Vacuum multistage steam ejectors

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. 

The efficiency of the condenser is reduced by poor air removal (and the presence of other noncondensable gases), so surface condensers usually are equipped with vacuum pumps but also may incorporate older style, single or multistage multielement, steam-jet air ejectors. Under most normal operations, the residual oxygen level is below 20 to 40 ppb 02. 

Jet ejectors require very little attention and maintenance and are especially valuable with corrosive gases that would damage mechanical vacuum pumps. For difficult problems the nozzles and diffusers can be made of corrosion-resistant metal, graphite, or other inert material. Ejectors, particularly when multistage, use large quantities of steam and water. They arc rarely used to produce absolute pressures below 1 mm Hg, Steam jets are no longer as popular as they once were, because of the dramatic increase in the cost of steam. In many instances where corrosion is not a serious consideration, they have been replaced by mechanical vacuum pumps, which use much less energy for the same service. 

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