POWER REQUIREMENT OF PUMP
The work done during a process depends on the path followed as well as on the properties at the
end states. Reversible work interactions lead to the maximum work output for work-producing
devices and the minimum work input for work-consuming devices. This theoretical energy in
converted to actual energy by means of thermodynamic efficiency that is determined by test. This
efficiency covers the error in the isentropic assumption and true mechanical losses. For reversible
steady-state flow, energy balance equation is reduced to:
𝑊
𝑟𝑒𝑣
= ∫ 𝜈𝑑𝑃
2
1
+ ∆KE + ∆PE
When the changes in kinetic and potential energies are negligible, this equation reduces to:
𝑊
𝑟𝑒𝑣
= ∫ 𝜈𝑑𝑃
2
1
Obviously, one needs to know ν as a function of P for the given process to perform the integration.
When the working fluid is incompressible, the specific volume ν remains constant during the
process and can be taken out of the integration. Then equation simplifies to:
𝑊
𝑟𝑒𝑣
=𝑉(𝑃
2
− 𝑃
1
)
It is obvious from this equation that the reversible steady-flow work is closely associated with the
specific volume of the fluid flowing through the device. The larger the specific volume, the larger
the reversible work produced or consumed by the steady-flow device. Therefore, every effort
should be made to keep the specific volume of a fluid as small as possible during a pumping process
to minimize the work.
The actual(shaft) work is defined by:
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