Pity the neglected air dryer: Much time and attention is paid to efficiently controlling air compressors, but often the control of air dryers gets ignored. If your plant has desiccant dryers, it's important to manage their operation effectively, too, in order to get the maximum return from your compressed-air efficiency efforts.
Consider, as an example, a large desiccant air dryer recently found in a fertilizer plant during a compressed air audit. The specifications for the dryer stated that typical compressed air consumption for cooling air purge should have been 2% of the compressed air flow through the dryer. But when measurements were taken this flow turned out to be ten times higher than expected. In addition, the purge was causing an extra compressor to run during times of peak system demand. The cause of this problem turned out to be both operator error and the way the air dryer was adjusted.
Why do you need desiccant dryers?
Air dryers are needed to remove water vapor from the compressed air produced by the air compressors. The flow of air from the compressors is almost always 100% saturated with water vapor and, in the case of air-cooled compressors, the air is slightly warmer than ambient conditions. If this wet air were released into the plant, the contained water would condense and collect in low spots along the plant piping. This water would rust the inside of iron pipes, pick up any dirt and oil that has collected in the piping, and (if not filtered or dried) deposit a mess inside your compressed-air-powered machinery or on your product.
Refrigerated air dryers can remove the water vapor from the compressed air, resulting in a dew point at system pressure of about 35 to 40 degrees F, which is adequate for most end uses inside a heated plant. But if the pipes are subject to temperatures below this dew point, water will condense or freeze inside the pipes. Therefore, lower dew points of -40 or lower may be required. In addition, industrial processes using compressed air occasionally will require a higher "instrument quality." Some processes may be sensitive to water vapor. For example, say a plant uses compressed air to transport a product that will explode and burn if it comes in contact with free water. In such a situation and with similar vapor-sensitive processes, desiccant-style dryers are needed.
How desiccant dryers work
Figure 1: Basic Heatless Desiccant Dryer Source: Compressed Air Challenge
Central to the operation of desiccant-style dryers are beds of some type of chemical that will remove water vapor from the compressed air by an adsorption process as the compressed air passes through it. The most common desiccant is activated alumina that is in bead form. Because this desiccant can hold only so much water vapor, there needs to be a way to regenerate it once the desiccant becomes saturated.
Desiccant dryers are designed with two possible paths for the compressed air through two identical desiccant beds that are located inside two pressure vessels, or the dryer towers. When one tower is drying the compressed air, the other is in a regeneration cycle. The dryer has controls and a system of valves that will switch towers on a regular basis without interrupting the flow of compressed air.
There are various common ways to regenerate the desiccant:
- “Heatless” dryers – In "heatless" dryers, the desiccant is regenerated by depressurizing the tower and redirecting a flow of already-dried compressed air through the desiccant beads. The word “heatless” is in quotations because to work most effectively, this dryer type needs the natural heat generated in the adsorption process to help with the regeneration process. This type of dryer typically uses a 10-minute regeneration cycle. The flow of compressed air required for operation of this type of dryer is 15% to 20% of the dryer rating.
- Externally heated – This type of dryer uses an externally mounted electric heater that provides hot compressed air for the regeneration process. Because heated air is more effective in driving off the water from the desiccant, a lower flow of purge air is needed – typically about 7.5% of the dryer rating. This type of dryer does not need the heat generated by the adsorption process to work effectively. Heated dryers use a four-hour regeneration cycle. Use of a heater is more efficient that using compressed air for regeneration, so this type of dryer is less energy intensive than heatless.
- Externally heated blower – This type of dryer directs electrically heated air into the desiccant during the regeneration cycle but uses ambient air from an electric blower instead of compressed air. Therefore, no compressed air is required during the regeneration of the desiccant, freeing it up to send to the plant. After the regeneration cycle is complete, the desiccant needs to be cooled. Hot desiccant does not effectively adsorb water vapor, so failure to cool the desiccant would result in a dew point spike. Because the use of moist ambient air for this cooling could contaminate the desiccant, most dryers use a flow of already-dried compressed air to remove the heat. This cooling purge is rated at 2% of the dryer rating, but this actually means a peak flow of about 8% will flow only during the cooling period (averaging 2%). Some new styles of dryers use other, more-efficient methods of cooling, such as closed loop air or water cooling. Where ambient conditions allow radiant cooling, or where dew-point spikes are not a problem, the cooling purge on some makes and models of dryers can be turned off.
- Heat of compression – If the system has only lubricant-free compressors, then the heat of compression produced by the air compressor can be used to regenerate the desiccant. The heat is directed to the air dryer by bypassing the compressor after-cooler or taking air off the first stage of the compressor. The basic types of heat of compression dryers do not produce a constant -40 dew point or lower, but they achieve a dew-point suppression of a certain number of degrees below ambient conditions, which can be acceptable in some cases. Some newer HOC dryers can achieve similar dew points as other desiccant types through the use electric heaters and/or a cooling purge, which decreases the energy efficiency of the dryer.
Why desiccant dryers can be inefficient