When the working condition of the air separation plant is stable, the cooling capacity of the unit keeps balance with the cooling capacity consumption, and the temperature, pressure, liquid level and other parameters of each part in the unit no longer change with time. The main cooling is the bond connecting the upper tower and the lower tower. The rising nitrogen from the lower tower is heated and condensed in the main cooling, and the reflux liquid oxygen from the upper tower is absorbed and evaporated in the main cooling. When the amount of recirculation liquid is equal to the evaporation, the liquid level remains unchanged.
When the processed air enters the tower, it has certain "moisture content", that is, a small part of it is liquid. Most of the air will be liquefied in the main cooling. As for the low-pressure air separation equipment, the air going into the tower is composed of the air at the cold end of the main heat exchanger and the air passing through the liquid purifier; as for the medium-pressure air separation equipment, it is composed of the air of expansion air and the air of throttling and depressurizing through the section-1 valve after the heat exchanger is discharged. Under normal circumstances, their comprehensive state of entering the tower has a certain "moisture content" (liquefaction rate ). The air condition of entering the tower is guaranteed by the heat exchange system and cooling system in the air separation plant.
When the cooling loss of the unit increases, the refrigerating capacity is insufficient, which reduces the air moisture content entering the tower. It is required that the amount of nitrogen condensing in the main cooling will increase, and the heat load of the main cooling will increase, correspondingly, the evaporation of liquid oxygen will also increase and the liquid oxygen surface will decrease; if the refrigerating capacity is too much, for example, when the working pressure of the medium pressure device is too high, the moisture content of the air going into the tower will increase and the heat load of the main cooling will decrease, when the evaporation of liquid oxygen decreases, the liquid oxygen surface will rise. Therefore, whether the cooling capacity of the unit is balanced or not should be reflected in the change of the main cold liquid surface first.
Of course, the main cooling liquid oxygen surface is the main sign of whether the cooling volume is balanced, not the only sign. Because the opening of the air throttle valve is too large or too small, it will change the liquid level of the lower tower, and then affect the change of the liquid oxygen level of the main cooling. However, this is not caused by the imbalance of cold volume, but by the improper distribution of liquid volume in the upper and lower towers, and the fluctuation of liquid level is also temporary.
