In this article, we’ll cover the relative humidity ratio in pneumatics, the amount of moisture in air based on relative humidity and temperature, and how to calculate the amount of moisture released by compressing air.
Air contains a certain amount of water as moisture. This amount is directly proportional to the air’s temperature. When air is cooled, the moisture inside it separates out as water droplets. This water is called condensate.
For pneumatics, this is important because the amount of water in the air depends not on pressure, but on the air’s temperature and volume. For example, in a system where the system pressure is Pe=6 bar, 7 times more air has been compressed. But the amount of water inside is the same as for 1 unit of air. As a result, in pneumatics it’s essential to separate condensate from the air using various equipment. An increase in the air’s temperature also means an increase in the amount of water. For example, 1 m3 of air contains 30 cm3 of water at 30°C, and 2 cm3 at 10°C.
Amount of Moisture in Air Based on Relative Humidity Ratio and Temperature
Atmospheric air contains certain amounts of moisture depending on the air’s relative humidity ratio (RH) and air temperature.
As temperature increases, the amount of moisture in the air increases.
In the chart alongside, we see the amount of moisture in the air based on relative humidity ratio (RH) and temperature.
Relative Humidity Effect
At a constant temperature, as the relative humidity ratio increases, the amount of water vapor the air can carry also increases proportionally.
Condensation Risk
When compressed air cools, the moisture it can no longer hold condenses into liquid; this can cause corrosion in pneumatic lines.
Use of Dryers
Dryer units are generally used to prevent moisture-related failures in sensitive pneumatic equipment.
Calculating the Amount of Moisture Released by Compressing Air
Example Problem:
A compressor raises 10 m3 of air, at a temperature of 20°C and a relative humidity ratio of 70%, to a pressure of 7 bar. What’s the amount of moisture released?
Solution:
The amount of moisture in air with RH 100% (fully saturated with moisture) is found to be 17.4 g/m3 at 20°C, as seen in the figure above. 10 m3 of air contains 17.4 x 10 = 174 g of moisture.
Since the problem states the air drawn in by the compressor has an RH of 70%, there is 174 x 70/100 = 121.80 g of moisture.
Since the air pressure is compressed from 1 bar absolute to 8 (7+1) bar absolute, the air’s volume decreases, and the moisture in the air condenses into water. When 10 m3 of air is compressed to 7 bar, the air’s volume is calculated with the following formula.
P2 . V2 / T2 = P1 . V1 / T1
(7+1) . V2 / 273+20 = 1 . 10 / 273+20
V2 = 1.25 m3
When 10 m3 of free air is compressed to 7 bar, its volume drops to 1.25 m3. The amount of moisture in the compressed air coming out of the compressor will be 1.25 m3 x 17.4 g/m3 = 21.75 g.
The amount of moisture released: 121.80 – 21.75 g = 100.05 g.
Related Questions
The amount of water vapor a given quantity of air can hold is determined by that air’s temperature. Even if the air is compressed, the total amount of water inside it doesn’t change; it’s just easier to notice condensation because the volume has shrunk.
A compressor operating in a hot climate draws in much more humid air than one in a cold climate. This is why the need for drying becomes even more critical for pneumatic systems operating in hot environments.
Related Posts
Maintenance and Repair Recommendations for Hydraulic Circuits 
Hydraulic Cylinder Force Calculator 
What Is a Pneumatic Muffler, What Does It Do? 
Screw Compressors: Construction, Types, Strengths, and Weaknesses 
Safety Rules for Workers on Hydraulic and Pneumatic Machines 
Flow Control (Speed Control) Valves 
Pneumatic Conditioners (FRL Units) 
General Safety Rules for Hydraulic Technicians