This thesis presents an experimental study of the inclination effect on pressure drop and flow regime during condensation of steam in a large flattened tube used in air-cooled condensers (ACC) for power plants. Steam Because of the creation of the visualization window, the tube width was less than that used in industry, and thus a model that accounts for the differences in tube geometry between the full and test tube was developed and used to convert experimental pressure drop results to the full-sized tube under the same operating conditions. A prediction of pressure drop performance of the same steam condensing system under vacuum condition was also discussed. The negative dependence of pressure drop on inclination angle also prevailed in both converted results in atmospheric condition and the predicted ones in vacuum condition.iii Acknowledgement