ENGINEERING MANUAL OF AUTOMATIC CONTROL
PSYCHROMETRIC CHART FUNDAMENTALS
46
The air at Points A and B has 4.0 grams of moisture per
kilogram of air. While the moisture content remains the same
after the air is heated to 24°C (Point B), the relative humidity
drops from 52 percent to 21 percent. To raise the relative
humidity to 50 percent at 24°C, find the new point on the chart
(the intersection of the 24°C dry-bulb line and the 50 percent
rh curve or Point C). The moisture content at this point is 9.3
grams of moisture per kilogram of dry air. Calculate the moisture
to be added as follows:
9.3 g/kg – 4.0 g/kg = 5.3 grams of moisture
per kilogram of dry air
Line B-C in Figure 16 represents this humidifying process
on the psychrometric chart.
Fig. 16.
At 24°C and 21 percent relative humidity, the psychro-
metric chart shows that the volume of one kilogram of air is
about 0.847 cubic meters. There are two ways to find the weight
of the air. One way is to use the volume to find the weight.
Assuming 4700 liters (4.7 m
3
) per second of air:
4.7 m
3
/s ÷ 0.847 m
3
/kg = 5.55 kilograms of air per
second
The other way is to use the density to find the weight. The
reciprocal of the volume provides the density as follows:
1 ÷ 0.847 m
3
/kg = 1.18 kilograms per cubic
meter
The weight is then:
4.7 m
3
/kg x 1.18 kg/m
3
= 5.55 kilograms of air per
second
If each kilogram of dry air requires 5.3 grams of moisture,
then the following moisture must be added:
5.55 kg/s x 5.3 g/kg = 29.4 grams of moisture
per second
Thus, a humidifier must provide 105.8 kilograms of water
per hour to raise the space humidity to 50 percent at 24°C.
STEAM JET HUMIDIFIER
The most popular humidifier is the steam-jet type. It consists
of a pipe with nozzles partially surrounded by a steam jacket.
The jacket is filled with steam; then the steam is fed through
nozzles and sprayed into the air stream. The jacket minimizes
condensation when the steam enters the pipe with the nozzles
and ensures dry steam for humidification. The steam is sprayed
into the air at a temperature of 100°C or higher. The enthalpy
includes the heat needed to raise the water temperature from 0
to 100°C, or 419 kJ plus 2256 kJ to change the water into steam.
This is a total of 2675 kJ per kilogram of water at 0 kPa (gage)
as it enters the air stream. (See Properties of Saturated Steam
table in General Engineering Data section). The additional heat
added to the air can be plotted on Chart No. 1 (Figure 17) to
show the complete process. In this example, air enters the
heating coil at 13 °C dry-bulb temperature (Point A) and is
heated to 32 °C dry-bulb temperature (Point B) along a constant
moisture line. It then enters the humidifier where the steam
adds moisture and heats the air to Point C.
Figure 17 also shows use of the protractor nomograph.
Assume the relative humidity of the air entering the
humidifier at Point B is to be raised to 50 percent. A
process line can be constructed using the protractor
nomograph. The total heat of the entering steam in
Kilojoule per kilogram is located on the enthalpy/
humidity ratio scale (∆h / ∆W) of the nomograph. This
value, 2675 kilojoules per kilogram, is connected to the
reference point of the nomograph to establish the slope
of the process line on the psychrometric chart. A parallel
line is drawn on the chart from Point B up to the 50
percent relative humidity line (Point C). The Line B-C
is the process line. The Line X-Y (bottom of the chart) is
simply a perpendicular construction line for drawing the
Line B-C parallel to the line determined on the
nomograph. Note that the dry-bulb temperature increased
from 32 to 33°C.
B
A
C4332
10°C DB
52% RH
SUPPLY FAN
4700 L/s
HEATING COIL
24°C DB
50% RH
24°C DB
21% RH
C
4.0 g/kg
9.3 g/kg
10°C DB
52% RH
24°C DB
21% RH
0.847 m /kg
50% RH
DA
MA
3
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