75 feet Ī = 3.14159 x 0.5625 = 1.77 Ft² Determining Air Flow in CFMĪfter obtaining the Flow Velocity and the Duct Cross Sectional Area from the previous two procedures, the Air Flow in CFM is determined by multiplying the two:Īir Flow in CFM (Q) = Flow Velocity in Feet Per Minute (V) x Duct Cross Sectional Area (A)Īn 18” diameter round duct with a Velocity Pressure of. The equation for square or rectangular ducts is:Īn 18” diameter round duct has a Duct Cross Sectional Area of 1.77 Ft²ġ8” diameter is 1.5 feet, therefore the radius is. There are two different equations for determining the Duct Cross Sectional Area, one for round ducts and one for square or rectangular ducts. Flow Velocity = 3,468 Ft/Min Determining Duct Cross Sectional AreaĪfter obtaining the Flow Velocity from the previous procedure, that figure is now multiplied by the Duct Cross Sectional Area to determine the Air Flow in CFM.ΔP = The Velocity Pressure measured by the pressure sensorĮxample: Measuring a Velocity Pressure of. √= Square root of the number to the right. The Flow Velocity is then determined with the following equation: 2: BAPI Zone Pressure Multi-Sensor (ZPM) measuring Velocity Pressure 1: BAPI Pitot Tube Assembly, includes Static and Total Pressure Probe Assemblies (ZPS-ACC12) Fig. If you connect the Total Pressure Probe to the HIGH port on a differential pressure sensor and the Static Pressure Probe to the LOW port on the differential pressure sensor, then the sensor’s output will be the Velocity Pressure, as shown in the figures below. The difference between the total pressure reading and the static pressure reading is the Velocity Pressure. A Static Pressure Probe, aligned at a right angle to the airflow, senses only the static pressure. The Pitot Tube Assembly includes a Static Pressure Probe and a Total Pressure Probe.Ī Total Pressure Probe, aligned into the airflow, senses the duct velocity pressure. The easiest way to determine Flow Velocity is to measure the Velocity Pressure in the duct with a Pitot Tube Assembly connected to a differential pressure sensor. To calculate Air Flow in Cubic Feet per Minute (CFM), determine the Flow Velocity in feet per minute, then multiply this figure by the Duct Cross Sectional Area.Īir Flow in CFM (Q) = Flow Velocity in Feet Per Minute (V) x Duct Cross Sectional Area (A) Determining Flow Velocity
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