More Than Just Hot Air
The atmosphere is air, which is a mixture of gases. Mainly it is an inert gas called nitrogen, mixed with around 20% oxygen, which is what we need, and other gases such as carbon dioxide, which is being blamed for everything, it seems. It also contains water vapour in varying amounts, dependent on the temperature and the humidity.
Oxygen is separated from air, to be used for a huge variety of very useful applications, such as medicinal, welding, metal cutting, and as part of the process, nitrogen is separated in prodigious quantities. It is a by-product of oxygen production. And here is the rub – the nitrogen produced has no moisture in it, and is inert – it will not support combustion – which requires oxygen.
One application touted for nitrogen is in tyre applications where either the moisture content of the air, or the risk of fire, makes a dry, inert gas very acceptable.
In our humid tropical summers, the typical tyre service air compressor should be fitted with an after cooler, which cools and therefore de-humidifies the air, provided the air reservoir is drained regularly. Unfortunately, not every compressor has them, and it is not uncommon for airlines to contain water.
A law in physics states that the total pressure is the sum of the partial pressures. If moist air under pressure gets hot, it will rise further in pressure than dry air.
Some pressure increase is normal under operating conditions, as the tyre gets hot due to flexing. 2 to 3 p.s.i (15 to 20 kPa) increase is normal for passenger radials. If it is much higher, the tyre is either under-inflated or overloaded. This is why all inflation pressures are specified as ‘cold inflation pressure’, and a check first thing before setting out is the only meaningful measurement.
Nitrogen is used to inflate heavy aircraft tyres, which are already operating at exceedingly high pressures, in the region of 230 p.s.i. It removes one source of risk. It also stops unauthorised service people adjusting tyre pressures, since they are unlikely to have a source of bottled nitrogen.
Some heavy truck fleets are using nitrogen filling, but the claimed advantages of more casing life appears to be stretching it a bit. What is does do is dissuade the truck driver from deflating the steer tyres to get a more comfortable ride, since steer tyres are usually inflated to 110 to 120 p.s.i. Many of the fuel tankers you see on the highway also use nitrogen inflated tyres, which appears worthwhile to the operators because of the lower fire risk.
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