A number of food processes involve heat transfer to solid-liquid mixtures. Continuous sterilisation plants consist of three sections : heating, holding and cooling. The system brings a flowing mixture to a suitable temperature, and keeps it at this temperature long enough to ensure the required sterility. Overprediction of the steriliser component sizes can result in excessive nutrient degradation and quality loss of the product. Experiments have demonstrated that particles and liquids can move at different velocities in real food systems. A one-dimensional computational model, which explicitly takes account of the variation in velocities, has been developed. The model has been used to study the variation in design length which results from solid-liquid slip. Steriliser sizes have been calculated for varying solid-liquid slip velocities and delivered solids concentration for a fixed flow rate. Plots of the variation of heat tube length as a function of slip velocity and solids concentration are presented as a function of particle Blot number, and the possible effects on the design of real systems suggested.