Multistage ejectors

For these curves, multistage ejectors are assumed to be provided with surface intercondensers (20°C cooiing water) efficiencies for noncondensing muitistage ejectors wiil be much iower. 

The capacity of an ejector is expressed as pounds per hour total of non-condensable plus condensables to the inlet flange of the unit. For multistage units, the total capacity must be separated into pounds per hour of condensables and non-condensables. The final stages are only required to handle the non-condensable portion of the load plus the saturation moisture leaving the intercondensers. 

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. 

Vacuum Pumps. The function of these pumps is to evacuate the drying chamber quickly without allowing the prefrozen material to melt—and thereafter to reduce the pressure progressively to the desired vacuum and maintain it at Ihis level by removing the noncondensablc gases. The vacuum equipment can be cither an oil-sealed rotary vacuum pump, or a multistage stream-ejector system. 

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 In many instances where corrosion is not a serious consideration, they have been replaced by mechanical vacuum pumps, which use much less energy for the same service. 

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. 

Steam ejector system, multistage with barometric condensers, 0.07-27 kPa-abs, steam pressure 0.7 MPa, and 29 °C water including two ejectors, barometric condenser and interstage piping. FOB cost = 28 500 at the ratio of the gas capacity (kg/h)/the absolute pressure (kPa) = 10 with n = 0.24 for the range 0.8-80. L-rM ... 

The staging efficiency is particularly true for steam jet ejectors because the motive steam for one stage becomes the load for succeeding stages. Interstage condensers can dramatically reduce steam consumption consequently all multistage jets should be provided with intercondensers. The reduction in operating cost completely overshadows the increase in capital investment. 

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