Central AC Plant (Chiller)

What sets a central plant apart from split units

Where a split air conditioning system places a compressor at or near each served space, a central chiller plant concentrates all refrigeration in one or two machinery spaces and distributes cooling as chilled water, typically at 6-7°C supply, through insulated piping to air handling units around the ship. This means only one refrigerant circuit to survey and leak-test instead of many, and it lets the plant run on partial load with one chiller and one on standby, but it also means a single chiller failure with no redundancy can lose cooling to the entire accommodation at once.

Main components

Compressor and refrigerant circuit

Screw or scroll compressors circulate refrigerant through an evaporator, where it absorbs heat from the chilled water loop, and a condenser, where that heat is rejected to seawater or to a fresh water intermediate circuit.

Chilled water circuit

Pumps circulate chilled water from the evaporator to air handling units and fan coil units throughout the accommodation and control spaces, returning warmed water to the chiller for re-cooling.

Air handling units

AHUs mix fresh and recirculated air, pass it over a chilled water coil, and distribute it through ductwork with dampers controlling the balance per zone or space.

Condenser cooling

Heat rejection uses either seawater cooling directly on the condenser or an intermediate fresh water circuit cooled by the ship's central cooling system, keeping raw seawater away from the refrigerant condenser itself.

Selection and sizing

Cooling load is calculated from accommodation area, occupancy, solar and equipment heat gain, and the design ambient conditions the ship is expected to operate in.

• Total cooling load in kW, typically split across two or more chillers for redundancy.
• Design ambient seawater and air temperature, since Gulf or tropical routes push chillers much harder than North Sea trades.
• Refrigerant type and its regulatory status (HFC phase-down schedules affect long-term availability of some refrigerants).
• Redundancy philosophy: N+1 chillers versus a single oversized unit.

Regulations and class requirements

Class rules require accommodation, wheelhouse and control spaces to be maintained within defined temperature and humidity ranges, and set minimum ventilation and air conditioning capacity for the vessel's intended trading area. Refrigerant handling falls under MARPOL Annex VI provisions on ozone-depleting substances and, depending on flag and refrigerant type, under regional F-gas type regulation for leak checking and logging of refrigerant top-ups. Periodic survey includes pressure testing of the refrigerant circuit and verification of leak detection arrangements in the machinery space.

Typical faults

What to look for in a supplier

• Confirmed capacity at the vessel's actual design seawater and ambient air temperature, not a standard test condition.
• Refrigerant choice with a clear long-term availability outlook given phase-down schedules.
• Redundancy and standby arrangements matching the ship's operating profile and accommodation size.
• Local service support or a training package, since chiller electronics and controls vary significantly between makers.

A slow, unexplained rise in condensing pressure over weeks is almost always condenser fouling, not a refrigerant fault, and it is cheaper to clean than to chase with refrigerant top-ups.