How to Choose the Best Air Compressor for Your Shop

In shopping for an air compressor, I am reminded of the advice of the Victorian writer, John Ruskin: “It’s unwise to pay too much but it’s unwise to pay too little. When you pay too much you lose a little money, that is all. When you pay too little, you sometimes lose everything, because the thing you bought was incapable of doing the thing you bought it to do.”

Make no mistake about it, budgetary considerations are important. In today’s challenging economy, they are more important than ever. But budget is just one of many considerations. In this article, I will review these considerations, which include compressor type, product specifications, such as horsepower, portability, and most important factor of them all – its intended use.

How an Air Compressor Works

I liken the way an air compressor works to the way I pack my clothes into my suitcase. Imagine taking eight cubic feet of clothing and forcing it into four cubic feet of space. Somehow I manage to compress the clothes and batten down the latches.

An air compressor does about the same thing – just more efficiently. It takes a given volume of air; let’s say eight cubic feet of air; and then it packs or compresses it into a much smaller space, about one cubic feet. By compacting or compressing the air, the compressor puts the air under pressure. Forcing the air into a space eight times smaller increases or multiplies normal air pressure, which is about 15 psi at sea level, by a factor eight to 120psi.

Types of Air Compressors

The mechanical design of air compressors differs from one model to another. The basic types or styles of compressors are piston compressors, which are also called reciprocating compressors, diaphragm compressors, rotary screw compressors and centrifugal compressors.

Regardless of the design, air compressors have the same basic functions. I learned to describe the operation of a compressor as “suck, squeeze and blow”. I apologize if this description conjures up some unsavory connotations, and offends anyone. For me, it was a good way to remember how the equipment works. The compressor sucks in air which is the intake phase; it squeezes the air in the compression phase; and it blows out the air in the exhaust phase.

Reciprocating Air Compressors

A reciprocating compressor is just a fancy name for a system that uses a piston to operate. As the piston of the compressor moves back and forth it forces outside air inside the air tank, without letting any of that air escape. As more and more air is forced into the tank the air pressure rises. While there are many different models of reciprocating compressor, they all operate using this basic piston principle.

One difference between single and two stage compressors is the amount of pressure that they can deliver. The maximum output in pound per square inch or psi for a single stage compressor is about 100 psi. Two stage compressors can deliver much higher pressures.

Reciprocating compressors uses pistons to compressors. Some of these compressors have only one piston. These are called single stage compressors. Other reciprocating compressors have two pistons and are referred to as two stage compressors.

Oil Filters

Nobody wants in oil in their paint or sandblast medium. That’s an advantage of an oil-less airbrush compressor. No oil! No contamination! No problems with fish eyes.

If oil is such a bad thing, why don’t they make all compressors oil-less? Without oil, compressors run hotter. That contributes to the potential water vapor problem in your air lines. It’s not the only problem, and certainly not the most important one. Running a compressor at higher temperatures shortens it useful life.

If you buy a compressor that is lubricated with oil, you will need an oil filter in addition to your water filter. Just having a water film, just won’t do. The oil filter traps any of the oil that gets into your air line, before it gets to your airbrush or spray gun.

Piston compressors come in two varieties: those with just one piston, called single stage compressors; and those with two pistons, called two stage compressors. Both types of compressors have air storage tanks. Here’s how they work. The piston compresses the air and pushes it into the storage tank. When the air pressure attains a specified level, the compressor motor shuts off. This gives the motor a period of respite, so it doesn’t overheat. After the pressure drops below the specified level, the motor kicks in again and the piston pumps up the air pressure in tank.

Two stage compressors are typically more expensive. This type of equipment is designed for heavy-duty commercial applications. If you need to use a compressor continually, you need to seriously consider this type of unit.

For a small shop or a do-it-yourselfer spraying in his garage, a reciprocating system is the most affordable and the most common type of compressor. The key questions, though, are: (1) will this style of compressor will provide you with the capacity you need? (2) Will it provide you with the useful life that you expect? (3) What will it cost to operate and maintain this type of compressor?

Reciprocating or piston-type of compressors are not the only type on the market. If you will be using a compressed air on a continuous basis, you will need a heavy-duty industrial unit, such as a rotary screw or centrifugal compressor.

Diaphragm compressors.

Diaphragm compressors work similarly to reciprocating or piston compressors. Instead of a piston pumping back and forth inside of a cylinder to compress the air, a diaphragm compressor uses a membrane that pulses back and forth. Among airbrush artists, diaphragm compressors are a popular choice, especially among novices, because they are relatively inexpensive. In physical size, these compressors are very compact and lightweight, which makes them very portable. This is perfect for the artist on the move, who must go to the job site versus doing the work in the comfort of his shop.

You get what you pay for, though. Diaphragm compressors are noisy. They usually don’t have an air storage tank, which means they are running all of the time. Not only are you subjected to constant noise, but because they are working all of the time, they run hot. The hot air that they pump is loaded with moisture vapor that can make its way to your airbrush. The length of time that you can use this type of compressor is limited, because it can overheat, which will also shorten the unit’s lifetime use. Diaphragm compressors also have insufficient power to deliver the volume of air at the air pressure required to operate anything more than a single airbrush.

Rotary Screw Compressors.

Instead of using a piston to compress air, a rotary screw compressor uses a rotor inside of a casing. The mechanism of a rotary compressors is encased or surrounded by oil. (Water-cooled units are also available.) The oil serves two functions. First, it cools the air. This cooling significantly reduces the problems associated with water vapor. Secondly, cooling compresses the air.

Because rotary screw compressors do not have a pumping mechanism like a piston type of compressor, the air pressure is constant rather than pulsing.

Although this type of compressor is often used for industrial applications, the physical size of the unit is not necessarily massive. Instead, these compressors are typically compact and can deliver a high volume of air at high pressure. If maintained properly, these units will provide years of service.

Centrifugal Compressors.

Centrifugal compressors use a centrifuge to compress air. A spinning blade rotates on a shaft to suck air into the compressor, where it is compressed and discharged. Unlike a reciprocating compressor, which uses the pulsing motion of a piston, a centrifugal compressor delivers a steady stream of air. This high capacity equipment is designed for industrial applications, rather than for shop use.

Other Parts Of The System.

In addition to the compressor, the other elements of the air supply system that you will need to consider and in some cases budget for include the storage tank, air hoses and piping, air regulators and gauges, and filters. Many shops use some type of piston compressor. In the plumbing for the system, you will need to allow for an air pressure regulator at each drop, a moisture filter, an oil filter and shut off valves.

Air Compressor Storage Tank.

If you buy a reciprocating or piston type of compressor, it will probably have an air storage tank. This storage or receiver tank provides a number of different functions. Storing pressurized air is the key function. By doing this, the storage tank is similar to a capacitor in an electrical system. The stored air provides a constant flow of air, eliminating fluctuations in air pressure.

“If you invest in a compressor, buy one with a storage tank,” says Julian Braet of Mr. J’s Sign Service in Paterson, NJ. “Without a tank, you can’t regulate the air pressure,” he says. “The pressure ebbs and flows with the pulsing of the piston.”

In purchasing a system, bigger the storage tank, the better, because the bigger the tank is, the more compressed air it can store at higher air pressures. The size of storage tanks is rated in gallons.

Just so you’re not confused, the air tank of a compressor is also referred to as a reservoir or a receiver. In selecting a compressor, pay attention to the size of the air tank. As the horsepower and the CFM of the compressor increases, so will the air tank increase correspondingly or at least it should. Generally, for every CFM of output capacity, you will need a gallon of air tank space.

The purpose of a storage tank is to supply you with an uninterrupted supply of air at a constant air pressure. If you are spray painting, the air stored in the tank prevents any peaks and valleys in the amount of air that your spray gun is supplied, so it functions properly.

The air tank functions as a holding tank of compressed air. It also performs another function. In this storage tank, the moisture-laden air cools down. As it does, the moisture condenses and water forms at the bottom of the tank. For this reason, you should periodically drain the air tank, which will help keep water out of the air lines.

Air regulator.

The purpose of the air regulator is to reduce the air pressure from the compressor to a specified amount. A good quality air regulator with a pressure gauge on the outlet side is a must. Just so there is no mistake, there is an inlet side for unregulated air and an outlet side after the air pressure has been regulated. These ports are not interchangeable. So that you don’t install the regulator the wrong way in the line, they usually have an air on the body of the unit indicating the direction of air flow.

Air Filters.

The air compressor isn’t just compressing air; it’s also compressing and condensing the moisture in the air. On a humid day, that’s a lot of water. If you’re sweating bullets in your shop, just think about what’s happening inside your compressor. When the ambient temperature is hot and the humidity is high, your compressor can produce gallons of moisture. The hot air exhausted from the compressor is also saturated with water vapor. As the air travels through an air line or , the hot air cools, condenses and forms droplets of moisture inside the pipe.

Along with water, the compressed air may also have picked up other contaminants, such as particles of rust and dirt. All of these contaminants can clog your spray gun or screw up your paint job. To prevent contaminant-related problems your compressed air system needs filters. Here’s how filters work. The air enters the inlet; it goes ‘round and ‘round, bouncing up against the walls of the filter, causing the water and other contaminants to flow to the bottom of the filter bowl. The purified air is then expelled through the outlet.

Green Side Up.

You have probably heard the joke about the dumb landscapers that needed to be reminded when laying sod: “green side up”. When installing an air filter there’s only one way to do it right. Pay attention to the arrow, usually marked on the cap of the filter or on the body of the unit. The arrow indicates the direction of the air flow. If you install the filter backwards, it will not work properly.

Air filters comes in various sizes. Here’s another rule about air filters: “size matters”. In selecting an air filter for your system, make sure that you install one that is big enough to handle the volume of air that your compressor can generate. If you don’t you will restrict the flow of air, decreasing its volume and pressure downstream after the filter. Don’t worry about installing an air filter larger than what you really need. It won’t hurt anything.

As the filter element inside the filter separates the contaminants from the air stream, it gradually fills up with waste matter at the bottom of the bowl.

This debris must be drained regularly or it will become clogged with contaminants. The drain for the filter is a little valve at the base of the bowl. Periodically the filter will need to be replaced.

Just as the filters in your car can become clogged with contaminants, the same can happen with the filter element in your air filter. Maintenance of air filters includes replacing filter elements.

In the bowl surrounding the filter elements, debris consisting of water and oil, will collect at the bottom of the filter. As this waste material collects, it should be drained using the valve at the bottom of the bowl. This gunk is considered “hazardous waste”. You should dispose of it according to local regulations.

Pipes and hoses.

Air from the compressor is hot and quite often is laden with lots of water vapor. A water filter is designed to trap water in a liquid state. It won’t, however, trap water vapor. This moisture can pass through the filter and end up in your paint, causing problems.

So how do you solve this dilemma? To trap the moisture, the hot air needs to chill out and condense as droplets. One way to do that is to use at least 25 feet of hose from your compressor to your water and oil filter. As an alternative to hosing, some people prefer to use metal piping for a neater installation. To further trap any moisture in the line, you can also include a riser and drain valve in the plumbing before the water separator.

Purchasing Considerations.

How do you select the right for your shop? You certainly don’t want to buy more compressor than you need. But then again, you don’t want to buy too little of one either and put yourself in a position of having to upgrade a year from now.

In selecting a compressor the primary consideration is having sufficient equipment capacity to satisfy your shop requirements. What capacity amounts to the ability of the compressor to supply the amount of air that you need at the required pressure.

If you are shopping for a compressor, bigger is better. Even if you don’t need maximum capacity that an industrial compressor can produce, you can always dial down the air pressure.

Any other considerations, while they may be important, are secondary. Some of these other considerations include equipment cost, operating expenses, maintenance requirements, physical size and noise level.

When you compare the specs for one compressor with another, be sure that you are comparing apples to apples. Here are some factors to consider:

Horsepower (hp). We rate the power of our car engines based onhorsepower, so why not rate compressors on the same standard? Don’t buy a compressor solely based on its horsepower rating. In the grand scheme of things, the horsepower of a compressor is not quite as important as you may think. It just refers to the power of the electric motor. What is much more important is the output of the compressor in cubic feet per minute or CFM at a specified pressure per square inch or psi.

If you are running a commercial sign shop, 10hp is ideal. You might be able to get by with a 5hp unit, but if you can afford it, go with the bigger model. As a general rule of thumb, compressors with a higher horsepower (hp) rating usually deliver higher psi. Another rule of thumb is that 1 hp will deliver 4 cfm at 90 psi.

Pressure Per Square Inch (psi). Air pressure is measured in pounds per square inch. It is critical, because the more viscous or thicker a paint is, the more pressure is required for airbrushing. Thin or low viscosity paints, can be sprayed at low pressures, such as 30 psi. Other paints, such as some water based paints, require pressures as high as 50 to 60 psi.

Cubic Feet Per Minute (CFM). The amount of air pressure that an air compressor produces is just one way to compare or rate a system. But it’s not the only way and certainly not the best.

Here’s one reason. When spray painting or sandblasting, air pressure is not enough, if your air compressor cannot produce the required volume of air required. A more useful standard for gauging the performance of a compressor is cubic feet per minute or CFM. CFM is a measurement of how much air, in cubic feet, that the compressor displaces or discharges per minute.

For the large industrial compressors, 10HP and larger, you can figure that 1HP will produce 4CFM @ 90PSI. That’s a great rule of thumb, but it doesn’t do you much good if you are buying a smaller compressor. In these cases, the rule of thumb is forget the rule of thumb and study the specs.

Noise. Most compressors are noisy. Really noisy. Thirty years ago I built a studio adjacent to a large compressor. Even though the walls were soundproofed, I could still heard the muffled sound of the compressor every time it kicked in. This is especially distracting for some airbrushartists, when they are trying to concentrate on their work.

For those who cherish peace and quiet, especially those that work out of their homes, you have a few options. You can buy a quiet airbrush compressor, such as those sold by Silentaire. Or you can decide not to use a compressor and instead use cylinders of CO2.

“You don’t have to buy the CO2 tank,” says Julian “Mr. J” Braet. “You just lease the tank and pay for the refills.” Tanks come in different sizes, such as 20lb. and 40lb.

Finally, you could locate the compressor in a room some distance from the one where you work. Each of these options is viable. The one that is best for you, depends on your own requirements.

Mr. J has his compressor in another building. That way he isn’t distracted when the compressor kicks in. In running air lines, he recommends running ½” pipe.

Portability.

If the job won’t come to you, and you must go to the job, you will need a portable unit that you can throw in the back of your truck.

Future Air Compressor Needs.

Before you make an investment in equipment, remember to gaze into your crystal ball and envision what the future may bring. If your business grows according to your plans, make sure that the compressor that you buy today, satisfies your needs three years from now or five years from now.

In deciding on the size of compressor that you will need for your shop, you need to consider your requirements today and in the near future. Operating a single spray gun is not a very demanding application, because typical requirements are only 5 to 10CFM. If you need to operate several spray guns, your requirements have multiplied.

In determining the right size of compressor for your shop, make a list of the tools that you will run with compressed air. Next to each tool, write down how much CFM the tool will require. Then tally up the numbers. In determining air consumption for a specific tool, you may need to get the information from the manufacturer, because air consumption can vary from one model to another.

After you determine what you will need, factor in a little extra. You will need at least another 10% additional capacity to compensate for pressure drop and air leaks.

Budget.

When buying a compressor, you need to look beyond the price tags. Resign yourself to the reality that you are going to have to spend at the very least more than $750. For that amount of money, you can buy a 5 to 6 HP compressor, producing about 12CFM at 100psi, with a 60 gallon storage tank. A compressor such as this will provide you with the minimum amount of capacity to run a spray gun.

If your needs exceed this, you will have to take the next step up. A compressor that can deliver 15CFM at 100psi with a storage capacity of 80 gallons will cost at least $1,200 to $1,500. This should be enough compressor for a small shop. Anything less and you are asking for trouble. Either the compressor won’t have capacity you need or it will wear out or burn up within a year or two. In the end, if you buy a cheap, you will get what you pay for and you will probably need to buy another compressor. By that time, maybe you will buy the unit that you should have bought in the first place.

Airbrushing And Spray Painting.

If you are a novice airbrush artist, there’s nothing wrong with a small compressor. You can buy a 1/5 hp or 1/8 hp oil-less unit for about $250.

Don’t get the idea that you’ll can also spray paint cars with a small compressor. Spray guns and air tools require more capacity, which is measured not just in air pressure, but more importantly is rated in terms of how many cubic feet per minute (CFM) a compressor can crank out.

Special Requirements for Sandblasting.

In selecting a compressor for sandblasting equipment, always buy a compressor that exceeds equipment requirements by 25% to 30%. For etching and carving glass or wood, you will typically need a compressor that can deliver 5 to 10 CFM @ 30 psi. Frugality does not pay off in this area. Pricking an underpowered compressor can result in two problems. The first is that you may not reach the blast pressure required. And the second problem is that you overwork the compressor and it prematurely fails.

In selecting the compressor, you will need to consider to key factors: pressure (measured in pound per square foot or psi) and volume (measured in cubic feet per minute or cfm). In buying a compressor, also consider all of the different possible uses for this equipment, such as spray painting, sandblasting and air powered tools.

For a standard pressure blaster, you will need a compressor with an output of at least 5 cfm at 40 psi. Pressure blasters aren’t the only type of sandblast equipment on the market. A much cheaper option is to buy a siphon blaster. If you buy a siphon blaster, you will need a compressor with a higher capacity. What you save in the cost of the blaster, will require a more costly investment in a compressor.

To operate a pressure blaster you will need at least a 5 hp compressor; a siphon compressor requires a 10 hp compressor with an output of 38cfm @ 80psi.

Sandblast specialist, Linda Roederer (also known as “that blasted glass girl”) believes that investing in a pressure pot blaster as well as the big compressor is the way to go. “Don’t skimp on the compressor,” she says. “It’s really exasperating to have to wait on air when you’re working on a larger piece.”

“When I am carving large pieces of glass, I need lots of pressure,” Roederer says. “In many cases, I am using 80 psi to frost a large architectural piece.”

After you decide on a compressor, you will also need a good filtering system, so the air supplied to the blaster is clean and dry. Why is that important? Moisture in your air will cause the dust to clog filters.

“I have a moisture separator at the tank and at the pressure pot to ensure that I don’t end up blasting with wet abrasive,” says Roederer.

Compressor Maintenance.

Do you remember the TV commercial in which an auto mechanic proclaims, “you can pay me now, or pay me later.” Just as regular maintenance is critical for your car, it is equally important for your compressor. Failure to follow the manufacturer’s preventative maintenance schedule can result in equipment failure.

Oil-free compressors, which have sealed bearings, are virtually maintenance free. For most commercial applications, these compressors are underpowered. To operate an airbrush, though, this type of unit provides sufficient air.

Compressors that are oil lubricated require periodic maintenance. Like it or not, you need to study the owner’s manual and religiously follow the manufacturer’s recommendations, before you use the compressor and screw things up.

Just as you need to regularly change the oil in your car, you will have to do the same with an air compressor. Running your compressor when the oil is low can result in the engine overheating. This can damage the motor. Some compressor motors also have filters, which should also be changed according to the manufacturer’s instructions.

Conclusion.

The key to making the right decision in buying an air compressor is to do your homework. Determine the needs of your business, anticipate future needs, collect all of the appropriate technical information and then analyze the specifications. Avoid the temptation to buy a lower price used compressors. Compressors have a life expectancy, which is determined by hours of use and how the unit was maintained. Rather than buying the previous owner’s problems, it’s usually better to invest in new equipment. A new compressor comes with fewer surprises and a warranty in case of problems. Finally, if you have questions when making your purchasing decision, don’t be afraid to ask your friends in the business for advice.