Nearly every commercial cooling system, from a small rooftop unit to a large centrifugal chiller, and every heat pump uses the same basic process: the vapour compression refrigeration cycle. It moves heat from a cold place to a warm place, against the natural direction of heat flow, by using a refrigerant that evaporates and condenses at convenient temperatures.
Understanding the cycle helps building owners make sense of efficiency ratings, helps engineers select equipment and helps technicians diagnose problems. This guide explains the components, the key measurements and the factors that determine efficiency.
The four main components
The cycle has four essential components connected in a closed loop.
- Evaporator: low-pressure liquid refrigerant absorbs heat from the air or water being cooled and boils into vapour. This is where useful cooling happens.
- Compressor: draws in low-pressure vapour and compresses it to a higher pressure and temperature. This is where most of the energy input occurs.
- Condenser: high-pressure, hot vapour rejects heat to outdoor air, cooling tower water or another medium, and condenses back into liquid.
- Expansion device: drops the pressure of the liquid refrigerant, which lowers its boiling temperature so it can absorb heat again in the evaporator.
The key idea is that a refrigerant's boiling point depends on pressure. At low pressure it boils at a low temperature, absorbing heat from a cold space. At high pressure it condenses at a high temperature, releasing heat to a warmer place.
Following the refrigerant around the loop
| Point in cycle | State | Pressure | Temperature |
|---|---|---|---|
| Leaving evaporator, entering compressor | Superheated vapour | Low | Cool |
| Leaving compressor, entering condenser | Superheated vapour | High | Hot |
| Leaving condenser, entering expansion device | Subcooled liquid | High | Warm |
| Leaving expansion device, entering evaporator | Mixture of liquid and vapour | Low | Cold |
Engineers visualize the cycle on a pressure-enthalpy (P-h) diagram, which shows how much heat is absorbed in the evaporator, how much work the compressor adds and how much heat the condenser rejects.
Superheat and subcooling
Two measurements are central to setting up and diagnosing refrigeration systems.
Superheat
Superheat is the temperature of the refrigerant vapour above its saturation (boiling) temperature at the same pressure, usually measured at the evaporator outlet or compressor suction. Some superheat ensures that only vapour reaches the compressor, since liquid can damage it. Too much superheat means the evaporator is starved of refrigerant and capacity is lost.
Subcooling
Subcooling is the temperature of the liquid refrigerant below its saturation temperature at the condenser pressure. Adequate subcooling ensures that solid liquid, without flash gas, reaches the expansion device. Low subcooling can indicate low refrigerant charge; high subcooling can indicate overcharge or restrictions.
Target values depend on the equipment and refrigerant. Technicians use manufacturer data to set charge and diagnose faults.
Expansion devices
- Fixed orifices and capillary tubes: simple and inexpensive, used in small equipment; they do not adjust to changing loads.
- Thermostatic expansion valves (TXVs): mechanically adjust refrigerant flow to maintain a set superheat.
- Electronic expansion valves (EEVs): controlled by a microprocessor, they allow precise control across a wide operating range and are common in modern chillers, VRF systems and cold climate heat pumps.
Compressor types
Commercial HVAC uses several compressor types, each suited to different capacities.
| Compressor type | Typical application | Notes |
|---|---|---|
| Reciprocating | Older and smaller equipment | Largely replaced by scroll in new HVAC equipment |
| Scroll | Rooftop units, small chillers, heat pumps | Reliable, few moving parts; often staged or variable speed |
| Rotary | Small heat pumps, VRF, mini-splits | Common with inverter drives |
| Screw | Medium to large chillers | Good for high lift and part-load with slide valves or variable speed |
| Centrifugal | Large chillers | Very efficient at large capacities; oil-free magnetic bearing options exist |
For more on how compressor type affects chiller selection, see chiller types: centrifugal, screw, scroll and absorption.
Lift and efficiency
Lift is the difference between condensing and evaporating temperatures (or pressures). The greater the lift, the more work the compressor must do, and the lower the efficiency. This leads to some practical principles:
- Raising the chilled water or supply air temperature raises the evaporating temperature and improves efficiency.
- Lowering the condenser water temperature or outdoor air temperature lowers the condensing temperature and improves efficiency.
- Dirty condenser coils, fouled tubes and poor airflow raise the condensing temperature and waste energy.
Efficiency metrics
- Coefficient of performance (COP): useful heat moved divided by energy input, both in the same units. A COP of 5 means 5 kW of cooling for each kW of electricity.
- kW per ton: common for chillers in North America; lower is better. COP equals 3.517 divided by kW per ton.
- EER and IEER: energy efficiency ratios in Btu/h per watt, used for packaged equipment.
Full-load ratings are only part of the picture; see part-load performance.
Heat pumps: the same cycle in reverse
A heat pump uses the same cycle, but the useful output is the condenser heat. A reversing valve switches the roles of the indoor and outdoor coils between heating and cooling. In heating, the heat delivered equals the heat absorbed from outdoors plus the compressor energy, which is why COPs above 1 are possible. As outdoor temperatures fall, lift increases and capacity and COP decline, which is the central design challenge for cold climate heat pumps.
Refrigerants and safety
Refrigerants are chosen for their thermodynamic properties, safety and environmental impact. Older CFC and HCFC refrigerants damaged the ozone layer and have been phased out. Many current HFCs, such as R-410A and R-134a, have high global warming potential and are being phased down under the Kigali Amendment, implemented in Canada through federal regulations. New equipment is moving to lower-GWP options, some of which are mildly flammable (A2L classification). See refrigerant regulations and low GWP options in Canada.
In Canada, CSA B52, Mechanical Refrigeration Code, governs the safe design, installation and operation of refrigeration systems, including refrigerant quantity limits in occupied spaces and machinery room requirements. Provincial regulations govern refrigerant handling and technician certification.
Frequently asked questions
Why does my rooftop unit lose capacity on very hot days?
Higher outdoor temperatures raise the condensing temperature, increasing lift. The compressor moves less heat and uses more power. Dirty condenser coils make this worse.
Can a system be charged by pressure alone?
Pressure alone is not reliable. Technicians use superheat or subcooling targets from the manufacturer, along with temperatures and pressures, to charge correctly.
What does A2L mean?
It is a safety classification under ASHRAE Standard 34 for refrigerants with lower toxicity and lower flammability. A2L refrigerants require specific design and installation precautions under applicable codes.
Key takeaways
- The cycle uses evaporation at low pressure and condensation at high pressure to move heat.
- Superheat and subcooling are the key measurements for charge and diagnosis.
- Lift drives compressor work; reducing lift improves efficiency.
- Heat pumps use the same cycle, with capacity falling as outdoor temperature drops.
- Refrigerant choice is shaped by the HFC phase-down, and CSA B52 governs safety in Canada.
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