PSYCHROMETRIC CHART FUNDAMENTALS
40
ENGINEERING MANUAL OF AUTOMATIC CONTROL
THE ABRIDGED PSYCHROMETRIC CHART
Figure 2 is an abridged form of Chart No. 1. Some of the
scale lines have been removed to simplify illustrations of the
psychrometric processes. Smaller charts are used in most of
the subsequent examples. Data in the examples is taken from
full-scale charts
The major lines and scales on the abridged psychrometric
chart identified in bold letters are:
—Dry-bulb temperature lines
—Wet-bulb temperature lines
— Enthalpy or total heat lines
— Relative humidity lines
— Humidity ratio or moisture content lines
—Saturation temperature or dew point scale
—Volume lines in cubic feet per pound of dry air
The chart also contains a protractor nomograph with the
following scales:
— Enthalpy/humidity ratio scale
— Sensible heat/total heat ratio scale
When lines are drawn on the chart indicating changes in
psychrometric conditions, they are called process lines.
With the exception of relative humidity, all lines are straight.
Wet-bulb lines and enthalpy (total heat) lines are not exactly
the same so care must be taken to follow the correct line. The
dry-bulb lines are not necessarily parallel to each other and
incline slightly from the vertical position. The purpose of the
two enthalpy scales (one on the protractor and one on the chart)
is to provide reference points when drawing an enthalpy (total
Fig. 2. Abridged Chart No. 1.
S
E
N
S
I
B
L
E
H
E
A
T
T
O
T
A
L
H
E
A
T
∆H
S
∆H
T
=
1.0
0.8
0.6
0.5
0.4
0.3
0.2
0.1
5000
3000
2000
1500
1000
500
-1000
-0.1
-0.3
-0.5
-1.0
-2.0
-4.0
1.0
2.0
E
N
T
H
A
L
P
Y
H
U
M
I
D
I
T
Y
R
A
T
I
O
=
∆ h
∆ W
4.0
0
∞
15
20
25
30
35
40
45
50
35
35
40
45
50
50
55
55
60
65
70
75
80
85
85
80
75
70
60
55
50
45
40
.030
.026
.022
.020
.016
.012
.006
90
100
110
10
15
20
25
ENTHALPY—
BTU PER POUND OF DRY AIR
60%
40%
14.0
14.5
15.0
30
SATURATIO
N TEM
PERATURE -
°F
DRY BULB - °F
WET BULB -
°F
REALTIVE HUMIDITY - %
VOLUME-CU FT PER POUND OF DRY AIR
HUMIDITY RATIO(W) - POUNDS OF MOISTURE PER POUND OF DRY AIR
M10306
.028
.024
.018
.014
.010
.008
.004
.002
80%
20%
13.0
13.5
45
60
95
105
120
12.5
115
40
35
40
45
50
55
60
65
70
75
80
85
65
35
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