When working with compressed air, it's important to keep in mind its moisture content. That’s because an unwanted level of moisture in your air system can create corrosion, mould, and other quality control issues. While many compressors come with pre-installed aftercooler and drying systems, there are many advantages of adding desiccant dryers to your setup.
In addition to the maintenance risks mentioned, industry air quality standards often require keeping compressed air below a certain pressure dew point (PDP). This term refers to the amount of moisture in your air system. Depending on your ISO 8573-1 class, you may need to meet strict PDP levels.
One of the main benefits of desiccant dryers is that they can achieve a PDP of -40°C or lower. This capability makes these dryers suitable for most applications requiring dry, clean air. If you're concerned about energy usage, you can apply to highly efficient models.
How desiccant dryers work
Desiccant dryers use hygroscopic material to remove moisture from your compressed air flow: this desiccant material ensures your air is as dried as possible. With this setup, the desiccant needs to be regenerated, and proper filter maintenance is required.
In a typical desiccant dryer, there are two towers filled with desiccant material. Compressed air moves into the first tower, where the adsorbent material removes the water vapour. During this process, the desiccant will become saturated. That is when the second tower will take over drying duties while the first regenerates its desiccant. That means a desiccant dryer continuously cycles between its two towers.
When performing maintenance on a desiccant dryer, there's no need to disconnect it from the air compressor. Everything can be cleaned, changed, and regenerated within the dryer. As mentioned, in order for this process to work smoothly, desiccant must be regenerated. There are four different ways of achieving this, explained below.
Regenerating the desiccant
Depending on your environment, budget, and energy needs, you'll find what you need with the following regeneration methods.
Purge regenerated adsorption dryers ("heatless type dryers"): As suggested by their name, these dryers regenerate desiccant with expanded/purged compressed air. They require approx. 15-20% of the dryer's nominal capacity at 7 bar(e) working pressure and work best for relatively small air flow demand.
Heated purge regenerated dryers: With these dryers, expanded purge air is heated with an electric heater. They limit the required purge flow to around 8% and use 25% less energy than heatless type dryers.
Blower regenerated dryers: Using a blower to move ambient air over an electric heater, these dryers regenerate wet desiccant beads via air flow. To cool the beads, approx. 3% of outgoing air is used to lower the temperature and remove water vapor. This process uses 40% less energy than heatless type dryers.
Heat of compression ("HOC" dryers): These dryers use heat generated from an air compressor. This hot air is reused to regenerate the desiccant. The most efficient type of regeneration, they can achieve a PDP of -20 °C without any additional heat/energy. They're also capable of reaching a lower PDP with extra heaters.
Maintenance and environmental concerns
With the various components involved in running a desiccant dryer, it's important to keep your filtration system clean. This point is especially relevant if you use an oil-injected air compressor.
To properly ensure you're removing oil correctly, you'll likely need an oil-water separator for your drying system. As environmental regulations increasingly become stricter, it's important you're disposing of oil correctly.
Undetectable to the naked eye, these contaminants are generally found in condensate and become separated during the drying process. You do not want this draining directly onto your floor or into a plastic container.
Choosing the right dryer
While this article focuses on desiccant dryers as an optimal solution for a compressed air system, there's other options worth exploring. Other popular methods include an aftercooler, a refrigerated dryer, and a membrane dryer.
While each has their own benefits, desiccant dryers are usually preferred due to their PDP capabilities. If you live in an area with relatively high humidity, a desiccant dryer is generally the best product for you. Similarly, if you work in a moist environment, you'll want to invest in one of these machines.
Of course, choosing the right solution is really based on your unique setup and needs. If you're uncertain about where to start, our team is happy to help. We can evaluate the best solution for your application(s).
