Page 173 - Fluid mechanics, heat transfer, and mass transfer

EJECTORS 151

An ejector may operate well in excess of these figures ➢ Estimate air or other gas leakage into the sys-
but the efficiency will usually decrease at points tem. Every effort should be made to keep it as
beyond these limits. tight as possible. Possible leak points can be
. Is it desirable to use higher pressure steam than the sealed with polystyrene, which produces an
design values in the operation of an ejector? excellent seal.
& No. It does not increase suction capacity. It just uses ➢ In-leakage of air to evacuated equipment de-
more steam. pends on the absolute pressure (in Torr) and the
3
volume of the equipment, V (in m ), according
& At pressures more than 25% above design pressure,
to
capacity will actually fall due to choking in the
venturi.
W ¼ kV 2=3 kg=h; ð5:26Þ
. What is the effect of using higher design steam pres-
sures in ejectors? where k ¼ 0.98, when P > 90 Torr; k ¼ 0.39,
& Decreased steam consumption (single and two-stage when P is between 0.4 and 2.67 kPa (3 and
ejectors). 20 Torr); and k ¼ 0.12 at p < 133.3 Pa (1 Torr).
& Decrease is negligible if steam pressure is >25 bar. & Blockage or Pressure Drop in the Lines
. “Ejectors are very sensitive to lower than design steam ➢ A shifted gasket can cause a pressure drop in the
pressures.” True/False? Comment. vacuum system.
& True. Even a small decrease (few millimeters of ➢ If the final atmospheric discharge piping becomes
mercury gauge pressure) leads to improperoperation. blocked, it could result in a pressure drop that
. What are motive steam pickup and break pressures for affects the entire vacuum system.
an ejector? What are their significances? & Insufficient Steam Pressure
& If the steam pressure is being increased from a region ➢ Under normal conditions, an ejector needs its full
of unstable operation, the point at which the ejector design motive pressure to operate.
first becomes stable is called the motive steam pickup ➢ In practice, an ejector works with steam that is fed
pressure. at a pressure up to 10% below the design pressure
& The pickup pressure is a direct function of the of the ejector.
discharge pressure. At the higher discharge pressure, & High-Pressure Steam
the ejector will regain its stability once the motive
➢ Running an ejector at a pressure substantially
steam pressure is increased to the pickup pressure.
above the design pressure decreases capacity,
& For everydischargepressure in an ejector there is also
mainly because of choking in the ejector
a minimum steam flow below which the operation throat.
will be unstable.
➢ It is safe to run an ejector at a pressure of about
& If the steam pressure is being decreased from a region
25% above its design pressure.
of stable operation, the point at which the ejector
➢ At higher pressures, the steam to the ejector should
becomes unstable is called its motive steam break
be throttled back to a pressure that is closure to the
pressure.
design pressure.
& The motive steam break pressure is below the motive
& Superheat in the Steam
steam pickup pressure for any given discharge pres-
➢ Superheat in the steam will increase specific vol-
sure and load. For this reason, the ejector operating
ume of steam.
with steam pressure between the break and the
pickup points may be stable or unstable depending ➢ Since an ejector nozzle has a fixed orifice, more or
on the direction of the steam pressure change. less steam will be passed, depending on the degree
of superheat.
. What are the common causes of problems arising in a
vacuum system operating with the use of ejectors? ➢ This can have the same effect as increasing or
decreasing the motive steam pressure.
& Air leakage: Air leakage often occurs through joints
➢ Too little superheat is not normally a problem,
or flanged connections, or through corroded or erod-
unless the ejector was designed for a large degree
ed parts.
of superheat.
➢ A hole measuring just a few millimeters of diam-
eter will cause sufficient air leakage, which da- & Excessive Moisture in the Steam
mages the vacuum, consequently increasing the ➢ Wet steam alters performance of the ejector. Ef-
need for ejector steam. ficiency decreases.