No matter in the starting working condition or in the normal operating condition, medium-voltage small air separation plant must pay great attention to the relationship between the working pressure of the process and the flow and expansion. The relationship between them should meet two conditions: one is to ensure the heat balance of the whole device and meet the demand of the equipment for cooling capacity; the second is to ensure that the second heat exchanger of the cold end of the main heat exchanger can work normally.
The working pressure of the air separation plant will directly affect the isotherm throttling effect of high pressure air and the cooling capacity of the expansion machine. That is to say, in order to ensure the normal operation of the air separation plant, when the high pressure changes, the distribution ratio of the flow and expansion volume should be adjusted timely.
If the operating pressure of the air separation plant rises, the isotherm throttling effect and the actual enthalpy drop of the expansion machine will increase. In order to balance the original Cold loss of the equipment, the expansion amount can be reduced accordingly. However, from the perspective of ensuring the normal operation of the second heat exchanger, it is required to increase the expansion and reduce the flow. Because after the air pressure increases, its specific heat capacity increases, and the amount of cooling needed to cool to the same temperature increases. If the positive flow air volume is too large, the second heat exchanger will not work normally.
When the operating pressure of the air separation plant decreases, in order to balance the normal cold loss of the equipment, the expansion volume should be increased to reduce the flow rate. At this time, in the second heat exchanger, due to the decrease of the flow section of the positive flow, the amount of nitrogen and oxygen that flow back does not change much, and the flow section air of the positive flow may be excessively cooled, while the phenomenon of "zero temperature difference" or "negative temperature difference" appears. It causes the temperature difference of the Heat end of the main heat exchanger to expand, and the reheat is insufficient and the cold loss to increase. In turn, it requires to increase the amount of expansion. In this way, the working condition of the second heat exchanger is worse, and as a result, the heat balance of the whole air separation plant is damaged.
At present, there is a view that to reduce the energy consumption of the air separation plant, as long as the operating pressure is reduced, the lower the operating pressure of the equipment, the better. Not so. The minimum operating pressure of the air separation plant must meet the cooling loss of the equipment and ensure that the second heat exchanger can work normally. There is a certain limit to reduce the operating pressure. Therefore, the pressure change of the air separation plant in the process of operation must be combined with the adjustment of flow and expansion volume to conduct comprehensive consideration. Only in this way can the air separation plant be ensured to operate under the optimal parameter conditions.
