In the last post, we left you hanging. We showed you how not to size up an air compressor and then made you wait for the correct way.
So what is the correct way to size a compressor?
The correct way is to determine the flow, pressure, duty cycle and ambient conditions.
- Flow & Pressure
In the USA flow is normally measured in cubic feet per minute (CFM). You may also see the flow rating in m3/min, l/s, or m3/hr. Basically it’s just the volume of air the compressor is supplying and how fast. A cubic foot is 7.48 gallons, so 1 CFM gives you 7.48 gallons of air every minute.
Pressure is normally measured in pounds per square inch (psi), but you may also see it as bar, mbar, kg/cm3, or MPa. I think most people reading this already understand the concept of pressure. If not, please read more here.
I mentioned flow and pressure together, because you need specific flow delivered at a specific pressure. Knowing your psi without your CFM is useless, and knowing your CFM without knowing your psi is useless. A compressor’s CFM rating is dependent on the psi it’s delivering. You must determine both.
Flow and pressure are inversely proportional, so with the same compressor as your pressure goes up, your flow will go down. You need to make sure the compressor produces the correct CFM at the correct pressure. Make sure you don’t look at the CFM “displaced” rating. You need the “delivered” CFM rating. So a compressor brochure might say 125 psi max and it delivers 10 CFM at 40 psi. If you needed 10 CFM at 90 psi for your application, then that compressor probably wouldn’t work. You have to call up the distributor or the factory to see what the delivered CFM at 90 psi would be.
Every piece of equipment you have that uses compressed air should have a rating of flow and pressure that it needs. This information should be in the manual or on the manufacturer’s website. If not, call them and find out. Some tools, like nailers, give you the amount of cubic feet used per cycle, instead of a CFM. In that case you must figure out how many cycles you can do in one minute. If you could put in a nail every 2 seconds, and the nailer is .3 cubic feet per cycle at 40 psi, then you’d be using 9 CFM at 40 psi. If you had a slow assistant and he can only do a nail every 10 seconds, then when he’s using the nailer it’s only 1.8 CFM at 40 psi.
You need to add up all of the CFM ratings and find out what psi you need. After that you must look at duty cycle.
- Duty Cycle.
We went over duty-cycle when talking about reciprocating compressors. If you missed it, scroll about 3/4 of the way down the page and read the section on duty cycle.
After figuring out the psi and adding up the CFM you need, determine how you’re using this air. Are you using this equipment all day long or are you using it for just a few hours a day? If you have a constant use application, then either oversize a piston compressor by 40% or get a rotary compressor.
Another part of duty cycle is looking at equipment that occasionally needs large bursts of air. Let’s say you have a machine that needs a large amount of CFM for just 30 seconds every hour, but the rest of your machines use much less. Do you need to buy the compressor for the CFM of the biggest machine? Probably not. That kind of problem can be solved with extra tank storage. Get a tank large enough to supply the air for that 30 seconds, and make sure the compressor can fill it up in less than an hour with all of the other things running, and you should be okay.
Vehicle maintenance shops that use diaphragm pumps to deliver oil and other fluids are good examples of this. Your double-diaphragm pump may need 60 CFM, but you only use that 60 CFM for 90 seconds at the most. Then it’s another 2-3 minutes before you use the next pump for the other fluid. So that’s 90 cubic feet used (60 CFM at 90 seconds) every 120-180 seconds. You’d only need a 30-45 CFM compressor with a properly sized tank to do that – you wouldn’t need 60 CFM. Pretty much all of the other applications for a maintenance bay are 10 CFM or less. Getting a 60 CFM compressor to run all day just for that is a waste. A smaller compressor with a bigger tank would do the job, cost less up front, and use less electricity.
As I stated before, duty cycle is one of the most important things to look at when sizing a compressor, but it’s often the most overlooked. How often you’re using the air is just as important as how much.
- Ambient Conditions.
I just said that duty cycle is often overlooked, but it’s not even in the ballpark of being overlooked when you compare it to ambient conditions. Have you ever been inside of a typical compressor room? People stick air compressors in some of the hottest, dirtiest rooms you can imagine. The only comparison I can think of is a boiler room, and I’ve seen compressors in boiler rooms (please don’t do that).
I’ve been in compressor rooms that were 140°F, I’ve been in compressors rooms where the dust in the room was over a foot thick (grain dust, concrete dust, or saw dust), and I’ve been in compressor rooms where the chemicals were so strong you could barely be in the room for 5 minutes without getting sick.
We understand that you may have to stick the compressor in these extreme conditions, because you have no other choice. However, if you’re going to put the compressor there, at least take that into account when you buy it.
The CFM rating on all air compressors is SCFM, which stands for standard CFM. A compressor manufacturer cannot list all of the possible conditions customers may have on its brochure. Because of this, the manufacturers got together and agreed upon a standard. That standard is 68°F, sea level (or 14.5 psi ambient pressure), and 0% humidity. If your conditions vary, and they probably do, then you must take that into account.
If you go up in altitude, higher in temperature, or higher in humidity, the compressor will give you less CFM than the SCFM rating on the brochure or tag. Altitude affects it the most. If you are high in altitude, then you need to oversize the compressor. A 100 SCFM air compressor in Denver will only give you 80-85 CFM.
Humidity and temperature affect the air treatment more than they do air compressors, and the effect is not as dramatic as going up in altitude. However, extremes need to be looked at. Here in South Florida seeing a compressor room with 100°F temperatures and 90% humidity is not uncommon. In that situation a 100 SCFM compressor will give you about 93 CFM. That’s only about a 7% derating, but if the compressor was sized for 90 CFM, and you had some pressure drop in filters or piping, or maybe there’s a small leak somewhere, then that 7% could make or break you.
If you like doing math, here is the formula:
Ps = standard pressure, psi
Pa = atmospheric pressure, psi
Ts = standard temperature, °F
Ta = ambient temperature, °F
PPwv = partial pressure of water vapor at ambient temperature
rh = relative humidity
ACFM is the actual CFM that you will get in the conditions you have.
An easier way is to contact your local compressor salesperson and have him/her figure it out. In addition to doing the math, they can use their experience and training to possibly find a better solution or know of a trick you can do to make sure everything works properly. Speaking for our company, we have guys that have been doing this over 30 years, have months of factory training and multiple industry certifications. Whatever you’re facing, they’ve seen it before and they know what works and what doesn’t.
- Other Factors.
There are a few other things to check for.
- Air treatment. We’ll cover compressed air treatment in different post, but filtration inherently has pressure drop associated with it. If you size them correctly, the pressure drop should be minimal, but take it into account anyway.
- Tank size. The bigger, the better. If you’re buying a piston compressor, then you probably have very little choice in tank size, and usually the tank it comes on is more than adequate. If you have applications that need a large burst of air you may want to get a surge tank. If you have a rotary screw compressor, then the rule of thumb is four gallons of tank storage per CFM of the compressor. You may need more or less, depending on duty cycle.
- Backup. If the compressor is essential to your business, you should always have a backup.
- Multiple compressor solutions. If you are a business that runs multiple shifts and those shifts vary in their air demand, then usually a multiple compressor solution is best. If you need over 100 CFM of air, then you also get in the area where a multiple compressor solution may be better than just one.
- Fudge Factor. You should always give your compressor a 10-15% “fudge factor.” What I mean by that is that things don’t always turn out as planned. You might get pressure drop in your piping; you might spring a leak; a filter could get clogged – who knows. As your equipment gets older, it may consume more compressed air (this is actually fairly common). Getting a compressor with 10-15% more capacity than you calculated is often a good idea to cover for the unexpected. It’s a safety cushion. Now we always preach energy savings and using the most energy efficient compressor you can get, and oversizing goes somewhat against that. However if your facility is big enough for that amount to matter, then advanced compressor controls and large tanks can make it so that the compressors only run in their most efficient state, and you won’t waste that 10%.
“….But it’s so much easier just to say I need a 20hp compressor!”
What’s easier – taking an extra hour or two to do things the right way or buying the wrong thing and then having your compressor break on you in the middle of production? If you have kids, I’m sure they may complain about doing their homework sometimes, but you make them do it. You know that doing their homework now results in deeper understanding of the subject matter later, plus they if they don’t do the homework, they might fail the class.
You reap what you sew. People spend days deciding what car they want to get. Sometimes they’ll put in hours of research. That’s a good thing, because your car is very important to your life. In the same way your air compressor is essential to your business.
Put in the extra work before you buy it, and you will reap the benefits of having the correct air compressor. It’s an expensive piece of machinery. Why wouldn’t you want to make sure you’re getting the right thing?
Additionally there are trained air compressor sales personnel, who have been to multiple week long classes on how to correctly size and configure compressed air systems. You should contact them and rely on their experience and training to get you the correct air system.