Object of Refrigeration
The object of refrigeration is to delay the spoilage which would naturally occur in certain products at uncontrolled temperatures. Many vessels have partial or total capability to carry refrigerated cargoes either "chilled" or "frozen". Chilled cargoes indicate those which are above, at or just below freezing point, such as fruits, vegetables, dairy produce and meat. They are usually cargoes which would "die" if frozen, but benefit from a low temperature to increase their life. Frozen cargoes are those which are kept below freezing point to ensure the prolonged life of the cargo, such as fish, meats, fowl and their processed by‑products.
The main causes of spoilage are:
Commodity Groups
Living Cargoes .
Fresh fruit, vegetables, eggs, bulbs, and cheese continue to ripen while in storage. Oxygen is absorbed and carbon dioxide given off. The process is slowed down by reducing the temperature, but the product must not be frozen for it will die and quickly rot.
Without adequate ventilation, C02would build up to harmful concentrations, although up to 2% may assist in prolonging storage life and in inhibiting moulds.
Cold air is delivered at close to the freezing temperature of the product, but always above it. A constant temperature is required for good storage, so exhaust temperatures should be carefully monitored. Fans should be regularly reversed to circulate chilled air in the opposite direction to promote a more concentrated temperature throughout.
The temperature range is from OOC up to 130C, as in the case of bananas where controlled ripening may be required.
The shipper will issue instructions for the carriage of his cargo, but if these are not given expert advice should be sought.
Non‑living Organic Cargoes
Principally meat, fish and poultry. They do not generate heat, produceC02, or require oxygen.
Storage life is prolonged by reducing the temperature sufficiently low to inhibit the development of inicro‑organisn‑m, which means a hard frozen condition.
Temperatures for hard frozen cargoes range down to ‑250C. However, lower temperatures may be required to allow for a rise in temperature to occur after discharge.‑ This information should be available before loading, as it may be beyond the ship's capability to reduce to these temperatures.
Inert Commodities
Examples are confectionery, pharmaceuticals, and x‑ray film. Carriage instructions must he carefully considered with reference to their storage and their effects upon other cargo. They are usually stable with long storage life under normal conditions, though they may be subject to chemical changes in extremes of temperature and humidity.
Temperature range will vary with the commodity but will normally be in the chilled range.
Basic Refrigeration Installation
Refrigeration is achieved by the evaporation of a liquid refrigerant, such as Freon. The heat required for this is absorbed from the environment. The boiling point of a liquid is dependent upon its pressure. By regulating the pressure, the evaporated refrigerant can be condensed again, at a higher pressure than the initial one.
Evaporator
Here the refrigerant evaporates because heat will be transmitted from the refrigerated hold to the evaporator, and the compressor will then suck the refrigerant gas from the evaporator, which will give rise to a low pressure, and thus a low boiling point (‑90C).
The incoming liquid (+I 70C) is at the regulator valve (controlled by adjustable thermostat), reduced in pressure, causing the liquid to try to boil, but because it cannot absorb any heat from the environment, the liquid withdraws heat from itself and some of it is transformed into vapour (‑90C).
This results in surrounding air being able to transmit its warmth to the evaporator.
Compressor
The refrigerant gas is sucked in from the evaporator, increasing pressure until it has such a high boiling point (e.g. +240C) that sea water or air can be used to convert it into a liquid again. It causes the gases to be superheated.
Condenser
The superheated gases are cooled by means of water cooling to their saturation point (in diagram +240C). The gas now condenses into liquid at +240C. Further passage through the condenser results in sub‑cooling (+I 70C) and afterwards it passes to the evaporator thus completing the circuit.
Construction of Cargo Compartments
A typical system of air circulation is shown.
At the ships side, between the frames and below deck beams, slabs of glass wool are used, held in place by wooden supports fitted over the beams.
Cover plates, usually of hardened hardboard are fitted and serve as the inside wall of the side trunks.
"U" section steel girders fitted as shown will support the aluminium. wall which is now the side of the hold, and the space between the aluminium. wall and the hardened hardboard.
Gratings
Gratings are placed on the hold floor and should fulfil the following requirements:
I . ‑To allow greatest possible through flow of air, but not be so high as to reduce cargo capacity.
2. Easily handled for cleaning.
3. Sufficiently strong to resist cargo pressure and point loads (FLT).
4. Hygienically acceptable.
5.Durable ‑ temperature/moisture.
Materials which have been used include wood, steel, plastic, and aluminium. Currently aluminium
or wood are the usual materials.
The evaporator and fans are installed close to the cross bulkhead. Gratings with battens running fore and aft are placed on the floor of the hold.
Behind insulated side walls, insulated air ducts slanting downwards have been fitted, and the cold air from the evaporator is fed through them, to the outlet openings beneath the gratings, passing upward through the cargo stow.
The return openings are fitted in the top of the cross partition between the evaporator and the hold. The fans extract hold air, and feed it to the evaporator, thus completing the cycle.
Insulation
Deal beams (5 x 15 cm) are fixed lengthways just over half a metre apart and the space between almost filled with insulating foam. Boards of waterproof plywood are then nailed to the beams, and foam injected through drilled holes to fill up the remaining space. The distance between the beams is compatible with the board sizes. A layer of polyester is then applied on the plywood and this is covered with another layer of plywood boards, forming a vapour proof layer.
Aluminium. girders are then placed athwartships, half a metre apart, and the gratings are laid on top of them. The cold air from the outlet openings is forced along these sections, in an athwartships direction.
Three types of gratings are shown, type (a) a "Warkaus" grating of wooden construction and (b) and (c) of aluminium, capable of withstanding the load pressures. "Warkaus" gratings, are cheaper than aluminiurn and are used on many reefers.
Frozen Cargo
Cargo carried in hard frozen condition to prevent bacterial growth. Temperatures around ‑180C, usually meat, or meat products, and fish. Reject cargo with soft or bloodstained wrappers.
Chilled Cargo
More extensive fridge coils or grids and greater air circulation is required than frozen cargo since a greater uniformity of temperature is required. Temperatures‑ vary between +70C and ‑100C depending on the cargo and satisfactory carriage up to 35 days is normal. This period can be extended by the addition Of C02 gas to the atmosphere of the hold to reduce the 02 content and thus slow decomposition and multiplication of bacteria. Cargoes carried include chilled beef, butter, oranges, apples and bananas. Temperature of carriage is specified by the shippers along with stowage. For example oranges are well ventilated to purge C02, and carried at 20C to 50C. The carriage of fruit or vegetables will usually require much greater control.
Points to Inspect in Cargo Prior to Loading
Refrigeration Log
Record the hold, air delivery and return temperatures on charts, or data loggers. These temperatures are part of the refrigeration log. The dispatcher may have placed sealed portable thermographs in the cargo, to be removed from the vessel by the consignee.
