A control strategy is proposed to control the internal fluid temperature at the outlet of a co-current heat exchanger by manipulating the inlet external fluid temperature. The dynamic model of the heat exchanger is given by two partial differential equations. Based on nonlinear geometric control, a state-feedback law that ensures a desired performance of a measured output defined as spatial average temperature of the internal fluid is derived. Then, in order to control the outlet internal fluid temperature, a control strategy is proposed where an external controller is introduced to provide the set point of the considered measured output by taking as input the error between the outlet internal fluid temperature and its desired set point. The validity of the proposed control design and strategy is examined in simulation by considering the tracking and disturbance rejection problems.