The air passes through the cooler or the switching heat exchanger. As the temperature decreases continuously, the moisture gradually flows out and deposits on the stuffing of the cooler or the fin of the plate heat exchanger in the form of water drops, snowflakes, etc, there is basically no water in the gas at-60℃. When the air temperature drops below-130℃, carbon dioxide is also gradually precipitated in the form of solid (dry ice), and there is basically no carbon dioxide when it reaches-170℃. The precipitation and freezing of moisture and carbon dioxide in the air in the switching heat exchanger provide good conditions for the safe and reliable operation of the air separation plant. But it has a negative effect on heat transfer, and it will also block channels and increase flow resistance. Therefore, these eluates should be removed timely and regularly. The method adopted is to take away the deposited water and carbon dioxide when the backflow gas (such as dirt nitrogen) passes through, so it is called "self-cleaning ".
Why can the backflow of sewage nitrogen take away the frozen water and carbon dioxide? This is because the dirt nitrogen from the Tower of the distillation tower is basically the unsaturated gas without water and carbon dioxide, so the water and carbon dioxide can be evaporated and sublimated (from liquid or solid to gas ), enter the dirty nitrogen. Although the temperature of the polluted nitrogen is lower than that of the normal air, the maximum content (saturation content) of water and carbon dioxide that can be contained in every cubic meter of the backflow polluted nitrogen is less than that brought in by the normal air, however, since the pressure of dirt nitrogen is much lower than that of the forward air, the actual volume flow is 3 ~ larger than that of the forward air ~ 4 times, so the actual capacity of water and carbon dioxide that can be contained is larger than that in the positive flow, which is possible to clean up all the deposited water and carbon dioxide to achieve the purpose of self-cleaning.
