The present invention relates to an arrangement comprising a waste heat recovery system (WHR-system) and a method for controlling the arrangement. The arrangement comprises an expansion tank having a constant inner volume, first cooling means configured to cool the working fluid in the condenser and a control unit configured to control the first cooling means such that the working fluid is cooled to a desired condensation temperature in the condenser during operation of the WHR system. The arrangement comprises further a sub-cooler arranged in a position downstream of the condenser and second cooling means configured to cool the working fluid in the sub-cooler, and that the control unit is configured to control the second cooling means such that the working fluid receives a determined subcooling in the sub-cooler during operation of the WHR system.