A method for preventing formation of droplets in a heat exchanger, in which a second medium transfers heat to a first. The method is performed by a controller which receives different temperature values (T1, T2, T3) and a pressure (P) value to be used for calculating a boiling point temperature value (TB) and determining a first temperature difference (ΔT1) and a second temperature difference (ΔT2). Generating a flow control signal, for controlling the flow of the first medium into the heat exchanger, based on the first temperature difference (ΔT1), the second temperature difference (ΔT2) and the first temperature value T1 and sending the flow control signal to a regulator device for controlling the flow of the first medium in the heat exchanger.