A centrifugal pump delivers 0.3 m³/s against a head of 30 m at 1400 rpm. The external diameter of impeller is 0.5 m and the outlet width is 0.05 m. If the manometric efficiency is 80%, find the vane angle at outlet.
Given data:
Discharge Q=0.3 \mathrm{~m}^3 / \mathrm{s}
Speed of pump N =1400 rpm
Head H_m=30 \mathrm{~m}
External diameter of impeller D_2=0.5 \mathrm{~m}
Width at outlet B_2=0.05 \mathrm{~m}
Manometric efficiency \eta_{\text {mano }}=0.80
Tangential velocity of impeller at outlet is given by
U_2=\frac{\pi D_2 N}{60}=\frac{\pi \times 0.5 \times 1400}{60}=36.65 \mathrm{~m} / \mathrm{s}
The velocity of flow at the impeller outlet is
V_{f 2}=\frac{Q}{\pi D_2 B_2}
=\frac{0.3}{\pi \times 0.5 \times 0.05}=3.82 \mathrm{~m} / \mathrm{s}
Manometric efficiency is found from Eq. (20.10) as
\eta_{m a n o}=\frac{H_{m}}{\frac{V_{w2}U_{2}}{g} }=\frac{g H_{m}}{V_{w2}U_{2}} (20.10)
\eta_{\text {mano }}=\frac{g H_m}{V_{w 2} U_2}
or 0.80=\frac{9.81 \times 30}{V_{w 2} \times 36.65}
or V_{w 2}=10.04 \mathrm{~m} / \mathrm{s}
The outlet velocity triangle is shown in Fig. 20.8.
From the outlet velocity triangle, we have
\tan \beta_2=\frac{V_{f 2}}{U_2-V_{w 2}}
or \beta_2=\tan ^{-1} \frac{3.82}{36.65-10.04}=8.17^{\circ}