Tracer experiments were conducted in a lab-scale cold flow model of 0.4 m by 0.2 m in cross section to investigate the solid mixing behavior of bubbling fluidized beds under continuous particle exchange with and without immersed tube bundle heat exchangers. The superficial gas velocity, the solid circulation rate, and the heat exchanger geometry were varied thereby. An established inductance measurement setup detecting pulse-injected ferromagnetic tracer particles was used to obtain the residence time distribution of the continuously exchanged solids. A profound mathematical routine was applied to calculate the particles' mean residence time and characteristic values describing solid mixing phenomena. In this study, it was shown that the bubbling fluidized bed mixing characteristics are similar to mixed flow but superimposed by dispersed plug flow to a greater or lesser extent depending on the geometrical configuration of the immersed tube bundle heat exchangers. Furthermore, the results show that solid mixing enhances with increasing superficial gas velocity, whereby short- circuiting and sluggish turnover of solids were observed at fluidization numbers below 5. It was shown that these phenomena are increasingly promoted as the tube spacing decreases, and in general, quantifying them fundamentally improves the understanding of the fluidized bed reactor design process.