Air conditioning and refrigeration lower and or maintain temperature below the ambient one by transfer of heat for human, regulation or commercial reasons.
Air conditioning system aims to provide a comfortable working environment. 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. 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 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 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.
Refrigerated cargo vessels usually require a system which provides for various spaces to be cooled to different temperatures. The arrangements adopted can be considered in three parts: the central primary refrigerating plant, the brine circulating system, and the air circulating system for cooling the cargo in the hold.
In refrigeration system, the refrigerant flow through the chiller splits into four circuits, each with its own expansion valve. The four circuits are used to control the amount of evaporator surface, depending on the degree of condenser loading at the time, thus giving greater system flexibility. The large oil separator is a feature of screw compressor plants and the circuit for oil return.
Each refrigerant circuit has its own evaporator within the brine chiller which results in totally independent gas systems. Each brine temperature is identified by a colour and will have its own circulating pump. The cold brine is supplied to the cargo space air cooler and the flow of this brine is controlled by the temperature of the air leaving the cooler. The cooler in the cargo space is arranged for air circulation over it and then through the cargo before returning. An arrangement of fans and ducting direct the air to the cooler and below the cargo. The cargo is stacked on gratings which allow the passage of cooled air up through the cargo.
Vessels designed for specific refrigerated container trades have built-in ducting systems. These can be in two forms: a horizontal finger duct system in which up to 48 containers are fed from one cooler situated in the wings of the ship or, alternatively, a vertical duct system in which each stack of containers has its own duct and cooler. This type of system is employed for standard containers having two port holes in the wall opposite the loading doors. Air is delivered into the bottom opening and, after passing through a plenum, rises through a floor grating over the cargo and returns via another section of the plenum to the top port. The connection between the duct arid containers is made by couplings which are pneumatically controlled.