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Piston Displacement
Piston displacement is the theoretical volume of air that is swept by a compressor’s pistons during a single minute. This calculation is based on the compressor’s physical design, accounting for its physical size and the speed of its various components, including:
- Cylinder size or bore
- The number of pistons
- Piston travel distance or stroke
- Compressor speed in revolutions per minute (RPM)
Quincy Compressor has been designing and manufacturing air compression technology since 1920. We provide tailored solutions for demanding applications with high quality, premium performance and rugged durability. Understanding key concepts like compressor displacement empowers you to make informed purchases, understand your equipment and improve your operations.
Here are some of the most frequently asked questions about piston displacement:
FAQs
Piston displacement is a theoretical number that only considers the compressor’s characteristics and will always be higher than the actual amount of air the machine delivers. Piston displacement is not the amount of usable air your compressor will produce. The real-world measurement is referred to as Actual Cubic Feet Per Minute (ACFM) or Free Air Delivery (FAD).
Even if piston or engine displacement reflects your machine’s capabilities, there are several factors that determine the final amount of compressed air:
- Heat of compression: Air heats up and expands as it is compressed. This means that less of it can be packed into the cylinder with each stroke.
- Pressure drops: Air loses pressure and density as it moves through the intake filter and valves.
- Valve configuration: The type of seal within the machine determines the amount of air that can leak past the piston rings or valves.
- Mechanical friction: The friction between the moving parts of the mechanism always dissipates a small amount of energy.
Not necessarily. Piston displacement is a measure of potential rather than real-life performance. It describes more of the compressor’s engine size rather than how well it works. Volumetric efficiency is the percentage of the theoretical displacement that the compressor successfully converts into usable, delivered air.
A well-designed, efficient compressor with a lower displacement can often produce more usable air. Features like cooling, advanced valve designs and tighter manufacturing tolerances allow for higher volumetric efficiency.
While displacement offers useful insight about the compressor’s engine capacity, it’s more useful to use FAD measurements to select the right model for your application. They reflect the proven, real-world delivery of how much usable air the compressor provides to power tools and equipment.
FAD is calculated by the manufacturer, using calibrated instruments in a laboratory environment to physically measure the volume of air exiting the compressor at a specific discharge pressure. To size the compressor correctly for your air tools, the FAD rating should be higher than the total Cubic Feet Per Minute (CFM) required by all the tools you plan to use simultaneously.