Abstract: A fluid cooled heat exchanger system comprising a heat exchanger, first conduit means connected to the heat exchanger, a pump for pumping a cooling fluid to the heat exchanger via the first conduit means, second conduit means for receiving fluid from the pump that is in excess of the heat exchanger requirements, and a valve which is so connected to the first and second conduit means that fluid that has passed through the heat exchanger via the first conduit means mixes in the valve with fluid that has by-passed the heat exchanger via the second conduit means, and the valve having a movable valve element which is moved off a valve seat by an amount dependent upon the flow of fluid in the second conduit means whereby the flow of fluid through the heat exchanger is maintained substantially constant.