The main heat exchanger and condenser evaporator in air separation unit play a key role in liquid production.The main heat exchanger uses the expanded low-temperature and low-pressure gas as the back-flow gas of the heat exchanger to cool the high-pressure positive air so that the temperature before expansion gradually decreases.At the same time, the temperature after expansion decreases gradually and correspondingly until the required temperature for liquefaction can be reached finally, partially liquefying the positive air.The cooling process of air separation unit at start-up stage is such a step-by-step cooling process.
The expanded air remains gaseous at very low temperatures due to its low pressure.For small, medium and high pressure oxygen generator, at the later stage of start-up stage, some liquid will be produced under the main heat exchanger and act as liquefier.For low-pressure air separation equipment, a liquefier is provided, which uses expanded low-temperature and low-pressure air to cool the positive high-pressure (about 0.6MPa) low-temperature air and make it partially liquefied.At the same time, the condenser evaporator also acts as liquefier in the later stage of start-up.The cold air entering the upper tower after expansion cools the low-temperature pressure gas from the lower tower in the condensation evaporator. Some of the air is condensed and then throttled to the upper tower, which further reduces the temperature and becomes part of the low-temperature and low-pressure reflux gas, thus gradually increasing the accumulated liquid amount.
