Vacuum devices, ejectors

The ejector is widely used as a vacuum pump, where it is staged when required to achieve deeper vacuum levels. If the motive fluid pressure is sufficiently high, the ejector can compress gas to a slightly positive pressure. Ejectors are used both as subsonic and supersonic devices. The design must incorporate the appropriate nozzle and diffuser compatible with the gas velocity. The ejector is one of the ( to liquid carryover in the suction gas. 

Figure 6-9C. Improving the maximum vacuum obtainabie by staging an ejector ahead of the suction of a iiquid ring pump or any other device that can handie water into the unit, uniess dry air is used. By permission, Graham Manufacturing Co., Inc.

Similarly, vacuums can be created when a blower, fan, compressor, or jet ejector removes gases from equipment. The magnitude of the vacuum attainable will be governed by the performance characteristics of the device. Other mechanisms for generating a vacuum, which have been demonstrated by industry experience, include the following. 

Volume lA hird Edition, which covers process planning, scheduling, and flowsheet design, fluid, flow, pumping of liquids, mechanical separations, mixing of liquids, ejector and vacuum systems, and pressure-relieving devices. 

For producing vacuum, reciprocating machines are effective for absolute pressures down to 10 mm Hg. Rotary vacuum pumps can lower the absolute pressure to 0.01 mm Hg and over a wide range of low pressures are cheaper to operate than multistage steam-jet ejectors. For very high vacuums, specialized devices such as diffusion pumps are needed. 

Two types of vacuum producing devices are used jet ejectors and mechanical pumps. Jet ejectors usually have lower initial costs, lower maintenance costs, but higher operating costs than equivalent mechanical systems. Mechanical pumps are more efficient users of energy. Jet ejectors require no moving parts and therefore are simplest of all vacuum producers. 

Current economics favor the use of mechanical vacuum pumps in most applications. Steam jet ejectors are economically superior to other pumping devices when their initial cost and simplicity compensate for their inherently low efficiency. 

In vacuum filters, vacuum can be easily produced either by the suction of an ordinary liquid pump or by a gas displacement device, such as a rotary vacuum pump or steam ejector. Depending upon the device, the effect is to find a driving force of up to about 12 p.s.i. (0.81 bar), which in many instances is sufficient to give vastly improved rates of filtration with all except the finest solids. 

Degasifier. Also referred to as the deaerator, this device used heat and vacuum to remove noncondensible gases (e.g., nitrogen and oxygen) from the cation effluent water. The degasifier has two vacuum systems a steam jet air ejector system that uses medium-pressure steam to create a vacuum and a system that consists of three vacuum pumps (DOE-RL 1990). 

Venturi-type ejectors (Figure 2-4) are popular for the delivery of chemical solutions. These devices use water pressure through a Venturi to produce a vacuum that draws the chemical solution into the water stream. This stream is then directed into the process water to deliver the chemical. Ejectors are simple and reliable as long as a pressurized water supply is available. It is important to use materials for all of these devices that are compatible with the chemicals being used. 

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