BT-9 Vertical Tube Falling Film Evaporation - State-of-the-Art Report

J. W. Palen

A brief survey is made of the state of the art of thermal design for vertical intube falling film evaporators. Earlier HTRI publications on this subject are HTRI Report BT-5 (May 1980) and HTRI Research Brief 3-5 (May 1986). These reports have not been replaced by this present report and should still be referred to for additional information. Several new references and increased understanding of some of the processes involved justify this extension.

Much information is now available on heat transfer and hydrodynamics for a water film. Some correlations are general and can be applied to other pure fluids. Mass transfer effects for mixtures have received little attention so far, except for absorption, but some articles are now emerging on this subject.

In addition to briefly summarizing the available literature, it was intended to bring together design equations which will permit a realistic estimate of the tubeside heat transfer rates and pressure drops for a falling film evaporator. Many of the methods presented, however, need additional data for confirmation and should be considered as a first estimate rather than final design calculations.

Even with approximate methods, it can be shown that the falling film evaporator has great advantages over other types of vapor generators in cases of low temperature difference, low operating pressure, and for viscous or heat sensitive fluids (see HTRI Research Brief 3-5). With the increasing energy shortage forcing more low DT operation, it is expected that more use will be made of falling film evaporators in the process industries. However, more reliable design methods will require additional research.