Details About Marine 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 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 accommodations 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. 

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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 systems are in general use, 

1] the single duct, 

2] the twin duct and 

3] the single duct with reheat

1] The single-duct system 

It is widely used on cargo ships. Several central units are used to distribute conditioned air to several cabins or spaces via a single pipe or duct. In warm climates, a mixture of fresh and recirculated 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 are controlled automatically at the central unit. Within the conditioned space control is by variation of the volume flow of air.

2] the twin duct 

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. 

3] the single duct with reheat

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 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.