Refrigeration equipment uses different devices depending on the cooling method, with the most widely used being vapor compression refrigeration. Key equipment includes compressors (see fluid conveyance machinery), condensers, evaporators, and throttle valves. Compressors are used to compress and convey refrigerant vapor, with reciprocating and centrifugal compressors being most commonly used. Condensers and evaporators are primarily used for heat exchange. Items release a certain amount of heat during cooling or freezing, and the surrounding structure of the refrigeration system also absorbs some heat during operation.