Air Compressor Selection

 

Authors: Tom Eddleman and Cliff Swanson

Compiled by: Jim Yount

Published:

Revised: August 03, 2005

 

Please view the following as a "work in progress".  If you have comments, additions, disagreements, etc, please e mail any of us, and we'll incorporate the changes. 

As far as equipment questions go this is one of the most frequently asked questions: What compressor do I need? It would be easy to say "Go get a 25 horse Acme 3 phase 220V and that will do the trickĒ. The only problem with that is it would be more than most people need and the resulting cost and electric bill would be cost prohibitive even if they could get 3 phase 220 in their area.

For those that are already sandcarving the answer is fairly obvious Ė Whatever youíre using now is what percent of what you really need?  If that 5 HP compressor thatís 10 years old is pumping all the time and not getting any rest is making you wait every 10 minutes for 5 minutes to fill up, then you either need a rebuild, repair some leaks, or get another compressor. If it was a 6.5 CFM when new itís less now through wear and maybe a new 8 CFM unit would get you going.   You may have decided that this is going to be the LAST compressor you buy, so reliability is a key issue, and you'd like to upgrade to a commercial unit. 

For those just starting into sandcarving, the question is more difficult.  You may not know yet just what your most demanding use will be for the compressor.   The higher cost of a seven horsepower two stage, cast iron professional reciprocating compressor ($1500 to $2000) may be prohibitive, and perhaps unnecessary if you later abandon sandcarving, or if your work will be totally confined to casual blasting, using small nozzles.  That $300 five horsepower 60 gallon oil less unit from Home Depot  looks tempting, but you know that most sandcarvers avoid oil less compressors like the plague.  Then, there's this new class of two stage consumer oil lubricated compressor that's about half the cost of a commercial unit.  So, where do you start?

This is a HUGE area, so we've broken it down into four parts:

How much air do I need?

What type of compressor should I buy?

What features should I select for durability?

What should I consider when installing the compressor?

 

How much air do I need?

Air Flow Rate - Cubic Feet per Minute

The real issue with regard to a compressor is how much air flow it will deliver at a given pressure.

It's very important that the delivery capacity of the compressor exceed the consumption, because itís unwise and unduly hard on a compressor to have the motor running continuously in an attempt to "just keep up" with the air demand.

Most compressors are rated in SCFM  (Standard Cubic Feet per Minute), rather than ACFM (Actual Cubic Feet per Minute).  This means that they are rated using certain criteria Ė (In the United States) Air at 14.696 Pounds per Square Inch (PSIA)/ 60 Degrees Fahrenheit (įF) / 0% Relative Humidity (RH).  If your application is different from this, for example if you live in Denver or Albuquerque at high altitude, you will need to de-rate the rating.  If you'd like to see how altitude effects pumping, Tom uses these charts.

 

Several choices will drive your air needs:

Pressure Pot versus siphon gun

Pressure pots work efficiently at much lower pressures and flow rates than do siphon guns. So the bottom line there is that a siphon system will require a compressor capable of delivering more air at a higher pressure than a pressure pot (the operating pressure of a siphon gun is generally in the 80-90 PSI range when blasting glass, whereas pressure pots are generally operated at 20-40 PSI for glass).

Nozzle size

Basically a larger diameter nozzle consumes more air than does a smaller diameter; AND you have to remember that grit will enlarge the nozzle diameter over time during blasting. So, the CFM requirement increases as a nozzle gets used, and eventually the nozzle must be changed. Remember that when you double the diameter, you quadruple the CFM required!

Some folks use ceramic nozzles; their very light weight makes it easier to do very delicate work.  While ceramic nozzles wear away much faster, their initial cost is less than carbide nozzles.   If youíre using ceramic nozzles on a large job, the wearing of the nozzle is something to contend with in respect to overall uniformity of the blast (due to the rapid eroding of the nozzle).  With carbide nozzles the wear of the nozzle is so minute that itís not much of a factor.

Most glass blasters start out with a 3/32" nozzle, and generally don't go larger than 1/8". A 3/32" nozzle requires 5 CFM @ 40 psi and the same nozzle requires 9 CFM @ 80 psi. If that same 3/32" nozzle enlarges to 1/8" because of grit wear during blasting, the air requirement of the now 1/8" nozzle is 9 CFM @ 40 psi, and 17 CFM @ 80 psi.

Nozzle size CFM @ 40 psi CFM @ 90 psi Suggested Compressor
  1/16 2 4 2 HP
  3/32 5 9 3 to 5 HP
  1/8 9 17 5 HP

data extracted from Dobbins Etched Glass Techniques and Design (see Resources section)

You'll find additional data in the lower right-hand corner on the second page of Tom's reference.   

Percent of Daily Use- Job size - continuous use versus intermittent

The amount of time that you spend daily at the blast cabinet or in the blast room does become a factor when youíre choosing a compressor. You can afford to wait a few minutes for a small compressor to build up more air and cool down if youíre doing an occasional ornament or sun catcher. If youíre doing larger work where youíre blasting for hours that wait time begins to feel like an eternity and a larger portion of your day is spent waiting. In that case you will want a compressor that keeps up with your work load.

By the time you crank these numbers, you'll probably need a compressor that will produce at least 8 CFM at 90 psi, and want at least a 60 gallon tank.  Folks who blast rocks, who do large pieces, have siphon guns, or who do long production runs will need more. 

 

What type of compressor should I buy?

 

Rotary Screw, Reciprocating, Centrifugal, Diaphragm and Oil less. There are others, but unless youíre to use this compressor in a plant turning out high volume work then weíll primarily focus on oil lubricated reciprocating.   We'll also mention oil less compressors, even though most people in the Sandcarving E-group that have used one wonít vouch for their longevity. (They are also REALLY LOUD.)

Hereís what Ingersoll-Rand has to say about reciprocating air compressors:

Reciprocating air compressors are positive displacement compressors. This means they are taking in successive volumes of air which is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by using a piston within a cylinder as the compressing and displacing element.

The reciprocating air compressor is considered single acting when the compressing is accomplished using only one side of the piston. A compressor using both sides of the piston is considered double acting.

The reciprocating air compressor uses a number of automatic spring loaded valves in each cylinder that open only when the proper differential pressure exists across the valve.

Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure in the cylinder is slightly above the discharge pressure.

A compressor is considered to be single stage when the entire compression is accomplished with a single cylinder or a group of cylinders in parallel. Many applications involve conditions beyond the practical capability of a single compression stage. Too great a compression ratio (absolute discharge pressure/absolute intake pressure) may cause excessive discharge temperature or other design problems.

Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations, may be packaged, and provide a wide range of pressure and capacity selections. 

 

Oil less (~$300)

 

Oil less compressors are designed with a low-friction coating on their moving parts to eliminate the need for oil, are generally less expensive than their oil-lubricated counterparts and don't require you to change the oil on a regular basis.  And, the air they produce is cleaner because there is no residue from the lubricating oil (although you should still use an inline filter).  These compressors are really LOUD (89 decibels and higher), and generally have duty cycles of 50% or less.  A few years ago, 60 gallon oil-less compressors were common; that "class" of higher-end consumer compressor has been replaced by the oil lubricated single stage compressor, generally with an all-aluminum compressor. 

 

Piston/ reciprocating oil lubricated (consumer units: ~$400 to $800; commercial units: ~$1000 to $2500 and up)

 

As the name implies, these compressors use oil for lubrication. They're significantly quieter than most oil less compressors and, if taken care of, can last a lifetime or two. You will need to change the oil periodically (once per year is enough for home shops, but follow your manufacturer's directions) and you'll need to install a high quality inline filter because of small amounts of oil that get in the air. Life expectancy generally runs anywhere from 2,000 to 6,000 actual pumping hours between rebuilds.  Both single and two stage units are available; they generally feature all cast iron construction for durability and noise reduction. 

 

What's new: consumer two stage units (~$800)

 

The most interesting news in air compressors is the recent availability for consumers of higher pressure, 2-stage machines. In the past, the maximum pressure developed by a compressor was around 125 psi. This was the pressure that the compressor would "cut out." Recently Porter-Cable, Campbell Hausfeld and others have introduced compressors capable of developing 175 psi. What's the big deal, you say? After all, you can't use any kind of glass gun at those pressures! Well, imagine that tank below the compressor. It will hold much more air at a higher pressure. You're essentially squeezing the air in tighter. Effectively, there can be as much as 140 percent more air delivered. According to Porter-Cable, their new 2-stage model CPLKC708V2, which has an 80-gallon tank and a cut-out pressure of 175 psi, has the equivalent air storage of a 194-gallon tank!  The "cut-in" pressure (the pressure level that kicks on the compressor), in compressors currently on the market, is  about 95 psi, while the new 2-stage compressors have a cut-in pressure of about 145 psi, so the compressor keeps a significant reserve for long blasting jobs.

 

Tank size and shape

 

Sandcarvers generally use fixed location tanks, and favor larger sizes.  Most of us therefore use vertically oriented tanks, of at least 60 gallons, with 80 preferred.

 

The full range of available compressors is staggering. There's a "buyer's guide" to compressors (dated 2001) here:

 

American Woodworker compressor guide

 

Ingersoll Rand also makes fine compressors, and should be investigated as part of any selection process.  For some reason, they're missing from this list.

 

Some typical compressors:

 

Table shows comparison of Porter Cable Oil Lubricated Compressors

 

Compressor Porter Cable 7.5 HP Porter Cable 7 HP Porter Cable 7HP Peak
Model: CPLMC7580V2C CPLKC7080V2 CPLC7060V
Price $1,499 $789 $396
Maximum Pressure 175 175 135
Stages: 2 2 1
Lubrication Oil Oil Oil
Cylinder body: Cast Iron Cast Iron (sleeve) Cast Iron (sleeve)
Crankcase and Head Aluminum Aluminum Aluminum
Voltage 240 240 240
Magnetic Starter Yes not required not required
Tank 80 Gallon 80 Gallon 60 Gallon
Duty Cycle Continuous Continuous Intermittent, no duty cycle listed
CFM 25 SCFM @ 100 psi 16.8 @ 90 psi 9.7 @ 90 psi

 

No, we're not necessarily pushing Porter Cable; but the range of features across different models in one manufacturer's product line makes for interesting reading. 

Horsepower

Notice that we havenít mentioned Horsepower ratings, since one manufacturer's 3 HP compressor may deliver the same CFM as another's 5 or 7 HP. The other thing is that most manufacturers advertise a compressor's PEAK HP rather than the actual RUNNING HP. When a compressor first starts up it draws more electrical current than when it's actually running. Inasmuch as the HP of an electric motor is evaluated by the amperage it draws, the peak HP is always greater than the running HP. So it's a bit pointless to evaluate the compressor by HP. This is especially true because the real job of the compressor is to deliver air at the CFM required by the tool it's running. However many HP a particular machine must muster to do that is really immaterial.

 

What features should I select for durability?

 

So, why do those "professional grade" compressors cost so much, and why would I want one anyway?

 

Durability and ease of maintenance distinguish commercial units. 

 

      Aluminum versus cast iron cylinders

Sandblasting per se can be an industrial activity that exceeds the air delivery and robustness of many of the "consumer grade" or "homeowner" compressors sold at the "big box" home improvement stores like Loweís or Home Depot. Some of the larger compressors sold in those stores are machines that are deliberately made with aluminum pump cylinders so that the prices can be kept below the more robust machines built with all cast iron pump heads. (Note that cast iron compressors are also being sold at these stores.) Despite the CFM ratings of some larger compressors sold in those stores, many of those machines are definitely NOT made to run at continuous duty as are more industrial cast iron compressors.

The bottom line is that even a large compressor equipped with a 60 gal tank that is rated at maybe 10-12 CFM @ 40 psi, will have to run for longer time periods than the machine is capable of sustaining without overheating. The result would be that the pump cylinders get quite hot, and, because theyíre made of aluminum, the heat would cause warping and structural changes in the metal that would eventually lead to early failure. This is in stark contrast to an industrial grade compressor, some of which are rated for continuous duty (meaning they can run all the time and not have heat failure) that are made of cast iron, and will likely cost more and have to be bought at a tool shop that specializes in industrial grade tools.

Porter Cable's hybrid cast iron sleeved, aluminum body compressor is interesting, in that it provides cast iron for durability, and aluminum for heat rejection.  If it proves as durable as most Porter Cable tools, it would be an interesting choice.  

Speaking of compressor design Ė another thing to consider: whether a twin cylinder compressor is of v-twin design or parallel design. With the v-twin design the cylinders are easier to uniformly cool.

Duty Cycle-

The duty cycle of an air compressor is what percentage of the time it is designed to run before it must rest and cool off. Each manufacturer lists the duty cycle in the specifications of that compressor. If he doesnít know, have him find out before purchasing the unit.

 By figuring what CFM youíll be blasting at (nozzle orifice size and  PSI) and the tank size, pressure cut on, and maximum pressure you can figure how often the compressor will be running and resting.  Some compressors are listed as continuous duty. For those you can expect to pay more than for a comparable CFM rating compressor with a duty cycle less than 100%.

Compressors generate a lot of heat in doing their job. The heat is tough on the pump components and they'll wear out quickly if the compressor runs all the time. If you don't want to over work your compressor, and risk burning it out, you MUST get a compressor that is capable of delivering about 50% more air than the tool youíre running will demand at its operating psi. So from the example above, if a person is using a pressure pot with a 3/32" nozzle requiring 5 CFM @ 40 psi to blast glass, then the compressor must deliver about 7-8 CFM @ 40 psi in order to meet the usage criteria of 50% excess air capability. As the nozzle enlarges to 1/8", the same compressor should deliver about 12-13 CFM @ 40 psi.

Cost

If you donít want to look back and say ď I wish I had bought a bigger or better compressorĒ now is the time to really think. If you get that cheaper compressor simply because of the cost factor itíll be a mistake.  We have all made this same  mistake. Take the time to weigh all the factors of  many compressors from many vendors.  Choose the one that fits you and get it. Itíll be the cheapest in the long run.

Cost is also not just the initial price of the compressor. Whatís its track record? Does it have a lot of maintenance? Does it require a special oil and if so is there a reason? Does it have a cheap looking inefficient air filter for the intake? Does it have a Baldor motor or is it something youíve never heard of made in China? These are considerations.    

Brand Name-

A brand name has a lot to do with parts and service. If you need a belt for an off brand compressor and canít find one locally will you be able to order it from the manufacturer? Can you get it worked on if need be?  Ingersoll Rand, Porter Cable, Campbell Hausfeld, and Quincy are four brands that have generally been well received.

Warranty

Last, but not least what warranty does the compressor that you chose have? Most compressors run for years with little or no work that needs to be done except for regular oil and filter changes. Occasionally a motor, compressor, or pressure switch will go out. Find out if itís covered and for how long.

Where should I install it?

Just when you thought you had this all figured out, we'll throw in some real food for thought.  Choosing a location for your compressor is at least as important as any other single consideration.

Noise.  If you have neighbors, or if you share your living quarters with anyone else, you better think this one through.  The 89 decibel staccato clatter from an oil less compressor can be deafening, or at least a major irritation (assuming you're partly deaf already).  This fact alone is enough to consider an oil lubricated unit.

Heat.  Compressors make heat.  Putting one in a closet is a sure way to overheat the unit, and damage it.  You will need air circulation

Draining,  Every year, folks die from compressor explosions in home shops.  They need to be drained regularly to avoid corrosion.  If it's tough to do, you probably won't do it.  Most of us install pipe and a ball valve, running the drain line to a convenient location, so we can open it at the end of the day.  Do this, it's cheap.

Another alternative is to install an automatic drain valve. At somewhere around $150 before installation theyíre somewhat expensive, but for the person that doesnít remember to drain his tank daily itís the solution to his problem. Tom uses one and swears by it.

Air quality.  Sandcarving generates fine abrasive dust.  If you discharge your exhaust system next to your air compressor, and don't filter it down to 0.3 microns, expect to replace your compressor sooner than you would like.

Maintenance.  Oil lubricated compressors require oil changes on at least an annual basis.  You will need access to do this.

Piping and filtering the air.  This is a section all by itself.  Tom will maybe tackle this when he gets a "round tuit".  Someone please send Tom a round tuit so he can enlighten us.

 Conclusion:

A simple fact is that we canít choose a compressor for you. Thatís something that you have to do for yourself. Weigh all the factors and get what you think you need. If you have to save up and wait a while youíll be better off. Itís a lot better than getting something that just wonít do the job.

Need more resources?  Think about folks in your area who have similar uses for compressors.  Probably an auto paint shop has similar demand for continuous duty, moisture free air.  See what works for them, and what maintenance sources are available in your area. 

 

 

 

 

 

 

 

 

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