Thermodynamics
PROPERTY DIAGRAMS AND STEAM TABLES
Pressure-Specific Volume (P-n) Diagram
A P-n diagram is another common
Figure 10 P-v Diagram for Water
type of property diagram. Figure
10 is the P-n diagram for pure
water.
A P-n diagram can be
constructed for any pure substance.
A P-n diagram is different from a
P-T diagram in one particularly
important way. There are regions
on a P-n diagram in which two
phases exist together.
In the
liquid-vapor region in Figure 10,
water and steam exist together.
For example, at point A, water
with a specific volume (nf), given
by point B, exists together with
steam with a specific volume (ng),
given by point C. The dotted lines
on Figure 10 are lines of constant
temperature. The quality of the
mixture at any point in the liquid-vapor region can be found because the specific volumes of
water, steam, and the mixture are all known. The quality can be found using the following
relationship.
n = xng + (1 - x)nf
x
n
n f
n g
n f
n
n f
nfg
where:
n
=
specific volume of the mixture (ft3/lbm)
x
=
quality of the mixture (no units)
ng
=
specific volume of the vapor (ft3/lbm)
nf
=
specific volume of the liquid (ft3/lbm)
nfg
=
specific volume change of vaporization (ft3/lbm) or vfg = vg - vf
Rev. 0
Page 43
HT-01
