In the food industry, heat treatment of highly viscous fluids in continuous processes is becoming more and more common, and the process should perform as a homogenous thermal treatment, in order to ensure quality and safety of the final product. To improve treatment homogeneity, geometric modifications can be used even in the laminar regime, to induce flow perturbation and mixing. The objectives of this work include: (i) Investigation of the residence time distribution (RTD) for industrial indirect Joule effect heaters (JEH), with smooth (ST) and modified (MT) tubes, (ii) Demonstration and quantification of the efficiency of the geometrical modifications, and (iii) Proposition of a single semi-empirical model including the flow regime (10 < Re < 2000) and tube diameters (18 and 23 mm). The results obtained confirm that the simple Dispersed Plug Flow (DPF) model is not adaptable to small Reynolds numbers. Further analysis demonstrates that certain geometrical modifications improve the treatment homogeneity by increasing the plug flow contribution and reducing the value of the reduced variance. These beneficial effects increase when the Reynolds number is increased, the nominal diameter is reduced, and modified tubes are used. The proposed model enables the prediction of the RTD in JEH with an accurate degree of confidence.