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Automatic direct expansion refrigeration system for cargo ships

Refrigeration is a process in which the temperature of a space or its contents is reduced to below that of their surroundings. Refrigeration of cargo spaces and storerooms employs a system of components to remove heat from the space being cooled. This heat is transferred to another body at a lower temperature. The cooling of air for air conditioning entails a similar process.

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.

ships machinery spaces
container ships machinery info
An automatic direct expansion refrigeration system is shown in Figure below. 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 is shown in the illustration.

Each primary refrigerant circuit has its own evaporator within the brine chiller which results in totally independent gas systems. There will probably be three such systems on a cargo or container ship installation. Since they are totally independent each system can be set to control the outlet brine at different temperatures. 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.

Automatic direct expansion refrigeration
Fig: An automatic direct expansion refrigeration system

For small refrigerated cargo spaces or provision rooms a direct expansion primary refrigerant system may be used . The twin circuit arrangement for each cooler (evaporator) provides flexibility and duplication in the event of one system failing. The back pressure valve maintains a minimum constant pressure or temperature in the evaporator when working a space in high-temperature conditions to prevent under-cooling of the cargo. If one space is operating at a low-temperature condition at the same time the back pressure valve would be bypassed. The liquid cooler illustrated in the diagram is necessary where an abnormal high static head has to be overcome between the machinery and the coolers. In this vessel the liquid is sub-cooled to prevent it flashing off before reaching the thermostatk expansion valve.

Containers which require refrigeration present particular problems. Where only a few are carried or the ship has no built-in arrangement for refrigerating containers, then clip-on or integral refrigeration plants would be provided. The clip-on or integral unit may be either air or water cooled. In the case of air cooled units adequate ventilation has to be supplied if they are fitted below decks. For water cooled units some sort of cooling water arrangement must be coupled up to each unit. Also an electrical supply is required for each type.

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.

Vapour compression cycle

Fig: Vapour compression cycle

Related Info:

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Basic principles of refrigeration system on board

Modern refrigerants for cargo ships refrigeration plant

Safety precautions for refrigeration plant and refrigerated compartments

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