Both absorption and compressor refrigerators use a refrigerant with a very low boiling point (less than 0 °F (−18 °C)). In both types, when this refrigerant evaporates (boils), it takes some heat away with it, providing the cooling effect. The main difference between the two systems is the way the refrigerant is changed from a gas back into a liquid so that the cycle can repeat. An absorption refrigerator changes the gas back into a liquid using a method that needs only heat, and has no moving parts other than the refrigerant itself.
The absorption cooling cycle can be described in three phases:
- Evaporation: A liquid refrigerant evaporates in a low partial pressure environment, thus extracting heat from its surroundings (e.g. the refrigerator's compartment). Due to the low pressure, the temperature needed for evaporation is also lower.
- Absorption: The now gaseous refrigerant is absorbed by another liquid (e.g. a salt solution), reducing its partial pressure in the evaporator and allowing more refrigerant to evaporate.
- Regeneration: The refrigerant-saturated liquid is heated, causing the refrigerant to evaporate out. This happens at a significantly higher pressure. The refrigerant is then condensed through a heat exchanger to replenish the supply of liquid refrigerant in the evaporator.
In comparison, a compressor refrigerator uses an electrically powered compressor to increase the pressure on the gas, and then condenses the hot high pressure gas back to a liquid by heat exchange with a coolant (usually air). Once the high pressure gas has cooled and condensed into a liquid, it passes through an orifice which creates a pressure drop, which causes the liquid to evaporate. The evaporation process absorbs heat, and the temperature of the refrigerant drops to its boiling point at the now low pressure.
Another difference between the two types is the refrigerant used. Compressor refrigerators typically use an HCFC or HFC, while absorption refrigerators typically use ammonia or water.