Air is like a sponge: It soaks up as much moisture as it can hold. When moisture-laden air is sucked into the intake of an air compressor and squeezed, like a sponge, it releases the moisture it has absorbed. If left untreated, this moisture will flow downstream with the compressed air. As it flows, it will gather the dust, rust and lubricant that exist on the compressed air piping walls and form a soupy mix that contaminates downstream equipment.
In an effort to prevent this contamination from occurring, compressed air is normally dried and filtered at various strategic points before it’s sent to plant end-uses. The type of air dryer and level of filtration varies, depending on the quality of compressed air required. (In general, the better the quality of air required, the more expensive it is to produce.)
Refrigerated air dryers and desiccant air dryers are two of the most common types.
Refrigerated air dryers…
These types of dryers cool air to near the freezing point of water using a refrigeration circuit and a heat exchanger. As the air cools, water condenses out of it and is removed via a water separator. This produces a dew point (the temperature at which the moisture within air starts to condense) of between 35 and 40 F. It is the refrigeration circuit in these dryers that consumes most of the energy; a smaller amount of energy is lost due to pressure differential. Rated specific power on these units is about 0.8 kW/100 cfm.
These types of dryers use a moisture-adsorbing material, such as activated alumina, to remove water molecules from the air stream. Most models incorporate two separate vessels containing desiccant: When one vessel is drying, the other is regenerating to remove adsorbed moisture. Once regeneration is finished, the dryer automatically switches sides. The regeneration process consumes most of the energy in desiccant dryers; a small amount of energy is lost due to pressure differential. These types of units typically produce compressed air with dew points of -40 F. Rated power consumed ranges from 2.0 to 3.0 kW per 100 cfm.
The key to energy efficiency of compressed air dryers is realizing that standard units consume near full power—even at light (or zero) loads. Because such dryers are usually sized for the worst-case scenario (i.e., the hottest, most humid day, when the compressor is at full load), the average loading at normal conditions is typically much less than the dryer rating. Thus, it’s desirable to select a dryer that can turn its energy down with reduced loading. A second benefit can be gained if the dryer has a low-pressure differential.
Cycling or thermal-mass refrigerated dryers reduce energy with reduced loading. For desiccant dryers, the use of dew-point controls or capacitive sensing of the desiccant moisture content will reduce wasted energy from unnecessary regeneration cycles. Often, the choice of these strategies will pay for themselves very quickly.
More information on this topic and others can be found in the Library section of the CAC Website, or in our Best Practices for Compressed Air Systems Manual. MT