Air Compressor Buying Guide

Air compressors are designed for both home and industrial uses. From inflating items around the home and powering air tools, to inflating tires and auto body work, air compressors can make a job quicker and easier to complete. They come in a range of sizes from small portable air compressors to large permanently installed industrial units.

So how do you choose the right compressor? Here are some items to consider:

• Gas or Electric Powered?
• How are compressors rated?
• What size compressor do I need?
• Piston or rotary screw, single or two-stage?
• What about air tanks?
• What maintenance requirements do I need to be aware of?

• Do I need in-line filters and dryers?

Gas or Electric Powered?

Gasoline Powered: Gasoline powered compressors are ideal for portable applications where you may not have an electric outlet readily available. Because they are gasoline powered, they do have emissions that require they be used outdoors or in a very well ventilated area. They are louder and generate more heat than electric compressors.

How are compressors rated?

Compressors are rated using 4 measurements: horsepower, CFM, air pressure, and duty cycle.

Horsepower is a measure of work done over time, and is represented as HP. Compressors are usually rated from 1.5 to 6.5 HP, with higher HP ratings for industrial use compressors. In general, the higher the horsepower rating, the higher the air pressure the motor can produce and higher workload the motor can handle. However with new technology, manufacturers are able to deliver higher CFM ratings with less HP. So, horsepower alone, does not give you the information you need to choose the right air compressor.

Air pressure is the amount of force exerted over an area, and is typically measured in pounds per square inch (psi). Air pressure rating tells how much pressure is generated and stored in the air tank. Typically, air compressors have an automatic shutoff once the maximum air pressure is reached in the air tank. Air pressure in the tank, though varies with use and is greatly affected by temperature (a rise in temperature increases the air pressure). So, the psi rating alone is not a reliable measure of the compressor’s performance, either.

The CFM rating is a measure of how fast the air comes out of the compressor, measured in cubic feet per minute (CFM). The greater the flow is – the greater the power. The CFM that is delivered, though, changes as the PSI varies. Some air compressor manufacturers will provide 2 CFM ratings – the maximum CFM and the working CFM. The “maximum” or “free” CFM rating gives the maximum CFM delivered under ideal conditions. The “working” or “delivered” CFM rating gives the CFM delivered at a specific psi. This is usually 90 psi, as it is the most common rating for air powered tools.

What size compressor do I need?

If you have the working CFM rating available, simply look at the CFM and PSI requirements of the equipment you’ll be connecting to the compressor. That will give you a minimum CFM requirement. If you plan to run multiple tools at the same time, add the CFM requirements of each to get your CFM requirement. That will be the absolute minimum air compressor CFM rating you look for. A good rule of thumb is to look for 1.5 times your minimum CFM requirements. This will provide a margin of error for line loss and small leaks.

Next, take the highest PSI requirement and bump that up as well. For example, if your largest tool requires 90 psi, you may want to consider a 100 psi compressor to ensure you always have enough air pressure to run your equipment. Don’t go overboard, however, as running air tools at pressures significantly higher than required may shorten their life.

Rotary Screw or Piston, Single or Two Stage?

There are two types of compressors – rotary screw and piston driven compressors. Both are available in two-stage models that compress air a second time for increased pressure and air flow.

Rotary screw compressors are good for 100% duty cycle operations (constant “on” operations, like those necessary for paint sprayers) and are quieter than piston driven compressors, but are more expensive to service and maintain. They use twin screws – like drill bits – side by side to force air to higher pressures.

Piston type compressors have one piston in single stage compressors, and two pistons in two stage compressors. The piston compresses the air and delivers it to a storage tank until a specified pressure (psi) is reached. The motor then shuts off. As air is depleted from the tank, the motor kicks back on to refill it.

Single stage compressors are normally found on light duty compressors with a maximum rating below 150 psi, and are adequate for most home users.

What about air tanks?

What maintenance requirements do I need to be aware of?

Regular maintenance of your air compressor will prevent premature equipment failure. Things to be aware of are fluid level requirements, the effects of heat, and eliminating moisture and contaminants Oil-Free Compressors have sealed bearings, and require less maintenance than oil-lubricated compressors. There is no need for periodic oil changes. These are typically found in light to general duty compressors.

Oil Lubricated compressors require the user to change the oil at manufacturer specified intervals. Most industrial-use compressors fall into this category. Some compressors are equipped with low-oil shutdown, which will prevent equipment damage caused by running with low oil levels. Note: It is imperative that oil lubricated compressors sit level when running to ensure proper lubrication of all cylinders and prevent scoring of the cylinders.

Heat is generated by the compressor motor, but is it also generated as a by-product of compressing the air. For larger stationary compressors, take this into account when deciding where to locate the compressor in relation to the work space. Proper venting of the heat can increase the compressor life.

Do I need additional in-line filters and dryers?

The use of in-line filters and air dryers can affect just two aspects of your business – the life of your compressor and the tools run by the compressor, and the end result of the jobs you perform. Sound significant? It is.

Compressed air contains concentrations of oil, water vapor, dirt and other contaminants that can damage air lines and equipment connected to the air compressor. Why? Air compressors suck in air from the atmosphere and compress it. Humidity and other contaminants are sucked in right along with the air, compressed and heated (as a natural by-product of the compression process) and sent into the storage tank. As the air, moisture and contaminants circulate through the compressor, they can cause damage to the compressor components. This creates even more particulates and contamination, and the end result can be a thick oily sludge, along with vapor containing water and compressor lubricants.

This moisture and the contaminants, when delivered along with the compressed air to your pneumatic equipment, can plug orifices, corrode parts, and cause equipment failures. When the compressed air is used in the finished product, as with spay guns, the contaminants can even degrade the quality of the finished product. Many manufacturers provide specifications on the air quality (temperature, humidity, and particulate counts) used to run their equipment.