FOOD ENGINEERING

Lesson 22. AIR BLAST FREEZERS

Module 2. Food freezing

Lesson 22

AIR BLAST FREEZERS

22.1 Introduction

Air-blast freezers are the most common type of food freezer. Individual product items are placed in a recirculation air stream within a room or tunnel. The air is circulated by fans, which are often associated with the evaporator coils providing cooling. These freezers can be simple, operating in batch mode with manual loading and unloading of the product (Fig. 22.1, 22.2) or more complex, with automated continuous operation (Fig. 22.3, 22.4). Continuous freezers are best suited to processing large volumes of product. They have lower labor costs, and generally provide more uniform freezing conditions, but are less flexible.

Many different air and product flow configurations can be used. Horizontal air flow is probably most common, but there are a number of designs using vertical air flow, often to avoid air bypassing the product (Fig. 22.5). In continuous systems, air and product flows can be cocurrent, countercurrent (Fig.22.5) or cross-flow (Fig. 22.3, 22.4). The latter two configurations are most common as air temperature rise is small, so the temperature driving force for cooling and thus the rate of heat transfer are maximized. Methods to present the product to the air depend on the size, shape and packaging of the product, and include trays, racks, trolleys, hooks, conveyors, and belts. A wide range of product types, sizes, shapes, and packaging types can be handled by such freezers.

The major advantages of air-blast freezers are their simplicity and flexibility. The disadvantages are that using air limits the rate of heat transfer at the product surface, requires the use of substantial fan energy, and to achieve uniform air distribution can be difficult. Also, further disadvantages are that: evaporative weight loss can be significant from unwrapped product; bulging of packaged product can occur; defrosting evaporator coils or other means of frost prevention is required which can disrupt freezer operation; and the refrigeration system must operate with a low suction condition due to the air-to-refrigerant heat exchange.

22.2 Still Air Freezers

The simplest type of freezer is one in which the product is placed in a refrigerated room that is usually used to store frozen product. The process is historically referred to as sharp freezing. The shelves on which the product is placed within the room may be directly refrigerated, and the product may be bulk-stacked. Air flow over the product is minimal and the freezing rate is slow. Also, heat removal from the freezing product may cause undesirable temperature fluctuations in adjacent stored product.

22.3 Air-Blast Room and Tunnel Freezers

These freezers are commonly used for medium to large products where the rate of freezing is limited by the size of the product. The product does not need to be regular in shape. The product is placed on trays in racks or suspended so that air flow is possible around each individual product item. In continuous tunnel freezers, it is usual to have a mechanical system moving racks through the tunnel in a cyclic manner, with device for automatic loading and unloading of the product. Belt freezers are similar but product is transported on a perforated conveyor belt. Schematic diagram of a continuous spiral belt freezer with vertical air flow. racks, and product arriving and leaving on a conveyor system (Fig. 22.3). For batch freezers, the racks are manually loaded and positioned in the room or tunnel (Fig. 22.1, 22.2). In a tunnel system, the air is confined to flowing in the cross-section where the product is located. Also, the product is spaced evenly so that uniform air distribution and high air velocity is more easily achieved for a low total air flowrate and fan power. While most continuous tunnel freezers are restricted to one product size and shape in order to optimize the product loading configuration and air flow distribution, a range of products can be processed in the same tunnel if a variety of rack sizes and tray spacing are used. In a blast room there is often less strict control of the air flow pathway and bypassing of the air around the product can more easily occur.

22.3 Belt Freezers

Belt freezers involve the product passing continuously through a tunnel freezer on a perforated belt (Fig. 22.4). The air flow is directed vertically up through the belt and product layer. There may be multiple belt passes. Such freezers are commonly used for small unwrapped products with uniform shape in which a free-flow individually quick frozen (IQF) product is desired. The air velocities are typically in the range 1 to 6 m/s and the layer of product can be partially fluidized. This creates high rates of heat transfer between the air and product. Even distribution of the product across the belt is important to achieve uniform air distribution and freezing rate. Product transfers from one belt to another and/or mechanical devices are sometimes installed to reduce clumping and to redistribute the product. The belt speed can be varied to cope with changing production rates but care must be taken to maintain a uniform thickness of product on the belt.

22.4 Spiral Belt Freezers

Spiral freezers are a specialized type of belt freezer in which a continuous belt is stacked in a spiral arrangement up to 50 or so tiers high (Fig. 22.5). They allow very long belts (long product residence times) in a compact area as long as sufficient overhead space is available. Therefore, they are suitable for processing products with longer freezing times compared with other belt freezers (e.g., larger products and packaged products for which the packaging

impedes heat transfer). The size of the product is limited by the distance between each spiral tier and the total height of the stack. Air flow can be either horizontal across or vertical through the belts. Recent design improvements have included self-stacking belts to reduce mechanical wear and maintenance, and cleaning-in-place of the belt and freezer.

22.5 Fluidized Bed Freezers

Fluidized bed freezers are only suitable for small unwrapped IQF products of uniform size and shape, such as fruits and vegetables for which the energy requirements for fluidization are not excessive. In a manner similar to belt freezers, air is directed up through a perforated plate and bed of the product but at a flow rate high enough to fluidize the product (Fig.22.4). The product is fed in at one end and overflows out of the freezer at the other. Fluidization

achieves good distribution of the product and prevents clumping, even with very wet incoming product, and the surface heat transfer is significantly enhanced. The product moves by flowing within the fluidized bed, but this can be aided by vibrating and/or sloping the air distribution plate. Individual items reside for different periods depending on the flow pattern in the bed. The average residence time is fixed by the feed rate and the volume of the bed, which is controlled by the height of the overflow weir. Fluidized bed freezers can be very compact because the small product size and high rates of convective heat transfer keep freezing times short.