Institute Traduccion.docx

1. -Thermodynamic Analysis of the Absorption Cooling Cycle A. Description of Typical Cycle In the absorption cooling cycle, as in the more familiar compression cycle, the useful cooling effect is obtained by boiling a refrigerant material in a heat exchanger or evaporator. The heat re-quired for this is obtained from the fluid, usually air or water, which is to be chilled. The operation of the absorption cycle is often misunderstood, however, because chethical processes are substi-tuted for the purely mechanical processes of the compression cycle. In air conditioning, the evaporator must operate in the 35°-50°F range for adequate humidity reduction, to reduce the volume of air circulated and prevent frosting of exchanger surfaces. This and other operating condition restrictions make of paramount importance the selection of -liquids for use in the cycles. The absorption cycle uses two fluid streams in a totally enclosed system. One is the refrigerant, which provides the cooling effect; the other is the absorbent, which conveys the refrigerant through part of the cycle. The major components of the system (Fig. f) are a generator, condenser; evaporator, absorber, and liquid-liquid heat exchanger. The refrigerant passes through all units; the absorbent is confined to movement through the generator, heat exchanger, and absorber. In operation, a mixture of absorbent and ref rig-erant is heated in the generator to boil off most or all of the refrigerant, which rises as vapor to the condenser. The generator and condenser op-erate at relatively high pressure, so the condensing temperature of the refrigerant is sufficiently high to permit rejecting the latent heat to outside air or cooling water. The liquid refrigerant is throt-tled to lower pressure so it will boil at relatively low temperature in the evaporator and thus

Fig. I.—Basic Absorption Cooling Cycle

absorb heat from the air to be cooled. The vaporized refrigerant passes to the absorber, where it dissolves in cool absorbent solution which has come to the absorber from the generator outlet. The cool solution, now rich in refrigerant, is pumped back to the generator to continue the process. No work is done on the system by external mechanical forces when solution is returned to the generator by gravity instead of pump, and energy can enter or leave the system only by flow of heat. Furthermore, in the absorption process the ref rig-erant material is liquefied and vaporized twice dur-ing the cycle, as compared with once in mechanical compression. The additional vaporization and con-densation are necessary to substitute chemical.

5 The thermal efficiency or coefficient of per f ormance (C.O.P.) of such a cycle may be defined as the ratio of the cooling effect to the energy input to secure this effect, that is, refrigeration by evaporator to heat input to generator : G.O.P. = 91/94 [Equation I ] For a mechanical compression refrigeration cycle, operating reversibly between T, and T,, the C.O.P. would be: C.O.P. = 91/ (92 — Q,) = TO/ L — T1) = Q1/W [Equation 2] where W is the work required in compression. In this case the coefficient of performance is the ratio of the refrigeration effect to the work done on the refrigerant. A comparison of Equations 1 and 2 shows that the heat absorbed in the generator of an absorption cycle is analogous to the work done by the compressor in a mechanical refrigeration cycle. If it is assumed that the cycle in Fig. 2 is revers-ible, then from a heat balance (first law of ther-modynamics) : Qi-F44=92±9. [Equation 3] In a reversible cycle, the net entropy change is zero, therefore : 41/T, + Q4/T4 = 92/12 + Q3/T3 [Equation 4] If the evaporator and condenser are considered components of a reversible Carnot cycle : 42/12 = 41/T, [Equation 5] and for the generator and absorber : Q4/14 — 93/13 [Equation 6] Solving for 91 /94 from Equations 3-6: C.O.P. = 4i/94 = 1, (14 — T3)/ T4 (T2 — T, ) [Equation 7] Equation 7 shows that the coefficient of performance increases with increase in the ratio of absolute temperature of the evaporator to absolute temperature of the generator. It is desirable to have a large temperature difference between the generator and absorber, and a small temperature

difference between the condenser arid evaporator. The effect of absorber and generator temperatures on the coefficient of performance is shown in Fig.