Calculating CFM with pressure means using measured pressure values together with fan performance data or duct equations to work out the airflow. This approach is common in industrial and mining ventilation, where direct velocity measurements are not always practical and where fan curves are available from the manufacturer.
The starting point is usually a set of pressure measurements. Using static pressure taps and a manometer or differential pressure transmitter, you measure static or total pressure at specific locations in the system—often at the fan inlet and outlet or across a duct section, filter or orifice plate. These pressures are typically expressed in Pascals (Pa) or inches of water gauge (in. w.g.).
One method to calculate CFM is to use an airflow measuring device with a known pressure–flow relationship, such as an orifice plate, Pitot tube grid or calibrated flow nozzle. For example, across an orifice plate, the volume flow is proportional to the square root of the differential pressure:
Q = K × √Δp
where Q is airflow, Δp is the measured pressure difference and K is a constant that depends on the orifice size, duct dimensions and air density. By measuring pressure and applying the correct constant, you obtain Q, which can then be converted into CFM.
Another method is to use the fan curve supplied by the manufacturer. Fan curves show the relationship between airflow (CFM or m3/h) and fan pressure (static or total) at a given speed. If you measure the fan’s static or total pressure rise in the installed system, you can locate that pressure on the vertical axis of the curve and read off the corresponding airflow on the horizontal axis. This gives you an approximate CFM value at the current operating point.
In duct and mine airway calculations, you can also combine pressure and friction equations. If you know the total pressure drop along a duct or roadway and have estimates for friction factors and loss coefficients, you can rearrange the equations to solve for velocity, and then use Q = V × A to obtain airflow. Although more involved, this method can be useful when fan curves are not available or when you are assessing a proposed system before installation.
It is important to ensure that units and air density are consistent when calculating CFM from pressure. High temperature, elevated altitude or gas composition changes can affect density and therefore the relationship between pressure and flow. Corrections may be needed for deep mines or hot process gas systems.
In summary, to calculate CFM with pressure you either use a device with a known pressure–flow formula, apply pressure readings to a fan curve, or solve duct and airway equations for velocity and then volume flow. In all cases, accurate pressure measurement and correct use of units are essential for reliable results.
