After-cooler for Air-Compressor – That Ain’t a Dryer!

I apologize in advance to my friends at Northern Tool + Equipment.  I’m about to pick on one of their otherwise solid marketing videos.  Watch it for yourself: http://bcove.me/ff77f8ps

What’s my complaint?  Can you guess?  Hint: It’s technical in nature, and since this blog focuses on the removal of contaminants from compressed air, my complaint is their misuse of the term “high quality air”.

Still not sure what I’m talking about?  Listen closely at 0:30.  This is where the narrator announces that the air compressor includes an after-cooler “ensuring dry high quality air”.

Nope.  Wrong.  Impossible.

And that has nothing to do with the quality of the compressor or the cooler.

It’s true that cooling is an important step – perhaps the most important step – toward ensuring high quality compressed air! This is especially true for piston compressors, which have high discharge temperatures.  In fact, cooling compressed air to within 10⁰ F of the ambient temperature will condense about 70% of the water in the air.  So the inclusion of a cooler on this or any air system is necessary.  But it’s not sufficient if high quality air is truly a requirement.

Make no mistake, a cooler alone does not yield high quality compressed air.

Cooling condenses lots of water.  Post-cooling, all that water is still in the air line!  Removing the liquid requires a moisture separator and a drain valve.  In small air systems (<20 HP) it’s common to see the receiver tank used to separate and collect liquids.  This may be tolerable in air systems with low duty cycles, but the harder and longer the compressor runs the less effective the tank-as-separator tactic becomes.  Serious air systems always use a dedicated compressed air moisture separator, with an automatic drain.

Okay, so now you’ve added a good moisture separator and drain.  (Good Separator = low pressure drop + high efficiency.)  Do you now have high quality air?

No.  Not yet.

Remember the air has been cooled, at best, to a few degrees above the ambient temperature of the room where the cooler is situated.  No cooler is 100% efficient.  This means as the air travels through the piping and expands within the tool, paint gun, blast cabinet, or pneumatic device, there’s going to be more cooling.  More cooling produces more condensation.

To prevent condensation in the piping and at the point(s) of use, an air dryer must be installed.  A dryer is any device that removes water vapor (not just liquid), lowering the dew point of the compressed air below its temperature.  Yes, I know that statement is a mouthful.  The concept of dew point is discussed elsewhere in this blog.  Or just google it.

There are a handful of different types of air dryers.  The right choice for you depends on many factors.  We’ve discussed that elsewhere.  And probably will again.  Suffice it to say that the dryer is a special piece of equipment whose function is never replaced by a cooler, separator, or filter.

Remember a dryer is usually paired with filters for removing compressor lubricants, mists, and solid particulates.

Now with the dryer and other components installed, you’ve got quality compressed air!   Here’s an illustration of one possible system layout for delivering clean dry air.

Any questions? Email me any time.

Finally, thank you to the marketing department at Northern Tool + Equipment for giving me a topic to write (rant?) about.  If you need to buy an air dryer, Northern Tool is a fine place to go shopping.