Selecting an air compressor seems so simple. All you have to figure out is how much air you need and decide how much you want to spend, right? But then you start delving into it and find all sorts of matters you have to decide on. And it seems you can’t decide on one without first deciding on something else. It’s a cat’s cradle.
For instance, how much air do you need? It depends. Will you be using a nail gun that uses quick bursts of air or a grinder that runs continuously? Those questions direct you to look first at the volume of the receiver tank.
Perhaps you need enough air to run a grease gun and a 1” impact wrench at the same time. That’s a matter of capacity. But the manual also says the wrench has to have 170 psi. In that case, you need to be looking at a two-stage compressor. A single-stage compressor might deliver higher cubic feet per minute (cfm), but top out on pressure at 150 psi. And you want to hold the weight and cost down — which points you to an electric unit — but you may be working in a remote area that demands a gasoline engine.
It can be maddening. But let’s look at the Decision Wheel (graphic) and take the points around it one by one. You can start at any point and move around it in any direction since all the issues eventually come up. But let’s start at the top:
Portability and flexibility needs: When looking at hand-carry or wheeled-air compressors, portability will be a given. The bigger concern will be ensuring the model you prefer also meets the pressure and capacity requirements of the job. With their low weight and compact configuration, these air compressors can go pretty much anywhere — with one condition (see next step).
Power source: The next obvious step involves considering where the compressor will be used. If you’re working in a remote area without access to electrical power either from the grid or from an on-site generator, your only choice is a gasoline engine model. That will give you complete mobility.
But there could be other issues that come into play, such as environmental restrictions — emissions in a confined space or noise abatement — that cancel out the advantage of convenience. The gasoline model also will have both a higher purchase price and the ongoing costs will be greater. But it will provide higher actual horsepower than its electric-motor counterpart.
Size matters
CFM required: Besides power, capacity will directly impact what the compressor will enable you to do. Begin by adding the cfm requirements for all the tools you plan to use simultaneously and then add another 30 percent of that to allow for unknown or uncommon compressor usage. You’ll find the cfm demand either on the tool itself or in the owner’s manual. Capacity or volume — cfm — can be figured three ways:
1) Displaced cfm is simply a mathematical calculation of the bore, stroke and rpm. But it does not take into account any of the important variables such as temperature, atmospheric pressure, humidity, friction or heat dissipation, and therefore it means almost nothing in the real world.
2) Standard cfm is a better measure of reality. It is the flow of free air in a standardized environment — such as 14.5 psi atmospheric pressure (the pressure at sea level), 68 F and 0 percent humidity. Since this is a standardized metric, it’s the best figure to use in comparing air compressors across the board, apples to apples.
But what about your own needs? You’re working in mile-high Denver in January and it sure isn’t 68 degrees. Or you’re at sea level, but it’s Louisiana in July and the temperature and humidity needles are on the peg.
3) Actual cfm is the number you need because it figures in the variables that apply to your own situation. It will give you the output of the pump for the actual working conditions. But actual cfm is a hard figure to get, precisely because it requires site-specific data and calculations that may be best left to an engineer.
So for selection purposes, your best bet is to compare air compressors based on the standard cfm ratings. Note, too, that cfm is often shown at various pressures. These numbers can be very useful to determine if a compressor produces enough volume for the application, but they can be confusing when you try to compare different compressors or compressors rated at different pressures. So, again, standard cfm is best: It levels the playing field.
One last word about cfm: add that 30 percent to provide a reasonable buffer against the unforeseen, but don’t exceed it. There’s no point in buying more than you will ever need. Just don’t buy less.
Required operating pressure: The cfm also will be important to know as you consider a single-stage compressor versus a two-stage model. Begin by listing the minimum operating pressure requirements for the tools you’re going to use, which will indicate whether you need a single-stage compressor or a two-stage. Single-stage compressors are fine up to 150 psi. If you go over that, you’ll need a two-stage.
A single-stage compressor will typically have a higher cfm rating because the cylinder is drawing in air and compressing it with every rotation, whereas a two-stage is compressing the air up to an intermediate pressure in one or more cylinder(s) and then passing it on to another cylinder to finish the job. Because the air is typically passed through an intercooler between stages, a two-stage compressor is more efficient at higher pressures.
Receiver tank: Once again, the capacity of the compressor tank, which is usually measured in gallons, depends on how you intend to use the compressor. If you’re running tools that require quick, concentrated bursts, such as air nailers, a small tank is best. If the tools are to be used for sustained periods, such as board sanders and grinders, you’ll want a larger tank. Think about it: it’s like blowing out a candle compared to blowing out a birthday cake covered with candles. You have to fill up your lungs for the birthday cake; the candle you can do with a puff.
Choose quality
Product features: Durability means longevity and longevity means cost-efficiency over time. Invest in quality up front and it will pay long-term rewards. So look for long-term durability features, such as a cast-iron cylinder, a heat-dissipating head, an efficient cooling system, structural protection for critical components and fittings, a heavy-duty steel frame and powder-coat or electrostatically-applied paint to resist chipping and wear. Anti-vibration feet will help keep the noise down and, more importantly, keep the compressor from rattling itself apart.
Parts and service support: How long is the unit designed to last? Look at job-site matters, too — size and weight, for instance. Is the listed weight of the unit actual weight or shipping weight? It seems obvious that the key number for the user would be actual weight, but often it’s the shipping weight that is shown, so check that.
And don’t skimp on 1/4” hose if you need 3/8” to handle the load of more tools or longer runs of hose. But be realistic about the actual needs, too. Make sure the larger hose will justify its extra weight and cost.
Asking questions helps determine quality factors that you can’t necessarily see. Are air compressors the primary or sole business of the supplier? Does the company make its own products or source them from a third party? How long has the company been in the air compressor business? (All we know about the future is what we know about the past, so look at the company’s history in this market.) Can the supplier answer all of your questions clearly and explain the subtleties that only an expert would know? What about the availability of technical help, parts and service and the distribution network?
Initial price and long-term costs: When you add it all up, what will you really be paying over time? Consider how you’re going to use this compressor, how often and how long. The big question in the selection decision is this: What would it cost you to be wrong?
Don’t buy less
Overall, the most critical issue to keep in mind is the job analysis. Every job application has its own requirements and therefore its own set of questions. You will need to make up a realistic checklist for your specific work situation, but these core topics on the Decision Wheel should lead you to a pretty good understanding of the problem you’re trying to solve.
Remember, it makes little sense to buy more than you need, but it makes no sense at all to buy less. What you’re buying ultimately is not an air compressor. What you’re really buying is a solution to your problem. You can try to get by with just one aspirin, but you’ll still have the headache. Therefore, analyze the decision points thoroughly in order to find the best solution for the present and future.
Dan Leiss is president of Jenny Products Inc. in Somerset, Pa.
This article originally appeared in the December 2011 issue.
