The objective of all cryogenic tunnel freezers is to quickly freeze or cool products using a cryogenic liquid in an efficient manner.
In its basic form, a cryogenic tunnel freezer consists of an insulated enclosure housing a conveyor belt to carry the products, an injection system to inject the cryogenic liquid, circulation fans to improve heat transfer with the products, and an exhaust system to effectively evacuate the excess gases.
Cryogenic tunnel freezers are ideal to process large quantities of products in a small footprint, while increasing product yield due to less dehydration and retaining product textures.
Although these are significant advantages over mechanical freezing systems, the most important concern is still product freezing cost. Therefore it is critical to design cryogenic tunnel freezers to operate as efficiently as possible, while incorporating the customer’s layout and required production capacities.
To get the most efficient cryogenic freezing system, the unit has to use as much of the available energy as possible from each pound of cryogen. The energy available in the cryogen will depend on a variety of factors; however the most important variables are the cryogen’s storage pressure and the exhaust vapor temperature of the freezer.
For carbon dioxide (CO2), about 85% of the refrigeration capacity will be obtained during the sublimation of the solid CO2 (latent heat) and 15% in the cold vapors (sensible heat).
For a nitrogen system, about 50% of the refrigeration capacity is obtained during the vaporization of the liquid (latent heat) and 50% from the cold vapors (sensible heat).
Once the refrigeration capacity of the cryogen is understood, one can begin to choose a type of freezing tunnel that will most efficiently utilise the selected cryogen.
A good distribution of CO2 throughout the tunnel freezer will be important to efficiently use the refrigeration capacity available in the CO2.
For a nitrogen system it will be necessary to strategically place the injection header to allow for a spray of liquid, and at the same time obtain a good recirculation of the nitrogen gases.
The efficiency of a nitrogen system is primarily reflected in the exhaust gas temperature. If the exhaust gas temperature is too low, we will see a reduction in the sensible heat of the nitrogen gases available to freeze the products, resulting in higher nitrogen consumption and a less efficient freezing process.
An efficient tunnel freezer allows for enough length to provide a proper heat exchange between the products to be frozen and the cryogen injected, while maintaining an exhaust temperature as close as possible to the product’s required equilibrated core temperature.
The most basic cryogenic tunnel freezer is a linear tunnel freezer, which consists of a single linear conveyor. A linear tunnel freezer can be configured either as a ‘counter flow’ or alternatively a ‘co flow’ system.
‘Counter flow’ refers to the fact that a given cryogen is injected at the exit end of the tunnel freezer and the gases are directed towards the entrance end of the freezer, in counter of the product flow. The exhaust gases would be extracted at the entrance side of the freezer that will provide the longest exposure to the product and therefore, create the most efficient system.
With the ‘Co flow’ principal, the cryogen is injected at the beginning of the tunnel and the gases travel with the direction of the product. Typically the exhaust gases are evacuated at the exit end of the freezer and will be much colder and therefore, the system will be less efficient.
This principal is mainly used with hot products, where it is important to cool the surface of the products quickly to reduce product dehydration.
gasworld would like to express its thanks to the CES Group, and both Hans Vanackere and Johnny Nuttin in particular, for providing this informative equipment profile.
Meeting any needs…
CES has made a variety of specially designed freezers over the past 22 years, to meet the needs of virtually any application.
Multi-pass freezers are another type of tunnel freezers that are designed to reduce floor space and boost throughput by housing multiple product conveyors in a single machine.
This design is in some cases more practical, especially when the required product dwell times or capacities are too high for a linear tunnel freezer, however they will be slightly less efficient due to the reduction in air flow and freezer length.
Flighted freezers are configured with multiple staggered conveyors to promote IQF freezing of bulk type products by continuously agitating the products. By exposing all sides of the bulk products to direct spray and ventilation it provides a consistent IQF product.
Dual belt freezers have two conveyor belts that operate side by side in one machine housing to allow processing of 2 different types of products with independent controls for belt speeds, ventilation and temperature. This is a unique setup that provides great flexibility for demanding applications.
Plate belt freezers house a belt configured of solid stainless steel plates allowing to process soft and delicate products. By eliminating the typical wire belt mesh we also eliminate product belt markings. Especially useful for high end products where product markings are unacceptable.
High Performance freezers are designed with multiple freezing zones to independently control airflow and nitrogen injection while maximising throughput and efficiency. These systems are built to extract the maximum available energy from each pound of cryogen and reduce the freezing cost.
Gas-Only freezers for products that are sensitive to freeze burn, prevent product damage by avoiding product shock from direct liquid contact. This has been a great solution for bakery applications and allow for a quick cooling while preventing any product damage.
CES also developed a range of snow removal systems to integrate in tunnel freezers that process moist products. Removing and preventing snow build-up on fan blades and temperature sensors allows for a longer and more efficient freezer operation.
Based on the specific production requirements, tunnel freezers can also be combined with immersion freezers or spiral freezers to get the best layout and capacities needed.
These Hybrid tunnel freezers are custom designed to get the most overall value.