β
Ar
Af
h = 1 -
hh A + hf A = 3141 W/m K
2
.
π
The enhancement ratio is
h
h
=
= 2.04 .
hp hh
Based on the envelope area over the fins, the average heat transfer coefficient is
h = h = 9501 W/m2 K .
The Beatty-Katz model predicts a lower value of h than the Webb et al. model. The
value of 9501 W/m2 K is about the same as the value as measured by Webb et al. for
condensation of R-11 on 748 fins/m tube (see Fig. 6 in Webb et al. 1985).
Experimental heat transfer coefficients
The review article by Marto (1988) provides a comprehensive listing and discussion of
references that report experimental heat transfer data for horizontal finned tubes. Conse-
quently, the brief review here will discuss the difficulties and uncertainties in experimen-
tal investigations, present some sample results, and assess the predictive capabilities of
the theoretical models in the light of experimental data.
Experimental investigations of film condensation on horizontal integral-fin tubes en-
tail many difficulties. For example, the presence of noncondensable gases, partial drop-
wise condensation or a substantial va-
10
por velocity near the tube can affect the
Steam
Wanniarachchi et al. (1985)
Tsat = 100 C; Tc = 24 C
data significantly, but is rarely brought
8
u c = 3.3 m/s
out in published work. In many cases,
the uncertainties in measurements re-
6
hfinned
main obscure. The technique used to de-
hplain
termine the average condensation heat
4
Honda et al.
transfer coefficient can itself introduce a
(1987)
10 to 15 % discrepancy between differ-
2
ent data. The lack of consistency in the
Beatty-Katz
Honda-Nozu
(1948)
(1987)
choice of the surface area on which is
0
based often confuses the end user. The
6
interpretation and use of experimental
Tsat = 48 C; Tc = 22 C
data must therefore be made with great
u c = 3.3 m/s
care. The difficulties and uncertainties
associated with the measurement of con-
4
densation on horizontal integral-fin
hfinned
tubes are discussed in detail by Marto
hplain
(1992).
2
Attention is now turned to the dis-
cussion of the experimental data. The
last fifty years of activity has generated
a vast amount of data for condensation of
0
2
4
6
various refrigerants and steam on tubes
δ (mm)
having different fin geometries and spac-
Figure 23. Effect of fin spacing on the enhancement
ratio for steam condensing on horizontal integral- ing. Only a representative sample can be
discussed here. Figure 23 shows the en-
fin tubes. Adapted from Honda et al. (1987).
37