Document E1EgYb4kQKvYQp4vDebYeQVR

662 CHAPTER 40 tile generator. At the same time it reduces cooling water re quirements. Cooling water is required at two points in the cycle. The cooling water is circulated initially through the absorber (to remove the heat of absorption of the water into the solution) 1965 Guide And Data Book EVAPORATOR ' RUMP Fig;"I. .-.' Components of Lithium Bromide Absorption System and then through the condenser (to remove the heat of the' load and of the heat source). This cooling water may be sap; plied by a cooling tower or other suitable source. A variation of this cycle is the use of a two-effect generator. Heat is supplied to a weak lithium bromide solution in the first effect generator. The solution boils; liberating water vapor and concentrating the solution. The partially concentrated lithium bromide then passes through a heat exchanger which serves as a second effect generator. The pressure in the second effect generator is lower than that of the first effect generator! At the reduced pressure, the solution boils at a temperature lower than that of the condensing vapor from the first effect generator. Chillers incorpdrating'this principle are available. A'lithium bromide^water cycle* is shown in separate dia grams for'water and lithium bromide in-Figs. 2 and 3, re spectively. Since water is used as the refrigerant in the absorption ma chine, the pressure-enthalpy diagram for water is shown in Fig. 2. Line W-X shows that at 40 F evaporating temperature and 120 F condensing temperature, the net latent heat avail able for refrigeration is the enthalpy difference of the vapor ised refrigerant water at 40 F (1079.3) saturated, and the con densed liquid at 120 F (87.9); or 991.4 Btu per ibof refrigerant water in circulation in the cycle. Line Y-Z shows that the net heat rejected by tire refrigerant water in condensing!at 120 F condensing temperature, is the enthalpy difference between`vapor superheated to 230 F and condensed water at 120 F or 1163.8 -- 87.9 = 1075.9 Btu per lb. Line Z-W shows that each pound of refrigerant water returning from the condenser to the cooler at constant enthalpy, partially vaporizes, thus becoming a mixture of ap proximately 93 percent water and 7 percent vapor by weight. The absorption refrigeration cycle cannot be shown com pletely on a pressure-enthalpy diagram for water. Water vapor at point X in Fig. 2 is absorbed into a water solution of lithium bromide, and separated as superheated vapor from the solution at point Y. To complete the cycle, reference must be made to a water vapor pressure-solution temperature- solution concentration chart for a solution of lithium bromide and water. . , When the concentration of-lithium bromide in water is zero the solution is pure water.'Temperature of the solution is the temperature of water, and its vapor pressure is that of 1 ^Absorption Air-Conditioning and Refrigeration Equipment 663 PERCENT LITHIUM BROMIOE BY WEIGHT IN SOLUTION ! Fig. 3 .... lithium Bromide Pressure-Temperature - Concentration Chart water vapor. At zero concentration, the water temperature- vapor pressure relation agrees exactly with standard steam tables. , lithium bromide is a neutral salt, soluble.in water. .When the weight concentration of the solution is zero, solution vapor pressure remains water vapor pressure. With lithium bro mide in the solution, the vapor pressure of the tolutionis;re;'' duced. To obtain* equal vapor pressures from pure water and from a lithium bromide solution, the pure water would have to be at a lower temperature than the lithium bromide solu tion. This difference in temperature may be called dew-point depression. These relationships are shown graphically in Fig. 3. The dew-point depression thus obtainable can be calcu lated. For example: the vapor pressure of a 100 F, 60 percent by weight, concentration lithium bromide solution is 0.23 in. Hg (see Fig. 3); the temperature of pure water having- the same vapor-pressure is 38 F. Thus, the dew-point depression is 100 -,38 = 62 F. ' Continuing the refrigeration cycle, refrigerant water vapor at point X (Fig. 2) is absorbed along line A-B (Tig. 3). As it is absorbed, it reverts to liquid water, giving up its heat of con densation and diluting the lithium bromide solution from its original concentration A to concentration B. Because -the heat of condensation would raise the solution temperature and stop the absorption process by raising the solution vapor pressure, cooling water is used to remove'heat from the ab sorber,' thus preventing rise of the solution temperature. - The solution at reduced concentration B now contains-the refrigerant water. This solution is pumped from the absorber through the heat exchanger to the generator,' entering at temperature and ^concentration, C. In the- generator,; the solution is heated and then boiled. This boiling process drives off refrigerant water vapor' and- leaves the solution aV the higher concentration D. The refrigerant again in .the form of water vapor, flows to the condenser, point Y (Fig. 2): The solution at the elevated temperature and concentration D, returns by gravity to the absorber,, gives up heat in; the liquid heat exchanger-and enters the absorber at tempera ture and concentration E. The heat given up by the hot solu tion, cooling from the elevated temperature at D to the lower temperature E, heats the weak solution from the'low tem perature at B to the higher temperature C. The solution at temperature and concentration A, which does -the-actual refrigerant absorbing is obtained by mixing solutions E and B. For simplification, the cycle is shown operating with four separate chambers. In practice, the chambers may be incor porated into two shells or even into a single shell.-ln the case of a two-shell machine/ the generator and condenser ore located in the upper shell (called the-high pressure shell, although-the pressure is approximately l-psia), and the evaporator and absorber are located in toe lower shell(called the low pressure shell, [where the pressure is approximately 0.15 psia). In the of a single?6kell machine,-the shell is physically divided into; two'zones,, with the generator and condenser in toe WpA pressure zone, and the absorber and evaporator in the low pressure zone. The solution and evapo rator pumps shown in Fig. 1 are not a part of-the thermo dynamic cycle. The small (3 to 25 ton) units operate on the hjtfis of natural circulation of the-various solutions. The pumps are used on large equipment only to reduce the physi cal dimensions of the machines. A purging device is included to remove any noncondensable gases from the machine, assuring maximum capacity efficiency at all times. The arrangement of. components-in a 3-ton direct-fired air con ditioner is shown in Fig. 4.'The arrangement of components in large tonnage indirect-fired, two-shell and single-shell water chillers is shown in Figs. 5 and 6. Coefficient of Performance The coefficient of performance (CP) has long been used to permit evaluation of the effectiveness of a refrigeration cycle. It is defined as the ratio of refrigerating effect (output) to toe heat equivalent of toe input. In toe case of an absorption cycle, it can be measured as toe ratio of the heat absorbed in the evaporator to the heat absorbed in the generator."