When replacement was due for a 20-year-old tunnel freezer in the Frosta plant in Lommatzsch, Germany, the company went for a freezer system from GEA.

The features that convinced them to choose the GEA system were, among others, a completely welded, insulated case in modular configuration and sequential defrost functions. Freshly harvested vegetables can now be processed faster at Frosta because the freezer, with its throughput of up to 17 metric tons per hour, offers 15% increase in output over its predecessor. Productivity gain is even higher, since the new system requires only one defrost cycle per week.

The Frosta company

Frosta plant
Frosta plant in Lommatzsch, Germany

Frosta is one of the largest producers of frozen foods in Europe. Since 1978, Frosta has specialized exclusively in deep-frozen food products. It is the market leader for frozen ready meals in Germany and for frozen fish in Poland. At its plant location in Saxony, it annually processes approx. 40,000 tons of vegetables. Peas and carrots represent the greatest amounts of vegetables processed: each approx. 13,000 tons per year. Third and fourth on the list are spinach and green beans. This mix of vegetables ensures that there is a virtually continuous stream of fresh goods for processing from spring until November.

Two-stage processing staggers the production processes

Whereas peas, Brussels sprouts, and green beans are frozen whole, spinach, white cabbage and carrots enter the freezer in chopped form. In this way, the sizes and weights of the products to be frozen range from delicate peas to carrot slices, a few millimeters thick. The vegetables are frozen in two parallel production lines in tunnel freezers, after which they are packed in large package units and are then placed in intermediate storage. The vegetables remain there until they are needed and removed from storage for production of the finished food products: various vegetable mixtures that are ready to be cooked. 

Evaluation of bids based on technical characteristics

Frosta replaced one of its two tunnel freezers in 2016. As Technical Director Michael Huttary explains, “This freezer was already 20 years old and caused difficulties with occasional malfunctions. We therefore decided in 2015 to replace it.” The company expected the new tunnel freezer to increase production capacity (the old freezer enabled 15 tons of peas per hour), and to easily allow adaptation to the various types of vegetables being processed. Frosta wrote the specifications for the new freezer system. They received four bids and finally decided for the GEA offer, since its tunnel freezer demonstrated a number of functional characteristics that the other manufacturers could not provide. In 2015, GEA – in close collaboration with the Frosta team – designed the tunnel freezer, installed it in 2016, and placed it into operation.

Working with insulated walls and floors

 Tunnel freezer processing peas
The new tunnel freezer processes up to 17 metric tons of peas per hour

The special features of the new GEA tunnel freezer included a fully welded stainless-steel enclosure that consists of modules. The walls and floors of this enclosure are thermally insulated. The unique modular design of the freezer simplifies system prefabrication, delivery to the factory floor, and assembly procedures. The thermal insulation prevents cold bridges, which eliminates the need for separate insulation and floor heating to protect the building foundation. Customized design enabled effective use of the restricted floor space, and increased freezer capacity to 17 tons per hour: 2 tons more than with the old system – no less than an approx. 15% increase (for peas).

An additional feature, welcomed by Frosta, is sequential defrost of the heat exchangers. This involves sequentially shutting down the 12 individual fans: one at a time, which allows flushing the shut-down heat exchanger with hot gas to remove frost. Since, the ventilation flap assigned to each of the fans will automatically close when the fan shuts down, this defrost process has virtually no influence on the temperature in the interior of the freezer. The n-1 design for the crusting freezing line and the finish zone ensures the required freezing duty, also during defrost. Michael Huttary explains: “In the front zone of the freezer, where the vegetables will emit moisture, defrost is more frequently required than at the rear.”

Heat exchanger design for maximum process efficiency

Heat exchanger fans
For sequential defrost procedures, single heat exchanger fans are shut down to feed them with hot gas and melt the frost layer

On the process side, icing of the heat exchanger is greater than on the fan side. To prevent the fins of the heat exchangers from frequent icing, fin spacing on the process side is approx. twice that of the fan side. The result is an optimal compromise between the frequency of the defrost cycles and the efficiency of the system. Closer fin spacing would theoretically enable enhanced heat transfer, but icing-up of the fins would take place more often. The consequence would be greater pressure drops – i.e., greater power consumption by the fans – and the heat exchangers would require more frequent defrost. Both these consequences would inhibit the overall efficiency of the process.

Automatic cleaning is necessary only once a week

It is possible to operate the tunnel freezer for six continuous days without cleaning. The automatic sequential defrost feature and other peripheral components such as the continuous belt-cleaning systems make this possible. Once a week, the tunnel freezer is completely defrosted and cleaned, for which a time window of 12 hours must be provided. The clean-in-place system (CIP) performs this cleaning in approx. 10 hours. The bottom of the freezer is slanted to assure reliable draining of the cleaning liquid. This prevents the collection of any water on the bottom of the freezer, as well as any increased icing of the heat exchangers.

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