What You Need to Know When Buying an Air Compressor
You want air power. Compressed air is a convenience in your home shop or garage, a necessity in a wide variety of businesses and a utility in industrial manufacturing environments.
The most common use of a reciprocating piston air compressor is to power air tools and you’ll find several different models of air compressors to fit a variety of situations. High-demand industrial compressed air is generally delivered through rotary screw air compressors. You’ll find more detailed information about rotary screw air compressors in our blog post about how to choose the right air compressor for your industry.
This air compressor buying guide will answer some of the questions you may have about buying an air compressor, let you know what to look for in features and design elements and provide you with other useful tips and recommendations.
To familiarize yourself with the terms concerning air compressors, turn to this short list of definitions that will be helpful for your next air compressor purchase:
Helpful Air Compressor Definitions
- Pounds per Square Inch (PSI) — the measurement of air pressure and force that is delivered by an air compressor. Higher numbers mean that a larger volume of air can be compressed in the tank.
- Pounds per Square Inch Gauge (PSIG) — the measurement of air pressure and force that is delivered by an air compressor relative to our atmospheric pressure at sea level, which is 14.7 PSI. Most gauges are calibrated to read 0 at sea level so the 14.7 psi of the earth’s atmosphere is not measured.
- Actual Cubic Feet per Minute (ACFM) — the actual volume of air pumped in one minute from a compressor running at its rated operating conditions of speed, pressures and temperatures.
- Standard Cubic Feet per Minute (SCFM) — the representation of the volume of air pumped in one minute at 60 degrees Fahrenheit (15.6 degrees Celsius). Some air compressors will be rated with SCFM to provide a more accurate working rating since air contracts and expands at different temperatures.
- Horsepower (HP) — the measurement of power the motor produces. The higher the number, the more work the air compressor can do to deliver a greater PSI and ACFM.
- Brake Horsepower —Horsepower delivered to the output shaft of a motor or engine, or the horsepower required at the compressor shaft to perform work.
- Indicated Horsepower —The horsepower calculated from compressor indicator diagrams. The term applies only to displacement type compressors.
- Intercooler — part of the air compressor that cools air between compression stages.
- Aftercooler — a heat exchanger used for cooling air discharged from a compressor. Resulting condensation may be removed by a moisture separator following the aftercooler.
- Pneumatic Tools — tools that operate by air pressure.
- Positive displacement compressors —Compressors in which successive volumes of air or gas are confined within a closed space and the space is mechanically reduced, resulting in compression. These may be reciprocating or rotating.
Light-Duty to Heavy-Duty Air Compressor Options
Here are three general categories that air compressors fall into.
Consumer-grade air compressors are generally described as pancake, hotdog or single-stage models. They are good for small household uses like inflating tires, sporting goods and other inflatables. Most have the power to operate small load air tools such as brad guns and staplers, but not much more.
These are a favorite for homeowners, hobbyists, carpenters or those in other professions that have occasional, low-load compressed air needs below 135 PSI and below 7 CFM. They are usually portable models that can be carried or wheeled around.
Professional-grade air compressors are designed for more demanding air tools and for regular intermittent use applications. They perform well for contractors, small auto shops and woodworking shops. You’ll find many models that give you features to use air more efficiently and for longer periods of time with a durable design.
Commercial/Industrial-grade air compressors are designed to provide a steady flow of compressed air for long periods of time. These compressors are built with high-quality components and come with additional features that use advanced technology to improve performance, energy efficiency and reliability.
Determine the Work Your Air Compressor Will Do
It would be difficult to buy the right air compressor for your needs without answering a few questions about its intended use. Here are some basic questions you’ll want to answer before you get into the finer details of buying an air compressor.
Questions to Answer Before Buying an Air Compressor
- Where will you be using your air compressor?
Is portability a priority for you? If you’d like to roll it to different areas in your shop or building or transport it to different locations then you’ll want to focus on wheeled designs. You may have to sacrifice portability for performance, as you’ll find that portable units will deliver less power than most stationary units.
You can use longer air hoses if your compressed air needs to reach longer distances — this will serve you better than having an air compressor that doesn’t deliver the needed CFM. Air compressors that are too small for their use will run more frequently, use more electricity and burn out quickly.
- Is there a dependable electrical supply?
This will help you decide whether you’ll be going with a gas or electric model. Using your air compressor with a generator is not recommended as the fluctuations in power delivered by a generator can cause serious damage to your air compressor. Most manufacturers will also deem this as improper use and will void any warranty on the compressor.
Long extension cords are not a good fit for an air compressor, since longer cords are less efficient at delivering the right amount of power to the unit. Your owner’s manual will provide specifics for using the correct length and gauge of a power cord with your air compressor. Remember that you can always get long air hoses to reach your working area instead of trying to extend electrical supply.
- What tools do you want to use with your compressor?
Determine what tools you will be using with your air compressor at one time. All air tools have an average CFM rating, so look at it to ensure you get an air compressor that will provide you with the power you need.
- What type of air compressor do you need?
A type of air compressor can be described in a number of ways:
- By the air generated — how compressed it is by the PSI rating or by a single-stage or two-stage design
- By the amount of air stored — the capacity of the tank measured in gallons
- By the work load — the rated HP of the motor and the volume of air being generated as measured in CFM
- By technology – rotary screw or piston reciprocating are the types most commonly made
Determining the Specifications for Your Air Compressor
- What is the maximum operating pressure (PSI) you’ll require?
Answering this question will let you know the PSI rating you need your air compressor to offer. This will also help in the decision of a single-stage or two-stage design among piston models. Two-stage models will have a higher PSI than single-stage compressors since a second compression of air will increase the PSI. To find out what PSI you’ll need, check the specifications of the air tools you’ll be using. The CFM rate will be noted at a specific PSI.
For example, if you check all the air tools you’ll be using, you would determine your air compressor requirements by the tool with the highest numbers. If your highest rated air tool was a blow gun with 2.5 CFM at 90 PSI then you would want to get an air compressor with a minimum 90PSI rating.
- What is the maximum air volume (CFM) you’ll require?
Pneumatic tools require different levels of air volume to operate properly. Note that most air tools are rated with an “Average CFM” which is usually based on a 25 percent duty cycle. If you’re using tools that will be used on a continuous basis and they are rated with an average CFM, multiply that number by four to get the continuous CFM. If you will have more than one air line from your air compressor, be sure to get your minimum CFM rating from all tools that could be used at one time, so you don’t underestimate your air power needs.
We recommend to increase the CFM total you get from the highest rated tool (or the total number of highly rated tools that could be used at the same time) by 30 to 50 percent to ensure that you have adequate performance under the most demanding applications you may encounter.
For example, if you had two air lines and your highly rated tools that could run at the same time are an impact wrench with a CFM of 7 and a spray gun with a CFM of 6 then you’d want to look for an air compressor that can deliver in the range of 16-20 CFM
Use a 50 percent markup of the CFM if you foresee an increase of your air supply needs in future years.
- What size tank will you need?
Air compressor tank sizes are measured in gallons and will affect the use of some tools. Typically, the more continuous-use air tools, like sanders and grinders, will demand a bigger tank than tools that are used intermittently, like staplers and nail guns. While there’s not any calculation of tool ratings to determine the most appropriate tank size, our general recommendation is to get the largest, practical size of tank you can afford that is available in the CFM and PSI requirements for your uses.
Bigger is better when you take motor strain and condensation into account. An air compressor uses its motor to fill up the tank with compressed air. When it is doing this more often than it was designed for, the motor will burn out quicker, shortening the life of the compressor. The longer the motor can go without switching on again, the less strain on the motor and the better return you’ll get on your investment.
When air is initially compressed, it is hot and holds moisture. If you have an air tank too small for your needs, the compressed air could be used before it has the opportunity to cool down, which causes condensation to build up in your air lines. This can damage your air tools and compressor. Larger tanks and other features such as intercoolers and aftercoolers allow the air to be cooled before flowing through the air line.
- What horsepower (HP) is needed for the motor of your air compressor?
The HP of a compressor isn’t directly connected to the performance of the air tools or delivery of air, but it is an important factor when deciding on what air compressor to buy. The HP is a rating for the motor or engine’s power. The motor creates the compressed air and delivers it into the tank. The higher the HP, the more efficiently the air tank will be filled, which reduces the recovery time.
Air Compressor Features Explained
When you’ve got all your numbers figured out and have a few good options for an air compressor, you may find that some of the features described are still unclear, making your decision difficult. Below are some of the common air compressor features with a brief explanation.
Belt drive or direct drive: The motor of most rotary screw type air compressors can be either belt driven or direct driven. Direct drive models have the motor directly connected to the crankshaft of the compressor. They have the ability to operate at lower temperatures and high-energy efficiency but are usually more expensive, difficult to maintain and noisier.
Belt-drive compressors have a belt that connects the motor to the air compressor pump with pulleys. They offer greater flexibility to adjust the air flow and pressure as needed. They are also less expensive to purchase, and are easier to install and maintain. These models are not ideal for severe temperatures or harsh conditions.
Auto start/stop or continuous speed or dual control: These features deal with the operation of compressed air production. Auto start/stop means the air compressor is set with a pressure switch that automatically turns the motor on when air is needed and turns it off when the tank is full. Continuous speed is for air compressors that run constantly and will adjust the air flow as the demand requires. Dual control offers the option to set the air compressor to either run in auto start/stop mode or continuous mode as needed.
Cast iron components: Cylinders are fairly standard in air compressor design but you will also notice piston style models with a cast iron crankcase, flywheel, crankshaft or valve seats, all of which add to the durability of the unit.
Vertical or horizontal tanks: Vertical tanks take up less space, which could be a deciding factor for you. The biggest benefit to horizontal tanks is the portability.
ASME parts: These parts or components have met the standards and codes set by the American Society of Mechanical Engineers.
Low oil protection: The air compressor is equipped with technology to shut itself down in the event of low oil levels, preventing downtime and expensive repairs due to low fluid levels.
Ball-valve tank drain: allows safe and easy draining with a simple quarter turn in the tank for frequent removal of any moisture.