Reefer cargo ship holds require continuous circulation of cooled air to maintain the desired temperature of the cargo. Contemporary reefer cargo ships have cooling by forced draught ventilating fans circulating the air from cooling batteries througout each cargo compartment and the cargo it contains. Normally, each lower hold compartment and each tweendeck compartment has two cooler batteries and several small axial fans mounted in a narrow, separate cooler space usually at the end of the compartment behind a light bulkhead of plywood boards or similar. In the past coolers were often mounted in deckhouses above the transverse bulkheads seperating each two holds. The air circulation or distribution may be arranged according to one of two different principles or systems as follows:

1. the air is circulated in a ductless system by fan pressure in a longitudinally flow underneath the longitudinally arranged gratings over the full breadth of the compartment. Through air holes in the gratings the air is pressed upwards and through the cargo and returned to the cooler compartment between the topside of the cargo and the underside of the overhead deck, and

2. The air is circulated by fan pressure through a number of downwards slanting air channels or ducts at each ship side between the side insulation lining and a bulkhead of plywood boards set in from the side. The trunks terminate underneath the, in this case, transversely arranged gratings. There the air flow is changed to a transverse direction, passes upwards through the air holes of the gratings and returns to the cooler compartment as mentioned for the first principle. Called the Robson System this second system with its air channels or ducts at sides obviously occupies more space in a compartment than the first mentioned system.

The air changes must be minimum 90 times per hour in modern all-round ships, whereas 30 to 60 times are sufficient on pure freezers. In addition to the air circulation a modest air renewal with fresh air of 1.5 to 2.5 times per hour is required for live cargo, but not for dead cargo in pure freezers. The ripening of bananas for instance causes formation of carbon dioxyde and ethylene, which must be removed. The fresh air fans are mostly placed above weather deck and the air is lead through vertical trunks to the individual compartment.

Reefer containership holds in ships provided with hatch covers have forced draught hold ventilation fans to remove the heat dispersion emanating from the integral reefer containers' refrigerating machinery.

Fruit juice carrier holds are provided with air coolers and ventilating fans for forced air circulation. They may be placed inside the individual holds or in deckhouses on the upper deck.

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General requirements for refrigerating plants are:

• compactness
• reliability
• controllability under different climatic conditions
• easy, fast, and reliable handling
• resistant to influences of marine environment (corrosion free)
• resistant to engine vibration and motions caused by seaway.

The dimensions of the refrigeration plant have to give consideration to the different factors of heat generation and absorption. According to A.M.Friis the heat transfer through the insulation of space is expressed by the formula

where:
- k=the transmission coefficient for the different exterior surfaces of the space, that is, ship sides, deck above and underneath and end bulkheads
- F=the areas of exterior surface
- t₀= the ambient temperature
- t_s= the temperature of the space;

• the insulation is typically dimensioned to obtain a transmission coefficient k=0.45 kcal/m² h °C(0.52W/K m²),
• the heat conveyed by the air circulation fans depends on the fan speed and multiple speed fans are use. With fruit cargoes the fans operate at maximum revolutions, since the cooling need is large due to the ripening heat or respiration heat developed. The fan heat energy is equal to the fan power. In brine plants the heat energy conveyed by the impeller of the brine pumps shall be included,
• the respiration heat of a fruit cargo, which depends on the fruit in question and its temperature,
• the cooling down of the amount of fresh air renewal required to keep the CO₂-concentration at a low level when fruits are carried, the maximum air renewal being 2.5 times the empty space volume per hour. Multiple speed fans are to be used,
• the heat to be removed during cooling down a cargo from the loading temperature to the transportation temperature. This is equal to the product of the specific heat of the cargo in question times the temperature difference times the amount of cargo. The cooling down time depends on the type of cargo, its condition when being loaded and during transportation and also the estimated transportation time,
• leakage heat emanating from leakages between the compartment and the surrounding exterior air. This is impossible to estimate and an assumed amount should therefore be included in a safety margin added to the calculated necessary cooling capacity, and
• the heat conveyed outside the hold space to the refrigerant if direct expansion or to the brine if indirect cooling. This is also to be included in the safety margin.

To counteract possble local heating of the cargo due to heat penetration from the exterior, a pressure exceeding the atmospheric pressure is maintained.

When a new reefer cargo ship is completed and the refrigeration machinery is tested the condition of the insulation is checked as well. If ice should appear in spots or areas on the exterior shell plating or uppermost decks in way of holds the insulation is not in order and must be rectified in those places.

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Refrigeration may be accomplished by either of two systems based on forced fan air circulation in the cargo hold space:

1. Direct cooling by direct evaporation of the refrigerant in coolerw mounted in the cooler spaces in the cargo compartments

Advantages

• the total plant is cheaper
• the total plant is lighter
• the system produces relatively more cooling

Disadvantages

• the increased risk of leakages in the long pipes between the compressor room and the individual cargo compartments and the consequent risk of damaging the cargo with the refrigerant.

2. Indirect cooling with brine where a heat exchanger for refrigerant-brine is installed in a separate brine room from where the brine is circulated to tall the coolers (evaporators) in the cargo spaces.

Advantages

• the refrigerant cannot harm the cargo in case of leakages since all piping containing the refrigerant are located in the compressor room and the brine room. That means short pipes where any leakage easily may be detected.
• the amount of refrigerant in the system is smaller than in the other system resuliting in lower costs for first fillings and later replenishments.

Disadvantages

• the temperature difference between the coolers (evaporators) and the air in the cargo spaces must be 4 to 6 °C bigger than with the other system,
• a brine pump is required,
• the weight of the entire system is higher than that of the direct cooling system due to the brine pump, the extra heat exchanger, the larger compressors needed to make up for the required bigger difference in temperature in the cargo spaces and also the weight of the brine which exceeds that of a refrigerant circulating in a direct cooling system.

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As refrigerant NH₃ (ammonia, also called R717) with indirect cooling with brine has been used for many years. But in the 1950s to 1980s CHF₂Cl (freon 22 also called R22) has been preferred since it was suitable for direct evaporation making the system simpler despite the higher first costs and replenishment costs of this refrigerant. However, du to the detrimental effect on the environment R22 and other HCFC refrigerants containing fluor are being phased-out gradually by legislation, but unfortunately not simultaneously in all countries. In new installtions a revival of the R717 (ammonia) is seen. Also R744 (CO₂) is expected to be used in the near future. Both are well-documented natural products whilst R22 and other newer candidates developed by the industry are synthetic and possess certain disadvantages.

Ammonia is, as mentioned, used in an indirect cooling system, that is with brine circulating in the evaporators in the cargo spaces. Brine is a 25 weight per cent dissolution of CaCl₂ in water. The CaCl₂ content must be sufficiently high to ensure the brine remains frost-free at a temperature not less than 10 °C lower than the evaporation temperature.