In this study we analyzed the effects of the hydrodynamic conditions and properties of the working fluid on the operation of external loop air lift bioreactors. In particular, the effects of design and operating parameters (e.g. reactor geometry, design of the gas sparger, superficial gas velocity, flow regime) as well as the fluid properties (liquid viscosity and addition of surfactants) on the liquid superficial velocity and gas hold-up were defined. Several correlations found in the literature based on theoretical models of fluid flow, as well as several semi-empirical and empirical correlations were tested and the applicability of all the proposed correlations was verified on the available experimental data. The most accurate correlations for the prediction of the gas hold up, liquid circulation velocity and slip velocity in each bioreactor operating regime were identified.