Tunnel freezer are used to quickly freeze and chill a variety of food products for use in restaurants, grocery chains, warehouse superstores, and other retail outlets. Tunnel freezers freeze meat cuts, burger patties, and other types of food, as well as bread or dough products and parbaked foods. They can be designed to produce a wide variety of products and are available in a number of different sizes.
The basic design of a tunnel freezer consists of a conveyor belt carrying the food product into a system that quickly and evenly freezes it. The product passes through a tunnel of circulating cold air, which is injected with cryogenic gases such as liquid nitrogen or carbon dioxide. The injection system and fans circulating the gas-chilled air ensure that all surfaces of the food product are quickly and thoroughly frozen.
Several types of tunnel freezers are used in the production of IQF food products. Some types of tunnel freezers are used as stand-alone freezers, while others are part of larger systems that include immersion or mechanical systems. The main differences between these different types of tunnel freezers are the type of conveyor belts they use, the temperature and pressure control systems, and how quickly they freeze products.
Most tunnel freezers are equipped with a conveyor belt made from a special material that is not damaged by the freezing process. These conveyor belts are usually stainless steel. The tunnels are also equipped with heat exchangers, which help to recirculate the cryogenic gas and keep it from being depleted by the evaporating heat in the product as it is being frozen.
Another important feature of a tunnel freezer is an oxygen balance system. This helps to prevent product clumping and agglomeration, as well as keeping the cryogen levels stable. Most of these systems have automatic temperature and pressure control systems that monitor the temperature, pressure, and flow of the cryogen to compensate for incoming or outgoing product temperatures, resulting in more consistent quality and higher yields (Tomlins 1995).
Some tunnel freezers are fitted with flighted conveyors, which gently tumble the individual food pieces while they are being frozen. This is particularly useful for high-value and often delicate IQF products, such as scallops, strawberries, and other fruits, chicken pieces, seafood, meatballs, and pizza toppings. This prevents the products from sticking together and allows them to be frozen more evenly, resulting in better quality and higher yield.
Many tunnel freezers are cooled with either liquid nitrogen or carbon dioxide. Both of these gasses are very effective at freezing products without damaging their cell structure. Carbon dioxide is an oxygen-free refrigerant that can inhibit bacterial growth in some products, however, this effect is not permanent and can be lost over time.
It is critical to understand the risks of working with oxygen-deficient gases like nitrogen and carbon dioxide. Employees who are not adequately trained or monitored could become disoriented, lose consciousness, or even suffocate if they accidentally inhale these gases. In order to minimize these risks, all tunnel freezers should be connected to a properly functioning gas supply system that can safely deliver the proper quantity of gas to the machine in an emergency.