Financial considerations will take into account both the capital and running cost of the equipment and also projected losses such as product damage and dehydration. Expensive freezers should therefore justify their purchase by giving special benefits and if these benefits are not worthwhile, they need not be considered.
Types of freezer
The three basic methods of freezing are:
- Blowing a continuous stream of cold air over the product – air blast freezers.
- Direct contact between the product and a refrigerated surface – contact or plate freezers.
- Immersion in or spraying with a refrigerated liquid – immersion or spray freezers.
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. However, because of this versatility it is often difficult for the buyer to specify precisely what he expects it to achieve and, once it is installed, it is all too easy to use it incorrectly and inefficiently.
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
Continuous air blast freezers may economically justify air speeds in excess of the above-recommended value. Continuous freezers are expensive and require a good deal of floor space. If the air speed is increased and the freezing time reduced, a smaller freezer will be required for a given freezing capacity. The savings in freezer costs may therefore justify the use of higher air speeds. Air speeds as high at 10 to 15 m/s may therefore be economically justifiable for continuous freezers. Higher airspeeds can also be justified when products have freezing times of less than about 30 mins.
Types of air blast freezer
There are many different designs of air blast freezer both for batch and continuous operation. Details are given of a number of types of air blast freezer in common use, with comment on their suitability for various products and methods of processing and also on their limitations.
Continuous air blast freezers
In this type of air blast freezer, the fish are conveyed through the freezer (on trucks or trolleys or they may be loaded on a continuously moving belt or conveyor) usually entering at one end and leaving at the other.
Batch air blast freezers
Batch air blast freezers use pallets, trolleys or shelf arrangements for loading the product. The freezer is fully loaded, and when freezing is complete, the freezer is emptied and reloaded for a further batch freeze. Apart from this difference in mode of operation, the batch freezer gives rise to bigger fluctuations in the refrigeration load than continuous or batch-continuous freezers (Figure 13).
This large fluctuation in refrigeration load means that the refrigeration system will require special control arrangements to cater for the variations. Capacity control or a multiunit system can be used or a competent engineer can manually control the system to match the load. Some refrigeration systems are also better suited to this type of variable load application than others.
It is seldom that fish processing can be arranged so that all the fish can be loaded into a batch freezer at the same time. Therefore, if each trolley or pallet is loaded as and when it is ready, the refrigeration peak load will be considerably reduced. This will make the operation similar to a batch-continuous process, but again, care should be taken not to place warm fish upstream of a partly frozen product.
The freezer shown in Figure 14 is a batch tunnel freezer with a push-through arrangement for two lines of trucks. If this design of freezer was used with a batch-continuous operation, warm fish might loaded upstream of partly frozen fish. This freezer should therefore only be fully loaded and operated as a batch freezer.
Another batch freezer arrangement is shown in Figure 15. In this model, the trolleys are loaded from the side of the freezer and the air flows across the three trolleys in line.
In some air blast freezers, the cooling coil can be at the same level as the working section (Figure 16). This is a fairly good arrangement since the cooler acts as a diffuser and evens out the air flow immediately before it is directed over the fish.