Page 390 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

358 Applied Process Design for Chemical and Petrochemical Plants
110 .......... ....---..----,
·-!� ......--....--5
0 c»l08t---..+---->4---+---I
....
� 106 r---+-----------t
80 Rotary piston "'
oil-sealed � 104t---+---+----+----1
o,
E
... � 102 R . . Sf p
a1smg eom r
::E
o 60 100
<(
..
af � 9ar=::==:t:::==:t:::::z::��-l
Unstable<
>
St o 96 2--+----r--+-t-T
c
't ! 94 Dropping Steam Press.
"' 40
...
:,
·[ 92 Break
Q)
20 Cl 9094 96 98 100 102
Design Capacity 1 %
Basis: 70"F sealing water
Figure 6-13. Effect of steam pressure on capacity tor constant sys-
100 150 200 250 300
PressJre, torr tem suction and back-pressure. By permission, P. Freneau, [4].
Figure 6-11C. Typical performance curves for steam jet ejectors, liq-
uid ring pumps, and rotary piston oil-sealed pumps. By permission, Suction Pressure
Ryans, J. L. and Roper, D. L. [24].
The suction pressure of an ejector is expressed in
absolute units. If it is given as inches of vacuum it must be
converted to absolute units by using the local or reference
barometer. The suction pressure follows the ejector
capacity curve, varying with the non-condensable and
vapor load lo the unit.
Discharge Pressure

-;fl. As indicated, performance of an ejector is a function of
�1401----+---4<�<\-----,l-#-,,-+ ...... -+--+---I backpressure. Most manufacturers design atmospheric
i120--t--iM---t----lir#-lr-+---Mi:--+---1 discharge ejectors for a pressure of 0.5 to 1.0 psig in order
Cl to insure proper performance. The pressure drop
(.)
..
c 100--+�-+-�t-1--N-r--+-
"'
·;;; through any discharge piping and aftercooler must be
a aO--tt--t---f-.,....,r---r--t--+---1 taken into consideration. Discharge piping should not
have pockets for condensation collection.
A= 150 lb. Pressure
60 ----+--+-tt--+-i (Design Basis) Figure 6-15 indicates the effect of increasing the single-
B =200 lb.Pressure
40 1--+-+--+-+-1t-,f--lC =250 lb. Pressure stage ejector backpressure for various suction pressures.
Figure 6-16 illustrates the effect of increasing the motive
steam pressure to overcome backpressure effects. When
this pressure cannot be increased, the nozzle may be
O I 34 5 67 8 redesigned to operate at the higher backpressure.
Absolute Pressure at Suction, In. MercurJ
Figure 6·12. Effects of excess steam pressure on ejector capacity. Capacity
By permission, C. H. Wheeler Mfg. Co.
The capacity of an ejector is expressed as pounds per
Effect of Superheated Steam hour total of non-condensable plus condensables to the
inlet flange of the unit. For multistage units, the total
A few degrees of superheat are recommended capacity must be separated into pounds per hour of con-
(5-15°F), but if superheated steam is to be used, its effect densables and non-condensables. The final stages are
must be considered in the ejector design. A high degree of only required to handle the non-condensable portion of
superheat is of no advantage because the increase in avail- the load plus the saturation moisture leaving the inter-
able energy is offset by the decrease in steam density [ 16]. condensers.