This study addresses issues which arise in the computational and experimental modelling of flow and heat/mass transfer in membrane distillation and other processes adopting spacer-filled channels (either planar or spiral wound), but have not been sufficiently clarified in the literature so far. Most of the argumentations presented are based on original computational results obtained by the authors by finite volume simulations; some literature results are also considered. The questions addressed regard the choice of scales for the reduction of data and the definition of dimensionless numbers (Re, f, Nu, Sh); the definition of average heat or mass transfer coefficients; the combined effects of the parameters that characterize the process (spacer pitch to channel height ratio 1/H, flow attack angle gamma and Reynolds number Re) and the applicability of simple correlations; the influence of the spacer's thermal conductivity. In regard to the influence of the parameters, Re, 1/H and gamma were found to interact heavily, making a separate-effect analysis impossible and power-law friction or heat/mass transfer correlations of little use. Thermal conduction in the spacer, even for low-conductivity polymeric spacers (lambda approximate to 0.15 Wm(-1) K-1), was found to be responsible for up to 10% of the total heat transfer. (C) 2017 Elsevier B.V. All rights reserved.