The technology combines cryogenic temperatures with atomized liquid nitrogen for higher heat transfer

The BOC Group has combined cryogenic impingement with atomized liquid nitrogen to maximize frozen-food production in minimum floor space. Food Engineering interviewed Michael Newman, technology development manager for U.S. food markets at BOC Gases (Murray Hill, N.J.), to learn about the technology.

FE: How would you define impingement heat transfer?

Newman: Basically, impingement is a form of convective heat transfer where air flow is perpendicular to the surface of the product. Unlike a conventional axial-flow fan, impingement generates very high, localized velocity jets through radiused holes or v-shaped slots. In cryogenic impingement, the jets are cold gas at temperatures as low as minus 250 F, resulting in a dramatic increase in convective heat-transfer coefficients to the surface of the product. At velocities of 20 to 30 meters per second, these jets break-down the surface boundary layer of gas which surrounds the food product. As you minimize that surface layer, you maximize the amount of convective heat transfer you can deliver to the product. When comparing the overall heat-transfer coefficients of cryogenic freezing tunnels, impingement heat transfer is typically three to five times that of a conventional tunnel utilizing axial-flow fans.

FE: How does cryogenic impingement differ from impingement freezing technology introduced several years ago?

Newman: What was available earlier was mechanical impingement freezing. The air flow principle in a cryogenic impingement freezer is the same as in a mechanical impingement freezer. But we've taken cryogenic impingement one step further. At the inlet zone of the freezer, we've combined high-velocity impingement airflow with atomized liquid nitrogen sprayed on the surface of the food product. This creates evaporative cooling which is accelerated by the high-velocity impingement airflow.

FE: What's the difference?

Newman: Evaporative heat transfer occurs as a result of the large amount of heat required to produce a phase change from liquid to gas. Extremely high heat transfer is achieved when this process occurs directly on a surface to be cooled. Liquid nitrogen applied to a warm food product creates very high evaporative heat-transfer rates,

This evaporative cooling with liquid nitrogen can be controlled and optimized by controlling droplet size, spray distribution and gas flow. Impingement gas flows accelerate the evaporation rate to increase the resulting surface heat transfer coefficient.

Marrying these two heat-transfer processes led to the development and commercialization of a cryogenic impingement freezer.

FE: How does freezer design integrate these two processes?

Newman: The full-scale impingement freezer is of modular design, with the first module combining atomized liquid nitrogen injection with impingement heat transfer from both above and below the product. All nitrogen for freezing is injected into this zone. Combining the two heat-transfer processes produces a heat transfer coefficient, or U factor, in that initial zone three times higher than in a conventional mechanical impingement freezer. Tests compared the difference in heat transfer coefficients between an impingement configuration with separate liquid spray zone and one that combines liquid spray with impingement. The combination of spray and impingement resulted in a 25-percent increase in overall heat transfer coefficient (4-module or 40-ft. unit).

FE: What follows the injection zone?

Newman: Subsequent modules are simply impingement gas-flow zones, with U factors comparable to mechanical impingement. Impingement heat transfer is applied from both sides of the product in each zone. Gas flows down the length of the unit, and can be fine-tuned by adjusting operating pressures in each of the independently-controlled freezing zones. Modules are 10 ft. in length and the freezer can be built to the desired length.

FE: What are the advantages of cryogenic impingement freezing to the frozen-food manufacturer?

Newman: First, reduced product dehydration. The combination of atomized liquid nitrogen and high-velocity impingement air flow crust-freezes the product almost instantly to minimize dehydration. Although we have yet to exactly measure dehydration loss, it's well below .08 percent as compared to .45 to .50 percent in a conventional tunnel freezer. That's one of the big benefits of cryogenic over mechanical.

Because you can run at much colder temperatures than a mechanical freezer, you can achieve higher production rates in smaller floor space. Typically we can achieve the same production rate in one-third the floor space of a conventional cryogenic freezer.

With the increased overall heat-transfer coefficient, one can either increase the freezing temperature to increase overall cryogen efficiency, or run at colder temperatures and dramatically increase the overall production rate. This relates to energy savings and, more importantly, to cryogen savings. There are many options because you can precisely control air flow, offering manufacturing flexibility.

Another benefit is the impingement flow pattern, which evenly distributes air flow across the length and width of the freezing zone. We apply these high-velocity impingement jets above and below the surface of the product and evenly across the width - something a conventional cryogenic freezer can't accomplish because the velocity profiles off conventional axial-flow fans are erratic and different.

FE: For what kinds of products is cryogenic impingement freezing best suited?

Newman: The process is best suited to products with high surface-to-weight ratios such as hamburger patties, chicken filets and fish filets. Tests show that products with a thickness of less than 20 mm freeze most effectively in an impingement heat-transfer environment. It can also be used to quickly crust-freeze thicker products, even IQF products such as shrimp. Products which tend to stick to a belt aren't as well suited, but just about any product you want to initially crust-freeze is a good candidate. We've also tested some bakery products, where the option to operate the system at warmer temperatures yet maintain a high production rate is an advantage because -- when freezing a dough product -- you can't lower the surface temperature beyond minus 55 degrees F without adversely affecting the yeast for rising later when thawed.

The impingement module might also be applied as a front-end "production booster" to a mechanical freezer, such as a spiral, to initially crust-freeze a thicker product before subjecting it to the longer retention time needed to thoroughly freeze the thicker product. Here you get the benefits of minimum dehydration and reduced energy costs because you can run the spiral at warmer temperatures.

FE: Has the technology been commercialized yet?

Newman: We're getting ready now for the official rollout. We introduced the technology in a low-key way at the International Poultry Show last January.

We currently have four machines installed in a burger plant, where they replaced conventional counter-flow nitrogen tunnels, and they're finding dehydration reduced from .45 to .50 percent to less than .08 percent. That's about a fivefold improvement in dehydration savings alone, equivalent to more than $250,000 per year. When you apply that to a higher-value product the benefits can be even greater.