HEAT TRANSFER DURING CONDENSATION OF R-134a OVER A HORIZONTAL INTEGRAL - FIN TUBE

Abstract

The surface condensers have wide applications in the refrigeration & air-conditioning, and other allied industries. Many augmentation techniques, to enhance the heat transfer in the surface condenser rate from the condensing vapour to the cooling water, have been reported in the literature. The size of condensers can be reduced by these techniques in a significant manner. A review of literature shows that the augmentation in condensing side heat transfer coefficient will enhance the heat transfer in the surface condensers. The heat transfer coefficient can be increased many folds by the simple technique of providing integral-fins over the tube surface. The conventional refrigerants are damaging the ozone layer and causing the global warming. The R-134a is a new environment friendly refrigerant with thermo-physical properties closer to those of R-12. R-134a has zero ozone depletion potential and ten percent global warming potential in comparison to R-12. Hence, R-134a is seen as a suitable replacement of R-12. Therefore, an experimental investigation has been planned and carried out to study the augmentation of heat transfer by enhancing the condensing side heat transfer coefficient and heat flux during condensation of the saturated vapour of R-134a over 6 horizontal tubes consisting of one plain tube and 5 integral-fin tubes with different fin heights. The investigation was initiated by acquiring data for the condensation of R-1 34a over a plain tube in an experimental set-up fabricated in the laboratory. The results for the condensation over a plain tube established the integrity of the experimental set-up and helped in the preliminary analysis of the data. Later on, the plain tube was replaced by five circular integral-fin tubes of 472 fpm fin density, one by one. The fin height of the finned tubes was 0.45 mm, 1.14 mm, 1.47 mm, 1.92 mm and 2.4 mm. The CIFT-1 with fin height of 0.45 mm out performed the other tubes, when the total tube surface area is taken into account. The enhancement factor of CIFT-1 has been found to be 2.95. When the finned tube surface area is calculated taking the root diameter of the tube into the account the enhancement factor is 5.74, however, the increase in the tube surface area due to finning is only 2.23 times. For the condensation of R-134a the temperature of condensing vapour has been kept at 312±0.5K (approximate temperature of refrigerant in the condenser of a refrigeration plant) and cooling water flow rate was varied from 400 kg/hr to 1250 kg/hr. In seventeen steps. So for each test-section the data were acquired for 18 runs, and hence,. a. total number of 108 runs were conducted •forthe entire investigation. An uncertainty analysis of the experimental results has also been carried out. The uncertainty in condensing side heat transfer coefficient for the condensation of R-134a has remained in the range of 7-11 percent.