Modern plate cooling systems differ little in principle from the first contact freezer patented in 1929 by Clarence Birdseye. Products in regular-shaped packages, such as ice-cream in flat cartons, are pressed between horizontal, flat, refrigerated plates (see Figure 14.3). These can be opened apart slightly to admit the product and are then closed by hydraulic rams to give close thermal contact. When freezing is complete, the plates open again to remove the packs. A horizontal plate freezer is shown in Figure 7.14(a). The vertical plate freezer (Figure 7.14(b)) is used for a loose product such as wet fish, which is packed into the gaps between the plates. When the freezing is complete, the product is removed as a solid block and may be of 75 mm or 100 mm thick.
Contact freezers are less costly to operate because they do not use fans for air movement. The cooling is accomplished by direct contact of product with a surface, which in turn is in direct contact with the refrigerant or secondary coolant.
Material to be frozen can be fully immersed in a cold liquid such as a brine. This is only suitable for wrapped product. Sodium chloride and glycol brines
Separated plates Closed plates
Figure 14.3 Arrangement of plate freezer (FRPERC)
Are not cold enough to get complete freezing, so this may be a first pre-cooling stage before a final air blast. Alternatively, liquid nitrogen (—196°C) or carbon dioxide (—78.5°C) can be sprayed onto the surface.
Posted in Refrigeration and Air Conditioning