, No. 2, 1938). 11 F. A. Eugtis (Industrial and Engineering Chemistry, May 1933,p. 77). ** W. 0. Walker and W. R. Rinelli: U. 8. Patent 2,145,203. _ * A. W. Dioiak, E. E. Hughes, and Minor Fujii: Determina tion of water content in Freon-12 circulating in a refrigerating system (Refrigerating Engineering, February 1954, p. 56. Also National Bureau of Standards Technical Bulletin 38, April 1954, p. 59). * T. W. Duncan: Moisture determination in refrigerator units (Refrigerating Engineering, December 1949, p. 1182).. ** H. M. Ebey: Determination of water content of Freon-12 by a dew point method (Refrigerating Engineering, JuJv 1949, p. 665). M B. J. Stork:-Certain test results for three desiccants in a Freon-12 system (Refrigerating Engineering, August 1949, p. 782). J. H. Bower (Bureau of Standards Journal of Research, De cember 1934, p. 241). * M. Shepherd (International Critical Tables, Vol. 3, p. 385). ** M. V. Dover and J. W. Marden (Journal of American Chemi cal Society, 39, 1917, p. 1609). " K. M. Newcum (Atr Conditioning and Refrigerating News, December 20, 1933, p. 12, and August 26, 1936, p. 17). M G. H. Clark: Dehydrators (Refrigerating Engineering, June 1936, p. 366). * V. E. Hall (Refrigeration Service Engineering, VoL 6, No. 12, 1938, p. 13). . W. McGovern (Refrigeration Service Engineering, Vol. 7, No. 4, 1939, p. 18). * W. O. Walker (Aiuui News Nates, Vol. 12, No. 5, 1948, p. 2; Vol. 12, No. 7, 1948, p. 2; VoL 13, No. 2, 1949, p. 2; Vol. 13, No. 4,1949, p. 3; Vol. 13, No. 5, 1943, p. 3; VoL 13, No. 6, 1949, p. 3). 0 W. O. Walker (Section 5, Refrigeration Service Engineers Society Service Manual, 1955, p. 521). M W. O. Walker and J. B. Hostettier: Water sorption charac teristics of silica gel, activated alumina, anhydrous calcium sulfate (Refrigerating Engineering, April 1956, p. 34).