Piston vs. Rotary Air Compressors. Which “Makes” More Water?

True or false?  A piston air compressor introduces more water to the compressed air system than a rotary air compressor.

This is false.  The amount of water that enters a compressed air system depends on ambient air conditions, not the compressor mechanics.

But there’s a good reason why a piston compressor – also called a reciprocating compressor – often seems to produce a lot more liquid water.  The reason involves heat!  Piston compressors run much hotter than rotary compressors.  Piston compressors usually discharge air at 250⁰F or hotter.  Rotary machines usually deliver air at less than 175⁰F.  (Compressor specialists, please correct me if I’ve got these temperatures wrong.)  Heat and the loss of heat (i.e. cooling) have a big impact on where and when water in a compressed air system condenses.

Let’s talk through a typical set of conditions.  Say it’s a mild summer day with 70⁰F ambient air and 60% relative humidity.  At these conditions, atmospheric air contains about .7 lbs. of water vapor per 1000 cubic feet.  The dew point – the temperature when condensation happens – works out to about 55⁰F.  In other words, if you set out your cold beer the can will become moist (if the can is 55⁰F or cooler).

Now the compressor sucks in this atmospheric air and reduces the volume by a factor of eight (assuming sea level and 100 PSIG discharge).   Air leaving the compressor contains eight times the amount of water vapor per actual cubic foot.  8 x .7 lbs. of water vapor per 1000 atmospheric cubic feet = 5.6 lbs of water per 1000 actual pressurized cubic feet.

Concentrating the water vapor elevates the dew point.  Under the example conditions, the new dew point is 125⁰ F.  You don’t need a very cool surface (beer can!) to begin condensing water.  Any heat loss below 125⁰F will cause liquid to form.

(Thank you Vaisala moisture calculator for helping generate these numbers!)

Here’s where the temperature differences between piston and rotary machines enter the discussion.  Remember that air coming from the rotary machine is relatively cool, around 150⁰F.  It’s already quite close to the dew point.  The air will quickly cool below the dew point of 125⁰F and liquid will collect in the receiver tank, liquid separators, and coalescing filters.

The air coming from a piston compressor, however, will be 250⁰F or greater!  All else being equal, this air may not cool below the dew point until it travels beyond the receiver tank and through air distribution piping.  That’s where liquid water will form, all of it coming out in your air tool or spray gun.

In turn you’ll be forgiven for believing that a piston compressor simply “makes” more water than its rotary cousin.  In truth each compressor “makes” the same amount of water, it’s just that the condensation is likely to occur in different places within the air system.

So if you’re using a piston compressor at or near the machine’s peak rated duty cycle, you should probably consider an additional cooler.  Otherwise you’ll be pushing really hot air through the receiver tank, and bear in mind that air treatment components (filters, dryers, etc.) are not effective at high temperatures.  (Yes, there are high temperature refrigerated dryers but what a travesty, using mechanical refrigeration when ambient air cooling is free!)

Ultimately the proper application of air treatment components will rid you of moisture related headaches, regardless of compressor type.   Still, every application is different, and there is no one size fits all solution for clean and dry compressed air.  We’re glad to talk you through the process of understanding what equipment will work best for your conditions.  Get a hold of us through the contact form on the left.