a refrigerator system is composed of four basic components. a compressor, a condenser, an expansion valve, and an evaporator. a refrigerant fluid is circulated between these four components. it enters the compressor as a vapor where work is inputted to increase its pressure. then it flows to the condenser where it becomes a liquid by rejecting heat to the surrounding medium (the air of the kitchen). then it enters the expansion valve and its pressure and temperature drop dramatically. the low-temperature refrigerant then enters the evaporator, which is in contact with the air of the refrigeration space. it absorbs heat from the food and vaporizes, and then flows back to the compressor to restart the cycle. heat engines do the exact opposite, they take in natural heat flow from a warm body, convert some of it to useful work, and then excess heat flows to a heat sink.