Page 402 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

370 Applied Process Design for Chemical and Petrochemical Plants

pounds per hour) which will enter the suction inlet of the steam jet refrigeration systems, degassing of liquids, high
ejector. It consists of the following vacuum distillation, evaporation, vacuum cooling and vac-
uum drying, or other systems where large volumes of con-
1. Air leakage from surrounding atmosphere. densable materials are to be removed at high vacuum. Fig-
2. Non-condensable gases released from gases original- ure 6-23 illustrates one application.
ly injected into the process for purge, products of
reaction, etc. Evacuation Ejector
3. Non-condensable gases, usually air, released from
direct contact water injection. An evacuation booster or "hogging" ejector is some-
4. Condensable vapors saturating the non-condensables. times used to remove air from a system on start-ups. Its
capacity is set to bring the system pressure down to near
Reasonable factors of safety should be applied to the operating conditions before the continuous operating
various loads in order to insure adequate capacity. Excess ejector system takes over. Figure 6-23 illustrates the instal-
ejector capacity can be handled by pressure control and lation of such a unit.
some adjustment in steam flow and pressure, but insuffi-
cient capacity may require ejector replacement. Factors of When an extra jet for this purpose is not desirable, the
2.0 to 3.0 are not uncommon, depending upon the par- secondary jet of a multiple system is often sized to have
ticular type of system and knowledge of similar system sufficient air removal capacity to pump down the system
operations. in a reasonable time.

Capacities of Ejector in Multistage System Load Variation

When the ejector system consists of one or more ejec- Figure 6-24 illustrates three different multistage ejector
tors and intercondensers in series, the volume as pounds designs, A, B, and C, which indicate that design A is quite
per hour of mixture to each succeeding stage must be eval- sensitive to changes in load above the design point.
uated at conditions existing at its suction. Thus, the sec- Designs B or C are less sensitive. The curve extended
ond stage unit after a first stage barometric intercon- toward point D indicates the capacity of the primary or
denser, handles all of the non-condensables of the system first stage when all the vapor is condensed in the inter-
plus the released air from the water injected into the inter- condenser; or if handling air or an air-vapor mixture, the
condenser, plus any condensable vapors not condensed in performance when the secondary jets have sufficient
the condenser at its temperature and pressure. Normally capacity to take all the non-condensables.
the condensable material will be removed at this point. If
the intercondenser is a surface unit, there will not be any The curve labeled A indicates performance at overload
air released to the system from the cooling water. when the air-handling capacity of the secondary stage is
limited. This condition arises as a result of design for
steam economy. If the capacity of the secondary jets is
Booster Ejector
larger, the performance along curve B or C can be expect-
ed. When the secondary jet capacity is limited as curves A,
Booster ejectors are designed to handle large volumes
of condensable vapors at vacuums higher than that B, or C indicate, a capacity increase brings a rise in suc-
obtainable with standard condensers using cooling water tion pressure when the load increase is mainly air or non-
at the maximum available temperature. They are usually condensables. The increase in pressure is less when the
used with a barometric (or surface) condenser and stan- load increase is due to condensables. This emphasizes the
dard two-stage ejectors. The booster picks up vapors from importance in sizing the secondary jets for ample non-
the process system at high vacuum (low absolute pressure, condensable capacity, and the importance of specifying
around 0.5 in. Hg abs) and discharges them together with the range and variety of expected conditions which may
its own motivating steam to a lower vacuum condition confront the system.
(compresses the mixture) where the condensable vapors Once a system has been evacuated to normal operating
can be removed at the temperature of the condenser conditions, it is possible for capacity to fall to almost zero
water. The non-condensable vapors leave the condenser, when the only requirement is air inleakage or small quan-
passing to the two-stage ejector system. This overall system tities of dissolved gases. Under these conditions, it is
allows a constant vacuum to be maintained in the process, important to specify an ejector system capable of stable
unaffected by the temperature of the cooling water. operation down to zero load or "shut-off' capacity. The
Booster ejectors are used with barometric and surface curve of Figure 6-24 represents such a system.