The actual airflow (CFM) your dust collector delivers in a real-world application will be significantly lower than the number advertised by the manufacturer, often being half of the advertised “free air delivery” once the system is installed.
Manufacturers typically quote a “free air delivery” CFM, which is the volume of air moved when no pipe or ductwork is attached.
Why Actual CFM is Lower
When you install ductwork, fittings, and blast gates, you introduce resistance (measured as Static Pressure) to the system.
Static Pressure (SP): This is the force the fan has to overcome to pull air through all the components (pipes, elbows, flex hose, filter bags, etc.).
CFM Drop: When resistance (Static Pressure) is put on a collector, its ability to move air (CFM delivery) decreases. The higher the SP in your system, the lower the actual CFM the fan can deliver to your tools.
How to Find Your Actual CFM
Instead of relying on the “free air” number, you must calculate the needs of your entire system first. It is important to first find out your system’s required CFM and RESISTANCE (Static Pressure), and then purchase a collector that can meet those requirements.
A dust collector must be able to overcome the piping resistance and pull the required air volume from the machines you need to operate simultaneously. You can use specialized tools and resources (like fan performance curves or system design manuals) to determine the necessary CFM and SP.