Handbook of air conditioning and refrigeration / Shan K

50
60
where k a � thermal conductivity of air, Btu/h�ft�°F (W/m 2 �°C)
D h � hydraulic diameter of cooled air passage, ft (m)
In Eq. (27.3) the hydraulic diameter D h is given as
where Aca � area of cooled air passage, ft2 (m2 Dh �
)
Pca � perimeter of cooled air passage, ft (m)
4Aca Pca sp
50 60 70 80
FIGURE 27.3 (Continued)
s
70
rp2
ex
rp1
(d)
80
r
op
(27.4)
On the wet air side, water is sprayed onto the wet airstream; however, because of the heat transfer
from the cooled air through the plastic plate to the wet air, the saturation process is no longer
adiabatic. Return air from the conditioned space (which becomes the wet airstream) is humidified
from point r to ex, as shown in Fig. 27.3d. According to Wu and Yellot (1987), the relative humidity
of the air exhausted from the indirect evaporative cooler is about 95 percent, and the change in the
dry-bulb temperature is rather small. Consequently, there is an increase in wet-bulb temperature and
air enthalpy due to the increase in latent heat.
In the plate heat exchanger, the cooled air and the wet air are in a crossflow arrangement. The
temperature of the saturated air film on the wet air side depends on the wet-bulb temperature of the
local wet airstream, and the wet-bulb temperature of the wet air gradually increases during the humidifying
process. The increase in the wet-bulb temperature of the wet air can be determined from
its enthalpy increase �h wet, in Btu/lb (J/kg), and can be calculated as
90%
80%
�h wet � h ex � h r � V˙ ca� cac pa(T o � T s)
o
AC SYSTEMS: EVAPORATIVE COOLING 27.9
V˙ wet� wet
60%
50%
40%
0.020
0.016
0.012
0.08
0.04
90 100
(27.5)