A GUIDE TO DRYERS

Types of dryers

Dryers are your most important line of defense against moisture in your compressed air and, therefore, against harmful corrosion and dangerous microorganisms.

The right technology can effectively safeguard your system and your end products, and therefore save you a lot of hassle and costs.

Obviously, it would be easy to simply use the most effective dryers, i.e. those that can help you meet even the most stringent air quality requirements, to provide your system with optimal protection.

However, that is not an economically viable option because drying air takes energy … and energy costs money. This means that the more you dry the air, the more energy you need and the more your operating costs go up. Furthermore, in many cases, you won’t have to meet these standards and a limited use of dryers will suffice to protect your compressed air system and equipment.

That is why your goal has to be to find the optimal balance between meeting your air quality requirements and keeping your expenses down.

So let’s look at your options.

Refrigerated (or refrigerant) dryers

Refrigerant dryers are the most common type of dryer. They consist of an air-to-air heat exchanger and an air-to-refrigerant heat exchanger. They remove moisture from your compressed air through condensation.

Refrigerant dryers can either be air-cooled or water-cooled. They cool the warm and wet air that comes from the compressor. In the first stage, this happens in the air-to-air heat exchanger and in the second stage in the air-to-refrigerant heat exchanger. As the temperature falls, the moisture contained in the air condenses and can then be drained from the compressed air with the help of a water drain.

The compressed air is then reheated to about room temperature in the air-to-air heat exchanger to lower the PDP of the outgoing flow. This prevents the formation of condensation on the outside of the piping system. This heat exchange between ingoing and outgoing compressed air also reduces the temperature of the incoming compressed air, and therefore reduces the required cooling capacity of the refrigerant circuit.
In order to be effective, the relative humidity of the compressed air should be below 50%.

We also distinguish between non-cycling, cycling and VSD refrigerant dryers.

Non-cycling dryers: The term “non-cycling” means that these dryers operate the refrigeration compressor continuously and utilize a hot gas bypass valve to redirect the refrigerant even at less than full load conditions.

Non-cycling dryers are a good solution for operations hoping to improve their compressed air quality while keeping investment costs down. They are very simple and reliable machines that come with minimal options in order to simplify both their design and operation. In short, these refrigerated dryers feature the lowest initial investment cost while providing dry and clean compressed air.

Non-cycling dryers are easy to install and simple to operate, which makes them the standard in terms of performance, quality and the ability to deliver desired outcomes. They are ideally paired with any rotary screw air compressor, while a high-temperature version is preferred and recommended for use with piston compressors. As the name suggests, “non-cycling” means that the dryer will run continuously, regardless of the compressed air load coming into the dryer. Therefore, they are less energy efficient than other options.

Cycling dryers: Unlike the non-cycling versions, cycling dryers use additional equipment such as thermal mass or frequency controllers, which allows them to turn on and off based on the compressed air demand coming into the dryer. As a result, they are more energy efficient.

While cycling dryers are more expensive to buy, their improved efficiency results in lower lifecycle costs – especially for operations with a fluctuating air demand. They are very reliable and offer the convenience of easy installation, a small footprint and a low noise level.

VSD dryers: If you have a fluctuating air demand, then dryers using a variable speed drive (VSD) offer the lowest operating costs by far and, therefore, also the lowest cost of ownership. Here, the motor speed of the compressor integrated into the dryer adjusts automatically to the air demand and therefore reduces energy expenses drastically.

Tip: The benefits of integrated dryers

You may be able to choose between a dryer integrated into the compressor canopy or a standalone model. In that case, the integrated version is often the better solution.A built-in dryer saves space, which is often a precious commodity in compressor rooms and on shop floors. It also reduces installation costs while offering a small, quieter and convenient solution for your compressed air needs. [End Box]

Desiccant (adsorption) dryers

A desiccant is a material that adsorbs moisture. One of the best-known desiccants is the silica gel contained in the small pouches included in the packaging of products in order to keep them dry.
Also referred to as a hygroscopic material, desiccant is used in dryers to adsorb the moisture contained in the compressed air. The air flows over the material, gives off its moisture and is thereby dried.

There are two main types of desiccant. Traditionally, desiccant dryers have used thousands of tiny beads to dry air. Now, however, Atlas Copco has introduced Cerades™, an innovative structured desiccant that makes dryers much more efficient and offers a slew of other benefits. You can find out more about Cerades below.

The exchange of water vapor from the moist compressed air into the desiccant causes the material to gradually be saturated with adsorbed water. As a result, the hygroscopic material has to be regenerated regularly to regain its drying capacity.

That is why adsorption air dryers are usually designed with a pair of drying vessels: The first dries the incoming compressed air while the second is being regenerated. When this process has been completed, the two vessels (also referred to as “towers”) switch tasks.
There are four ways to regenerate desiccant. The method used determines the type of adsorption dryer. More energy-efficient types are usually more complex and therefore generally also more expensive to buy.

Purge regenerated adsorption dryers (“heatless-type dryers”): These dryers are best suited for smaller air flow rates. Their regeneration process takes place with the help of expanded compressed air (“purged”) and requires approx. 15-20% of the dryer’s nominal capacity at 7 bar(e) working pressure.
Heated purge regenerated dryers: These dryers heat up the expanded purge air by means of an electric air heater and hence limit the required purge flow to around 8%. This type uses 25% less energy than heatless-type dryers.
Blower regenerated dryers: Ambient air is blown over an electric heater and brought into contact with the wet desiccant in order to regenerate it. A small portion of the outgoing air, approx. 3%, is then used to cool down the heated beads and remove the water vapor. This results in an energy consumption that is 40% lower than of heatless-type dryers.
Heat of compression dryers (“HOC” dryers): In HOC dryers the desiccant is regenerated by using the available heat of the compressor. Instead of evacuating the compressed air heat in an aftercooler, the hot air is used to regenerate the desiccant. This type of dryer can provide a typical pressure dew point of -20°C without any energy being added. A lower PDP can also be obtained by adding extra heaters.

Meet the future of desiccant dryers

As we have learned, it usually takes a desiccant dryer to achieve the lower pressure dew points and meet the more stringent air quality standards.

While traditional desiccant dryers get the job done, they have some drawbacks. For example, they contain thousands of tiny beads of desiccant.

Pushing the compressed air through this mass of beads to dry it requires quite a bit of energy. In addition, the desiccant can decompose, creating a fine dust that is a health and environmental hazard. Finally, the decaying beads could also lead to an air quality reduction. Another downside of desiccant dryers using beads is that they require frequent maintenance.

Now, Atlas Copco has addressed these drawbacks with the introduction of Cerades™ – the world’s first solid desiccant.

Cerades desiccant dryers consume less energy, offer better air quality, require less maintenance and they eliminate health and environmental hazards.

Instead of forcing the compressed air through thousands of beads, Cerades allows it to flow through straight structured tubes. Because the air meets no resistance, the pressure drop is much lower when using this process and, as a result, a lot less energy is required to operate the dryer.
But the much lower operating costs are just one of the many benefits.

A better, safer dryer

Cerades dryers eliminate the fine dust that is created in traditional dryers when the compressed air forces the desiccant beads to bounce around and, as a result, gradually decompose. If that dust is not filtered out, it can contaminate the air and downstream equipment. Having these dust particles removed results in higher operating costs.

Worst of all, this dust is a health and environmental hazard because it circulates in the ambient air when the desiccant is replaced.

Because Cerades decomposes at a much slower rate, it eliminates this dust problem. That further reduces costs and delivers users ISO 8573-1:2010 Class 2 air purity for particles without any extra filtration.

In addition, the lack of bouncing beads also makes Cerades vibration-resistant, which allows it to be mounted horizontally and ensures continuous operation in rigorous applications, for example in the transportation industry.

Membrane dryers

These dryers are quiet, are easy to operate, have no moving parts, consume little power and require minimal service (mainly filters upstream of the dryer).

Membrane technology

They use a process called “selective permeation” of the gas components the air contains. The dryer consists of a cylinder that houses thousands of tiny hollow polymer fibers with an inner coating. Using their selective permeability, these fibers remove water vapor.

This is how membrane dryers work: As wet compressed air enters the cylinder, the membrane coating allows water vapor to permeate through the membrane wall and collect between the fibers. In the meantime, the dry air continues through the fibers in the cylinder at almost the same pressure as the incoming wet air. The permeated water is vented to the atmosphere outside of the cylinder.

The temperature and humidity of the incoming air defines the performance of membrane. Instead of providing a fixed dew point at the outlet, they deliver a dew point suppression. Their design is very simple and reliable. They have no moving components, which makes them 100% maintenance free. However, the average purge consumption of these types of dryers is around 12%.

Calculate the moisture in your air

Drying air – what else you need and what else you need to know

Aftercoolers and drains

In addition to your dryer, you may also need an aftercooler. What is that?

An aftercooler is a heat exchanger that cools the hot compressed air to precipitate the water that otherwise would condensate in the piping system. Both water-cooled and air-cooled aftercoolers are equipped with a water separator with automatic drainage. Aftercoolers should be placed close to the compressor.

Water separators collect approximately 80-90% of the condensation water. A common value for the temperature of the compressed air after passing through the aftercooler is approx. 10˚C above the coolant temperature. However, this can vary depending on the type of cooler. An aftercooler is used in virtually all stationary installations.

All Atlas Copco compressors are equipped with an aftercooler. However, production facilities with extremely high ambient temperatures might need additional cooling. Add-on aftercoolers prevent excess moisture from entering your compressed air system.

Drains are another important part of compressed air systems. However, they are often ignored, which can be a very costly mistake.

While the other air treatment equipment can capture the moisture in your air system, it is the drains that make sure it is purged from the system.
Drains can be installed in many different places – from dryers or aftercoolers to filters and the point of use.

To optimally protect your system, Atlas Copco offers a wide range of top-quality drains. Here are your different options:

Automatic drains: These drains get rid of the water that collects at the lowest point of the compressed air system. In addition to getting the job done automatically, their patented design also ensures that only minimal maintenance is required.

Timer drains: These drains remove the condensate using a solenoid valve in combination with an electronic timer. They allow operators to pre-select the timing and length of the drain cycles. This minimizes the compressed air loss. Timer drains are a cost-effective solution that are compact, easy to install and fully automatic.

Electronic drains: An intelligent drain function monitors the build-up of condensate and only removes it when necessary. This helps avoid unnecessary compressed air loss.

Can you use over-compression?

Over-compression may be the simplest method of drying compressed air. It is a process where the air is first compressed to a higher pressure than the intended working pressure. As a result, the concentration of the water vapor increases.

The air is then cooled and the water is separated as a result. Finally, the air is allowed to expand to the working pressure, which results in a lower PDP.

So is over-compression for you? That depends. Because this process consumes a lot energy, it is only suitable for very small air flow rates before the costs get prohibitively expensive.

How to properly dispose of the condensate

If you use oil-injected technology, then your condensate will include traces of oil. In that case, you must be aware of how to properly dispose of the condensate in a responsible manner in order not to violate any environmental laws.

The tiny particles of oil in the condensate are not visible to the naked eye. Still (or rather, because of that) they require proper disposal. Doing this incorrectly would harm the environment and could lead to fines and a loss of reputation.

As anybody knows who has visited a recycling center in recent years or followed the wealth of new environmental regulations, there are many rules concerning the disposal of waste, so don’t let your compressed air supply leave you exposed to violating any of them.

Have a walk around your compressed air equipment: you should be able to see the condensate drains appearing from the back of the compressors and dryers on your site. Take a look at where these are piped to. Ideally you should see them all going to an oil-water separator and then off to a foul drain.

If they are going straight from a drain onto the floor or just into a standard plastic container then this should raise a red flag. Oil-water separators are very easy to install. Be mindful that, even with the correct equipment in place, there are many different regulations around the disposal of condensate and the rules can vary from region to region.