Humidifying Process; Cooling Process - Honeywell AUTOMATIC CONTROL Engineering Manual

PSYCHROMETRIC CHART FUNDAMENTALS
1. Draw diagonal lines parallel to the constant enthalpy lines
from Points A and B to the enthalpy scale.
2. Read the enthalpy on the enthalpy scale.
3. Calculate the enthalpy added as follows:
Total heat at Point B – total heat at Point A = total
heat added.
24.4 – 17.1 = 7.3 Btu per pound of dry air
Since there is no change in moisture content, the total heat added
is all sensible. Whenever the process moves along a constant
moisture line, only sensible heat is changed.

COOLING PROCESS

The cooling process removes sensible heat and, often, latent
heat from the air. Consider a condition where only sensible
heat is removed. Figure 12 illustrates a cooling process where
air is cooled from 90F to 70F but no moisture is removed. Line
A-B represents the process line for cooling. The relative
humidity in this example increases from 50 percent (Point A)
to 95 percent (Point B) because air at 70F cannot hold as much
moisture as air at 90F. Consequently, the same amount of
moisture results in a higher percentage relative humidity at 70F
than at 90F. Calculate the total heat removed as follows:

HUMIDIFYING PROCESS

BASIC PROCESS
The humidifying process adds moisture to the air and crosses
constant moisture lines. If the dry bulb remains constant, the
process involves the addition of latent heat only.
Relative humidity is the ratio of the amount of moisture in
the air to the maximum amount of moisture the air can hold at
the same temperature and pressure. If the dry-bulb temperature
increases without adding moisture, the relative humidity
decreases. The psychrometric charts in Figures 13 and 14
illustrate what happens. Referring to Chart No. 2 (Fig. 13),
outdoor air at 0F dry bulb and 75 percent rh (Point A) contains
about 0.0006 pounds of moisture per pound of dry air. The
0.0006 pounds of moisture per pound of dry air is carried over
to Chart No. 1 (Fig. 14) and a horizontal line (constant moisture
line) is drawn.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
COOLING COIL
90°F DB
50% RH
37.9 BTU/LB
33.3 BTU/LB
Total heat at Point A - total heat at Point B =
total heat removed.
37.9 – 33.3 = 4.6 Btu per pound of dry air
This is all sensible heat since there is no change in moisture
content.
A
0°F DB
75% RH
Fig. 13. Chart No. 2.
44
SUPPLY FAN
70°F DB
95% RH
AIRFLOW
A
B
70°F DB 90°F DB
95% RH 50% RH
C1836
Fig. 12.
0.0006 LB/LB
C1837