AC Plant

What the AC plant covers

The AC plant is the central refrigeration package that produces the cooling, typically a vapour-compression system with one or more compressors, a condenser cooled by seawater, an evaporator, and the controls that tie them together. It is distinct from the distribution side of the system, such as ducting, fan coil units and diffusers, and from the refrigerant handling components covered elsewhere in this category: the plant is where the actual thermodynamic work happens, everything downstream just moves the result around the ship. Most marine plants are chilled-water systems, where the plant chills water that is then pumped to air handling units throughout the accommodation, rather than direct-expansion systems that circulate refrigerant itself to remote units.

Main components

Compressor

Reciprocating compressors remain common on smaller plants; screw compressors dominate larger installations for their smoother output and lower vibration. Most vessels carry two or more compressors so the plant retains partial capacity if one unit is down for maintenance.

Condenser

Shell-and-tube, seawater-cooled, rejecting the heat absorbed by the refrigerant to the sea. Fouling on the seawater side is the single biggest efficiency killer over a plant's life.

Evaporator and chilled water circuit

The evaporator chills fresh water, often with glycol added for freeze protection where the plant also serves low-temperature duties, which is then circulated by pumps to air handling units and fan coils throughout the accommodation and control spaces.

Controls

Pressure and temperature switches, capacity control by cylinder unloading or variable-speed drive on newer plants, and safety cut-outs for high discharge pressure, low suction pressure and low oil pressure.

Selection and sizing

• Total cooling load in kW, calculated from accommodation volume, occupancy, solar gain and equipment heat load in control rooms, not simply scaled to cabin count
• Redundancy - number and capacity split of compressors so the plant still meets a workable load with one unit out of service
• Refrigerant type, driven by current environmental regulation rather than by cost alone
• Seawater cooling capacity and condenser fouling margin for the vessel's warmest trading area

Regulations and class

Refrigerant choice is constrained by the Montreal Protocol phase-out schedule and, for newer ships, by regional F-gas type regulation affecting high-GWP refrigerants; class societies require refrigerant leak testing and logging of refrigerant charge. SOLAS Chapter II-2 requirements for control room and machinery space habitability indirectly drive minimum cooling capacity for those spaces, since crew endurance in an emergency depends on control spaces remaining usable.

Typical faults

What to look for in a supplier

• Compressor spares and service network coverage in the vessel's actual trading routes
• Current refrigerant type compliant with the regulatory phase-out schedule, not a soon-to-be-restricted refrigerant
• Documented cooling capacity at the vessel's design seawater temperature, not just a nominal rating
• Clear service history and condition survey if the plant is being sourced second-hand for a retrofit

Track condenser seawater-side fouling with discharge pressure trends rather than waiting for a nuisance trip - by the time the high-pressure cutout activates, capacity has usually already been marginal for weeks.