How does a ship's refrigeration plant work?

A marine refrigeration plant runs on exactly the same vapour-compression cycle used ashore, but the equipment has to survive constant vibration, list, and roll, and a seawater-cooled condenser instead of an air-cooled one, so its design and maintenance routine are shaped by that environment. The compressor, usually reciprocating on smaller plants and screw type on larger cargo reefer installations, draws refrigerant vapour from the evaporator side at low pressure and temperature and compresses it to a high-pressure, high-temperature gas. That hot gas is condensed against seawater in a shell-and-tube condenser, rejecting its heat to the sea and turning back into a high-pressure liquid, which is why a fouled or scaled condenser tube is one of the most common causes of high head pressure and poor cooling.

The high-pressure liquid then passes through a filter-drier and sight glass, where the engineer checks for moisture, bubbles (an early sign of low charge or a restriction), and the general health of the refrigerant, before reaching the thermostatic or electronic expansion valve. This valve throttles the liquid into the low-pressure side, where the sudden pressure drop causes flash evaporation and a sharp fall in temperature. In the evaporator inside the cold room, this cold refrigerant absorbs heat from the space and boils off completely into vapour, ideally with a small amount of superheat, which the expansion valve is set to maintain so that liquid refrigerant never reaches the compressor and causes slugging.

Routine watchkeeping on a reefer plant means logging suction and discharge pressures, condensing and evaporating temperatures, superheat and subcooling, oil pressure on the compressor, and the state of the sight glass, and comparing these against the plant's normal figures for the ambient and load conditions. A low suction pressure with a warm room usually points to low refrigerant charge or a restriction; high discharge pressure points to a fouled condenser or air in the system; short-cycling or icing on the evaporator often means the thermostat or defrost cycle is out of adjustment. Refrigerant leaks matter both operationally, because lost charge degrades cooling, and environmentally, since many marine refrigerants are controlled substances under MARPOL Annex VI and international ozone and greenhouse gas agreements, so leak checks and proper recovery during servicing are a standing requirement, not an optional extra.