| 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)
- Decide the location of blower
- Measure the available space for blower
- Finalize the path of air flow. (how the air generated by
blower will be guided).
- Sketch the layout. Mark blower, ducts, bends, equipment/
process.
- Finalize the exact value of air flow required at space of
application of air.
- Determine the temperature of air entering the blower and at
various stages in duct, equipment/ process.
- 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.)
- Calculate the pressure drop in the ducting.
- Now you have values of Air Flow & Pressure Drop.
- 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.
- 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).
- When you want to place blower far from systems using duct
then Foot Mounted Blower is recomended.
Example
AIR VENTILATION BLOWER SYSTEM
FOR OFFICE SPACE
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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