Liquid nitrogen or liquid carbon dioxide are purchased and kept in a pressurized storage vessel. The cryogen is piped as a liquid into the freezer unit and applied directly to the product in a variety of modes depending on the cryogen, freezer type or food product. However, the cryogen is a consumable product and except in very unusual circumstances, can only be used once.
Cryogenic freezing systems are characterized by a moderate capital investment, minimal preventive maintenance cost, and a smaller and more flexible commitment of plant space (cryogenic freezers can be installed or removed over a weekend). However, liquid nitrogen or liquid carbon dioxide pricing and availability vary based on the geographic location of the processor, and the cost of refrigeration purchased this way can be more than triple the cost of mechanically produced refrigeration.
In contrast to mechanical freezing, cryogenic freezing tends to “crust” the outside of the product and prevent (insert pictures) excessive moisture loss. Depending on the product, this could be viewed as a quality advantage.
Major Types of Cryogenic Tunnels
Flat Belt Tunnel Freezer
The belt is flat the entire length of the freezer. Can be LN2 or CO2
Multi-pass Tunnel Freezer
The belt is flat the entire length of the freezer, but there are three (or more) stacked belts. At the end of the first tier, the product is discharged to the second tier and returned to the entrance end of the freezer and discharged again onto the third tier and conveyed back to the exit end of the freezer. Can be LN2 or CO2
Flighted Belt Tunnel Freezer
The conveyor system is made up of three or more short inclined belts, which tumble the product from one flight to the next. This is excellent for small food products that tend to freeze together when touching. By keeping the individual pieces of food product in motion, large volumes of small food products such as diced meats, berries or vegetables can be individually quick frozen (IQF). Can be LN2 or CO2
Food product is conveyed through a bath of liquid nitrogen. It is the fastest heat transfer rate available in cryogenic freezing, but is characterized by lower cryogen efficiency rates (higher nitrogen usage) than those achieved with other types of tunnels. However, LN2 immersion freezers mated to spiral freezers (either cryogenic and mechanical) have proven to be highly effective in increasing productivity and yield (reduced dehydration costs) and preventing damage to the bottom of the food product from the spiral belt. LN2 only.
The outward appearance of the spiral is that of a large square box. Inside, the belt spirals around a center drum, each layer a few inches higher. Product is normally fed into the bottom of the spiral, and exits at the top of the box. Although heat transfer rates are slower than other cryogenic equipment, this unit can contain as much as 450 feet of belt, allowing for very high production rates. Can be LN2 or CO2
Cabinet (Batch) Freezer
Looks like a large cabinet. Food product is loaded onto trays, slid into a rack, and the rack is pushed into the freezer. Rack and product is removed from cabinet after freezing is complete. The unit is takes up very little floor space, but production is usually limited to a few hundred pounds per hour. Although this is not an inline freezer, its relative low cost and versatility make this unit a favorite of start-up processors and new product development projects. Can be LN2 or CO2
A long, inclined, rotating tube. Using gravity and internal design to convey the product, this unit is used for small food products that tend to freeze together when touching. Like the flighted freezer, it produces an IQF product. CO2 only
How LN2 Works in a Cryogenic Tunnel Freezer
Most straight belt nitrogen tunnels (the most common type) take advantage of the refrigeration value of converting liquid nitrogen to vapor (Latent Heat of vaporization, 86 BTUs per lb), and then blow the very cold vapor over and over the product in order to remove as much refrigeration as possible before exhausting the vapor from the freezer (a process known as “vapor-stripping”). Flat belt LN2 tunnels are designed so that the LN2 is sprayed on the food product at the exit end of the freezer, and the cold vapor forced back towards the entrance of the freezer. In this way, the available refrigeration in the vapor is used most efficiently.
How CO2 Works in a Cryogenic Tunnel Freezer
Liquid CO2 acts very differently in a freezer than liquid nitrogen. CO2 is piped to the tunnel as a high pressure liquid (300 psi), but once it exits the injection orifice, it instantaneously expands into a mixture of gas and tiny dry ice solid particles (at -109F). The dry ice solid, commonly referred to as dry ice “snow” is driven into the surface of the food product, where the heat from the food product rapidly causes the dry ice to “sublimate” or phase directly from a solid into a gas. The refrigeration effect of CO2 occurs as a result of the latent heat of sublimation (246 BTUs per lb of solid CO2 or more commonly represented as 120 BTUs per lb of liquid CO2). Where nitrogen tunnels are able to use refrigeration from both the vaporizing of liquid and warming up the vapor, CO2 tunnels are primarily designed to use refrigeration from dry ice snow sublimation. A CO2 flat belt tunnels look much the same as a convention LN2 tunnel, except that LCO2 is injected on the product immediately after it enters the tunnel and almost continuously for about 70% of the length of the tunnel.
How CO2 and LN2 Work in Spiral Freezers
As a rule, nitrogen and carbon dioxide are used very similarly in a spiral freezer. Both cryogens are injected across the belt from adjacent manifolds. Both liquid nitrogen and CO2 snow particles are vaporized almost immediately, and maximum use is made of the refrigeration in the vapor form by extensive air movement from fans located primarily along the outer edges of the belt (vapor stripping). This freezer type makes highly efficient use of both cryogens.