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Single-duct & twin-duct marine air conditioning system for cargo ships

Requirement of ships air conditioning system
Ships travel the world and are therefore subject to various climatic conditions. The crew of the ship must be provided with reasonable conditions in which to work regardless of the weather. Temperature alone is not a sufficient measure of conditions acceptable to the human body. Relative humidity in conjunction with temperature more truly determines the environment for human comfort.

Relative humidity, expressed as a percentage, is the ratio of the water vapour pressure in the air tested, to the saturated vapour pressure of air at the same temperature. The fact that less water can be absorbed as air is cooled and more can be absorbed when it is heated is the major consideration in air conditioning system design. Other factors are the nearness of heat sources, exposure to sunlight, sources of cold and the insulation provided around the space.

An air conditioning system aims to provide a comfortable working environment regardless of outside conditions. Satisfactory air treatment must involve a relatively 'closed' system where the air is circulated and returned. However, some air is 'consumed' by humans and some machinery so there is a requirement for renewal. Public rooms and accommodation will operate with a reduced percentage of air renewal since the conditioning cost of 100% renewal would be considerable.

Galleys and sanitary spaces, for instance, must have 100% renewal, but here the air quantities and treatment costs will be much smaller. Systems may however be designed for 100% renewal of air although not necessarily operated in this way. Noise and vibration from equipment used in the system should be kept to a minimum to avoid a different kind of discomfort. Three main types of marine air conditioning system are in general use, the single duct, the twin duct and the single duct with reheat, The single-duct system is widely used on cargo ships .

Direct-expansion refrigeration system for an air cooler

Fig: Direct-expansion refrigeration system for an air cooler

Several central units are used to distribute conditioned air to a number of cabins or spaces via a single pipe or duct. In warm climates a mixture of fresh and recirulated air is cooled and dehumidified (some water is removed) during its passage over the refrigeration unit. In cold climates the air mixture is warmed and humidified either by steam, hot water or electric heating elements. The temperature and humidity of the air is controlled automatically at the central unit. Within the conditioned space control is by variation of the volume flow of air.

The twin-duct system provides increased flexibility and is mainly used on passenger ships . A central unit is used with cooled dehumidified air provided through one duct. The other duct is supplied with cooled air that has been reheated. Each treated space is provided with a supply from each duct which may be mixed as required at the outlet terminal. In cold climates the preheater will warm both supplies of air, resulting in a warm and a hot supply to each space.

The 'single duct with reheat' system is used for vessels operating in mainly cool climates. The central unit will cool and dehumidify or preheat and humidify the air as required by outside conditions. In addition, before discharge into the treated space a local reheating unit will heat the air if required, depending upon the room thermostat setting.

The refrigeration system used in the central unit is shown in Figure . A direct-expansion system is shown using a reciprocating compressor, sea water cooled condenser and a thermostatically controlled regulating valve. The air to be cooled passes over the evaporator or cooler. The cooling effect of the unit may need to be reduced if there is no great demand and the hot gas bypass system provides this facility.

Maintenance of the above systems will involve the usual checks on the running machinery and the cleaning of filters. Air filters in the central units are usually washable but may be disposable. The filters should be attended to as required, depending upon the location of the ship.

Air Conditioning system requirement

All equipment must be maintained in good condition as per maker's recommendations. Drains and scuppers on heat exchangers and plummer boxes must always be clear. The Chief Engineer has to verify that the air is correctly distributed.

Accommodation doors must be closed at all times, while air conditioning is in operation. Air conditioning must be maintained at such a level to be beneficial to the crew, with a correct difference, external/internal temperature and humidity degree, which indication Chief Engineer will find on maker's instructions.

Excessively cool accommodation results in high consumption and discomfort to crew and passengers. It should be noted that operative air conditioning is fundamental to the welfare of the crew. Accordingly any repairs that are required to be made to the air conditioning system should be considered as important and advice/assistance requested from the management office if required.

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