Food Engineering

Lesson 23. PLATE FREEZERS AND IMMERSION FREEZERS

Module 2. Food freezing

Lesson 23

PLATE FREEZERS AND IMMERSION FREEZERS

23.1 Plate Freezers

Plate freezers consist of a series of parallel flat plates through which a coolant is circulated (Fig. 23.1). The plates can be mounted either horizontally or vertically. A hydraulic system is used to both open the space between plates for loading and unloading, and to close the plates so that effective contact with the food product occurs during freezing. Spacers or limit stops between plates and a pressure relief valve in the hydraulic circuit can be used to prevent the product being crushed unevenly or excessively flattened during plate closure. Vertical plate freezers are best suited to freezing unpackaged deformable products such as fish and meat. Blocks are formed by direct gravity feeding of the product between the plates. Plate heating and block ejection systems are required to remove the block at the end of the freezing process and cleaning may be required before reloading. Horizontal plate freezers are commonly used for either product packed into rectangular cartons or product formed into rectangular shapes by metal molds or trays. Although automated systems have been developed where individual plates are opened in order to facilitate simultaneous loading and unloading of rows of product from feed conveyors and continuous operation, plate freezers are more often manually loaded and operate in batch mode. For efficient operation, uniform and effective contact between the plates and the food product surface is important. This can be achieved by a high packing density of the product (low void space) within the package and/or by application of a moderate pressure to the plates. For packaged products, design of carton dimensions ensuring low voidage and minimal head space is crucial for good heat transfer. The major advantages of plate freezers are that: the rate of freezing is high even for packaged products; the product has very consistent size and shape and can be easily bulk stacked with high packing density and stability for subsequent transportation; they are very compact; infrequent defrosting of the plates is required; the total heat load and energy use are lower than for air systems (no fans and less infiltration and air interchange loads); and if evaporating refrigerant is used directly in the plates, the refrigeration system can operate at a higher suction temperature. The major disadvantages of plate freezers are the high capital cost, especially if they are automated to run continuously, and the limitation on product types that can be handled.

23.2 Immersion Freezers

In immersion freezers the product is immersed directly in, or sprayed with, a cold liquid such as a brine or glycol. The product is usually packaged to prevent cross-contamination between the liquid and the product. Products with irregular shapes are easily handled. Although high rates of freezing can be achieved, these types of freezers are now seldom used except for some fish, meat, and poultry products. The liquid is refrigerated either by circulation through a heat exchanger or by cooling coils and/or a jacket built into the liquid tank.

23.3 Cryogenic Freezers

The most commonly used cryogens are liquid nitrogen (LN₂) and liquid carbon dioxide (LCO₂). Chlorofluorocarbons (CFCs) such as CFC-12 are no longer in use due to their ozone layer depletion effect. The cryogens have low boiling points, –196°C for LN₂ and –79°C for LCO, giving large temperature differences and high rates of heat transfer. Other important properties of such cryogens are that they are colorless, odorless, chemically inert, and nontoxic in normal concentrations. Therefore they are safe for direct contact with food. The product is either sprayed with, or immersed in, the cryogen at atmospheric pressure. Special care must be exercised with CO₂ because it forms a low density snow. Cryogenic freezers can operate continuously with the product being conveyed through a tunnel (Fig.19.2), but other

configurations are also used. Cryogenic freezers are generally only used for small to medium sized products because in larger products the rate of freezing is limited by heat transfer internal to the product. While the higher rate of freezing should lead to a higher quality product than using other freezer types, it is often forgotten that long-term storage negates these benefits. It has been shown that after about a month of conventional frozen storage, the quality of products was independent of the type of freezing system and rate of freezing used. LN₂ and LCO₂ are usually delivered as a high-pressure liquid, rather than being produced on-site, and are vented to the atmosphere after use. The cryogen storage system is a significant cost component. Effective insulation and/or refrigeration of the storage tank is necessary to prevent excess heat ingress and cryogenic loss. For LN₂ , the system losses are typically up to 1% of stored volume per day. For LCO₂ , the higher temperatures for the same storage pressure mean that a small supplemental mechanical refrigeration system can eliminate losses completely. For efficient use of the cryogen, the product and cryogen flows are usually countercurrent and the cryogen vent temperature is kept reasonably close to ambient conditions (–50°C to 0°C). Because of the very high rates of heat transfer achieved, a product temperature equilibration stage is commonly included. Even then the product surface temperature will usually remain significantly colder than the center temperature at the freezer exit.

The main advantages of cryogenic freezers are: high rates of freezing achieved by the very cold temperatures and low refrigerant-to-product surface heat transfer resistance (resulting in lower weight loss and higher quality); ease of operation; compact size; low cost of the equipment; rapid installation and start-up; mechanical simplicity; and low maintenance cost. The main disadvantage is the high cost of the cryogens. Cryogenic freezing (or alternatively liquid immersion freezing) can be used for rapid

crust freezing with completion of freezing in an air-blast freezer (often called combined cryomechanical systems). These systems aim to achieve the optimum balance between freezer operating costs and product moisture and quality loss. The principle is that most of the heat removal is performed by the mechanical system giving low operating costs while the initially rapid surface temperature pull-down minimizes product moisture and quality loss.