Efficiency at high temperature
Q - Does pump efficiency increase with increase in liquid temperature?
A - Yes. Two reasons why efficiency increases at higher liquid temperature:
1. The increase in temperature reduces the pump running clearances due to
thermal growth. For example the impeller wear rings will expand more than the
case wear rings and will therefore reduce the wear ring clearance. This will result
in lower internal leakage and, consequently, in higher pump efficiency.
Another example, a single stage pump whose clearance is reduced by 0.005"
from its standard clearance may gain an efficiency improvement of one-half to one
point depending on pump size and type. On multi-stage pumps the efficiency gain
can be twice as much.
2. The liquid viscosity is reduced at higher temperature. The reduced viscosity
results in lower disc and hydraulic skin friction losses. According to Hydraulic
Institute the pump efficiency, at elevated temperature, can be calculated from the
no = 1-(1-nt)[Vo/Vt]^n
no = efficiency at operating temperature, in decimal
nt = efficiency at test temperature, in decimal
Vo = kinematic viscosity at operating temperature
Vt = kinematic viscosity at test temperature
n = exponent established by pump manufacturer, 0.05-0.10
F: efficiency temperature
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