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

47.5 kJ/kg
A
28.5 kJ/kg
MA
C
12 C DB
9.5 C 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 47.5 kilojoules per
kilogram 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 –10.5 kilojoules per kilogram
of dry air.
c. Using the determined values, calculate the mixed
air enthalpy:
(2/3 x 47.5) + (1/3 x –10.5) = 28.2 kilojoules per

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 13 C dry bulb to 30 C 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 3.8 grams of moisture per kilogram
of air. Determine the total heat added as follows:
HEATING COIL
30 C DB
13 C DB
12% RH
40% RH
Fig. 10. Fan System with Heating Coil.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
RA
24 C DB
17 C WB
9.4 g/kg
6.5 g/kg
0.7 g/kg
C4324
kilogram of dry air
SUPPLY FAN
AIR FLOW
C4326
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 9.4 grams of moisture per kilogram 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 0.7 grams of moisture per
kilogram of dry air. Also, project this value on to
Chart No. 1 as shown in Figurre 9.
c. Using the determined values, calculate the mixed
air moisture content:
(2/3 x 9.4) + (1/3 x 0.7) = 6.5 grams of moisture
5. Using the enthalpy value of 28.2 kJ/kg and the moisture
content value of 6.5g, plot the mixed air conditions, Point
C, on Chart No. 1, Figure 9, by drawing a horizontal line
across the chart at the 6.5g moisture content level and a
diagonal line parallel to the enthalpy lines starting at the
28.2 kilojoules per kilogram of dry air enthalpy point. Point
C yields 12 C dry-bulb and 9.5 C wet-bulb temperature.
6. Read other conditions for the mixed air (MA) from Chart
No. 1 as needed.
40 kJ/kg
22.8 kJ/kg
A
13 C DB
40% RH
43
per kilogram of
dry air
3.8 g/kg
B
30 C DB
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12% RH
Fig. 11.