Document DXw52kap5vVYdXOpn093BRYQ

r 16 CHAPTER 1 1965 Guide And Data Book - variable, concentration. For systems using water as the re frigerant, it is pftrtifnlflrly essential to control the evaporator pressure, to prevent freering of the refrigerant. Control of concentration is generally accomplished by stor ing some of the pure liquid refrigerant out of the system when a more concentrated absorbent is required, and releasing it to the system when a more dilute absorbent is required. The ' storage volume may be a receiver between the condenser and ! evaporator, a receiver to store unevaporated refrigerant pass ing through the evaporator, or it may be the evaporator itself. Control of the storage is usually attained by means of some form of valve, actuated by either evaporator temperatiire or ' pressure, or, in some cases, by the temperature of the heat ' sink. The control of concentration may improve the coeffi cient of performance slightly, as it avoids operation .with ' excessively low evaporator temperatures, but its primary purpose is to keep the evaporator operating in the desired and safe temperature range. Other auxiliary equipment mainly consists of operating and safety controls. Cycle Analysis Although the analysis of the ideal cycle by means of the Carnot relation is useful for determining the limiting condi tions, a more detailed analysis of a real cycle is necessary for, design purposes. Such an analysis generally consists of heat Kalanrea and material halftruieg around the various compo nents as well as around the system as a whole. In a system having a nonvolatile absorbent (Fig. 28) heat and material balances can be set up to enable the designer to calculate both the quantities of heat and quantities of fluid flowing in each of the various components. The various fluid streams are numbered 1 through 12 for convenience. The equations are set up on the basis of one pound of refrigerant Reprinfecf by pormitaoe from IGJ wumJi BvBoHa No. I 4, Ths Ab(prption Cooling Process. 1957, fostifuf* of Get Tedvtafogy. Fig. 21 ...Enthalpy-Concentration Diagram for ' Lithium Bromide-Water Combination* A relatively recent innovation** is the incorporation of the multiplexed principle in the generator of the absorption system. Since the boiling point of the solution in the generator depends upon the pressure above it, it is possible, by the ju dicious selection of pressures, to arrange two or more generators in series so that the vapor expelled from the first or highest pressure generator can supply , its heat of condensa tion to cause the solution in the second generator to boil, and the process is repeated through the series. External heat has to be applied only to the first generator. It is obvious that such a system can do much to improve the CP. Because of the highly concentrated solutions involved, the boiling tempera tures of any two adjacent generators, or effects, are far apart, and it is therefore necessary to use liquid heat exchangers between the effects to realise the gain in CP. Some form of concentration control is generally desirable. In an absorption system, the evaporator pressure, and its temperature, is a function of the concentration and tempera ture of the absorbent solution. In general, the designer will not have control of the temperature of the absorbent solution because its temperature will depend upon the temperature of the heat. sink. Therefore, to have any control over the evaporator pressure, the designer must work with the other Fig. 22 .... Enlarged Enthalpy-Concentration Diagram for Lithium Bromide-Water Combination* Thermodynamics and Refrigeration Cycles flowing through the evaporator per unit of,time. For the gehanalysis, it is assumed that beat gains and heat losses of the system are zero, except for those occurring at designated heat exchange surfaces. Heat balance around whole system: + 9 + 9 " ? -- " 0 (20) Heat balance around evaporator: q. + kt - ht - 0 (21) Heikt balance around absorber: - $1 + h\ + Mhlt - (M + l)ht = 0 .'. -(22) Heat balance around generator: q, + (M + l)h, - Mhn - ht - 0 ; (23) Heat balance`around condenser: - ? -- A. -f A, - 0 (24) Heat balance around liquid heat exchanger: SUM - ^(Ai. - An) - +. D(A, - A7) (25) . Heat balance around refrigerant precooler: q^t " Aj -- A* * A --.A* (26) where q, -- heat flow to evaporator, Btu per unit time. q, -- beat flow to generator, Btu per unit time. qp -- beat flow to pump, Btu per unit time. qt -- heat flow from absorber, Btu per unit time.. -- heat flow from condenser, Btu per unit time." * enthalpy of ith stream, Btu per pound. < 17 WClCHT %L.B fig. 24 .... Specific Heat of Aqueous Solutions of Lithium Bromide M weight of absorbent solution entering absorber,* pounds per pound of refrigerant.. 9<u)'" heat exchange in liquid heat exchanger, Btu per unit time. q,e -- heat exchange in refrigerant preeooler, Btu per,unit time. . The .heat equivalent of the energy supplied to the pump may be evaluated'by considering that the pump is lifting (Af +. Impounds of solution from the absorber pressure to the generator pressure. If P, is the generator pressure, and Pa the Fig. 23 .... Specific,Gravity of-Aqueous Solutions of Lithium Bromide - - fig. 25 .... Viscosities of Aqueous Solutions of Lithium Bromide