Page 167 - Fluid mechanics, heat transfer, and mass transfer

EJECTORS 145

The inert gas acts as a diluent for flammable or & Can handle wet, dry, or corrosive vapor mixtures.
corrosive gases, besides helping to control internal & Can handle condensable loads.
temperatures.
& Suitable for handling practically any type of gas or
& Dry pumps are sensitive to process upsets and should vapor or for handling wet or dry mixtures or gases
be carefully selected to ensure maximum run time. containing sticky or solid matter.
& In most models, difficult to repair or rebuild. & Tolerance for entrained liquids, even in slug form and
& Expensive than either ejectors or liquid ring pumps. intermittency.
. What is a scroll pump? & Their efficiencies are reasonable to good and
& Scroll pump uses a rotating plate shaped into a spiral sustainable.
(involute curve), which moves within a second sta- & No special start-up or shutdown procedures required.
tionary plate, shaped as a similar spiral. Gas moves & Simple and easy operation. Quiet and stable opera-
within the spiral passage, from the outside of the tion within design range.
spiral to the center where it is exhausted through a & Higher reliability in severe services.
valve.
& Lowest capital cost among vacuum producing
& Multiple stages can be used to provide pressures
devices.
down to 0.01 mmHg abs, with pumping capacities & Installation costs are relatively low when compared
3
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limited to less than 1.4 m /min (50 ft /min).
to mechanical vacuum pumps.
& This type of pump is limited for applications limited
& They have no moving parts, hence, maintenance is
to clean gases because of the tortuous flow path.
low and operation is fairly constant when corrosion is
. What are the applications of dry vacuum pumps?
not a factor.
& Used when contamination of the process fluid is to be
& Long life.
avoided, when solvent recovery is the main objective
& Corrosion damage is easily repaired at low cost.
or when emissions must be kept low.
Ejectors are available in many materials of construc-
& In cases where there is risk of liquid carryover from
tion to suit process requirements.
the evacuated vessel, use of knockout pots, filters, or
& No stuffing box sealing is required.
other similar devices are required to avoid pump
& Space requirements are small.
damage.
& Can be mounted in any orientation.
& Used in semiconductor industry for etching of wafers
and chemical vapor deposition requiring pressures of . What are the disadvantages of ejectors?
0.01–1 mmHg abs.. Oil-sealed pumps require lubri- & Very low efficiency (1–20%).
cation with inert, fluorine-based fluids for protection & Efficiency is high in its application for pumping
from corrosive gases (e.g., HCl) and harsh conditions boiler feed water by steam in an injector, where heat
(condensation/precipitation of hard solids such as of steam used for pumping is added to steam space in
AlCl 3 and SiO 2 ) of fabrication. These involve high the boiler, thus recovering the heat.
costs for liquid ring pumps for such applications. & Head developed is low.
. What are the most important parameters that affect
& Dilutes fluid pumped through mixing.
vacuum pump selection?
& Requires steam at medium pressures (2–4 barg) as
& Suction pressure.
the motivating fluid.
& Capacity required for the process.
& The effluents (condensates) from the ejector are
contaminated with process fluids and require to be
treated to meet pollution control requirements.
5.2 EJECTORS & Can be noisy. May require discharge silencers or
sound insulation.
. What are the incentives for the use of ejectors?
. What are the applications of ejectors?
& They develop any reasonable vacuum needed for
& Steam jet ejectors are used on all kinds of distillation
industrial operations.
units, vacuum deaerators, evaporators, crystallizers,
& All sizes are available to match any small- or large-
oil deodorizers, steam vacuum refrigeration, flash
capacity requirements.
coolers,condensers,vacuumpandryers,dehydrators,
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& Largest throughput capacities, more than 30,000 m /
vacuum impregnators, freeze dryers, vacuum filters,
min, handled in relatively small equipment sizes. and stream degassing and vacuum melting of metals.