ENERGY, WORK, AND HEAT
Thermodynamics
Example:
Determine the specific P-V energy of 15 lbm of steam at 1000 psi in an 18 ft3 tank.
Solution:
Using Equation 1-14
Pn
PV
m
Pn
(1000 lbf/in.2) (144 in.2/ft2) (18 ft3)
15 lbm
Pn
172,800 ft lbf/lbm
Specific Enthalpy
Specific enthalpy (h) is defined as h = u + Pn, where u is the specific internal energy (Btu/lbm)
of the system being studied, P is the pressure of the system (lbf/ft2), and n is the specific volume
(ft3/lbm) of the system. Enthalpy is usually used in connection with an "open" system problem
in thermodynamics. Enthalpy is a property of a substance, like pressure, temperature, and
volume, but it cannot be measured directly. Normally, the enthalpy of a substance is given with
respect to some reference value. For example, the specific enthalpy of water or steam is given
using the reference that the specific enthalpy of water is zero at .01°C and normal atmospheric
pressure. The fact that the absolute value of specific enthalpy is unknown is not a problem,
however, because it is the change in specific enthalpy (Dh) and not the absolute value that is
important in practical problems. Steam tables include values of enthalpy as part of the
information tabulated.
Work
Kinetic energy, potential energy, internal energy, and P-V energy are forms of energy that are
properties of a system. Work is a form of energy, but it is energy in transit. Work is not a
property of a system. Work is a process done by or on a system, but a system contains no work.
This distinction between the forms of energy that are properties of a system and the forms of
energy that are transferred to and from a system is important to the understanding of energy
transfer systems.
HT-01
Page 18
Rev. 0
