Understanding Cryogenic Freezing Tunnels
Cryogenic tunnel freezers are suited for processing large quantities of products in a small footprint while increasing product yield.
The objective of all cryogenic tunnel freezers is to quickly freeze or cool products in an efficient manner using a cryogenic liquid. Suited for processing large quantities of products in a small footprint, cryogenic tunnel freezers can increase product yield compared to alternative technologies. Cryogenic tunnel freezers also result in less product dehydration while retaining product textures.
Although these are significant advantages over mechanical freezing systems, the most important concern for many food processors remains product freezing cost. It is critical, therefore, to design a cryogenic tunnel freezer to operate as efficiently as possible while incorporating the processor’s layout and required production capacities. In its basic form, a cryogenic tunnel freezer consists of four primary parts:
- An insulated enclosure that houses a conveyor belt to carry the products.
- An injection system to inject the cryogenic liquid.
- Circulation fans to improve heat transfer with the products.
- An exhaust system to evacuate excess gases.
To get the most efficient cryogenic freezing system, it is necessary 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 percent of the refrigeration capacity will be obtained during the sublimation of the solid CO2 (latent heat) and 15 percent in the cold vapors (sensible heat). For a nitrogen system, about 50 percent of the refrigeration capacity is obtained during the vaporization of the liquid (latent heat) and 50 percent from the cold vapors (sensible heat). Once it is understood where the refrigeration capacity of the cryogen being used is found, it is possible to begin to choose a type of freezing tunnel that will best and most efficiently utilize the cryogen.
The most basic cryogenic tunnel freezer is a linear tunnel freezer, which consists of a single, linear conveyor.
A good distribution of CO2 throughout the tunnel freezer is 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 primarily is reflected in the exhaust gas temperature. If the exhaust gas temperature is too low, there will be a reduction in the sensible heat of the nitrogen gases available to freeze the products. This results in higher nitrogen consumption and a less-efficient freezing process.
For a nitrogen system, it is necessary to place the injection header strategically to allow for a spray of liquid and, at the same time, obtain good recirculation of the nitrogen gases.
Comparing Freezing Tunnel Designs
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 products’ 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 counterflow or co-flow system.
Counterflow refers to the fact that cryogen is injected at the exit end of the tunnel freezer. The gases are directed toward the entrance end of the freezer, in the counter direction of that of the product flow. The exhaust gases are extracted at the entrance side of the freezer. A counterflow cryogenic freezer provides the longest cryogen exposure to the product and, therefore, creates the most efficient system.
In some cases, a multi-pass freezer design is more practical such as when the required product dwell times or capacities are too high for a linear tunnel freezer.
With the co-flow freezing tunnel design, the cryogen is injected at the beginning of the tunnel. 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. For this reason, the system will be less efficient. This design is used primarily with hot products where it is important to cool the surface of the products quickly to reduce dehydration.
A variety of special designs also are available to meet the needs of virtually any application. Multi-pass freezers are another type of tunnel freezer. They are designed to reduce floor space and boost throughput by housing multiple product conveyors in a single machine. In some cases, this design is practical such as when the required product dwell times or capacities are too high for a linear tunnel freezer. In comparison to counterflow designs, however, multi-pass freezing tunnels are slightly less efficient due to the reduction in airflow and freezer length.
Flighted freezers are configured with multiple staggered conveyors to promote individually quick frozen (IQF) freezing of bulk-type products by continuously agitating the products. By exposing all sides of the bulk products to direct spray and ventilation, the flighted freezer design provides a consistent IQF product.
Dual-belt freezers have two conveyor belts that operate side by side in one machine housing. They allow processing of two different types of products with independent controls for belt speeds, ventilation and temperature. This is a unique setup that provides flexibility for demanding applications.
Plate-belt freezers house a belt configured of solid stainless steel plates, allowing processing of soft and delicate products. By eliminating the typical wire-mesh belt, product belt markings also are eliminated. This type of freezer is especially useful for high-end products where product markings are unacceptable.
Multi-pass freezers are designed to reduce freezer floor-space requirements and boost freezer throughput by housing multiple product conveyors in a single machine.
High performance freezers are designed with multiple freezing zones to independently control airflow and nitrogen injection while maximizing throughput and efficiency. These systems are built to extract the maximum available energy from each pound of cryogen and reduce the freezing cost.
For products that are sensitive to freeze burn, gas-only freezers prevent product damage by avoiding product shock from direct liquid contact. This has been a good solution for bakery applications, and gas-only freezing tunnels allow for quick cooling while preventing any product damage.
A range of snow-removal systems can be integrated into tunnel freezers that process moist products. Removing and preventing snow buildup on fan blades and temperature sensors allows for a longer and more efficient freezer operation.
Based on the specific production requirements, tunnel freezers also can be combined with immersion freezers or spiral freezers to get the best layout and capacities needed.
Given the range of freezing tunnel designs, it is clear there are designs for nearly any food freezing process. PC