Air has all sorts of properties, if we can understand how it behaves then we can understand the problems and complexities of compressing it. First of all let's consider it's properties and try to get rid of some misunderstandings.
Air has its own weight. It's quite easy to demonstrate this by weighing an empty balloon on a set of accurate scales. Then pump the balloon up with a bicycle pump and weigh it again. The balloons got heavier. The reason it's got heavier is because the balloon is now full of compressed air. If air didn't have any weight, then the balloon with compressed air in it would weigh the same as the empty balloon. Because its heavier, we've demonstrated that air has weight.
Because the weight of air varies with pressure and temperature it has to be defined accurately. Meteorological offices and aviation textbooks use the following figures.
Air is under pressure; this is caused by gravity. If gravity didn't exist, then air pressure would not exist. Air would also be weightless, although it would still have mass.
Just a quick note to explain the difference between mass and weight. Here on earth we assume weight and mass to be the same, in our normal day to day lives we don't need to appreciate the difference.
If you were up in space and floating around weightless, you would quickly appreciate the difference. Imagine launching two satellites from earth, each weighing a couple of tonnes. Once up in space these satellites would be weightless. If something went wrong and they bumped into each other and you were between them, you'd get squashed. Even though they are weightless in space, they retain their mass - and it's the mass that squashed you.
The reason for this pressure is because there is so much air stacked up on top of it. If you were higher up, say in and aeroplane, the air pressure outside the 'plane would be much lower.
The reason for the reduction in pressure is because there is less air stacked up on top at these high altitudes.
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. This is the basis of the Characteristic Equation
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.
Neither of these conditions require you to specify a moisture content, this is because it's not necessary. We are defining a volume and it doesn't matter if you fill the volume with air or seawater, it's still only a volume. The volume will be constant whatever you put in it.
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.
Whenever people talk about air compressors, they always refer the volume of the compressed air back to atmospheric conditions. For example, if a compressor delivers 1000 Scfm, it means that the compressed air occupies a much smaller space inside the pipe, but if you could magically un-compress the air back to Standard conditions then it would fit inside a box measuring 10ft x 10ft x 10ft.
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%, (or its 100% saturated, it means the same thing).
A sponge half full of water can take on board just as much again. This sponge has a relative humidity of 50%. And so on, and so on.
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. Its a little bit like dissolving salt in water. You can only dissolve just so much salt into a pint of cold water, but you can dissolve a lot more salt into boiling water. However, when the boiling water finally cools down, the dissolved salt will come out of solution and crystallise.
Air acts in the same way. Hot air holds much more water than cold air, however if you cool down hot air which has a 100% RH, the water vapour condenses out into liquid.
The amount of water vapour contained in air can be measured in one of several ways:
You can see this every day, leaves getting whipped up by the breeze and being blown down a road. In an air compressor the velocity is usually ignored, unless the velocity becomes significant.
Pressure is made up of two components, Static head and Velocity head. The sum of these two produce a Total Head. In a reciprocating compressor or screw compressor the Velocity Head is ignored because it is insignificant when compared to the Static Head. In a turbo compressor the static head on the impeller is insignificant compared to the velocity head. For the technically minded, this is why the area under a PV diagram does not reflect Work Done when carrying out calculations on TurboCompressors.
Air is a mixture of gasses, mainly nitrogen and oxygen. The typical composition of natural air is as follows.
Component. |
Mass% (dry air) |
Volume % (dry air) |
Oxygen |
23.14 |
20.9476 |
Nitrogen |
75.52 |
78.084 |
Argon |
1.288 |
0.934 |
Carbon Dioxide |
0.048 |
0.0314 |
Hydrogen |
0.000003 |
0.00005 |
Neon |
0.00127 |
0.001818 |
Helium |
0.000073 |
0.000524 |
Krypton |
0.00033 |
0.000114 |
Xenon |
0.000039 |
0.0000087 |
Air also contains water vapour and hard matter such as dust, microbes and pollen. These variables depend upon climatic conditions, which vary worldwide. The table therefore reflects the European average dry gas content of air, which may vary slightly in your area.
When we look at compressor performances we have to take into account the moisture content of air. This dramatically affects the performance of an air compressor for the following reasons.
The mean molecular weight of dry air is approximately 28.97.
The molecular weight of water is only 18.
This means that volume by volume, moist air is lighter than dry air.
When you buy a compressor, the manufacturer may quote you a power absorbed figure based upon a 65% RH value. However, the power absorbed by the same compressor handling dry air will be higher, because dry air is heavier than wet air.
To confuse things a little further, the amount of energy required to heat up dry air is less than the amount to heat up the same volume of wet air. This is because of the difference in Specific Heat between dry air and water vapour.
Everything has a Specific Heat. This is the amount of energy that is required to heat up a given mass of stuff compared to the amount of energy required to heat up the same mass of pure water. If we keep the pressure the same, the Specific Heat is termed Cp.
Cp for 100% water saturated air at atmospheric pressure is about 2000 Joules/kg deg K.
Cp for dry air at atmospheric pressure is 1020 Joules/kg deg K.
In real terms this means that it takes almost twice the amount of energy to heat up the water vapour in wet air, than it would take to heat up an equal number of molecules of dry air. This is another factor that has a direct effect upon the performance of an air compressor.
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2007-11-15