Thermodynamics
SECOND LAW OF THERMODYNAMICS
A pump is designed to move the working fluid by doing work on it. In the application of the
first law general energy equation to a simple pump under steady flow conditions, it is found that
the increase in the enthalpy of the working fluid Hout - Hin equals the work done by the pump,
Wp, on the working fluid.
(1-28)
Hout
Hin
Wp
(1-29)
m(hout
hin)
wp
where:
Hout
= enthalpy of the working fluid leaving the pump (Btu)
Hin
= enthalpy of the working fluid entering the pump (Btu)
Wp
= work done by the pump on the working fluid (ft-lbf)
= mass flow rate of the working fluid (lbm/hr)
m
hout
= specific enthalpy of the working fluid leaving the pump (Btu/lbm)
hin
= specific enthalpy of the working fluid entering the pump (Btu/lbm)
= power of pump (Btu/hr)
wp
These relationships apply when the kinetic and potential energy changes and the heat losses of
the working fluid while in the pump are negligible. For most practical applications, these are
valid assumptions. It is also assumed that the working fluid is incompressible. For the ideal
case, it can be shown that the work done by the pump Wp is equal to the change in enthalpy
across the ideal pump.
W
p ideal
= (Hout - Hin)ideal
(1-30)
ideal=
(hout - hin)ideal
(1-31)
wp
m
where:
Wp
= work done by the pump on the working fluid (ft-lbf)
Hout
= enthalpy of the working fluid leaving the pump (Btu)
Hin
= enthalpy of the working fluid entering the pump (Btu)
= power of pump (Btu/hr)
wp
= mass flow rate of the working fluid (lbm/hr)
m
hout
= specific enthalpy of the working fluid leaving the pump (Btu/lbm)
hin
= specific enthalpy of the working fluid entering the pump (Btu/lbm)
The reason for defining an ideal pump is to provide a basis for analyzing the performance of
actual pumps. A pump requires more work because of unavoidable losses due to friction and
fluid turbulence. The work done by a pump Wp is equal to the change in enthalpy across the
actual pump.
Rev. 0
Page 81
HT-01
