Page 172 - Fluid mechanics, heat transfer, and mass transfer

150 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

systems. However, ejectors designed for a vacuum of vapor to be condensed, then discharged to atmo-
5 mmHg abs or less should have steam that is sphere through a barometric leg or tailpipe of suffi-
completely dry or with a few degrees of superheat. cient length to overcome the atmospheric pressure.
. “Use of higher steam pressures increases steam con- & A means of cleaning up or otherwise disposing the
sumption in an ejector.” True/False? water that has become contaminated by process
& False. Results in decreased steam consumption in material is often required.
single or two-stage ejectors. Decrease in steam con- & In direct contact condensers, high or low water flow
sumption is negligible if steam pressures are more rates can cause problems in the vacuum system. High
than 24 barg (>350 psig). water flow could flood the condenser, increasing
. “Ejectors are very sensitive to lower than design steam pressure drop, and therefore, the back pressure on
pressures.” True/False? Comment. the stage discharging into it. Low water flow may not
be distributed properly, allowing condensable load to
& True. Even a small decrease in steam pressures (by
bypass into the following ejector and resulting in a
few units) from design values lead to improper
poor vacuum.
operation.
& The surface contact condenser permits main con-
. How can one detect steam used in an ejector is wet and
denser cooling water to be used as cooling water
what are the remedies to prevent wet steam entering an
through inter- and after-condensers, for energy and
ejector?
process-water conservation.
& Fluttering of the needle on a steam pressure gauge
. How does a steam jet water chiller work?
during operation is a sign that steam flowing is wet.
& Steam jet water chiller works on the principle of
& Testing the steam with a throttling calorimeter, which
evaporative cooling.
is a constant enthalpy device that measures steam
pressure and temperature. When used in combination & To cool water quickly (chilling) evaporation process
with a Mollier Chart, a reading of steam quality is must be performed under vacuum.
obtained. & The process is generally completed in a multistage
& Maintaining few degrees of superheat in the steam ejector system, which normally involves two to four
supply. chilling stages.
& Installing a steam separator in the steam line as close & A typical chiller employs several booster ejectors,
to the ejector as possible. one or more per chilling stage.
& Keeping all steam piping, and the steam separator, & The booster section pulls a vacuum, evaporating
completely insulated to help prevent the formation of water and entraining dissolved noncondensables and
wet steam. leakage air and then discharging into a condenser,
which while operating under vacuum, condenses
. What are the effects of changes in process loads on the
motive steam and water vapor, using cooling water.
performance of ejectors?
. What is compression ratio for a steam jet ejector? What
& A change in process load will have a direct effect on
is the range of compression ratios for these units?
the ejector system. Any increase in load will result in
a higher absolute pressure. & Compression ratio of an ejector is defined as the ratio
of (discharge pressure of the jet)/(suction pressure).
& Any increased noncondensables will travel through
the system, affecting the following ejector stages. & Compression ratios are normally in the range of
Discharge pressures of each stage will increase to the 3:1–4:1. Some times, these values can be as high
point of a breakdown in operation. as 8:1.
. What is the effect of rise in temperature of cooling water & Serious problems can result in the operation of
used in condensing-type ejectors? a steam jet ejector if the compression ratio falls
below 2:1.
& When inlet water temperature increases above design
maximum, loads to the following stage increase, . What is overall compression ratio for a multistage
resulting in a poorer vacuum at that stage. If the ejector?
affected stage is the last stage, then thevacuums of all & Overall compression ratio is the product of compres-
the preceding stages could also be affected. sion ratios of individual stages working in series.
. Evaluate direct and indirect contact condensers in a . What are the tolerable operational limits with respect to
condensing ejector system. deviations from design capacities of steam jet ejectors?
& In direct contact (countercurrent, barometric design) & A reasonable range of operation for good efficiency
condensers, cooling water is mixed directly with the would be from 50% to 115% of the design capacity.