Q: What keeps the refrigerator cold inside?

A: Refrigerators and air conditioners cool by evaporation, the same process that cools us when we are wet. In these chilling systems evaporation and condensation occur in a controlled, closed cycle. Evaporation and condensation are phase changes which absorb and emit heat. An evaporating liquid absorbs heat which remains with the vapor as latent heat and is released only when condensation returns the vapor to the liquid state. In the natural environment water is the working fluid in heat transfer. Evaporation from the surface cools it and adds water vapor to the air. When the moist air rises it cools and the vapor condenses to form clouds as it releases heat into the atmosphere.. In chilling systems evaporation and condensation are controlled by confining the working fluid and controlling its flow through a circuit which contains a mechanical evaporator and compressor where the phase changes take place. At the evaporator the liquid absorbs heat. From there the vapor flows to the compressor where the heat is released as condensation occurs. The liquid is repressurized by the compressor and flows back to the evaporator where the cycle repeats. The heat must be exhausted outside the cooled area. A refrigerator exhausts heat into the kitchen and warms up the air slightly. Air conditioners exhaust heat into the outside air so the box containing the mechanism sits half inside and half outside the window. Energy must be continuously added to the compressor to work against the natural flow of heat from hot to cold. Insulation restricts the flow of heat back into the cooled area and increases cooling efficiency. The working fluid of chilling systems must have certain properties to make the cooling process both efficient and safe. It must be able to condense under pressure at high temperature. It must be non toxic and non corrosive, readily available , inexpensive, and it must be environmentally friendly. Most modern chillers use chlorinated fluorocarbons, or CFCs as a working fluid. These chemicals have been implicated as a major factor in damaging stratospheric ozone and are being replaced with more benign substances.

Richard Brill is assistant professor of science at Honolulu Community College where he teaches earth and physical science. Send questions to him at Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817 or email to rickb@hcc.hawaii.edu

Chilling ©1995 Richard C. Brill