Ejector PrincipleEffective pumping action without moving parts
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| To explain how an ejector operates as a vacuum pump, a simple ejector mounted on a vacuum vessel will be used. Even though an ejector operates continuously, the illustrations are broken into stages for simplicity. |
STAGE 1 High pressure steam is fed, at relatively low velocity, into the motive fluid connector, 1 .
- Nozzle Head (Steam Chest)
- This is simply a nozzle holder. It connects the nozzle to the high pressure steam line and aligns it with the diffuser. (see Stage 3)
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- Motive Fluids
- Steam is usually used as the motive fluid because it is readily available. However an ejector can be designed to work on other gases or vapors if their thermodynamic properties are known.
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- Water and other liquids are sometimes good primary fluids as they condense large quantities of vapor instead of having to compress them. Liquids will also handle small amounts of non-condensable gases. (see hydro-jets)
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STAGE 2 The motive high pressure steam enters the nozzle and issues into the suction head as a high velocity, low pressure jet.
- Nozzle
- This is a device for converting the pressure and temperature energy of high pressure steam or other fluid into velocity energy. Of course, steam will issue at a high velocity from any hole in a high pressure pipe line; but it will not be well directed nor will its energy be sufficiently developed.
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- Suction Head
- This is the vacuum chamber and connects to the system being evacuated. The high velocity jet issues from the nozzle and rushes through the suction head.
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STAGE 3 A low absolute pressure at 2 (inside suction head) entrains all of the adjacent gases in the vacuum vessel, accelerates them to a high velocity and sweeps them into the diffuser.
- The Diffuser
- The process in the diffuser is the reverse of that in the nozzle. It compresses a high velocity, low pressure jet stream into a high pressure, low velocity stream.
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STAGE 4 In the final stage the high velocity stream, passing through the diffuser, is compressed and exhausted at the pressure of the discharge line. |
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A closer look at the diffuserA simple tube could be used as a diffuser but efficiency is too low for most applications. A well engineered and well made convergent-divergent diffuser will convert the velocity back into pressure with the greatest effectiveness. |
In the converging section of the diffuser mixing becomes complete and there is some rise in pressure. When a supersonic stream enters the straight section of the diffuser a very sharp rise in pressure takes place along with a slowing of the stream. This is known as compression shock. In the diverging section of the diffuser pressure is built up to the exhaust line and velocity is lowered to where it is just sufficient to keep the mass moving. |
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