Honeywell AUTOMATIC CONTROL, Engineering Manual

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PSYCHROMETRIC CHART FUNDAMENTALS
45
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Fig. 14. Chart No. 1.
The outdoor air (0F at 75 percent rh) must be heated to a
comfortable indoor air level. If the air is heated to 70F, for
example, draw a vertical line at that dry-bulb temperature. The
intersection of the dry-bulb line and the moisture line determines
the new condition. The moisture content is still 0.0006 pounds
of moisture per pound of dry air, but the relative humidity drops
to about 4.5 percent (Point A, Fig. 14). This indicates a need to
add moisture to the air. Two examples of the humidifying
process follow.
EXAMPLE 1:
Determine the amount of moisture required to raise the
relative humidity from 4.5 percent to 35 percent when the air
temperature is raised from 0 to 70F and then maintained at a
constant 70F.
Figure 15 provides an example of raising the relative humidity
by adding moisture to the air. Assume this example represents
a room that is 30 by 40 feet with an 8-foot ceiling and two air
changes per hour. Determine how much moisture must be added
to raise the relative humidity to 35 percent (Point B).
To raise the relative humidity from 4.5 percent (Point A) to
35 percent (Point B) at 70F, the moisture to be added can be
determined as follows:
1. The moisture content required for 70F air at 35 percent
rh is 0.0056 pounds of moisture per pound of dry air.
2. The moisture content of the heated air at 70F and
4.5 percent rh is 0.0006 pounds of moisture per pound
of dry air.
3. The moisture required is:
0.0056 – 0.0006 = 0.005 pounds of moisture per
pound of dry air
Line A-B, Figure 15, represents this humidifying process on
the psychrometric chart.
C1838
A
70 ° F DB
4.5% RH
0.0006 LB/LB
FROM CHART 2
Fig. 15.
The space contains the following volume:
30 x 40 x 8 = 9600 cubic feet
Two air changes per hour is as follows:
2 x 9600 = 19,200 cubic feet per hour
This amount of air is brought into the room, heated to 70F, and
humidified. Chart No. 2 (Fig. 13) illustrates that outdoor air at
0F has a volume of 11.5 cubic feet per pound. The reciprocal
of this provides the density or 0.087 pounds per cubic foot.
Converting the cubic feet per hour of air to pounds per hour
provides:
19,200 x 0.087 = 1670 pounds of air per hour
For the space in the example, the following moisture must
be added:
1670 x 0.005 = 8.5 pounds of water per hour
Since a gallon of water weighs 8.34 pounds, it takes about one
gallon of water per hour to raise the space humidity to 35 percent
at 70F.
EXAMPLE 2:
Determine the moisture required to provide 75F air at
50 percent rh using 50F air at 52 percent rh.
In this example, assume that 10,000 cubic feet of air per
minute must be humidified. First, plot the supply air Point A,
Figure 16, at 50F and 52 percent rh. Then, establish the condition
after the air is heated to 75F dry bulb. Since the moisture content
has not changed, this is found at the intersection of the
horizontal, constant moisture line (from Point A) and the
vertical 75F dry-bulb temperature line (Point B).
SUPPLY FAN
10,000 CFM
70 ° F DB
35% RH
B
A
0 ° F DB
75% RH
HEATING COIL
70 ° F DB
4.5% RH
4.5% RH
35% RH
0.0056 LB/LB
0.0006 LB/LB
70 ° F DB
FROM CHART 2
C1839
DA
OA