Refrigeration & Equipment Engineering

Unit 8 - Irradiation

8.5.Liquid nitrogen freezer
In this freezer, the product is brought into direct contact with the refrigerant. The fish on the stainless steel conveyor belt initially come into contact with the counter current flow of nitrogen gas at a temperature of about -50^oC. As the fish progress through the freezer, the temperature of the cooling gas progressively falls to -196^oC. This initial stage of cooling in the gaseous nitrogen partially freezes the fish and also preconditions the product before it is passed below the liquid spray where freezing is completed by the boiling liquid. During this pre-cooling stage, up to 50 percent of the product heat is extracted and the remainder of the heat transfer takes place in the small area below the sprays. After leaving the spray zone, the last stage in the freezer is used for the fish temperature to reach equilibrium before the fish are discharged. If fish are cooled directly below the sprays without preconditioning, they are physically damaged due to the sudden change in temperature which sets up thermal stresses within the fish.

The main advantage of the nitrogen freezer is that freezing is very quick and the physical size of the freezer is correspondingly small. The freezer is operated without the need for compressors, condensers or coolers; therefore maintenance requirements are minimal and the power required to operate the freezer is very low. Liquid nitrogen cannot be retained economically in a pressure vessel and continuous venting is required to keep the contents cool and the internal pressure down. One estimate given is that 0.5 percent of the stored contents is lost each day by this method. In addition, about 10 percent has been estimated to be lost during the transfer of liquid from the tanker to the storage vessel although the customer is not charged directly for this loss. This method of freezing is more expensive than most others, being at least four times more costly than conventional air blast freezing, or even higher if the freezer is only used intermittently with partial loads.

Although the freezer is small and there is no refrigeration machinery requirement, storage space and access is required for the liquid nitrogen tank. The main disadvantage of this type of freezer in most developing countries is that delivery of nitrogen would be expensive and there may be no guarantee of regular supplies.