THERMODYNAMIC CYCLE OF THE SHIPBOARD AIR CONDITIONING SYSTEM AND LOADING MECHANISMS

Shipboard air conditioning (AC) units use the vapour-compression refrigeration cycle with R134a, R407C, R410A or similar refrigerant (older plant R22). The cycle comprises four processes:

• Compression (1-2): Low-pressure refrigerant vapour from the evaporator enters the compressor where it is compressed adiabatically to a high pressure and temperature, becoming superheated vapour.
• Condensation (2-3): The hot high-pressure vapour passes to the condenser, where it is cooled by sea water (or chilled water/air) and condenses to high-pressure liquid, rejecting heat to the cooling medium.
• Expansion (3-4): The high-pressure liquid passes through the thermostatic expansion valve (or orifice), an isenthalpic throttling process in which pressure and temperature drop sharply, producing a cold mixture of liquid and flash vapour.
• Evaporation (4-1): The cold refrigerant absorbs heat from the air blown through the evaporator (heat is taken from the air), boiling/latent heat absorption, so the air is cooled and the refrigerant leaves as low-pressure vapour to re-enter the compressor.

The compressor continually circulates refrigerant, transferring heat from the cooled space (evaporator air side) to the sea water at the condenser.

Key components and functions

• Compressor: Raises refrigerant pressure and provides the driving circulation; may be reciprocating, scroll or screw type.
• Condenser: Rejects heat; sea-water cooled shell-and-tube or plate type.
• Expansion valve (TX valve): Throttles refrigerant, controls the degree of superheat at evaporator outlet and regulates refrigerant flow to match cooling load.
• Evaporator: Direct-expansion air-cooling coil (or chilled-water/brine system) where refrigerant absorbs heat from the air.
• Thermostats, HP/LP cut-outs, solenoid shut-off valves and capacity-control devices complete the plant.

Cooling load on the compressor (heat to be removed from the plant spaces) varies with ambient conditions, passenger complement, solar gain and external heat. To avoid the compressor over-refrigerating (cycling frequently) or running inefficiently, the following mechanisms are used:

• Capacity (unloader) control: On multi-cylinder reciprocating compressors, capacity control by loading/unloading cylinders - a solenoid valve admits discharge pressure to hold the suction valves open on selected cylinders, so those cylinders do not compress gas (reducing effective capacity), while the remaining cylinders carry the load. Loading restores full cylinder operation when demand returns.
• Hot gas bypass / cylinder unloading by means of a valve: Bypasses a controlled amount of gas from discharge to suction, unloading the compressor while it continues to run.
• Multiple compressors operating in sequence: Starting and stopping individual compressors or loading successive machines to match total load.
• Expansion valve regulation: The TX valve continuously modulates refrigerant flow so the plant tracks the evaporator load; as load falls the valve closes, and as it rises the valve opens, maintaining the set superheat and efficient evaporator usage.
• Thermostatic on/off control on smaller plant: A thermostat cycles the compressor, unloading it (stopping) at the set-point.
• Speed control (Variable Frequency Drive) on screw/centrifugal compressors: Adjusts compressor speed to match the load, providing the most energy-efficient part-load operation.
• Water-flow and brine-temperature control: Modulating chilled water/brine flow or chilled water temperature set-point adjusts the cooling delivered to the spaces.

By properly matching compressor capacity and refrigerant flow to the cooling load, the system maintains steady space temperature and humidity, avoids excess cycling, saves power and protects the machine.