Talk:Energy Efficiency Reference/Industrial/Compressed Air/Technology Primer

Air Dryer

Air dryers are included in most compressed air systems. Many industrial applications require air with low moisture content. For examples, pneumatic controls typically require dry air. In colder climates moisture-laden air can lead to ice which blocks or breaks the lines. High moisture content can also lead to corrosion in any compressed air system. Drying requirements and the volume of the air dictate the type and size of air dryer required. Typical air dryers either use refrigeration or desiccant remove moisture from the air. Aftercoolers provide initial cooling of hot compressed air from over 150 degrees F. Water cooling can be more effective by using cooling tower water near the wet bulb temperature. Air from the compressor room is often warmer than outside air. Aftercoolers condense some water vapor in the compressed air, but usually not adequately for air tools or pneumatic controls. Consider uses for the waste heat. Avoid city water cooling flowing down the drain. Refrigerated Air Dryers are able to drop the dew point (the temperature at which the air becomes completely saturated and moisture in the air beings to liquefy) of compressed air to 35-50 degrees F. Often these are all that is required and are typically the most economical means of drying air. Refrigerated air dryers work by cooling the compressed air with a refrigeration system. Moisture condenses out of the cooled air and is captured at the dryer. Operating costs are roughly $5.00 to $8.00 per million cubic feet of air. This associated with 130 kWh per million cubic feet of air. Desiccant Air Dryers can reach a lower dew point than refrigerated dryers; down to -150 degrees F. Desiccant air dryers consist of two desiccant bends through which the compressed air flows. A control system of two desiccant beds through which the compressed air flows. A control system channels all of the air through one bed while the other is regenerated. The regeneration process varies among desiccant dryers and has a large impact on operating costs. All systems blow dry compressed air across the desiccant bed to regenerate it and then purge the air and moisture to the atmosphere. Some models heat the dried air first to increase its capacity to absorb moisture. Heat types have better efficiencies because the energy required to heat the air to 300 degrees F is less than the energy required to compress and dry the additional air that would otherwise be necessary to dry the bed. Heated air dryers require only about 1-7% of the total compressed air to purge the desiccant bed, while non-heated dryers require 15% or more. Desiccant dryers are more expensive to purchase, maintain and operate than refrigerated air dryers, but to achieve lower dew points. These are often the only option. Operating costs range from $15 to $30 million cubic feet of compressed air. This is associated with 300 to 500kWh per million cubic feet of air. This cost of the compressed air used to regenerate the dryer. The extra compressed air required can be huge cost. Membrane Dryers are relatively new to the marketplace. They use a semi-permeable membrane that allows dry air to pass through holding back the water vapor. These dryers are easy to maintain the achieve dew points as low as 35 degrees F, however, they cause a 9-10% drop in system capacity as much compressed air is lost along with the water vapor in the membrane system. The operating costs for this type of air-drying are minimal, as the membranes need to be replaced infrequently.