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Understanding blowers

   How to select air blower?

   Mistake in air blower selection will not only cause poor performance of blower & whole system but also may result in high initial investment and unnecessary electricity bill.
 In this section we will explain step by step procedure for selection of correct specification of blower for industrial application.
  Readers who have already installed system can also follow the same procedure and find out if they have the correct blower in place. (or if there is any opportunity for improvement)

  1. Decide the location of blower
  2. Measure the available space for blower
  3. Finalize the path of air flow. (how the air generated by blower will be guided).
  4. Sketch the layout. Mark blower, ducts, bends, equipment/ process.
  5. Finalize the exact value of air flow required at space of application of air.
  6. Determine the temperature of air entering the blower and at various stages in duct, equipment/ process.
  7. Find out the pressure drop in line of air flow other than ducting. Such as the pressure drop in the equipments were air is delivered, suction filters etc.
       (If possible calculate/ ask the equipment manufacturer for the value of pressure drop.)
  8. Calculate the pressure drop in the ducting.
  9. Now you have values of Air Flow & Pressure Drop.
  10. Normaly upto 80Deg. C. air temerature entring blower, direct drive can be used. (when using TEFC F Class Induction motor). For temperature above 80Deg. C, belt/ coupling drive is recommended.
  11. When mounting blower on some equipment you can use flange mounted blower. (in case of paint shop many times blower is directly mounted on top of paint shop).
  12. When you want to place blower far from systems using duct then Foot Mounted Blower is recomended.


 Let us Consider an office space located at top floor of building which requires circulation of 10000cubic feet of air per minutre (cfm) for maintaining  comfort  condition.
 So the volume flow rate required by the blower is 10000cfm.
 If the blower is placed inside the office, it will require considerable amount of expensive office space and also the noise of blower will cause discomfort for employees. So it is not possible to put blower inside the office.
 Lets consider that the blower is placed at the roof of the building and the air from the blower is transmitted via GI Ducting into the office.
 10000cfm of air generated by blower will enter into the ducting. However Ducting will offer resistance to the flow of air  which is called pressure drop. Amount of resistance (pressure drop) depends on the cross section area of duct, total length and  bends.

 Lets select the duct cross section of 0.5ft x0.5ft = 0.25sq. ft flow area
 Then velocity of air flow through duct wil be   = 10000cfm/ 0.25sq. ft = 40000ft/min.
 This is very high velocity and will cause tremendous pressure drop. We can say that selecting such a diameter will cause choking of 10000cfm of air. i.e. too little space to pass 10000cfm of air. The above paragraph brings understanding about RESISTANACE TO FLOW/ PRESSUR DROP. So 0.5ft x0.5ft cross section is rejected.

 Lets select the duct cross section of 2ft x2ft = 4sq. ft.
 Flow Velocity thru duct = 10000cfm/ 4sq. ft =2500ft/min
 Which is acceptalbe. With this velocity,  duct length and bends total pressure drop can be calculated. Consider the total pressure drop of 2inches of water coloum = 54mm wc = about 540pascal
 Fig. Shows The layout of Air Ventilation System.
 Following are final air blower specification
 1. Volume Flow Rate = 10000cfm
 2. Total Pressure = 2inches of water coloum
  3. Air Temperature = 20deg. C.

 If you provide the above three values to air blower manufacturers they will provide you further details of power consumption, required motor rating, cost, type and efficiency of air blower.

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