Air Conditioning Processes; Heating Process - Honeywell AUTOMATIC CONTROL Engineering Manual

28.2
BTU/LB
A
19.8
BTU/LB
MA
C
53.3°F DB
49°F WB
FROM CHART 2
Fig. 9. Example B, Chart No. 1
3. Calculate the mixed air enthalpy as follows:
a. For the return air, project a line parallel to the
enthalpy line from Point A to the enthalpy scale
on Figure 9. The value is 28.2 Btu per pound of
dry air.
b. For the outdoor air, project a line parallel to the
enthalpy line from Point B to the enthalpy scale on
Figure 8. The value is 3.1 Btu per pound of dry air.
c. Using the determined values, calculate the mixed
air enthalpy:
(2/3 x 28.2) + (1/3 x 3.1) = 19.8 Btu per
pound of dry air

AIR CONDITIONING PROCESSES

HEATING PROCESS

The heating process adds sensible heat to the system and
follows a constant, horizontal moisture line. When air is heated
by a steam or hot water coil, electric heat, or furnace, no
moisture is added. Figure 10 illustrates a fan system with a
heating coil. Figure 11 illustrates a psychrometric chart for this
system. Air is heated from 55F dry bulb to 85F dry bulb
represented by Line A-B. This is the process line for heating.
The relative humidity drops from 40 percent to 12 percent and
the moisture content remains 0.0035 pounds of moisture per
pound of air. Determine the total heat added as follows:
HEATING COIL
85°F DB
55°F DB
12% RH
40% RH
Fig. 10. Fan System with Heating Coil.
RA
75°F DB
62.5°F WB
0.0094 LB/LB
0.00648 LB/LB
0.00065 LB/LB
C1832
SUPPLY FAN
AIR FLOW
C2056
PSYCHROMETRIC CHART FUNDAMENTALS
4. Calculate the mixed air moisture content as follows:
a. For the return air, project a line from Point A hori-
zontally to the moisture content scale on Figure 9.
The value is 0.0094 pounds of moisture per pound
of dry air.
b. For the outdoor air, project a line from Point B
horizontally to the moisture content scale on
Figure 8. The value is 0.00065 pounds of moisture
per pound of dry air. Also, project this value on to
Chart No. 1 as shown in Figure 9.
c. Using the determined values, calculate the mixed
air moisture content:
(2/3 x 0.0094) + (1/3 x 0.00065) = 0.00648 pounds
of moisture per pound of dry air
5. Using the enthalpy value of 19.8 and the moisture content
value of 0.00648, plot the mixed air conditions, Point C,
on Chart No. 1, Figure 9, by drawing a horizontal line
across the chart at the 0.00648 moisture content level
and a diagonal line parallel to the enthalpy lines starting
at the 19.8 Btu per pound of dry air enthalpy point. Point
C yields 53.3F dry-bulb and 49F wet-bulb temperature.
6. Read other conditions for the mixed air (MA) from Chart
No. 1 as needed.
24.4
BTU/LB
17.1
BTU/LB
A
55°F DB
40% RH
43
ENGINEERING MANUAL OF AUTOMATIC CONTROL
0.0035 LB/LB
B
85°F DB
12% RH C1835
Fig. 11.