Interstitial Velocity Calculator
What is Interstitial Velocity?
Interstitial velocity is the speed at which air moves between the filter bags inside the baghouse. This is different from the main airflow—it refers to how air travels in the small spaces between filters.
How to Use This Calculator
You can then use the buttons below the calculator to email yourself a PDF summary of the results or request a free system sizing consultation from our engineering team.
⦿ Enter the ACFM (Actual Cubic Feet per Minute): Input the total volume of air flowing through your baghouse.
⦿ Enter the Baghouse Side Length and Width (inches): Input the internal dimensions of the baghouse housing.
⦿ Enter the Bag Diameter (inches): Provide the precise diameter of a single filter bag.
⦿ Enter the Number of Bags: Input the total quantity of filter bags inside the collector.
⦿ Calculate: Click the calculate button to evaluate the speed of the air moving upward between the filters. You can then use the buttons below the calculator to email yourself a PDF summary of the results or request a free system sizing consultation from our engineering team.
Understanding the Formula
Interstitial Velocity = ACFM ÷ ((Length × Width − π × (Bag Dia ÷ 2)² × # of Bags) ÷ 144)
Interstitial velocity is the speed at which air moves vertically between the filter bags inside the baghouse (unlike can velocity, which focuses on the area below the bags).
If the interstitial velocity is too high, dust that pulses off during the cleaning cycle will not be able to overcome the upward draft to fall into the hopper. Instead, the dust remains suspended and is immediately drawn back onto the bags. This phenomenon causes severe issues, including constant high pressure drops, excessive compressed air consumption, and severely shortened filter life.
This formula calculates the gross cross-sectional area of the baghouse housing, subtracts the physical area taken up by the bags themselves to find the true “open area”, converts that figure to square feet (by dividing by 144), and divides the ACFM by that open area to give the velocity in feet per minute (FPM).