Refrigeration Cycle Reference
Why this matters
Every air conditioner, heat pump, refrigerator, freezer, dehumidifier, and ice maker uses the vapor-compression refrigeration cycle. Knowing what each component does, what the refrigerant looks like at each point, and how to recognize abnormal conditions is the foundation of HVAC and refrigeration diagnostics. Without this mental model, you're swapping parts based on guesses.
The four core components
- Compressor - raises refrigerant from low pressure / low temperature vapor to high pressure / high temperature vapor. Mechanical work input.
- Condenser - rejects heat from the hot high-pressure vapor; refrigerant condenses to liquid. Outdoor in AC; back of fridge in appliances.
- Metering device (TXV / EEV / piston / capillary tube) - drops pressure of the liquid refrigerant. Liquid flashes partially to vapor as pressure drops.
- Evaporator - absorbs heat from the conditioned space; refrigerant boils from low-pressure liquid to low-pressure vapor. Indoor coil in AC; freezer coil in appliances.
The cycle: vapor in compressor → high-pressure vapor to condenser → liquid out of condenser → liquid through metering device → low-pressure liquid into evaporator → low-pressure vapor back to compressor.
The four states of refrigerant in the cycle
At any moment, refrigerant in the system exists in four distinct thermodynamic states:
State 1 - Low-pressure vapor (compressor suction):
- Pressure: low side (e.g., 118 psig for R-410A at 40 °F saturation)
- Temperature: slightly above saturation = superheated vapor
- Location: from evaporator outlet to compressor inlet
State 2 - High-pressure vapor (compressor discharge):
- Pressure: high side (e.g., 365 psig for R-410A at 110 °F saturation)
- Temperature: well above saturation (typically 150-200 °F) due to compression heating
- Location: compressor outlet to condenser inlet
State 3 - High-pressure liquid (condenser outlet):
- Pressure: high side (slightly less than discharge due to condenser pressure drop)
- Temperature: at or below saturation = subcooled liquid
- Location: condenser outlet to metering device inlet
State 4 - Low-pressure mixture (metering device outlet):
- Pressure: low side
- Temperature: at saturation (flash boiling)
- Quality: typically 15-25% vapor, 75-85% liquid
- Location: metering device outlet to evaporator inlet
Each state has its own measurable signature. The diagnostic process is reading these signatures.
Pressure-temperature (P/T) saturation relationship
In a sealed refrigerant system at equilibrium, pressure and temperature of the saturated mixture are locked together - knowing one tells you the other.
R-410A example P/T saturation:
| Pressure (psig) | Saturation temp (°F) |
|---|---|
| 50 | 0 |
| 75 | 19 |
| 100 | 33 |
| 118 | 40 |
| 145 | 50 |
| 180 | 62 |
| 240 | 80 |
| 300 | 95 |
| 365 | 110 |
| 425 | 121 |
| 500 | 132 |
When you read 118 psig on the suction gauge of an R-410A system, the evaporator coil is at 40 °F (the boiling point of R-410A at that pressure). When you read 365 psig on the discharge gauge, the condenser coil is at 110 °F.
Each refrigerant has its own P/T chart - R-22, R-410A, R-32, R-454B, R-134a are all different.
Common refrigerants
R-410A - long-dominant residential and light commercial AC and heat pump refrigerant since the early 2000s. Higher operating pressures than R-22. GWP = 2088, being phased down.
R-32 - single-component HFC. Lower GWP (675) than R-410A. Becoming common in newer Mitsubishi, Daikin equipment.
R-454B (Solstice N40, Puron Advance) - blend of R-32 + R-1234yf. Low GWP (466). Replacing R-410A in 2024-2025 new equipment.
R-1234yf - automotive AC refrigerant; extremely low GWP. Some commercial use.
R-134a - older automotive and small commercial refrigeration. Being phased down.
R-22 - original residential AC refrigerant. Phased out for new equipment (2010); still serviced in existing systems with reclaimed or recycled R-22.
R-600a (isobutane) - used in newer refrigerators (hydrocarbon refrigerant). Very low GWP but flammable.
R-717 (ammonia) - industrial refrigeration. Toxic, but excellent thermodynamic properties.
EPA Section 608 certification required for technicians who service refrigeration systems. Different certification levels (I, II, III, Universal) cover different equipment.
Superheat and subcooling - the diagnostic measurements
Superheat (SH) = suction line temperature − saturation temperature at suction pressure
Tells you how much heat the refrigerant absorbed BEYOND just boiling. Healthy SH on a fixed-orifice / piston system is 10-15 °F at design conditions; on a TXV system, SH is whatever the valve sets (usually 8-12 °F) and is NOT the charge indicator on TXV.
Subcooling (SC) = saturation temperature at liquid pressure − liquid line temperature
Tells you how much the liquid was cooled BELOW its condensing point. Healthy SC on a TXV system is 8-12 °F (manufacturer-specified). High SC = overcharge; low SC = undercharge or restriction.
See HVAC Superheat and Subcooling Method Reference for full charging procedure.
Common cycle abnormalities
Low charge:
- Low suction pressure, low discharge pressure (both shifted down)
- High superheat (less refrigerant boiling, suction line stays hot)
- Low subcooling (less liquid available, condenser still has vapor mixed in)
- Compressor amps below normal
- Customer symptom: not cooling enough, ice on suction line possible (severe undercharge)
Overcharge:
- High suction pressure, high discharge pressure
- Low superheat (excess refrigerant boiling, suction line stays cold)
- High subcooling (excess liquid in condenser bottom, more cooling)
- Compressor amps above normal
- Customer symptom: high bills, possible compressor damage, condensate at outdoor unit
Air in system (non-condensables):
- Discharge pressure abnormally high
- Subcooling normal or low
- Performance degraded
- Caused by inadequate vacuum during charging
- Fix: recover, evacuate properly (>500 microns), recharge
Dirty condenser coil:
- High discharge pressure
- Subcooling high (heat can't escape, condenser stays hot)
- Compressor amps high
- Customer symptom: high bills, possible compressor overload tripping
Dirty evaporator / low airflow:
- Low suction pressure (less heat in, coil too cold)
- Possibly icing on coil at extreme cases
- Suction line cold or sweating excessively
- Customer symptom: poor cooling, ice in coil
Metering device problem (TXV stuck):
- Stuck closed: low suction pressure, high superheat, low subcooling
- Stuck open: low superheat, high suction, flooded evaporator
- Compressor liquid slug risk if stuck open
References
- ASHRAE Handbook - Refrigeration
- EPA Section 608 (refrigerant handling, recovery, certification)
- Manufacturer service manuals (Carrier, Trane, Lennox, Goodman, Rheem)
- ACCA Manual SR (refrigerant charging standards)
- AHRI 700 (refrigerant specifications)
- Manuall internal: HVAC Superheat Subcooling Method, Refrigerant Charging Procedures, Vacuum Evacuation Procedures