3.3.1.2. Plate freezers

3.3.1.2. Plate freezers

Plate freezers can be arranged with the plates horizontal to form a series of shelves and, as the arrangement suggests, they are called horizontal plate freezers (HPF). When the plates are arranged in a vertical plane they form a series of bins and in this form they are called vertical plate freezers (VPF). Modern plate freezers have their plates constructed from extruded sections of aluminium alloy arranged in such a manner as to allow the refrigerant to flow through the plate and thus provide heat transfer surfaces on both sides. Plate freezers are fitted with hydraulic systems which move the plates together and apart.

Horizontal plate freezers

At present in India all the processing plants have HPF. Capacity of this freezer varies between 500kg to 1ton/load. Each plates are connected with pressure resistant hose on both ends, i.e. suction line and discharge line. 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 plates must be completely free from frost or ice and dried before the freezer is used again.

Vertical plate freezers

The main advantage of this type of freezer is that fish can be frozen in bulk without the requirement to package or arrange on trays. 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 1070 nm x 535 mm. Other dimensions however, can be produced in which the thickness can vary from 25 to 130 mm, and it will depend on the fish to be frozen. The maximum weight and dimensions are also limited by the physical effort required from the operator to lift the block, and by the ease with which it can be handled 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 contact with the plates. Fish such as cod and haddock produce compact blocks with a block density of approximately 800 kg/m3. It has certain operational disadvantages.

Defrost is essential to prevent fish sticking to the plates which are at a temperature below 0oC, and thus failing to form a compact block. Freezing times are longer due to the poor contact being made with the plates and because of the lower block density, more storage space is required for a given quantity of fish.

Last modified: Sunday, 25 December 2011, 10:24 AM