# Is it possible to store moist air at high pressure?

Is there any way to store moist air inside a high pressure tank?

Let's suppose that I have humid air (30%) at room temperature and I want to store it at 300Bar in a tank. when I extract the fluid from the tank (isothermal expansion) to bring it back to ambient pressure will I keep the same percentage of humidity?

In most cases where it is necessary to store high pressure air to be used in a system that requires humid air, a cylinder is used where ambient air is stored which then pass into a humidifier. I'd like to know if it is possible, conceptually, to remove the humidifier and directly store moist air in some way

There is one importing principle to consider. Air does not store vapor. Space does. Less space means less vapor capacity, no matter how much of air is there. (*)

If air is being compressed, the partial vapor pressure increases. It it reaches the saturated vapor pressure at given temperature, water vapor starts to condensate. When air is then expanded, its absolute and relative humidity decreases, according to the expansion ratio.

E.g. Lets have 1 cubic metre of compressed air with 100% relative humidity. If it expands to 2 cubic metres and temperature raises back to the initial temperature, its relative humidity will be 50 %.

So compression provides liquid water and expansion needs liquid water to restore humidity.

If there is compressed air with cold water and if pressure drop e.g. 20 to 1 atm) then its relative humidity suddenly drops from 100% to 5% (when air warms up back after adiabatic cooling down.). Water exposed to low humidity air will start slow evaporation.

It will not help much in humidity restoration, unless some active evaporation technique is involved a/o the air release is slow, passing evaporation unit.

(*) Exactly speaking, saturated vapor pressure and therefore vapor capacity of space very slighly increases with total pressure. But it is negligible in context of your question.

Note that absolute humidity is in g/m3 - it is intensive, not extensive variable. Rather, the vapour content = absolute humidity * volume.

• let's see if I understand correctly, the absolute maximum humidity depends only on the temperature and the volume, if I compress the humid air, I reduce the volume and therefore the absolute maximum humidity. therefore as I compress the water it condenses inside the tank. I have another question, if I have two fluids in the high pressure tank (air and condensed water) if I abruptly reduce the pressure a certain percentage of liquid water should turn into vapor? – Patrick Danzi Mar 15 at 10:38
• Sudden decompression would also typically be accompanied by a change in temperature, which would also reduce the rate of evaporation. – Michael Seifert Mar 15 at 19:18
• @michaelseifert Sure, I have implied it in the note about temperation of the final air volume. – Poutnik Mar 15 at 20:04
• @PatrickDanzi It will turn into vapor but only slowly. No matter how fast or slow you draw the first air that comes off will be at 1.1% RH. Draw it slow enough and it will eventually rise to 100% RH--but slow enough is measured in days. Probably lots of days. – Loren Pechtel Mar 15 at 23:46