Vacuum Equipment

A variety of chemical processes operate at pressures below atmospheric. Most (vacuum distillation, evaporation, drying) require rough vacuums down to 1 mmHg. Freeze drying typically requires more vacuum, and some electronics processing requires very high vacuums (on the order of 10-7 mmHg.

A variety of equipment is available to supply these vacuum needs:

This list is arranged in order of increasing efficiency and cost (ejectors are cheapest, but least efficient). Ejectors are probably preferred for systems that are corrosive, contain entrained solids, or are prone to slugs of liquid; ring pumps are best for high discharge pressures and pumping condensibles; and mechanical pumps for surges of noncondensibles.

Vacuum Ejectors

Steam jet ejector Steam jet ejectors are often used to pull vacuum on surface condensers, evaporators, etc. A high pressure, motive, fluid (usually steam) enters the ejector chest through a nozzle and then expands. This converts its pressure energy to velocity. The increased velocity causes reduced pressure, which sucks in and entrains gas from the suction. The diffuser section then recompresses the mixed steam/gas stream to some intermediate pressure. The exhaust is then sent to a condenser which quickly condenses the steam at a low pressure and temperature so that the volume quickly decreases.

Ejector systems have no moving parts; thus, they are designed for optimum performance at a single set of conditions.

A key performance measure is the compression ratio: the ratio of the discharge pressure to the suction pressure (note that the pressure of the motive steam is not included). A single ejector stage can achieve compression ratios up to 8:1, although values in the 3:1 to 5:1 range are more typical. The discharge pressure is set by the condenser pressure -- minimum pressure is the condensing pressure of steam at the vapor outlet temperature.

3 stage ejector system Compression ratios can be increased by using several stages. In this arrangement, vacuum is pulled on each condenser by a second ejector. This results in a lower vacuum on the process.
Number of Stages Suction Pressure (lowest)
1 75 mmHg
2 12 mmHg
3 1 mmHg
4 0.2 mmHg
5 0.02 mmHg
6 .002 mmHg
The average compression ratio for a system is best approximated as the overall compression ratio to the 1/NS power (NS is the number of stages).

References:

  1. Croll, S.W., "Properly Speecify Vacuum Systems", Chemical Engineering Progress, 92(1): 48-49, January 1996.
  2. N.P. Lieberman and E.T. Lieberman, A Working Guide to Process Equipment, McGraw-Hill, 1997, pp. 185-191.
  3. McCabe, W.L., J.C. Smith, P. Harriott, Unit Operations of Chemical Engineering, 5th Edition, McGraw-Hill, 1993, pp. 212-213.
  4. Ryans, J.L and S. Croll, "Selecting Vacuum Systems", Chemical Engineering, December 14, 1981, pp. 72-90.

R.M. Price
Original: 4/3/98
Revised:

Copyright 1998 by R.M. Price -- All Rights Reserved

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