VAPOR CONDENSATION OF R-600aOVER SINGLE HORIZONTAL INTEGRAL-FIN TUBES

Abstract

Shell-and-tube heat exchangers are the most common equipments used in many industries like refrigeration and air-conditioning industries, petrochemical industries, process industries, atomic and thermal power plants and many other industries. Condenser designed for executing the present investigation comes in this category of heat exchanger. The optimum design of tubes not only saves the energy but also reduces the size of condensers in a significant manner. The present investigation is an endeavour in this regard. The experimental investigations had been performed to augment the condensing side heat transfer coefficient of refrigerant R-600a vapor over plain and integral-fin tubes by varying different parameters like fin-specification, wall sub-cooling temperature and coolant flow rate. Firstly, experiemental investigation had been performed over plain tube thenafter experiments over integral-fin tubes were carried out so that significant comparisons would be done between plain and integral-fin tubes. Condensation heat transfer coefficients of refrigerant R-600a (iso-butane) over a single horizontal smooth tube of diameter 19 mm were measured at different wall sub-cooling temperatures. Like other refrigerants, condensation heat transfer coefficients of R-600a showed the same trend with wall sub-cooling that external condensation heat transfer coefficients decrease as wall sub cooling temperatures increase. Based upon the Data taken, different graphs were plotted varying different parameters to show their dependency on other parameters. The experimental data were validated by comparing them against the standard model for condensation over plain tube. Nusselt’s model was used as the standard model for validating the experimental results. Further, condensation heat transfer coefficients of refrigerant R-600a (iso-butane) over five integral fin tubes of different fin-densities (945, 1024, 1102, 1181, and 1260 fins per meter) were determined at vapor temperature of 39 ± 0.5°C with different wall sub-cooling temperatures in a range of 3-12°C. Like other refrigerants, condensation heat transfer coefficients of R-600a showed the same trend with wall sub-cooling where condensation heat transfer coefficients decrease with the rise in wall sub cooling temperatures. The experimental data were also validated by comparing them against different models. 1102 fpm integral fin tube shows the highest heat transfer characteristics among all tubes with EF = 5.1 whereas 1260 fpm integral fin tube shows the lowest with EF= 4.1. Rudy-Webb and Webb-Rudy-Keidersky models overpredict the experimental results iv whereas Beatty-Katz and Owen et al. models underpredict the experimental values. Condensation heat transfer coefficients of refrigerant R-600a (iso-butane) over three different spined integral fintubes (SIFTs) were determined at vapor temperature of 39 0C. One of three SIFTs was made by cutting 40 axial slots on CIFT and other two were made by cutting axial slots on upper and lower half of the CIFT known as partially spined integral fin-tubes (PSIFTs) with spine upper half (suh) and spine lower half (slh) respectively. Also, outside condensation heat transfer coefficients of refrigerant R-600a were calculated at the vapor temperature of 39°C with wall sub-cooling temperatures of 3-9°C or, 3-9 K under heat flux of 33-87 kWm-2 Outside condensation heat transfer coefficients of R-600a were measured by modified Wilson plot technique varying the coolant flow rate for five integral fin tubes of different fin-densities (945, 1024, 1102, 1181 and 1260 fpm). This technology is a method of measuring heat transfer coefficient indirectly without measuring the tube wall temperature. The objective of this study is to provide a brief understanding of condensation heat transfer characteristics of R-600a (iso-butane) over single horizontal integral fin-tubes, calculated by Modified Wilson Plot technology. This study also showed variations in heat transfer characteristics calculated by this technology with coolant Reynold Number. The modified Wilson plot technique underpredicts HTC by 6 to 17 % for plain tube, 12 to 28 % for CIFTS and 15 to 35 % for SIFT and PSIFTS. . SIFT (1102 fpm) and PSIFT (slh) enhances heat transfer characteristics more than CIFT and PSIFT (suh) with EF= 5.86 and EF= 5.85 for SIFT and PSIFT respectively. From the present investigation, following correlation has been developed in non-dimensional form having different dimensionless numbers to determine the condensing side heat transfer coefficient during condensation of R-600a over CIFTs and SIFTs: CN = 0.056 Re -1/3 We 0.67 Y The agreement of experimental data with this correlation lies in a range of ± 20 % and the prediction of Honda and Nozu model with the proposed correlation lies within the range of +23 % to -25 % for the condensation of R-600a on circular integral-fin tubes (CIFTs).