|18.104.22.168 Integral Unit (Integrated Unit):|
|Containers having their own integral refrigeration unit are thus more widely used. These integral units are connected to the on-board power supply system (Fig. 87). All that they require is a slot with a suitable electricity supply.
Such a supply can easily be provided on ocean-going vessels, in port, at terminals and in the shipper's and receiver's warehouses. Clip-on units may additionally be fitted to many container types, in which case they serve only to supply the refrigeration unit with electricity.
The most commonly used type of refrigerated container has an integral refrigeration unit (integral unit container). Air flow in the integral unit container is the same as that in the porthole container (see Fig. 88).
Due to the refrigeration unit fitted onto their end face, clip-on containers are longer than integral containers, while the latter have a smaller cargo capacity. Close to the refrigeration unit, there is a ventilation flap which, depending upon its setting, allows a proportion of the circulating air to escape, so drawing fresh external air into the container (see Fig. 89).
Such controlled fresh air exchange allows harmful metabolic products from goods of vegetable origin, such as carbon dioxide or ethylene, to be removed and oxygen to be supplied. The flap should not be opened more than necessary as the incoming external air must additionally be cooled, so increasing the load on the refrigeration unit.
To ensure adequate circulation of the cold air, the floor is provided with gratings (see Fig. 90). Fig. 91 shows an incident of loss. In addition, the side walls of the container are "corrugated", which ensures satisfactory air flow there too. Fig. 92 shows several corrugations. When packing the container, suitable free space of approx. 15-20 cm must remain above the cargo in order to permit air flow here as well (see Fig. 92).
A mark indicates the maximum admissible cargo height. In usual container types, fresh air exchange is currently still adjusted manually; a microprocessor-controlled method has, however, been developed which makes it possible to carry out ventilation in accordance with a predetermined program. Using this method, it is possible, for example, not to begin ventilation until 72 hours have elapsed, in order to remove any heat which has been absorbed during packing. Ventilation may then be provided at specific time intervals in order to minimize any associated increase in temperature.
Use of additional intelligent sensors makes it possible to measure the carbon dioxide and/or oxygen content of the cooling air and adapt ventilation to the specific requirements for the particular product. Since ethylene production is proportional to carbon dioxide production, this ripening gas can also be successfully monitored in this way.
The doors constitute a weak point in both integral units and porthole containers. Wear to rubber door gaskets or improper handling may result in the doors no longer closing correctly, so that they are no longer sealed against rainwater and the like.
During transport of chilled and frozen goods, water ingress may lead to cargo spoilage or to ice formation in the door area (see Figs. 93-95).
In addition, refrigeration capacity has to be increased to compensate for losses due to cold air leakage. The temperature of the cold air which is blown in is recorded, for example, by means of a Partlow recorder or data logger (see TIS).
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