What to check on a AC Compressor (for HVAC).
Starting air and service air compressors compress atmospheric air for pneumatic use. An HVAC compressor is a refrigeration compressor: it compresses a refrigerant gas (commonly R-134a or R-407C on older tonnage, R-513A or R-1234ze on newer installations moving away from higher-GWP refrigerants) as part of a closed vapour-compression cycle serving the accommodation air handling units. It never handles atmospheric air directly and runs in a sealed circuit with an evaporator, condenser and expansion device.
What sets an HVAC compressor apart from other shipboard compressors
Starting air and service air compressors compress atmospheric air for pneumatic use. An HVAC compressor is a refrigeration compressor: it compresses a refrigerant gas (commonly R-134a or R-407C on older tonnage, R-513A or R-1234ze on newer installations moving away from higher-GWP refrigerants) as part of a closed vapour-compression cycle serving the accommodation air handling units. It never handles atmospheric air directly and runs in a sealed circuit with an evaporator, condenser and expansion device.
Main components
Compressor types in service
Reciprocating semi-hermetic compressors remain common on mid-size vessels for their serviceability — cylinder heads can be opened for valve work without breaking the refrigerant circuit at the crankcase. Scroll compressors appear on smaller accommodation plants for their lower vibration and fewer moving parts, but are fully hermetic and cannot be internally repaired on board; a failed scroll unit is replaced whole.
Crankcase heater and oil management
A crankcase heater keeps refrigerant from migrating into the oil during standstill, since refrigerant dissolved in oil on start-up causes violent foaming and can strip bearing lubrication in seconds. Oil separators downstream of the discharge return entrained oil to the crankcase rather than letting it circulate and coat the condenser and evaporator surfaces.
Controls and safety cut-outs
High- and low-pressure switches, an oil differential pressure switch on larger units, and discharge temperature protection together stop the compressor before a fault develops into a burnout. These are not optional extras — a compressor running without working pressure protection is one blocked condenser away from a locked rotor.
Selection / sizing
- Cooling load of the accommodation spaces served, in kW, against the compressor's rated capacity at the design condensing and evaporating temperatures.
- Refrigerant type, driven by what is already in use on the vessel and by phase-down schedules for higher-GWP refrigerants that flag states are progressively adopting under the Montreal Protocol framework.
- Semi-hermetic versus fully hermetic, weighed against the crew's ability to carry out valve or bearing work at sea versus simply carrying a spare unit.
- Electrical supply — voltage, phase and starting current against the vessel's switchboard capacity, particularly for direct-on-line starts on larger units.
Regulations and class
MARPOL Annex VI and the Montreal Protocol/Kigali Amendment framework govern which refrigerants can be used and how leaks must be logged; vessels carrying more than a threshold refrigerant charge need a refrigerant management plan and log book entries for every top-up. Class rules require periodic survey of the HVAC system's pressure vessels and safety devices, and accommodation ventilation capacity itself is checked against SOLAS habitability requirements at build, though the compressor itself is not separately class-approved equipment in most cases.
Typical faults
| Fault | Consequence |
|---|---|
| Crankcase heater failed or left switched off during layup | Refrigerant migrates into oil, causing violent foaming and bearing damage on the next start |
| Condenser sea water side fouled with marine growth | High discharge pressure trips the compressor repeatedly or drives up power consumption |
| Refrigerant undercharge from a slow leak | Low suction pressure causes short-cycling and eventual loss of cooling capacity |
| Moisture ingress after an open repair without proper evacuation | Acid formation in the refrigerant circuit that attacks windings and can cause a burnout |
What to look for in a supplier
- Confirmed refrigerant compatibility with what is already charged in the system, since retrofitting a different refrigerant is a full re-commissioning job, not a drop-in swap.
- Availability of spare valve plates and gaskets for semi-hermetic units, since a full replacement compressor is a significant spare part to carry.
- Documentation for the vessel's refrigerant management plan and leak log, matched to flag state reporting requirements.
- Electrical characteristics matched exactly to the existing starter and switchboard protection, not just the nameplate kW.
Never open a refrigerant circuit without recovering the charge into a certified cylinder first — venting refrigerant to atmosphere during a compressor swap is both an environmental discharge that must be logged and, on vapour-compression systems, an easy way to introduce moisture that ruins the repair.