Where Does Water Come From in an Air Compressor Tank?

Graph of temperature versus max water storage capacity of air.

Anyone who owns an air compressor knows that one of the regular maintenance chores is to drain the tank to release water. This is essential to prevent eventual corrosion of the tank.

Water accumulates naturally as air is compressed. Air can only hold a certain amount of water at a given temperature before it starts to condense out. Relativity humidity (RH) is defined as the amount of water vapour in air as a percentage of the maximum amount of water that air can hold at a specific temperature. So 100% RH means air is saturated and 0% RH means it is totally dry. As temperature increases, air can hold more water per unit volume and vice versa. When air is compressed, it takes up less space, and because there is a limit to the amount of water a certain volume can hold, it will eventually condense out if volume is reduced sufficiently. Water vapour density in the metric system is usually specified in g/m3.

Question:
2 cubic metres of air at 20° C is at a water vapour density of 7 g/m3. (50% RH at this temperature) The air is compressed by a factor of 10 and allowed to cool back to 20 deg C. How much water condenses out?

 

Answer:
The amount of water in the original air is 2 m³ x 7 g/m³ = 14g

When it is compressed, the new volume is 2/10 = 0.2 m³

At 20 deg C, the max amount of water the air can hold is 15 g/m³ (see graph below)

But 0.2 m³ can only hold 0.2 x 15 g/m³= 3g

So the amount of water that condenses out and collects in the air tank is 14 - 3 = 11g