Compressor Water - Here's Why

Compressor water - here's why! Why do your compressed air mains and the drop legs to your various applications have water flowing through them? This page is starting you on the journey of understanding how compressors generate water, what that water does to your compressed air system, and leads to solutions (pun intended) to rid your air system of water's unwelcome presence.

Why does water spray out your airbrush gun, creating all sorts of fish eyes when you are nearly completed a perfect air brush project?

Here Is Why.Picture this. It is a real hot, sticky, sultry summers day. The R.H. (relative humidity) exceeds the 85% mark today, and the air feels wet - your breath is labored, there is no breeze, and your skin sticks to itself. Moving or working outside becomes increasingly unpleasant. At an R.H. of 85%, the air you are walking through and breathing and working in is holding 85% of the water vapor it is capable of holding.

Now, take that air at 85% relative humidity from the outside of the plant and suck it into the intake port on your compressor. The compressor will take that free air which is at one atmosphere / or one bar / or at about 14.7 PSI at sea level, and 85% R.H, and it will compress it.

All three of these pressure measurements are the same. One bar is the same as one atmosphere which is 14.7 PSI actual. It is 14.7 PSI actual. The pressure may shows zero because the gauge has been adjusted at the factory to ignore the actual ambient pressure. The gauge registers zero even though the actual air pressure is 14.7 PSI., or one bar, or one atmosphere.

Your compressor has a high cut in set point of around 120-150 PSI, a common range of high pressure set points for many industrial air compressors.

120 PSI is approximately 8 bar or about 8 atmospheres, 150 PSI is about 10 bar.

I will use a 120 PSI pressure level for this example.

So, for each cubic foot of usable compressed air at 120 PSI, the compressor is ingesting about 8 cubic feet of free air, scrunching all of those cubic feet down into the space of one. One cubic foot of space inside the tank now had 8 times the air compressed into it.

With 8 cubic feet of air in the space of one cubic foot, and each of those 8 cubic feet containing the 85% R.H., the relative humidity of the resulting compressed cubic foot will well exceed 100%.

We know that when the R.H. outside the plant in the air we breathe gets to around 100%, the atmosphere cannot hold any more moisture, and it rains.

For most folks, that usually happens on weekends! :-)

It's Raining In Your Air Tank

Water here's why - Random Rain Drops Sketch

When the relative humidity inside your compressor receiver gets to 100%, which happens almost as soon as the compressor kicks in and starts compressing air, then it begins to rain inside your compressor tank / receiver as well. As long as the R.H. is above 100%, water vapor will condense into liquid water inside the compressor tank.

And as the compressor runs, it rains, and rains, and as your compressor kicks on and off with the downstream applications demanding more and more compressed air, it rains and rains even harder in the tank, starting to fill the air tank of your compressor with a river of water.

The amount of water will, of course, be relative to the humidity of the air ingested, the downstream demand for compressed air, and the size of the compressor. The larger the compressor and the higher the downstream demand for compressed air, the higher the volume of water generated by the compressing of air.

"A 200 HP compressor operating in a climate of 60 degrees F with 40 percent relative humidity will generate approximately 50 gallons of condensate a day. However, that same compressor operating in a climate of 90 degrees F with 70 percent relative humidity will generate approximately 260 gallons of condensate a day."


If the R.H. of the free air outside is higher than about 15%, and it is almost everywhere in the world all the time, every time your compressor is running it is raining inside the receiver. That is the nature of the beast. You cannot avoid generating water when you compress air.

compressed Air is Hotter Air!

On top of that, the air in your receiver is getting hotter, the longer the compressor runs.

Next time you are in a plant that is using a lot of compressed air, carefully touch the compressor receiver. You may find that it gets quite hot.

As the air temperature inside the receiver get higher, the compressed air is able to hold more water in vapor form. It becomes super saturated. Instead of holding 100% of the moisture it can normally carry, air that is super saturated with water might hold 10% more, for example. That super saturated, hot and unstable compressed air in your receiver has to go someplace, and that is right down the air mains to your air-using tools.

Did you ever have an air line blow out of the fitting in your face? If you have experienced that joy you know that compressed air really is pent up energy, and when it lets go it is explosive in nature.

In your plant or home workshop, somewhere downstream of your compressor, you have an application that suddenly demands air. An air valve opens, the trigger on an air tool is pulled, someone starts to clean a part with a blow gun, and all of a sudden there is a rapid decompression in the tank as air flows down the mains and out the air tool.

Hot Compressed Air Cools

And now the hot, super-saturated air from your tank is suddenly cool again, as it races along the air mains and air lines to the air tools.

As it escapes the receiver the compressed air roils up the water in the receiver bottom taking drops of water and contamination towards the low pressure area. Compressed air moving from your compressor tank can take that soup of contaminants with it in droplet form into your plant air lines.

What happens to compressed air as it expands from the tank into your air mains and lines? It cools.

What happens to the airs ability to hold moisture as it cools? It lessens.

What's the R.H. of the hot compressed air leaving your receiver? Well over the 100% mark, as it has been supersaturated by the hot conditions in the receiver.

What is this a recipe for? It rains in your air lines too!

Water Is A Problem?

The combination of water and the soup of contamination from the receiver, couples with any contaminants in the main air lines (rust from the pipes / pipe dope, etc.) to send a slurry of crud down through your air valves, to the actuators, or into your air tools. When the air-using equipment stops, and when the operating-elevated temperatures cool, the crud-soup dries. It hardens into a varnish-like consistency that effectively stops the operation of some of your control and actuator equipment. Next time you go to start the machine, it may not start, or it may not even run!

Remember how many times you've had to tap a reluctant air valve to get it to fire, or smacked the air tool on the bench to get it going? :-) This is why. The crud-soup has dried out and it is sticking the valve components or the tool parts together.

Water Costs Money

This costs you money in lost production, through the time necessary for maintenance staff to diagnose and resolve the problem, and the loss of some of the pneumatic components themselves, due to contamination build up inside, and the shortened life expectancy as a result.

That is why water in your compressed air lines is a problem!

And that is why you need to condition compressed air before use. To do that, you will want to have a greater understanding of compressed air conditioning and compressed air treatment. This link takes you to a page on this site.

Got a question about water - heres why? Here is the forum.