In this investigation, the problems of two-phase flow in horizontal pressure-influenced return lines and sloped gravity-influenced condensate return lines of steam heating systems were studied. A number of experimental tests for different flow configurations were conducted in order to provide measured values for correlating with theoretical models.

The use of the open channel flow equations and the tables developed through solution of these equations make it possible to model the condensate carrying capacity for sloped returns and fully flooded wet returns. The use of tables developed from a modified Lockhart-Martinelli procedure make it possible to predict the condensate flow rates in horizontal closed systems. By selecting a pressure drop across the steam trap, the system designer can then find the necessary pipe size and slope or pressure difference.