Fluidization in IQF freezing
Fluidization is the process during which a granular material is converted from a static or solid state to a dynamic or fluid state. In individual-quick freezing (IQF), fluidization is referred to as the ability of the product to act as a liquid under the influence of air currents, moving and advancing inside the freezer towards the outfeed.
So, why is fluidization important?
Fluidization separates and individually freezes each piece of product in the freezer. Fluidization can help processors achieve the best freezing result with the most efficient consumption of resources. Each type of product has different characteristics such as firmness, brix level, water content, etc., therefore these would need a different type of fluidization for optimal result. Processors need to learn how to adjust the fluidization to achieve the best appearance and separation.
There are three main types of fluidization in IQF freezing—circulating fluidization, bubbling fluidization and fixed bed fluidization—each used in a different part of the freezer.
Circulating fluidization lifts the product from the bed under strong, cold airflow and is usually used for crust-freezing in the first stages of freezing.
Bubbling fluidization, also called semi-fluidized bed, is a lighter version of the circulating fluidization, and partially lifts the product from the bedplate. This is usually the second stage of the freezing process.
Lastly, the fixed bed fluidization is usually used in the last part of the IQF freezer tunnel, as it finish-freezes the product. This type leaves the product resting on the bed, making it suitable for very brittle products.
How to create the right fluidization?
There are a few main elements in the IQF freezer playing a central role in obtaining the right fluidization in each part of freezer—the fans, the bedplate and the overall freezer design.
Important to note is that not all type of fans can help in creating the right fluidization. The vane axial fans, for instance, allow the efficient use of frequency converters. This translates into a 100% controlled speed of fans, which can be increased or decreased depending on the product. IQF freezers that do not present the possibility of using frequency converters must find a way to bypass the extra airflow, which translates in vast, unnecessary energy waste in both the freezer and the refrigeration plant.
The pressure drop, which is created on the bedplate, depends on the strength of the airflow generated by the fans and on the design of the bedplate on which the product is landing. The size and distribution of the holes in the bedplate are of crucial importance. A thick bedplate is needed for allowing thicker holes.
The last important factor is the aerodynamics of the freezer body. The way the freezer body is built and designed directly influences the degree of fluidization control. Minimum parts and details to obstruct the airflow is key to a truly adjustable fluidization for each product.