Dr. John B. Gorrie

Evaporation of a liquid in a system is an endothermic process - it requires heat to convert a liquid to vapor.  That heat comes from the surroundings - the rest of the liquid.  The temperature of the resevoir of liquid left unevaporated falls as energy goes into the evaporating liquid. Thus, Gorrie evaporated water from a reservoir of water, insulated from outside heat.  The temperature of the remaining water falls - and when it reaches the freezing point, ice forms.  The formation of ice is exothermic and continues the process that energy flows, now from the freezing liquid into the continued vaporization process. Think about this process someday on a ski lift.  Watch the liquid water, mixed with air, freeze instantly as it jets from the snow guns.

Subsequently, engineers and scientists worked to reduce this concept to a practical mechanical refrigerator. In such an apparatus:

1. A gaseous refrigerant is compressed from a lower to a higher pressure. Some of the energy supplied by the compressor transfers to the gas as heat.   The figure below shows that the gas temperature rises to 45^oC.
2. The heat is removed from the system to the surroundings in a condenser by, for example, forcing the gas through a tube exposed to room-temperature air.  The refrigerant liquifies.
3. Passage of this liquid through an expansion valve lowers the pressure exerted on it by its own vapor pressure, thereby causing some of the liquid to evaporate and cool the remaining liquid as a result. Expansion of the gas further lowers its temperature (Joule - Thomson Effect).
4. In the evaporator the cold liquid-vapor mixture removes heat from the surroundings as it completely vaporizes. The now-gaseous refrigerant returns to the compressor, completing the cycle.

This cycle is known as the Clausius-Rankine cycle. A thermostat regulates the operation of the compressor to turn it off when the temperature in the cold chamber decreases to a pre-determined set point. A change in state (gas <--> liquid), rather than a chemical change, has taken place.

Many engineering developments were needed, such as designing a compressor, condenser-evaporator components and an expansion valve. Materials of high and low thermal conductivity had to be selected for the condenser-evaporator and cold chamber, respectively. An expansion valve was needed to maintain a high enough pressure on the condenser side for the refrigerant to liquify and yet permit its partial vaporization into the evaporator section. Of course, to be truly practical, a small AC electric motor had to be developed to drive the compressor.