The compression-refrigeration cycle: four stages and how heat pumps reverse it

One-line orientation

The vapor-compression refrigeration cycle drives conventional air conditioners, refrigerators, and heat pumps. Learn the four steps and how a heat pump reverses the heat-flow direction.

Key points

• Four stages (refrigeration / cooling mode):
  1. Evaporation — low-pressure liquid refrigerant enters the evaporator coil (indoors); absorbs heat from the room and vaporizes, cooling the air.
  2. Compression — the compressor raises the vapor’s pressure and temperature.
  3. Condensation — hot high-pressure vapor enters the condenser coil (outdoors); releases heat to the outside air and condenses back to liquid.
  4. Expansion — the liquid passes through an expansion valve; pressure drops sharply, the refrigerant cools, and the cycle restarts.
• Heat pump reversal: a reversing valve swaps the roles of the two coils. In heating mode the indoor coil becomes the condenser (releases heat inside) and the outdoor coil becomes the evaporator (extracts heat from outside air or ground).
• Air-source heat pump: uses outdoor air as the heat source (heating) or heat sink (cooling). Efficiency degrades in extreme cold (hard to extract heat) and extreme heat (hard to reject heat).
• Geothermal (ground-source) heat pump: uses the ground or groundwater as the exchange medium. Ground temperature is relatively stable in a ~50–60°F range regardless of season, so efficiency is more consistent and the system performs well in climates where air-source systems struggle.

Confusions / comparison

Compression refrigeration — one loop, four components

SCHEMATIC LOOP

The loop crosses between a cold low-pressure side and a hot high-pressure side.

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Key

• blue zone = cold · low pressure
• warm zone = hot · high pressure

Evaporator absorbs heat; condenser releases it. The component names describe the refrigerant's state change.

Heat-pump heating uses the same four components; a reversing valve swaps which coil is indoors.

Stage	Component	Refrigerant state	What happens to heat
Evaporation	Evaporator coil (indoors)	liquid → vapor	Absorbs heat from room — room cools
Compression	Compressor	vapor, pressure ↑	Temperature rises — no heat exchange yet
Condensation	Condenser coil (outdoors)	vapor → liquid	Releases heat to outdoors
Expansion	Expansion valve	liquid, pressure ↓	Temperature drops — ready to absorb again

	Air-source heat pump	Geothermal heat pump
Heat exchange medium	Outdoor air	Ground or groundwater
Climate sensitivity	High — less efficient in extreme cold or heat	Low — stable ~50–60°F ground temp
Installation cost	Lower	Higher (drilling / ground loops)
Long-term efficiency	Moderate	High

Related

→ HVAC: balance temperature (when heating vs cooling kicks in) · HVAC distribution systems (all-air / all-water) · Thermal: degree days (measure heating/cooling demand over a season).