[et_pb_section bb_built=”1″][et_pb_row][et_pb_column type=”4_4″][et_pb_text _builder_version=”3.2.1″ custom_margin=”-40px|||”]<\/p>\n
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.<\/p>\n
Air Flow in CFM (Q) = Flow Velocity in Feet Per Minute (V) x <\/span>Duct Cross Sectional Area (A)<\/p>\n <\/p>\n 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. The Pitot Tube Assembly includes a Static Pressure Probe and a Total Pressure Probe.<\/p>\n A Total Pressure Probe, aligned into the airflow, senses the duct velocity pressure and the static pressure, which equals the total pressure. A Static Pressure Probe aligned at a right angle to the airflow senses only the static pressure. The difference between the total pressure reading and the static pressure reading is the Velocity Pressure.<\/p>\n 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\u2019s output will be the Velocity Pressure, as shown in the figures below.<\/p>\n [\/et_pb_text][et_pb_image _builder_version=”3.2.1″ src=”https:\/\/www.kele.com\/content\/wp-content\/uploads\/2018\/05\/Screen-Shot-2018-05-29-at-8.54.55-AM.png” show_in_lightbox=”on” use_overlay=”on” overlay_icon_color=”rgba(255,255,255,0.43)” hover_overlay_color=”rgba(0,0,0,0.41)” hover_icon=”%%51%%” \/][et_pb_text _builder_version=”3.2.1″]<\/p>\n The Flow Velocity is then determined with the following equation:<\/strong><\/p>\n \u00a0<\/b> \u00a0V = 4005 x <\/span>\u221a\u0394<\/span>P<\/p>\n \u00a0 \u00a0V = Flow Velocity in feet per minute<\/p>\n \u00a0 \u00a0\u221a <\/span>= Square root of the number to the right<\/p>\n \u00a0 \u00a0\u0394P = The Velocity Pressure measured by the pressure sensor<\/p>\n Example:<\/strong> Measuring a Velocity Pressure of .75\u201d W.C. equals a Flow Velocity of 3,468 Ft\/Min.<\/p>\n \u00a0 \u00a0V = 4005 x <\/span>\u221a<\/b><\/span>0.75<\/p>\n \u00a0 \u00a0\u221a<\/b><\/span>0.75 = 0.866 \u2022 4005 x <\/span>0.866 = 3,468 \u2022 Flow Velocity = 3,468 Ft\/Min<\/p>\n [\/et_pb_text][et_pb_text _builder_version=”3.2.1″]<\/p>\n After 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.<\/p>\n There are two different equations for determining the Duct Cross Sectional Area, one for round ducts and one for square or rectangular ducts.<\/p>\n The equation for square or rectangular ducts is:<\/strong><\/p>\n \u00a0<\/b> \u00a0A = X x <\/span>Y<\/p>\n \u00a0 \u00a0A = Duct Cross Sectional Area<\/p>\n \u00a0 \u00a0X = Duct height in feet<\/p>\n \u00a0 \u00a0Y = Duct width in feet<\/p>\n The equation for a round duct is:<\/strong><\/p>\n \u00a0 \u00a0A = \u03c0 <\/span>x <\/span>r2 <\/span><\/p>\n \u00a0 \u00a0A = Duct Cross Sectional Area<\/p>\n \u00a0 \u00a0 \u03c0 <\/span>= 3.14159<\/p>\n \u00a0 \u00a0r = radius of duct in feet<\/p>\n Example:<\/strong> An 18\u201d diameter round duct has a Duct Cross Sectional Area of 1.77 Ft2<\/span><\/p>\n \u00a0 \u00a0A = \u03c0 <\/span>x <\/span>r2 <\/span>or A = 3.14158 x <\/span>.5625<\/p>\n \u00a0 \u00a018\u201d diameter is 1.5 feet, therefore the radius is .75 feet \u2022 r2 <\/span>= 0.752 <\/span>= 0.5265 \u2022 \u03c0 <\/span>= 3.14159<\/p>\n \u00a0 \u00a0A = 3.14159 x <\/span>0.5625 = 1.77 Ft2 <\/span><\/p>\n [\/et_pb_text][et_pb_text _builder_version=”3.2.1″]<\/p>\n After 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:<\/p>\n<\/h2>\n
Determining Flow Velocity<\/strong><\/h2>\n
Determining Duct Cross Sectional\u00a0Area<\/strong><\/h2>\n
Determining Air Flow in CFM<\/strong><\/h2>\n