In this chapter, our recent work on the entransy dissipation theory and its application in heat convection and the heat exchanger design is reviewed. In that work, the thermodynamic basis of the entransy dissipation theory is established. It is shown that the entransy is a state variable and the second law of thermodynamics can be described by the entransy and entransy dissipation. Then this entransy dissipation theory is applied to the convective heat transfer and heat exchanger design. The local expression of the entransy dissipation rate is obtained. The extremum principle of entransy dissipation rate is proposed. The expressions of the entransy dissipation caused by heat conduction and fluid friction in heat exchanger are derived. The optimization design of heat exchanger is discussed.