These ARSs utilize a combination of water (as the refrigerant) and lithium bromide (as the absorbent), as the working fluid. These systems are also called absorption chillers and have a wide range of application in air conditioning and chilling or precooling operations and are manufactured in sizes from 10 to 1000 tons, leading to the lowest evaporation temperature of 4°C (with a minimum pressure of 0.8 kPa) because the water is used as the refrigerant. In practical applications the temperature is 5°C. Low-pressure steam is the main energy source for these H2O-LiBr absorption systems. Despite their COPs less than unity, cheap energy can make these systems economically competitive with much higher COP values for vaporcompression systems. In practical H2O-LiBr ARSs the evaporator and absorber are combined in a shell at the lower pressure side and the condenser and generator are combined in another shell at the higher-pressure level. A liquid-liquid heat exchanger is arranged to increase system efficiency and hence to improve the COP. Its operating principle is the same as that of other ARSs. In the H2O-LiBr ARS, crystallization (which is a solidification of the LiBr) appears to be a significant problem. The crystallization lines are shown on the pressure-temperature and enthalpy-concentration charts. Dropping into the crystallization region causes the formation of a slush, resulting in blockage of the flow inside the pipe and interruption of the system operation. In order to prevent this problem, practical systems are designed with control devices to keep the condensation pressure artificially high. Note that absorption chillers are classified into two categories as follows:
• Single stage (single effect) ARS: Units using low pressure (135 kPa or less) as the driving force. These units typically have a COP of 0.7.
• Double stage (double effect) ARS: Units are available as gas-fired (either direct gas firing, or hot exhaust gas from a gas-turbine or engine) or steam-driven with high pressure steam (270 to 950 kPa). These units typically have a COP of 1.0 to 1.2. To achieve this improved performance they have a second generator in the cycle and require a higher temperature energy source.