LIQUID air can now be used to chill the atmosphere safely, thanks to a technique that prevents oxygen building up as the air evaporates, thus preventing a fire hazard. This technique will allow liquefied gas to be used to cool places where people work or where animals are kept.

BOC Gases Europe, a company based in Guildford, UK, which makes industrial gases, decided in 1993 that it was worth developing air as a refrigerant, despite concerns about flammability. "People told us we were crazy," says Steve Waldron, the general manager of food marketing at BOC.

The company has developed a computer-controlled venting system that regulates gas pressure and composition inside cylinders of liquid air. Specially developed sensors monitor the balance of the nitrogen-oxygen mixture, in both the liquid and the vapour, as the cylinder empties. The technique will allow liquid air, cooled to -191^oC and stored in cylinders, to be piped through nozzles into warehouses or refrigerated lorries. By regulating the rate of discharge, the air temperature can be kept as low as -30^oC.

Usually, when liquid air begins to vaporise, the nitrogen boils faster than the oxygen. The oxygen content of the remaining liquid rises, and the oxygen-rich atmosphere produced when this liquid is piped through a nozzle and vapourised can cause materials to burst into flames. BOC has tested the system at a poultry farm, where the number of chickens dying from heat stress in the summer fell from six per cent to just two per cent.

The system can make the temperature drop unusually fast, by up to 2^oC per minute. "This is around five to ten times faster than orthodox mechanical cooling systems," says Waldron.