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Methods of refrigeration used in general cargo ships

Why refrigerate? Refrigeration is essentially the removal of heat through the process of evaporation. We choose to refrigerate commodities such as fruits and vegetables because we want to prolong their “practical shelf life” – the time from harvest until the product loses its commercial value.

Humidity is also a very important factor in the conservation of many fruits and vegetables. By shipping products in our Controlled Atmosphere containers, the humidity inside the containers can be increased, thus minimising any weight loss in the fresh produce.

Temperature is not the only important part of the equation… but it is the key element to successful refrigerated transport.

Technically speaking, the internal biological and chemical processes of fresh produce, such as respiration, will continue after harvesting. This means that the product absorbs oxygen (O2) and releases carbon dioxide (CO2) and ethylene (C2H4). This is a process that generates heat.

Lowering the temperature reduces the respiration, and consequently the heat, considerably. Therefore, temperature is the most important factor when prolonging the practical shelf life. As high concentrations of CO2 and ethylene can deteriorate the commodities, these gases must be removed and replaced with fresh air through the ventilation system. Ethylene production is especially high in fresh produce such as apples, peaches, apricots, avocados and pears.

Many perishable cargoes are carried in refrigerated compartments on dry cargo ships, and there are an appreciable number of vessels specifically designed for carrying refrigerated cargo only.

Stowage of refrigerated cargo

Chilled meat cargo is hung from the strengthened deck stiffening members, and the tween deck height is arranged to provide space below the hung carcasses for the circulation of air. Frozen meat is stacked in the holds of the ship. Fruits and vegetables are stowed in a manner which permits an adequate flow of air to be maintained around the crates, etc.

As a rule the refrigerated rooms in general cargo ships are made rectangular to keep down insulation costs.

Refrigeration systems on board

Brine made by dissolving calcium chloride in fresh water will have a freezing point well below the desired temperatures of the refrigerated compartments. Cold brine may be pumped at controlled rates to give the correct working temperature, and it is led from the evaporator of the refrigerating machine to pipes at the top of the cold compartment. The brine absorbs heat from the compartments and returns to the evaporator where it is again cooled and recirculated.

Air must be continually circulated where fruit is carried to disperse any pockets of carbon dioxide gas given off by the ripening fruit. The brine is then led into grid boxes and air drawn from the bottom of the compartments by fans is blown over the brine grids into the compartments via trunking arranged along the ceiling.

Refrigerated ship midship section
Refrigerated ship midship section

Methods of insulation

As the steel hull structure is an excellent conductor of heat, some form of insulation must be provided at the boundaries of the refrigerated compartments if the desired temperatures are to be maintained economically. Cork, glass fibre, and various foam plastics in sheet or granulated form may be used for insulating purposes, also air spaces which are less efficient.

Glass fibre is often used in modern ships as it has a number of advantages over the other materials; for example, it is extremely light, vermin-proof, and fire-resistant, and it will not absorb moisture. On the decks and particularly at the tank top the insulation must often be load-bearing material, and cork might be preferred, but fibreglass can be supported by tongue and grooved board linings and wood bearers.

The thickness of the insulation depends on the type of material used and the temperature to be maintained in the compartment. However the depth of stiffening members often determines the final depth. Insulating material is retained at the sides by galvanized sheet steel or aluminium alloy sheet screwed to wood grounds on the frames or other stiffening members .

Insulation on the boundaries of oil tanks, e.g. on the tank top above an oil fuel double bottom tank, has an air space of at least 50mm between the insulation and steel. If a coating of approved oil-resisting composition with a thickness of about 5mm is applied the air gap may be dispensed with. Suitable insulated doors are provided to cold rooms in general cargo ships, and in refrigerated cargo ships the hold and tween hatches may be insulated. Patent steel covers or pontoon covers may be filled with a suitable insulating material to prevent heat losses.

A particular problem in insulated spaces is drainage, as ordinary scuppers would nullify the effects of the insulation. To overcome this problem brine traps are provided in drains from the tween deck chambers and insulated holds. The brine in the trap forms an effective seal against ingress of warm air, and it will not freeze, preventing the drain from removing water from the compartment .

General Cargo in Refrigerated Chambers

The most important consideration when stowing general in refrigerated chambers is that of avoiding damage to insulation, air-trunking and gratings. Where practical, soft packages and cases should be stowed in the hatch wings while bulky or unprotected heavy cargo should be stowed in the hatch square. These latter should be clean lift as dragging out from the wings and ends can cause damage to insulation.

Advantage follows if the floor or ceiling in squares of hatchways is protected by means of stout boards or sheathing. Sizeable packages, having flat surfaces (cases in preference to bales), should be selected for stowing against the pipe battens at sides and ends, so as to minimise the risk of cargo protruding into the grids and, by exerting pressure on the piping, cause the same to leak at the joints.

The permanent battens should be in good order or, in the absence of such, battens should be provided for the protection of the piping and cargo, such to be placed vertically, close spaced and "stopped" in position.

When bale, bag or other goods liable to damage from sweat or rust stains are carried, the piping as well as other ironwork should be covered with kraft paper, etc.

Goods or liquids such as creosoted materials, aniline oils, essential oils, cargoes with a phenolic base, chemicals, etc., which leave behind strong odours, which cannot safely be dissipated by ventilation, should not, on any account, be stowed in refrigerated chambers. Some of the most serious claims for damage and consequent costs for getting chambers again fit for refrigerated cargoes have been through phenol taint. All dusty cargoes liable to choke air passages should be avoided. Heavy packages should be stowed on the ceiling with dunnage or bearers in order to avoid damage to ceiling and insulation by evenly distributing the weight over the maximum surface.

Other useful articles :
  1. Reefer cargo stuffing

  2. it is essential that all products are treated correctly prior to stuffing. Even though the temperature, ventilation and humidity are all optimal during the entire voyage, products will only arrive in perfect condition if the pre-treatment has been performed correctly. Successful shipping begins at the product sourcing area.
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  3. Growing demand for container refrigeration

  4. On deck refrigerated containers are generally serviced by clip-on air cooled electric motor drive cooling units. The units are plugged into the ships electrical system by way of suitable deck sockets.
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  5. Frozen products packaging requirement

  6. Proper packaging procedures will help protect frozen cargo during transport. Frozen products do not require air holes in the top and bottom of the cartons. Air flowing around the load is sufficient to remove heat that has penetrated the container. The cartons should be stacked directly on top of each other to take advantage of their strength in the corners.
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  7. Packaging & stowage guideline for reefer cargo

  8. Packaging plays an important role when it comes to protecting the cargo. The packaging material must be able to support a stacking height of up to 2.4 metres (7’10’’). The material should be able to withstand humidity without collapsing, and should allow the passage of an adequate vertical airflow through the cartons in order to maintain the desired temperature..
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  9. Choice of packaging for various commodities

  10. Goods should be well stowed within the package, evenly distributed and properly secured. Items completely filling the case or carton contribute to the strength of the whole package. Items which do not completely fill the package must be cushioned against shock or vibration.
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  11. How to keep cargo fresh ?

  12. Proper ventilation of fresh, chilled products is necessary to remove the heat, carbon dioxide and other gases produced by the cargo. Heat is removed by continuously circulating the internal air, whereas carbon dioxide and other gases are removed by replacing the internal air supply with cooled fresh air..
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  13. Loading precautions for refrigerated cargoes

  14. Refrigerated cargoes include meat carcases, carton (packed) meat, fruit, cheese, butter, fish and offal. Ships are specifically designed for their carriage, with separate spaces in holds and ’tween decks, each fitted with suitable insulation and individual control of ventilation. Ordinary general cargoes may be carried in the spaces at other times, the temperature being regulated accordingly for the type of cargo being carried.
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  15. Role classification societies maintaining seaworthiness of vessels

  16. classification societies publish rules and regulations which are principally concerned with the strength of the ship, the provision of adequate equipment, and the reliability of the machinery .
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  17. Periodic survey requirement by classification societies

  18. To maintain the assigned class all steel ships are required to be surveyed and examined by the Society’s surveyors at regular periods. The major hull items to be examined at these surveys only are discussed here..
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