In an ejector, motive steam is passed through a converging-diverging nozzle to achieve high velocity. The purpose of the nozzle is to convert the thermal energy (enthalpy) of the steam into kinetic energy.

When steam flows through a nozzle, its pressure and temperature decrease, while its velocity increases. For the steam to entrain the suction fluid effectively in an ejector and compress it to a higher discharge pressure, the motive steam jet must have a very high momentum. This requires a high velocity.

Understanding Steam Velocity in Ejector Nozzles

A converging-diverging nozzle is designed to accelerate a fluid flow to supersonic speeds, provided the pressure ratio across the nozzle is sufficiently low (below the critical pressure ratio). For the typical operating conditions of motive steam in an ejector, the pressure drop across the nozzle is large. This large pressure drop is necessary to produce the high-velocity jet required for the ejector to function efficiently.

The flow through the converging section accelerates up to sonic speed at the throat (the narrowest point). If the exit pressure is low enough relative to the inlet pressure, the flow continues to expand and accelerate in the diverging section, reaching supersonic speeds at the nozzle exit.

Therefore, for a steam ejector to operate as intended, the motive steam expanding through the nozzle needs to achieve a velocity significantly higher than the speed of sound at the exit conditions. This high kinetic energy allows the motive steam to entrain the suction fluid and carry it to the mixing section, where momentum transfer occurs.

In summary, the design and operational requirements of a steam ejector nozzle are specifically aimed at producing a high-speed jet, which is typically at supersonic velocity.