Document 5VGvY7Z3vZLZogdnMM8DJbdV

660 CHAPTER 39 .,1965-Guide Arid'Data- Book 4 U 1 Ss , i CHILLED WATEft\. TEMPERATURE \ s f ' POUNDS OF'sTEAU'nR TON HOUR ' Rg. 7 .u.. Effect of Steam Pressure on Steani' ` Demand at J 00'F Condenser Temperature ' Rg. 8`..Effect'of Steam PressureoriSteam1 ,!Demand at' 110 F Condenser Temporature ating, effect) per ton.of refrigeration-will likewise increase: This characteristic is valuable for a quick pull-down at start? up and forabsorbing high, sudden loads which may be thrown on the system.' If-the refrigeration effect-is-reduced1 by throttling the steam, the'ejector is liable to break if'the con densing temperature'continues to remain high.--This'is'be cause the lower kinetic energy in throttled motive steam is insufficient'-to permit compression over the design:pressure ratio. It is: therefore better to operate the system intermit tently 1 by means of automatic-controls set-to maintain.the chilled water temperature within fixed limits than to* throttle the-steam. . '-T I mj The typical characteristics of the steam-jet'refrigeration indicate that'its economy of operation is-very- sensitive, to condensing temperature, particularly when the;evaporator temperature is 50 For lower,'as is common in air-conditioning applications.! ............... *.,-?!- j -of'-'-j Becausethe motive steam of ejectors flows through*a criti cal-flow nozzle,, its steam consumption does not change with load* variations except by on-off or throttling -oontroh* The latter is not a preferred control method. ` -.-i.j -'`When'a refrigeration system must utilize condensing tem peratures over 100 F, it is usually not-economical to> use :a steam jet in- place !of'an absorption machine or 'mechanical refrigeration unit Evaporative or barometric- condensers oh steam-jet units can usually maintain? condensing` tempera^ tures-below-100 F.1 This enables the 'steam-jet cycle to be usedeconomically in comfort air-conditioning applications whichiwould--be uneconomical if a surface condenser wereen*- ployed. An- undesirable characteristic of the' steam' ejector limits limited ability to operate over temperature-lifts (condensing temperature minus evaporator temperature)'much' above'its design temperature lift.-An ejector willoperate satisfactorily at design lift or lower, but usually if operated at lifts about' 5 F above design, .the ejector becomes xinsteble and will break, Le., completely stop pumping-vapor.from the evapora tor. The margin of stable lift over the desgn.value can, within limits, be made broader only at the expense of rapidly in creasing steam consumption. In order to be.competitive with other refrigeration cycles the steam jet typically has only 3 to 5 F deg of stable lift'abbye design.1 Controls;cari be provided to shut the system off just belowi the break!temperature, so that the break does not occur.,-- . f < Another control procedure' at hi^ lifts-is',to provide an over-ride control oh the chilled water*temperatiire thermostat that will raise its set temperature iri'prdpqrtiori'to any mr in condensing temperature, in' effect maintaining a Annatant lift dining those rare'operating' conditions that exceed design expectations. A lowering dftmotive sieam pressure,below the design.value will also, decrease the ability, of the ejector to provide any lift.margin.! With'properly applied controls^ modern steam-jet ejectors, rarely if evei break in sendee.' The use of multiple or parallel ejectors is an effective of load control of the steamy etsystem.-As cooling load drops, ejectors are sequentially deactivated or shut off. This provides a series of load change steps (equal to number of ejectors). Load control within a given 6tep can be obtained by varying tiie chilled water or evaporator temperataire/.thecondeiising temperature, or by throttling the.inlet' steam'supply to'the ejector. .......... In order to start a steam-jet refrigerationlimit; the air ej&s tor must evacuate the'system to operating vacuum before the main jet-ejector is activated andanychiiled water produced. REFERENCES ! : 1 Elliott Spencer:.New development in steam'vacuum refrigera tion (ASHRAE Transactions, VoL 67,1961, p. 339). * N.. H. Johannesen: Ejector.totoiy-and'experiments. (Tranj- aetions of the Danish Academy of Technical .Sciences, A_TJB. 1, 1951, p. 1). l. : *.R. Royda and E. Johnson: .The fundamental principles of the steam ejector {Proceedings, 'Institutidn~of Mechanical Enomeert; VoL 145-1941; p'.il93). > -:i > 4 J. :Wiegan:. Bemessung von, DampfttrahbfrdiehtemAVXtl--- Forachungeheft 40L 1940,. ` * T. H. Keenan,* E. P. Neumann, ahd.F. Ldstwerk: An invests gation of ejector design by- analysis'and experiment'(Journal of Applied'Mcchanics;-VoL 17/1950; p. 299).'. ' > * * A. H. Shapiro:'Dynamics and Thermodynamics of Compressible Phud Flou (Ronaldfrnps Co., NewjYork, .VoL I,,1953). J. H. Keenan and E.' P.Neumann:.'A-simple air* ejector (ASME Transactions', VoL" 64/1942, p.A75)." '*- 'T;. 1 H.`G. Elrod Jr: The theory-of ejectors(A5Jf Transaction*, VoL 67, 1945, p. A170). . ...... L. F. WhitneyThe.mercury ejector refrigerator (Rermasa- atino Engineering, VoL 24/1932, o'. 143)i ' > S. S. F3chaiekef and H. 'Hoge:'JetH3b'mpresadn efficiencies as influenced by the nature of the driving and driven *** (Journal of the Aerospace Sciences;. VoL ;27, .4960, p. 63). u E. P. Neumann and' F. Lustwerkfjet pump refrigeration . system (U. S. Patent 2,653,356, 1953)..: , rtSSZS CHAPTER 40 ABSORPTION AIR-CONDITIONING AND REFRIGERATION EQUIPMENT Bose Cydes, lithium Bromide-Water Cyde, Coefficient ofPerformance, influence of Condenser and Chilled Wafer Temperrs- tares and Heat Source Temperature, Ammonia-Water Cyde, Basic Equipment, Evaporators and Coolers, Condensers, Generators, Control of Generator and Absorber, Auxiharies, limit and Protective Controls, Equipment Sizing and Selection --- fj^HERMODYNAMICALLY, the absorption cycle is the I oldest known cycle for producing a refrigeration effect; mechanically, , it has the simplest operating cycle of the ays-. frWnq m common use today. Whereas a detailed knowledge of., the thermodynamics of the absorption cyde'remaiha the re?/ spoliability of the equipment designer, an understanding of' the cycle will assist tiro air-conditioning engineer to determine the relative merits of different refrigerating cycles. .' II `This chapter has been divided into three parts: the first reviews the basic absorption cycles discussed in Chapter '1;. the second describes basic equipment, and givesdata on per-1 forxnance and operating characteristics; and the third suggests equipment giving and selection criteria. BASIC ABSORPTION CYCLES The. absorption cycle and the mechanical compression cyde.have in common the evaporation and. condensation.of. a refrigerant liquid, these processes occurring at two pressure, levels withlh the unit. The two cycles differ, in that the ab sorption cycle uses a heat-operated generator to produce, the., presure differential where the mechanical compression cycle- uses a compressor; the absorption cycle substitutes physico- . chemical processes for the purely marJianinal processes of the ' compression cycle. Both cycles require energy for operation:; heat in the absorption cyde, mechanical energy in tbe.com?,, pression cycle. A complete thermodynamic analysis of the absorption cyde b relatively complex. Several excellent discussions of the many different absorption cycles may be found in Reference 1. A dpt*sm analysis is not necessary to obtain ah nndprt4nHing of tiie operating prmdples of the cycle. The importance of good thermal properties in refrigerants and absorbents is evident; it is also important that the re frigerant and absorbent have certain and physical' properties. Experience and research''have established that tiie properties of an ideal refrigerant, absorbent,' and. re- : frigerant-ebsorbent combination are:, 1. lie ideal refrigerant will have: (a) vapor pressure ebarao- .. tenstic8 that will permit boiling at 35-50 F and; condensation ' 100 F or above, at pressures not far from atmospheric, (b) high : mural temperature,' (c) large latent heat of vaporization, - (d):kw specific brat, (e) low molecular weighVand (f) chemical - stability. . , 2. lie %deal absorbent is a liquid under operating conditions,' and will have: (a) high boiling point, (b).low.viaeosity, (c) low toecific heat, and (d) chwniral rtdriKty. -. ...i, nspouSmEtr for'iUs dnpter b anicned to TC &S, AbaorptlDo ' 3. He refrigerant-absorbent combination will: (a) have a high degree of negative deviation from Raoult's law, (b) give a solution with low specific heat and low viscosity, (c) be noa-cor- 1 roeive, and (d) yield a solution with as email a heat of dilution as ia Compatible with the other properties. Of the many combinations that have been tried, only the lithium bromide-water and the ammonia-water cycles remain in common use in air conditioning equipment. In addition, ammonia-water absorption equipment is often used in large tonnage industrial applications requiring low temperatures for process work. LITHIUM BROMIDE-WATER CYCLE Fig. 1 shows, diagrammatically, the components of a lithium bromide-water cycle unit Diagram A shows two.closed, connected vessels. The non toxic hygroscopic salt (lithium bromide) solution is in one, water in the other. Water vapor in the evaporator is attracted to and absorbed by the salt solution in the absorber. The water remaining in the evaporator is cooled by the consequent boiling of some of the evaporator water. To utilize the re frigeration effect, a coil is placed in the evaporator as shown in diagram B. A pump circulates refrigerant water from tiie evaporator sump to a spray header to assure that the sur faces of the coil are wet at all times, assuring high beat-trans fer rates. The evaporation of the water at the surface of the cooling coil chills the water in the coil as it is circulated, in a dosed circuit, to the load. These two vessels operate under a high vacuum (approx. 0.15 psia) and deliver chilled water at temperatures as low as 38 F. The evaporator temperature is kept above 32 F so that the refrigerant water will not freeze. The absorbed water vapor continually dilutes the salt solution, reducing its ability to absorb additional'water vapor. To maintain the salt solution in the absorber at the proper con centration, a generator is added to the system, as, shown in diagram C. The weak (dilute) solution is pumped from the' absorber sump to the generator where heat boils off a portion of the refrigerant water, concentrating the solution. This strong (concentrated) solution is then returned to the ab sorber to continue the cyde. The water vapor refrigerant released from the weak solution in the generator is now con densed in the condenser (added in Diagram D) and returned to the evaporator. A heat exchanger is added to the system to conserve heat in the cycle by ing the hot strong (concentrated) salt solution from the generator to preheat the cooler weak (dilute) soluf tion from the absorber. This improves?the.iefficiency, of|the;: system by reducing the input required to heat the solution in .661