-
h N = a Rel n
(100)
where the condensate Reynolds number Rel equals 4ml / lL . The constants a (W/m2 K)
˙
and n for the four geometries tested together with values of h N for Rel = 100 are given
below:
1024 standard integral fin: a = 12.90 103, n = 0, h N = 12,900 W/m2 K
Tred-26D: a = 269.90 103, n = 0.576, h N = 18,956 W/m2 K
Turbo C: a = 257.80 103, n = 0.507, h N = 24,885 W/m2 K
GEWA-SC = a = 54.14 103, n = 0.22, h N = 19,657 W/m2 K.
The 1024 standard integral fin exhibits zero row effect (n = 0) for the same tube using R-
113. The results for h N show that the Turbo C tubes gives the best heat transfer perfor-
mance. GEWA- SC and Tred-26D tubes perform comparably. The lowest performance is
displayed by the 1024 standard integral-fin tubes.
Theoretical models for the effect of tube bundle geometry have been expounded by
Ishihara and Palen (1983), El-Meghazy (1986) and Honda et al. (1987).
The combined effects of interfacial shear and tube bundle geometry with integral-fin
tubes have been studied for vapor downflow by Smirnov and Lukanov (1972). Their
results show that the effect of tube bundle geometry is more pronounced in finned tubes
than in plain tubes. The paper by Webb (1984) reviews the work done on the effects of
interfacial shear and tube bundle geometry for both plain and integral-fin tubes. The
paper recommends that designers of shell-side refrigeration condensers should maintain
a certain minimum vapor velocity in all regions of the tube bundle.
Effect of tube thermal conductivity
The bulk of the experiments described in Experimental Heat Transfer Coefficients pertain
to condensation on copper tubes. Because copper has a high thermal conductivity, the "fin
efficiency" effects due to the temperature drop in the fin are small. However, this effect
can become more pronounced if tubes of lower thermal conductivity are employed. To
determine the effect of thermal conductivity on the performance of horizontal integral-fin
tubes, Huang et al. (1994) condensed steam and R-113 on tubes made of copper, brass, and
bronze. Each tube had a root diameter of 12.7 mm, with rectangular fins 1 mm thick and
spaced 0.5 mm apart. Four tubes of each material were tested with fin heights of 0.5, 0.9,
1.3 and 1.6 mm. For comparison, plain tubes (outside diam. = 12.7 mm) of each material
were also tested.
The measured data expressed as the enhancement ratio are given in Table 3. As one
might have expected, the effect of lowering the thermal conductivity is to reduce the
enhancement ratio. This effect is stronger for steam than for R-113. Consider, for example,
the condensation of steam on copper and bronze tubes with 1.6-mm-high fins. The en-
Table 3. Effect of tube material on enhancement ratios, k = 315
W/m K, 112 W/m K, and 78 W/m K for copper, brass and
bronze, respectively.
Fin height
Steam
R-113
(mm)
Copper
Brass
Bronze
Copper
Brass
Bronze
0.5
1.74
1.50
1.50
3.16
3.15
2.96
0.9
1.90
1.63
1.43
4.24
4.35
3.90
1.3
2.05
1.68
1.37
4.60
4.72
4.28
1.6
2.40
1.77
1.39
5.16
5.09
4.91
40
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