A reefer container is designed to maintain cargo at the pulp temperature prevailing at the time of stuffing. Although the container machinery over a period of time can bring cargo delivered at too high a temperature down to (or closer to) the designated temperature, this is not the primary function of a Reefer Containers at BSL Offshore.
If a container is loaded with a cargo where the pulp temperature exceeds the carriage temperature stipulated by the shippers, the "warm" cargo will cause the temperature of the delivery air to rise very rapidly when passing up and through the cargo. Eventually, the return air may reach a temperature level whereby the refrigeration machinery cannot cool it down sufficiently prior to re-circulating it as delivery air. In this event, the tracking pattern on the chart or logger will show a temperature higher than that of the temperature control setting. The delivery/return air differential will in most cases narrow as the continuous circulation of air, being cooler than the cargo, brings the cargo temperature down towards the desired level. Any rise in return air temperature will be arrested as the refrigeration unit begins to run in standard operational mode.
In cases where the stuffing temperature is higher than the stipulated carriage temperature, the refrigeration unit will cool down the surface layer of the cargo relatively quickly (within days). However, the centre of the stow will not reach the desired temperature for a considerable period of time. The temperature of a cargo stuffed into a refrigerated container should not, in general, deviate by more than 3ºC (5ºF) from the specified carriage temperature. Chilled cargo (excluding bananas) should not deviate by more than 0.4ºC (1ºF). This does not mean that even these deviations should be encouraged; the objective is to receive and deliver the cargo at the carriage temperature.
During the operation of a refrigeration unit, a layer of ice will form on the evaporator coils depending on the temperature set, the temperature of the cargo, the amount of fresh air ventilation and the cargo humidity. The unit periodically enters a phase where heat is produced by a series of electrical bars, allowing defrosting to take place. At such times, all fans are turned off automatically in order to prevent heat from entering the cargo compartment.
However, the return air temperature sensor is so closely located to the refrigeration machinery that the temperature record will inevitably register some of this rise. The record will therefore display periodic temperature increases in keeping with the defrost periods. It must be stressed that these increases, which are conspicuous on paper chart recorders, have no immediate effect on the actual temperature of the cargo and are not an indication of an unstable refrigeration unit. Electronic loggers usually indicate the timing and duration of defrost periods in addition to temperatures.
If, as described in the previous section, a cargo is loaded into a container in a "warm" condition exceeding the specified temperature, the refrigeration unit will automatically work to bring the cargo temperature down towards the correct level. This unintentional strain on the unit may result in a heavier accretion of ice on the evaporator coils, leading to an increase in the defrost patterns recorded.
Recorder charts do not identify refrigeration unit defects, but do give useful indications of correct operation. Data logger records may give detailed information about system faults in addition to set point, delivery and return air temperatures. Container temperature recording systems do not usually record actual cargo temperature, only air temperatures, but cargo temperature may be recorded by shippers' loggers within the stow.
Malfunction of a reefer unit
Should a refrigeration unit cease to operate, the chart or logger will register a gradual but steady rise in temperature to the point where eventually the ambient temperature is recorded. Again, the sensor will record an air temperature and the record will not accurately reflect the true position regarding the cargo itself. The cargo will be reasonably well protected from the influences of the external air temperature by the surrounding insulation.
There are many other situations where the record may not be a precise representation of the temperature or condition of the cargo within. These examples are given merely to warn that conclusions should not be drawn automatically from the temperature tracking pattern alone.
In general, refrigerated commodities may be divided into two distinct categories:
Many chilled cargoes (e.g. fruit) are regarded as a "live" cargo since they continue to respire post harvest and as such are susceptible to desiccation (wilting and shrivelling). This is not the case with commodities such as chilled meat or cheese. The minimum fruit carriage temperature is usually no lower than -1.1ºC (30ºF). Frozen cargo is regarded as "inert" and is normally carried at or below -18ºC (0ºF).
However, both categories are highly perishable and require care in handling to ensure arrival in optimum condition. In chilled commodity transportation, the ventilators are normally left in an open position, with a limited number of exceptions (eg meat, chocolate, film, chemicals, dairy products, and controlled atmosphere shipments). Some cargoes may require controlled humidity (eg flower bulbs). It should be remembered in such cases that many refrigeration units are only capable of reducing humidity within the cargo space and the settings should be applied accordingly. Those units which can increase humidity may incorporate water tanks with special cleaning and hygiene requirements to avoid contaminating the cargo. Controlled atmosphere carriage involves the use of specialized containers capable of substituting the oxygen levels with nitrogen and carbon dioxide in order to extend the post harvest shelf life of the product. This method is suited to many soft and stone fruits, but requires specialist knowledge to determine the most appropriate gas concentration levels.
Stowage inside the Reefer Container
Correct stowage is extremely important to the carriage of containerized reefer cargo. However, this is seldom under the control of the carrier, who often receives a sealed container "said to contain" a specific cargo. With frozen cargo, the objective is to provide a circulation of cold air around the cargo to reduce the possibility of temperature variations at the boundaries (eg walls, floor and roof). With chilled live cargoes (eg fruit and vegetables), the air flow must be allowed to permeate up and through the cargo stow, removing product heat, carbon dioxide, ethylene (if present), moisture and other residual gases in the process.
Cargo must never be loaded above the red line marked inside the container. This space must always be left to allow an uninterrupted flow to the front air intake. The ideal stowage pattern should permit free movement of delivery air whilst restraining any movement of the cargo. Adequate space must be left above and below the stow to allow free air circulation.
Frozen products require a very simple stowage arrangement provided they are loaded at the specified carriage temperature. This can be achieved by a solid block stow, with no space between the stow and the container walls. When carrying frozen cargo, the fresh air ventilation hatches must always be closed.
It is important to ensure that the cargo stow covers the entire floor area, projecting beyond the rear floor restrictions of the "T" bars in order to prevent the air short circuiting, and to facilitate an effective flow of return air. In larger containers, if the cargo volume is less than the space in the container, the stow should be of uniform height. As stowage plays an important part in maintaining the quality and security of the cargo during transit, it is essential that specialist advice is sought should there be any doubts when a cargo is booked.
Reefer Containers Tips
Reefer Containers – Providing Fresh Produce Worldwide
Reefer Containers Tips
Reefer Containers – Providing Fresh Produce Worldwide