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.