Entry & progression
On a merchant ship this is a support system. On a yacht it is the system that defines your life. If the guests are hot, you are the problem — whatever else is wrong with the boat.
The cycle, and where it goes wrong
- Compressor — raises pressure and temperature of the vapour.
- Condenser — rejects heat to seawater; the vapour becomes liquid.
- Receiver / liquid line / drier — stores and cleans the liquid.
- Expansion device — drops the pressure; the liquid begins to boil.
- Evaporator — absorbs heat from the space; the liquid becomes vapour.
Every fault is a fault in one of those five. Diagnose with pressures and temperatures, not with intuition. Suction pressure, discharge pressure, superheat and subcooling will tell you which component has failed, every time, if you take the readings honestly.
The four readings, and what they say
| Fault | Suction | Discharge | Signature |
|---|---|---|---|
| Undercharge / leak | Low | Low | High superheat, bubbles in the sight glass, low subcooling |
| Overcharge | High | High | Low superheat, high subcooling, liquid back to the compressor |
| Dirty condenser / no seawater | Normal-high | High | High subcooling, hot condenser, high-pressure trip |
| Blocked drier or expansion valve | Low | Low-normal | Frosting at the restriction, high superheat, starved evaporator |
| Iced evaporator / no airflow | Low | Low | Low superheat at the coil, poor cooling, ice |
WARNING — Liquid does not compress Liquid refrigerant returning to a compressor will destroy it — bent rods, broken valves, washed-out bearings. Overcharge, a flooded evaporator, a failed expansion valve or a compressor started with a cold sump full of dissolved refrigerant will all do it. Check superheat before you blame the compressor, and use the crankcase heater.
The chilled water plant
Most modern yachts run chilled water: a central plant makes cold water, and fan coil units in each cabin take heat out of the air. It means the refrigerant is confined to the plant room, and it means your daily problems move to the water side — pumps, air locks, strainers, three-way valves, thermostats, and the fan coil unit whose condensate tray is full and dripping through a guest's headlining.
- Air in the chilled water loop kills a whole zone. Vent it properly, at the highest points, and check the expansion tank pressure.
- Fouled seawater side on the condenser is the single most common cause of a high-pressure trip in the tropics. Strainers, then tubes.
- Condensate drains block with dust and biological growth. A blocked drain floods a cabin, and floods it into the ceiling of the cabin below.
- Legionella and biofilm live in warm stagnant water. This is a health matter, not just a comfort one.
Provisions, freezers and the galley
Cold rooms fail slowly and are discovered late. Alarm the temperatures, trend them, and check the door seals and the heater tapes on the freezer door — a freezer door frozen shut gets forced, and then it never seals again.
CAUTION — Refrigerant handling is regulated, and the rules vary by country Refrigerant recovery, leak reporting, handling licences and permitted gases are governed by environmental law that differs between jurisdictions — and none of it is specified in Marine Order 72. Do not assume the practice you learned in one country is lawful in another. Check the requirements applying to your flag, your class and the country you are working in.