as the high pressure steam(P), known as ‘primary fluid’, expands and accelerates through the primary nozzle (i), it fans out with supersonic speed to create a very low pressure region at the nozzle exit plane (ii) and hence in the mixing chamber. According to the differences of pressure of two positions, higher-pressure vapour,which, can be called the ‘secondary fluid’ (S), can be entrained into the mixing chamber. The primary fluid’s expanded wave was thought to flow and form a converging duct without mixing with the secondary fluid. At some cross-section along this duct, the speed of secondary fluid rises to sonic value (iii) and chokes. This cross-section was defined by Munday and Bagster as the ‘effective area’.