Handbook of air conditioning and refrigeration / Shan K

50
50
Figure 27.9 shows the performance of an indirect-direct two-stage evaporating cooling VAV system
if 1 percent dry-bulb and mean coincident wet-bulb (1 percent DB/MWB) temperatures, 1 percent
wet-bulb and mean coincident dry-bulb (1 percent WB/MDB) temperatures, and 1 percent
dew point and mean coincident dry-bulb (1 percent DP/MDB) temperatures of Denver, Colorado,
are selected.
From ASHRAE Handbook 1997, Fundamentals, in Denver the outdoor design condition based
on 1 percent wet-bulb temperature and the mean coincident dry-bulb temperature T o � has a dry-bulb
temperature of 80°F (26.7°C) and 63°F (17.2°C). In an indirect evaporative cooler, if its effectiveness
is 0.60, the temperature of cooled air discharged from the cooler is
T� ca,l � 80 � 0.6(80 � 63) � 80 � 10.2 � 69.8�F(21.0�C)
AC SYSTEMS: EVAPORATIVE COOLING 27.25
Draw a line from point ca,l� that parallels the thermodynamic wet-bulb temperature line. With the
saturation efficiency of the direct evaporating cooler � sat � 0.9, the cooled air leaves this direct
evaporating cooler with a dry-bulb temperature of 60.3°F (15.7°C) and a relative humidity of 95
percent.
If the space temperature is maintained at 75°F (23.9°C) and if the sensible heat ratio of the
space conditioning line is 0.9, the space relative humidity can be determined from the psychrometric
chart to be 60 percent. The supply temperature differential used to calculate the supply volume
flow rate is �T s � 75 � 60.3 � 14.7°F (8.2°C).
Similarly, the performance of the indirect-direct two-stage evaporating cooling system using 1
percent dry-bulb temperature and the mean coincident wet-bulb temperature, also 1 percent dew
point temperature and the mean coincident dry-bulb temperature can be calculated as follows:
s
s�
60
s�
ca,l�
60
T o �
70
ca,l
ca,l�
70
r�
r�
r
80
80
T o �
90
90
T o
100
100
To , �F
w, lb/lb
FIGURE 27.9 Comparison of 1 percent dry-bulb, 1 percent wet-bulb, and 1 percent dew point temperatures in Denver,
Colorado.
80%
60%
50%
40%
30%
20%
0.016
0.012
0.08
0.04