Handbook of air conditioning and refrigeration / Shan K

27.10 CHAPTER TWENTY-SEVEN
Effectiveness
where hex, hr � enthalpy of wet air at points ex and r, Btu/lb (J/kg)
To, Ts � temperature of cooled air at points o and s, °F (°C)
volume flow rate of cooled air and wet air, cfm (m3 /s)
�ca, �wet � cooled air and wet air density, lb/ft3 (kg/m 3 V˙
ca,V˙
wet �
)
cpa � specific heat of moist air, Btu/lb�°F (J/kg�°C)
The average temperature of the saturated film on the wet air side T s,a, in °F (°C), is approximately
equal to the average water temperature in the water sump T w,s, in °F (°C). According to actual
observation, T w,s is about 3°F (1.7°C) higher than the wet-bulb temperature of the return air for this
indirect evaporative cooler. As with Eq. (10.21) in Sec. 10.8, the surface heat-transfer coefficient on
the wet air side h wet can be calculated as
(27.6)
In Eq. (27.6), h dry indicates the sensible heat-transfer coefficient from the wetted surface when it is
dry; it can be calculated as in Eq. (27.3):
where k wet � thermal conductivity of wet air, Btu/h�ft�°F (W/m�°C)
D h,w � hydraulic diameter of wet air passage, ft (m)
L wet � length of wet air passage, ft (m)
(27.7)
As the heat flow in an indirect evaporative cooler is often in a crossflow pattern, the performance of
an indirect evaporative cooler is mainly determined by its effectiveness. Usually, the cooled air has
a smaller heat capacity rate than the wet air. Therefore, the indirect evaporative cooler effectiveness
� in is defined as follows:
(27.8)
where Tca,e, Tca,l � temperature of air to be cooled entering and leaving indirect evaporative cooler,
°F (°C)
T� wet,e � wet-bulb temperature of wet air entering indirect evaporative cooler, °F (°C)
Peterson and Hunn (1992) and Peterson (1993), based on experiments in the performance of indirect
evaporative coolers, recommended that the effectiveness of an indirect evaporative cooler in
either outdoor air or mixing air applications be represented by the following relationships:
and from Eqs. (15.36) and (15.37)
h wet �� m�
c pa � h dry
h dry � 0.023� k wet
D h,w � Re0.8 Pr 0.4
� in � T ca,e � T ca,l
T ca,e � T� wet,e
� in �
C � C min
C max
1
1 � C min / C max
� C ca
C wet
C ca � 60 V˙ ca� cac pa
C wet � 60 V˙ wet� wetc sat
(27.9)
(27.10)
(27.11)