In processing equipment in which falling films are employed, the break up of the him may be a limitation on the effective use of the surface for transfer operations; A model has been developed which enables the geometry of fluids travelling down the inside or outside of vertical tubes to be calculated. Once the geometry of the film or rivulet flow has been found, the associated heat or mass transfer conditions can be calculated. Flow geometries can be calculated for any flowrate and it is expected that this theory will be of Value in the optimisation of falling film equipment. (C) 1997 Elsevier Science Ltd.