Thermodynamics
ENERGY, WORK, AND HEAT
Example:
Determine the specific internal energy of 12 lbm of steam if the total internal energy is
23,000 Btu.
Solution:
Using Equation 1-13.
u
U
m
u
23,000 Btu
12 lbm
u
1916.67 Btu/lbm
Specific P-V Energy
In addition to the internal energy (U), another form of energy exists that is important in
understanding energy transfer systems. This form of energy is called P-V energy because it
arises from the pressure (P) and the volume (V) of a fluid. It is numerically equal to PV, the
product of pressure and volume. Because energy is defined as the capacity of a system to
perform work, a system where pressure and volume are permitted to expand performs work on
its surroundings. Therefore, a fluid under pressure has the capacity to perform work. In
engineering applications, the units of P-V energy, also called flow energy, are the units of
pressure times volume (pounds-force per square foot times cubic feet), which equals foot-pounds
force (ft-lbf).
The specific P-V energy of a substance is the P-V energy per unit mass. It equals the total P-V
divided by the total mass m, or the product of the pressure P and the specific volume n, and is
written as Pn.
(1-14)
Pn
PV
m
where:
P
=
pressure (lbf/ft2)
V
=
volume (ft3)
n
=
specific volume (ft3/lbm)
V
m
m
=
mass (lbm)
Rev. 0
Page 17
HT-01
