LIQUID air can at last be used to chill the atmosphere safely, thanks to a
technique that prevents oxygen building up as it evaporates and causing a fire
hazard. This could allow liquefied gas to be used to cool places where people
work and animals are kept.
The technique will allow liquid air, cooled to 鈭�191 掳C 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 鈭�30 掳C.
Usually when liquid air begins to vaporise, the nitrogen boils off faster
than the oxygen. The oxygen content of the remaining liquid rises, and the
oxygen-rich atmosphere produced when this is piped through a nozzle and
vaporised can cause materials to burst into flames.
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BOC, which makes industrial gases, decided in 1993 that it was worth
developing air as a refrigerant, despite concerns about flammability. 鈥淧eople
told us we were crazy,鈥� says Steve Waldron, the general manager of food
marketing at BOC Gases Europe, in Guildford, Surrey.
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. By following a careful venting
procedure, BOC says it has perfected a system that keeps the oxygen content
stable at around 18 per cent by volume. 鈥淚n essence, we stop the nitrogen
boiling off,鈥� says Waldron.
BOC has tested the system at a poultry farm, where the number of chickens
dying from heat stress in the summer fell from 6 per cent to just 2 per
cent.
The system is suitable for temporary refrigeration depots, such as warehouses
where supermarkets store fresh fruit in the summer, or in stores where
refrigeration is needed only once a week. The system can make the temperature
drop unusually fast, by up to 2 掳C per minute. 鈥淭his is around 5 to 10 times
faster than orthodox mechanical cooling systems,鈥� says Waldron.
The gas could also be used in lorries that have to deliver a single
refrigerated cargo to many addresses, such as deliveries of frozen food to
shops. This is impractical if liquid nitrogen is used because the driver has to
wait at each stop for fresh air to replace the nitrogen in the wagon, before
going in to retrieve the goods.
