Abstract: A standalone flow rate controller for controlling flow rate of cooling or heating fluid through a heat exchanger by providing a control signal to a flow valve actuator so as to open or close a flow valve in response thereto. A first temperature sensor measures an entry temperature of the cooling or heating fluid flowing into the heat exchanger and a second temperature sensor measures an exit temperature of the cooling or heating fluid flowing out of the heat exchanger. A control unit is responsive to a temperature difference between the entry temperature and the exit temperature for adjusting the control signal so as maintain the temperature difference substantially constant.