The temperature condition adjustment of the cooler (or switching heat exchanger) is based on the central temperature. The "middle" mentioned here does not refer to the central position of geometric dimension, but refers to the temperature on the barrel (or switching channel) near the central air extraction or the outlet of circulation. It is also related to the location where the thermometer is installed. The purpose of temperature condition adjustment is to keep the temperature difference between cold end and hot end within the specified range. Then, why should the temperature in the middle prevail? We need to take a look at the relationship between the temperature of the middle part and the temperature difference of the cold end and the temperature difference of the hot end, and what rules the temperature changes along the height direction of the cooler (or the switching heat exchanger) follow.
The temperature condition change of the cooler (or switching heat exchanger) is generally caused by the change of the flow of positive and recirculating gas or its inlet temperature. For example, if the amount of recirculating air increases and the inlet temperature of the cold end remains unchanged, then the cooling volume appears insufficient and the air cannot be cooled to the original required temperature. That is, the outlet temperature of the air at the cold end will rise, and the temperature difference at the cold end will expand. At the same time, the air temperature on each section along the height of the refrigerator will increase, so the heat transfer temperature difference will increase, and the transmitted cooling volume will increase. The temperature of the outlet temperature at the hot end and the temperature of each section along the height of the refrigerator will also increase when the reflow gas releases more cold, the temperature difference at the hot end will decrease (when the air inlet temperature remains unchanged ). In this way, the heat transfer temperature difference will rise, but it is impossible to return to the heat transfer temperature difference before the working condition is changed, which is larger than the original one, so the cooling amount released by the return gas will still increase a little. However, due to the increase of air volume, the amount of cooling absorbed by every 1 kg of air decreases, otherwise it is impossible to reach a new heat equilibrium relationship. This will inevitably lead to the expansion of temperature difference at the cold end and the decrease of temperature difference at the hot end. Therefore, although the average value of heat transfer temperature difference increases, the temperature difference in the upper half of the cooler still decreases, but it only increases in the lower half. This causes that the distribution ratio of the cooling quantity transmitted by the positive and recirculation gas flowing through the stuffing (or fin) is that the upper part decreases and the lower part increases; the temperature drop of the air in the upper part decreases, The lower part of the temperature drop increases. In general, the temperature drop of air still decreases, but there are some changes in the distribution proportion of temperature drop. In this way, the temperature of the middle part is bound to rise, and it is much higher than that of the cold end, because the temperature drop of the air in the lower part increases.
Thus it can be seen that the rise of temperature in the middle part reflects the rise of temperature at the cold end, the expansion of temperature difference at the cold end and the decrease of temperature difference at the hot end. Moreover, the change range of temperature in the middle part is larger than that in the end part. The change of end is 1℃, and the change of middle part is about 10℃.
If the amount of return gas increases, and the positive flow air volume and its inlet temperature remain unchanged, then the situation is opposite to the above. That is to say, the temperature of the middle part decreases, the temperature of the cold end decreases, the temperature difference of the cold end decreases and the temperature difference of the hot end enlarges. And the change range of temperature in the middle part is also larger than that in the end part.
In conclusion, the change of temperature in the middle part can reflect both the cold end and the temperature condition of the hot end, and the change is significant and easy to detect. In addition, when adjusting the intake gas or circulation gas, there is a sudden change in the proportion of positive and return flow gas at the intake or circulation exit, and the change of temperature is relatively severe. Therefore, the temperature in the middle part is the most sensitive and representative. During the operation, we often adjust according to the temperature in the middle part, and keep the temperature in the middle part at the numerical value which is suitable for the specified temperature difference between the cold and hot ends.
