Compressor Quality Is All About CFM

Although it may appear to be a simple purchase decision, selecting the correct stationary air compressor for your shop can get complicated fast. The simple solution is to work with a well-known dealer and buy a reputable brand of compressor. A good dealer can help narrow the vast field of compressors available today and assist in its installation. Also, getting a compressor from a well-known manufacturer offers peace of mind as well as longer-lasting, more extensive warranty coverage, and easy access to repair parts.

Still, it pays to understand what you need in the way of compressed air before shopping. Start this process by calculating how much air is needed based on the pressure (in pounds per square inch or psi) and flow (in cubic feet per minute or cfm) needed by your air tools.

Of these two measurements, cfm is the more important standard for selection.

Sizing Up a Compressor

To size a compressor, you must calculate how much cfm your tools need. You can find the cfm requirements for a tool either on the tool itself or in its owner's manual, advises Stephen Lazos of Ingersoll Rand.

This doesn't mean you have to look up the cfm requirements on all of your air tools. Instead, select the tools that consume the most air such as large impact wrenches (a ½-inch wrench consumes 3 cfm while a 1-inch impact sucks up 10 cfm), grinders, or paint sprayers. A paint sprayer is the biggest air hog of all. Some paint pots consume up to 18 cfm.

The other consideration when calculating air needs is whether multiple tools will be used at the same time, Lazos says. If this is the case, then add up the cfm consumed by the most common tools in use at one time. This is the absolute minimum cfm to look for in a compressor.

Next, multiply this number by 1.5 to provide a margin of error in case some of the tools are leaking.

Once you have both of these figures, choose a compressor that can meet the cfm as well as psi requirements at the same time. However, some manufacturers list the flow of their compressors as having displaced instead of delivered or free cfm. Displaced cfm is the airflow produced by a compressor working in a perfect environment at 100% efficiency, explains Richard J. Kinch of truetex.com. No compressor was ever made that runs 100% efficient. Therefore, buy a compressor by its delivered cfm rating.

Horsepower Considerations

So where does a compressor's horsepower figure into these calculations? Horsepower rating is often the most prominent feature promoted on the unit. Some off-brand compressors boast a high horsepower rating, but they actually generate a lower cfm because the pumps they employ are smaller.

While horsepower is related to compressor performance, it is not nearly as important as cfm when comparing compressor performance. The rule of thumb is that 1 hp. should produce 3.5 cfm (for two-stage reciprocating compressors) to 5 cfm (for vane and screw compressors).

Also, be wary of a compressor whose horsepower is listed as peak instead of running. Peak horsepower is the surge of power needed to start the motor and can be 1.5 to 3 times the running power needed.

Tank Size

Like horsepower, the tank size of a compressor has little to do with its cfm performance. Obviously, the bigger the tank, the longer it takes to consume the compressed air it holds. The downside of this is that once the air is depleted, it takes longer for the tank to refill.

Manufacturers typically fix tank size to the cfm production of an air pump, although you can seek larger size tanks. If more air storage capacity is needed at a later date, you can always plumb a storage tank into your system. In fact, some farmers operating in large shops have taken to locating a secondary tank at the opposite end of the building (from the compressor) to hold reserve air. This eliminates the lag time for air to travel from the compressor to distant outlets.

Compressor Types

Until the last decade, stationary compressors in farm shops consisted almost entirely of two-stage reciprocating industrial units. Recently, rotary compressors have begun to appear in farm shops, as well.

Both types are positive-displacement compressors that generate air pressure through a pump that's split into two sides of suction and discharge. These two sides (or cavities) expand and decrease as air is pulled into the suction compartment and released on the discharge side during the compression stroke.

Positive displacement is common among reciprocating and rotary screw air compressors because of its suitability to compressing small amounts of air into high pressures. Compressors can quickly disperse the heat resulting from compression, too.

Reciprocating compressors employ one or two cylinders to pump air. Rotary screw compressors employ two or more interlocking screws that draw air into the system.

Price Difference

The noticeable difference between the two compressor types is the price.

Rotary compressors are much more expensive. For example, a single-phase rotary compressor capable of turning out 18.5 cfm will cost from $4,200 to $4,800.

A comparable reciprocating compressor will run you $1,500 to $1,800.

The other noticeable difference between the two compressors is that rotary units are much quieter in operation.

In terms of performance, two-stage industrial-grade reciprocating compressors are capable of generating airflows as high as 100 cfm. Rotary screw units can reach 1,500 cfm with a single stage and 2,500 cfm or more with two stages.

Finally, rotary screw compressors have a longer life – 20 to 30 years. A reciprocating compressor has a life expectancy of 10 to 20 years (depending on the quality of manufacturing).

What About Stages

Stages were mentioned previously in comparing compressors. When it comes to reciprocating units in farm shops, the two-stage compressor is the rule.

A two-stage compressor is more efficient, typically by 15%, vs. a single-stage compressor.

"You might see 4 to 5 cfm per horsepower at 100 psi, instead of 3 or 4 cfm," Kinch notes.

Multistage compressors are designed to supply multiple pneumatic tools and those that require more than 120 psi. With more than one cylinder, the air is first compressed in the initial cylinder and then it is compressed in each successive cylinder to achieve higher levels of power.