The more water is allowed to evaporate, the more evaporation latent heat is absorbed, and the lower the temperature of cooling water out of the cooling tower. Therefore, the higher the temperature at the outlet of the sewage nitrogen, the better the cooling effect of the water cooling tower. Of course, the outlet temperature of sewage nitrogen can not be higher than the inlet temperature of water. According to the weight of different fractionating tower, select appropriate steel wire rope, hang the tower flat on the sleeper, and carry out air tightness test; Tie the steel wire rope to the lower part of the upper cylinder shell, and pad it with about 10 mm wood around to prevent deformation of the shell, and then lift it to the specified position with a crane; It is possible to obtain lower temperature than the environment by artificial means. However, it needs to cost a certain amount of energy (work, electric energy, etc.) to achieve. This process of artificially obtaining low temperature is called "refrigeration".
Because this characteristic was not considered in advance in a large domestic air separation unit, the expander was always in the condition of expansion at high temperature after the start-up of the air separation unit. The temperature difference at the hot end of the non pressurization channel of the heat exchanger was expanded to 10 ℃, resulting in large cold loss and slow liquid accumulation in the tower, which took five days to start up. For the unit group with low temperature in the middle, turn down the product control valve to reduce the product throughput, that is, reduce the cooling capacity, so that the temperature in the middle can rise. The distribution of product throughput should be adjusted according to the middle temperature, but the total amount of product should not be changed. When the temperature of each unit group is unbalanced, the two groups with the highest and lowest temperature in the middle should be adjusted first.
In order to avoid the increase of hydrocarbon concentration in liquid oxygen, according to the material balance, the air separation plant manufacturer needs to extract part of liquid oxygen from the main cooler and extract part of hydrocarbon from the main cooler. The minimum amount of liquid oxygen extracted is equal to 1% of the gas oxygen output, and then gasified separately. For the air separation unit with liquid air subcooler, part of the liquid in the lower tower and main cooler shall be drained before starting to prevent the liquid air subcooler from blocking; When the cold end temperature reaches - 150 ℃, the circulation flow should be reduced as much as possible to prevent carbon dioxide from blocking the circulation channel; In the switching heat exchanger, the positive and backward gas flow through different channels at the same time to exchange heat directly. Therefore, under normal conditions, it is basically a stable heat transfer process, that is, the temperature of each section of the heat exchanger (the temperature of positive flow and backflow gas, as well as the temperature of baffles and fins) does not change with time.
Under the condition of 3 ℃ temperature difference at the hot end of the large capacity air separation plant, the incomplete heat exchange loss accounts for about half of the total cooling loss. If the temperature difference increases by 1 ℃, the total cooling loss will increase by about 16%. In order to make up for the increased cold loss, it is required to increase the expansion capacity of the expander, which will affect the work of the whole device. In the process of operation, pay attention to the change of temperature difference at the hot end, and take corresponding measures to prevent the expansion of temperature difference and avoid exceeding the design value. Under the condition of constant water vapor content, due to the decrease of temperature, the original unsaturated water vapor in the air can become saturated vapor, and the excess water will be separated out. The temperature at which water vapor reaches saturation is called the dew point.
