The most common measurement of compressed air water content is dew point. Dew point is the temperature where air - or any gas - is saturated with water and moisture will begin to condense. In other words, it’s the point where dew begins to form. Dew point is always stated as a temperature. Simply put, dew point is the temperature where condensation begins.
In compressed air applications, pressure is critical when discussing dew point. Compression and expansion of air affect its dew point. Generally speaking, compression increases dew point, and expansion (i.e. de-compression) lowers dew point. For example, consider compressed air leaving a dryer at 200 PSIG with a pressure dew point of -40º F/C @ 200 psig. If the pressure is eventually reduced to 100 psig, the pressure dew point will fall to -50 ºF @ 100 psig. If the air is further expanded to 5 psig, the pressure dew point becomes -77º F @ 5 psig.
For this reason, the phrase pressure dew point (PDP) is commonly used. This term usually refers to the dew point of the compressed air at full line pressure.
Conversely, the phrase atmospheric dew point refers to what the dew point would be if fully depressurized to atmospheric conditions.
Many industrial users of compressed air specify a pressure dew point of -40⁰ F/C at line pressure. This is especially so in process industries, facilities where air lines will be exposed to cold ambient air, and systems where compressed air interacts with sensitive instruments and processes.
In rare cases, a pressure dew point lower than -40⁰ F/C @ 100 psig will be specified. We normally see this requirement in instances where the end-user is conveying chemicals that react with trace amounts of moisture, micro-electronics manufacturing, some food processing facilities, and cryogenic (super-chilled) applications. ISO 8573.1 is a common standard for defining compressed air quality. ISO 8573.1 dew point class 1 calls for a pressure dew point of -94 ºF PDP @ 100 PSIG.
Most off-the-shelf regenerative desiccant dryers, both heated and heatless, will deliver a -40º F/C @ 100 psig average pressure dew point.
But how does one obtain a continuous ultra-low pressure dew point? Here are some useful guidelines.
1. You probably won’t use a heated dryer. It’s well known that most industry standard heated dryers will involve a dew point spike at tower change over. This can be eliminated through design modifications, but that will drive up capital and operating costs.
2. Use a heatless dryer – but only at the point of use or for that portion of the air system where an ultra-low pressure dew point is truly required.
3. Continuously operate the heatless dryer. To obtain very low dew points, the dryer needs to run continuously. This means 24/7. Demand switching and purge shut-off features cannot be used.
4. De-rate the heatless dryer by 20% for -100º F. So if the standard dryer is rated for 100 SCFM @ 100 PSIG for -40º F/C PDP, it will be rated for 80 SCFM @ 100 PSIG for -100º F PDP.
5. Fast cycle the heatless dryer. Most heatless dryers operate on a 10-minute NEMA cycle, 5 minutes per tower. For an ultra-low dew point a heatless dryer will need to operate on a 4-minute NEMA cycle, 2 minutes per tower. Unfortunately, this will result in rapid aging of valves and desiccant. Expect the desiccant to last only 1-2 years on a NEMA 4 cycle.
6. Turn up the purge on the heatless dryer to about 22.5%.
7. Verify the operating pressure will be at least 100 PSIG. Heatless dryers function on the principle of pressure swing adsorption. The driving force of regeneration (the swing) is a differential between line pressure and regeneration pressure. If that differential is less than 100 PSIG, you'll never meet a -100º F PDP requirement.
8. Operate the dryer in a clean, temperature-controlled, dry indoor area with minimum outlet piping to avoid back pressure on the purge line. Again, outlet piping on the purge line creates back pressure on the regenerating tower, thus inhibiting the all-important swing.
9. Have a good after-filter. The fast cycling often creates elevated levels of desiccant dust.
As you can see, driving down the pressure dew point from -40º F/C to -100º F takes considerable effort. It can be costly too, both in terms of energy use and increased maintenance costs. This is why I suggest only pursuing this dew point at the point of use or in isolated portions of the plant. Trying to achieve a pressure dew point of -100º F throughout the plant is likely to be needlessly wasteful and very difficult to maintain.