Freezing Technology

3.3.1.1. Air blast freezers

The advantage of the blast freezer is its versatility. It can cope with a variety of irregularly shaped products and whenever there is a wide range of shapes and sizes to be frozen, the blast freezer is the best choice. Before going on to describe the various types of air blast freezer, it is necessary to deal with some of the basic principles of air blast freezer design and operation.

Designing air blast freezers

The use of air to transfer heat from the product being frozen to the refrigeration system is probably the most common method used in commercial refrigeration. The natural convection of the air alone would not give a good heat transfer rate; therefore, forced convection by means of fans has to be introduced. To enable the product to be frozen in a reasonable time the air flow rate should be fairly high. Also, in order to obtain uniform freezing rates throughout the freezer, the air flow requires to be consistent over each fish or package.

Frost build-up on the cooler is also more prolific on the upstream coils; therefore a cooler with a large frontal area will be able to operate longer before a defrost is necessary.

Continuous air blast freezers

The belt has to be flexible, easily cleaned, non corroding, suitable for use in direct contact with food and should not interfere unduly with either the freezing time or adversely affect product quality. Stainless steel mesh link belts or chain link belts are mainly used for this purpose but they have certain disadvantages. Apart from being expensive, they affect the appearance of the product. If fish are loaded directly on the belt, the crinkled or indented appearance of the frozen product is not always acceptable. Open mesh belts can also give rise to difficulty when removing the product after freezing and some weight loss may be incurred due to slight physical damage. Skin-on fillets can usually be removed quite easily but skinless fillets and fish portions can stick to the belt and cause unacceptable weight losses.

Continuous belt freezers can be constructed with either cross-flow or series-flow air circulation. In the series-flow arrangement, the direction of air flow must be such that the coldest fish meet the coldest air. The design of the belt entry and exit must keep the rate of air infiltration to a minimum.

Fluidized and semi-fluidized freezers

One type of air blast freezer fluidizes the product with a strong blast of air from below. The product then behaves like a fluid and when poured into the trough at the input, it moves along the length of the freezer without mechanical assistance and over-flows at the output. This type of freezer has not had a wide application for fish or fishery products. Small cooked and shelled shrimp is one of the few fish products that have been successfully frozen by this method.