Article # 12: Compress Air vs. Blower powered air knives

Both air sources have a useful place and this is meant to be a simple comparison of the two options.  It is also a general comparison and not meant to be absolute to any application or circumstance. 

Many factories have one or more large compressors supplying air to the entire plant from a central system.  This makes a lot of sense from a maintenance and noise point of view.  It also provides for staged usage to meet current demands for air during peak and slow work hours.  Compressed air is easy to plumb to a new location.

Over years of growth companies have added compressors as the demand has increased and to provide a back-up in case a compressor fails.  I don’t think anyone would disagree that the reason air is compressed to 125 psi or more is because it is needed at that pressure to operate pneumatic equipment.  Typically it is also conditioned by filtering out condensed water and by adding oil for the tools it is used to operate.  One of the problems with compressed air systems is that it is so easy to add another blowoff.  For example, some parts that were dipped or washed needed blown off so a hand nozzle was added.  In another location they needed to remove the excess coolant from a machined part so another blow off was added.  Over time these kinds of air stations increased with production and more blow-offs were added as needed because the air was easily accessible and thought to be free.  Unfortunately, not only is that air not free, it is horrendously expensive.  We have worked with people who do energy audits at manufacturing plants.  They all agree that compressed air is the most expensive utility in any plant.  We are told that one 1/8” diameter nozzle blowing at 100 PSI 24/7 costs approximately $20,000 a year to operate.   With that kind of cost, they are as concerned with tracking down and fixing the many small leaks in the system as anything else.  They also all agree that the worst possible thing you can do with compressed air is to use it to blow something off.  The reason is that compressed air is simply the wrong tool for the job.  It’s like trying to drive nails with a pipe wrench.  You can do it, but you’re going to hurt yourself in the process.

There are several reasons for this.  The first reason has to do with pressure.  An air compressor is designed to provide air at pressures of 100 PSI and greater.  Most blowoff applications can be done at 2 or 3 PSI.  That means that all the energy used to compress the air from 3 PSI to 100 PSI is wasted.  That means that something like 97% of the energy used is wasted.  The higher the pressure the compressor produces, the worse the situation becomes.  Some of them operate at 250 PSI.  Another aspect of this waste is that a lot of compressors are reciprocating; pistons moving back and forth in cylinders.  This means that half the time the compressor is operating it is pushing that piston without producing any compressed air.  Blowers are rotary.  They are producing pressurized air all the time.

The second reason has to do with flow, how much air the compressor puts out.  A typical 15 horsepower, single-stage air compressor can put out 113 cfm at 100 PSI.  (Taken from a particular manufacturer’s published data)  A 15 horsepower blower can put out 1,000 cfm at 3 PSI (while operating at 80% efficiency).  Let’s return to that 1/8” nozzle we mentioned earlier.  At 3 PSI, one of those nozzles use 3.17 cfm.  The 15 hp compressor, with proper piping and regulators could feed a maximum of 35 of those.  If you added more, the pressure would start to drop and you would not get the same performance.  The 15 hp blower could feed 315 such devices without losing pressure.  The blower is capable of doing 9 times as much as the air compressor.   The blower fits in a space 30” x 16” x 27” and weighs 280 lbs.  The air compressor requires a space 84” x 67” x 72” and weighs 3,440 lbs.

If the comparison stopped here, it would be enough to convince you not to use compressed air for blowoff.  But, this is not the end of the expense for compressed air.  As mentioned above, compressed air is almost always treated to remove condensed water and to add oil.  Both of these are done to protect air tools powered by a compressor.  That treatment is expensive, and since blowoff devices use much more air than the typical air tool, the bulk of that treatment expense should be assigned to the blowoff.

In addition to all of this, now you are blowing off your product with air that has had oil deliberately added to it.  If your product is unpainted steel, this might be an advantage.  For everything else, it’s a problem.

Above, we looked at a 1/8” nozzle and discovered that a blower could do nine times as much as a similar sized compressor.  We can’t always provide that big an advantage, so let’s work through a more realistic comparison.  A typical air knife system might consist of two 12” air knives with 0.045” gaps.  Most drying applications can be done at 28,000 fpm air velocity.  If I were trying to set up such a system, a 7-1/2 hp blower would not be big enough.  I would have to specify a 10-hp blower, and instead of 28,000 fpm, I would be getting more like 34,000 fpm and using 255 cfm. The motor would be pulling about 7.2 kW.  To provide the same cfm using an air compressor operating at 100 PSI would require (acc. to Machinery’s Handbook) approximately 40 hp, 29.8 kW.  The blower-powered system would cost $6200.  If you are paying $0.06/kWh for electricity, you would save about $1.356/hr.  Operating two shifts a day, 40 hours a week, the electricity cost savings would pay for the system in a little over thirteen months.   After that, you would be saving about $6,000 a year.  This does not include the cost of treating the air, which is entirely absent for the blower-powered system.  The blower-powered system has a reasonable life expectancy of more than five years and there are similar systems out there that have been running for ten years.  The blower I chose is a relatively inefficient blower.  With a more efficient blower, more constant operation, or a higher electricity cost, the situation is even better.

In addition to just being expensive, using a compressed air nozzle can actually create problems.  Because of the high pressure, the air is moving very fast.  And the air is dense because it has been compressed to six or ten times its normal density.  When this high-energy blast of air is targeted at a small area, a lot of momentum is transferred.  Some of the liquid you are trying to remove may be atomized, that is, it may be blown completely off of the surface and into the air.  If you blow off water or oil with a high pressure nozzle it may remove the oil or water; it may also make a terrific fog that spreads water or oil over a wide area.  We think it makes more sense to use a controlled air curtain at 2 or 3 psi to remove the liquid like a squeegee on a window.  More air at a lower velocity gives you the same transfer of momentum, but in a much more controlled fashion. At the same time it reduces the work load on the plant compressor and uses a fraction of the power to do the same job and do it better.