Calculating Air Flow Volume with Differential Pressure Transmitters
Calculating CFM in your control system using a differential pressure transmitter is significantly less costly than using air flow stations or flow meters. This post explains how to use a differential pressure transmitter and simple math to calculate CFM using the below formula:
CFM = FPM x Duct Cross Sectional Area
Step 1
Calculate Flow Velocity in FPM
Finding the Flow Velocity in feet per minute (FPM) is the first step. To find the Flow Velocity, use this 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. The PT samples multiple points across its length - see YouTube video for complete details of PT installation, port designation, and recommended length.
The “H” port of the PT 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 output from the DLP or MLP2 is the Velocity Pressure used in the equation.
For example: If a Velocity Pressure of 0.45" 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 √0.45
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
Step 2
Determine the Area of the Duct
Next, we need to determine the 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 = X x Y
The equation for a round duct is:
A = π x r²
A: Duct Cross Sectional Area
X: Height in feet
Y: Width in feet
r²: Radius of the duct in feet
If we have a round 14" diameter duct, the radius is half that, or 7" which converts to 0.585 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 0.585²
A = 1.07 sq. feet
Step 3
Calculate Air Flow Volume (CFM)
Now that we’ve calculated Flow Velocity (2686 FPM) and Duct Cross Sectional Area (1.07 square feet) we can calculate Air Flow in CFM for our 14" diameter round duct using the equation:
CFM = FPM x Duct Cross Sectional Area
CFM = 2,686 x 1.07 sq. feet
CFM = 2,874
Air Flow Volume = 2,874 CFM
ACI SENSORS NEEDED
DLP
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.
MLP2
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.
PITOT TUBES
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.
