The middle temperature difference between the control units is generally not greater than 3 ~ 5 ℃; the middle temperature should not be too low, and the cooling capacity should not be too much concentrated in the main heat exchanger, resulting in the increase of the temperature difference at the hot end. When the temperature of the cold end reaches the air liquefaction temperature, the cooling capacity should be transferred to the distillation system to make the distillation system cool enough, accumulate liquid as soon as possible and establish the distillation system;The air tightness test of fractionator has been done before leaving factory, but the equipment may be damaged due to vibration during transportation. Therefore, air tightness should be checked again before installation. In addition, if severe frosting is found on the shell of the air separation tower after operation, or after overhaul, air tightness inspection is also required.DEAR Air separation equipment.The higher the thermal conductivity is, the stronger the thermal conductivity is. The thermal conductivity of different materials is different. For the same material, it is also related to its structure, density, humidity, temperature, pressure and many other factors. Table 27 lists the approximate values of the thermal conductivity of some commonly used materials.
DEAR Air separation equipment.The moisture content of the air into the tower is determined by the distillation system according to the heat balance of the tower. Because the heat transfer temperature difference of the main heat exchanger is larger than that of the switching heat exchanger, the heat transfer area needed to transfer the same heat can be reduced, so the length of the non switching plate fin heat exchanger is generally shorter than that of the switching heat exchanger.DEAR Air separation equipment.Because the backflow gas is less than the positive flow gas, the cold energy can be released as much as possible, and the temperature of the backflow gas out of the accumulator will increase, resulting in the reduction of the temperature difference between the cold end and the hot end of the accumulator. Large temperature difference at hot end, insufficient reheating and large loss of cooling capacity. Too large temperature difference at the cold end will affect the self removal of carbon dioxide, increase the resistance too fast, and shorten the operation cycle of the whole air separation unit.
DEAR Air separation equipment.If the heat transfer surface of the condensing evaporator is insufficient, its heat load will be reduced, that is, the heat transfer will be reduced. Therefore, the evaporation of liquid oxygen will be reduced, and the condensation of gas nitrogen will be reduced accordingly. This will directly affect the distillation conditions of the upper and lower columns. At the same time, the air intake of the lower tower will be reduced, the air can not be fed, and the oxygen production will be reduced.DEAR Air separation equipment.In the start-up stage, the time of passing through the water and carbon dioxide freezing zone is prolonged, and the temperature difference at the cold end of the switching heat exchanger is not controlled within the allowable range (in the start-up stage, the temperature range gradually decreases with the decrease of temperature), which will cause the increase of water and carbon dioxide impurities into the air separation tower. If the start-up process of an air separation unit is interrupted or started for many times, the temperature of the accumulator or the switching heat exchanger will rise and a large amount of carbon dioxide will be brought into the tower, thus shortening the operation cycle.
DEAR Air separation equipment.When a full annular gas is added, it can reduce the temperature difference at the cold end and ensure self-cleaning; the throttling effect of the air throttled by the bypass valve can recover its cooling capacity through the sewage nitrogen channel; the cooling capacity of the plate heat exchanger will move down and expand the freezing area. But it has the side effect of enlarging the temperature difference at the hot end and increasing the cooling loss.DEAR Air separation equipment.The centralized cooling method, also known as the complete cooling method, is also one of the start-up operation methods of the air separation unit with the freezing self cleaning process. This method is to use the cold capacity of the expander to cool the accumulator to the operating temperature, that is, the temperature of the cold end reaches - 170 ～ - 172 ℃, so that it can completely pass through the freezing stage of water and carbon dioxide, and then cool other equipment, accumulate liquid and adjust the distillation condition.
