The cold box of the air separation plant is equivalent to the shell of a low temperature container. Whether it is medium or large air separation plant or small air separation plant, in order to reduce the cooling loss of the unit, the structure of the cold box is developing towards sealed type. That is, the cold box is almost the welding structure of skeleton and cover plate except the overhaul hole.
The equipment is in a low temperature environment after starting, and the thermal insulation materials (such as Pearl sand) in the cold box are filled with air. When the temperature decreases, the air volume will shrink, which will form a vacuum state. In order not to cause the cold box to be sucked out due to external pressure (atmospheric pressure), the tower should be filled with some nitrogen or dirty nitrogen to maintain a micro positive pressure. However, this pressure is difficult to control. If the pressure is too high, it will lead to bulging belly.
In order to prevent the cold box from deformation, now one or two safety devices are often installed on the top of the cold box, which are convenient for gas to enter and exit and can keep a certain pressure inside the cold box, and they are called cold box respirators. It is actually similar to a bidirectional safety valve. When the pressure in the cold box is higher than a certain pressure, a part of the gas will be discharged automatically, which is very safe as people breathe.
However, the structure of the respirator is complex, the manufacture is troublesome and the operation is inconvenient. It has been gradually replaced by the breathing tube filled with silica gel and connected with the atmosphere. When the pressure of the cold box is high, the gas can be discharged through the breathing tube; when the pressure is reduced, the inhaled gas is the air dried by silica gel, which will not bring in water. The pressure inside the cold box is always slightly higher than that of the atmosphere, which not only protects the cold box from deformation, but also prevents the internal Pearl sand from reducing the heat insulation performance due to damp.
