to avoid pressure concerns when condensation occurs (see below). In comparison, a compressor based heat pump works by pumping refrigerant gas from an evaporator to a condenser. This reduces the pressure and boiling temperature in the evaporator and increases the pressure and condensing temperature in the condenser. Energy from an electric motor or internal combustion engine is required to operate the compressor pump. Compressing the refrigerant uses this energy to do work on the gas, increasing its temperature. The warm, high pressure gas then enters the condenser where it undergoes a phase change to a liquid, releasing heat to the condenser's surroundings. Warm liquid refrigerant moves from the high pressure condenser to the low pressure evaporator via an expansion valve, also known as a throttling valve or a Joule-Thompson valve. The expansion valve partially vaporizes the refrigerant cooling it via evaporative cooling and the resulting vapor is cooled via expansive cooling. (This is a combination of Joule-Thompson cooling and work done by the expanding gas, both at the expense of the internal energy of the gas) The cold, low pressure liquid refrigerant will now absorb heat from the evaporator's surroundings and vaporize. The resulting gas enters the compressor and the cycle begins again. Simple salt and water system A simple absorption refrigeration system common in large commercial plants uses a solution of lithium bromide or lithium chloride salt and water. Water under low pressure is evaporated from the coils that are to be chilled. The water is absorbed by a lithium bromide/water solution. The system drives the water out of the lithium bromide solution with heat. Water spray absorption refrigeration Another variant, uses air, water, and a salt water solution. The intake of warm, moist air is passed through a sprayed solution of salt water. The spray lowers the humidity but does not significantly change the temperature. The less humid, warm air is then passed through an evaporative cooler, consisting of a spray of fresh water, which cools and re-humidifies the air. Humidity is removed from the cooled air with another spray of salt solution, providing the outlet of cool, dry air. The salt solution is regenerated by heating it under low pressure, causing water to evaporate. The water evaporated from the salt solution is re-condensed, and rerouted back to the evaporative cooler. Single pressure absorption refrigeration A single-pressure absorption refrigerator takes advantage of the fact that a liquid's evaporation