Plate freezers now for continuous freezing

A horizontal freezer under construction. The plates are generally bigger than in a vertical freezer and they are used to freeze fish, fish fillets and crustaceans.

Dybvad Stål Industri makes plate freezers for the production of frozen blocks that are used by fish, and other food, processing companies. The company designs and manufactures manual or automatic vertical and horizontal freezers that it supplies to processing plants on land as well as on board fishing vessels in countries round the world.

In a quiet corner in north east Jutland is the town of Dybvad (pop. 628), home to Dybvad Stål Industri, a company established in 1969 that specialised at the time in steel components. A decade later building on the experience and abilities gained working first with steel and later with aluminium DSI manufactured its first plate freezer and in 1997 the company decided to pull out of component fabrication altogether and concentrate on plate freezers. Today the company has one of the widest ranges of plate freezer models in the world. As the name suggests the basic unit of plate freezers are two hollow extruded aluminium plates which contain the refrigerant. The material to be frozen is filled into the cavity between the two plates. As the plates are in direct contact with the material the freezing is relatively rapid and results in a block, the thickness of which corresponds to the distance between the plates. This distance can be adjusted depending on the size of the block that needs to be produced.

Raw material quality determines that of the frozen product

Freezing is a way of locking in the freshness of the fish and preventing its deterioration. By freezing products it is possible to prolong their shelf life substantially and if the freezing is done properly and with care the quality of the product will hardly be affected. For optimum quality however, the fish should be frozen as soon as possible after it is caught and should be kept refrigerated while waiting to be frozen. The quality of the final product is determined by the quality of the raw material – the quality of poorly handled raw material will not improve after being frozen. Once frozen, fish, if stored properly, can safely be shipped halfway around the globe to be thawed, processed, and then frozen again and sent to the market. Proper freezing and storage are among the factors that have contributed to the development of a processing industry in China and other parts of Asia that depends on frozen raw material in the form of whole round or headed and gutted fish from the US and Europe.

Fish is typically frozen either individually (individually quick frozen) or in blocks. Plate freezers are used for the manufacture of blocks. They are made to ensure the blocks are not too thick as this will negatively influence the efficiency of the freezing. Small pelagic fish are often the raw material for the production of blocks. However, their high fat content makes it necessary to glaze the blocks to prevent the fish from coming in contact with atmospheric oxygen and the fats within the fish from getting oxidised (rancid). The finished blocks are often wrapped in plastic for the same reason, before being packaged in cardboard cartons.

Most of the production operates on board fishing vessels

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At DSI plate freezers are not just manufactured, but also designed and developed. The company can offer a range of vertical and horizontal plate freezers that can be used wherever there is a demand for the production of blocks. We work very much with the fishing industry, says Peter N Christensen, the Sales Director at DSI, roughly 60% of our production is intended for fishing vessels, and we serve both the small vessels that have one or two freezers on board as well as the big Dutch pelagic trawlers that may have 50 freezers in operation and can process 400-500 tonnes of fish per day. The remaining 40% of the production goes to the on shore processing industry, where it is used to freeze fish fillets or crustaceans. Building plate freezers is a specialised operation that calls for specialised knowledge and tools so the barriers for entry are relatively high. Plate freezing is among the most effective ways of freezing, both in terms of freezing speeds as well as energy consumption, says Mr Christensen, which is also why DSI decided to focus on this type of freezing. Installation costs for a plate freezer are somewhat higher than, for example, for a blast freezer, on the other hand a plate freezer requires less space, which is an advantage particularly in the constrained space on board a vessel.

A plate freezer is designed to produce blocks, so for any other product form, for example, individual pieces or pre-packaged goods, a different kind of freezer is required. Blocks because of their regular shape and size are very easy to handle and store and can be produced in both the horizontal and vertical plate freezers that DSI manufactures. In vertical freezers the product is placed in the vertical chambers between the freezing plates, while in the horizontal freezers the product is placed in a carton or tray between the plates. In both cases the final product is a block of regular dimensions that can be removed from the freezer automatically, semi-automatically, or manually, and placed in storage. Both vertical and horizontal freezers are available in a range of configurations that can produce blocks of different lengths and widths and the thickness of the block can also be varied. The ability to produce blocks of different sizes is useful because block sizes can vary depending on the market they are being produced for. In Russia blocks are one size, while in the EU they are another and for North America and Asia the sizes can be different. When fulfilling an order DSI will typically work closely with the client providing drawings and sharing the layout of the equipment so that the client can incorporate it into his facility on land or aboard the vessel.

Carbon dioxide substitutes conventional refrigerants

The expertise at DSI is in designing and building freezers rather than in refrigeration technology. That said, the company is fully capable of advising on the capacity of the refrigeration systems that are needed to power its freezers. When selling its freezers the company relies on a network of refrigeration companies and contractors around the world, who are responsible for actually connecting the freezer to the refrigeration system. These people function as agents and are responsible for the marketing, sales and after sales service in the different areas and they give the orders to DSI from the client. Today many of the contracts that the company has won have been because vessels are being renovated or because old freezers are being phased out and replaced with new machines that use newer and more environmentally compatible refrigerants. The first freezers were powered by CFC (chlorofluorocarbon) gases, which were discovered to have highly destructive effects on the environment, in particular on the ozone layer which shields the earth from the ultraviolet rays from the sun. CFCs were replaced by HCFCs (hydro chlorofluorocarbons) and HFCs (hydro fluorocarbons),
which were initially thought to be better alternatives. However, these too were found to have unwelcome side effects particularly in their global warming abilities. Today there is a push to use natural refrigerants including carbon dioxide, ammonia, and hydrocarbons such as propane and butane, none of which have an impact on the ozone layer and only a minimal effect on global warming if at all.

Freezers using the old refrigerants are thus slowly being phased out in some parts of the world providing DSI with opportunities to install new equipment that use more environmentally friendly refrigerants. Many of the company’s freezers use carbon dioxide, a refrigerant that it began testing many years ago, after research showed that it offered several advantages compared with conventional refrigerants both with regard to the environment and in terms of efficiency. In addition to carbon dioxide the company is also using ammonia, and Freon, an HFC.

Greater automation for improved safety

Developments at DSI also include systems that automate the removal of the blocks from the freezers thereby reducing the time needed between batches and which are sought after on board vessels where conditions can be particularly harsh. After removing the blocks from the freezer the system places them on a conveyor which carries them away to the packaging line. However, one of the disadvantages of vertical freezers compared with tunnel or spiral freezers is that they are not continuous, that is, product cannot be frozen as it leaves the processing line. Plate freezers need to be fully loaded, the product is frozen, and then the blocks are removed before it is filled up again, each operation in a discrete step. To address this issue DSI has developed a horizontal freezer where a single work station can be opened, loaded, closed, and the contents frozen, opening up the possibility of continuous freezing. Innovative developments like this as well as leaner manufacturing practices to keep down costs, combined with high quality products and flexible customer service have enabled DSI to compete with manufacturers from countries with lower cost levels. Peter N Christensen believes that if DSI can maintain these features the company will remain attractive to clients seeking freezing solutions in the years ahead.