How to Improve Plate Freezer Freezing Efficiency
The freezing temperature of quick-freezing equipment is generally -30°C ~ -40°C, suitable for rapidly freezing food materials such as blocks, granules, and slices. Food quick-freezing equipment can be classified by cooling method or by overall structure into tunnel type, belt type, fluidized bed type, spiral type, etc.
1. Plate Freezer
The plate freezer uses the direct-contact freezing method. The housing is wrapped in insulating material, and the active components are a set of hollow plates that act as evaporators. Refrigerant is passed through the hollow interior, and brine may also be filled in the plate tube gaps. The food to be frozen is pressed between two adjacent plates, with tight contact between the food and the plates. The contact pressure is generally 7~30 kPa, and the heat transfer coefficient can reach 93~120 W/(m²·K).
Both sides of the freezing plate are in contact with the food (or freezing pan), and the inner cavity serves as a passage for the refrigerant. There are several forms as follows:
(a) Assembled plate with shaped tubes: The inner cavity is rectangular, with dovetail grooves and tongues on the two sides respectively, and several shaped tubes are assembled to form a plate.
(b) Welded plate with rectangular seamless tubes (Figure 14-21): The freezing plate is welded together from rectangular seamless steel tubes or aluminum tubes, and the manufacturing process is simple. Among them, steel plates are rigid, readily available, and resistant to deformation. Meanwhile, aluminum tube-assembled plates are lightweight and have good heat-transfer properties.
The plate freezer operates intermittently, with a long operation cycle, large cold loss during loading and unloading, and relatively low production capacity. Generally, multiple freezers are used. Poor contact between the food and the evaporator plates increases thermal resistance, drastically reducing the freezer’s production capacity. However, it can operate at room temperature and is suitable for freezing meat, aquatic products, and pressure-resistant packaged foods with relatively small thickness (e.g., thickness less than 50 mm). It is widely used for the quick freezing of fish.
Based on plate arrangement, plate freezers are divided into two types: horizontal and vertical.
A. Horizontal Plate Freezer
The horizontal plate freezer is a cabinet-type structure, with freezing plates installed horizontally and generally consisting of 6 to 16 plates. The positions between the plates are controlled by a hydraulic device. After the food to be frozen is placed in trays and inserted between two adjacent plates, the hydraulic cylinder is activated to bring the food into close contact with the freezing plates. To prevent food from being crushed, limit blocks are installed between adjacent plates.
The refrigerant supply system of a common horizontal-plate freezing device is a flooded system. The liquid-vapor separator is installed at the top of the device. The upper part of the liquid-vapor separator 2 is connected to the suction line, and the lower part is connected to the liquid-collecting pipe 7 and the vapor-collecting pipe 1 of the evaporator plate group. Flexible hoses connect the two collecting pipes to each plate, allowing the plates to move up and down.
The inner lining of the hose is made of low-temperature-resistant butyl rubber, with a 2- to 3-layer nylon or polyester braided core, and the outer layer is covered with a metal protective braid. The connecting pipes may also be detachable, with union joints. Regardless of whether flexible hoses or detachable connecting pipes are used, valves should be installed between the collecting pipes and the plates to facilitate maintenance and replacement.
The horizontal plate freezing device facilitates quick-freezing after the food is arranged as required, and the resulting frozen blocks have an orderly tissue structure.
B. Vertical Plate Freezer
The structural principle of the vertical plate freezing device is similar to that of the horizontal plate freezing device, but the freezing plates are arranged in parallel in a vertical state, as shown in Figure 14-24. The device generally has about 20 plates. The food to be frozen is generally poured directly into bulk containers between the plates, enabling convenient operation. It is suitable for freezing small mixed fish and meat by-products. The methods for discharging the frozen product from the plates include top-in-bottom-out, top-in-top-out, and top-in-side-out. Movements such as plate movement, frozen block lifting, and ejection are driven and controlled by a hydraulic system.
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