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posted by  TFerguson on 7/24/2008 12:39:00 PM  |  status: Closed  

Pump Efficiency

Course Textbook Chapter Problem
Fluid Mechanics Fluid Mechanics Fundamentals and Applications (Cengel and Cimbala) 5 24
Question Details:


Water is pumped from a lake to a storage tank 20m above at a rate of 70 L/s while consuming 20.4kW of electric power. Disregarding any frictional losses in the pipes and any changes in kinetic energy, determine a) the overall efficiency of the pump-motor unit and b) the pressure difference between the inlet and exit of the pump.

 - I figured I'd be using the equation  ηpump = (ΔEmech, fluid)/(Wshaft, in) do determine efficiency. I just can't seem to find a good explanation on using height difference and volumetric flow rate to determine (ΔEmech, fluid) in my book. Does anyone have an idea how to do this?

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posted by rooya on 7/26/2008 6:37:15 AM  |  status: Live
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Response Details:
 
H=20m
Q=70L/s
Q=70*10^-3 m3/s
(a)
Power input Pin=20.4KW
Power output Po=γ*Q*H
 Po=(9810*70*10^-3*20)/1000 =13.734KW
Pump Effiency η =Po/Pin = 13.734/20.4
 η =0.673
(b) Applyong Bernoulies equation at points 1 & 2
 
 
P1/γ +V21/2g +Z1 = P2/γ +V22/2g +Z2
P1/γ-P2/γ =Z2-Z1
P1-P2 = γ*(20-0)
P1-P2 =196200 N/m2
 
 
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