Compressed Air Dryer Sizing – Some Basic Questions to Ask

When I receive a phone call from a someone having problems with water in compressed air, the first thing I ask is, “What dew point do you need?”  Or more commonly, “What problem are you trying to solve?”

As we learned in a recent blog posting, dry compressed air is a concept with a wide range of meanings and dew point signals how dry compressed air is.

After learning how dry the customer’s air needs to be, we now need to establish a flow, temperature and pressure.  In another blog posting, I quoted from a customer’s e-mail to the effect of “we do not know flow or pressure, but could you give us an estimate?”  The response to this kind of vague application info has to be a kind but firm “not yet.”  Knowing operating flow rates and pressures is essential to sizing a compressed air dryer.

Many times, an end-user is simply not familiar with how an air dryer works and does not know what to ask.  Fortunately, the customer had the wisdom to call before acting.  Although there may be no such thing as a dumb question, I have given many dumb answers.

One mistake I’ve made several times is asking the customer the size of his compressor.  After he returns to the phone, huffing and puffing following a trip to the compressor room, the response is “84 inches long by 36 inches wide.”

Eventually I learned to ask the customer for the horsepower of the motor on the machine.  I then explain how you can get up to 5 SCFM for each horsepower.  From there, sizing a dryer is a lot simpler.

When asking for the operating pressure, be sure to get both the high and low values.  You size air treatment equipment for the low value but want to be sure that the high value is within the design range of the filter or dryer.  Think safety!   It is important to get both values.

Sometimes the water problem is at a point-of-use and does not involve the whole compressed air system.  Often a customer will want to size a dryer or filter based on the pipe diameter going to the point-of-use device or to the area being supplied.  This approach risks seriously under or overestimating the size of the air treatment equipment needed.

Most production machinery lists air consumption in the specifications or operating manual.  Digging through old files to find a manual may be tedious, but it’s a sure way to avoid a sizing mistake for the filter or dryer.  Plan B is to put a flow meter in the line and actually measure what is going through it.  In most applications, the price of a flow meter is cheap insurance versus spending money for equipment that's too big or small and not getting the job done properly.

Temperature is the final critical area.  We get very fussy about this because of the large changes in moisture content due to temperature and the consequences for a 10° or 20°F error in either direction.  The old time method of touching the pipe with your hand is not that accurate at best.  I cannot remember the number of times that someone said that it had to be less than 100°F, as it didn’t feel too warm to the touch.  If it feels warm to the touch it is over your 98.6° body temperature (zombies not included).

The quickest and most effective way to measure compressed air temperature is with one of the infrared devices that are available at home supply stores for a very reasonable price.  In olden times, these were priced out of reach of most customers and sales people, but not now. The good part is that you do not need to get close to or actually touch that filthy compressor or piping for an accurate reading and you have a digital result.  For some reason, digital readings seem to be more trusted than analog.

As always, the technical people here at Van Air Systems are more than willing to share their experiences (good and bad) so that you and your customer gets what he or she needs.  Warning: this may be different than what he or she wants.

Call or send an e-mail at any time, and we glad to work with you.  800-840-9906 or info@vanairsystems.com