The double-pipe heat exchanger ,whose two kinds of different size diameters of straight pipe to form a concentric sleeve,the pipe Ends with the U-shaped fitting, connecting them into a row, and to arrange according to actual needs. It have some features:The counter-current mode operation can be carried out,;Because the inside and outside pipe’s diameter can be made smaller,so rate of flow of inside and outside be raised and the heat transfer coefficient of both sides of heat transfer surface be raised. This heat exchanger adapt to Fluid Flow smaller and Required heat transfer surface smaller.