The air-conditioning (AC) unit on board consists of:

• A refrigeration plant (compressor, condenser, expansion valve, evaporator) that cools the air.
• An air-handling unit: a fan (blower) that draws in fresh/ recirculated air, passes it over the cooling coil (evaporator) and/or a heating coil, and delivers the conditioned air to the spaces through the ducting.
• A filter to clean the air.
• A humidifier/ dehumidifier arrangement to control the humidity.
• Controls (thermostat, humidistat) to regulate the temperature and humidity.

Operation: The fan draws in air, which is filtered, then cooled (and dehumidified) by passing over the cooling coil (where the refrigerant evaporates, absorbing heat), and/or heated by the heating coil, and the conditioned air is delivered to the spaces. The temperature and humidity are controlled by the thermostat/ humidistat regulating the cooling/ heating.

• When air is cooled below its dew point, moisture condenses on the cooling coil, dehumidifying the air. The condensate is drained away. Thus, cooling the air reduces its humidity (the air is dehumidified as it is cooled). If the humidity is still too high, additional dehumidification (e.g. a reheat coil or a desiccant) may be used to lower the humidity further.

(ii) How discomfort caused by the excessive drying effect of heated air is overcome

• When air is heated, its relative humidity falls (the air becomes dry), causing discomfort. This is overcome by humidifying the air - adding moisture (e.g. by a humidifier/ steam injection, or by spraying water into the air stream) to raise the humidity to a comfortable level. The humidistat controls the humidifier to maintain the desired humidity.