The Refrigeration Cycle Explained

A basic overview of the vapor-compression cycle used in most HVAC systems.

Understanding the Vapor-Compression Cycle

The vapor-compression refrigeration cycle is the most common method used for air conditioning and refrigeration. It's a closed loop system where a working fluid, called a refrigerant, undergoes phase changes (liquid to gas and back) to absorb heat from one location and release it in another.

Diagram of the Vapor-Compression Refrigeration Cycle

A simplified diagram of the four main components of the refrigeration cycle.

The Four Main Components and Stages:

  1. Compressor:
    • The refrigerant enters the compressor as a low-pressure, low-temperature superheated vapor.
    • The compressor "squeezes" the refrigerant, increasing its pressure and temperature significantly. It leaves as a high-pressure, high-temperature superheated vapor.
    • This is where most of the system's energy input occurs.
  2. Condenser (Outdoor Coil in AC):
    • The hot, high-pressure vapor from the compressor flows into the condenser.
    • Here, it releases heat to the surrounding environment (e.g., outdoor air or water). As it cools, it condenses from a vapor into a high-pressure, warm liquid.
    • A fan often helps move air across the condenser coil to facilitate heat rejection.
  3. Expansion Valve (or Metering Device):
    • The high-pressure liquid refrigerant flows through the expansion valve.
    • This valve creates a significant pressure drop, causing the refrigerant to rapidly cool and partially flash into a low-pressure, low-temperature mixture of liquid and vapor.
    • It controls the amount of refrigerant entering the evaporator.
  4. Evaporator (Indoor Coil in AC):
    • The cold, low-pressure liquid/vapor mixture enters the evaporator.
    • Here, it absorbs heat from the space to be cooled (e.g., indoor air). As it absorbs heat, the liquid refrigerant boils (evaporates) and turns into a low-pressure, low-temperature superheated vapor.
    • A fan blows air across the evaporator coil to transfer heat from the air to the refrigerant.
    • The refrigerant then returns to the compressor to repeat the cycle.

Note: This cycle can be reversed in heat pumps to provide heating. Understanding this cycle is fundamental to diagnosing HVAC problems and appreciating how air conditioning and refrigeration work. Key concepts like superheat and subcooling are critical for ensuring the system is operating correctly and efficiently.