Characteristics of liquid nitrogen
Liquefied nitrogen-- liquid nitrogen is a colorless, transparent, low-viscosity gas with stable chemical properties. The boiling point of liquid nitrogen at atmospheric pressure is 77.35K. It does not react chemically with poultry food ingredients and meets the requirements of food hygiene. It is an ideal fast-freezing preservative. Nitrogen is about 78% in the air and is therefore inexhaustible in the atmosphere surrounding the Earth. It can't be used.
Mechanism of quick freezing and preservation of liquid nitrogen
Under atmospheric pressure, when the liquefied nitrogen changes to a gaseous state in a liquid state, the average latent heat of vaporization per kilogram of liquefied nitrogen is 199.29 kJ, and the heat absorbed from 77.35 K to 273 K is 202 kJ. . Liquid nitrogen quick freezing is the use of this ultra-low temperature endothermic property. Liquid nitrogen dispersion technology is used to disperse liquid nitrogen into gas directly to the desired bird body. At this time, the temperature difference is as high as 200K or more, and strong heat exchange can be performed to enable the bird body. Instant freezing, generally complete the freezing process in about 20 minutes, 30 times faster than the ordinary freezing method. Due to the short freezing time, the generated ice crystals are very small, the freezing speed is higher than the vapor permeation rate inside and outside the cells, and the ice crystals are generated inside and outside the cells, which have no destructive effect on the cells, so the original color, fragrance, taste and freshness can be maintained after thawing. The state is recognized as a first-class product in the world.
      Liquid nitrogen quick freezing process and device
The liquid nitrogen quick-freezing device suitable for quick-frozen poultry has a tunnel shape and is transported by a special stainless steel mesh belt. The bird body is placed on a stainless steel conveyor belt, and the frozen zone and the temperature equilibrium zone are sequentially frozen as the movement moves through the pre-cooling zone. After being sent out from the other end of the quick-freezing machine, the liquid nitrogen tank is placed outside the box, and is introduced into the equipment through a special stainless steel pipe. The liquid nitrogen system is carried out by the equipment to disperse the liquid nitrogen and pass through the tunnel, and the heat is vaporized and overheated. The nitrogen temperature is still very low, generally around 58K. At this time, the nitrogen is sent to another equipment for pre-cooling by the fan. Generally, the frozen chicken of 1 kg needs to consume 0.4 kg of liquid nitrogen.
      The liquid nitrogen tank storage tank is standardized by professional factory. The internal structure is metal vacuum interlayer, and the outside is also covered with polyurethane insulation material. The pressure is generally within 8KG, equipped with safety valve and liquid level gauge. Safety measures such as pressure gauges, the pressure value in the system can be freely adjusted to ensure that the specified value is not exceeded. The amount of liquid nitrogen stored in the tank can be expressed by the liquid level indicator. After use, the tank is filled with liquid nitrogen through the filling valve .
   Characteristics of fresh-keeping device for liquid nitrogen quick-freezing machine
1. Freezing quality and high quality
The main factor affecting the frozen quality of poultry foods is the size of ice crystals formed in their cells. The color, aroma and taste are directly related to the amount of damage in the cells during freezing. The old-fashioned freezing method takes a long time to form. The large size of the ice will deform and damage the cells, while the freezing time of liquid nitrogen is short and has a quick freezing property. This can be avoided completely. The ice crystals formed are very small, so the original freshness, color and flavor can be maintained. Nutritional value, and does not ignite or burn in the process of low temperature storage, can be stored for a long time; simple structure, safe and reliable. According to the analysis, the moving parts of the whole set are only the moving mechanism of the conveyor belt and the simple axial flow fan. Unlike the ordinary freezing system, the structure is complicated and there are many mechanical moving parts, which reduces the possibility of malfunction and the system work pressure is low. No high voltage equipment; easy to operate. It can be seen that the device is a belt conveyor and a straight line continuous operation. The operator only needs to operate a simple button outside the box and transport the frozen product outside the inlet and outlet; the power consumption is small. The device relies on liquid nitrogen phase change refrigeration and internal energy conversion to achieve the purpose of quick freezing and fresh keeping. Unlike ordinary refrigeration systems, it needs to consume a large amount of electric energy to drive refrigeration machinery refrigeration when freezing, so from the principle of refrigeration, it is No need to directly consume electrical energy; clean and no pollution, no noise. The freezer box is composed of assembled heat-insulating plates, and the inner and outer plates are made of stainless steel, which is easy to disassemble and easy to clean, and the reloading is also rapid. The conveyor belt that is in direct contact with the bird body is also made of stainless steel, which is more convenient to clean.