Gases such as oxygen and nitrogen are also used in high volumes in liquefied form in industry, medicine and the food sector. The most widely used industrial process for liquefying gas goes back to the established Linde process. In the first stage, air is liquefied in the cryogenic air separation unit. To this end, ambient air is drawn in through a filter, compressed and traces of carbon dioxide, hydrocarbons and water vapour are separated in absorbers. The heat produced is then dissipated through pre-cooling with water. In the downstream expansion in the low temperature part of the plant (cold box), the end temperature of the air falls below the ambient temperature. Already expanded gas flows result in further cooling of the compressed air to liquid form in a heat exchange process. Because of the various pressurisation levels and the extremely low temperatures, the different boiling points of the individual air components oxygen (-183 °C), nitrogen (-196 °C) and argon can be approached, and thus separation is possible in a multistage process of the following distillation column (rectification) by means of counterflow distillation. Depending on the type of air separation unit, the end products are transported to the customer in product tanks as liquefied gases or by pipeline as gases.

Safe and reliable operation of an air separation unit requires monitoring ­ and measuring equipment for process parameters that can withstand extremely low temperatures. Specially equipped measuring devices can even withstand the condensation of water from moist air. In addition, highest purity is important for the end products. KROHNE has a wide range of sophisticated measuring equipment – for example for level, flow and (differential) pressure measurement - especially for cryogenic media. These devices maintain their metrological availability, even with a gas content of the liquefied gases of up to 100%. And thanks to our innovative ultrasonic flowmeters with integrated temperature and pressure sensors, the measured volume flow – whether gaseous or liquid – can always be converted to standard conditions. We also have a range of variable area flowmeters for the reliable measurement of all air products, for example in analysis. These can even fulfil the maximum requirements ex-works, for example by X-ray weld inspection or helium leak testing.