The use of liquid nitrogen as a refrigerant to freeze food directly has been developed internationally since the 1960s and gradually matured in the 1980s, and is now widely used for all kinds of food except vegetables. The principle of liquid nitrogen freezing is to spray low-temperature liquid nitrogen directly on the food, the use of its atmospheric pressure vaporization of the very low temperature (-196 ℃) and direct vaporization of the material surface of the high coefficient of heat transfer, so that the food quickly and y frozen. Freeze 1kg of food, liquid nitrogen consumption of 0.7-2kg, depending on the variety of materials and different sizes, nitrogen in the air content of 75%, liquid nitrogen is generally a by-product of oxygen production, by the chemical reduction in the atmosphere, no pollution of the environment. Liquid nitrogen quick-freezing machine for the tunnel type, insulation board for polyurethane foam sandwich layer, double-sided stainless steel panel, material transfer for the stainless steel mesh belt, driven by the speed motor, the equipment can be lifted and lowered, can be elevated in the maintenance, cleaning. The tunnel consists of pre-cooling section, spraying section and homogenizing section.
The pre-cooling section is mainly to pre-cool the material moderately to make full use of the sensible heat of liquid nitrogen vaporization, and to avoid too large a temperature difference in deep-freezing, so that the thicker materials have a tendency to crack; the spraying section is under the pressure of the liquid nitrogen tank, through the nozzle to the surface of the material directly spray droplets to carry out rapid freezing, and the homogenization section to make the surface of the material is not fully vaporized droplets can be fully vaporized. The vaporized low-temperature nitrogen is pumped out by the exhaust fan, and the material is stirred with the low-temperature nitrogen on the stainless steel mesh belt and sucked by the fan and the exhaust fan to carry out the countercurrent heat exchange. Due to the short freezing time, and mainly by liquid nitrogen direct vaporization heat transfer, so the frozen product dry consumption is much smaller than the air-cooled method. At present, in foreign countries, liquid nitrogen freezing method has been widely used in seafood, whole fish, meat pie, split meat, pies, hamburgers and other seasoned foods, as well as ice cream hardening and other fields.