Adjustment and control of spatial temperature difference in constant temperature cold storage area

Adjustment and control of spatial temperature difference in constant temperature cold storage area

The constant temperature cold storage is a special storage facility for storing fruits, vegetables, fresh eggs, flowers and other perishable and rotten products. Reasonable regulation and strict scientific temperature control are the key to ensuring the quality of commodity storage in constant temperature cold storage. Since the constant temperature cold storage objectively has regional temperature difference and spatial temperature difference, and can not maintain the absolute average and constant optimum temperature, this negative effect directly affects the overall quality of the refrigerated goods and the economic benefits of the enterprise. Therefore, effective technical measures must be taken to adjust and strictly control the temperature difference of the regional space, to minimize the temperature difference between various areas and different spaces in the constant temperature cold storage, so that all the stored goods are in the optimal temperature state. . This is also a major technical issue in the overall management level of constant temperature cold storage, which deserves further research and discussion.

First, the main reason for the formation of regional space temperature difference

There are several reasons for the formation of regional spatial temperature differences:

1. The structure of the commodity cold storage room; the commercial cold storage room of the constant temperature cold storage is a space composed of length, width and height. The length, width, height and low of the space are the basic reasons for forming the temperature difference of the regional space. Although the structure of the refrigerating compartment is designed to take into account the influence of the structure on the temperature difference, it is not technically possible to achieve a relatively uniform spatial temperature in the refrigerating compartment. In general, the larger the volume of the refrigerating compartment, the more difficult the convection and exchange of hot and cold air, and the greater the temperature difference of the regional space.

2. The physical specific gravity of hot and cold air; in the commodity storage environment, the temperature and temperature are determined by the exchange mode and frequency of hot and cold air. In the process of convection of hot and cold air, the hot air has different physical specific gravity, hot air. Gathered in the upper part of the refrigerating room, and the lowest point of the air temperature is often in the bottom layer of the refrigerating room. According to this physics principle, the temperature difference between the upper and lower spaces is inevitably formed in the refrigerating room. The higher the design height of the refrigerating room, the greater the temperature difference between the upper and lower spaces. .

3, the cold source is relatively concentrated; the general constant temperature library refrigeration equipment mostly uses box type air cooler, and the evaporation temperature of the air cooler evaporator is mostly designed around -15 °C, and the evaporator is usually fixed in one direction of the refrigerating room At a certain point, the cold source in the cold storage room is mainly derived from the evaporator, which forms a relative concentration of the cold source. Although the cooling fan is designed to take a long-distance forced air supply, it is often due to the long distance, large resistance, and large loss of air volume, which cannot achieve the purpose of temperature equalization, thus inevitably causing the temperature difference in the length region in a certain area. Some large refrigerated rooms have a length of 40 meters. Due to the distance, the temperature difference in the area is sometimes quite large. 

4. The stacking form of the commodity; the commodity is the main source of heat in the cold storage room. The variety, quantity, time, and density, height, width, and packaging form of the goods in the warehouse are all factors that cause the temperature difference in the area. In addition to cooling the air temperature in the refrigerated storage room, the cooling must continuously cool the heat emitted by the product due to the respiration. Due to the unreasonable unreasonable form of the stacking code, the heat dissipated during the storage of the product cannot be cooled in time, and the hot and cold air cannot be cooled. Effective exchange and circulation inside and outside the cargo pile will also cause temperature differences in the regional space, which will directly affect the storage effect of the goods. 

5. Other reasons; there are many other reasons for the formation of space temperature difference in the constant temperature cold storage area, for example: the difference in insulation performance of various parts in the cold storage structure, the existence of various cold bridges, the sunny side of the refrigerating room, and the overload of the power equipment in the warehouse. The operation, the frequent opening of the warehouse door, the irregular frosting process, etc. are all direct and indirect causes of the spatial temperature difference in the constant temperature cold storage area.

Second, the situational analysis of regional spatial temperature difference in the cold storage room

The temperature difference of the regional space in the cold storage cold storage room is complicated, and the forms are different. It presents a variety of situations. Taking the constant temperature library of Xingtai Fruit Company as an example, a brief analysis of the regional spatial temperature difference in the cold storage room is made. Xingtai Fruit Company is a 3,000-ton constant temperature library. A 300-ton cold storage room has a building structure of 40 meters long, 15 meters high and 5.5 meters high, with an area of ​​600 square meters and a space volume of 3,300 square meters. Two sets of box-type air coolers of 350 square meters are set according to design requirements. The temperature index is 0 ° C, which belongs to the larger refrigerated room. In the actual storage and temperature control process, most of the space in the refrigerating room can not reach or exceed the specified index temperature. According to actual observation, the maximum regional spatial temperature difference can reach 3 °C ~ 4 °C, and the minimum regional space temperature difference is also 0.50 ° C ~ 1 ° C. We can analyze the formation and change of regional spatial temperature difference from the following aspects to understand its regularity.

1. The temperature difference between the refrigerating compartments; the temperature difference in the refrigerating compartment is the most significant manifestation of the spatial temperature difference in the area. Under normal circumstances, the temperature difference of the length of the warehouse is almost increased by 1 ° C for each length of the warehouse, and the temperature difference is longer. The larger, especially the temperature limit is reached, and the temperature difference of the length of the zone is more prominent after the temperature of the reservoir begins to rise. At this time, if the temperature control index is 0 ° C, the usual temperature observation position is set at the center of the refrigerating compartment. After the regional temperature measurement, the temperature at the nearest end from the chiller is -2 ° C, and the temperature farthest from the chiller is at At about 2 °C, at this time, the temperature in one region exceeds the specified index by 2 °C, and the temperature in the other region is lower than the specified index temperature by 2 °C, and the total length temperature difference reaches 4 °C.

2, high and low space temperature difference; the average space height of the refrigerated room is 4 meters, the highest is up to 6 meters, the product stacking height usually accounts for 70-80% of the net height of the warehouse, storing goods in such space height, due to cold The difference between the high and low temperature of the space generated and formed by the physical specific gravity of hot air is also obvious. The temperature difference is observed at the height of the 5.5 m refrigerated room. The temperature at the highest point and the lowest point is generally not less than 1.5 to 2 ° C. The temperature difference per height meter is mostly around 0.3 ° C. After the liquid supply is stopped at the specified temperature, the product is distributed. The heat is gradually rising, and the difference between high and low temperatures will continue to increase.

3. The temperature difference of the space between the length and the height; the storage space is the highest temperature in the farthest area of ​​the cooling fan, the lowest temperature in the nearest area, the highest temperature in the highest storage space, the lowest temperature in the lowest place, and the highest temperature in the whole storage space should be At the highest point at the farthest point, the lowest temperature zone should be at the lowest point closest to the chiller, and the terminal of the junction area of ​​the height and length of the storage space is the maximum difference of the zone temperature difference.

4, the temperature difference between the inside and outside of the product stack; goods must be stored in the cold storage room, which forms a certain volume and a certain density of commodity piles. Sometimes in order to effectively use and increase the capacity of the library, it is necessary to artificially increase the density and increase the volume. According to the relevant regulations and design requirements, the space utilization rate of the refrigerating room should reach 80%. In such a high space, the goods are stored. Although the fan performs forced convection of hot and cold air, due to the resistance of the packaging and the poor air passage, The cold air can not enter the stack smoothly, and the heat generated by the breath can not be dissipated in time. After stopping the cooling, the natural circulation of the hot and cold air is extremely difficult to achieve temperature equilibrium. Due to the above reasons, the temperature difference between the inside and outside of the commodity pile is formed; the larger the stack of goods in the refrigerated compartment, the larger the temperature difference between the inside and outside of the stack, and vice versa. According to tests, a product with a length of 30 meters, a width of 5 meters and a height of 4 meters, the temperature difference between the center temperature of the pile and the outer temperature of the pile is often above 1.5 °C, and the temperature difference per depth meter is about 0.25 °C. .

5, the temperature difference between the inside and outside of the package; the goods in the refrigerated room are stored in different shapes of packaging, and some goods to prevent the loss of water and storage and preservation, but also in the packaging of polyethylene plastic film bags and Silicon window storage bags, most of which are made of single-fruit wrapping paper and bagging to prevent bumping and squeezing. The outer packaging and inner packaging of the goods are made of heat-insulating materials, which forms an artificial insulation layer. It directly hinders the conduction and convection of cold and hot air between the cold air in the warehouse and the goods in the package, and the heat of the goods in the package is not easily dissipated, resulting in a temperature difference between the temperature inside the package and the temperature outside the package. According to the test, a carton package containing 15 kilograms of apples, in the environment of the refrigerator temperature of 0 ° C, the temperature inside the carton is often higher than the same region temperature of 0.5 ° C.

6. The temperature difference between air and commodity; the air temperature in the refrigerating room is the direct standard for measuring the temperature of the warehouse. The various temperature control indicators specified in various commodity storage refer to the air temperature index of the refrigerating room. The commodity temperature usually refers to the surface temperature of the product. In the same and different regional spaces, there is often a temperature difference between the air temperature in the refrigerating compartment and the surface temperature of the commodity, because the merchandise constantly dissipates heat due to respiration during storage, and the commodity temperature is always higher than the air temperature around the merchandise. Typical temperature differences are typically between 0.5 and 1 °C. The degree of internal temperature of the product is also higher than the surface temperature of the product depending on the type, shape and size of the stored product.

7. The temperature difference changes during the frosting; the evaporator in the refrigerating room needs to use the well water and the hot ammonia to carry out the frosting. Although the frosting is not carried out every day, during the frosting process, the temperature difference of the regional space in the cold storage room will be The opposite change occurred, when the hot spot has been transferred to the evaporator, because the external water and hot ammonia are much higher than the temperature inside the library, the water temperature and hot ammonia temperature are generally above 10 °C, and the frosting time should last 20 ~40 minutes, at this time the temperature around the evaporator is usually higher than other areas of 2 ~ 3 ° C or even higher, directly affecting the temperature balance within the library.