Collection: Fixed Speed Rotary Screw Air Compressors

An unloaded rotary screw compressor still draws 20 to 35 percent of its full load power. Everything about buying a fixed speed machine comes back to that number. It is cheap to own when it spends its time loaded, and expensive when it does not.

Fixed speed is right when demand is steady and sits at 70 percent or more of the compressor's rated CFM. Production lines, a blast cabinet that runs all day, a busy shop on two shifts. The machine stays loaded, you skip the cost of a drive, and there is one less thing on the package to fail in year eight.

Where it falls down is swing. If your load jumps from 30 CFM to 120 CFM and back every few minutes, a fixed speed unit loads and unloads all day, and short cycling is what kills airends, contactors and inlet valves. Two fixes: add storage, or buy variable speed. Plan on 4 to 10 gallons of receiver per CFM, toward the high end if you have a big intermittent draw.

The sizing mistake

People total every tool on the floor at 100 percent duty, add a safety factor on top, and buy twice the compressor they need. Almost nothing in a shop runs continuously. Add up what genuinely runs at the same time, at the pressure you actually hold, then add 15 to 20 percent for leaks and growth. An oversized fixed speed machine costs more up front, costs more to run, and short cycles on top of it. Tell us which tools you are feeding and we will work the numbers with you.