Figure 1 Schematic of a typical batch air-blast carton cell.
Refrigeration evaporators are positioned at the opposite end of the room to the load/unload door and are designed to match the cross-section of the product storage area. Pallets holding 1 to 2 tons of product (i.e. 36 to 72 cartons, each weighing about 60lb) are loaded into the blast cell by forklift. Air is usually forced through the evaporators into the product storage area and recirculated above a false ceiling (this is known as forced draught air flow).
Freezer dimensions vary from 1 to 3 storage bays wide, from 1 to 3 storage bays high and from 2 to 8 storage bays long. Freezer capacities range between 200 to 2000 cartons, but are typically between 600 to 1200 cartons.
Most plants aim for a 48 hour production cycle with blast cells, which allows between 36 and 44 hours for freezing once loading and unloading times are taken into account. Therefore, the primary task of the blast cell is to reduce all product to pre-specified centre temperature within the available freezing time.
In practice, however, product freezing times often exceed the desired time. Usually, this is caused by plant or refrigeration designers underestimating the importance of certain design and operational factors when sizing the evaporators.