Regulation and control of space temperature differ

2022-08-18
  • Detail

Regulation and control of regional space temperature difference in constant temperature cold storage Abstract: due to the objective existence of regional temperature difference and space temperature difference in constant temperature cold storage, it can not maintain the absolute average and constant optimal temperature. This negative effect directly affects the overall quality of refrigerated goods and the economic benefits of enterprises. Therefore, effective technical measures must be taken to adjust and strictly control the regional space temperature difference, so as to minimize the temperature difference in various regions and different spaces in the constant temperature cold storage, so that all the stored goods are in the best temperature state

key words: constant temperature cold storage area temperature difference space temperature difference

constant temperature cold storage is a special commercial storage facility for storing fruits, vegetables, fresh eggs, flowers and other perishable goods. Standard and reasonable, strict and scientific temperature control is the key to ensure the quality of goods storage in constant temperature cold storage. Because the constant temperature cold storage objectively has regional temperature difference and space temperature difference, it cannot maintain the absolute average and constant optimal temperature. This negative effect directly affects the overall quality of refrigerated goods and the economic benefits of enterprises. Therefore, effective technical measures must be taken to adjust and strictly control the regional space temperature difference, so as to minimize the temperature difference in various regions and different spaces in the constant temperature cold storage, so that all the stored goods are in the best temperature state. This is also a major technical topic of the overall management level of constant temperature cold storage, which is worthy of further research and discussion

first, the main reasons for the formation of regional space temperature difference

the formation of regional space temperature difference mainly has the following reasons:

1. The structure of commodity cold storage room; The commodity cold storage room of constant temperature cold storage is a space composed of length, width and height. The length, width and height of the space are the basic reasons for the formation of regional space temperature difference. Although the design of the structure of the cold storage room takes into account the influence of the structure on the temperature control difference, it cannot technically make the regional space temperature in the cold storage room reach a relatively balanced level. Generally, the larger the volume of the refrigeration room, the more difficult the convection and exchange of cold and hot air, and the greater the temperature difference in the regional space

2. Physical specific gravity of cold and hot air; In the commodity storage environment, what determines the temperature is the exchange mode and frequency of cold and hot air. In the exchange and convection process of cold and hot air, due to the different physical proportion of cold and hot air, hot air accumulates in the upper part of the cold storage room, and the lowest point of air temperature is often at the bottom of the cold storage room. According to this physical principle, the temperature difference between the upper and lower spaces must be formed in the cold storage room, and the higher the design height of the cold storage room, The greater the temperature difference between the upper and lower space

3. The certification objects of cold source are all product quality supervision and inspection institutions and other laboratories that provide fair data to the society; Such as various product quality supervision and inspection stations, environmental testing stations, centers for Disease Control and prevention, and so on; Generally, the refrigeration equipment of the constant temperature storehouse mostly adopts the box type air cooler, while the evaporation temperature of the evaporator of the air cooler is mostly designed at about -15 ℃, and the evaporator is usually fixed at a certain position in one direction of the cold storage room. The cold source of the cold storage room mainly comes from the evaporator, which forms the relative concentration of the cold source. Although the long-distance forced air supply measures are adopted in the design of the air cooler, it is often unable to achieve the purpose of temperature balance due to long distance, large resistance and large air loss, which inevitably leads to the temperature difference in the length of a certain area. Some large refrigerated rooms are 40 meters long, and the regional temperature difference is sometimes quite large due to the long distance.

4. Stacking form of commodities; Commodities are the main source of heat in the cold storage room. The variety, quantity and time of goods entering the warehouse, the density, height and width of goods stacking and the packaging form of goods are all factors that lead to the regional space temperature difference. In addition to cooling the air temperature in the cold storage room, refrigeration also needs to constantly cool the heat emitted by the respiration of goods. Due to the unreasonable and unscientific stacking form, the heat emitted by goods during storage cannot be cooled in time, and the cold and hot air cannot be effectively exchanged and circulated inside and outside the cargo pile, which will also cause the temperature difference in the regional space, thus directly affecting the storage effect of goods.

5. Other reasons; There are many other reasons for the formation of the space temperature difference in the constant temperature cold storage area, such as the difference in the thermal insulation performance of various parts in the cold storage structure, the existence of various cold bridges, the sunny side of the cold storage room, the overloaded operation of the electrical equipment in the warehouse, the frequent opening of the warehouse door, the nonstandard frost flushing process, etc., all of which are the direct and indirect reasons for the space temperature difference in the constant temperature cold storage area

II. Situation analysis of regional space temperature difference in the cold storage room

the formation causes of regional space temperature difference in the cold storage room of constant temperature cold storage are complex, with different forms, showing a variety of situations. Now, taking the constant temperature storage of Xingtai fruit company as an example, this paper makes a brief analysis of the situation of regional space temperature difference in the cold storage room. Xingtai fruit company is a 3000 ton constant temperature warehouse. The building structure of a 300 ton cold storage room is 40 meters long, 15 meters wide, 5.5 meters high, with an area of 600 square meters and a space volume of 3300 square meters. According to the design requirements, two 350 square meter box type air coolers are set, and the specified temperature control index is 0 ℃, which is a relatively large cold storage room. In the actual storage temperature control process, most of the regional space in the cold storage room can not reach or exceed the specified index temperature. According to the actual observation, the maximum regional space temperature difference can reach 3 ℃ ~ 4 ℃, and the minimum regional space temperature difference is also 0.50 ℃ ~ 1 ℃. We can analyze the formation and change trend of regional space temperature difference from the following aspects to understand its regularity

1. Length temperature difference between refrigerated rooms; The length temperature difference of the cold storage room is the most significant manifestation of the regional space temperature difference. Generally, the length of the warehouse increases by 10 meters, and the length of the area temperature difference almost increases by 1 ℃. The longer the length, the greater the regional temperature difference. In particular, when the specified temperature index is reached, the cooling is stopped, and the temperature of the warehouse begins to rise, the regional length temperature difference is more prominent. At this time, if the temperature control index is 0 ℃, and the temperature observation position is set in the center of the cold storage room at ordinary times, after the regional temperature measurement, the temperature at the nearest end from the cooling fan is -2 ℃, while the temperature at the farthest place from the cooling fan is about 2 ℃. At this time, the temperature in one area exceeds the specified index by 2 ℃, the temperature in the other area is lower than the specified index by 2 ℃, and the total length temperature difference reaches 4 ℃

2. Temperature difference between high and low space; Generally, the space height of the cold storage room is as low as 4 meters and as high as 6 meters. If the stacking of goods is not zero, the height of clicking the hardware conditioning button usually accounts for 70 ~ 80% of the net height of the warehouse. In such a space height, the space high-low temperature difference caused and formed by the physical proportion of cold and hot air is also relatively obvious. Observe the temperature difference at the height of the 5.5-meter cold storage room. The temperature at the highest point and the lowest point is generally not less than 1.5 ~ 2 ℃, and the temperature difference per meter is mostly about 0.3 ℃. After the specified temperature is reached and the liquid supply is stopped, the heat emitted by the commodity gradually rises, and the high-low temperature difference will continue to increase

3. The presentation form of spatial temperature difference in the area where the length and height intersect; The temperature in the farthest area of the storage space from the cooling fan is the highest, the temperature in the nearest area is the lowest, the temperature in the highest part of the storage space is the highest, and the temperature in the lowest part of the whole storage space is the lowest. The terminal of the contact area of the height and length of the storage space is the maximum difference of the regional temperature difference

4. Temperature difference inside and outside commodity stacking; Goods must be stacked in the cold storage room, which forms a stack of goods with a certain volume and density. Sometimes, in order to effectively use and improve the storage capacity, it is necessary to artificially increase the density and volume. According to relevant regulations and design requirements, the space utilization rate of the cold storage room should reach 80%. When storing goods in such a high space, although the fan carries out the forced convection of cold and hot air, the cold air cannot smoothly enter the stack due to the resistance of the packaging and the poor air passage, and the heat generated by the breathing of goods cannot be released in time. After the temperature is stopped, It is extremely difficult to achieve temperature equilibrium only by natural circulation of cold and hot air. Due to the above reasons, the regional temperature difference inside and outside the commodity stack is formed; The larger the stack of goods in the cold storage room, the greater the regional temperature difference inside and outside the stack, and vice versa. According to the test, for a 30 meter long, 5 meter wide and 4 meter high commodity stack, the temperature difference between the central temperature of the stack and the outer temperature of the stack is often more than 1.5 ℃, and the average temperature difference per meter of depth is about 0.25 ℃

5. Temperature difference inside and outside the package; The goods in the cold storage room are stored in packages of different shapes. Some goods need to be padded with polyethylene plastic film bags and silicon window fresh-keeping bags in order to prevent wilting and water loss and storage and preservation. Most goods also need to be wrapped with single fruit paper and bagged to prevent bumping and extrusion. The outer and inner packages of goods are made of thermal insulation materials, which forms an artificial thermal insulation layer, It directly hinders the conduction and convection of the cold air in the warehouse and the cold and hot air between the goods in the package, and the heat of the goods in the package is not easy to be emitted, resulting in the temperature difference phenomenon of higher temperature in the package and lower temperature outside the package. It is measured that when a carton containing 15kg apples is packed, the temperature in the carton is often 0.5 ℃ higher than that in the same area when the storage temperature in the cold storage room is 0 ℃

6. Temperature difference between air and commodity; The air temperature in the cold storage room is the direct standard for measuring the storage temperature. Various temperature control indicators specified in the storage of various commodities refer to the air temperature indicators in the cold storage room. Commodity temperature usually refers to the surface temperature of commodities. In the same and different regional spaces, there is often a temperature difference between the air temperature in the cold storage room and the surface temperature of the goods, because the goods constantly emit heat due to respiration during storage, and the temperature of the goods is always higher than the air temperature around the goods. Generally, the temperature difference is usually 0 Between ℃. According to the type, shape and size of the stored goods, the internal temperature of the goods is also higher than the surface temperature of the goods to varying degrees

7. Temperature difference change during frost flushing; The evaporator in the cold storage room needs to be defrosted with well water and hot ammonia. Although the defrosting is not carried out every day, during the defrosting process, the temperature difference in the area space in the cold storage room will change in the opposite direction. At this time, the hot spot area has been transferred around the evaporator, because the external water and hot ammonia are much higher than the temperature in the warehouse, the water temperature and hot ammonia temperature are generally above 10 ℃, and the defrosting time will last for 20 ~ 40 minutes, At this time, the temperature around the evaporator is usually 2 ~ 3 ℃ or higher than that in other regional spaces, which directly affects the temperature balance in the warehouse

III. Influence of regional space temperature difference on storage effect

constant temperature cold storage has strict requirements on temperature. All kinds of commodities have their own temperature control indicators. Generally, the fluctuation range should not exceed 1 ℃, while garlic moss is more strict, and the requirement is less than 0.5 ℃. Due to the objective existence of regional space temperature difference, the commodities stored in constant temperature cold storage will inevitably be adversely affected, and its harm mainly includes the following aspects

1. Due to the existence of temperature difference in regional space, some or most of the commodities are at a temperature lower or higher than the specified index temperature, resulting in higher commodity storage temperature in some regional spaces and lower commodity temperature in other regional spaces

Copyright © 2011 JIN SHI