Freezing Technology

3.3.1.3. 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 pre-cooling stage of the freezer, the gaseous nitrogen partially freezes the fish and up to 50 percent of the product heat is extracted. The product then passes below the liquid spray where freezing is completed by the boiling liquid. The last stage in the freezer provides a few minutes for the fish temperature to reach equilibrium before the fish are discharged.

The main advantage of the liquid nitrogen freezer LNF 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 must be retained in a vacuum insulated pressure vessel with continuous venting to keep the contents cool and the internal pressure down. It is found 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. This method of freezing is more expensive than most others. It is four times more costly than conventional air blast freezing. 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 like India is that delivery of nitrogen could be expensive and there may be no guarantee of regular supplies.