AHU (Air Handling Unit)

What sets the AHU apart

A chiller plant produces chilled water or refrigerant; a fan coil unit conditions a single cabin or small space locally. The AHU sits between the two, drawing in a mix of fresh and recirculated air, passing it across cooling and heating coils fed from the central plant, filtering it, and pushing it through ductwork to accommodation, wheelhouse, control rooms and engine control spaces. Because one AHU commonly serves an entire deck or zone, a fault in the unit or its controls affects many spaces simultaneously rather than one cabin.

Main components

Filter section

Removes dust and salt particulate from incoming air before it reaches the coils; filter class is chosen for the space served, with finer filtration for control rooms with sensitive electronics.

Cooling coil

Chilled water or direct refrigerant coil that removes heat and, as a side effect, dehumidifies the air stream as moisture condenses on the coil surface.

Heating coil

Electric, or hot water/steam fed from the ship's heating system, brings supply air temperature up in cold conditions or reheats dehumidified air to the target comfort level.

Fan section

Supply fan moves conditioned air into the ductwork; larger installations include a separate return or exhaust fan to balance pressure across the served spaces.

Casing and insulation

Insulated panel casing prevents condensation on the outer surface and limits heat gain or loss between the coil section and the ductwork connection.

Selection and sizing

Sizing starts from the required air flow rate in cubic metres per hour for the served spaces, combined with the cooling and heating capacity needed to hold design temperature and humidity under the worst expected ambient condition, whether that is tropical heat and humidity or Arctic cold. Fresh air ratio is set to meet ventilation requirements for occupied spaces while managing the energy cost of conditioning outside air. Duct static pressure capability of the fan must match the actual duct run length and number of outlets, since undersizing here shows up later as poor air distribution at the far end of the system.

Regulations and class requirements

Class rules set minimum ventilation rates and temperature/humidity design conditions for accommodation and working spaces. Where AHU ductwork crosses fire zone boundaries, fire dampers rated to the boundary's fire integrity are a mandatory part of the installation, and their operation is checked during survey and fire drills. Control room and engine control space AHUs may carry additional filtration or pressurization requirements to keep out smoke and dust during an incident.

Typical faults

• Fouled filters restricting air flow, forcing the fan to work harder while delivering less conditioned air to the spaces served.
• Coil fouling or corrosion reducing heat transfer efficiency, showing up as spaces that never quite reach setpoint despite the plant running continuously.
• Fan bearing wear producing vibration and noise long before outright failure, often ignored until the fan seizes.
• Condensate drain blockage, allowing water to build up in the casing and either overflow into the ductwork or promote corrosion and mould growth.
• Fire damper seizure from corrosion or lack of exercise, which can fail either open, defeating the fire boundary, or closed, cutting off ventilation to occupied spaces.

What to look for in a supplier

• Filter class matched to class society requirements for the specific space category served.
• Corrosion-resistant coil coating suited to the vessel's operating environment, particularly for units drawing marine air directly.
• Fire damper integration certified to the fire boundary rating of the ducting it penetrates.
• Fan and motor sizing documentation matching the actual duct static pressure of the installation, not a generic catalogue selection.

Check the condensate drain before chasing a cooling complaint; a blocked drain backing water into the casing mimics several other faults and gets missed because nobody looks at the bottom of the unit.