R-113 data of Luu and Bergles (1979) and of Said and Azer (1983), and the R-11 data of
Venkatesh (1984) to develop a general heat transfer correlation. This correlation was
claimed to predict 71% of the data points within 30%. An analytical model to predict
condensation heat transfer in internally finned tubes was proposed by Kaushik and Azer
(1989), but the model remains unvalidated against experimental data, and as such can
only be recommended with caution.
The information on pressure drop correlation for condensation in internally finned
tubes is limited. Kaushik and Azer (1990) used the data of several workers and developed
a correlation using the least squares regression technique. This correlation predicted 68 %
of the data for steam and R-113 within 40 %. Sur and Azer (1991) proposed an analytical
model, which is claimed to predict the effects of changing the number, height, and thick-
ness of fins on the pressure drop.
So far, the discussion has been focused on condensation of pure vapor. When internally
finned tubes are used in the condensers of vapor compression refrigeration systems, the
refrigerant vapor is often mixed with small quantities of compressor lubricating oil. This
situation has been studied by Schlager et al. (1990a,b) in a two-part paper. In the first part
(1990a), the authors reviewed the heat transfer and pressure drop correlations for single-
phase and two-phase flows in smooth and internally finned tubes. They found that there
was no published correlation for flow of refrigerant-oil mixtures in finned tubes. This
conclusion prompted them to embark on an experimental program to study the evapora-
tion and condensation of refrigerant-oil mixtures in smooth and internally finned tubes.
The results of this study revealed that the presence of oil lowers the condensation heat
transfer coefficient for both smooth and finned tubes. The degradation of heat transfer
performance increases as the oil concentration increases. The pressure drop in finned
tubes was also found to increase with the increase in oil concentration.
The discussion in this section clearly indicates a need for better understanding of the
condensation process in internally finned tubes, so that accurate theoretical models for
heat transfer and pressure drop can be developed. There is also a need for correlations,
which are more generally applicable and not confined to specific fluids and operating
conditions. Similarly, more work is needed to understand and predict condensation of
refrigerant-oil mixtures in internally finned tubes.
MICRO-FIN TUBES
The micro-fin tube is currently the most popular enhancement device for residential
air-cooled air conditioners (window and central) and automotive refrigerant condensers.
This tube geometry was first developed by Hitachi Cable, Ltd., and described in a patent
by Fujie et al. (1977). The tube is commercially available in diameters between 4 and 16
mm. The original design, trade named Thermofin, has now been superseded by Thermo-
fin EX, Thermofin HEX, and Thermofin HEX-C. The last design, Thermofin HEX-C, was
specially developed for condensation enhancement applications. Figure 27, adapted from
(a)
(b)
Figure 27. Cross sections of Hitachi Thermofin tubes: (a) Thermofin,
(b) Thermofin EX; (c) Thermofin HEX; and (d) Thermofin HEX-C.
(c)
(d)
Adapted from Yasuda et al. (1990).
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