Steam Ejectors

Operating Principles
The steam ejector is a simple device consisting of three basic components,
a motive steam nozzle, a suction chamber and a mixing diffuser.
The motive steam enters the ejector through the motive fluid nozzle
and is expanded into the mixing diffuser converting pressure energy into
velocity energy. The steam entrains the suction gases and the mixture
proceeds through the mixing diffuser where some of the velocity
energy is reconverted back into pressure energy, enabling the mixture
to be discharged at a higher pressure than the suction pressure. The
achievable compression ratio is highly dependent upon the quantity of
motive steam, therefore the practical compression ratio limit for a single
stage unit is defined by economics. Larger capacity ejectors often utilise
multiple steam nozzle technology, this has a significant benefit is steam
usage when compared with single nozzle units.

Multi Stage Steam Ejector Systems
Further efficiency gains can be made by utilising multi stage systems in
favour of single stage models, in general it is more efficient in specific
energy terms to use multi stage ejectors, this requires the use of intercondensers
to remove the operating steam so that only the incondensable
gases pass to the second stage reducing the amount of steam required.
The diagram illustrates typical vacuum levels that can be achieved
economically for a specific number of stages.

Multi Stage Steam Ejector Performance
Multi stage steam ejectors enable lower suction pressures to be achieved than can be produced by a single stage unit. The inter-stage condensers economise on the use of motive steam as the operating steam from the upstream stage of the system is condensed leaving only the saturated non condensable gas to be handled by the next stage. Non condensing designs can be considered for some applications where the 1st stage discharges directly into the 2nd stage without a condenser, this increases the load on subsequent stages but reduces the initial cost.
The inter-stage condenser can be either a direct contact or surface type either of which can be positioned at barometric height to enable gravity draining, or at low level which requires a condensate extraction pump to remove the condensed vapours.

Direct contact or jet condenser
These operate by condensing the steam through direct contact with the cooling water, jet condensers typically use less cooling water than surface type but as the condensed vapours are in direct contact with the cooling water the treatment of the resulting effluent must be considered.

Surface condensers
Shell and tube type surface condensers can also be used as inter and after condensers, the key benefit being that as the cooling water is not in contact with the condensate then the potential effluent problem is greatly reduced.

Hybrid Systems
Multi stage systems can be designed utilising a liquid ring pump instead of the final stage ejector, this enables large vapour flows to be handled by the steam ejector system whilst significantly reducing the steam consumption and operating costs.