The vapor-compression refrigeration cycle is the process that moves heat from a cold space to a warmer one using a compressible fluid. It has four stages: compression, condensation, expansion, and evaporation. A typical comfort-cooling system absorbs about 3.5 kW of heat per ton of refrigeration while consuming roughly 0.8 to 1.2 kW of compressor power, giving a coefficient of performance of 3 to 4. This is the same cycle behind refrigerators, chillers, and heat pumps, differing only in temperature lift and refrigerant.
The Four Components in Sequence
In the compressor, low-pressure vapor at about 0.4 to 0.6 MPa is raised to 1.2 to 1.8 MPa, lifting its saturation temperature above the ambient so heat can be rejected. The condenser then dumps that heat to outdoor air or water, condensing the refrigerant to a liquid at 35 to 55 °C. The expansion valve drops the pressure to 0.3 to 0.6 MPa, and the evaporator boils the liquid at 2 to 10 °C, absorbing heat from the conditioned space. The four steps repeat thousands of times per hour, each cycle carrying a packet of heat uphill against the temperature gradient.
Refrigerant State Points
The table maps the four stages to the thermodynamic state of the fluid. These temperatures and pressures are typical for an R-410A or R-32 air-conditioning loop and shift with the chosen refrigerant and the design lift, so treat them as planning anchors rather than exact setpoints for a specific machine.
| Stage | Refrigerant State | Typical Pressure | Typical Temp. |
|---|---|---|---|
| Evaporator out / compressor in | Low-P superheated vapor | 0.4 – 0.6 MPa | 2 – 10 °C |
| Compressor out / condenser in | High-P superheated vapor | 1.2 – 1.8 MPa | 60 – 85 °C |
| Condenser out / valve in | High-P liquid | 1.2 – 1.8 MPa | 35 – 55 °C |
| Valve out / evaporator in | Low-P liquid + vapor mix | 0.3 – 0.6 MPa | 2 – 10 °C |
Coefficient of Performance
COP equals heat moved divided by work input. A residential split system at AHRI conditions (evaporator 7 °C, condenser 35 °C) commonly shows a COP of 3.0 to 4.0, meaning 3 to 4 units of cooling per unit of electricity. Every 1 °C rise in condenser temperature can cut COP by 2% to 4%, which is why clean coils and good airflow matter more than many users realize. A dirty condenser that loses 5 °C of heat rejection can quietly erase 10% to 20% of a system's efficiency.
Why the Lift Determines Efficiency
The pressure lift between evaporator and condenser sets the compressor work. A system designed for a 5 °C evaporator and 40 °C condenser has a smaller lift than one serving –10 °C freezing with a 45 °C condenser, so the latter needs 20% to 40% more specific work. Selecting the highest evaporator temperature the process allows is the cheapest efficiency gain available, which is why medium-temperature cooling is always preferred over low-temperature cooling when the product permits it.
Subcooling and Superheat
About 3 to 5 K of condenser subcooling protects the expansion valve from flash gas and adds 2% to 4% capacity; 5 to 10 K of evaporator superheat keeps liquid out of the compressor, protecting it from slugging. Both are set by the charge and the metering device. Too little superheat risks liquid floodback that can destroy a compressor in a single season, while too much starves the evaporator and drops capacity by 5% to 15%.
When to Use Vapor Compression
- Use standard vapor compression for almost all comfort cooling and most process chilling from –40 °C up to about 10 °C evaporator temperature.
- Use cascade or two-stage compression when the evaporator must reach below –40 °C and a single lift would be inefficient or exceed compressor limits.
- Use heat-pump mode when you want the same loop to deliver heating, reversing the valve so the indoor coil becomes the condenser and the COP for heating reaches 3 to 5.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
Common Failure Modes
Loss of charge is the leading fault: a 10% to 20% refrigerant shortfall can drop capacity by 15% to 30% and raise compressor discharge temperature past 100 °C, risking burnout. Restricted expansion devices, fouled condensers, and non-condensable air in the loop each push the high-side pressure up by 50 to 200 kPa, cutting COP and shortening compressor life. A yearly pressure and temperature check at the four state points catches most of these faults before they become failures.
Refrigerant Choice and Practical Note
Although the cycle is the same, the refrigerant sets the exact pressures and temperatures in the table. R-32 and R-410A are common in comfort cooling with evaporating pressures near 0.8 to 1.0 MPa at 5 °C, while R-134a sits lower at about 0.35 MPa for the same temperature, which changes compressor sealing and piping cost. Lower-GWP alternatives such as R-32 (GWP about 675) are displacing R-410A (GWP about 2,088) in new equipment, and CO₂-based transcritical systems reach discharge pressures of 8 to 12 MPa that need stiffer components and careful relief sizing.
The engineering point is that the four-stage cycle and the COP logic above hold for every fluid; only the state-point numbers shift, so designers should re-read the manufacturer's pressure-enthalpy diagram rather than reuse another refrigerant's setpoints when retubing or retrofitting a machine. A common field error is charging an R-32 system to an R-410A weight, which overfills by 20% to 30% and pushes discharge temperature past 100 °C, or undercharges and starves the evaporator by 10% to 15% of capacity, so the nameplate charge and the correct subcooling of 3 to 5 K must be followed exactly regardless of how familiar the cycle looks.
FAQ
What are the four stages of the vapor-compression cycle?
The cycle has four stages: compression raises low-pressure vapor to high pressure, condensation rejects heat and liquefies the refrigerant, expansion drops the pressure through a valve, and evaporation boils the liquid while absorbing heat from the space. These repeat continuously, moving heat from cold to warm against the natural gradient, and they are the basis of every refrigerator, chiller, and
What COP can I expect from an air conditioner?
A typical comfort system shows a coefficient of performance of 3.0 to 4.0 at standard rating conditions, meaning it moves 3 to 4 units of heat per unit of electricity. COP falls 2% to 4% for every 1 ยฐC the condenser runs hotter, so coil cleanliness and airflow directly protect efficiency and capacity.
Why does condenser temperature matter so much?
Condenser temperature sets the high-side pressure and therefore the compressor lift. A 5 ยฐC rise can lower COP by 10% to 20% and add 50 to 200 kPa to discharge pressure. Keeping the condenser coil clean and the fans effective is usually the highest-return maintenance action on any vapor-compression machine, because it attacks the single largest efficiency lever in the
Can the same cycle provide heating?
Yes. In heat-pump mode a reversing valve swaps the indoor and outdoor roles so the indoor coil becomes the condenser, releasing heat. Heating COP commonly reaches 3 to 5, better than electric resistance heating by a factor of three or more. Defrost cycles briefly reduce output in freezing outdoor conditions, but annual heating efficiency still far exceeds direct electric resistance.