Steam Jet Vacuum Systems

Figure 2-44. Friction factor for streamlined flow of air at absolute pressures from 50 microns Hg. to 1mm Hg. By permission, Standards for Steam Jet Ejectors, 3rd. Ed., Heat Exchange Institute, 1956 [54] and Standards for Steam Jet Vacuum Systems, 4th Ed., 1988. Note f on same basis as Figure 2-3 [58].

Standards for Steam Jet Ejectors, 3rd ed.. Heat Exchange Institute, 1956, and Standards Jor Steam Jet Vacuum Systems, 4th ed., 1988, Cleveland, Ohio. 

Note that in Figure 6-8 and Table 6-2 the letter designations of the stages conform to the latest Standards of the Heal Exchange Institute for Steam Jet Vacuum Systems [11]. The letter designates the jet s stage position in the system. 

HEI (2007) Standards for Steam Jet Vacuum Systems, 6th edn, Cleveland, Ohio. 

Figure 23-21 Controlling pressure in steam jet vacuum systems.

Figure 23-26 Economic comparison of mechanical and steam jet vacuum systems at two electricity costs.

Heuristic 46 For pressures down to 10 torr and gas flow rates up to 10,000ft /min at the inlet to the vacuum system, use a liquid-ring vacuum pump. For pressures down to 2 torr and gas flow rates up to 1,000,000 ft /min at the inlet to the vacuum system, use a steam-jet ejector system (one-stage for 100 to 760 torr, two-stage for 15 to 100 torr, and three-stage for 2 to 15 torr). Include a direct-contact condenser between stages. 

Heuristic 47 For a three-stage steam-jet ejector system used to achieve a vacuum of 2 torr, 100 pounds of 100 psig steam per pound of gas are required. 

Steam-Jet Systems. Low pressure water vapor can be compressed by high pressure steam in a steam jet. In this way, a vacuum can be created over water with resultant evaporation and cooling water, therefore, serves as a refrigerant. This method frequently is used where moderate cooling (down to 2°C) is needed. The process is inefficient and usually is economically justified only when waste steam is available for the motive fluid in the steam jet. 

Uses of Ejectors For the operating range of steam-jet ejectors in vacuum applications, see the subsection Vacuum Systems. ... 

A vacuum condenser has vacuum equipment (such as steam jets) pulling the noncondensibles out of the cold end of the unit. A system handling flammable substances has a control valve between the condenser and Jets (an air bleed is used to control nonflammable systems). The control method involves derating part of the tube surface by blajiketing it with noncondensibles that exhibit poor... 

Cracking imposes an additional penalty in a vacuum unit in that it forms gas which cannot be condensed at the low pressures employed. This gas must be vented by compressing it to atmospheric pressure. This is accomplished by means of steam jet ejectors. Ideally, it would be possible to operate a vacuum pipe still without ejectors, with the overhead vapors composed only of steam. In practice, however, leakage of air into the system and the minor cracking which occurs make it necessary to provide a means of removing non-condensibles from the system. In addition to the distillation of atmospheric residuum, the lube vacuum pipe still is also used for rerunning of off specification lube distillates. 

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. 

Vacuum capacities and operating ranges, table, 344, 355 Ejectors, 344, 357 Integrated systems, 344 Liquid ring pumps, 344 Rotary lobe blowers, 344 Rotary piston pumps, 344 Rotary vane pumps, 344 Vacuum equipment, 343 Applications diagram, 352 ASME Code, 344 Pumps, 382 Steam jets, 357 Vacuum flow,... 

Other pieces may have to be elevated to enable the system to operate. A steam jet ejector with an intercondenser that is used to produce a vacuum must be located above a 34 ft (10 m) barometric leg. Condensate receivers and holding tanks frequently must be located high enough to provide an adequate net positive suction head (NPSH) for the pump below. For many pumps an NPSH of at least 14 ft (4.2 m) H2O is desirable. Others can operate when the NPSH is only 6 ft (2 m) H2O. See Chapter 8 for a method of calculating NPSH. 

Barometric condenser systems can be a major source of contamination in plant effluents and can cause a particularly difficult problem by producing a high-volume, dilute waste stream [8]. Water reduction can be achieved by replacing barometric condensers with surface condensers. Vacuum pumps can replace steam jet eductors. Reboilers can be used instead of live steam reactor and floor washwater, surface runoff, scrubber effluents, and vacuum seal water can be reused. 

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