Document wDbdQ5qpd66zaeK9MeVokqw5Q

664 CHAPTER? 40^ ' r v ' 1965 Guide And Data Book they are carried out nor how small the temperature dif ference between the evaporator and the tyinHono^' absorber may be. It should be noted, however, that the (CP) fells off slowly as these temperature differences are tncr?ififnj This assists in maintaining high part-load efficiencies. These characteristics are very different from those of compression Ti(Tt r,) (CP) T<(Tt - TO- (1), .:7V, " evaporator temperature, Rankine.. 7t ".condenser temperature, Rankine. Tt * absorber temperature, Rankine. Tt " generator temperature, Rankine. - : < : -............... ...|? Actual cycles are never as efficient as the ideal cycle. Every difference between the actual and ideal cycle producesa ther modynamic loss. As a result,"the (CP) of actual absorption cycles is generally less than' one, no matter how efficiently Influence of Condenser Water Temperatures.' An analysis of the cycle shows that unit efficiency improves with a reduction in condenser water temperature. The con densing temperature is almost a direct function of the condenser water temperature; hence, an increase in .the con denser water .temperature will result in an almost* identical increase in the condensing temperature. With a fixed evapo rator temperature, an increase in the condensing' temperature will result in a higher temperature condensate having a greater enthalpy. This condensate, upon'its return .to-the evaporator, has a reduced refrigerating effect The net result of an increase in condenser water temperature is a reduction in capacity, and an increase in the energy input requirements. If the condenser, water temperature continues.to rise, the capacity of the unit continues to' decrease. Since the lithium bromide unit is operating under a high vacuum,- no pressures higher than atmospheric can develop. This eliminates the need for a high head-pressure safety cut-out. Absorption machines using the lithium bromide-water cycle continue to pro duce some refrigeration even with unusually high cooling water temperatures. On the other hand, if the temperature of the condenser water should decrease, the capacity of the .unit would increase. It is desirable to limit variations in capacity resulting from vari ations in-the temperature of the entering condenser water, as may .exist with a cooling tower. The installation of an auto matic 3-way valve between the cooling tower and the con denser is recommended. Tliis valve will either recirculate fig. 6 .. Diagram of One-Shell Lithium Bromide Cycle Water Chiller.'-' fig. 7 .... Performance Characteristics of Lithium Bromide Cycle Water Chiller ' / Absorption Air-Conditioning and Refrigeration Equipment 665 water, mix condenser and cooling tower water, or deliver(100 percent tower water, to.hold the water tempera ture within design limits; : :- ' In the modem lithium bromide absorption system, the con denser water'flow'rate is held constant throughout the entire operating, range of the chiller. Fig. 7 shows the performance of a typical lithium bromide cycle water chiller with respect to condenser cooling water temperatures. Influence'of Chilled Water Temperature ' .-Further.analysis of.the lithium bromide-water cycle also reveals the advantage of maintaining the chilled water tem perature as high as the load will permit. The chilled water temperature follows evaporator: temperature- very closely; hence an increase.in the required temperature of .the chilled wa& requires ah almost identical increase in the evaporator temperature. With/a fixed .condenser temperature,' the en- ti&lpy of tire refrigerant to the evaporator remains unchanged.' If the, evaporator, temperature rises,1 the leaving enthalpy' increases,' thereby increasing the refrigerating effect. This results in ah increase in the refrigerating capacity of the unit,, and at' the same time a reduction of `energy input'require ments. .. .., ' - .' The chilled water temperatures may be dictated by the de-: humidification 'requirements of the load. .High' chilled water temperature, if otherwise acceptable, will increase the ca pacity1'of the machine for sensible cooling, but will reduce the capadty of the machine for latent cooling. . If required, chilled water temperatures as low as 38 F can' readily be produced in chillers using the'lithium bromide- water'cycle. Fig. 7 illustrates the performance'of a typical lithium broinide^cycie water chiller with respect to chilled water temperatures. Influence of Temperature of the Heat Source , The absorption cycle requires heat for operation. This heat may be supplied by direct firing, or it may be supplied indi-, rectly by steam, hot water, or a number of other hot fluids. At any giveu.load, the heat input requirement is' fixed, without regard to the nature of the hot medium. ' With a decrease in the temperature of the source,;there is a' decrease in'the'amount of refrigerant water vapor boiled out of the strong solution in the generator. This reduces the re frigerating capacity of the unit:' Fig. 8 shows the performance of a typical lithium bromide cycle water chiller with respect to steam pressure. (Steam pressure is generally used to indi- * For UfWu Broewl*. Cydo W<rf*r Chflter _ ... fig. 8 .... Effect, of Steam Pressure,on Capacity* cate steam conditions.) When the unit is-to be operated on hot water, or other hot fluids, the manufacturer should be consulted for specific performance, information. Because direct-fired units are generally operated m-off, the temperature of the heat source is not significant with respect to capacity control. The unit will run a greater or leser portion of the time, corresponding closely to the ratio of the actual load to the unit capacity. This kind of operation is primarily used in residential and small commercial applica tions. AMMONIA-WATER CYCLE Thermodynamically, the ammonia-water absorption cycle is identical to the lithium bromide cycle discussed earlier in this chapter. In the ammonia-water cycle, however, water plays a different role. Here it acts as the absorbent. Ammonia is the refrigerant, and consequently lower temperatures may be reached. The other principal operating difference between the two cycles is the pressure range in which the cycles operate. The lithium bromide-water cycle operates under a very high vacuum, while the ammonia-water cycle operates in a pressure range comparable to that of compression systems. Fig. 19 of Chapter 1 shows the properties of aqueous-ammonia solu tions. Although the major components of all absorption systems are the same, the characteristics of specific refrigerant-' absorbent combinations may require auxiliary devices in order' to improve the operating efficiency of the cycle. In the am monia-water cycle, the volatility of water under the genera-; tor conditions is the major consideration. The proportion of water in the ammonia-water. vapor leaving the generator depends on the composition of the liquid and its temperature. In a simple cycle, no provision is made for removing water from the vapor before it enters the condenser. Hence, the water passes through the condenser to the evaporator and, being almost nonvolatile under evapo rator conditions, it concentrates as evaporation proceeds, thus raising the boiling point of the remaining refrigerant: To keep such an evaporator operating, the accumulated water would have to be purged to the absorber, either continuously or periodically, as a liquid of High ammonia content. Two devices commonly used to reduce the amount of water reaching the condenser and evaporator in the actual ammonia absorption cycle are the analyzer and the rectifier. The' analyzer consists of a chamber through which the vapors leaving the generator pass in oouriterfiow contact with the weak solution flowing to the generator. A bubble column^ parking, or a series of baffle plates, may be used to make the' vapor-liquid contact more effective. As the vapor passes up ward through the analyzer it is enriched with ammonia, as well as cooled, and the solution is heated. Thus,, the vapor going to the condenser is cooler and richer in ammonia, and the, heat input required at the generator is decreased. The vapor flows from the analyzer into the rectifier. The rectifier is a heat exchanger set before the condenser (or it may be toe inlet to the'condenser) arranged in a manner to enable'! ite condensate to drain to the analyzer or the' generator^ The' warm refrigerant vapor from the analyzer is cooled by the weak solution flowing to the generator. ......... 1 As in toe case of the lithium bromide-water cycle, it is uni versal practice to conserve heat in the cycle by the use of a beat exchanger which uses the hot strong solution from' the generator to preheat the weak solution from the absorber. This improves the .cycle efficiencyby reducing the heat input required. Less beat has to be carried away, reducing heat rejection requirements.. Still .further improvements! in toe efficiency of the operating cycle may '.be obtained, by! sub1.