Page 385 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Ejectors and Mechanical Vacuum Systems 353

Al I .... 40 PSIG motive.
Terminate discharge
pipe below liquid level
or •dd a couple of
JET PUMP elbows or a swing check
valve to provide enouah
back pressure for
ejector to "seal".
OPERATING PRESSURE

VESSEL TO BE
EVACUATED
SUCTION
LIFT
j
��--�-
PrN_su,.. bet- 1 • H1. 1bsolut1 m11 Ill obt1lnld.
____
Figure 6-98. Typical Jet (gas) system application. By permission, Penberthy, Inc.

Performance Factors

Steam Pressure

The motive steam design pressure must be selected as the
lowest expected pressure at the ejector steam nozzle. The
unit will not operate stably on steam pressures below the
design pressure [16].
Recommended steam design pressure:

= minimum expected line pressure at ejector nozzle minus
10 psi

This design basis allows for stable operation under
minor pressure fluctuations.
An increase in steam pressure over design will not
Figure 6-9C. Improving the maximum vacuum obtainable by staging increase vapor handling capacity for the usual "fixed
an ejector ahead of the suction of a liquid ring pump or any other capacity" ejector. The increased pressure usually decreases
device that can handle water into the unit, unless dry air is used. By capacity due to the extra steam in the diffuser. The best
permission, Graham Manufacturing Co., Inc.
ejector steam economy is attained when the steam nozzle
and diffuser are proportioned for a specified performance
um. Determine the absolute pressures at the entrance to [8]. This is the reason it is difficult to keep so-called stan-
the ejector and at the bottom of the column. dard ejectors in stock and expect to have the equivalent of
Ejector inlet, absolute pressure = 30 - 27.5 = 2.5 in. a custom designed unit. The "throttling type" ejector has a
mercury family of performance curves depending upon the motive
Columns bottom, absolute pressure = 30 - 23 = 7 in. steam pressure. This type has a lower compression ratio
mercury across the ejector than the fixed-type. The fixed-type unit
is of the most concern in this presentation.
Pressure Drop at Low Absolute Pressures For a given ejector, an increase in steam pressure over
the design value will increase the steam flow through the
Refer to Chapter 2, Flow of Fluids nozzle in direct proportion to the increase in absolute