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Water in Compressed Air Calculations | Air Compressor Guide

# Water in Compressed Air Calculations

By: Cas | Posted on: 27-03-2018

# Water in Compressed Air Calculations

To continue on the topic of ‘water in your compressed air’ (the other two articles are here and here), let’s examine how we can calculate the amount of water that is generated in a typical compressed air system. I promise, you will be surprised! ## What influences the amount of water in a compressed air system

There are a few things that influence the amount of water in your compressed air system. The main factors are the intake relative humidity and temperature, the end pressure and the end temperature. The intake relative humidity and temperature gives us the total amount of water vapor that enters the compressed air system. In other words, we can calculate that absolute humidity of the incoming air in grams per cubic meter. The output pressure and temperature gives us the maximum amount of water vapor that the compressed air can hold, again in grams per cubic meter. If the incoming water vapor is more than what the compressed air can hold, some of it will condense into liquid water, once compressed. If the holding capacity of the compressed air is big enough to hold all the incoming water vapor, then no water vapor will condense into liquid water: your compressed air stays dry. ## Static example

Let’s first take a look at a static example. By this I mean we will compressed a fixed amount of air, as if we were in a laboratory. Let’s say we have a 7 bar compressed air system. Since we normally talk relative pressures in compressed air land, that’s 7 bar relative pressure or 8 bar absolute pressure. This means we need to compress the ambient air 8 times (from 8 m³ to 1 m³ for example). The pressure will increase from 1 bar absolute to 8 bar absolute. Air Compression:- Volume decreases 8 times. From 8 m³ to 1 m³

• Pressure increases 8 times. From 1 bar to 8 bar (absolute)

But we don’t just compress air. We compress air that holds some water vapor. The total (absolute) amount of water vapor of the incoming air depends on the temperature and the relative humidity of that air. As we compress the air, the water vapor that was present in the 8 m³ is now squeezed into 1 m³. (we assume the temperature constant. In reality it isn't, but if we give the compressed air time to cool down, it will again be the same temperature as it was before compression). Now let’s see what happens with the water vapor. Let’s say we had air of 20°C and a relative humidity of 50%. Air of 20°C can hold 17 g/m³ (grams per cubic meter). Since our air is 50% relative humidity, it holds half of that: 8.5 g/m³. Now let’s do some calculations... Compressed air created We compress the air to 7 bar relative pressure. That’s 8 bar absolute pressure. In the process we reduced the volume 8 times. Our 1 m³ intake air becomes 1/8 m³ compressed air, or 0.125 m³ compressed air. Water content of the compressed air Each cubic meter of inlet air contains 8.5 grams of water vapor. We compress the air: we push 8 cubic meters into 1 cubic meters of compressed air. This means our compressed air has a water content of 8.5 * 8 = 68 g/m³ (grams of water per cubic meter of air). Maximum water vapor content in compressed air The hold capacity of air depends on it’s temperature. This means that a cubic meter of ambient air of 20°C can hold the same amount of water vapor as a cubic meter of compressed air of 20 °C. The pressure only has a tiny bit of influence on the water vapor holding capacity of the air. We can forget about it here. We already saw that 1 cubic meter of 20°C can hold a maximum of 17 grams of water vapor. Our compressed air of 20°C can hold the same 17 grams of water vapor maximum. Water vapor vs liquid water in our compressed air As we see, the water content of the compressed air is bigger than the maximum water vapor holding capacity of the air! We could say the relative humidity is 400% (impossible!) - Max = 17 g/m³

• Actual = 68 g/m³

The difference will condense into liquid water, it ‘rains’ in our compressed air system. The result is that we have 17 gr of water vapor (the maximum, 100% relative humidity, or saturated air), and we have 68 – 17 = 51 grams of liquid water in our compressed air. All in one table: Before compression After compression Pressure [bar, absolute] 1 8 Volume 8 m³ 1 m³ Temperature [°C] 20 °C 20 °C (after cooling down) Max. humidity [absolute] 17 g/m³ 17 g/m³ Actual humidity [absolute] 8.5 g/m³ 68 g/m³ Actual Humidity [relative] 50% 400% Total water present 68 g 68 g Water as vapor 68 g 17 g Liquid water 0 g 51 g So for every 8 cubic meters of ambient air that we compressor, we generate 1 cubic meter of compressed air at 7 bar (relative pressure, 8 bar absolute pressure), with a water vapor content of 17 g/m³ . And we generate 51 g of liquid water. That’s 0.051 liters. ## Compressed Air System Example