Chiller Unit

What Defines a Chiller Unit

A chiller is the central refrigeration machine that cools water, or a glycol brine on vessels trading in freezing conditions, which is then pumped to air handling units, fan coil units and cargo cooling coils throughout the ship. This centralised approach is what separates a chiller plant from self-contained split or window air conditioning units: a single chiller serves many spaces through a chilled water loop, so its capacity, redundancy and controls matter far more than on a standalone unit. Most marine chillers use a vapour-compression cycle with a shell-and-tube or plate evaporator and condenser, though absorption chillers using waste heat appear on some vessels where exhaust or steam heat is abundant.

Main components

Compressor

Reciprocating, screw or scroll compressors depending on capacity; screw compressors dominate larger marine chiller plants for their reliability and capacity control range.

Evaporator and condenser

Shell-and-tube or plate heat exchangers; the evaporator cools the chilled water circuit, the condenser rejects heat to sea water or to a fresh water cooling system on vessels avoiding direct sea water in the refrigerant circuit's heat exchangers.

Expansion device

A thermostatic or electronic expansion valve meters refrigerant flow into the evaporator, matching it to the actual cooling load.

Refrigerant and controls

HFC refrigerants, commonly R134a or R407C/R410A blends, circulate in a sealed system, with a control panel managing compressor staging, capacity control and safety cut-outs for high pressure, low pressure and low oil pressure.

Selection and sizing

• Cooling capacity in kW or refrigeration tons, calculated from accommodation heat load, outside air load, and any process or cargo cooling duty served by the same plant
• Number of compressors and redundancy: most vessels run two or more chillers so one can be serviced while the other carries load
• Chilled water supply temperature and flow rate, matched to the air handling units and fan coils downstream
• Refrigerant type, weighed against phase-out schedules for high-GWP refrigerants under evolving environmental rules
• Condenser cooling medium - direct sea water versus a closed fresh water central cooling system, which reduces fouling and corrosion of the condenser

Class and regulatory requirements

MARPOL Annex VI and the Montreal Protocol framework restrict the use of ozone-depleting refrigerants, which is why older R22 plants have been phased out in favour of HFC or, increasingly, lower-GWP refrigerants. Classification societies require accommodation and habitability notations, where they apply, to specify minimum cooling capacity per occupied space, and periodic survey covers pressure vessel and piping inspection, refrigerant leak testing, and safety device function tests. Crews handling refrigerant must follow class and flag rules on recovery and logging of refrigerant quantities to avoid uncontrolled venting.

Typical faults

• Refrigerant leakage - failed shaft seals or corroded joints slowly lose charge, reducing capacity and eventually tripping the low-pressure cut-out
• Condenser fouling - marine growth or scale on sea-water-cooled condenser tubes raises condensing pressure and cuts efficiency
• Compressor short-cycling - low refrigerant charge or a faulty pressure switch causes frequent starts, accelerating motor and bearing wear
• Oil migration - poor oil return at part load starves the compressor of lubrication over time
• Chilled water pump failure - loses circulation to the whole distribution loop even though the chiller itself is healthy

What to look for in a supplier

• Documented capacity and efficiency figures at the actual sea water or ambient design temperature for the vessel's trading route
• Redundancy configuration matched to the accommodation and process cooling load, not undersized to save first cost
• Compatibility with the vessel's chosen refrigerant strategy, including any planned retrofit path away from higher-GWP gases
• Spare parts and service network covering compressor overhaul kits, since a compressor rebuild is the most common major repair over a chiller's life

Track condenser approach temperature (condensing temperature minus sea water inlet temperature) over time; a steady rise with no change in sea water temperature is the earliest reliable sign of condenser fouling, well before the high-pressure cut-out ever trips.