Refrigeration Cycle 101

We try to remember the refrigeration cycle components in the following order: compressor, condenser, metering device, and evaporator. We also try to remember the compressor as the pressure increaser, the condenser as the heat rejector, the metering device as the pressure dropper, and the evaporator as the heat absorber. The refrigerant absorbs heat in the evaporator, is pressurized in the compressor, rejects heat in the condenser, and is depressurized in the metering device.

The backbone of the refrigeration cycle is the ideal gas law, which states that pressure and volume are equal to mass and temperature. In the refrigeration circuit, we’re constantly manipulating the temperature, volume, and pressure of the refrigerant. The pressure is the key; although the refrigerant absorbs and rejects heat, we get the most out of it by pressurizing it and depressurizing it. The refrigeration cycle’s main goal is to get heat into the refrigerant and out of it again.

The compressor is fed cool, low-pressure gaseous refrigerant via the suction line and discharges hot, high-pressure vapor via the discharge line. When it compresses refrigerant, it forces that refrigerant to occupy a smaller space (volume) by heavily increasing the pressure.

Discharge gas goes down the discharge line and enters the condenser coil, which rejects heat. When the condenser rejects enough heat, the vapor refrigerant cools enough to become liquid. (Some reject heat to fluid like water or glycol via a heat exchanger.) Most typical residential HVAC condensers are air-cooled condensers.

Liquid refrigerant travels to the metering device via the liquid line. Some common metering devices are pistons, TXVs, EEVs, and capillary tubes. These devices drop the pressure of the refrigerant. Depressurizing refrigerant allows it to occupy a larger space. Some of the liquid refrigerant may flash off into gas as well.

From there, the refrigerant enters the evaporator, where it absorbs heat from the air. The evaporator coil is quite cold, and as the cool liquid refrigerant absorbs more heat, it transforms into a vapor. The refrigerant leaves the evaporator as a cool, low-pressure vapor and goes through the suction line to the compressor. From there, the cycle repeats itself.