Falling-film heat exchangers used in desalination systems: A review

AbstractInfluence of the heat exchanger shape on the heat transfer efficiency investigated. Heat transfer efficiency of two vertical pipe-in-pipe heat exchangers, one with a straight wall and the other with a twisted wall were compared. Both heat exchangers worked on the principle of liquid falling film. Electric transformer oil was used as the heated liquid and water vapor at atmospheric pressure was used as the heating agent. Oil was just heated, not evaporated during the experiments. Oil inlet and outlet temperature were measured at six different oil mass flows. Heat flux and heat flux density in the straight- and twisted-wall heat exchangers were compared. Mathematical model was built to verify the possibility of the oil outlet temperature prediction.

The heat exchangers for ice formation on tube essentially consists of cold pipes submersed in stagnant water or in a cross flow of water. The heat exchanger considered here is a falling film one. Water falling film falls down over a set of vertical in-line cold horizontal tubes. The falling film main modes are droplets, jets, and sheet depending on its flow rate. The tubes are internally cooled by a controlled subzero temperature coolant. The coolant passes through the pipes in parallel. Water falling film freezes gradually outside the test tubes. The quantity of ice formed on the test tubes is observed, photographed, and measured at different times for different falling film modes. It has been noticed that the rate of ice formation decreases with time as ice accumulates on the test tubes. The overall heat transfer coefficient decreases as more ice accumulates on the test tubes.

This paper presents a state-of-the-art study of a heat transfer process in liquid spraying heat exchangers placed in a vacuum chamber. The experimental case studied here describes the behavior of a falling film evaporation and condensation mode on horizontal tube bundles. The study aims to obtain the heat transfer coefficient and its correlations by means of a mathematical model.