In a plate freezer it is metal, not air, that does the cooling. Refrigerant evaporates at −35 to −40 °C inside channelled aluminium plates; the product is placed between two plates and a hydraulic system presses the plates against the product with light pressure. Heat passes from both faces of the product directly into the metal. Because heat transfer by contact is much stronger than to air, the freezing time is shorter and no fan energy is spent. The product comes out as regular, compact blocks of even thickness.
There are two types. In the horizontal plate freezer the product is loaded in trays or cartons between plates arranged like shelves; it is used for fish fillets, shrimp, minced meat, spinach and ready-meal blocks. In the vertical plate freezer the product is poured from above into the compartments between the plates; whole fish, offal and meat trimmings are frozen as unpackaged blocks. It is common on fishing vessels and in slaughterhouses. Horizontal types are loaded by hand or automatically; in vertical types the blocks are discharged from below or from the side after defrost.
Block thickness is between 25 and 100 mm, and the time increases almost with the square of the thickness: a 50 mm block reaches −18 °C at the centre in about 1.5–2 hours, a 100 mm block in 3.5–5 hours. One load is usually 500–1,500 kg. The product must lie fully against the plates; a half-full tray, bulging packaging or an air gap left in between lengthens the time noticeably. Regular, compact blocks also save space in storage and transport.
On the refrigeration side, because of the low evaporating temperature, two-stage compression, a screw compressor with an economiser or a CO₂ cascade system is used; in large plants the plates work with pumped liquid feed. After every cycle a short hot gas defrost is carried out to release the blocks from the plates. The surface of the plates is kept clean and free of ice; because the flexible hose connections move together with the plates, they are checked regularly for leaks. A plate freezer is not suitable for irregularly shaped products; those are frozen in air blast systems.