The Refrigeration Cycle: Four Components
Compressor — pressurises low-pressure refrigerant vapour from the suction line, raising its temperature and pressure. The heart of the system; most failures trace back here eventually.
Condenser — rejects heat from the high-pressure refrigerant to the ambient air or water. The refrigerant desuperheats, condenses, and subcools as it passes through. A dirty or undersized condenser raises head pressure and stresses the compressor.
Expansion device — TXV or EEV on most modern systems, capillary tube on smaller/older equipment. Creates the pressure drop that allows the refrigerant to evaporate in the evaporator. Controls superheat at the evaporator outlet.
Evaporator — absorbs heat from the conditioned space as the low-pressure refrigerant evaporates. Operates below the space temperature. Airflow restriction across the evaporator is one of the most common causes of poor performance.
Refrigerant Classifications
HFCs (R410A, R134a, R404A) — zero ozone depletion, but high GWP. Being phased down under F-Gas regulations. Still widely in service on existing equipment.
HFOs and blends (R32, R454B, R1234yf) — low GWP alternatives. R32 and R454B are A2L (mildly flammable), requiring spark-free certified tools. R1234yf is the automotive standard.
Natural refrigerants (R290, R744, R600a) — ultra-low GWP. R290 (propane) is A3 (highly flammable) — strict charge limits and safety requirements apply. R744 (CO₂) requires specialist high-pressure equipment.
Zeotropic blends (R404A, R454B) — mixtures that experience temperature glide during phase change. Must be charged as liquid to prevent fractionation — vapour charging alters the blend composition.
Key Diagnostic Parameters
| Parameter | Normal Range | What It Tells You |
|---|---|---|
| Evaporator superheat | 6–12°C | Refrigerant charge level and TXV operation |
| Condenser subcooling | 3–8°C | Heat rejection efficiency and charge level |
| Compression ratio | 2:1 to 8:1 | System loading and condenser/evaporator performance |
| Discharge temperature | Refrigerant-dependent | Compressor health, superheat, oil condition |
Superheat and subcooling are the two most useful diagnostic numbers on any system. If both are within range, the charge is correct and the expansion device is working. If superheat is high and subcooling is low, the system is undercharged. If superheat is low and subcooling is high, it's overcharged or the TXV is flooding.
Common Faults and What They Indicate
Compressor short-cycling — low refrigerant charge, expansion device hunting, or low-pressure cutout tripping. Check charge and superheat first.
Extended run times, poor cooling — dirty condenser or evaporator, low charge, or ambient conditions beyond design. Check airflow across both heat exchangers before touching the refrigerant circuit.
High head pressure — dirty condenser, non-condensable gases (air in the system), overcharge, or high ambient. Non-condensables don't condense — they sit in the condenser and raise pressure without contributing to cooling.
Low suction pressure — undercharge, restricted TXV, blocked filter-drier, or low evaporator airflow. Work through these systematically before recovering refrigerant.
Moisture in the system — indicated by a wet filter-drier, acid in the oil, or a TXV that freezes intermittently. Full recovery, filter-drier replacement, deep evacuation, and recharge required.
Service Fundamentals
Evacuate to below 500 microns and hold-test before charging. Charge by weight, verify by superheat and subcooling. Use a dedicated micron gauge — manifold gauges aren't accurate enough at low vacuum levels. Replace the filter-drier every time the system is opened. Document refrigerant quantities for F-Gas compliance.
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