1) A Pelton wheel is required to develop 6 MW under a head of 300 m. I...
Solution:
Given:
Power to be developed(P) = 6 MW
Head(H) = 300 m
Speed of the wheel(N) = 550 rpm
Jet ratio = 10
Overall efficiency(η) = 85%
Cv = 0.98
Ku = 0.46
i) Diameter of wheel(D):
The power developed by the Pelton wheel is given by the formula:
P = ρQgHη
where, ρ = density of water, Q = quantity of water, g = acceleration due to gravity
We know, Q = (Cv)(A)(N)
where, A = area of the jet
Jet ratio = D/d = 10
where, D = diameter of the wheel, d = diameter of the jet
So, D = 10d
We also know, Ku = (π/4)(d^2)/(π/4)(D^2) = d^2/D^2
So, D = d/√Ku
Substituting these values in the formula for power, we get:
P = (ρCvπ/4)(d^2)(N)(g)(H)(η)/Ku
6 × 10^6 = (ρCvπ/4)(d^2)(550/60)(9.81)(300)(0.85)/0.46√Ku
On solving, we get:
D = 2.7 m
ii) Quantity of water required(Q):
From the equation Q = (Cv)(A)(N), we can find the area of the jet:
A = Q/(CvN)
We know, A = (π/4)(d^2)
Substituting these values, we get:
d = √(4Q/(CvNπ))
We know, D = 10d
Substituting these values in the expression for power, we get:
P = (ρCvπ/4)(d^2)(N)(g)(H)(η)/Ku
On substituting the given values, we get:
6 × 10^6 = (ρ × 0.98 × π/4) (4Q/(CvNπ)) (550/60) (9.81) (300) (0.85) / 0.46 (√Ku)
On solving, we get:
Q = 115.7 m^3/s
iii) Number of jets:
We know, Q = (Cv)(A)(N)
Substituting the given values, we get:
115.7 = 0.98 (π/4) (d^2) (550/60)
On solving, we get:
d = 0.377 m
We know, D = 10d
Substituting the value of D, we get:
D = 3.77 m
Area of the jet, A = (π/4)(d^2) = 0.111 m^2
Area of the water passage = πD^2/4 = 11.24 m^2
Number of jets = Q/(CvA) = 115.7/(0.98 × 0.111) = 1058
Therefore, the diameter of the wheel is 2.7 m, the quantity of
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