Supply Fan

The Job a Supply Fan Does

A supply fan draws air from open deck and delivers it into a space under positive pressure, the mirror image of an exhaust fan's job. Engine room supply has to cover two separate demands at once: combustion air for the main engine and auxiliaries, and general cooling of the space, which is why engine room supply capacity is checked against engine air consumption figures rather than a simple air-change target. Accommodation supply units condition the air first, usually with a heating and/or cooling coil upstream of the fan, before distributing it through ducted diffusers. The positive-pressure side of the ventilation balance is what defines this fan's role, whatever space it happens to serve.

Main Components

• Impeller and casing – centrifugal for most ducted supply duties, sized against the full system resistance including any coil.
• Inlet filter – washable or disposable, protects the coil and downstream ductwork from salt and dust.
• Heating and/or cooling coil – steam, thermal oil or electric heating and a chilled water or refrigerant cooling coil on accommodation units.
• Weather louvre and intake screen – on the open-deck inlet, keeps out rain and spray while limiting pressure loss.
• Silencer – where the unit serves accommodation spaces with noise limits.
• Distribution ductwork – with volume control dampers and diffusers set to balance airflow between outlets.

Selection and Sizing

Engine room supply fans are sized to exceed the combustion air demand of the main and auxiliary engines running together, plus enough margin to remove radiant heat from the space; two fans each rated at roughly half the total demand is a common arrangement so one can be taken out of service for maintenance without losing all ventilation. Accommodation units are sized against occupancy and recognised air-change standards, with filter class and coil duty added on top. Static pressure has to include coil pressure drop, which is often the largest single loss in the system and easy to underestimate at the design stage.

Regulations and Class

SOLAS Chapter II-2 requires that ventilation supplying combustion air to machinery spaces can be stopped, automatically or by a remote control located outside the space, as part of the fixed fire-extinguishing system release sequence, so a fire cannot draw on an unlimited air supply. Emergency stops must be positioned where they remain accessible if the space itself becomes untenable. Class machinery rules set out the redundancy expected for engine room supply capacity.