Document o0JwvwpNXaXRpjdDD2J9686o

fi fI ! 'i :r J 710 CHAPTER 43 R. j. Yoder and' B. F.- Dodge:-Heat transfer coefficients of boiling F-12 ;(Refrioesatinq ENoWExarao,. February-1952, W. F.-Witsig, Q. W. Penny,'and J. X: Cyphers: Heat transfer rates to' evaporating Freon-12 in' a horizontal' tube evaporator (RErBIGEBATXHQ EltQlNBEBmO/AugtBt IMS, p. 153). < - J,' L. G. Seigel,. W. I*. Bryan,.and M. C. Huppert: Heat transfer rates for refrigerant bolting in horizontal tube .evaporators (ASHVE THANaacrrONs,. VoL.55, 1949, p. 83). '. . . ' * Mart Rfltrt>r;'p<ai tr*nrfpr nitpa from heated horizontal tubes tO (RimiOERATINO EnGINEEBINO, January 1956, p. 35). . .. J. E. Myera and D. L.':Katz:B<nZrnj; Coefficients Outside Horizontal Tubes (University of Michigan, Ann Arbor, Michigan). - " 8.-Levy:' Generalized' correlation of;boiling heat ^transfer (ASME Pqper, No. 58-HT-8, 1958). i. uE. Forester and R. Greif: Heattransfer to a boiling liquid: mechanisms and correlations (A6ME Paper No. 58-HT-ll. 1958). "W; H. McAdams: Beat Trtmimiatum (McGraw-HiUJB6ok Co:, New-York, 1954,3rd ed., p/202). v - " V ' ; - o C. T. Shields:-Brine film resistance to heat flow of calcium ffhlArjdft/y>dinm; chloride, and ethylene glycol (RersiGEnaTpra Engineering, September 1951; p.'880). ~ w Heat Exchanger Tube Manual. (Scovill .Manufacturing Co., 1957,3rd ed.). ' ` ' ................... .1965 Guide And .Data Book - u W. H McAdams: Heat Transmission (McGraw-Hill Book Co.,' New York, 1950, 2nd ed:). R. B. Williams and D. L. Katz: Performance of finned tube in shell and tube heat exchangers (ASME Transaction*, VoL 74, 1952, p/.1307). . 17 J. H. Perry: Chemical Engineers Handbook (McGraw-Hill Book Co., New York, 3rd ed., 1950, p. 1696).. BIBLIOGRAPHY , Bo Pierre: Varmeovergongen vid kokande koldmedier; horisontella'ror {Kylleknisk Tidskrift No. 3, May 1957, p. 129). 7 Bo Pierre: The coefficient of neat transfer for boiling Freon-12 in horizontal tubes (5. F. Review, VoL 2, No. 1, 1955, p. 55, English) (published by A. B. Svenska, Flaktfabriken, Stockholm 7,' Sweden).' R. C. Martinelli and D. B. Nelson: Prediction of pressure drop during forced-circulation boiling of water {ASME Transactions, Vol. 70,1948, p- 695). M. Altman, R. H. Norris, and F. W. Staub: Local and average heat-transfer and pressure-drop for refrigerants evaporating in horizontal tubes {ASME Transactions, Vol. 82, Series C, August 1960, p. 189, Discussion, p. 196). IU M CHAPTER 44 COMPONENT BALANCING IN REFRIGERATION SYSTEMS Performance Diagrams, Effects of Pressure Drop, Effect of Other Variable Factors, Trouble Diagnosis, System Performance ONE of tire principal aspects of the design of a refrigera which the evaporator performance is expressed in Fig. 1. tion system, either in factory-built units or in field- Part A of Fig. 2 shows the amount of heat that must be re assembled systems, is the selection of the components, such moved by the condenser to condense the refrigerant and sub as the compressor, condenser, evaporator, refrigerant flow cool it 10 F deg. Part B of Fig. 2 shows the corresponding re- control device, fan, motor, and controls. frigerating capacity of the refrigerant circulated by the com Some components can be selected individually, based en pressor, based on the stated superheat and sub-cooling. tirely on definable operating requirements, and without re Fig. 3 shows the capacity of the condenser to transfer heat gard to interaction with other components in the 'system. from the refrigerant to the condenser cooling air, as a function However, most components must be selected so that when of condensing temperature and condenser entering air tem they are operated together as a system, the net result is as re perature, with the M-Tnft sub-cooling of refrigerant as used in quired. Fig. 2. In general, more than one combination of components will; Since condensing temperature is common to the compressor meet the performance requirements of the system. Examina and condenser, Figs. 2 and 3 are plotted against this variable tion of several such combinations must be made to determine to permit easier superposition of the two separate performance which is best suited to the particular application for which the' diagrams. In Fig. 4, the diagram of Fig. 3 has been superposed system is being designed. on the diagram of Part A of Fig. 2. The intersections of the lines of constant entering air temperature with the lines of PERFORMANCE DIAGRAMS A simplified evaporator-compressor-condenser combination for an air-cooled direct expansion system will illustrate the methods used to determine the system performance. The capacity of each of the components can be defined in. terms of certain variables. Some of these variables are internal to the system, such as refrigerant temperatures in the evapo rator and condenser. Others are external, such as the air flow, quantity and air temperatures entering the npndi^nggr and evaporator. Determination of the performance of the three components operating together as a system involves the elimination of the internal variables by the process of simultaneously, solving constant evaporator, temperature indicate the condensing temperatures that will result from operation of this particular compressor and condenser as a* combination. The lines of constant condenser entering-air temperature can be plotted on Part B of Fig. 4 by transferring each point of intersection in the upper portion vertically downward to the corresponding evaporator temperature line on Part B of Fig. 4. Part B of Fig. 4 is then re-plotted on Fig. 1, using the com mon variable of evaporator temperature, resulting in the diagram shown in Fig. 5. It is now possible to determine the refrigerating capacity of the system for a given combination of evaporator and condenser entering-air temperatures, the. two external variables. three equations, each equation expressing the performance of one of the components. Since these equations <*An become very complex, an equivalent graphical method is often simpler. This graphical method offers the further advantage that the pic torial representation makes it readily possible to visualize- tiie relative importance of the major variables. Thus, the de- oguer is able to make quick estimates of the effects of modifi-.. cations in the performance characteristics of each component.' The development of the graphical method is illustrated by Figs. 1 to 6. Rg- 1 shows the total capacity of the evaporator plotted frgfcinst the saturation temperature of the refrigerant for- several air inlet wet-bulb temperatures and with a constant air flow. Fig. 2 shows the performance of the compressor.plotted ?ffaint the suction saturation temperature and .condensing temperatore, at tire same suction,inlet superheat as that for Air 6ow-1600 efts. StipOnt-10 F dvg. fig. 1......... EvaporatorPerformance,Diagram.; 711