Compressor water can be a problem. For anyone that’s ever used a compressor for any length of time, they know that the process of compressing air generates water. Lots of water.

Farther along this page there is information about why compressors generate water!

What’s the big deal with compressor water; water that the pump has forced into the compressor tank and through there into into your compressed air lines, anyhow?

Fish Eyes Is One Compressor Water Problem

You will know the answer to that if you have sprayed fish eyes onto your paint project from a compressed air paint gun. Water exiting the gun with the air / paint spray lands on the surface of the object being painted, and prevents the paint from adhering. That leaves the characteristic paint-free dot ~ a “fish eye” is the term the industry calls this problem!

The image is an example of what fish-eyes can do to a paint job.


image of fish eyes on paint job

“Fish eyes” in your paint job is one by-product of water in the compressed air line.

Not that having to refinish your paint job isn’t a big enough problem, but what else does water do? See the pages on this site about what water in your compressed air lines does.

If you’ve had a chance to look at the other pages relating to water in your compressed air lines, you have a good understanding now of where the water comes from, and what it can do to your compressed air systems. Water can be a real problem for the compressed air user.

Getting Water Out!

The process of air preparation and de-watering your compressed air is a continuum. By that I mean that you can have bone dry and pristine clean air for every application in your workshop or plant and spend a fortune getting the air that way.

Yet there’s not much point in spending the money necessary in preparing your compressed air to that level of dryness and cleanliness if your application does not require it.

You Decide How Much Treatment

Decide for yourself (or let your compressed air applications decide for you) how dry and clean your compressed air has to be, and only use the financial resources and equipment to get your air to that level. This will save you money!

Draining the receiver and modifications to how the receiver(s) are installed can help remove water at the source. Sensible plumbing of your compressed air supply lines can resolve a lot of the problems with compressor generated water.

Now, if you have installed your air mains sensibly, and you have ensured that your compressor receiver(s) are being drained regularly, and you still have a water problem at your end application, it is time to take compressed air treatment up a notch by adding some more equipment designed with the express purpose of ridding your compressed air of water in vapour or free water form.

Aftercoolers and refrigerant dryers may need to be used to remove water from the air stream.

So far, on this page we’ve talked about:

  • Where water in your compressed air system comes from
  • Problems that this water causes
  • Corrective and simple plumbing ideas to help reduce water in the compressed air system
  • Aftercoolers
  • Refrigerant dryers

Now. there’s another level of compressed air drying you can move up to when you need absolutely “bone dry air”. Desiccant dryers may be required if you require compressed air that is dessert dry.

Before you start on the journey to improving the quality of your compressed air through air treatment, you absolutely need to know your clean, dry air end point. How clean, how dry, how pristine does your compressed air supply have to be?

Do you need instrument quality air for the whole plant or do just certain areas need highly treated air, and most a basic level of water removal?

Thousands upon thousands of dollars can be spent to purchase air preparation equipment and even more will be spent on the energy costs to power that equipment. You do not have to spend the money if your application doesn’t demand air of that quality!

Did you know that… “Typically for a 1000 SCFM compressed air system, (nominal 250 HP) the annual volume of Condensate is about 80,000 gallons” Source:

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.” Source:

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. There is lots of information about that on this site, too!

Cooling compressed air

If natural cooling of your compressed air does not resolve the water issues, and you have still got water dripping from your air valve exhaust ports and blowing through your air tool onto your work place, you have got to take it up a notch, in terms of air treatment.

Compressed air de-watering options

There are a number of pieces of equipment that you can purchase and install to cool and dry your compressed air.

I have listed ideas below, roughly in order of lower cost to higher cost. That cost might be reflected in higher purchase price, or higher operating costs, depending on the solution selected.


The first piece of equipment to consider is an aftercooler, a water jacket style, also known as a Shell & Tube type aftercooler.

This style uses cold water to remove heat from the air stream and to cool the compressed air, allowing moisture to condense out.

The compressed air from the discharge on the compressor flows into the after cooler and through one of a number of tubes inside the shell.

Outside the tubes, cold water is flowing throughout the shell, either in the same direction as the air flow in the inside pipe, or in the opposite direction depending on the brand and specifications.

As the warm, wet air from the compressor passes through the tubes, heat is radiated out of the compressed air, through the tube, and is removed by the cold water. Water condenses out of the compressed air stream as a result.


water solutions option 3 - Water Jacket Aftercooler

One of the benefits of this type of water jacket cooling system is that it is low technology. Installation is non-complex, and it – overtly at least – does not consume energy to function.

In earlier times it was common to have water piped from the plant water supply into the water jacket, and out the other end to a sewer. Water was cheap and abundant… then!

Water Is Not Cheap Any More

However, water costs and sewage costs have put an end to that practice as companies are charged first for the water, and then again for the sewer costs for that volume of water.

As a consequence, and for good environmental reasons, the shell & tube type of cooler now normally recirculates the same water in an endless loop.

Thisrequires a pump, which uses energy, more complex control circuitry, and – depending on the temperature of the water as it exits the cooler, perhaps a cooling tower of some sort, all of which adds significantly to the equipment capital cost, and may – too – involve the consumption of higher levels and costs of energy to run.

A shell & tube type compressed air cooler starts out as being uncomplex and low-tech, but could grow quickly into a much more complex and expensive solution.

Refrigerant Air Dryer

Another post-receiver air cooler / dryer is a refrigerant system.

It functions similarly to your home refrigerator, with a compressor and cooling coils, but rather than pumping heat out of a closed box like happens with the fridge, the refrigerant air cooler funnels compressed air through cooling radiator(s) inside the dryer that decrease the temperature of the compressed air.

The refrigerant dryer lowers the temperature of the compressed air so that water vapor in the air condenses out where you want it to, and not in the air lines or air tools where you do not.

What You Need To Know About Selecting A Refrigerant Dryer

Things you’ll need to know to get your refrigerant dryer sized are:

  • temperature of the compressed air entering the dryer
  • maximum flow of compressed air through the dryer
  • electrical supply voltage(s) available

The refrigerant dryer will provide a range of drying predicated on how much air going through it and at what temperature the compressed air is as it enters the cooling coils.

No Benefit To Under Sizing A Dryer

If you under size the refrigerant dryer – purchase a smaller model than necessary to save up-front costs – you will find that the unit cannot provide the compressed air cooling needed, and you undermine the purpose of the unit in the first place. Money saved here will cost you in the maintenance of and the replacement of air using components downstream.

It is important to ensure accuracy in sizing the unit, or err on the side of caution and oversize the dryer a bit. The downside of getting an oversized refrigerant dryer is the cost goes up, both in terms of purchase and operating the unit.

Still Need An Aftercooler?

It is quite possible that there will still need to be a compressed air aftercooler – such as a shell & tube style – installed before the refrigerant dryer. In high demand air applications the air temperature coming to the refrigerant dryer from the compressor would be too high for the dryer capacity without pre-cooling the air.

Conditioning or treating compressed air is a continuum. You will need to add additional equipment to bring the plant air supply to a level of dryness needed by the equipment you are powering with compressed air.

Another form of dryer used where the air application demands it, is the desiccant dryer.

Desiccant Dryers

Some applications require the compressed air to be absolutely bone dry. The compressed air will have a down to the minus temperatures, in some cases down to -100 deg. F. That’s dry air.

What that means for the compressed air with a dew point that low is that as long as the temperature in the system and ambient air is higher than that -100 deg. F, no more water can condense out of that compressed air anywhere in the plant.


water-solutions-option4 - desiccant dryer

Desiccant dryer and chemical desiccant

There are costs associated with this type of system, and, as I always say, you only want to treat your air to the level of dryness that you need. Going further will make dryer air yet add cost, and if the system does not require compressed air that dry, there will be no real benefit for your compressed air using applications to pay for greater dryness.

Desiccant dryers can be relatively small, in-line type, or twin tower units with regenerative capabilities for high compressed air-flow systems.

Simple, in-line desiccant dryers are not complex, relatively low cost, are easy to install, and work! They contain a desiccant chemical that is consumed as water is adsorbed.

Installation of a Desiccant Dryer

Install the in-line desiccant filter immediately just before the application in which you are using compressed air.

Just upstream from the desiccant dryer I recommend that you install a general purpose compressed air filter. This G.P. filter will remove any free water before it can enter into, and perhaps overload, the in-line desiccant dryer. It will also remove airborne contaminants that could, in time, coat the chemical in the dryer, rendering it ineffective.

Yes, the desiccant will remove free water too, but in so doing it will rapidly deplete the desiccant chemical. The in-line desiccant dryer should be installed to remove water vapor, and to dry the air to a dew point below the temperature of the application and ambient air. If it does this, there should be no more condensation downstream from the desiccant dryer.

Draining the Desiccant Dryer

Some in-line desiccant dryers need to be drained regularly. Their use results in a pool of water and desiccant chemical in the bottom of the housing. If this filtrate is not drained regularly, it will ultimately flood the dryer, and destroy it’s capacity for drying air.

Check the manufacturers specifications to determine how best, and how frequently, to drain a particular desiccant dryer.

Refills Needed Regularly

The in-line desiccant dryers will require periodic charging with the desiccant material, and this means that you have to know when to do it, and of course someone has to do it.

This is an important maintenance issue. It is one of the down-sides of the in-line desiccant systems, particularly in companies that have more maintenance work to do in the plant than their down-sized maintenance staff can cope with.

Refill the in-line desiccant dryers regularly, or you will soon see water appearing in your tools and equipment.

Twin Tower Desiccant Dryer

If an application calls for high volumes of very dry air there is another alternative in desiccant dryer products.

Water solutions option 4 - Compressed Air Dryer
The twin tower desiccant dryer has two desiccant dryer towers built into the one system.

Periodically, based on time, or perhaps a moisture sensor in the compressed air system, the air entering the dryer will change flow paths from one desiccant tower to the other.

Once air is flowing through the new path to be dried, dry air from that tower is then used to regenerate or dry the desiccant charge in the now unused tower.

After a certain time period, the valve shifts again, and the compressed air flows to the tower with the now-dry desiccant, and that new supply of dry air is used to regenerate the desiccant in the other tower.

These twin tower desiccant units are sized based on the expected flow demands of your compressed air system and the level of dryness required by the compressed air equipment in the shop.

Heated Or Heatless

One type of dryer is classified as heatless, meaning that no external heat is required or needs to be generated to help dry the desiccant.

Other similar-in-concept dryers heat the air to speed the drying of the desiccant.

The choice between them depends on your plant air demand, the level of dryness required, the speed with which the towers need to regenerate, and of course, the cost to buy and run.

In time, these twin-tower units will require maintenance, including recharging of the desiccant medium. Yet, they do provide well-dried compressed air for the whole plant, negating the need for installing multiple, in-line units throughout the whole facility, along with the need to maintain these multiple desiccant dryers.



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