(i) Accommodation Air Temperature Control

The air temperature is controlled to a specific set point, typically around 24°C. A thermostat senses the temperature in the accommodation spaces. When the temperature rises above the set point, the thermostat signals a solenoid valve to open, allowing refrigerant to flow through the cooling coil. This cools the air passing over the coil. Conversely, when the temperature drops, the solenoid valve closes, stopping the cooling process.

The evaporator pressure is also a key factor. The system is designed so that the refrigerant's saturation pressure corresponds to the desired temperature. For example, if the desired temperature is 24°C, the system might be set to maintain an evaporator pressure of 4.5 bar, where the refrigerant's saturation temperature is 24°C. The thermostatic expansion valve (TEV) senses the superheat at the evaporator outlet and adjusts the refrigerant flow to maintain this pressure and thus the desired cooling temperature.

(ii) Humidity Control within Comfort Limits

Humidity is controlled using a humidistat, which measures the relative humidity (RH) of the air. The comfort zone for RH is generally between 30% and 50%.

• To reduce humidity (dehumidify): If the RH is too high, the air is overcooled below the set temperature (e.g., to 18°C). At this lower temperature, the air reaches its dew point, and excess moisture condenses out. This condensate is then drained away. The now cool, dry air is then passed through a reheater to bring its temperature back up to the set point of 24°C, which in turn lowers its RH to within the comfort zone.
• To increase humidity (humidify): If the RH is too low, the humidistat activates a humidifier. This device sprays a fine mist of water (often freshwater) into the air stream, increasing the air's moisture content until the desired RH is achieved.

(iii) Contribution to Ship's Main Plant Efficiency

A well-maintained accommodation air conditioning system can contribute to the ship's main plant efficiency in several ways:

• Reduced electrical load on the main generators: A more efficient air conditioning system requires less power to operate. This reduces the load on the ship's generators, which are often powered by auxiliary engines. A lower generator load means less fuel consumption for these engines.
• Heat Recovery: Some modern systems are designed to recover waste heat from the main plant's cooling systems. This recovered heat can be used for the reheater or other heating purposes on the ship, reducing the need for additional heating sources and thereby saving energy.
• Crew Comfort and Performance: A comfortable working and living environment helps maintain crew morale and performance. A well-rested and alert crew is less prone to making errors, which can prevent costly operational mistakes and improve overall plant efficiency and safety.