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.
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
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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)
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.