A GUIDE THROUGH DIFFERENT FREEZING TECHNOLOGIES
A guide through different freezing technologies
For many food processors, choosing the right freezing equipment is the greatest challenge at the start, as the choices seem wide and many times confusing. This guide will try to give a better understanding of the differences between the many types of freezers out on the market.
As a start, it is important to mention that freezing time is one of the most important parameters in the freezing process. The freezing time represents the time needed to decrease the temperature of the product’s thermal centre to a specific temperature below zero and the reason why it is so important is because it determines the quality of the final product and the operation costs.
The International Institute of Refrigeration has defined a number of factors which influence the freezing time and those include: the shape and dimensions of the product, the in-feed and out-feed temperatures, the temperature of the refrigerating medium, the surface heat transfer coefficient of the product, the change in the total heat content in the system and the thermal conductivity of the product. Therefore in order to get the optimal freezing time and by extension, the optimal freezing results, the right equipment should be chosen for specific products.
There are 9 main types of different freezers and most of them have more than one naming:
- The air blast freezers / cold storage freezers
- The cartoon freezers / box freezers
- The spiral belt freezers
- The fluidized bed freezer / IQF freezer (or tunnel freezers)
- The immersion freezers / brine freezers
- The plate freezers / block freezers
- The contact belt freezers
- The impingement freezers / flat product freezers
- The cryogenic freezers
Air Blast Freezer or Cold Storage Freezer
Probably the oldest type of freezers, these are using still or forced air as a medium and the product is kept static in freezing storage rooms. Still air freezer or cold storage is the simplest method with the lowest investment costs. It is most suitable for large or unprocessed products, however it is the slowest freezing method. Forced air freezer is the improved version of cold storage and it is using convection to circulate cold air in the freezing room. However, with this technology the freezing time is still long as the airflow is not sufficiently controlled in the freezer leading to low surface heat transfer. As it is the cheapest freezing method, air blast freezers are used on a wide range of products.
In some alternative versions of this method, the product is placed on rack trays and is frozen inside the cold storage with the help of cold air circulation. The trolleys or racks can be moved and replaced manually or by specialized trucks, however the high manpower needed for operation is one of the main disadvantages to this method. The advantage though is that it can be suitable for many types of products.
Cartoon Freezer/ Box Freezer
Cartoon freezers, also called box freezers are mechanically complicated freezers, based on relatively simple freezing concept. Products already packaged and placed in boxes are sorted, transported and stored mechanically by automated mechanism, placing them on shelves in a storage with cold blasts of air. These boxes are then left for usually long amounts of time, being ultimately unloaded also mechanically. The whole mechanism is automated and needs very limited manpower however it is not suited for many types of products.
There is a number of freezing technologies which are using belt as transportation mean of the product. The main advantage of the technologies using belts is the extremely accurate control of transportation process, while the disadvantage can range from maintenance requirement, difficulty of cleaning or belt marks left on the product. We will further present the main types of freezers using belts for product transportation.
Spiral Belt Freezer
In this case, the belt is bent around a central supporting structure, maximizing the belt surface in a limited space. The spiral belt freezer can be a good solution for gentle products as it minimizes product damage at transfer points. This technology also presents the advantage of very large amount of square metres on a limited footprint allowing high capacity and continuous production with limited man power. However the spiral belt freezer still presents the disadvantages of the belt.
Fluidized Bed Freezer or IQF Freezer (Tunnel Freezer)
Sometimes referred to as tunnel freezer in South and North America, the fluidized or IQF freezer is based on a belt or a perforated bed on which the product is fluidized with strong vertical airflow which is passing from beneath the bed/belt. This is the most complex freezing technology as the product is not statically frozen thus multiple variables such as shape of the product, aerodynamics, ripeness, firmness or water content of the product will all influence the freezing result.
The advantage of an IQF freezer is its high air velocity which is suspending the product in the cold air stream therefore separately freezing each piece of product without creating lumps. This technology is specifically optimal for unpacked, small items which tend to lump such as shrimp, peas, diced meat or diced/sliced fruits and vegetables. In addition, this is a very quick process therefore dehydration of the product is being minimized. There are already several solutions in terms of bed types depending on the producer, ranging from plastic or wire conveyor belts to removable and perforated bed plates.
The next 3 types of freezers are part of the same bigger category called the Contact Freezers. When it comes to heat transfer rate, the contact freezers are some of the most efficient types as the product becomes in direct or indirect contact with the freezing medium. Therefore we can categorize them into direct contact freezers also called immersion freezers and the indirect contact freezers which can be of 2 types: plate freezers or contact belt freezers. We will further describe each of these types of technology.
Immersion Freezer or Brine Freezer
In the immersion freezer the product is completely emerged in a tank with a cooled freezing medium which can be chemicals or mixtures with salt or sugar. Immersion method is the fastest freezing method and it is commonly used as a pre-treatment of large products in order to create a frozen layer before the product is exposed to longer freezing time, in order to avoid dehydration. The challenge with this method is that the solution becomes quickly diluted with the product which can change the process speed and efficiency.
Plate Freezer or Block Freezer
This is the most commonly used contact freezer and it uses a method of pressing the product between two hallow plates with a refrigerant circulating inside those plates. However, this type of freezers is only suited for regular shape products or small products block-frozen in a liquid (e.g. seafood). This technology is used as a first step in the processing of seafood as a fast freezing solution for conservation purposes.
Contact Belt Freezer
Consisting of a single band or double band on which the product is laid, the contact belt freezer is usually used for freezing thin product layers such as egg yolks, soups, fruit pulps, or other very soft products.
Impingement Freezer / Flat Product Freezer
The impingement freezer is moving the product through the freezer on a belt or wire mesh while high velocity air is blown on the product. This freezing method is most suitable for flat products such as hamburgers. However this technology can be challenging for too big or too small products.
This technology consists of exposing the product to an atmosphere of aprox. -60°C through direct contact with liquefied gases, usually nitrogen or carbon dioxide. There two main types of cryogenic freezers: the ones using immersion or dipping of the product into the medium and those which are spraying the medium on the product. In both types the product is continuously transported on a belt to the other end of the freezer. Usually those cryogenic freezers using dipping method are consuming more medium but are getting better freezing results, while the spraying method insures lower medium usage, therefore lower costs. The advantages of the cryogenic freezers is their small size, mobility and the good freezing results. However, high cost of the liquefied gases and constant dependency on suppliers is seen as an important drawback for this type of freezers, especially for processors located in remote areas.
All in all, the selection of the right freezer is not simple and cannot be limited to the “cost-to-freeze per kg” rational. Every food processor has to carefully balance all the important elements such as the cost of investment, operating costs, regulatory compliance costs, product type and desired freezing quality – all being important in determining the optimal freezing solution for the processor.