Improve compressed air quality with an adsorption dryer
Did you know all atmospheric air contains some amount of water vapor? When air is compressed, its water concentration increases. To avoid future problems with your air compressor set-up, it's important to treat this moist air. In this article, we discuss how an absorption dryer and a desiccant / adsorption dryer work.
What is absorption drying?
Absorption drying is a chemical process in which water vapor becomes bound to absorption material. The absorption material can either be a solid or liquid. Sodium chloride and sulfuric acid are frequently used, meaning the possibility of corrosion is worth considering. This method is unusual and involves high consumption of absorbent materials. The dew point becomes lowered to a limited extent.
What is a desiccant or adsorption dryer?
The general working principle of desiccant air dryers is simple. Moist air flows over hygroscopic material (typical materials used are silica gel, molecular sieves, and activated alumina) and becomes dry. When water vapor changes from moist compressed air into hygroscopic material or "desiccant," the desiccant becomes gradually saturated with adsorbed water.
Therefore, it's important to regenerate the desiccant regularly to regain its drying capacity. Adsorption air dryers are typically built with two drying vessels for that purpose. The first vessel will dry the incoming compressed air while the second one regenerates (Similar to the workings of a nitrogen generator). Each vessel ("tower") switches tasks when the other tower is completely regenerated.
The typical pressure dew point (PDP) these dryers achieve is -40 °C, making them suitable for providing very dry air. There are 4 different ways to regenerate the desiccant, and the method used determines the type of adsorption dryer. More energy-efficient types are usually more complex and, consequently, more expensive to purchase.
Regenerating the desiccant
- Purge regenerated adsorption dryers ("heatless type dryers"). These dryers are best suited for smaller air flow rates. The 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 with an electric air heater, limiting the required purge flow to around 8%. This type uses 25% less energy than heat less 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. With this type of dryer, no compressed air is used to regenerate the desiccant material. Therefore, the energy consumption is 40% lower than for heat less type dryers.
- Heat of compression dryers ("HOC" dryers). In HOC dryers the desiccant regenerates using the available heat of the compressor. Instead of evacuating the compressed air heat in an after-cooler, the hot air is used to regenerate the desiccant. This type of dryer can provide a typical PDP of -20 °C without any energy added. A lower PDP is also possible by adding extra heaters.
Heat of Compression dryers
Guaranteed separation and drainage of the condensation water must always be arranged before adsorption drying. If the compressed air is produced with oil-lubricated compressors, an oil separating filter must also be fitted upstream of the drying equipment.
In most cases, it's required to use a particle filter after adsorption drying. Heat of Compression (HOC) dryers can only be used with oil-free compressors since they produce heat at sufficiently high temperatures for dryer regeneration. A special type of HOC dryer is the rotary drum adsorption dryer.
This type of dryer has a rotating drum filled with desiccant. With this, one 1/4 sector becomes regenerated with a partial flow of hot compressed air (at 130–200 ˚C). Regenerated air is then cooled, and the condensation is drained before air is returned via an ejector device into the main compressed air flow.
The rest of the drum surface (three-quarters) is used to dry the compressed air coming from the compressor after-cooler. A HOC dryer avoids compressed air loss, and the power requirement is limited to that required for rotating the drum. For example, a dryer with a capacity of 1000 l/s only consumes 120 W of electrical power. In addition, no compressed air is lost and neither oil filters nor particle filters are required.