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Air Flow Measurement Procedure

Reference standard for these instruction are CTI ATC 143

Air Flow –

is the amount of air movement across a plan. The primary cause of airflow is the existence of pressure gradients which directly related to altitude, temperature, and composition. Airflow is created by the natural means of wind and circulation, or it can be created artificially by the mechanical means of a fan or blower unit.
vane-type-anemometer

Instrument for flow measurement -

There are two basic instruments for airflow testing of cooling towers: Anemometer or Pitot tube. The purpose of each is to measure the velocity at a point. After a number of points are measured to obtain a representative or average velocity. The total airflow is calculated using the net discharge area of the fan.

Anemometer –

is a device for measuring speed of air, for different application various type of anemometer are use but for fan airflow measurement mainly vane type anemometer is used which basically a calibrated propeller. Speed of the rotating vane of the anemometer is converted into electric power, which is converted into the passing air speed.


Where to Measure – Measurement of flow can be exercise at any plan of the flow path but to avoid any miss representation due to leakage it is a better practice to measure flow closest to the fan it can also be measured at succession or delivery but it should cover the entire path of air without any disturbance by measuring team. And also safety of instrument and human should be kept in mind. So in general it is a common practice to measure air flow at delivery side.
For air flow calculation we need net free area at plan of measurement, it is area from where flow passes in this be consider the total area of the casing/fan ring and remove the area of any blockage in the flow path it could be hub, beam or any additional structure available on measuring plan.
Net free area is - Net Area = Casing Area – Hub Area or Aerodynamic hub/Seal Disc whichever is higher

Calculate point of measurement

For air velocity sample reading – it is recommended to have ring (also known as equal area band) in each of four quadrants in the measuring plan. Minimum number of points for a fan would be 20, four points in each of 5 equal area bands. A typical air-flow test requires measuring velocity at the centre of each equal area bands in four quadrants. If fans are more than 20 ft. diameter, 10 equal area bands should be considered. Up to 20 ft. fan we need 5 equal area Band in Four Quadrants i.e. 20 reading. Above 20 ft. fan we need 10 equal area Band in Four Quadrants i.e. 40 reading.

Preparation before Test-

  • Measure fan ring diameter at measuring plan also measure hub/seal disc diameter
  • Calculate net free area.
  • Calculate distance from edge of fan ring or stack to centre of 40 equal area bands. Note there will typically be 40 but could be less on small fans.
  • Measure from centre of anemometer for each of the distances and mark a ring on the anemometer handle with tape or marker.
  • Clamp protractor on anemometer handle so that when anemometer is resting on a horizontal surface, protractor reads "zero" degrees.
  • Prepare data sheet with four quadrants (NE, SE, NW, SW) allowing space for at least 40 readings and an average for each quadrant.

Test Procedure

  • Set up a work location in the first quadrant.
  • Move anemometer to tick mark (closest to ring).
  • Anemometer taking 16 second averages of velocity, record three runs (each position).
  • Repeat for each position each quadrant.
  • The Air Flow report is given below-

Visit Example tab to see an actual example of manual calculation of CTI airflow measurement point

or Visit Support tab for information and user instruction of your online support tool

Air Flow Measurement Example

For example we have fan diameter of 9753 mm and 9814 mm is Fan ring Diameter and 1500mm of Hub Dia-
“DA” Discharge area/Gross Area = 0.7854D² or π /4 D²
0.7854 X (9814) ²
DA = 75645483.7 mm²
“BA” Hub Area/Block Area =0.7854D² or π /4 D²
0.7854 x 1500 ²
BA = 1767150 mm²
• Net Free Area or Discharge Area(NFA) = Gross Area – Hub Area
75645483.7 - 1767150
73878333.7mm²
• Area / Band (AB) = Net Free Area / ( 2 * Number of Bands)
Note – We are taking 10 band if diameter is more than 20 feet.
73878333.7/ (20)
3693916.7 mm²
Area to centre Band 10 = DA – Net area Band
= 75645483.7 – 3693916.7
A10 = 71951867 mm²

Radius to centre of Band R10 = ѴA10/π
= Ѵ71951867 / π
R10 = 4786 mm

Area to centre of Band 09 = A10 - (2* AB)
= 71951867– (2 * 3693916.7)
A9 = 64564033.6 mm²

Radius to centre of Band R9 = ѴA9/ π = Ѵ64564033.6/ π
R9 = 4534 mm

Area to centre of Band A8 = A9 (2* AB) = 64564033.6 - (2* 3693916.7)
A8 = 57176200.2 mm²

Radius to centre of Band R8 = ѴA8/ π = Ѵ57176200.2 / π
R8 = 4267mm

Area to centre of Band 7 = A8 - (2* AB) = 57176200.2 – (2* 3693916.7)
A7 = 49788960.8mm²

Radius to centre of Band R7 = ѴA7/ π
= Ѵ49788960.8/ π
R7 = 3981 mm

Area to centre of Band 6 = A7 - (2* AB)
= 49788960.8– (2* 3693916.7)
A6 = 42401127mm²

Radius to centre of Band R6 = ѴA6/ π = Ѵ42401127/ π
R6 = 3674 mm

Area to centre of Band 5 = A6 - (2* AB) = 42401127.4 – (2* 3693916.7)
A5 = 35013293.6mm²

Radius to centre of Band R5 = ѴA5/ π = Ѵ35013293.6/ π
R5 = 3339 mm

Area to centre of Band 4 = A5 - (2* AB) = 35013293.6 – (2* 3693916.7)
A4 = 27625460.2mm²

Radius to centre of Band R4 = ѴA4/ π = Ѵ27625460.2/ π
R4 =2966 mm

Area to centre of Band 3 = A4 - (2* AB) =27625460.2 – (2* 3693916.7)
A3 = 20237626.8 mm²

Radius to centre of Band R3 = ѴA3/ π = Ѵ20237626.8 / π
R3 = 2538 mm

Area to centre of Band 2 = A3 - (2* AB) = 20237626.8 – (2* 3693916.7)
A2 = 12849793.4mm²

Radius to centre of Band R2= ѴA2/ π =Ѵ12849793.4 / π
R2 =2023 mm

Area to centre of Band 1 = A2 - (2* AB) = 12849793.4– (2* 3693916.7)
A1 =5461960mm²

Radius to centre of Band R1= ѴA1/ π =Ѵ5461960/ π
R1 = 1318 mm

Check = (A1 –AB) should be equal to (Hub area)
5461960 – 73878333.7
1768043.3 mm² (Check)
So following are 10 points of 4157 mm length of net free area-
Band Centre point Radius(mm) Distance From Fan Ring(mm)

  • R10 4786 121
  • R9 4534 373
  • R8 4267 640
  • R7 3981 926
  • R6 3674 1233
  • R5 3339 1568
  • R4 2966 1941
  • R3 2538 2369
  • R2 2023 2884
  • R1 1318 3589

User Instruction to use our tool for calculation of radious as per CTI method

Open CTI Point Calculation Tool

> Now enter the diameter of measuring section (it is not always the diameter of fan if you are measuring on a diffuser then you should find the area of measuring plan on the diffuser)

> Inter the area of hub/ aerodynamic hub/seal disc whichever is large.

Unit of both area should be same and answer of distance/radius will also remain in same unit.