Pressure drop measurements for R-22 condensing in 12.7-mm-diameter micro-fin tubes
have been reported by Schlager et al. (1990c). Their results show that the pressure drop in
micro-fin tubes is somewhat higher than that of the plain tube. Because of the uncertain-
ties in the pressure drop measurements, the relative differences between the different
microfin tubes could not clearly be established.
The effect of small concentrations of oil in R-22 condensation in micro-fin tubes has
been studied by Schlager et al. (1989). The presence of oil generally adversely affects the
heat transfer and pressure drop in micro-fin tubes.
Despite the proven effectiveness of the micro-fin tube in practice, detailed work on
understanding the enhancement mechanism involved is lacking. Webb (1994) has pro-
posed that both vapor shear and surface tension have important roles to play in augment-
ing the heat transfer process in micro-fin tubes. There is a great need for experimental
work on fluids other than refrigerants so that general purpose correlations for heat trans-
fer and pressure drop in micro-fin tubes can be developed.
CONCLUDING REMARKS
This report described the progress that has been achieved with the analysis and design
of extended surfaces for applications involving condensation. The report covered conden-
sation of pure vapor, as well as the condensation of moisture from humid air. The simple
models based on Nusselt-type correlations furnish the basic information about the perfor-
mance of single fins. However, a more detailed picture about the condensate film charac-
teristics and the thermal response of a fin is obtained with conjugate models. Consider-
able progress has been made in understanding and predicting condensation on horizontal
integral-fin tubes. On the other hand, the existing correlations for predicting the perfor-
mance of internally finned tubes are not only limited in number but also unsatisfactory in
many cases. The micro-fin tube, which is currently very popular with condenser design-
ers, remains virtually unexplored both theoretically and experimentally.
The bulk of the information appearing in the literature pertains to analysis. The various
analyses have established general guidelines for designing fins for condensation applica-
tion, but the designer is still left to exercise judgment for designing condensing extended
surfaces. This is true of both single fins and an ensemble of fins. Despite 60 years of
studies on horizontal integral-fin tubes, these enhanced surfaces are still designed based
on past experience. Basic design questions as to the optimum fin shape, size, and number
for a specified condensation rate and vapor-tube-coolant combination are still a challenge
for researchers. The development of analytical and design tools for the micro-fin tube is
still in its infancy.
LITERATURE CITED
Acharya, S., K.G. Braud and A. Attar (1986) Calculation of fin efficiency for condensing
fins. International Journal of Heat and Fluid Flow, 7: 9698.
Adamek, T.A. (1981) Bestimmung der Kondensationgrossen auf Feingewelten oberflachen
zur auslegun optimaler Wandprofile. Warme-und Stoffubertragung, 15: 255270.
Adamek, T.A. and R.L. Webb (1990) Prediction of film condensation on horizontal inte-
gral-fin tubes. International Journal of Heat and Mass Transfer, 33: 17211735.
tubes. Chemical Engineering Progress, 44: 908914.
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