Document LJ8LoRwYYpXn63k2y3MZ2Go73

700 CHAPTK 42 Fig. .29 .. ^ Typical Rating Curves for Evaporative Condensers ' increasing'the effectiveness of an oil separator which can' be located between it and the condensing coil. The design may differ horn that'of 'the condenser coil.because of the lower transfer heat rate on the refrigerant side.'.. ':' Water Treatment In many areas the make-up water is virtually saturated with low solubility salts. To withstand the further concentra tion which' is inevitable in a recirculating system,' polyphos phate chemicals are used to enable water to become' somewhat supersaturated withorut preripitation of scale-forming solids. Principles of water treatment'are given in Chapter 15/ Condenser Location ; . Evaporative condensers located indoors can be used to yen; tilate tiie machine room or may be connected by ductwork,to the. outside. In cold climates,-.it may'be desirable to.provide suitable dampers to prevent lowering of the temperature of the machine room below a workable condition, or to provide doors to prevent lowering of temperature of. adjacent work areas. Air.contaminated with food dusts will often cause excessive sHme formation arid should be exhausted by duct connections to the outdoors.,Condensers.located in dirty or.dusty areas should be protected by filtezB in .the air intake. Evaporative condensers located outdoors may be protected from freezing during cold weather by placing the pump nd water tank in a heated area below the condensers, as 6hown in Fig. 30. Hie piping can be arranged so that each .-time, the pump stops, all the water drains back into the'Vater tanif "and thus avoids freezing.1-Electric heaters in the Sump pan, con trolled by a thermostat, have also been used to prevent freez ing. A steam coil immersed-in the sump of the evaporative condenser' is another effective for preventing freezing during winter operation. Quite often, steam may be available if the building andtherefore economical to use. - -> u < - If- the'condenser-must operate during cold weather, autOr matic'dampers should be provided to avoid'excessively-low head pressure and to prevent'freesing'during-eztreme'caiH ditions.1 Long discharge ducts tend to^condense -moisture from the nearly saturated air leaving an evaporative oondenser> This is 1965 Guide And 'Data1 Boole especially true if,the ducts pass through a cool space such as a basement. This rendition should be avoided by! keeping,the ducts, as short as possible. It may. be necessary,to some, condensation'even with insulated ducts and ,to'provide drainage at points'where no damage, will .result. Obviously' the.elimmatora must be highly.effective. ,,.! ' Capacity Modulation V. ` -.To assure the proper operation of expansion valves or other refrigerant control' 'devices,'! it' is:ofteri:-necessary to avoid extremely low condensing pressures-such as mightiresult dur ing winter operation .with -an evaporative .condenser'. Ca pacity reduction,'controlled, by a .pressureiiswitch; can'be obtained^in*.several-waysi^by: (1). intermittent operation of thevwater-drculating pump,-^-intermittent operation-of the fan, 1(3) intermittent operation ofrboth the pump and fan; (4) placing a modulating or a two-position:damper < in- tKw air stream to reduce the' air flow.-.The intermittent operation of the water,pump is definitely not recommended, since'a thin fiim'of scale is deposited each time the pump is'stopped and the water is evaporated from the tube surface. After continued cycling, a thick deposit of scale will result. Of the other three methods, the use of a modulating or a two-position damper is often-most desirable sinceTrt may be used to prevent freezing m cbld weather/For cold weather operations, the pump arid a sump tank should be located in a Heated area. PART IV: LIQUID RECEIVERS In systems^using water-cooled condensersj:.it is common practice to'use: the condenser as the receiver/but, in many cases, it is advisable and necessary to use a separate,receiver. It is.desirable to have the receiver, or condenser;wibn one-is 'used as the receiver, of such a.size that .it can hold the entire refrigerant charge.. However,, this .is not always possible, es: pedally.on.systems with.large flooded.evaporators!.In,all cases: (1) the miaimtim receiver yolume;should.be such that it can absorb the seasonal fluctuations in liquid refrigerant requirements; (2) the iiqmd seal.in the.receiver'must neverbe Condensers- ' - 761 broken, and (3) the liquid level should never.build up into the condenser while it is.in operation! Although' the' receiver, is a part of the high side, Us size is determined by the'design and operation of the bw side. .... ir. Air-cooled condensers have a smaller storage volume for refrigerant and, on systems where storage for the entare charge of refrigerant is necessary, they require a separate receiver. Receivers also provide storage for the extra refrigerant charge for systems with head pressure control that requires flooding of the coPHpns*r coil to reduce capacity during winter opera tion. On systems using air-side head pressure control, a re ceiver is not required for operation of the head pressure control- .... On systems in which the air-cooled condenser is designed with an integral subcooling coil, a receiver must not be included between the outlet of the subcooling coil and the expansion valve, unless provisbn is made to valve off the receiver and bypass the liquid around it during system -operation. A receiver will operate with liquid and refrigerant vapor in the vessel, making it impossible to maintain a flooded subcooling coil. Furthermore, subcooled liquid entering the receiver will immediately condense a part of the vapor revert to the saturated condition. A receiver may be located between the condenser outlet and the inlet to an independent subcooling coil A liquid seal in the subcooling coil can then be maintamed so that no loss of subcooling occurs. In systems using water-cooled condensers, the receiver is normally located near the condensing unit in the equipment room. In systems using air-cooled condensers, the receiver is preferably located in the equipment room,--but may be located outdoors at the air-cooled condenser. In cases where the receiver is located so. that the receiver pressure may exceed the condenser pressure, e.g., on roof tops or in attic installations, the receiver should be vented to the condenser. - In some instances it may also be neoessary to insulate' the receiver or to provide a ventilated housing over the receiver. When a flooding type head pressure control for an air cooled condenser is used, the receiver is preferably located indoors. This minimizes liquid subcooling and bw receiver pressures on shut-down during winter operation, making it easier to get the compressor on the line without cycling on low pressure cut-out. If a vent line is used, it should be valved off daring winter operation. REFERENCES 1 W. EL McAdams: Seat Transmission (McGraw-Hill Book Co., New York, 1954, p. 219). ` D. L. Katz, R. E. Hope, 8. C. Datsko, and D. B. Robinson: Condensation of Freon-12 with finned tubes (Refrigerating Engineebing, March 1947, p. 211, and April 1947, p. 315). * A. P. Kratz, H. J. MacIntire, and R. E. Gould: Heat Transfer m Ammonia Condensers, Part III (University of Illinois Engi neering Experiment Station Bulletin No. 209, June 1930). * Standard 4-55, Performance of Ammonia Condensers, Hori zontal Closed Shcll-and-Tube Type (Air Conditioning and Re frigeration Institute, 1953). 1 Standards of the Beat Exchanger Institute, Tubular Ex changer Section (1937). * H. B. PownaD: Application of heat transfer data to practical design of horizontal shell type refrigerant condensers (Refriger ating Engineering, January-1940, p. 27). 1 K. Iinge: International Bulletin of Information, Annex 1, December 1933. * -W. L. Badger: Beat Transfer and Evaporation (Chemical Catalog Co., New York, 1928). * Justin Neuhoff: Condensing surface and air quantities for inteSrl horsepower air-cooled air conditioning equipment (Rbfbigebatino Engineering, September 1956, p. 53). -u O. J. Nussbaum and Mark Lichtenstein: Operating economy of air-cooled refrigerant condensers (ASHRAE Journal,' March 1962, p. 75). : u ASHRAE Standard Methods' of Testing for Rating Mechani cal-Draft Air-Cooled. and Evaporative . Condensers (ASHRAE Standard 20-60, I960). -r BIBLIOGRAPHY F. R. Zumbro: Test of a vertical shell and tube type ammonia condenser {Refrigerating Engineering, August 1926, p. 49). F. Ophuls and G. A. Horne: Recent improvements in refriger ating apparatus (Refrigerating Engineering, July 1924, p. 1). W. Jones: Cooler and condenser heat transfer with low pressure Freon refrigerant (Refrigerating Engineering, June 1941, p. 413). A. B. Stickney: Log M.T.D. at a glance (Refbigebatino Engi neering, October 1935, p. 208). D. L. Katz and J. M. Grist: Condensation on six finned tubes in a vertical row (ASMS Transactions, VoL 70, 1948, p. 252). R. B. Williams and D. L. Katz; Performance of finned tubes in shdl-and-tube heat exchangers (ASME Transactions, VoL 74, 1952, p. 1307). K. O. Beatty, Jr. and D. L. Katz: Condensation of vapors on outside of finzied tubes (Chemical Engineering Progress, VoL 44, No. 1, 1948, p. 55). D. B. Robinson and D. L. Katz: Condensation of refrigerants on finned tubes (Heating and Ventilating, November 1947, p. 83). R. M. Armstrong: Heat transfer and pressure loss in small commercial sheD-ond-finned tube heat exchangers (ASME Transactions, VoL 67, 1945, p. 675). D. Q. Kern: Process Beat Transfer (McGraw-Hill Book Co., New York, 1950). R. E. Cherne: Cost of operation of refrigeration used for air conditioning (Refrigerating Engineering, December 1943, p. 421). J. D. Lovely: Operating costs of air-cooled residential air con ditioners (Refrigerating Engineering, January 1954, p. 35). O. J. Nussbaum: Adaptability of air-cooler, condensers for large tonnage systems (Refrigeratino Engineering, March 1954, p. 43). S. F. Gilman, L. A. Kali, and E. P. Palmatier; The operating cost of residential cooling equipment (ASHVE Transactions Vol. 60, 1954, p. 525). O. J. Nussbaum; Air-cooled condensers for large tonnage (Industrial Refrigeration, March 1955, p. 13). Residential Air Conditioning--^ summary report of the Austin Village project (National Association of Home Builders Research Institute, May 1956). Warren Vieesman: How, when mH where to use air-cooled condensers (Heating, Piping and Air Conditioning, September 1956, p. 84). M. D. Irwin: New fifld application problems of air-cooled con densing equipment (Refrigeratino Engineering, September 1956, p. 48). H. R. Musson: Air cooled condensers for supermarkets (Canadian Refrigeration and Air Conditioning, October 1956, p. 28). P. W. Wyckoff: What is an air conditioning horsepower? (Finish, November 1956, p. 29). R. A. Gonzales: Operating cost of air-cooled equipment (Re frigerating Engineering, August 1956, p. 58). R. A. Gonzalez: How power usage varies for summer air condi-. tioning of identical-residences (Refrigerating Engineering, January 1957, p. 35). D. E. Kramer: Refrigerant control in air-cooled condensers (Refrigerating Engineering, July 1958, p. 41). J. L. Wolf: Economic evaluation of condensing methods (ASHAE Journal Section, Heating, Piping and Air Condition ing, August 1958, p. 135). O. J. Nufflbaum: Air-cooled condensers (ASHAE Journal Section, Heating, Piping and Air Conditioning, September 1958, p. 155). R. C. Edwards: Air condensers with gravity flow (ASHAE Journal Section, Heating, Piping and Air Conditioning, October 1958, p. 123).