Answer: Most air compressors are equipped with aftercoolers designed to lower the compressed air's temperature and remove some water vapor from the airstream. But an aftercooler can't remove all the moisture contained in ambient air.For instance, a 25-horsepower compressor delivering 100 cfm at 100 psig can produce 18 gallons of water per day at fairly standard conditions of 90°F ambient temperature and 50% relative humidity. An aftercooler will remove approximately 75% of this moisture, leaving about 6.2 gallons of water per day to run through the air-supply system. Engineers suggest two ways to remove this moisture. One way is to further lower the compressed air's temperature after it's passed through the aftercooler, causing water vapors to condense. Refrigerated and thermal mass dryers cool the compressed air as it flows through refrigeration circuits. However, the refrigeration systems don't work well under dusty or below-freezing conditions.
A better choice for concrete producers is the second dryer style, which employs desiccants to remove moisture. This type of dryer is more common in concrete production plants because it is effective in winter weather, when the pipeline temperature drops below 35°F. Manufacturers offer single- and double-tower desiccant dryers that remove moisture differently.In a single tower dryer, absorbent desiccant beads attract vapor. Because the beads dissolve slowly, they require occasional replenishment. In a twin-tower style dryer, water is captured with adsorbent desiccants. Rather than dissolving upon contact, these desiccants hold the water until it is purged by either internal heaters or countercurrent airflow. Since the tower must be removed from service during purging, each dryer has two desiccant-filled towers. While one is drying compressed air, the other is being regenerated for the next drying cycle.Managers with plants using rotary vane compressors usually avoid twin-tower dryers because airstreams from those compressors often contain higher-than-normal quantities of oil. If this oil is not removed, it will damage the desiccant and cause other hazards. Producers interested in reducing airstream water should consult air dryer manufacturers for help in sizing and designing their systems.Reference
- Frederick Sitter, "Selecting an air dryer for dust-collection systems,"Concrete Journal, October 1995, pp. 806-809.