Calculating air flow volume (CFM) in your PLC or building management system based on output from a differential pressure sensor costs just a fraction of what you might spend on expensive air velocity or CFM monitors. This blog post explains how to use the output from a differential pressure sensor and simple math to find the variables in the following equation used to calculate Flow Volume:
CFM = FPM x Duct Cross Sectional Area
Finding the Flow Velocity, typically expressed in feet per minute (FPM), is the first step in filling in our equation variables. To find the Flow Velocity, we use the equation:
FPM = 4005 x √ΔP (The square root of the Velocity Pressure)
The Velocity Pressure value will be provided by either ACI’s DLP or MLP2 differential pressure transmitter paired with a PT Differential Pitot Tube installed in the duct. The PT is an ABS plastic pitot tube that comes in 3”, 5.2”, 7.5”, 9.7” lengths. The insertion depth should cover as much of the width of the duct as possible without touching the opposite side. There are multiple sampling points throughout the span of the PT with the number of sampling points depending on the length of the PT.
The “H” port of the PT Pitot Tube will connect to the HIGH port of the differential pressure transmitter, and the “L” to the LOW. The difference between the total pressure reading, monitored at “H” port of PT, and the static pressure monitored at the “L” port, is the Velocity Pressure. The DLP or MLP2 differential pressure transmitter output provides the Velocity Pressure to be used in our equation.
For example: If a Velocity Pressure of .45” W.C. is measured by our pressure transmitter and entered into our equation, we see that the Flow Velocity is 2,686 Feet Per Minute (FPM).
FPM = 4005 x √.45
FPM = 2,686
Our Flow Velocity solution of 2686 FPM can now be inserted into our equation used to calculate Flow Volume in CFM:
CFM = 2,686 x Duct Cross Sectional Area
Next, we need to determine our Duct Cross Sectional Area.
There are two equations for determining a Duct’s Cross-Sectional Area. One to be used for square or rectangular duct and the other for round duct.
The equation for a square or rectangular duct is:
A (Duct Cross Sectional Area) = X (height in feet) x Y (width in feet)
The equation for a round duct is:
A (Duct Cross Sectional Area) = π x r (radius of the duct in feet)²
If we have a round 14” diameter duct, the radius is half that, or 7” which converts to .585 feet (7”/12”).
Substituting our values into the equation we see that the Duct Cross Sectional Area equals pi, or 3.14159 times our radius, .585 squared giving us a solution of 1.07 square feet.
A = π x .585²
A = 1.07 sq. feet
Now that we’ve calculated our Flow Velocity (2686 FPM) and Duct Cross Sectional Area (1.07 square feet) we can calculate Air Flow in CFM for our 14” diameter duct using our equation.
Air Flow in CFM = Flow Velocity in Feet Per Minute x Duct Cross Sectional Area
CFM = FPM x Duct Cross Sectional Area
CFM = 2,686 x 1.07 sq. feet
CFM = 2,874
Air Flow Volume = 2,874 CFM
The A/DLP Series uses a piezoresistive, silicon sensing element which senses differential pressure and provides an analog output. The hinged cover is easily opened using the locking tab on the side of the enclosure. This allows for easy access to the zero function and field selectable ranges and outputs. The A/DLP Series is available in 0.25% and 0.50% accuracy and has pitot tube, din-rail, and LCD options for installation and operating flexibility.
DLP Datasheet »
The A/MLP2 Series pressure transmitter incorporates a durable piezoresistive, silicon micro-machined sensing element to enable very low-pressure measurements. The pushbutton zero feature provides field adjustability and its small size uses minimal panel space. In addition, the terminal block is removable and the enclosure features an integral DIN-rail mounting bracket.
MLP2 Datasheet »
The 3”, 5.2“, 7.5” and 9.7“ pitot tubes are designed to sense the differential inlet pressure in the inlet section of VAV and Fan Terminal Boxes. Pitot tubes can be used to measure the velocity pressure when mounted facing into the air stream.
Pilot Tubes Datasheet »
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