Cryogenic air separation plants produce pressurized oxygen gas at 150 bar and 99.8% purity to fill oxygen cylinders directly. It is either produced by compression in oxygen compressor (external compression) or pumping and vaporizing liquid oxygen in main heat exchanger by incoming air (internal compression). Small and medium-scale plants differ in the types of main heat exchanger, compression and expansion devices owing to limitations of market availability. Levelized cost of production of oxygen gas is about 9% less in external compression, though internal compression process is far more fire-safe. Design and operating parameters applicable for internal compression plants supplying oxygen at 40 bar cannot be extended to 150 bar plants due to the differences of fluid properties between super-critical and sub-critical oxygen. UA (surface area times overall heat transfer coefficient) of main heat exchangers, flow and pressure of incoming air are optimized for minimum capital and operating costs. If the chosen flow and pressures deviate substantially from the optimum points, size of main heat exchanger may even increase five-fold. Ideally, 27–29% of total air intake should be compressed between 110 and 100 bar in a medium-scale internal compression plant to keep levelized cost of production at the minimum.

低温空气分离设备可产生150 bar的加压氧气和99.8％的纯度，以直接填充氧气瓶。它既可以通过氧气压缩机中的压缩（外部压缩）产生，也可以通过进入空气的主热交换器中的液氧泵送和汽化（内部压缩）来产生。由于市场可用性的限制，中小型工厂的主要热交换器，压缩和膨胀设备的类型有所不同。尽管内部压缩过程具有更高的防火安全性，但在外部压缩过程中，氧气生产的平均成本降低了约9％。由于超临界和亚临界氧气之间的流体特性不同，适用于以40 bar供氧的内部压缩设备的设计和运行参数无法扩展到150 bar设备。主热交换器的UA（表面积乘以总传热系数），流入空气的流量和压力经过了优化，以最小化投资和运营成本。如果选择的流量和压力大大偏离最佳点，则主热交换器的尺寸甚至可能增加五倍。理想情况下，在中等规模的内部压缩工厂中，应将总进气量的27–29％压缩在110至100 bar之间，以将生产的平均成本保持在最低水平。