Thermodynamics
FIRST LAW OF THERMODYNAMICS
The forms of energy that may cross the control volume boundary include those associated with
the mass (m) crossing the boundary. Mass in motion has potential (PE), kinetic (KE), and
internal energy (U). In addition, since the flow is normally supplied with some driving power
(a pump for example), there is another form of energy associated with the fluid caused by its
pressure. This form of energy is referred to as flow energy (Pn-work). The thermodynamic
terms thus representing the various forms of energy crossing the control boundary with the mass
are given as m (u + Pn + ke + pe).
In open system analysis, the u and Pn terms occur so frequently that another property, enthalpy,
has been defined as h = u + Pn. This results in the above expression being written as m (h +
ke + pe). In addition to the mass and its energies, externally applied work (W), usually
designated as shaft work, is another form of energy that may cross the system boundary.
In order to complete and satisfy the conservation of energy relationship, energy that is caused
by neither mass nor shaft work is classified as heat energy (Q). Then we can describe the
relationship in equation form as follows.
(1-22)
m(hin
pein
kein)
Q
m(hout
peout
keout)
W
where:
=
mass flow rate of working fluid (lbm/hr)
m
hin
=
specific enthalpy of the working fluid entering the system (Btu/lbm)
hout
=
specific enthalpy of the working fluid leaving the system (Btu/lbm)
pein
=
specific potential energy of working fluid entering the system (ft-lbf/lbm)
peout
=
specific potential energy of working fluid leaving the system (ft-lbf/lbm)
kein
=
specific kinetic energy of working fluid entering the system (ft-lbf/lbm)
keout
=
specific kinetic energy of working fluid leaving the system (ft-lbf/lbm)
=
rate of work done by the system (ft-lbf/hr)
W
=
heat rate into the system (Btu/hr)
Q
Rev. 0
Page 59
HT-01
