Water, here's why. Why there's water in your air lines.

Why do your compressed air mains and the drop legs to your various applications have water flowing through them? Why does water spray out your airbrush gun when you're nearly completed a perfect painting job?


Here's why!

Picture this. It's a real hot, sticky, sultry summer's 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’s capable of holding.

Let’s 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.

120 PSI is approximately 8 bar or about 8 atmospheres.

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 them down into the space of one. The same cubic foot of space, but now with 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! :-)

When the relative humidity inside your compressor receiver gets to 100%, which happens almost as soon as the compressor kicks in, then it begins to "rain" inside your 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 even more compressed air, it rains and rains even harder, filling the bottom of your receiver with huge amounts of water.

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

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

Next time you’re in a plant that’s using a lot of compressed air, carefully touch the compressor receiver. You’ll be surprised at how hot it gets.

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.

That super saturated, hot and unstable compressed air in your receiver has to go someplace!

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

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 there’s a sudden decompression in the main line from your receiver as the air flows with cyclonic force to the area of lower pressure.

The air pressure in the plant air mains drops in response, and the air inside the compressor receiver reacts by exploding towards the lower pressure area too.


And now it's cool again?

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. So, air moving from your compressor will take that “soup” of contaminants with it in droplet form into your plant air lines.

What happens to compressed air as it expands? It cools.

What happens to the air’s 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’s been supersaturated by the hot conditions in the receiver.

What’s this a recipe for? The hot moisture laden air cools rapidly as it transits the plant air lines, and as a result, it rains in your air lines!


Why is this 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 equipment stops, when the operating-elevated temperatures cool, when the “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 won’t 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 bendh to get it going? :-) This is why. The "varnish" has dried and it's sticking the valve components together.

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

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

And that’s why you need to “prepare” the air. To do that, you’ll want to have a greater understanding of Compressed air preparation / air treatment.