Abstract: When continuously heat treating a liquid the flow of which varies, incoming liquid is preheated by already heated liquid and is then heated to a predetermined temperature-time-effect by a medium giving off heat in a second step. The flow of incoming liquid is sensed and controlled by a pump (1) with a variable capacity. The amount of heat needed to obtain the desired heating effect is influenced by a control unit (9). The heating effect is adjusted to the liquid flow by increasing/decreasing the temperature of the medium giving off heat or increasing/diminishing the flow of this medium from a heat store (6).