Author Topic: Week of 6/12/17 Fluids, Pumps  (Read 1290 times)

FastTrack Guru

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Week of 6/12/17 Fluids, Pumps
« on: June 12, 2017, 07:03:16 AM »
Question: Select the most efficient horsepower pump that can lift water against a head of 25 feet with a volumetric flow rate of 5 ft3/s.                                 

(Assume pump efficiency = .72, and ɣ = 62.4 lb/ft3. Neglect all friction losses)

 
(A) 10 horsepower
 
(B) 19 horsepower
 
(C) 30 horsepower
 
(D) 20 horsepower

Kevinator

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Re: Week of 6/12/17 Fluids, Pumps
« Reply #1 on: June 12, 2017, 07:40:43 AM »
Isnt the density supposed to be in mass per volume?  Where did these units come from? thxs

FastTrack Guru

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Re: Week of 6/12/17 Fluids, Pumps
« Reply #2 on: June 15, 2017, 05:26:08 AM »

Refer to page 106 of the FE Handbook: Pump Power Equation: Top Left on Page.
 
Power (ft lb/s) = Q • ɣ • h • / n
 
Givens:
 
Q = 5 ft3/s
 
h = 25 ft
 
n = .72
ɣ = 62.4 lb/ft3               

(Note: if ɣ was not given in the problem, simply look it up on the table on page 110 Properties of Water) If they gave you the temperature of the water, use it to find the specific weight for that temperature)

Therefore:
 
Power (ft lb/s) = (5 ft3/s • 62.4 lb/ft3 • 25 ft) / .72
 
Power (ft lb/s) = 10,833.33 ft lb/s       
 
However, be careful! The equation solved for the power in ft lb/s, not horsepower.
 
Let’s convert to horsepower:
 
Refer to page 2 of the FE Handbook: Conversion Factors:
 
10,833.33 ft lb/s • (1.818 Χ 10–3)
 
= 19.69 horsepower   (therefore, select the 20 horsepower pump)
 
The answer is (D) 20 horsepower.

FastTrack Guru

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Re: Week of 6/12/17 Fluids, Pumps
« Reply #3 on: June 15, 2017, 05:27:21 AM »
These units are usually correct.. Just make sure that the Q is a volumetric flow rate , cubic feet / sec, and all the units will cancel out. 

Shelly_Brouton

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Re: Week of 6/12/17 Fluids, Pumps
« Reply #4 on: June 15, 2017, 05:34:25 AM »
The ole horsepower conversion !  I haven't seen that since school.   Bringing back bad memories..  I always thought the following conversion was helpful : 1 HP = 550 ft lb / sec (write it on your front cover!)