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The Journal of the Royal Institute of Thailand °“√‡¥◊ Õ¥¢Õß “√∑”§«“¡‡¬Á π R-134a ∫√‘ ÿ ∑∏‘Ï „πÕÿ ª°√≥å ·≈°‡ª≈’Ë ¬π§«“¡√â Õπ·∫∫∑à Õ´â Õπ∑à Õ√à «¡»Ÿ π¬å ·∫∫¢¥‡ªì π‡°≈’ ¬« Vol. 32 No. 4 Oct.-Dec. 2007 804 Abstract Boiling of Pure R-134a in a Helically Coiled Concentric Tube in Tube Heat Exchanger Maitree Polsongkram Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Laboratory. (FUTURE) Department of Mechanical Engineering, King Mongkut’s University of Technology Thonburi Somchai Wongwises Associate Fellow of the Academy of Science, The Royal Institute, Thailand; Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Laboratory. (FUTURE) Department of Mechanical Engineering, King Mongkut’s University of Technology Thonburi The two-phase heat transfer coefficient and pressure drop of pure HFC-134a boiling inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786 m long helically coiled double tube with refrigerant flowing in the inner tube and heating water flowing in the annulus. The inner tube is made of smooth copper tubing of 9.52 mm outer diameter and 8.3 mm inner diameter. The outer tube is made from smooth copper tubing of 23.2 mm outer diameter and 21.2 mm inner diameter. The heat exchanger is fabricated by bending a straight copper double-concentric tube into a helical coil of six turns. The diameter of coil is 305 mm. The pitch of coil is 35 mm. The test runs are done at average saturation boiling temperatures ranging between 10 and 20 o C. The mass fluxes are between 400 and 800 kg m -2 s -1 . The effects of heat flux, mass flux and, boiling temperature on the heat transfer coefficients and pressure drop are also discussed. It is found that the percentage increase of the average heat transfer coefficient and the pressure drop of the helically coiled concentric tube-in-tube heat exchanger, compared with that of the straight tube-in-tube heat exchanger, are in the range of 30% to 35% and 10% to 70%, respectively. Key words : two-phase flow, boiling, heat transfer coefficient, pressure drop, refrigerant, helically coiled tube