Refrigeration & Equipment Engineering

Unit 5 - Freezers

5.2.4.Vertical plate freezers

The main advantage of using this type of freezer is that fish can be frozen in bulk without any previous requirement to package or arrange on trays. The plates form what is in effect a bin with an open top and fish are loaded directly into this space. This type of freezer is therefore particularly suitable for bulk freezing and it has also been extensively used for freezing whole fish at sea. The maximum size of block made by this method is usually 1070mm x535 mm. Other dimensions of block, however, can be produced an block thickness can vary from 25 to 130 mm. The block dimensions selected will depend on the fish to be frozen and also the maximum block weight that can be readily handled by the operators. Maximum block; dimensions and weight are limited by the physical effort required from the operator to lift the block, and by the ease with which he can handle the block so that damage to the fish is kept to a minimum. In most cases, fish can be loaded between the plates without wrappers and water need not be added either to strengthen the frozen block or improve the contract with the plates. Fish such as cod and haddock produce compact blocks with a block density of approximately 800 kg/m³.

With fatty fish such as herring, it has been found advantageous to use wrappers and add some water to fill the voids in the block. Fatty fish do not form blocks which are as firm and strong as blocks made from lean fish especially during seasons when the oil content of the fish is high. Water added to the block helps to strengthen the block, protects the fish during subsequent handling and reduces the effects of dehydration and oxidation during cold storage. Well formed, rigid blocks are particularly important when freezing at sea. The frozen product is handled under particularly adverse operating conditions and poorly formed blocks would result in al high percentage of broken blocks and loose fish. Machine filleting or splitting of the fish, for instance, may be difficult if fins and tails are broken. Wrappers have therefore been used when freezing fatty fish in VPF to protect the exposed fish on the outside of the block. A wrapper that has been found suitable for this purpose is a single layer paper bag, coated internally with polyethylene, and shaped to fit the space between the freezer plates. Wrappers made from polyethylene and other plastic materials were found to be difficult to handle and constituted a danger when stacked due to slipping.

Fish frozen with wrappers will inevitably require a longer freezing time due to the insulating properties of the wrapping material. Some types of wrapper would have a considerable effect on freezing time but the material described did not increase the freezing time by a significant amount.

Vertical plate freezers are defrosted to release the blocks of fish after each freeze. Fish in VPF are in direct contact with the plates and the force required to release the blocks without a defrost would be excessive and result in plate damage. The defrost time for a VPF need not exceed 3 or 4 min if a suitable supply of defrost gas or hot liquid is available. If a primary refrigerant is used in the plates, a hot gas defrost is generally used and when there is a multiple freezer installation, the freezers are defrosted in turn with the other freezers in operation providing the necessary refrigeration load for the compressor. When a secondary refrigerant is used, a reservoir of hot liquid has to be maintained and pumped through the plates to displace the cold liquid present. With this arrangement, it is possible to return the bulk of the cold liquid to the low temperature reservoir at the start of defrost, and also return the warm defrost liquid to the hot liquid reservoir for reheating at the start of the next freeze. This arrangement reduces the quantity of liquid interchanged at each defrost but even with this system provision must be made to maintain the liquid charges in both the cold and hot systems at the correct level.