April 16, 2019
Water vapor in compressed air is inevitable, but if not treated it can damage your equipment, tools or final products if moisture passes through the air stream. To remove water vapor from compressed air, both adsorption (desiccant dryers) and refrigeration (refrigerated dryers) can be used in for different applications and different conditions.
A desiccant or adsorption dryer uses desiccant material to adsorb and remove the humidity from compressed air. With this method, a pressure dew point as low as -100 °F can be reached. A desiccant dryer should be used when the ambient temperature goes below freezing point, to avoid ice forming in pipes and applications. These dryers maintain a stable pressure dewpoint of -4℉, -20℉, -40℉ or -100℉ and are used in applications where moisture content in the air is critical to equipment operation or product quality. There are 3 main technologies for desiccant dryers to remove water vapor:
Heatless dryers scavenge dry air from the outlet of the dryer to regenerate one of the towers. These dryers provide the lowest initial cost and have a 3 to 10 min cycle time with ≈15 to 18% purge consumption on -40° versions.
Heated dryers work on the same principal as a heatless dryer but a heater is added to the regeneration system to improve regeneration efficiency. These typically have a higher initial cost than heatless but have a very low life cycle cost. 8 hour cycle time = longer valve life and lower maintenance cost. Heated dryers are good for dirty or wet environments where blower purge dryers can not operate.
Heated Blower Purge dryers are similar to externally heated however a blower is added to utilize ambient air to regenerate the towers. The average purge consumption is 2% of the dryers capacity and an 8 hour cycle time = longer valve life and lower maintenance cost.
Refrigerated air dryers use the same cooling principal as typical air conditioning units in homes across the world. Using the refrigeration system and passing compressed air through a heat exchanger, the air is cooled to approximately 40 °F. As the air cools, water droplets begin to form in the air due to condensation much like the water droplets that form on the outside of a cold drink. The moisture laden air then undergoes a mechanical separation process where the liquid or “condensate” is separated from the air stream. The air is then rewarmed using the incoming air which lowers the relative humidity of the air and traps any remaining moisture in a vapor form. The simplest form of water vapor removal using a refrigeration system to cool the air approximately 40℉. As the air leaves the dryer is rewarmed to decrease the relative humidity of the air and ensure no liquid water coalesces out of the air stream.