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Ventilation of Refrigerated Cargo Holds

These require a carefully controlled air-replacing system for each individual space. Cooled air is supplied to the refrigerated hold where it gains heat from ripening cargoes and entrains the gases produced. This air is then exhausted and a careful balance must be maintained between inlet and exhaust gas quantities, regardless of the outside climatic conditions.

In figure this is achieved by drawing outside air down to a bank of cooler tubes via a central unit. The dehumidified air then passes into the cargo holds. The exhaust gases are drawn from the hold through ducts to the central unit and then returned to the outside atmosphere.

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Ventilation of double-bottom tanks is provided by means of an air pipe situated remote from the filling pipe and usually at the highest point in the tank to avoid unventilated pockets. The air pipe is led up to the weather deck to a gooseneck or patent type of head. Air pipes from fuel tanks are positioned in low risk areas and have flame screen gauzes fitted (see figure).

Cargo Tanks

Ventilation of cargo tanks avoid overpressure or partial pressure conditions which could occur during loading and unloading of cargo. Temperature fluctuations during a voyage could have a similar effect. Vapor pipelines from the cargo hatch are led to pressure/vacuum relief valves which are usually mounted on a standpipe some distance above the deck. Individual vent lines are fitted for each tank on large tankers and a common venting line is led up a mast or Sampson post on smaller vessels.

Design Criteria

The criteria include the recommended ventilation system, the temperature rise which is the maximum difference between exhaust air and supply air temperature (6oC for the galley and toilets), and the maximum air change which is the time in minutes for the supply of a quantity of air equal to the space volume (2 for the battery room, 10 for the emergency generator room, 1 for the galley, 4 for the toilets and showers).

In designing a ventilation system, the aim of the designer should be to run the duct in as nearly a straight line as possible, avoiding sharp bends, abrupt changes in duct sizes or shapes to minimize excessive pressure losses. Cross-sectional areas of ducts should be large enough to permit the air to flow at moderate velocities to avoid power waste and to reduce noise. Maximum duct velocity of 10 m/sec for areas where quiet operation is essential and 18 m/sec for areas where quiet operation is not essential, are good values.

When smaller ducts are taken off the main supply duct to serve individual spaces, the air quantity is divided in proportion to the area of the duct.

Construction Details

Ducts may be constructed of galvanizes sheet steel in order to withstand corrosion and vibration, or ducts may be constructed of aluminum in order to save topside weight. Vertical and horizontal ducts in general cargo holds are usually constructed of 6.5 mm and 4.8 mm plates respectively.

Because of headroom requirements, most ducts are rectangular; round ducts being used only in the smaller sizes. Usually a heavy section of ductwork is

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welded into the penetrated structure where structural compensation is required. Hand holes, access holes, and portable sections are provided to permit cleaning, painting, and inspection. Ducts passing over electrical equipment are made watertight. Ducts are made with either riveted, welded, or hook seams and are airtight.

To maintain habitable temperature in machinery spaces and for the removal of fumes, spot cooling with large quantities of air at high terminal velocities, 13-15 m/s, is used at operating stations and other strategic locations. Exhaust terminals should be located in the vicinity of heat-producing equipment, and on the upper levels of the machinery space.

Ventilation Components

a) Types of Fans

Axial flow fans are used widely because of compactness and high efficiency and are well adapted for ventilation of cargo spaces, machinery spaces and other spaces where noise is not a significant consideration. Centrifugal fans are used for ventilation where quiet operation is desired and also for galleys, battery room exhaust, and areas where explosive vapors are removed, where the motor is not to be located in the air stream. Propeller fans are used in bulkhead installations and sometimes in a cowl for machinery space supply and exhaust systems where the pressure is small.

b) Weather-Terminal Openings

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These include cowls, goosenecks, mushrooms, louvers and air lift boxes on deck or in bulkheads. All of these devices are fitted with wire mesh for rat-proofing. Weather terminals should be so located that exhaust air or stack gasses do not contaminate supply air.

c) Interior Terminals

High velocity directional terminals are used in galleys, pantries, laundries, machinery spaces, and similar heat-producing spaces where spot cooling is desired. Slotted outlets may be used in front of galleys hoods and switchboards. Ceiling or wall type diffusers are used for ventilated living spaces, with terminal

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velocities so as to provide diffusion and throw, without objectionable air movement and air noise in the space.

Terminals for holds and storerooms are merely open-ended ducts or a cut in the side of the duct fitted with wire-mesh screens for rat proofing.

d) Exhaust Terminals

These are located close to heat sources and are usually an open-ended duct covered with wire-mesh or grilles where appearance is important. Exhaust inlet velocities should be about 5-8 m/s in living spaces and up to 10 m/s in other places.

e) Dampers

They are sometimes used to control the volume of air delivered at terminals. Manually operated dampers must be provided on passenger ships at the weather opening in all ventilating systems to shut off the passage of air in the event f fire.

Automatic dampers are required in exhaust ducts over potential sources of fire in galleys and are required to operate by melting a fusible link.

Air Conditioning Systems

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These are used to modify the outside fresh air to improve the ships interior environment by heating, cooling, dehumidifying, and contaminant removal processes. Air conditioning is used almost exclusively for living spaces and sometimes for spaces containing electronic equipment. Conditioning of the air is accomplished by a cooling medium of chilled water or Freon and a heating medium of steam, hot water or electricity.

Types of Systems

a) Combination heating and cooling systems. b) Cooling systems. c) Dehumidification: for preventing moisture damage or condensation of

moisture on cargo and internal structures of all those portions of holds suitable for carrying dry cargo.

Design Criteria

Shipboard air conditioning systems are usually designed to maintain inside air temperatures ranging from 24oC to 29oC dry bulb, and a relative humidity of 50% with an outside air temperature of 35oC dry bulb and 28oC wet bulb during the summer season and inside air temperature of 21oC dry bulb with an outside temperature of -18oC dry bulb during the winter season.

The criteria for cargo hold dehumidification is normally to maintain the dew-point of atmosphere within the hold at a minimum design depression of 6oC dew-point below the surface temperature of the cargo or ships structure.

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