If you can’t see water in compressed air, does that mean it’s dry? This is not a trick question, but the answer does depend on the meaning of the word dry. The dictionary lists a lot of definitions for dry. But none is completely helpful for understanding what it means to have dry compressed air.
In Jurassic times, having “dry air” meant installing a separator or drop leg to remove enough liquid so that you’d be operating an air system rather than a high pressure water line. As pneumatic equipment got more sophisticated and sensitive, the phrase “dry air” came to mean simply no visible liquid in the compressed air system. Indeed many air system operators still insist that the absence of liquid signals a dry compressed air system.
For a long time there were no official standards of dryness and few people actually measured moisture content. If there was not visible liquid, all was well. For a few critical applications, you’d hear customers talk about dew point, but this was rare.
Over time, industry groups began to formally define and quantify the meaning of “dry” compressed air. This moved many conversations from the realm of opinion to the realm of measurement. The first published standard that I recall was from the Instrument Society of America (ISA) in standard S7.3. It defined dry air as having a pressure dew point 18°F / 10°C below the lowest temperature the system would encounter, but not higher than 35°F. Many people consider the ISA “instrument air” standard as requiring a pressure dew point of -40 °F, but I think this is due to a paragraph that shows -40°F only as an example, and it gets mistaken for the standard itself. I won’t bore you here with the details, but will gladly do that upon request, as I am an acknowledged authority in the field of boring people.
The other group that offers a standard for dryness is the International Standards Organization (ISO). In standard 8573.1, various dew points classes are defined, and a relatively precise moisture level may be specified in a range from -94F-70C to +50F/+10C. This ISO standard is readily available online.
For many compressed air systems, simply preventing condensation is adequate. Sometimes, an ambitious customer will want to get the air “bone dry.” What does that mean? If that means having a -40°F pressure dew point then a careful conversation needs to occur. Many times compressed air does not need a dew point of minus anything, especially when the plant is operating routine equipment, tools, and cylinders in a climate controlled environment. To be sure, in some applications a low dew point is justified, but over drying compressed air is costly and wasteful.
Some old school operators are still around who’ll say things like, “I need a filter to dry my compressed air stream.” This is where I need to deliver a quick lecture on dew point, humidity, and why dry air on the suction side of compressor comes out saturated. Most importantly, I must explain why a filter cannot remove water vapor.
I recently worked on an 10 SCFM application where the operator wanted a dew point of -110°F. Why such an extremely low level of moisture? Yes, a dryer can be designed and operated to meet the requirement. But at what cost? Apart from the dryer and the compressor, the monitoring equipment needed to verify performance would be terribly expensive.
As always, the best thing for the customer is making sure that he gets just the right amount of dryness and not too much or too little. Just right, as Goldilocks might say.