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Cooling of refrigerated containers - How a reefer container works ?

Container cooling components :
Systems designed for the cooling of refrigerated containers employ trunkings arranged so that containers stowed in stacks between built-in guide rails, can be connected to the suction and delivery air ducts of the ship's refrigeration plant by bellows pieces operated pneumatically.



The air is cooled either by brine or direct expansion batteries and the containers are arranged so that one cooler can maintain a stack of containers at a given temperature. The temperature of the return air duct for each container is monitored. Provision of a cooler and trunking system for maintaining container temperatures must also be provided at container terminals.

Individual containers with their own refrigeration plant are connected to the 440 or 220 a.c. sockets provided on deck. These containers may be arranged for ships' systems with either 440 or 220 V by provision of a direct connection for a 220 V supply to the self-contained refrigerator and a 440V connection through a step down transformer.



Air cooler fans

Fans may be either centrifugal or of the propeller type; the air circulation systems being based on a pressure requirement of about 50 mm W.G. (water gauge). All of the electrical energy of the fan motors is dissipated in the form of heat and has to be removed by the refrigerating plant. Fan output should be variable so that it can be reduced as heat load diminishes. There was no problem with d.c. motors but with a.c. either the motors are two speed, or each cooler has a number of fixed speed fans which can be switched off individually to suit the load. In the latter case, provision must be made to blank off the stopped fans to prevent air loss.

Reefer Container
Fig:Reefer Container

The capacity of the fans is determined by the number of air changes per hour required in the cargo chambers, and this is influenced by the maximum calculated heat load. In a system using air coolers and fans, all the heat load must be carried away by the circulating air and the difference between delivery and suction air temperatures is directly proportional to the weight of air being circulated. Since the temperature difference is limited by the allowable temperature spread in the cargo chambers and maximum temperature spread in the cargo chambers and maximum load can be estimated, the selection of suitable fans is straightforward.

In most installations the number of air changes required per hour, based on an empty chamber, varies between 40 for dead cargoes such as frozen meat or fish and 80 for fruit cargoes, such as bananas which evolve heat freely.


Instruments

It is essential to measure and log the temperature of refrigerated cargo to ensure that the correct conditions are maintained and also to provide a record should there be complaints from a shipper. Mercury or spirit thermometers suspended from screwed plugs in vertical steel tubes with perforations hung in the cold chamber have been replaced by remote reading devices.

Electrical resistance and electronic self-balancing thermometers use the principle of the Wheatstone bridge. The former rely on a galvanometer to indicate a balance. In the latter the unbalanced current causes an electric motor to adjust the resistance. All necessary cargo temperature readings are obtained on modern reefers and container ships on a data logger which makes an automatic record.

The temperatures and pressures relating to refrigerant gas and liquid, cooling water, brine and the ambient are also required. Most of these are obtained from direct reading instruments.


Carbon dioxide measurement

Carbon dioxide concentration in the cargo chamber is important when fruit or chilled beef is carried. The electrical CO2 indicator operates on the principle that CO2 is a better heat conductor than air. A sample of air with CO2 content, is passed over platinum resistance wires carrying a constant heating current. Between the sample chambers CO2 is absorbed to give a differential reading. The wire temperature is less when CO2 content is higher. The temperature difference is detected on a Wheatstone bridge circuit through a suitably calibrated milliammeter which gives a direct CO2 reading.


Defrosting

This very necessary operation presents no difficulty when the cooling medium is brine. All that is required is a brine heater with brine pump and circuits to circulate hot brine through the coolers.

In direct expansion systems, defrosting can be effected by separate electric heaters installed in the evaporator grids or by providing a means of bypassing the condenser so that hot gas from the compressor circulates the evaporator directly.

Heat leakage and insulation The total load on a cargo refrigerating plant is the sum of:
  1. surface heat leakage from the sea and surrounding air;
  2. deck and bulkhead edge leakage from the same sources;
  3. heat leakage from surroundings into system pipes;
  4. heat equivalent of fan and some brine pump power;
  5. cooling of cargo not precooled at loading;
  6. respiratory heat of live cargoes;
  7. heat introduced by air refreshment of live cargoes.

The load arising from 1, 2 and 3 can be much reduced by the efficient use of insulation. A number of materials are used for this including slab cork, glass and mineral wools, expanded plastics, aluminium foil and polyurethane. The latter, although generally most costly, is the best insulator, having the lowest coefficient of conductivity, with the further advantages of being impervious to air leaks and almost impervious to the passage of vapour, when the material is foamed in situ, Materials which contain CFCs should not be specified. Some rigid urethane foams (polyisocyanates and polyurethanes) and expanded polystyrene or phenolics may contain CFCs. These materials are used for their low thermal conductivity, high resistance to the passage of vapour, good mechanical properties and ease of construction. They can be produced so as to be free of CFCs but with higher thermal conductivity.

All of the materials mentioned have to be enclosed by linings for protection and the prevention of air leakage. The design and construction of the linings makes a greater contribution to the efficiency of an installation.


Insulation test

The heat balance test which replaced an earlier unsatisfactory version, was introduced by the major Classification Societies in 1947. In this trial, temperatures in the refrigerated spaces are reduced to a specified figure and then after a lapse of time sufficient to remove all residual heat from the insulation and structure, the spaces are maintained at constant temperature for at least six hours by varying the compressor output. During this period all temperatures and pressures, speeds and electrical consumption of compressors, fans and pumps are carefully logged and the compressors' output is noted from appropriate tables.

From this information it is possible to compare the efficiency of the insulation with the theoretical estimate made during the design stage and also to decide whether or not the installation can maintain these temperatures in maximum tropical sea and ambient conditions. Obtaining the theoretical estimate entails taking each external surface of the individual chambers separately and considering all factors affecting the heat leakage. These factors include the pitch, depth and width of face of all beams, frames and stiffeners buried in the insulation, the type of grounds securing the linings, the presence of which have their effect in reducing the effective depth of the insulation. Hatches, access doors, bilge limbers, air and sounding pipes also have their effect on heat leakage and must come into consideration. It should be noted that in these calculations the laboratory value of the insulation is generally increased by about 25% to allow for deficiencies in fitting.

It has been found that the overall co-efficient of heat leakage in well insulated installations can vary between 0.454 W/m2/C for 'tweendecks in small lightly framed ships and 0.920 W/m2/C for fully refrigerated moderate sized ships having deep frames with reverse angles. Where there are also buried air ducts, the effective depth of the insulation may reduce to little more than zero.

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  • Reefer container
  • Individual containers with their own refrigeration plant are connected to the 440 or 220 a.c. sockets provided on deck. Systems designed for the cooling of refrigerated containers employ trunkings arranged so that containers stowed in stacks between built-in guide rails, can be connected to the suction and delivery air ducts of the ship's refrigeration plant by bellows pieces operated pneumatically......




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