The Rating Game

Know what to look for in a new compressor.

Jenny Compressor1

Know what to look for in a new compressor.

By Dan Leiss

Whether looking to purchase a new wheeled, portable air compressor or a compressor to mount in the back of a service vehicle, there are plenty of options to choose from. It may seem, in fact, that there are too many options. What makes the situation more complicated is that a true side-by-side comparison between compressors is difficult to conduct, since manufacturers often rate their specifications differently. There are, however, a few simple factors that can assist in finding a good compressor-even when manufacturers seem to be playing the rating game.

Coming to Terms

With compressors, there are three main specifications that determine performance: cubic foot per minute (cfm), pound per square inch (psi) and horsepower (hp). The cfm measures the volume of air a compressor delivers. The psi indicates the amount of pressure behind the air. The hp, of course, represents the power the engine or motor produces to turn the pump.

It seems simple enough, but, unfortunately, things can get more complicated because there are multiple methods for rating a compressor's cfm and hp. There are other considerations, such as service factor and duty cycle, that can come into play when determining performance. People, therefore, should come to terms with the jargon so they aren't misled into making poor purchasing decisions.

Jenny Compressor1
There are three main specifications that determine compressor performance-cfm, psi and hp.

Speaking Volumes

The first and most commonly misunderstood compressor rating is cfm. In theory, this could be determined by multiplying the cylinder bore by the stroke and revolution per minute (rpm). The formula, however, only calculates displaced cfm. Because of factors such as atmospheric pressure, temperature and friction, compressors produce less air than this number implies.

To more accurately measure performance, other rating systems have been developed. Standard cfm, for example, gives the unit's volume according to certain conditions, such as 14.7 pounds per square inch absolute (psia), 60 degrees ambient temperature and zero percent relative humidity. Actual cfm is another rating system that calculates the true output of the compressor at real-life operating conditions.

Because there is no industry standard for rating cfm, manufacturers are basically free to advertise the number they prefer. Some companies choose to be more realistic by rating their compressors with the actual cfm, while others state the displaced cfm because it's the larger number.

Despite the rating method, the most important point is to not compare apples with oranges. In other words, a unit advertised with a displaced cfm shouldn't be directly compared with another stating the actual cfm. In this case, the best approach is to ask for the actual cfm of the first unit and then make a fair comparison of the compressors' air volumes.

Jenny Compressor2
By becoming familiar with the components and rating systems of a compressor, better purchasing decisions can be made.

The Driving Force

Another specification that often gets confused is hp. For gas-powered compressors, this isn't such an issue. The hp unit ratings are fairly straightforward, making comparisons easier. In general, the ideal solution is to look for an engine with more than enough power needed to turn the pump. Otherwise, if the engine is underpowered for the unit, it will constantly run under high stress and lead to premature failure.

Electric motors, on the other hand, aren't as well understood. A person might believe he or she has a 5.5-hp unit when it really operates at 1.5 hp. This is the difference between peak and continuous hp.

Peak hp is the maximum output a motor can produce. The motor, however, only achieves this level during startup when the start windings are engaged. Once the motor reaches normal rpm, the start windings disengage and the motor runs at its continuous hp rating, which can be five to seven times less than peak hp. If the motor was to operate at its peak hp longer than the initial startup period, it could quickly overheat and would probably experience a short service life.

Similar to rating a compressor with displaced cfm, some manufacturers will advertise the peak hp because it's a larger number. Others will state the continuous hp. Again, be sure to use the same rating system when comparing units.

Jenny Beltdrive
A belt drive helps prevent damage to the compressor.

In addition to hp, the service factor can further infer how a motor will operate under load. This number indicates the percentage of continuous hp at which the motor can safely operate. If the service factor is 1.15, for example, the motor can run without issue at 115 percent of its rated continuous hp. The higher the service factor, the better the motor is at handling various operating conditions.

In addition, take note of the operating rpm. Most electric motors found on compressors will operate around 1,725 or 3,400 rpm. The slower-rpm units generally hold more value, since they operate quieter, generate less heat, and produce less wear and tear than faster motors.

Quality Checks

Even if a unit appears to be adequately rated for the task, there are more items to check. By taking note of other compressor qualities, you can get a better sense of whether the compressor will meet expectations.

First, understand that the compressor's duty cycle will affect its performance on the jobsite. All air-cooled piston compressors generate enough heat that they eventually need to rest to avoid overheating. Some compressors, however, can last longer than others. The best ones on the market have duty cycles up to 80 percent, meaning they can run 80 percent of the time and rest for the other 20 percent. On the other end of the spectrum, lower-quality units may only have duty cycles of 10 to 20 percent.

Another item to consider is the drive system. Is the compressor direct driven or belt driven? Keep in mind that direct drive units might be less expensive, but belt drive compressors generally last longer and are easier to maintain.

Jenny Pump
Duty cycle represents the length of time a pump can safely operate before starting to overheat.

In a direct drive system, the pump will turn at the same rate as the crankshaft, which is faster than necessary. In addition, direct drives are susceptible to more damage because the crankshaft is coupled directly to the pump. If the pump fails, the motor is likely to break down with it, or vice versa.

Belt drive units increase the ratio of how many revolutions the crankshaft makes to how many times the pump turns. In fact, the belt can slow the pump to nearly half the speed of the motor, which greatly reduces wear and tear. If the motor or pump fails, the other component is likely to remain unharmed and the damaged part can easily be replaced.

Next, analyze the overall construction of the compressor. Are the components made from heavy-duty metal, or are there plastic shields and guards? Are the grease fittings located in protected areas so they aren't easily damaged? And, if the compressor is belt driven, is the belt fully enclosed by a heavy-duty guard? These questions can be answered by simply looking at the compressor, but other features aren't as visible. You should ask, for instance, about the construction of the pump's cylinders. Cast-iron cylinders and linings are regarded as the most durable, while aluminum alternatives are subject to much more wear and tear.

Last But Not Least

Finally, the serviceability of a compressor can add great value. Some manufacturers design their products with wear parts that can easily be replaced. As long as routine maintenance is kept up, these units are likely to withstand thousands of hours of use. And, if they break down, the compressors can usually be repaired on site or taken to certified dealers. Other products on the market are better suited for noncommercial use and are practically disposable. Once something goes wrong, there often are no servicing dealers or replacement parts available to remedy the problem.

Jenny Pressure
The psi indicates the amount of pressure behind the air.

No matter what your compressor needs are, you'll find a wide variety of products that claim to have the solution. You can, however, weed out some of the options by understanding the various rating methods and using that knowledge to compare compressors on a level playing field. You can then narrow the selection even more by taking into account additional features that reflect the construction quality. So, even if manufacturers try to play the rating game, you can walk away with a clear winner-a compressor that will run for years, producing plenty of air to meet your needs.

About the author: Dan Leiss is president of Jenny Products Inc. For more information, please visit

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