HEFAT2003 2 nd International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics 23 – 26 June 2003, Victoria Falls, Zambia Paper number: LA2 HEAT TRANSFER COEFFICIENTS IN A HORIZONTAL HERRINGBONE TUBE DURING IN-TUBE CONDENSATION Adriaan Lambrechts 1 , Leon Liebenberg 2 , and Josua P Meyer 3 1 Master’s Degree Student, 2 Senior Lecturer Rand Afrikaans University 3 Professor and Head: Department of Mechanical and Aeronautical Engineering University of Pretoria, Pretoria 0002 Tel: +27 12 420 3104, Fax: +27 12 362 5124 E-mail: jmeyer@up.ac.za ABSTRACT An experimental investigation into the heat transfer characteristics of horizontal smooth, micro-fin and herringbone tubes during in-tube condensation was done. The study focused on the heat transfer coefficients of the tubes with refrigerants R-22, R-134a and R-407C. The herringbone tube results were compared to the smooth and micro-fin tube results. The average increase in the heat transfer coefficient when compared to the smooth tube was found to be as high as 322% with maximum values reaching 336%. When compared to the micro-fin tube, the average increase in heat transfer coefficient was found to be as high as 196% with maximum values reaching 215%. A new unified correlation was also developed to predict the heat transfer coefficients in a herringbone and micro-fin tube. The correlation predicted the semi-local heat transfer coefficients very accurately with 97.6% and 96.4% of the data points falling in the ± 20% region for the herringbone and micro-fin tube respectively. The average heat transfer coefficients were also very accurately predicted with all the data points falling in the ± 20% region for both tubes. The trend of the new correlation also fitted the data very accurately and the conclusion was made that the correlation is very accurate and could be successfully used in practice. INTRODUCTION In-tube condensation heat exchangers are of great importance in the refrigeration, automotive and process industry. The demand for more compact systems, higher energy efficiency, lower material costs and other economic incentives have led to the rapid development of better and more effective heat exchangers [1]. These heat exchangers have enhanced surfaces, displaced enhancement devices, swirl-flow devices and surface tension devices to enhance the heat transfer coefficient [2]. This means that a lot of research is currently being done and still needs to be done on enhanced heat exchangers. Furthermore, the amendment of the Montreal Protocol [3] which focuses on the phase- out of all chlorinated compounds has meant that substitutions for traditional refrigerants such as R-22 have to be used. Replacement refrigerants such as R-407C have additional problems due to the zeotropic characteristics of the refrigerant, which means extensive heat transfer degradation during condensation. The problem is thus that additional work with new refrigerants in heat exchangers needs to be done. Examples of new heat exchangers are micro-fin tubes and more recently herringbone tubes, shown in Figure 1. Micro-fin tubes are tubes that are internally grooved at different spiral angles as is shown in the figure. Micro-fin tubes have an increase of up to 200% in heat transfer coefficients and a pressure drop increase of up to 50% when compared to conventional smooth tubes [1]. In 1983, 175 references could already be found on enhanced tubes with extended surfaces (such as micro-fin tubes) [2]. Liebenberg [4] showed 32 of the most recent publications on micro-fin tubes along with their experimental parameters. The conclusion can therefore be made that a lot of work has been done on micro-fin type heat exchangers. Herringbone tubes, which are the latest development in heat exchanger technology, have an increase in heat transfer coefficient of up to 200% and a pressure drop increase of between 50% to 60% when compared to micro- fin tubes during condensation [5]. According to Goto et al. [6], herringbone tubes increase the heat transfer coefficient twice as much when compared to micro-fin tubes. Miyara et al. [7] found that the heat transfer coefficient of a herringbone tube was higher than that of a micro-fin tube in the high mass velocity region by as much as two times whilst it had slightly lower values in the lower mass velocity regions. The aforementioned articles are the only known publications pertaining to herringbone tube heat exchangers and the conclusion can be made that not much research has been done on herringbone type heat exchangers. Miyara et al. [7] is also the only known correlation for predicting the pressure drop and heat transfer coefficients of a herringbone tube heat exchanger. However, the correlation makes use of inside wall