Mechanical Engineering - Hydraulic Machines - Discussion
Discussion Forum : Hydraulic Machines - Section 1 (Q.No. 1)
1.
Power required to drive a centrifugal pump is directly proportional to __________ of its impeller.
Discussion:
34 comments Page 2 of 4.
RAM said:
9 years ago
Power = density * acceleration due to gravity * discharge * head loss.
Where Q = A * V = area * velocity.
Area= (pi/4) *d^2.
Velocity= (pi * d * N/60).
Head loss= (friction * length * velocity^2)/(2 * g * d).
= (f * length * (pi * d * N/60) ^2)/(2 * g * d).
Assume f, length and N are constant.
Head loss proportional to diameter.
Power = constant * d^4.
Where Q = A * V = area * velocity.
Area= (pi/4) *d^2.
Velocity= (pi * d * N/60).
Head loss= (friction * length * velocity^2)/(2 * g * d).
= (f * length * (pi * d * N/60) ^2)/(2 * g * d).
Assume f, length and N are constant.
Head loss proportional to diameter.
Power = constant * d^4.
Anurag Adhikary said:
9 years ago
There are so much of answers if we look into this from different laws and formulas. What answer should we choose in the exams?
Is someone there to give the most appropriate answer?
Is someone there to give the most appropriate answer?
Anonymous said:
9 years ago
I think C is the right answer.
Prashant said:
9 years ago
The relationship can be explained by the Affinity laws as described below:
Law 1. With impeller diameter (D) held constant:
Law 1a. Flow is proportional to shaft speed:
(Q1/Q2) = (N1/N2).
Law 1b. Pressure or Head is proportional to the square of shaft speed:
(H1/H2) = (N1/N2)^2.
Law 1c. Power is proportional to the cube of shaft speed:
(P1/P2) = (N1/N2)^3.
Law 2. With shaft speed (N) held constant:
Law 2a. Flow is proportional to impeller diameter:
(Q1/Q2) = (D1/D2).
Law 2b. Pressure or Head is proportional to square of impeller diameter:
(H1/H2) = (D1/D2)^2.
Law 2c. Power is proportional to the cube of impeller diameter:
(P1/P2) = (D1/D2)^3.
where
Q is the volumetric flow rate (e.g. CFM, GPM or L/s),
D is the impeller diameter (e.g. in or mm),
N is the shaft rotational speed (e.g. rpm),
H is the pressure or head developed by the fan/pump (e.g. psi or Pascal), and
P is the shaft power (e.g. W).
Law 1. With impeller diameter (D) held constant:
Law 1a. Flow is proportional to shaft speed:
(Q1/Q2) = (N1/N2).
Law 1b. Pressure or Head is proportional to the square of shaft speed:
(H1/H2) = (N1/N2)^2.
Law 1c. Power is proportional to the cube of shaft speed:
(P1/P2) = (N1/N2)^3.
Law 2. With shaft speed (N) held constant:
Law 2a. Flow is proportional to impeller diameter:
(Q1/Q2) = (D1/D2).
Law 2b. Pressure or Head is proportional to square of impeller diameter:
(H1/H2) = (D1/D2)^2.
Law 2c. Power is proportional to the cube of impeller diameter:
(P1/P2) = (D1/D2)^3.
where
Q is the volumetric flow rate (e.g. CFM, GPM or L/s),
D is the impeller diameter (e.g. in or mm),
N is the shaft rotational speed (e.g. rpm),
H is the pressure or head developed by the fan/pump (e.g. psi or Pascal), and
P is the shaft power (e.g. W).
Krishan kaushik said:
9 years ago
According to affinity law, discharge is proportional to rpm, head is proportional to the square of the head, &power is proportional to the cube of rpm. On the other hand, discharge is proportional to shaft diameter, head is proportional to the square of diameter & power is proportional to the cube of shaft diameter.
Subrata said:
9 years ago
Not right.
The correct answer Will be d^5.
The correct answer Will be d^5.
Mahi ss said:
9 years ago
The right answer is P proportional to D^5.
(1)
Ravi said:
9 years ago
It is 1/4 of the diameter.
Sreejith said:
9 years ago
Power is proportional to fifth power of diameter.
Dipanjan said:
8 years ago
I agree @Sreeejith.
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