Achiller is a cooling unit that refrigerates circulating water. they are often used in cooling systems that provide central air conditioning in large buildings. chillers allow cool air to be piped throughout a building. in a chiller, water is cooled to about 45º f and circulated throughout a building. this chilled water is referred to as a secondary refrigerant. leaks would be more likely to occur if standard refrigerant was pumped through hundreds of feet of piping to each floor of a multistory building. chilled water also costs less to circulate than refrigerants. when water passes through a chiller's evaporator section, the temperature of the water is reduced. as it circulates through the building, it collects heat, usually about 10º f. the warmer water then cycles back to the chiller where the heat is removed. the chilled water is then re-circulated through the building system, providing the secondary refrigerant for central air conditioning. there are two basic categories of chillers, the compression cycle chiller and the absorption chiller. compression chillers use a compressor to create the differences in pressure inside the chiller needed to boil and condense the primary refrigerant. an absorption chiller employs a salt solution with water and heat to boil and condense the refrigerant. what is the main idea of the whole document