Learn & Understand

Inside the Cooling Machine: The Refrigeration Cycle

An air conditioner performs a small miracle every summer: it makes a room colder than the world outside, seemingly moving heat from a cool place to a warm one. It does this with a four-step loop — the vapor-compression refrigeration cycle — that exploits how a fluid absorbs and releases heat as it changes pressure and phase.

The Working Fluid

At the heart of the cycle is a refrigerant, a fluid chosen because it boils and condenses at convenient temperatures when its pressure is manipulated. By making the refrigerant evaporate where you want to absorb heat and condense where you want to release it, the machine ferries heat from inside a building to outside. The fluid loops endlessly, changing between liquid and vapor.

Four Steps Around the Loop

The cycle has four components. The compressor squeezes the refrigerant vapor, raising its pressure and temperature. The condenser, outdoors, lets that hot high-pressure vapor shed heat to the outside air and condense to liquid. An expansion device then drops the liquid's pressure, cooling it sharply. Finally the evaporator, indoors, lets the cold refrigerant absorb heat from the room's air and boil back to vapor — and the loop repeats.

Pressure Is the Trick

The cycle works because a fluid's boiling point depends on its pressure. High-pressure refrigerant condenses and releases heat even at outdoor temperatures; low-pressure refrigerant boils and absorbs heat even at cool indoor ones. By moving the fluid between high and low pressure at the right places, the machine arranges for it to soak up heat inside and dump it outside — cooling the room in the process.

Measuring the Result

The efficiency of this cycle at peak conditions is captured by the EER — the cooling delivered per unit of electrical power at a fixed, hot outdoor temperature. Because it is measured at demanding conditions rather than averaged over a season, EER speaks to how the machine performs on the hottest days, when the cycle is working hardest to move heat against the largest temperature gap.

Calculating Efficiency

To compute a unit's EER, use the EER Calculator. Compare seasonal efficiency with the SEER Rating Savings Calculator, and gauge heat pump performance with the COP Calculator.