Steam-jet air ejector

Three-stage air ejector with internal diffusers

A steam­jet ejector may be used to withdraw air and dissolved gases from the condenser. In each stage of the steam­jet ejector, high-pressure steam is expanded in a convergent/divergent nozzle. 

The steam leaves the nozzle at a very high velocity in the order of 1220 m/s and a proportion of the kinetic energy in the steam jet is transferred, by interchange of momentum, to the body of air which is entrained and passes along with the operating steam through a diffuser in which the kinetic energy of the combined stream is re­converted to pressure energy. 

The maximum pressure ratio that can be obtained with a single stage is roughly 5:1 and consequently, it is necessary to use two or even three stages in series, to establish a vacuum in the order of 724 mm Hg, with reasonable steam consumption.

There are a variety of ejector designs in service that work on the same principle. Older units have heavy cast steel shells that serve as vapor condensers and also contain the diffusers. These are arranged vertically, the steam entering at the top. 

More recent designs have the diffusers arranged externally and the vapor condenser shell is somewhat lighter in construction. Horizontal and vertical arrangements can be found and some units are arranged as combined air ejectors and gland steam condensers.

Horizontal element two-stage air ejector

                          Horizontal single-element two-stage air ejector

An air ejector that has been commonly used, The unit comprises a stack of U­tubes contained in a fabricated mild steel condenser shell on which is mounted a single-element two-stage air ejector. 

The condensate from the main or auxiliary condenser is used as the cooling medium, the condensate circulating through the tubes whilst the air and vapor pass through the shell. 

The high-velocity operating steam emerging from the stage ejector nozzle entrains the non­condensables and vapor from the main condenser and the mixture discharges into the inter (or first stage) condenser. 

Most of the steam and vapor is condensed when it comes into contact with the cool surface of the tubes, falls to the bottom of the shell, and drains to the main or auxiliary condenser. The remaining air and water vapor are drawn into the second-stage ejector and discharged to the after (or second-stage) condenser. 

The condensate then passes to the steam drains tank and the non­condensables are discharged to the atmosphere through a vacuum retaining valve which is fitted as a safety device to reduce the rate of loss of vacuum in the main condenser if the air ejector fails. 

It is mounted on a pocket built out from the second stage condenser and consists essentially of a light stainless steel annular valve plate that covers ports in a gunmetal valve seat. When the pressure inside the after condenser exceeds atmospheric pressure the valve lifts and allows the gases to escape to the atmosphere. 

A relief valve is fitted on the first stage condenser shell of the twin-element unit. The ejector stages consist of monel metal nozzles in mild steel holders discharging into gunmetal diffusers. Expansion is allowed for by sliding feet at the inlet end