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PSYCHROMETRIC CHART FUNDAMENTALS
46
ENGINEERING MANUAL OF AUTOMATIC CONTROL
The air at Points A and B has 0.004 pounds of moisture per
pound of air. While the moisture content remains the same after
the air is heated to 75F (Point B), the relative humidity drops
from 52 percent to 21 percent. To raise the relative humidity to
50 percent at 75F, find the new point on the chart (the
intersection of the 75F dry-bulb line and the 50 percent rh curve
or Point C). The moisture content at this point is 0.009 pounds
of moisture per pound of dry air. Calculate the moisture to be
added as follows:
0.009 – 0.004 = 0.005 pounds of moisture per
pound of dry air
Line B-C in Figure 16 represents this humidifying process on
the psychrometric chart.
Fig. 16.
At 75F and 21 percent relative humidity, the psychromet-
ric chart shows that the volume of one pound of air is about
13.58 cubic feet. There are two ways to find the weight of the
air. One way is to use the volume to find the weight. Assuming
10,000 cubic feet of air:
10,000
÷
13.58 = 736 pounds of air
The other way is to use the density to find the weight. The
reciprocal of the volume provides the density as follows:
1
÷
13.58 = 0.0736 pounds per cubic foot
The weight is then:
10,000 x 0.0736 = 736 pounds of air per minute
If each pound of dry air requires 0.005 pounds of moisture,
then the following moisture must be added:
736 x 0.005 = 3.68 pounds of moisture per minute
This converts to:
3.68 x 60 minutes = 220.8 pounds per hour
Since one gallon of water weighs 8.34 pounds, the moisture to
be added is as follows:
220.8
÷
8.34 = 26.5 gallons per hour
Thus, a humidifier must provide 26.5 gallons of water per hour
to raise the space humidity to 50 percent at 75F.
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 212F or higher. The enthalpy
includes the heat needed to raise the water temperature from
32 to 212F, or 180 Btu plus 970 Btu to change the water into
steam. This is a total of 1150 Btu per hour per pound of water
at 0 psig 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 55F dry-bulb temperature (Point A)
and is heated to 90F 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 Btu
per pound is located on the enthalpy/humidity ratio scale
(
∆
h /
∆
W) of the nomograph. This value, 1150 Btu per
pound, 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 90 to 92F.
B
A
C1840
50
°
F DB
52% RH
SUPPLY FAN
10,000 CFM
HEATING COIL
75
°
F DB
50% RH
75
°
F DB
21% RH
C
0.004 LB/LB
0.009 LB/LB
50
°
F DB
52% RH
75
°
F DB
21% RH
13.56 CF/LB
50% RH
DA
MA
Summary of Contents for AUTOMATIC CONTROL
Page 4: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL iv ...
Page 6: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL vi ...
Page 11: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS 1 CONTROL SYSTEMS FUNDMENTALS ...
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Page 46: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS 36 ...
Page 66: ...PSYCHROMETRIC CHART FUNDAMENTALS 56 ENGINEERING MANUAL OF AUTOMATIC CONTROL ...
Page 128: ...ENGINEERING MANUAL OF AUTOMATION CONTROL ELECTRIC CONTROL FUNDAMENTALS 118 ...
Page 158: ...MICROPROCESSOR BASED DDC FUNDAMENTALS 148 ENGINEERING MANUAL OF AUTOMATIC CONTROL ...
Page 210: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL BUILDING MANAGEMENT SYSTEM FUNDAMENTALS 200 ...
Page 440: ...ENGINEERING MANULA OF AUTOMATIC CONTROL INDIVIDUAL ROOM CONTROL APPLICATIONS 430 ...
Page 516: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL GENERAL ENGINEERING DATA 506 Notes ...
Page 517: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL GENERAL ENGINEERING DATA 507 Notes ...
Page 518: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL GENERAL ENGINEERING DATA 508 ...