Steam ejector
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Steam Ejectors are the simplest means available today for vacuum - raising applications. The Steam Ejector utilizes high pressure steam to compress low pressure vapours or gases. This creates a vacuum in a vessel or chamber to which the ejector is connected.
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[edit] Description
Steam at pressure Pi expands through a nozzle and exits at very low pressure (Px) and at very high velocity. The low pressure induces a flow of load vapours at a pressure (P1) into the ejector. The two streams i.e. the low pressure, high velocity steam from the nozzle, together with the slower, entrained load vapours, will mix as they converge into the throat of the ejector. Upon leaving the throat, the gases slow down and regain pressure to a pressure P2.
The load gases have been compressed from their original pressure P1 to a new pressure, P2. We define the compression ration for this ejector as P2/P1.
If Ws kg/hr motive steam are required to entrain and compress Wl kg / hr of load vapours, we define the entrainment ratio for this ejector as Wl/Ws. These ratios are the key parameters in ejector design.
[edit] Multi stage ejector sets
In practice, for suction pressure below 100 mbar absolute, more than one ejector will be used, usually with condensors between the ejector stages. Condensing of motive steam greatly improves Ejector Set efficiency. Both barometric and shell-and-tube surface condensors are used for this purpose.
[edit] Construction
Ejector sets are fabricated in Carbon Steel, Stainless Steel, Titanium, PTFE, Carbon and others.
[edit] Railway applications
In the days of steam, ejectors were very widely used on British railway locomotives for creating a vacuum to operate the brakes. Locomotives usually had a Large ejector, which was used only for the initial brake release, and a Small ejector which ran continuously to maintain the vacuum. Ejectors are heavy consumers of steam so the small ejector was sometimes replaced by a reciprocating pump for economy.
