States of Matter and Phase Change

A substance can exist as a solid, a liquid or a vapor. Moving between those states absorbs or releases a large amount of heat without changing temperature. Refrigeration is built entirely on that fact.

Boiling a pound of water absorbs roughly a thousand BTU, while heating that same pound one degree absorbs one BTU. Phase change moves far more heat per pound of fluid, which is why a small line set can carry the capacity of a whole house.

In the evaporator the refrigerant boils and absorbs indoor heat. In the condenser it condenses and gives that heat up outdoors. The compressor and metering device simply set the pressures so those changes happen at the temperatures you want.

1. 1Low pressure liquid refrigerant enters the evaporator and begins boiling as it absorbs heat from the air.
2. 2Once fully boiled, any further heating is superheat, which protects the compressor from liquid.
3. 3The compressor raises the pressure so the vapor will condense at a temperature above outdoor ambient.
4. 4In the condenser the vapor gives up heat, condenses to liquid, and cools further as subcooling.
5. 5The metering device drops the pressure and the cycle repeats.

• Liquid returning to the compressor because the refrigerant never finished boiling, which can damage valves and dilute oil.
• A coil that ices over because the boiling temperature dropped below freezing and moisture froze on the fins.
• Non-condensable gases left in the system after poor evacuation, which raise head pressure and upset the relationship between pressure and temperature.

Every air conditioner, heat pump, refrigeration rack and dehumidifier uses phase change. Steam and hydronic heating use the same principles in the opposite direction.