Chapter hvac engineering Fundamentals: Part 1 Introduction

476
Figur
e
19.1
The
ASHRAE
psychrometric
chart.
(
SO
UR
CE
:
Copyright
2001,
American
Society
of
Heating,
Refrigerating
and
Air
Conditioning
Engineers,
Inc.,
www
.ashrae.org.
Reprinted
by
per
-
mission.)
Engineering Fundamentals: Part 4
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Engineering Fundamentals: Part 4
477
Figure 19.2
A state point on the psychrometric chart.
dual-duct or multizone system. The ASHRAE chart is a Mollier-type
chart. On a Mollier chart, a mixing process may be shown as a straight
line connecting two initial state points (Fig. 19.3, points
A
and
B
). The
mixture state point
C
will be on the line located such that it divides
the line into two segments with lengths proportional to the two initial
air masses. The mixture point will be closer to the initial point with
the larger mass. In the figure, if the volume at point
A
is 7000 ft
3
/ min
and the volume at point
B
is 3000 ft
3
/ min, then line
AC
will be 3 units
long and line
BC
will be 7 units long. The state point values for
C
can
then be read from the chart. They can also be calculated from the
tables, but the graphical solution is much faster unless a high degree
of accuracy is required.