Honeywell AUTOMATIC CONTROL, Engineering Manual

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PSYCHROMETRIC CHART FUNDAMENTALS
42
ENGINEERING MANUAL OF AUTOMATIC CONTROL
EXAMPLES OF AIR MIXING PROCESS
The following examples illustrate use of the psychrometric
chart to plot values and determine conditions in a ventilating
system. The examples also show how to obtain the same results
by calculation. Example A requires only Chart No. 1. Example
B requires both Charts No. 1 and 2 since the outdoor air
temperature is in the range of Chart No. 2.
EXAMPLE A:
Plotting values where only Chart No. 1 (Fig. 6) is required.
C1834
C
A RA
75 ° F DB
62.5 ° F WB
MA
62
° F DB
B
OA
36
° F DB
40% RH
Fig. 6. Example A, Chart No. 1.
In this example:
1. A fixed quantity of two-thirds return air and one-third
outdoor air is used.
2. The return air condition is 75F dry bulb and 62.5F wet
bulb.
3. Outdoor air condition is 36F dry bulb and 40 percent rh.
To find the mixed air conditions at design:
1. Plot the return air (RA) condition (Point A) and outdoor
air (OA) condition (Point B).
2. Connect the two points with a straight line.
3. Calculate the mixed air dry-bulb temperature:
(2/3 x 75) + (1/3 x 36) = 62F dry bulb
4. The mixed air conditions are read from the point at which
the line, drawn in Step 2, intersects the 62F dry-bulb line
(Point C).
EXAMPLE B:
Plotting values when both Chart No. 1 and Chart No. 2 are
required.
In this example, a ventilating system (Fig. 7) is used to
illustrate how to plot data on Chart No. 2 and transfer values to
Chart No. 1. Chart No. 2 is similar to Chart No. 1 except that it
covers the –40 to 50F temperature range. This is the temperature
range immediately below that of Chart No. 1. Note that there is
an overlap of temperatures between 35F and 50F. The overlap
is important when transferring values from one chart to another.
RA
DA
OA
N.C.
SUPPLY
FAN
C2055
Fig. 7. Example B, Ventilating System.
This example illustrates mixing two different air conditions
with no change in total heat (enthalpy). Any changes in the
total heat required to satisfy space conditions are made by
heating, cooling, humidification, or dehumidification after the
air is mixed.
In this example:
1. A fixed quantity of two-thirds return air and one-third
outdoor air is used.
2. The return air condition is 75F dry bulb and 62.5F wet
bulb.
3. Outdoor air condition is 10F dry bulb and 50 percent rh.
To find the mixed air condition:
1. Plot the outdoor air (OA) condition on Chart No. 2,
Fig. 8
.
Fig. 8. Example B, Chart No. 2.
2. Plot the return air (RA) condition on Chart No. 1, Fig. 9.
B
C1833
3.1 BTU/LB
0.00065 LB/LB
OA
10
°
F DB
50% RH