Often times we may be up against a wall in what compressor to choose for our needs. Some hobbyists may require only a 2 HP 10 gallon compressor while others have greater needs for a 50 HP unit complete with air dryers and aftercoolers. The choices inbetween the two are astounding. Rotary screw, piston type,oil-less..... it's enough to make your head spin sometimes. Let's focus on the two main types of compressors first, shall we?
Rotary Screw ~ Traditionally cooled and lubricated through glycol, this compressor offers the ability to create massive amounts of compressed air and a rapid response to loss of pressure. The system basically works by taking two rotating shaft assemblies and forcing air via means of constant flux of negative space between the "pattern" or design of the shaft. Another form of this method is seen on "blowers" for automotive use. While they may take slightly more power to produce the volume of air that it's cousin the piston style does... it makes up for it's appetite with noise reduction and low serviceability.
Reciprocating Piston ~ You've no doubt seen this compressor everywhere, no doubt. The true workhorse of the industry. This design most resembles that of your common automotive engine in many ways. A motor spins the crankshaft of the air end. One piston draws air down through valves while the other exhausts it into a tank reserved for when you need compressed air. Forever runninf in this fashion, air is moved along at a hefty pace from the intake to the exhaust of the "compressor" until a certain pressure is achieved. Once said pressure is achieved the limit switch tells the elcetric motor to turn off and stop the process. No doubt there are varying types of this compressor in use all over the world, but this is the basic principal of the machine. What the resiprocator lacks in style and grace that it's cousin the rotary screw has, it makes up in longevity. It's not uncommon to see this style of machine in use for well over 3 decades with simple rebuilds along the way. Slightly more noisy, this compressor is like the enrgizer bunny. It keeps going and going and going.....
With that said, here are some handy rules for estimating when looking for a compressor:
1. Air compressors are normally rated to deliver 4 to 5 CFM per horsepower at 100 PSIG discharge pressure.( non 110V estimates. Lower voltage is approx 1-2 CFM per HP)
2. Depending on the the size of the system, compressed air costs about 25 to 42 cents per thousand cubic feet of free air ingested by the compressor (including operating and maintenance costs).
3. A 50 horsepower compressor rejects approximately 126,000 BTU per hour for heat recovery.( they make heat. adjust accordingly for your HP needs)
4. Motor amperage draw: 1 Phase 115V - 10 Amps per HP
230V - 5 Amps per HP
3 Phase 230V - 2.50 Amps per HP
460V - 1.25 Amps per HP
In the general "6 levels of compressed air quality", powder coaters generally fall in the middle at level 3. That is to say you will need : compressor, refridgerated compressed air dryer and an oil removal filter. This system removes 99.999% of oil aerosols and any solid particles larger than .025 microns.
A good general rule of thumb to follow when researching your needs for a compressor is as stated : (requirements now + 20% growth) + 20% demand factor. That is to say..... if you need 25CFM (cubic feet per minute) now to supply your needs, add 5CFM for a future growth and 6 CFM for demand of draw. This equals 36 CFM requirement MINUMUM when shopping around for a new compressor (again, these numbers are used for an example purpose only. Your needs will be different). It's often a common mistake that we see something "on sale" only to find out perhaps a year down the road that we should have spent the extra money because you can't operate all of your equipment now that you've expanded. Or even worse.... you can't powder coat when somebody is blasting,therefore creating a loss of billable hours within your facility.
After all is said and done, it boils down to "do your homework". A compressor is a tool used to make money with, bottom line. If you buy cheaply made.... it will invaruiably break down on you at the worst possibly moment and you'll have to repair or re-purchase anyways. I suggest you calculate for your needs and abilities and then for your budget. Shop around. Wait for the sale. Ask that friend of your what he's doing with that old compressor he never seems to use
. I hope that helped somebody out there in your decision to purchase a machine that will service your needs for quite a while to come. ..... Russ