Oxygen bubble evolution in the cooling circuit of a polymer-electrolyte-membrane (PEM)-electrolysis is a severe technological problem as it highly influences heat transfer and the performance of the process. Bubbles arise due to an oxygen super-saturation of the process water. A force balance model for the calculation of the departure diameter at flow boiling is applied to the described scenario. For this purpose, the reformulation of the force due to bubble growth is carried out. Based on the derived model, the influence of the mean liquid velocity, the nucleation site diameter, the contact angle and the inclination angle on the bubble departure diameter is evaluated in a sensitivity analysis. Furthermore, regression equations are derived for a fast calculation of the departure diameter without the need of solving the force balances.