Dry compressed air is so very important to the operation of a compressed air system. Having free water in your pipes causes rust to form in the internal surfaces of iron piping — which mixes with dirt, water and lubricant to form a black sticky mess that can travel down steam and contaminate your tools and machinery.
This is why it is wise to keep an eye on the compressed air dew point. Keeping the dew point of the compressed air — the temperature at which moisture will form — well below the coldest temperature to which the piping will be exposed can eliminate the condensation of water in the pipes.
Very often, staff will use the display on their refrigerated dryer as a reference to determine if the quality of the air is adequate … and in most cases this is fine. But you should be aware that most refrigerated dryer displays show the internal temperature of the dryer, and not the actual dew point.
At first glance at the above photograph, we might assume this dryer is operating properly, with dew point of around 34° F, but a secondary test around the back of the dryer revealed problems. When staff opened the drain on the dry receiver tank, they found it to be half full of water. The water drain on the dryer had failed, allowing oily water that normally condensed due to the cold temperature in the dryer to pass through to the system rather than being expelled out the dryer drain. No drain had been installed on the dry receiver. One application within the plant was receiving big slugs of pressurized water rather than compressed air.
A similar thing can happen with desiccant air dryers. Sometimes the dew point monitor (if one is installed) on the dryer control can become out of calibration. If this monitor is part of the control system of the dryer, then very poor dew point or inefficient operation can result. If no dew point monitor exists, then the results can be hit and miss.
Consider a dryer at a food products processing plant. The compressor internal air/oil separator had failed, contaminating the dryer dew point probe with moisture. No matter what the actual dryness of the air, the probe read the same reading, -114.8° F (see the photograph at right).
This erroneous reading caused the dryer to produce excessively wet air whenever it was placed in dew point control mode. As such, the dryer had to be set to fixed cycle mode, costing an extra $6,000 per year in operating costs. A replacement probe cost a mere $1,500.
With some dryer dew point displays, it is important to realize that seeing is not believing!