Owing to influence of many factors

the exhausteam pressure (back pressure)is frequently varying.Determining the influence of back pressure variation upon thermal efficiency of the unit has very important significance to analyse operation and energy-saving of the unit.Two methods for determining influence of back pressure variation upon thermal economic efficiency have analysed and compared in detail

pointing out individual merits and weakness

as well as the conditions of application for each of them.Through calculation of concrete examples

it is believed that the equivalent enthalpy method is more conveniet and accurate calculation method

and has wide applicability

satisfying the requirements of calculation in engineering.