The temperature (hot end temperature, and air compressor under the condition that the cooling effect of the last-stage cooler or nitrogen-water precooler is related) and the temperature of the return gas into the cooler (the temperature of the cold end is related to the temperature of the heat exchanger such as the outlet supercooler and the liquid purifier) remains unchanged, cooler the temperature difference of the hot end and the temperature difference of the cold end (both refers to the difference between the temperature of the forward air and the return air participating in the switching) are mutually dependent. If the hot end temperature difference, cold end temperature difference will is too small; the hot end temperature difference is too small, cold end temperature difference will is too large. This is decided by the cooling balance of the positive and return gas of the cooler.
When the ratio of return flow to positive flow volume is larger than the normal value, for example, if the circular flow volume is too large or the medium extraction volume is too large, the cooling volume that the return flow gas can transmit to the positive flow volume is larger, while the positive flow air volume is relatively less than the return flow air volume, which causes the excess cooling volume of the return flow gas. Therefore, if the temperature of the backflow gas outlet cooler decreases, the temperature difference at the hot end will be larger. The amount of cooling absorbed by the forward air is more than that absorbed by the normal air when the ratio between the back flow and the forward flow volume is higher than that of the normal air. Therefore, the temperature of the air outlet cooler decreases, resulting in the reduction of the temperature difference at the cold end.
If the ratio of the return flow to the positive flow volume is smaller than the normal value, for example, the circular flow volume decreases or the medium extraction volume decreases, then the positive flow volume is larger than the return flow volume, the amount of cooling needed to be absorbed by the forward air is more than the amount of cooling that can be released by the backflow air. That is to say, if the amount of cooling is insufficient, the temperature of the air coming out of the cooler will rise, resulting in a larger temperature difference at the cold end. As the return flow volume is relatively small for the positive flow volume, the cooling volume can be released as much as possible, and the temperature of the return flow gas should rise when it comes out of the cooler, which causes the temperature difference between the cold and hot ends of the cooler to shrink.
The temperature difference at the hot end is large, the reheat is insufficient, and the cold loss is large. The temperature difference at the cold end is too large, which will affect the self-cleaning of carbon dioxide, increase the resistance too fast and shorten the running cycle of the whole air separation unit. Therefore, the temperature difference between the hot end and the cold end should be well controlled during operation. When adjusting, not only should the temperature difference of the cold end be kept within the range of the self-cleaning maximum allowable temperature difference, but also try to reduce the temperature difference of the hot end, both of which should not only focus on one side. The adjustment of temperature working condition is usually based on the central temperature, which is to change the central pumping volume (or circular flow), product oxygen or nitrogen flow rate, air volume and other methods to change the proportional relationship between return flow and positive flow, control the temperature difference between the hot end and the cold end within the allowable range.
