Incomplete cold loss in heat exchange of the air separation unit is caused by the fact that the backflow low temperature gas cannot be reheated to the temperature of the positive flow air entering the heat exchanger when it exits the hot end of the main heat exchanger. Therefore, the greater the temperature difference between the hot end of the return flow gas and the positive flow air heat exchanger, the less reheating, the more unused cooling capacity, and the greater the incomplete heat exchange cooling loss. Therefore, the cold loss due to incomplete heat exchange is proportional to the temperature difference at the hot end.
The backflow low-temperature gas is composed of several streams of gas that have been separated into product oxygen, product nitrogen and sewage nitrogen. The temperature difference between them and the positive flow air at the hot end is not exactly the same, and the flow rate and specific heat capacity are also different. When calculating the incomplete heat exchange loss, the total incomplete heat exchange loss should be calculated separately and then added. Since the amount of dirty nitrogen is the largest, the temperature difference at its hot end has the greatest impact on the incomplete loss of heat exchange. If the temperature difference at the hot end of each stream of backflow gas is equal, the sum of their air volume is equal to the amount of positive flow air. At this time, when the temperature difference at the hot end increases by 1 ℃, the incomplete heat exchange loss will increase by 1.31kJ/m3, which will increase the total cold loss of the device by more than 10%. Therefore, it is of great significance to minimize the temperature difference at the hot end to reduce the total cold loss of the device.
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