Purge air flow on an uncontrolled desiccant dryer is based on the nameplate rating, not the amount of compressed air flowing through the dryer. The purge flow is metered by an orifice or cracked open valve that takes a fixed flow of air from the tower that is regenerating. The cfm of air flowing thru the dryer does not change the purge flow unless there is a dew point or moisture control controlling the dryer.
Example: On a 1000 cfm fixed cycle dryer that is only running at of its rated or 500 cfm, the purge flow will still be 15% of the nameplate rating or 150 cfm, this would mean the real purge flow is 30% of flow.
Purge flows can change often, this is a manual adjustment that is done during a specific part of the dryer cycle, on most dyers the purge flow is controlled by a ball valve based on the pressure reading on a gauge, over time these valves can become misadjusted and purge exhaust ports can become plugged causing back pressure which changes the purge flow.
Dewpoint controls save energy by adjusting the dryer cycle time and purge time, letting the desiccant get fully saturated with moisture before the controls will let the dryer purge.
Southwire has two Gardner-Denver 250HP compressors rated at 1100 CFM each for a total of 2200 CFM, they have a 2700 CFM ZEKS desiccant dyer. The purge flow is 15% of rated flow or 405 CFM. When they have the dryer on-line and working correctly they are putting 2200 CFM into the dryer and the dyer is using 15% of that flow for purge air, so they are losing 405 CFM to purge flow which comes to about 18.5% of their compressor output. Instead of getting 2200 CFM like they need, they are getting 1795 CFM and that means when the dryer is on line the plant does get enough air to run.
International Paper, dealing with another distributor, replaced two Gardner-Denver 75HP compressors and a 650cfm refrigerated dryer with one 150HP Gardner-Denver compressor rated at 620 CFM and a 700cfm Airtek desiccant dryer. They found they could not run their plant because the dryer was using 15% of flow or 105cfm for purge flow; which meant they only had 515cfm of air flow. They had to buy an additional 200HP compressor to have enough air flow for the plant.
What dryer you use in your compressed air system can save energy and saving energy can lower your operating cost.
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