Experimental and Numerical Analysis of a PCM-Supported ...

[58] evaluated the cooling tower air-side effectiveness ζ evap in terms of a saturation
specific heat capacity C s :
1
exp NTU 1
C
r,
evap
evap
(4.70)
1
Cr,
evap
exp
NTU1
Cr,
evap
where C r,evap is the gas to water heat capacity flow ratios;
C m g C
min
s,
evap
Cr, evap
(4.71)
Cmax
mhwc
w
Braun [58] linearized the air saturation enthalpy over the temperature, as depicted in
figure (4.8) below, for analytical determination of the exit conditions from the cooling
tower. Then, he estimated an average value of saturation specific heat C s as the
average slope between the inlet and outlet water states;
C
h
h
s,
w2
s,
w1
s,
evap
(4.72)
T
w2
Tw
1
where h s,w1 is the enthalpy of saturated air at exit water conditions.
Figure (4.9) [Singham (56)] illustrates the effect of water film temperature on the
enthalpy difference between the bulk air and saturated air at the interface along the
axial direction of a counter flow cooling tower (h s -h g ). The air operating line begins at
a point having an enthalpy (h g,in ) corresponding to the inlet wet bulb temperature
T WB . The vertical distance between the two operating lines represents graphically the
enthalpy difference (h s -h g ) (as in the integrand of equation (D.4) in Appendix D).
According to the air-side effectiveness, ζ a definition, the maximum possible air-side
heat transfer would occur when the vertical distance (h s -h g ) at the air exit approaches
zero.
4.5.3 Dehumidifier efficiency
Since the humidifier and condenser are very similar, it is not surprising that the
condenser effectiveness ζ cond can be defined in a fashion similar to its counterpart
evaporative cooler. When the condenser is regarded as a parallel heat exchanger,
its effectiveness can be expressed as:
93