Continuous Belt Air Blast Freezer:
Contact-plate and Air blast freezers are the two most common kinds used in industry. Contact-plate freezers do not keep the versatility of the air-blast freezers. The former may only be utilized to freeze frequently shaped blocks. The afterwards can he utilized for a variety of irregular shaped plus small sized products like peas, shrimp, French fries, etc., that results in individual quick-freezing (IQF) of each piece. Air blast freezers may be freezing tunnel type, belt freezers or fluidized bed type freezers. Immersion freezers utilized liquid freezants at atmospheric pressure, the correspondent temperatures being - 196°C with N2 and - 30°C with R 12.
To attain quick freezing in air blast freezers, the air flow rates must be high given fig illustrates the effect of air velocity on freezing time. A high air velocity shall mean shorter freezing time however more fan power. A finest air speed of 5 m/s is suggested for most reason. In constant air-blast freezers, even higher speeds, of the order of 10 to 15 m/s, might also be economically working as that would result in smaller freezer length and lower cost because of reduced freezing time.
Influence of Air Velocity on Freezing Time
Above figure illustrate the schematic diagram of an air-blast freezer in which a moving belt carries the product. The belt may also be replaced through trolleys and the product stacked in multilayered trays.
Continuous Belt Air-Blast Freezer with Cross-Flow of Air
In a well-designed freezer, the fan power is of the order of 25 to 30 % of the refrigeration load. If the air-flow rate is very big, the fan power can be higher. As a consequence of small dehumidified growth and high air flow the temperature go up of air over the product will be very small. An average goes up in air temperature of approximate 1.5-2°C is allowable.
Air-blast freezers along batch operation utilized or shelves trolleys for loading the product. Those having continuous operation make utilization of conveyors or belts. The latter are utilized only if the product may be frozen rapidly, the freezing time need to not exceed in 30 minutes, or else the length of the freezer shall become very high.