A wire manufacturer used a refrigerated air dryer to condition the air from their three compressors. The dew point required in the plant was near 35° F, and the maintenance staff were confident of the proper operation of the dryer — because the onboard dew point reading on the dryer control panel said that the readings was less than the specified level.
Yet, on occasion, their production personnel complained about water in the compressed air piping. This was strange, because the dryer constantly showed a good reading. How could this be?
A compressed air service provider was hired as part of an energy efficiency effort. One of the auditor’s checks was to ensure that all readings were good with the compressed air dryer and that internal components were operating correctly.
The auditor checked the air dryer drains. Pressing the test button, he noticed that one of the two dryer drains was passing excessive water — in fact, after 10 operations of the dryer test button, a flow of water was still flowing from the drain! A check of the actual dew point of the air at the dryer discharge with a portable meter showed that the dew point of the air was at 70° F — yet the dryer dew point display showed 34° F.
This dryer was a unit that used a cold coalescing filter to remove the water from the air stream within the dryer. A check of the manual showed a warning code CF that was flashing on the dryer display panel meant Change Filter.
The dryer filter was clogged and was not properly separating the water from the airstream, allowing moisture to pass through the dryer. Plant personnel had noticed the code but nobody bothered to look up the meaning.
It is important to realize that most refrigerated dryer dew point indicators show the temperature of the evaporator within the dryer, not the actual dew point produced. If the dryer separator or condensate drains are not doing their jobs, excessive moisture will pass undetected to downstream piping, with the potential to cause moisture problems.
The best protection against this type of problem is good maintenance — as well as installation of a secondary dew point probe with alarm at the discharge of the compressor room.