Moisture in compressed airlines can be a serious problem and certainly a challenge to overcome. With air being compressed, water is a natural by-product that can make its way into the air stream. During the cooling, the water moisture condenses and is mixed into the compressed air that’s delivered to your pneumatic tools or devices.
Some water may be okay for a few applications but too much can be a significant problem, causing internal damage like rusting or affecting your tool’s performance. This is where the need for compressed air-drying methods comes in, and this article will provide you with all the common drying methods used across the compressed air industry.
Table of Contents
- Why Dry Compressed Air?
- The Most Common Air-Drying Methods
- How to Minimize Moisture in Your Compressed Air System
- FAQs (Frequently Asked Questions)
Why Dry Compressed Air?
So, why is drying compressed air so important? Not only can water content in the air cause damage to your compressed air system, but it is likely to cause other contaminants to slip into your airstream. For instance, moisture condensate can lead to the corrosion and rusting of the air receiver tank, and your piping network. If the inside of these crucial compressor features is allowed to corrode, rust will eventually form and begin to flake off.
When the rust debris enters the airstream along with the moisture condensate, filters and other components will have to work harder than normal, and their lifespan quickly reduces. If they fail or clog and begin to let small particulates or contaminants through to reach your air tools, you’re in trouble!
Not only will moisture condensate along with other contaminants cause the premature wear of your pneumatic tools, but they can significantly affect the quality of your projects. Let’s say you’re carrying out a painting job using a spray painter, if moisture condensate or any form of contaminant entered the air stream, your finish will result in areas that don’t dry or have evident patchiness. Now, you don’t want this right?!
This isn’t the only job that will be affected by moisture, all will be affected one way or another. It may be minimal, but it’s going to lower the efficiency and quality of every project.
The premature wear of your compressed air system and its components, or the more frequent maintenance required to keep them from breaking due to moisture content, is undoubtedly going to cost you a lot of money! To avoid these additional costs, along with potential downtime, it is strongly advised to have some form of air-drying system or method in place.
The Most Common Air-Drying Methods
Generally, the amount of water content contained in compressed air is a function of the temperature and pressure of the air, so to remove the moisture we must look at ways to change the temperature or pressure. The most common ways of removing or significantly reducing the amount of water content inside your compressed airlines include the following:
- Deliquescent Air Dryers
- Desiccant Air Dryers
- Membrane Type Dryers
- Refrigerant Dryers
- Storage Tank Cooling
Each one of these six methods for drying air is unique and hold their own advantages and disadvantages when compared against each other. Air aftercoolers, storage tank cooling, and membrane-type dryers are three of the most common methods for drying compressed air in mobile applications. While deliquescent air dryers, desiccant air dryers, and refrigerant dryers are typically found in stationary, industrial-like settings.
So, let’s look at each of them in greater detail!
Aftercoolers use the principle of heat transfer that occurs between two bodies of different air temperatures until an equilibrium is reached between them. They typically use this principle to balance the temperature of compressed air using atmospheric air, while some additional drying also occurs within its process.
Heat transfer can occur in three different ways, which is compressed air drying, typically occur simultaneously
Aftercoolers are a type of heat exchange used to help minimize the moisture within a compressed air system and cool the compressed air. By reducing the temperature of the compressed air, you can cause the water and oil droplets to precipitate out of the air and collect the contaminants, where you can then drain them with a moisture separation device or drain trap. The drains can either be opened manually, or they can be automatic so that they open at timed intervals.
The aim is to have the aftercooler being located as close to the compressor outlet as possible. They are a very popular choice for both mobile and stationary air compressors, and there are two main types:
- Air-Cooled Aftercoolers
- Water-Cooled Aftercoolers
You’ll find that an air-cooled aftercooler looks and acts quite like a car radiator. The difference is that the hot compressed air enters the bottom of the air-cooled aftercooler and tube system, and discharges through the upper discharge port into a moisture separator, rather than filling the interior tubes with coolant as a car radiator does.
As the heat transfer of compressed air goes to atmospheric air, the heat from the compression stage and resulting moisture is removed from the compressed air and carried out of the system.
There are, of course, different versions of air-cooled aftercoolers, some more effective than others. There’s a type that uses electrically powered 12V or 24V fans to push air through its system. The tubes on the aftercooler have fins, or metal plates between them to help increase the surface area and dissipate the heat more effectively.
Due to air-cooled aftercoolers being so effective, straightforward to install, and such an easy source, they are often found being used with mobile air compressor systems.
Water-cooled aftercoolers do exactly what their counterparts, air-cooled aftercoolers, do but with more control of the discharge air temperatures. The key difference is that water-cooled aftercoolers use a flow of liquid coolant typically in a shell and tube or plate-fin design heat exchanger to absorb the heat of compression from the compressed air volume.
Water-cooled aftercoolers are more common to find in industrial systems due to their sometimes very big size.
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- Efficient heat transfer
- Easy to source
- Pretty straightforward to install
- The fan-less systems don’t require electricity
- They reduce heat and moisture in a compressed air system
- Heat recovery is difficult
- Water-cooled types require a high volume of air
Deliquescent Air Dryers
Deliquescent air dryers are a common way of using the absorption process to dry compressed air. With the absorption process, incoming compressed air enters the vessel near the base and passes through a mechanical separation section. During this section, the free liquids and solids drop to the bottom of the vessel due to expansion, and this constitutes a pre-drying of the air.
Air then proceeds through a desiccant bed of deliquescent materials like water-soluble salts or shotted urea. These chemicals can condense water vapor as they deliquesce the liquid. Absorption will occur until the deliquescent materials become completely consumed, and then they must be replaced. For more information on them, visit our
Deliquescent Air Dyer Advantages
- Do not need electricity
- Initial cost is low
- No moving parts
- Simple operation
Deliquescent Air Dyer Disadvantages
- Deliquescent material must be added to or replaced when it absorbs, which the costs of replacement and disposal add up
- Dewpoint suppression tends to be on average between 20°F and 30°F
- Downtime required to replace deliquescent material
- Ecological problems with the disposing of dissolved deliquescent material
- Parts of the deliquescent material may solidify in the bed and cause channels for the air to by-pass most of the drying material, reducing its efficiency
Desiccant Air Dryers
Desiccant air dryers work by lowering the dewpoint of compressed air via adsorption. They use a desiccant material to adsorb water vapor onto its surface using any of the three following very common materials:
- Activated alumina
- Molecular sieve
- Silica gel
The process of adsorption will begin as the water vapor moves into an area of lower water vapor concentration in the pores of the desiccant. Once inside the pores, water vapor molecules build up on the surface of the desiccant due to the natural attraction of water vapor to its surface.
As these water vapor molecules gather, the vapor eventually changes its phase and becomes a liquid. This process will continue if the concentration of water vapor in the air exceeds the concentration in the desiccant pores.
The water remains on the surface of the desiccant until it is stripped off via a process called reactivating or regenerating, hence, why desiccant dryers are also commonly referred to as regenerative air dryers. When the water is stripped, the desiccant can be used again, and again.
Another common name for regenerative dryers is dual tower desiccant air dryers, which are very common in compressed air applications. Desiccant air dryers are available as heated or heatless models that offer a continuous supply of dry compressed air by using two identical towers that each containing a bed of desiccant beads.
The reason for there being two towers is optimum efficiency. As one tower is at work, adsorbing the water vapor in the compressed air, the other tower is regenerating its desiccant.
But how is the regeneration of the desiccant accomplished? The expansion of dried air near atmospheric pressure is directed across the wet desiccant bed. This swing in pressure produces expanded air, otherwise known as purge air or purge flow, which has a very low water vapor concentration.
Let’s think about a hypothetical situation where the air is used for purge flow is 80°F and has a dewpoint of around -40°F at 100 PSIG. When this purge air is expanded from 100 PSIG to a couple of pounds of pressure and heated to temperatures as high as 600 °F, the moisture-holding capability of this superheated, dry expanded air becomes extremely high.
Now, when this purge air is passed counter currently through the wet desiccant bed, the vapor pressure of the hot air is so low in comparison to that of the desiccant that the moisture moves from the desiccant (high vapor pressure) to the hot purge air (low vapor pressure). The purge air stream is then able to simply carry the water vapor out of the air dryer.
Dual tower desiccant air dryers are most used to dry instrument air and process air, and applications where airlines are exposed to very low ambient temperatures. You will often come across dewpoints in the range of -40°F to -100°F but it may also be possible to get a dewpoint even lower than this!
Desiccant Air Dryer Advantages
- Ability to operate at very low dewpoints in temperatures below freezing
- Capable of delivering extremely dry air that will meet ISO quality classes 1, 2 & 3
Desiccant Air Dryer Disadvantages
- High operational costs
- Initial purchasing costs
- Maintenance costs
Membrane Type Air Dyers
Membrane-type air dryers operate on the principle of selective permeation through a membrane. Compressed air is passed through a bundle of tiny holy (polysulfone) membrane fibers, water vapor and a certain amount of compressed air flow diffuse through the semi-permeable membrane walls while the dried air continues downstream.
The differential pressure on the inside and outside of the hollow fibers separates the water vapor from the compressed air and purges it out of the housing by the sweep of purge air. Membrane dryers can only be used on clean, oil-free air.
So, if you have an oil-lubricated compressor, you must install a coalescent filter ahead of the membrane dryer to remove any oil or aerosol contaminants from the compressed air stream. If you don’t, these will likely block the permeation of the fibers and reduce the efficiency of the dyer itself.
These dryers are for point-of-use applications and are sized for low capacities in comparison to the other types of dryers presented in this article. Membrane dryers are capable of being connected in parallel to increase their capacity beyond that of a single dryer. Membrane dryers are used in various railway and bus applications, and other larger vehicles like firefighting and agricultural vehicles.
Membrane Type Dryer Advantages
- Ability to operate in severe weather conditions
- Ability to operate in corrosive and explosive environments
- Dewpoint suppressions range from -40°F to +40°F
- Do not require a power source
- No consumables to replace
- No moving parts
Membrane Type Dryer Disadvantages
- Requires oil-free air
- Some models will consume about 15-20% of purge air to work effectively
- They reduce the oxygen content of the compressed air so cannot be used for breathing air applications
Refrigerant dryers are typically available as either cycling or non-cycling types. Both use a refrigeration system to cool the compressed air to a temperature close to freezing, to condense out as much water as possible.
Most refrigerant dryers provide a pressure dew point of 35°F, however, cheaper models that have a smaller refrigeration unit will be rated for dew points of around 50°F. Hot compressed air the air-to-air heat exchanger and flows down the inner tube of a refrigeration system. This incoming hot air is re-chilled by the air traveling in the outer tube, which itself has been cooled by the refrigeration section.
As the air cools, water vapor condenses into liquid droplets. These condensed liquid droplets are then removed from the air stream in a separator which then automatically drains the discharge with the air of an automatic condensate drain trap.
The last step is for the air to pass through the secondary side of the air-to-air heat exchanger, where it can then be reheated by the incoming hot air. This reheating of the hot air is to prevent downstream pipe sweating as well as the benefit of increasing the air-effective volume, which enables it to do more work.
It generally takes longer for a pipe that is exposed to cool atmospheric temperatures to drop from the re-heated temperature to a point less than 35°F.
Refrigerant Dryer Advantages
- Constant dewpoint of 35°F to 50°F
- Low maintenance requirements and costs (no chemicals to replace)
- No after filter required
Refrigerant Dryer Disadvantages
- Lowest dew point available is around the 35°F mark
- Some dryers are prone to have refrigerant leakages
Storage Tank Cooling
The final method of drying compressed air uses an air receiver tank to turn some of the moisture present in compressed air into water droplets as the air comes from the compressor, or even from the aftercooler.
The moment air leaves the compressor of the aftercooler and enters the compressor’s air receiver tank, it encounters the cooler steel walls of the tank, usually at ambient temperature. So, at this point, moisture starts to condense out of the compressed air as the air begins to chill due to its contact with the ambient temperature walls.
When the air has been stored in the tank for a long enough period, the temperature of the air in the tank will match ambient temperatures and no more moisture will be able to condense out. Therefore, the air in the receiver tank is 100% saturated at a dewpoint equal to atmospheric temperature, which results in the formation of water droplets.
These water droplets collect at the bottom of the tank, and it is important to drain the tank after each use to block the condensation and moisture leading to the formation of rust and scale inside of the tank which can end up in your air stream. In colder conditions, the water may even freeze. All these potential issues can cause blockages in your system, and premature wear of components or pneumatic tools attached downstream.
The storage tank cooling method is automatically employed on air compressor systems with an air receiver tank. It’s an inexpensive and simple solution, however, the efficiency of it will depend greatly upon the amount of time air is stored in the tank before use and some applications will still require an aftercooler.
If you’re drawing air pressure through your pneumatic tools as soon as the air is stored in the tank, then it’s likely that this method hasn’t had a chance to get into full swing.
Storage Tank Cooling Advantages
- Air receiver tanks are easy to source methods, mostly because your air compressor probably already comes with a tank attached
- Air receiver tanks are relatively inexpensive
- Simple and straightforward, all you must do is drain the tank if it’s not automatic
Storage Tank Cooling Disadvantages
- Not the most effective nor efficient way to dry air
- Requires tank to be drained regularly
- Tanks are likely to take up significant amounts of storage space
How to Minimize Moisture in Your Compressed Air System
Because it is not possible to eliminate moisture getting into compressed air systems, and although we have the methods presented above for dealing with the said situation, we can still take steps into minimized the unnecessary moisture in the workspace.
Even when working indoors, there will be a certain level of moisture in the present air surrounding you and your air compressor. Luckily we have multiple steps you can take to minimize this moisture content:
- Clean air conditioning filters if your building uses them because clogged or broken filters can lead to excess humidity. Have them check and changed regularly by a professional
- Drain the air compressor regularly to get rid of debris which includes excess moisture. The drain needs to be emptied to avoid rust, clogs and improve performance so remember to do this after each use
- Eliminate standing water immediately in the room which is home to your compressor – this includes areas that are wet from leaks or spills
- Ensure you have adequate ventilation by making sure the room you’re working in has airflow that can help expel naturally-occuring moisture
- Fix any leaks and structural issues in your walls, windows, plumbing and flooring immediately otherwise they may lead to undetectable standing water, creating additional moisture in the room
- Install insulation in your buildings walls, floors and around all windows and doors. This insulation will help keep your space at a more consistent temperature as well as prevent cool or damp air from seeping indoors and causing water to build up in air compressors
- Invest in a dehumidifier to help reduce the amount of humidity and moisture in the air. This will also help prevent mold and mildew formation and keep energy costs low
- Keep an eye on weather forecasts and plan ahead. Humidity in the air will provide excess moisture in your compressed air lines. If you are able to schedule operation for a time of day that is not as humid or when dew point is lowest. Go the extra mile and check which day is going to be the best to carry out your operations
- Replace your air compressor when necessary. If you are having trouble with excessive moisture, and you can’t fix it, maybe that is a sign that the compressor is no good and you need a new one
- Schedule preventive maintanance checkups to make sure your compressor runs effectively for as long as possible. Prventive maintenance can allow you to catch issues before they grow into more significant and costly problems later on. Allowing you to minimize the risk of you having to halt operation for repair
- Use both standard fans and exhaust fans to help try the air in your workshop. Under the assumption that the additional airflow won’t hinder your operations or quality of your product – install them in the area where compressed air is used
- Use a larger air compressor or multiple smaller systems to get the job done. Some small compressors are likely to overheat as it tries to keep up with demand, and with this increased temperature comes an increase in formed moisture
- Use and certainly maintain compressor filters to separate contaminants including water and moisture from your air stream. High-quality air filtration systems fitted correctly can be a god send, just ensure you inspect and clean them regularly and replace them when necessary
FAQs (Frequently Asked Questions)
To get moisture out of compressed air you either need to apply one of the different methods of drying compressed air, or install water separators, traps and filters along the lines to ensure that your pneumatic tools and devices are receiving the highest-quality air possible.
The most popular methods for drying compressed air include using aftercoolers, desiccant air dryers, deliquescent air dryers, membrane type dryers, refrigerant air dryers, and the storage tank cooling method.
It’s important to dry compressed air to remove moisture content from your airstream. Moisture condensate along with other contaminants not only cause the premature wear of your pneumatic tools, but they also significantly affect the quality of your projects.
If you have any questions regarding drying compressed air, please leave a comment below, with a photo if applicable, so that someone can help you!