Refrigeration & Equipment Engineering

Unit 5 - Freezers

5.2.3.Horizontal plate freezers
The two main uses for this type of freezer are the freezing of prepacked cartons of fish and fish products for retail sale and the formation of homogeneous rectangular blocks of fish fillets called laminated blocks for the preparation of fish fillets. The thickness of package or block frozen is 32 to 100mm and the freezer can; readily adapt from the thicker to the thinner package provided the range required is made known to the supplier at the time of purchase. Horizontal plate freezers need not be defrosted after each freeze.

There is no direct contact between the fish and the freezer plates when freezing by this method since the fish is always packaged before freezing. If the operator is also careful not to spill water on the plates during loading and unloading, the freezer may be operated with only a light brush between each freeze to remove surface frost, and a proper defrost may only be necessary once or twice per day. A hot gas defrost arrangement is the quickest method to defrost an HPF, but even with this method, it may take 30 min or more. The defrosted plate must be completely free from frost or ice and dried before the freezer is used again.
 
Horizontal plate freezers intended to be operated with a hot gas defrost are fitted with additional pipe work which allows the cold refrigerant to be discharged from the bottom of the freezer as the defrost proceeds. Without this special pipe work and operating valves, a hot defrost would clear the top plates only and leave the cold refrigerant in the plates at the lower levels. As in all hot gas defrost systems, the refrigeration system must have an adequate load to provide sufficient hot gas for an effective defrost. This system would therefore be better applied when there are two or more freezers operated from a common refrigeration system and each freezer will then be defrosted in turn while the others are in operation.
 
Horizontal plate freezers are often run without a defrost during the working day and the doors are left open overnight to allow the plates to defrost. This, however, is usually insufficient and water is hosed on the plates to assist the defrost procedure. Defrosting is easier when a secondary refrigerant is used or when the freezer operates with a pump circulated primary refrigerant. With these systems, a reservoir of warm liquid can be made available for pumping through the plates. An HPF will only operate correctly if good contact is made on both the top and bottom surfaces of the pack or tray to be frozen.If the product is frozen from one side only due to poor contact on the upper surface, the freezing time could be three or four times as long as the time achieved with good contact on upper and lower surfaces. The plates of the HFP are closed by means of al hydraulically operated piston to make contact with the upper surface of the product. The pressure applied can readily be varied to suit the product but 0.5 kg/m² is common and is increased by a factor of two as the fish expands during freezing.