Why and How to Dry Compressed Air

December 14, 2022  |  Daniel Launay

Many of us have to use dry air in our professional activities. Everyone is more or less familiar with the damage caused by just a few water particles in the air... And I am not just talking here about auto body painters, but in a very broad way, about all the applications subject to the use of machines prohibiting any trace of moisture in compressed air. This is the case with welding, laser cutting, "clean room" installations, etc.   

At first glance, you may think that good filtration is enough to solve the water problem. Although some filtration systems retain droplets of a water-oil emulsion, we are still far from eliminating water vapors! This is where the air dryer comes into play, more precisely the :   


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How a compressor air dryer works   

Why do I have water in my air?  I have often heard this question, or rather this cry of the heart... and even more often in the hottest countries. In a desert, you have to dig very deep to find the few drops of water that will barely quench your thirst.  

In fact, and contrary to what we imagine, the hotter it is, the more water there is in the air. Just look at a cactus with all its spines and apparently in great shape. Did you know that the cactus has no roots? It lives by absorbing water vapor from the surrounding atmosphere. The temperature in the desert is so hot that water cannot condense on the ground. The water evaporates immediately and remains suspended in the air. The cactus has adapted to this particularity.   

On the other hand, in the Far North, the opposite happens. It is so cold that the smallest particle of water vapor suspended in the air is immediately frozen to the ground. Because the water freezes on the spot, the air is extremely dry.  

When we understand this principle, we also understand the system of air drying called "by refrigeration": we will simply cool the air by making it circulate in a refrigerator, then recover the water that has become liquid and let the air go back to the pipe quite dry. 

The principle of the refrigerant air dryer  

The description of a refrigeration air dryer is pretty simple. Think of two pipes: the first carries a refrigerant gas (which generates cold), and the second is reserved for compressed air coming from the compressor.     

The refrigerant gas and compressed air are always separate (and at very different pressures), but the metal wall of the heat exchanger allows the transfer of heat from one to the other. The two pipes intersect at times in the form of a spiral.    

So the compressed air cools down and the gas warms up, ready to start another compression cycle. 

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Obviously, when the air temperature drops, the water in the air condenses. All that remains is to separate this moisture, condensed into liquid, through a recuperating filter and eliminate it by purging.    

The air is then dry enough to no longer cause condensation unless it is cooled below 4°C. This is called the dew point: the temperature below which the air must be cooled to condense water droplets.    

Since the dryer discharges water in liquid form, it is impossible to cool the air below zero degrees, otherwise, the water would form ice. This would soon block the flow of air and perhaps even swell enough to split the heat exchanger. This is the functional limitation of refrigeration dryers. They can never get the dew point of the compressed air lower than 2°C. Some applications require an even lower dew point, such as -40°C.  This is achieved with another type of dryer, the desiccant dryer.  

What do I need to know before investing in a compressor air dryer?    

Now that you know the basics of how a dryer works, you still need to choose and install one. To do this, you'll need to know a few key things that will help you make an informed choice.    

The air flow to be dried    

You need to know the maximum flow rate of your compressor (CFM or m3/h). If your dryer is undersized, then you will definitely find water in your compressed air network. The dryer must be calibrated to the maximum flow rate of the compressor.    

Compressed Air Pressure    

Pay attention to the pressure of the compressed air. All machines on the market are offered with an air pressure setting of 90 PSI (7 BAR). If your compressor is supplying air at a different pressure, either higher or lower, you should find the conversion table in the dryer brochure to recalculate the exact flow rate that can be handled. Do not neglect this calculation!    

The temperature of the compressed air    

The temperature at which the compressed air enters the dryer should always be as cool as possible. For example, if you store very hot food in a refrigerator, it will be more difficult for the refrigerator to reach the required cold level.    

I recommend that you always install a water separator filter before the dryer. The separator provides two very positive elements: cooling and pre-cleaning of the compressed air at an affordable price.    

Ambient temperature in the compressor room    

Another important point is the ambient temperature of the room where the dryer will be installed. If the temperature is too high, cooling will not be optimal and the efficiency of the dryer will be greatly reduced. Insulate and ventilate the room as needed.    

When choosing a dryer, check the specific constraints of your industry, such as those related to the food, medical or pharmaceutical world. In these cases, another type of air dryer may be required (an adsorption dryer for example), but the constraints are then well specified.  

Where to install the dryer    

During my career, I have often been asked where to install a dryer. Should it be installed before or after the tank?  

Some people will tell you that it is always before the tank, while others, equally confident, will tell you the opposite. In this case, who is right? Both possibilities exist and each can be quite viable depending on the circumstances. We will explore both possibilities, as well as a third option to consider.    

Why install the dryer before the air tank    

Placing the air dryer before the tank means it will be installed right after the compressor. 

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The compressor capacity must match the air requirement. Otherwise, since the volume required is greater than the capacity of the compressor, the air outlet temperature will rise very quickly. You will then have to reduce the amount of water, for example by interposing a water separator.     

The water separator will not only treat the air by removing many contaminants but also cool the air by a few degrees to allow the dryer to work properly.    

Installed before the tank, the dryer protects the tank from oxidation and extends the life of the tank. Dry air also means no rust particles from the tank that could damage tools.    

When a dryer is placed before the tank, its work will take place in a very linear fashion. In fact, it will only treat the air when the compressor itself is working, in regular sequences, allowing my dryer to rest quietly between two work periods.    

When working in a small or medium-sized workshop, some tools alone often represent a peak in consumption during use. Therefore, general consumption will be very irregular, going from everything to nothing. The tank, placed between the dryer and the network, will then play the role of a buffer absorbing these irregularities and, in so doing, protecting the dryer from these jolts. The dryer will only be better off, and my treatment will be more efficient! 

To remember if you place the dryer before the tank    
  • Since the air inlet temperature to the dryer is higher, the compressor must be properly sized.    
  • The tank receives drier air, which protects it from oxidation and increases its longevity.    
  • The dryer operates in a linear fashion, under the same cycle as the compressor.  
  •  This location protects the dryer from variations when air consumption is irregular.    
  • The dryer handles more condensate, so it is important to install a water separator between the compressor and the dryer.     

Why install the dryer after the air tank   

The second option is to install the dryer after the air tank. 

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The air coming from the compressor is humid and therefore generates condensation in the tank. This means that the tank takes on some of the gaseous particles and that the dryer does not have to handle the entire contaminant load. This location is therefore more viable if the dryer is ever undersized for air production.  

Caution! In all cases, do not forget to install a purge system at the bottom of the tank. 

Positioning the dryer after the tank means connecting it directly to the network and therefore to consumption. Therefore, the consumption of the downstream machines will have to be regular as the dryer is less tolerant to pressure variations. In general, air consumption is more stable in large companies than in small and medium-sized workshops.    

A dryer that is subjected to inconsistent air flow causes dew point drift, preventing stable settings. As a result, traces of water are sometimes found after drying, which can affect the air quality.    

If the compressor capacity is too low or the consumption has increased over the years, the tank protects the dryer from constant refilling of hot air. This gives the dryer more time to rest, increasing its longevity.    

To remember if you place the dryer after the tank   
  • The tank collects a significant amount of condensation and reduces the load on the dryer.   
  • The air temperature at the dryer inlet is lowered, which also reduces the dryer load.    
  • Installation after the tank is not recommended when the air demand is highly variable.       

A 3rd option: install the dryer between two tanks    

Many companies choose to install the dryer between two tanks. Despite the higher cost, this configuration offers considerable advantages: efficient dryer operation, maximum air quality, and tolerance of irregular compressed air demands.    

Complete with an air filter     

I always recommend installing a cyclonic air filter downstream of the dryer. It is an inexpensive investment compared to the benefits it provides. This filtration element will allow you to remove a lot of condensates, thus relieving the dryer and cooling the air at the same time.  It should ideally be installed in a well-ventilated, cool space.    

Things to remember about the location of the dryer    

After examining the possibilities, it is clear that there is no ready-made solution. The key is to analyze the situation. And no matter what your neighbor does, each case is different. You will have to make your own choice, taking into account all the elements mentioned above. And of course, if you have any doubts, don't hesitate to contact a technical advisor at Topring... you can be sure that he or she will be able to give you a personalized assessment and suggest the best solution.    

Some tips on refrigerant dryers    

  • Follow the installation guidelines, especially with regard to air circulation around the dryer. Do not block the dryer vents, otherwise, the dryer will not cool efficiently.    
  • Always install a pipe bypass in case you have to work on your dryer for a long time. This will not be a large investment but may pay off when you need it.    
  • Install your dryer in a location away from dust. The condenser screen must always be kept clean to ensure efficient operation.    
  • Even a head-end dryer does not allow you to eliminate the purge points along the pipe.    
  • If you can't invest in a large machine for your entire shop, install one or smaller dryers at the most critical point(s) in your system. This will protect your sensitive machinery and delicate work. 

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How a regenerative air dryer works   

In terms of compressed air treatment, workshops are often rather well-equipped. However, adsorption drying is not well known. Yes, I said ADSORPTION and not ABSORPTION!   

ABsorption or aDsorption   

To put it simply, a lump of sugar ABSORBS the water in which it is immersed... Whereas a sponge ADSORBS the puddle of water on which it is placed. What's the difference, you ask? When sugar absorbs water, its very structure changes upon contact with the liquid. Basically, it dissolves... The sponge keeps its solid nature. That's the difference.   

Drying by absorption   

An absorption dryer is based on the principle of an agent that captures all the moisture contained in the air and quickly disintegrates. Its major disadvantage is that the machine must be regularly refilled with an absorbent product, otherwise, once the material has disintegrated, the treatment will be completely useless. This process requires constant attention.  

Drying by adsorption   

In the case of adsorption, an agent is used that captures water without changing its own structure. As with the sponge, you just have to empty it of its water so that it becomes an adsorbent again. It is not more complicated than that. 

The principle of the regenerative dryer 

secheurs-dl_blogue_2022-11_EN_image06If we stay with our example of the sponge, the idea is to use two of them. While one is adsorbing water, the other is being squeezed to empty its contents, and so on. The regenerative dryer does not use a sponge. Its sponge is called a desiccant, more precisely activated alumina. This alumina comes in the form of small white balls. We will therefore use two tanks, each one filled with these small beads. We use activated alumina simply because it is an element that is extremely water-hungry and its resistance to dissolution is exceptional. So, it is the product we need in such a use. To operate continuously, we will have one tank in the treatment phase and the other in the emptying phase.    

A very fast process    

Everything happens very quickly. A treatment and regeneration cycle takes an average of two minutes. The regeneration phase takes place in two parts: A drying time of about one and a half minutes, followed by a gradual increase in pressure, where the air to be treated starts to enter, for thirty seconds. In this way, the beads are not rushed.    

A potential energy loss    

This type of dryer can cause a loss of more or less 15% of the dried compressed air. The lost air is re-injected into the tank during regeneration, which is called the sweep flow. This means that if you need a specific volume of air for an application, you will need to supply 15% more than the volume. If your industry requires even drier air, such as the food industry, you will need to increase the sweep. The loss of compressed air could then be as much as 30% of the volume processed!    

A sometimes expensive solution    

It is therefore clear that regenerative dryers, although extremely efficient, are also very expensive to operate. The use of these devices should be reserved for specific situations. Regenerative dryers are sometimes needed for applications that require a very dry airflow with a dew point down to -40 C. Think of hospitals, pharmaceutical plants, and even outdoor work in winter.    

Beware of contaminants    

To protect the alumina beads, adsorption drying of compressed air requires a total de-oiling treatment. If the beads are covered with a film of grease, they can no longer do their job of adsorption. For this purpose, a micron filter (1 micron) and a submicron filter (0.01 micron), also known as a coalescing filter, must be installed as pre-filtration.    

It is recommended that the tank be pressurized gradually before starting the treatment, to avoid rushing the beads. Otherwise, the beads could release fine dust that would mix with the treated air stream. You can avoid any future problems with bead dust by installing a micron filter at the outlet of the machine. This way, you will work with very dry, oil-free, and dust-free air!    

Maintenance of the regenerative dryer    

Concerning the maintenance of the dryer, the main operation remains the change of the alumina balls. On some models, an electronic panel warns you when it is time to change the balls. The maintenance is extremely simplified by means of easy-to-use interchangeable cartridges.    

In general, the dew point displayed with this type of treatment is -40°C. This means that the amount of water remaining in the air is at a level that could be found at the North Pole in cold weather. The air is very dry in the sense that all the water it could have contained is frozen to the ground in the form of ice.    

A last alternative    

Finally, in cases where the air flow supplied by the compressor does not allow the important loss due to the sweeping of the regenerative dryers, there are also systems which carry out the regeneration using a jet of hot air at low pressure supplied by a fan. This blower purge desiccant dryer system is more complex and more expensive to purchase but is more energy efficient. It is available on dryers from 250 to 6250 SCFM. But then, of course, the electricity bill will increase accordingly...    

In conclusion    

You should never compare a refrigeration dryer with a regenerative dryer. They are two very different devices. The refrigeration dryer is intended for general treatment, while the adsorption dryer is reserved for certain specific industries. An adsorption treatment may be necessary for extremely cold areas.   

Check out Topring's selection of refrigerant air dryers and regenerative air dryers. 

About the Author
Originally from Britain, is a “retired’ professional who now has all the freedom in the world to do the projects his is passionate about! Daniel was an Export Manager for 37 years for Prevost in France, a company specializing in the connection, air treatment and transport of compressed air. During his career, he was in charge of export development and training sales teams. He has a solid experience in the field of compressed air.
Daniel Launay