By considering channel flow as boundary-layer flow, it opens the possibility of applying the Prandtl boundary-layer equation to the total flow area. By this means, integral equations for continuity, momentum, and energy may be derived which in turn release correction factors for these quantities. The correction factors obtained are comparable to those which are well known from boundary-layer theory, i. e. , displacement-thickness, momentum-thickness, energy-thickness. Problems of shock and friction losses are dealt with, taking into account the velocity-profile correction as an example for using the integral expressions.