CASE STUDY
• A V-belt centrifugal fan is supplying air to a chemical process .
The performance test on the fan gave the following parameters.
– Ambient Temperature : 400C
– Density of air at 00C : 1.293 kg/M3
– Diameter of discharge duct : 01 meter
– Velocity pressure measured by pitot tube in discharge duct : 47mmwc
– Pitot tube co-efficient : 0.9
– Static pressure at fan inlet : 22 mmwc
– Static pressure at fan outlet : 188 mmwc
– Power drawn by motor coupled with fan : 68 KW
– Belt transmission efficiency : 95 %
– Motor efficiency at operating load : 90%
• Air Velocity = (Cp x (√2 x g x ∆p xϒ))/ϒ
– Cp : Pitot tube co efficient
– G : Acceleration due to gravity (g=9.81)
– ∆p : Velocity pressure in discharge duct.
– ϒ : Corrected gas density
– ϒ : 273/(273+ 40) *1.293 =1.128 kg/M3
• Air velocity = 0.9 x (√2 x 9.81x 47 x1.128)/1.128
= 25.73 m/sec
• Volume of air = (π/4xd2) x Air velocity
= (π/4x(1) 2) x 25.73
= 20.21M3/sec
• Power input to fan shaft = 68 x 0.9 x 0.95 = 58.14 Kw
• Static fan efficiency :
= (Air flow quantity(M3/sec) x ∆p)/ (102 x Fan shaft power)
= (20.21 x( 188- (-22)) / (102 x58.14)
= (20.21 x210)/(102x58.14)
= 0.7156 x100
= 71.56 %
• Due to modification in chemical process only half of the operating flow will be required in future. This is to be effected by damper control method.
• The fan characteristics curve shows that the total static head developed by the fan will be 333mmwc and static fan efficiency will be 59% by damper control method.
• Find out what will be the annual saving at 8000 hours operation per year and energy cost of Rs.6.5 per unit.
• Assume that the motor efficiency and belt transmission efficiency remains the same
Flow required = 20.21/2 = 10.1M3/sec
∆p =333 mmwc Fan efficiency =59 %
Fan shaft power = ????
• Static fan efficiency :
= (Air flow quantity(M3/sec) x ∆p)/ (102 x Fan shaft power)
0.59 = (10.1 x333) /(102 x Fan shaft power)
• Fan shaft power = ( 10.1 x333 ) /(102 x 0.59)
= 55.9 KW
• Motor power Input = 55.9/(0.95 x0.9) = 65.4 KW
• Saving in Power = (68 –65.4) = 2.6 KW
• Annual Energy saving = 2.6 x 8000 =20800 KWH
• Annual cost saving in Rs.= 20800 x 6.5
= Rs.135200
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