Air usually contains some Water Vapour

Date: 2015-10-07; view: 520.

Volume

Temperature

Air has temperature, an obvious statement really. Like most things around us, air expands when it gets hot and contracts when it gets cold. Heat up an empty can and then put the lid on it. When the can cools down it collapses. The reason for this is that the air inside has cooled and it now occupies a smaller volume. This also means that the pressure inside is lower than the pressure outside, which has in turn caused the can to collapse. We have just demonstrated that Temperature has an effect on Volume, and that Volume has an effect on Pressure.

An important thing to remember is that whenever we use Temperatures and Pressures in a calculation, they are always absolute values.

Another really obvious statement is that air occupies a specific volume. This volume is inter-related with pressure and temperature. If you squeeze air into a smaller space the air gets hotter. This is easily demonstrated when you pump up a bicycle tyre. The harder you pump, the hotter the air gets and the hotter the hand pump gets.

Because the amount of air contained within a box will vary with temperature and pressure, it is necessary to qualify the temperature and pressure. For this reason we have developed Standard Volume and Normal Volume.

Standard Volume is measured at standard reference conditions, which for compressor performance testing has been defined as 20 deg C, at 1 bar absolute pressure. This is also sometimes called the Standard Temperature and Pressure condition although technically it is incorrect as Standard Reference Conditions replace the obsolete term STP.

Normal Volume is measured at normal temperature and pressure conditions which has been defined as 0 deg C, at 1 bar absolute pressure. This is also called the Normal Temperature and Pressure condition.

The amount of air contained within a Standard cubic metre (or standard cu ft) is different to the amount contained within a Normal cubic metre (or normal cu ft). Even though the volume is the same, the weight will be different because air at Normal conditions is denser than air at Standard conditions.

Life gets a bit more complicated. Air behaves a bit like a sponge, if there's any water around it will try to absorb it. Like a sponge it can only hold just so much water before it becomes saturated. Again like a sponge, if you squeeze it (compress it) the water will drip out.

A dry sponge doesn't have any water in it; therefore it has a relative humidity of 0%.

A soaking wet sponge can't take in any more water because it's already saturated. Therefore this sponge has a relative humidity of 100.

If you pick up the saturated sponge and squeeze it, water drips out. If you dip it in a bucket of water, keeping it squeezed, it doesn't absorb any water out of the bucket. We've demonstrated that by compressing the sponge we have reduced its ability to hold water. Although a squeezed sponge holds less water than an un-squeezed sponge, the squeezed sponge is nevertheless 100% saturated.

Air acts in the same way. Compressed air can't hold as much water vapour as atmospheric air of the same temperature.

Hot air also has the ability to hold more water than cold air, however if you cool down hot air which has a 100% RH, the water vapour condenses out into liquid.