Evacuation Ejector

When an extra jet for this purpose is not desirable, the secondary jet of a multiple system is often sized to have sufficient air removal capacity to pump down the system in a reasonable time. 

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The prevacuum technique, as its name implies, eliminates air by creating a vacuum. This procedure faciUtates steam penetration and permits more rapid steam penetration. Consequendy this results in shorter cycle times. Prevacuum cycles employ either a vacuum pump/steam (or air) ejector combination to reduce air residuals in the chamber or rely on the pulse-vacuum technique of alternating steam injection and evacuation until the air residuals have been removed. Pulse-vacuum techniques are generally more economical vacuum pumps or vacuum-pump—condenser combinations may be employed. The vacuum pumps used in these systems are water-seal or water-ring types, because of the problems created by mixing oil and steam. Prevacuum cycles are used for fabric loads and wrapped or unwrapped instmments (see Vacuum technology). 

For example, assume that it is desired to evacuate air at 2.94 IbFin" with a steam ejector discharging to 14.7 IbFin" with available steam... 

To estimate the time required for an ejector to evacuate a system from atmospheric pressure down to the design pressure, assume that the average air handling capacity during the evacuation period is twice the design... 

If this approximate evacuation period is too long, it may be shortened by adding a larger last stage to the ejector or by adding a noncondensing ejector in parallel... 

The VPS overhead consists of steam, inerts, condensable and non-condensable hydrocarbons. The condensables result from low boiling material present in the reduced crude feed and from entrainment of liquid from the VPS top tray. The noncondensables result from cracking at the high temperatures employed in the VPS. Inerts result from leakage of air into the evacuated system. Steam and condensable hydrocarbons are condensed using an overhead water-cooled condenser. The distillate drum serves to separate inerts and non-condensables from condensate, as well as liquid hydrocarbons from water. Vacuum is maintained in the VPS using steam jet ejectors. 

An evacuation booster or hogging ejector is sometimes used to remove air from a system on start-ups. Its capacity is set to bring the system pressure down to near operating conditions before the continuous operadng ejector system takes over. Figure 6-23 illustrates the instal-ladon of such a unit. 

Once a. system has been evacuated to normal operating conditions, it is possible for capacity to fall to almost zero when the only requirement is air inleakage or small quantities of dissolved gases. Under these conditions, it is important to specify an ejector system capable of stable operation dow n to zero load or shut-off capacity. The curve of Figure 6-24 represents such a system. 

Figures 6-26A and B give q pical good estimating selection curves for single-stage ejectors. Table 6-9 gives evacuation factors.

Example 6-10 Size Selection. Utilities and Evacuation Time for Single Stage Ejector... 

This assumes dry air with no condensables and negligible pressure drop through the system to the ejector. Also, the jet air handling capacity is assumed approximately twice the design capacity, and air inleakage during evacuation is negligible. 

Example Using Penberthy Model U Ejector for Evacuation Time... 

Discharge pressure of pump, psia = Intake pressure of pump Mth closed intake, psia = Final pressure in system, in. Hg abs Gas constant, = 1544/mol weight Pump speed, revolutions (or strokes) per second Pump speed, liters/sec Pump speed at P ", liters/sec Pump speed at P/, liters/sec Temperature, °R = 460 + °F Evacuation pump dowmtime, min Evacuation pump downtime, sec Ambient air temperature, °F Temperature of mixture at ejector suction, °F Temperature of steam on downstream side of nozzle, °F... 

Wa = Air inleakage resulting from metal porosities and cracks along weld lines, lbs/hr Wm = Ejector capacity at final evacuation suction pressure, lbs/hr... 

Dusts, particle sizes, 225 Dusts, hazard class, 521-523 Explosion characteristics, 524 Efficiency, centrifugal pumps, 200 Ejector control, 380 Ejector systems, 343, 344, 351 Air inleakage, table, 366, 367 Applications, 345 Calculations, 359-366 Chilled water refrigeration, 350 Comparison guide, 357, 375 Evacuation lime, 380, 381 Charts, 382 Example, 381 Features, 345... 

Air/water vapor mixture, chart, 364,365 Air/water vapor, 359 Capacity at ejector suction, 369 Capacity for process vapor, 362 Evacuation time, 371, 380 Load for steam surface condenser, 367 Non-condensables, 362, 363 Size selection, 371 Steam pressure factor, 373 Steam requirements, 372 Steain/air mixture temperature, 361 Total weight saturated mixture, 362 Capacity, 358 Discharge, pressure, 358 Effect of excess steam pressure, 358 Effects of back pressure, 359 Effects of wet steam, 356 Inter-and-after condenser, 351 Load variation, 370 Materials of construction, 347 Molecular weight entrainment, chart, 360 Performance, 358, 370, 375 Relative comparison, 357... 

Instead of rotary pumps, large water jet, steam ejector, or water ring pumps can be used. For batch evacuation, and the production of hydrocarbon-free fore vacuum for sputter-ion pumps, adsorption pumps (see Section 2.1.8.1) are suitable. If the use of oil-sealed rotary vane pumps cannot be avoided, basically two-stage rotary vane pumps should be used. The small amount of oil vapor that backstreams out of the Inlet ports of these pumps can be almost completely removed by a sorption trap (see Section 2.1.4) Inserted In the pumping line. 

Steam jet ejectors are used primarily to evacuate equipment but also as pumps or compressors. They are discussed in Section 7.7. 

A four-stage ejector is to evacuate a system to 0.3Torr. The compression ratio in each stage will be... 

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. 

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