Contrary to compression refrigeration machines, which need high-quality electric energy to run, ammonia-water absorption refrigeration machines use low-quality thermal energy. Moreover, because the temperature of the heat source does not usually need to be so high (80-170°C), the waste heat from many processes can be used to power absorption refrigeration machines. In addition, an ammonia-water refrigeration system uses natural substances, which do not cause ozone depletion as working fluids. For all these reasons, this technology has been classified as environmentally friendly (Herold et al., 1996; Alefeld, 1994).
The NH3-H2O system is more complicated than the LiBr-H2O system, since it needs a rectifying column to assure that no water vapor enters the evaporator, where it could freeze. The NH3-H2O system requires generator temperatures in the range of 125°C to 170°C with an air-cooled absorber and condenser and 80 to 120°C when water cooling is used. These temperatures cannot be obtained with flat-plate collectors. The coefficient of performance, which is defined as the ratio of the cooling effect to the heat input, is between 0.6 and 0.7.
The single-stage ammonia-water absorption refrigeration system cycle consists of four main components—condenser, evaporator, absorber, and generator—as shown in Figure 6.27. Other auxiliary components include expansion valves, pump, rectifier, and heat exchanger. Low-pressure, weak solution is pumped from the absorber to the generator through the solution heat exchanger operating at high pressure. The generator separates the binary solution of water and ammonia by causing the ammonia to vaporize and the rectifier purifies the ammonia vapor. High-pressure ammonia gas is passed through the expansion valve to the evaporator as low-pressure liquid ammonia. The high-pressure transport fluid (water) from the generator is returned to the absorber through
High-pressure refrigerant vapor -■*-
? Expansion ^ valve
Low-pressure refrigerant vapor
FIGURE 6.27 Schematic of the ammonia-water refrigeration system cycle.
the solution heat exchanger and the expansion valve. The low-pressure liquid ammonia in the evaporator is used to cool the space to be refrigerated. During the cooling process, the liquid ammonia vaporizes and the transport fluid (water) absorbs the vapor to form a strong ammonia solution in the absorber (ASHRAE, 2005; Herold et al., 1996).
In some cases, a condensate pre-cooler is used to evaporate a significant amount of the liquid phase. This is, in fact, a heat exchanger located before the expansion valve, in which the low-pressure refrigerant vapor passes to remove some of the heat of the high-pressure and relatively high-temperature (~40°C) ammonia. Therefore, some liquid evaporates and the vapor stream is heated, so there is additional cooling capacity available to further sub-cool the liquid stream, which increases the COP.
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