Research and application of the hottest wet cold s

2022-10-23
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Research and application of wet cooling storage technology in fruit and vegetable preservation

Abstract: This paper discusses the energy storage refrigeration technology and theory, focuses on the application and research of wet cooling storage technology in fruit and vegetable preservation, and expounds its technical characteristics, advantages and application and development prospects

key words: application of wet cold storage and fresh-keeping technology

introduction

at present, the storage and fresh-keeping food processing of agricultural products in China is in the development stage, There is an urgent need for a large number of fresh-keeping and storage technologies and the establishment of the city "With the platform serving enterprises, China's food experts pointed out in their prediction of the food processing market that the scale of China's agricultural products and food processing depends on the proportion of the application of fresh-keeping storage technology and equipment, the capacity of fresh-keeping storage and the length of the warehouse's turnover period. This shows that the level of fresh-keeping storage technology and equipment, and the scale of application directly reflects the scale of China's agricultural products and food processing and production And level. That is, whether fruits and vegetables are sold or exported or processed, storage and preservation are involved, which is not only a basic condition, but also a crucial link

there are many kinds of storage and preservation technologies and methods. At present, the technologies that have been adopted include: wet cold preservation technology, controlled atmosphere technology, micro freezing technology, decompression technology and radiation treatment technology, air ion preservation technology, chemical agent treatment technology, etc. Based on the principle of energy storage technology, this paper mainly analyzes and discusses the prospect of research and application of wet cooling (cold storage) preservation technology

1 energy storage technology

the current energy storage technology mainly uses the sensible and latent heat effect or chemical reaction process of the state change of the working medium to store energy. Every substance has a certain heat capacity. When the form of the substance remains unchanged, the substance will absorb or release heat. Theoretically, all substances can be applied to energy storage technology, but in fact, substances with larger specific heat capacity are applied. At the initial stage of the application of energy storage technology, which can also reduce pollution, the most widely used is cold and hot water energy storage technology. Cold and hot water energy storage uses water with low price, convenient use and large specific heat capacity as the energy storage medium, and uses the sensible cooling of water for energy storage. Its advantages are: less investment, simple equipment, easy maintenance. The disadvantages are: the temperature difference of energy storage is small, the density is small, and it is not possible to store a large amount of energy

1.1 cold storage technology

cold storage technology uses the characteristics of absorbing or releasing heat when substances change phase to store or release energy. At present, it mainly includes ice cold storage technology and eutectic salt energy storage technology

ice storage technology is the most widely used cold storage technology at present. It uses the phase change latent heat of ice to store and release the cooling capacity. Because the phase change latent heat reaches 335kj/kg when solid ice is transformed into liquid water, the unit mass energy density of ice storage is much higher than that of water storage. Therefore, the volume of the cold storage tank required by ice storage is much smaller than that of water storage, and it is easier to arrange in or around the cold storage. Due to the low temperature of ice water, the supply of ice water and the air supply volume of air conditioning system can be reduced under the same air pressure load. The investment and operation cost of equipment system using ice storage are lower than that of water storage

the disadvantage of ice storage system is that the freezing point temperature of water is 0 ℃. Considering the heat transfer temperature difference, the evaporation temperature of the refrigeration system must be below - 8 ℃. For the ice storage system with indirect heat exchange of refrigerant, the evaporation temperature of the refrigeration system is even lower. Due to the low temperature of refrigeration, ice storage tank and ice water pipeline, the thickness of insulation layer must be increased in order to avoid condensation outside the ice tank and pipeline and reduce the transfer of environmental heat. The technical requirements of ice storage system are high, especially the recently proposed ice mud type storage refrigeration system, which makes the design and control of ice storage refrigeration much more complex than water storage refrigeration

in order to meet the requirements of low temperature for the accuracy required by flexible packaging, ice storage refrigeration technology can adopt general lead screw and trapezoidal lead screw to meet the requirements of low temperature, and overcome the shortcomings of increasing refrigeration energy consumption and reducing COP value due to low evaporation temperature. Eutectic salt with solid-liquid phase change temperature higher or lower than water can be used to store energy. Eutectic salts are mainly composed of inorganic salts, water, coagulants and stabilizers. At present, eutectic salt materials with phase transition temperature of about 8 ℃ are mostly used in cold storage and refrigeration systems

due to the particularity of high phase transition temperature of eutectic salt, the refrigeration capacity can be increased by 25% - 30%, COP value can be increased by about 15% and refrigeration energy consumption can be greatly reduced after being used in ice storage refrigeration system. The disadvantage of using eutectic salt is that the latent heat of phase change is low. When storing the same amount of cooling, the volume of eutectic salt cold storage is larger than ice cold storage, but smaller than water cold storage

1.2 working principle

when the electrical load is at a low point (i.e. power consumption at night), the ice storage refrigeration system starts and starts the cold storage work. According to the principle of thermal work cycle, the cold storage technology can be divided into four basic cycles due to the different working media. In Figure 1, (a) cycle is mainly applicable to the cold storage working cycle with water or other high boiling substances as refrigerant. (b) and (c) in Figure 1 are applicable to the energy storage working cycle with ammonia or other low boiling points as refrigerant. (d) in Figure 1 is a working cycle that converts heat energy into refrigeration and storage

since the density of cold storage depends on the type of refrigerant and the concentration difference of solution, the key of cold storage technology is the energy storage density of working medium. The greater the energy storage density, the smaller the cold storage equipment, the less the investment, and it is also easy to be adopted. The energy storage density can be calculated with the following formula: cdestor = CP· Δ T + P · L · IPF formula: CP is the specific heat of the energy storage medium; Δ T is the available temperature difference of sensible heat storage; P is the density of the energy storage medium; L is solid/liquid phase change heat; IPF originally means ice making rate, which is extended here as crystallization rate

for cold water storage, the temperature difference that can be cited is only 6 ~ 11 ℃, and the storage density is only 25 ~ 46kj/kg. For ice storage, the IPF value of different storage systems is between 20% and 75%, the available temperature difference is 10 ~ 12 ℃, and the storage density is 110 ~ 295kj/kg. At present, the storage density of other storage methods is lower than that of ice storage

2 application of wet cooling storage technology in fresh-keeping storage

wet cooling storage technology, namely ice storage technology, is a new fruit and vegetable fresh-keeping technology that was not popularized and used abroad in the 1980s. Because more than 80% of the damage of metal parts of this technology is caused by fatigue, the technology has better adaptability than the storage methods such as controlled atmosphere storage and ordinary cold storage, so it has been widely used in Europe and the United States and other economically developed countries

2.1 characteristics and advantages of wet cold storage and fresh-keeping technology

wet cold storage and fresh-keeping technology integrates precooling, high humidity and low temperature storage of fruits and vegetables. Using the cold storage technology and the price difference between peak and valley electricity, the ice made at night is forced to exchange heat and mass between the high humidity cold air and the gas in the fresh-keeping warehouse through heat and mass exchanger, water pump and fan, so as to realize the cooling of the cold storage and the high humidity fresh-keeping of fruits and vegetables, Thus, the water content of fresh-keeping products, that is, the original quality of fruits and vegetables, is maintained, and the shelf life is prolonged. Among many fresh-keeping technologies, the wet cooling cold storage and fresh-keeping technology has a small installed capacity (the installed capacity can be reduced by 1/3 compared with the same cold storage), and the investment is less, because the eutectic salt technology can make the cold storage temperature of ice reach – 8 ℃, which meets the temperature requirements for fresh-keeping of fruits and vegetables without being injured by freezing

fruits and vegetables are prone to mildew and decay in high humidity environment. Before warehousing, they can be soaked in ozone water to achieve the purpose of sterilization and disinfection and prevent mildew and decay

Jia Jun, Hu Weixiang, Junhong, Ren Changqing (Tianjin agricultural machinery testing and identification station of Tianjin Agricultural Machinery Bureau)

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