Picking a Compressor

Buying an air compressor looks easy at first glance – just look at the tank size, right? But as one explores the options, things can get a little confusing. For example, is it better to go oiled or oil-free? Should the tank be a pancake, twin, horizontal, or stationary vertical? Do I want a single- or two-stage? Does it have to be loud? Is gas really a substitute for electricity? What’s better – rotary screw, scroll, or reciprocating piston pump? And if it’s reciprocating, how many pistons?

Enough with the questions. Let’s find some answers.

The answer depends on how old the unit is. Newer oil-free units have sealed-in lubricants that are designed to last for the life of the machine, and they do. The technology has made huge leaps over the last decade, and today’s sealed compressor motors are every bit as reliable and long-lasting as ones that require maintenance. They also tend to be quieter than oiled ones, and they don’t seize up or get sludgy in extreme cold.

At the other end of that argument, oiled units are probably a better choice in extremely hot weather because they dissipate heat faster and have built-in rust resistance. Oiled units are perhaps better suited for use in the shop, while oil-free is often a better choice on the jobsite. But nowadays, there really isn’t much of a difference. If anything, oil-free may deliver slightly cleaner and drier compressed air, but filtering is still a great idea for both options.

The issue here is tank size and shape. If the unit is going back and forth to jobsites, it needs to be light and small enough for one person to move. But it also needs to be large enough to support the tool.

Manufacturers have discovered different ways to give small portable compressors enough storage capacity so the motor isn’t constantly cycling. The more air storage, the less frequently the pump needs to go to work. The dual tank, where two cylinders are stacked vertically or horizontally, is one way to give a woodworker more capacity, and it may stay a little cooler simply because there is more surface. When tanks are stacked on top of each other, a slim motor is usually mounted beside them. A handle emerging from between the tanks and the motor gives the operator a good sense of balance when carrying it. The pancake tank is just as efficient, but the weight is distributed differently, and this configuration often has a wider footprint. Wide and low works better on a sloped roof than tall and narrow. But the bottom line is that tank shape doesn’t make a whole lot of difference. Some shops get past the frequent pumping issue by using a shop compressor to charge portable tanks (just the tank, no motor). Lifting a tank up a ladder is a lot easier than carrying a motor, too.

A two-stage compressor will compress the air twice, so it’s usually under higher compression in the storage tank. The standard single-stage piston compressor only reduces the volume of air once and then sends it into the storage tank. The dual-stage unit squeezes it again (usually up to 175 psi or thereabouts), and then runs it through a cooler and on into the tank.

Whether it’s one- or two-stage has nothing to do with the number of cylinders in the pump.

Two-stage compressors are more expensive, are usually stationary, and are most often used to run equipment that uses a lot of compressed air such as sanding or spray booths, or to run several tools simultaneously.

The drone of a charging air compressor can be heard for long distances and can be particularly annoying in modern shops when people are trying to talk on a phone. Homeowners have a special hatred for pump/motor sounds, so loud compressors are especially unwelcome on remodels when the client is living or working in the space.

Normal conversation is somewhere around 60 decibels (dBs). Anywhere over 90 dBs can be harmful, especially over time. A shop vac usually runs in the high 80s, which is where most experts tell us to start wearing hearing protection.

Standard reciprocating compressors are inexpensive but noisy because they have so many moving parts. Scroll and rotary compressors are spendy but quiet. The easiest and least expensive way to reduce noise is to use a long air hose and park the compressor around a corner somewhere. Inside the woodshop, rigid pipe or flexible hose can be used to locate the compressor out on the loading dock or maybe in a basement. It’s not a great idea to leave it outside in extreme temperature (hot or cold). A shop owner thinking about building a box for the compressor needs to keep in mind that heat build-up will hurt the machine and may cause a fire, so that’s not advised. However, some manufacturers do offer units with a housing around the motor to reduce noise. They also put foam or rubber between various assembled parts to reduce vibration. A woodworker can try wrapping foam shoes around the feet of a compressor, and this may help reduce overall vibration.

Depending on a shop’s location, natural gas may be a less expensive operating option than electricity. This is usually an option for larger, fixed compressors. Gas compressors need to be in a ventilated or outdoor space, and they run most efficiently when running at the low end of their full capacity.

A gas compressor is essentially a generator and compressor all in one. Some larger units can be connected to the woodshop’s HVAC system to recycle heat being generated during compression. From an environmental viewpoint, running a large compressor on natural gas may have a smaller carbon footprint than plugging into a coal-fired electric system. Portable natural gas compressors need to be run by well-trained and careful operators, and they need to be parked where their loud noise emissions won’t bother the neighbors.

Electric compressors are a little less expensive to buy, safe indoors, and don’t stink of exhaust fumes, and are easier to manage and operate. But they devour electricity and are dead in the water if there’s a power outage. So, for larger shops and big onsite jobs, natural gas certainly has a lot of appeal.

Which kind of compressor a shop buys has a lot to do with budget and workload. Most small woodshops start with a portable reciprocating piston pump compressor because they’re inexpensive and widely available. They can be purchased with a single or dual piston, depending on the volume of air a shop requires.

Rotary screw compressors are quiet, efficient, powerful, and a bit more expensive. These machines have very few moving parts, so they run cooler and quieter. If you see a road crew running jackhammers, their compressor is probably a rotary screw unit because it can deliver large amounts of high-pressure air over a long period of time. This technology, where two spiral rotors mesh together and compress the air inside a cylinder, is best suited to large compressors.

A scroll compressor is usually regarded as the quietest and most efficient type. Also called a scroll pump, it generally has one fixed spiral and another moving one that is located inside the stationary one. Think of a rolled-up sheet of paper, and then a second sheet rolled into it. They’re quite common in household HVAC pumps because they save energy, have a long useful lifespan, and run quietly. The biggest challenge with these machines is the initial cost. They also have a lower capacity than other types of compressors.

As with buying any machine, the most vital part of the process is locating an informed salesperson who inspires confidence and trust, and who understands the woodshop’s requirements.  

This article was originally published in the July 2023 issue.