Dryer Series: Five Techniques for Removing Moisture from Compressed Air

Today, we begin a short series of blog posts on dryer technology. Be sure to tune in every Thursday for the next few weeks to learn about the ins and outs of drying compressed air.

All atmospheric air contains water vapor: more at high temperatures (think summer humidity) and less at lower temperatures (think dry air in winter). Compression causes the temperature of the air to increase and as a result, the water concentration in the air also increases. Therefore, after the air is compressed it must be dried in order to avoid problems that could be caused by having too much water in the air pipes or in connected parts.

There are five techniques for removing moisture from compressed air: cooling plus separation, over-compression and membrane, absorption and adsorption drying.

Cooling plus separation

Cooling plus separation is exactly how it sounds. The hot compressed air is cooled, which allows a large amount of water to condense. Once the water is condensed it can then be separated from the air. This technique occurs through an after-cooler and if being used, a refrigerant dryer. Almost all stationary compressor installations use after-coolers and most modern compressors have after-coolers built directly into the machine.

Over-compression

Over-compression is similar to the cooling plus separation technique in that the air is compressed, cooled and the resulting water is separated from the air. However, in this technique, the air is compressed to a higher pressure than the intended working pressure; after separation, the air is allowed to expand to the working pressure so it can be used in the intended process. Though this method is the easiest for drying compressed air, it is only suitable for very small flow rates because it consumes a large amount of energy.

Membrane drying

This technique uses the process of selective permeation of the gas components in the air to separate the water vapor out. Thousands of tiny hollow polymer fibers with an inner coating line the dryer’s cylinder, thus creating a membrane. The water vapor in the air is collected between the fibers while the dry air passes through the fibers and onto its intended process.

Absorption drying

Absorption drying is a chemical process in which water vapor is bound to an absorption material such as sodium chloride or sulfuric acid. This technique is not often used because of the possibility of corrosion and the high consumption of absorption materials.

Adsorption drying

In this technique, moist air flows over a hygroscopic material or “desiccant” and is dried. Typical materials used are silica gel, molecular sieves and activated alumina. The desiccant is eventually saturated with adsorbed water and needs to be regenerated regularly to maintain its drying capacity. Because adsorption dryers can achieve a pressure dew point of -40ºC, this technique is used in many critical applications.

Follow along as we continue to explore the technology behind dryers over the next few weeks. In the meantime, contact us with questions or subscribe to receive our weekly recap email. And, if you're really interested in diving deep into dryer technology and methods, contact us for a free copy of the Compressed Air Manual.