The Vapour Compression Cycle
Four components, one continuous loop:
- Compressor — draws low-pressure vapour from the suction line and compresses it to high pressure, raising its temperature. The energy input that drives the whole cycle.
- Condenser — rejects heat from the high-pressure refrigerant to ambient air or water. Refrigerant desuperheats, condenses, and subcools as it passes through.
- Expansion device — TXV, EEV, or capillary tube. Drops the pressure and temperature of the liquid refrigerant before it enters the evaporator.
- Evaporator — absorbs heat from the conditioned space as the low-pressure refrigerant evaporates. Operates below the space temperature.
Compressor Types
Reciprocating — piston and cylinder. Good part-load performance. Common in smaller commercial and light industrial applications.
Scroll — spiral elements, quiet, high volumetric efficiency. Standard in residential and light commercial splits.
Rotary — eccentric rotor. Compact, smooth operation. Common in small splits and window units.
Screw — intermeshing rotors, continuous compression. Used in large commercial chillers and industrial applications.
Inverter/variable speed — any of the above with VFD control. Matches capacity to load, significantly reduces energy consumption at part load.
Expansion Devices
TXV (thermostatic expansion valve) — maintains constant superheat via a temperature-sensing bulb on the suction line. Standard on most commercial systems.
EEV (electronic expansion valve) — stepper motor controlled, faster response, more precise. Standard on inverter systems and VRF.
Capillary tube — fixed restriction, no moving parts. Used on small domestic appliances and older equipment. Sensitive to contamination.
Refrigerant Quick Reference
| Refrigerant | GWP | Class | Typical Application |
|---|---|---|---|
| R410A | 2,088 | A1 | Existing residential/commercial splits |
| R32 | 675 | A2L | New residential splits (post-2025) |
| R454B | 466 | A2L | New commercial equipment |
| R134a | 1,430 | A1 | Automotive AC, chillers |
| R1234yf | 4 | A2L | Automotive AC (post-2017) |
| R290 | 3 | A3 | Small appliances, heat pumps |
| R744 (CO₂) | 1 | A1 | Commercial refrigeration, heat pumps |
A2L = mildly flammable. Requires spark-free certified tools. A3 = highly flammable. Strict charge limits and safety requirements apply.
Key Diagnostic Parameters
| Parameter | Normal Range | What It Indicates |
|---|---|---|
| Evaporator superheat | 6–12°C | Charge level, TXV operation |
| Condenser subcooling | 3–8°C | Heat rejection, charge level |
| Compression ratio | 2:1 to 8:1 | System loading, condenser/evaporator performance |
Common Faults: Quick Diagnosis
High head pressure — dirty condenser, non-condensables, overcharge, high ambient.
Low suction pressure — undercharge, restricted TXV, blocked filter-drier, low evaporator airflow.
Short cycling — low charge, TXV hunting, pressure cutout tripping.
Extended run times — dirty heat exchangers, low charge, ambient beyond design.
Moisture in system — wet filter-drier, acid in oil, intermittent TXV freeze. Full recovery, drier replacement, deep evacuation, recharge.
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