The Basic Refrigeration Cycle
Mechanical refrigeration is the process of moving heat from a location where it is unwanted to a location where it can be rejected. Understanding this process begins with heat transfer, changes of refrigerant state, and the four major components of the refrigeration system.
This section provides the foundation for the rest of the Air Conditioning / Refrigeration course. Before studying compressors, condensers, evaporators, metering devices, and refrigerant piping individually, you should understand how those components work together as one continuous refrigeration cycle.
What You Will Learn
The refrigeration cycle is built around a simple idea: refrigerant absorbs heat at one location, carries that heat through the system, and rejects it somewhere else. Pressure changes allow the refrigerant to boil and condense at temperatures useful for air conditioning and refrigeration.
Heat Transfer
Understand why heat moves, the difference between sensible and latent heat, and how refrigeration equipment moves heat rather than creating cold.
The Four Major Components
Identify the compressor, condenser, metering device, and evaporator and understand the basic purpose of each component.
Refrigerant State
Follow refrigerant as it changes between liquid and vapor while circulating through the refrigeration system.
High Side and Low Side
Recognize the high-pressure and low-pressure portions of the refrigeration system and the components that separate them.
Heat Absorption
Understand how refrigerant boiling inside the evaporator absorbs heat from the conditioned space.
Heat Rejection
Understand how refrigerant condensing inside the condenser releases heat to the surrounding environment.
Refrigeration Moves Heat
Heat Is Absorbed
The evaporator absorbs heat from indoor air, refrigerated products, or another material that needs to be cooled.
Heat Is Transported
Circulating refrigerant carries the absorbed heat through the refrigeration system.
Heat Is Rejected
The condenser transfers heat from the refrigerant to outdoor air, water, or another available heat sink.
Cooling occurs because heat is removed from the conditioned space. The refrigeration system provides a method of absorbing that heat at a low temperature and rejecting it somewhere else at a higher temperature.
Basic Refrigeration Cycle Lessons
Complete these lessons before moving into the individual component sections. They establish the terminology and system relationships used throughout the remainder of the course.
Introduction to Mechanical Refrigeration
Begin with the fundamentals of heat, heat transfer, sensible heat, latent heat, and the basic purpose of a mechanical refrigeration system.
Heat Transfer
Sensible Heat
Latent Heat
Mechanical Refrigeration
The Refrigeration Cycle
Follow refrigerant through the compressor, condenser, metering device, and evaporator while learning where pressure changes, heat absorption, heat rejection, evaporation, and condensation occur.
Condenser
Metering Device
Evaporator
Refrigerant Flow
Follow Refrigerant Around the System
Compressor
The compressor receives low-pressure refrigerant vapor and compresses it to a higher pressure and temperature. It also creates the pressure difference necessary to circulate refrigerant through the system.
Condenser
The condenser rejects heat from the refrigerant. As heat is removed, the refrigerant condenses from vapor into liquid while remaining on the high-pressure side of the system.
Metering Device
The metering device controls refrigerant flow into the evaporator and creates the major pressure drop between the high-pressure and low-pressure sides of the refrigeration system.
Evaporator
The evaporator absorbs heat into the refrigerant. The refrigerant boils as it absorbs heat and leaves the evaporator as vapor before returning toward the compressor.
The Cycle Continues
After leaving the evaporator, refrigerant returns to the compressor and the process repeats continuously while the system operates.
Pressure Divides the Refrigeration System
High-Pressure Refrigerant
The high side begins at the compressor discharge and continues through the condenser and liquid line to the inlet of the metering device.
- Compressor discharge
- Condenser
- Liquid line
- Metering-device inlet
Low-Pressure Refrigerant
The low side begins at the outlet of the metering device and continues through the evaporator and suction line to the compressor inlet.
- Metering-device outlet
- Evaporator
- Suction line
- Compressor inlet
The compressor raises refrigerant pressure as refrigerant moves from the low side to the high side. The metering device creates the pressure reduction as refrigerant moves from the high side to the low side.
Evaporation Absorbs Heat — Condensation Rejects Heat
Liquid → Vapor
Low-pressure refrigerant absorbs heat in the evaporator. Much of this heat causes the refrigerant to change from liquid to vapor rather than simply increasing its temperature.
Vapor → Liquid
High-pressure refrigerant rejects heat in the condenser. As latent heat is removed, refrigerant changes from vapor to liquid.
Changing refrigerant pressure changes its saturation temperature. The refrigeration system maintains a low enough pressure for refrigerant to boil while absorbing heat in the evaporator and a high enough pressure for refrigerant to condense while rejecting heat in the condenser.
Concepts You Will Use Throughout the Course
Saturation
Liquid and vapor can exist together at a specific pressure and corresponding saturation temperature.
Evaporation
Refrigerant absorbs latent heat while changing from liquid to vapor.
Condensation
Refrigerant rejects latent heat while changing from vapor to liquid.
Superheat
Once all refrigerant has become vapor, additional heat raises the vapor temperature above its saturation temperature.
Subcooling
Once all refrigerant has become liquid, additional heat removal lowers the liquid temperature below its saturation temperature.
Pressure-Temperature Relationship
Refrigerant saturation temperature is directly related to refrigerant pressure and is fundamental to refrigeration-system operation and diagnosis.
Where the Course Goes Next
Once the complete refrigeration cycle is understood, the course examines each major part of the system in greater detail.
Start With Heat and Mechanical Refrigeration
Begin with the fundamentals of heat transfer and learn why a refrigeration system is fundamentally a machine designed to move heat.