In this paper electro-hydrodynamic (EHD) flows are investigated theoretically with results presented for the flow field in model electrostatic precipitators (ESP's). The resulting flow field is shown and explained qualitatively. Calculations with a k−ε-model show the influence of the mean flow velocity and the importance of the inhomogeneity of the electric field distribution at the inlet and outlet of an ESP. Furthermore a perturbation analysis is presented, leading to a simple differential equation of the Helmholtz type. This allows a more detailed view of the important mechanisms which form the secondary flows as well as a means to get a very fast estimation of the resulting flow field.