Technical Bulletin 3
Dehumidification and the
Psychrometric Chart
I
R ELATIVE HUMIDITY
NTRODUCTION
The psychrometric chart has been well documented in a wide
variety of technical textbooks and journals. This technical
bulletin will not attempt to cover the chart in detail, but, will
highlight those features of the chart which apply to refrigerant
type dehumidification applications. It will define the terms
which form the nucleus of properly applying a dehumidifier.
Relative humidity is a misapplied term. It is often used in
place of absolute humidity. The key is the word “relative.” To
understand this concept, a law of nature must be Reviewed.
Air is a compressible fluid and its volume is represented by
the following equation:
v = K(T/P)
V = Volume
T = Temperature
P = Pressure
K = Constant
HE CHART
49
Figure 1 shows a typical psychrometric chart. Dry Bulb
temperatures are shown on the chart as vertical lines.
The horizontal lines represent Dew Point temperatures. Lines
representing Wet Bulb temperatures are the straight diagonal
lines sloping downward from left to right. The curve forming
the top edge of the chart is called the “saturation curve.”
Air in a condition that falls on any point along this curve is
totally saturated with moisture. Any additional moisture
added could not be absorbed and would remain in a liquid
state as condensation. The sweeping curved lines that
follow the saturation curve are relative humidity lines
expressed as percentages. These lines represent the
degree of volume displaced by moisture with respect to the
total air volume.
48
80
42
RH
80
%
RH
%
70
RH
id
110
100
90
ulb
%
e
m
120
tB
60
y
ti v
Hu
5
130
0%
Te
mp
80
era
RH
tur
es
70
RH
%
40
60
30
%
RH
50
45
40
13
40
12
20%
RH
35
30
30
25
H
10% R
20
Dew Point Temperatures
10
0
20
25
30
35
40
12.5 CU. FT.
45
50
55
60
13.0 CU. FT.
65
70
DRY BULB °F
75
80
13.5 CU. FT.
85
90
95
100
14.0 CU. FT.
105
GRAINS OF MOISTURE PER POUND OF DRY AIR
75
RH
%
90
70
33
32
31
65
it
140
Dry Bulb Temperatures
TIO
RA
TU
38
SA
37
AT
PY
36
AL
35
TH
EN
39
N
40
(B
41
TU
/L
B)
43
44
45
170
14
12
11
10
9
180
46
47
105
We
60
e
la
Re
100
150
30
29
19
15
n
95
7
8
ti o
rv
90
160
28
27
26
25
24
23
22
21
20
18
17
S
16
Figure 1
at
a
u5r0
Cu55
14.5 CU. FT.
85
34
T
As the air temperature increases, its total volume increases
and decreases on reduction of temperature. Pressure has the
opposite effect. As pressure increases volume decreases.
This is important to understand because water damage
occurs at an absolute humidity concentration regardless of its
relative humidity. This is known as the constant Dew Point
Temperature.
Water, however, is not compressible. Therefore given a
specific amount, it will always occupy the same amount of
volume.
S ENSIBLE AND LATENT HEATING AND COOLING
There are four types of energy changes when heat of moisture
is added or removed. Sensible heat occurs when heat is added
without the addition or reduction of moisture. Sensible cooling
is the reverse. Latent heat, also known as humidification, is the
addition of moisture without changing the dry bulb temperature.
Latent cooling or dehumidification is the removal of moisture.
Figure 3 shows how these are displayed on the chart.
49
Figure 2 illustrates how this applies to the psychrometric
chart. As moisture laden air is heated or cooled the air volume
changes but the moisture does not. Thus there is a change in
relative humidity, without a change in actual water content.
48
95
100
105
46
180
170
(B
N
80
42
40
TIO
39
RA
TU
75
38
140
130
%
35
60
%
90
RH
32
80
%
31
RH
%
70
65
88 GR.
90
25
RH
%
60
24
80
23
%
50
RH
21
%
40
20
18
RH
40
60
50
16
19
17
70
%
55
30
15
92°F
%
RH
50
40
40
9
12
10
13
11
12
14
45
20%
RH
35
30
30
25
H
10% R
20
7
8
78°F
60
70°F
27
26
28
29
30
100
22
Figure 2
110
10
0
20
25
30
35
40
12.5 CU. FT.
45
50
55
60
13.0 CU. FT.
65
70
DRY BULB °F
75
80
13.5 CU. FT.
85
90
95
100
14.0 CU. FT.
105
GRAINS OF MOISTURE PER POUND OF DRY AIR
RH
80
120
%
70
33
34
EN
TH
AL
PY
AT
SA
40% RH
37
60% RH
150
36
80% RH
160
41
TU
/L
B)
43
44
45
70°F
90
47
92°F
78°F
14.5 CU. FT.
85
TECHNICAL BULLETIN 3
Dehumidification and the Psychrometric Chart
Rarely will these occur as shown but will rather be a mixture of
them. A refrigerant dehumidification system is a combination of
sensible and latent cooling and sensible heating. First the
system cools the air to reduce the dry bulb temperature to the
dew point. Then latent cooling reduces the absolute humidity
and finally the air is reheated increasing its dry bulb temperature.
Figure 4 graphs this process.
By obtaining the starting and finishing grains per pound, the
amount of moisture to be removed can be calculated. The
amount of moisture to be removed is the difference between
these two values known as ΔGR.
Figure 5 shows how a dehumidification system was sized.
The ambient design was 91°F dry bulb and 78°F wet bulb. The
desired indoor value was 80°F dry bulb and 50% relative
humidity. The outside ambient has a moisture content of
124 grains and the indoor design has 78 grains. Thus the
required moisture removal rate is 124-78 = 46 grains per
pound of dry air.
D EHUMIDIFIER SIZING
49
To properly apply a dehumidification system, the amount
of moisture to be removed must be calculated. For most
applications the only information available is the dry bulb and
relative humidity or dry bulb and wet bulb temperatures. The
psychrometric chart is used to plot these two values by finding
their intersection and then following the horizontal line to the
right to determine the moisture content in grains per pound.
48
14.5 CU. FT.
90
95
100
105
180
46
47
85
(B
N
80
42
40
39
TIO
TU
75
SA
37
AT
PY
AL
35
TH
130
120
80
%
31
RH
32
110
100
RH
%
70
65
29
RH
%
60
25
27
26
28
90
D
60
24
80
23
%
50
RH
22
70
%
40
60
16
15
30
%
RH
50
40
40
9
12
10
13
11
12
14
45
20%
RH
35
30
30
25
H
10% R
20
7
8
RH
50
19
17
20
18
21
55
10
0
20
25
30
35
40
12.5 CU. FT.
45
50
55
60
13.0 CU. FT.
65
70
DRY BULB °F
75
80
13.5 CU. FT.
85
90
95
100
14.0 CU. FT.
105
GRAINS OF MOISTURE PER POUND OF DRY AIR
RH
A
%
B
90
70
33
34
EN
C
36
Sensible Heating
Sensible Cooling
Humidification
Dehumidification
140
30
A.
B.
C.
D.
150
38
RA
Figure 3
160
41
TU
/L
B)
43
44
45
170
49
Figure 4
48
14.5 CU. FT.
90
95
100
105
180
46
47
85
170
/L
TU
80
41
(B
N
40
IO
39
AT
UR
38
75
SA
T
37
AT
130
AL
36
PY
140
35
TH
150
120
RH
80
%
31
RH
32
110
A
65
70
%
RH
30
100
%
RH
28
90
60
27
26
QS
25
Entering Air
60
24
80
RH
23
%
50
22
70
40
16
15
D
RH
60
C
Leaving
Evaporator
Coil
45
14
12
30
%
Reheated
Air (Supply)
RH
WA
GR
WB
50
40
40
13
11
12
10
9
%
B
50
19
17
20
18
21
55
GRAINS OF MOISTURE PER POUND OF DRY AIR
90
%
70
33
34
EN
QL
160
42
B)
43
44
45
Total Cooling A-B
Sensible Cooling C-B
Latent Cooling A-C
Specific Humidity A (room air)
Specific Humidity C (supply air)
Total Temperature Rise B-D
WA-WB
29
=
=
=
=
=
=
=
QT
QT
QS
QL
WA
WB
ΔT
ΔGR
20%
RH
35
30
8
30
25
10% RH
7
20
10
0
20
25
30
35
40
50
45
55
60
65
75
70
DRY BULB °F
13.0 CU. FT.
80
85
90
95
100
105
14.0 CU. FT.
13.5 CU. FT.
T
49
12.5 CU. FT.
48
14.5 CU. FT.
90
95
100
105
47
85
180
46
Figure 5
80
40
(B
N
39
TIO
37
75
38
RA
TU
SA
AT
140
130
WA
91°F DB/78°F WB
35
36
PY
AL
TH
120
RH
80
%
31
RH
32
110
RH
70
%
65
90
RH
%
60
25
27
26
28
29
30
100
60
24
%
23
50
%
R
50
H
80
80°F DB
22
70
40
%
RH
60
50
16
19
17
20
18
21
55
15
3
0%
RH
WB
50
40
40
13
12
ΔGR
45
14
12
11
10
9
GRAINS OF MOISTURE PER POUND OF DRY AIR
%
90
70
33
34
EN
Constant
Total Air Volume
WA-WB
7000 GR/LB Air
8
=
=
=
=
150
20%
RH
35
30
30
25
10% RH
20
7
4.5
CFM
ΔGR
7000
41
TU
LBS/HR = 4.5 x CFM x GR
7000
160
42
/L
B)
43
44
45
170
10
0
20
25
30
35
40
12.5 CU. FT.
45
50
55
60
13.0 CU. FT.
65
70
DRY BULB °F
75
80
85
90
13.5 CU. FT.
95
100
105
14.0 CU. FT.
8300 West Sleske Court
Milwaukee, WI 53223
(414) 357-7400
FAX: (414) 357-8501
103 09/97