Purpose: to maintain a comfortable, healthy environment in the accommodation/public spaces by controlling temperature, humidity and air freshness (ventilation) - heating or cooling the air, and providing filtered, fresh air. It also dehumidifies in hot humid conditions.

Main components: compressor, condenser (with fan/cooler), thermostatic/expansion valve, evaporator coil, air handling/ fan unit, filters, ducts (supply and return), heating battery/ re-heater (for winter or humidity control), fresh-air intake, and the duct grilles. Refrigerant circuit: compressor -> condenser -> expansion device -> evaporator, closed loop.

Layout and working: Fresh and recirculated air is drawn through filters into the central air-handling/l every coil unit, where a fan blows it over the evaporator (in cooling mode) which removes heat and moisture and drops the air temperature; the cooled air is then distributed through supply ducts to each cabin via grilles, and return air is ducted back. In heating mode the heat is supplied by a heating battery (hot water or electric) instead of/by passing the coil. A thermostat/humidistat controls the system and the temperature is regulated.

Function of the unloader and its importance: the compressor unloader is a capacity-control device (e.g. a cylinder-head unloader valve / suction by-pass or a cylinder unloader that lifts the suction valves when unloaded) that reduces the compressor's effective displacement. Its importance: it prevents the sudden on/off cycling and slugging under light load, reduces motor starting overload, prevents evaporator frosting/short cycling, saves energy and ensures stable temperature control - by reducing capacity when the cooling load falls and increasing it when the load rises, so the compressor does not overload the motor or surge on start-up.

The marine A/C system operates on the vapour-compression refrigeration cycle.

Using a pressure-enthalpy (P-h) diagram: the cycle has four processes (1-2-3-4 on the diagram):

• 1-2: Compression. Refrigerant vapour is compressed from the suction pressure (low) to the discharge pressure (high); work is put in, enthalpy rises and temperature rises - the refrigerant leaves as hot, high-pressure vapour.
• 2-3: Condensation. The hot vapour flows through the condenser where heat is rejected to the cooling medium (air/sea water); the refrigerant gives up latent heat, condenses to a liquid at high pressure, with decreasing enthalpy at constant pressure.
• 3-4: Expansion (throttling). The liquid passes through the expansion (thermostatic) valve, dropping to the low pressure (flash gas formed, enthalpy essentially constant).
• 4-1: Evaporation. The low-pressure liquid/vapour mixture flows through the evaporator where it absorbs heat from the air, evaporating and cooling the airstream; the vapour returns to the compressor and the cycle repeats.

Role of each component: compressor - raises pressure/temperature of the vapour (heart of the cycle); condenser - rejects heat and liquefies the refrigerant; expansion valve - drops pressure so the refrigerant can evaporate at the required low temperature and control flow; evaporator - absorbs heat from the air to cool it. The cycle provides both cooling and (via the heating battery) heating as required.