Why does the air separation plant need to fill the cold box with lazy gas during operation?

The cold-keeping (heat insulation) material is filled in the cold-keeping box of the air separation unit, while the interspace between particles of the cold-keeping material (Pearl sand, etc.) is filled with air. After the operation of the air separation plant, the equipment in the tower is in a low temperature state, and the temperature of the cooling material decreases accordingly. As the volume of the internal gas shrinks, negative pressure will be formed in the cold box. If the cold box is strictly sealed, it will be easy to make the box body get sunken under the effect of internal and external differential pressure. If the cold box is not tightly closed, the external wet air is easy to invade, which makes the cold-keeping material become damp, the cold-keeping effect becomes worse, and the cold-running loss of the air separation plant increases. The general cold-keeping material is Pearl sand, whose thermal conductivity is about 0.040 W/(m · ℃); while the thermal conductivity of ice is 2.2 W/(m · ℃ ), visible to increase more than 50 times. Therefore, in order to prevent the wet air and the moisture in the air from invading due to condensation in the pipe and the wall of the cold box, when the air separation unit is running, dry lazy gas (nitrogen or dirty nitrogen) should be filled into the cold box ), keep micro positive pressure in the refrigerator, about 200 ~ 500Pa.