One of the most effective techniques for augmenting in-tube condensation and evaporation is the use of internally finned tubes, specifically, tubes with small spiral fins, referred to as "micro-fin" tubes. The characteristic that distinguishes micro-fin tubes from other forms of internally finned tubes is that the fins are much smaller and more numerous. The present experimental study was divided into two phases, each of which involved the design, construction, and operation of two different test facilities;In the first phase, the effects of several geometrical parameters on two-phase thermal-hydraulic performance using a smooth tube and nine different micro-fin tubes were studied with R-113 as a test fluid on rather short (i.e., 41 in. or 1.0 m) length test sections. The geometrical parameters investigated were peak shape, valley shape, fin height, number of fins, and spiral angle. The micro-fin tube with 60 spiraled fins of 0.008 in. (0.20 mm) fin height, with a round tip and a flat valley shape, resulted in the best heat transfer performance for both evaporation and condensation;The local single-phase and evaporation enhancement factors (increase in heat transfer or pressure drop over the reference smooth tube) for 12.5 ft (3.81 m) long test sections were obtained by modifying the R-113 test apparatus. The results indicated comparable evaporation heat transfer enhancement factors for both short and long test sections;In the second phase, local single-phase and evaporation enhancement factors for R-22 were obtained using 12.1 ft (3.68 m) long electrically heated test tubes. A reasonable similarity was found between the local evaporation data for the two different refrigerants (i.e., R-113 and R-22). Finally, the R-22 test apparatus was modified to determine the average evaporation and condensation enhancement factors using a 12 ft (3.65 m) long shell-and-tube heat exchanger test section with refrigerant flowing in the tube and the water in the annulus;In summary, since these micro-fin tubes have excellent heat transfer performance (maximum enhancement factor of 3.8) with low penalties on the pressure drop (maximum enhancement factor of 1.8), they can be effectively used in refrigeration and air-conditioning equipment, specifically for heat pump applications.