Page 168 - Fluid mechanics, heat transfer, and mass transfer

146 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

& Steam vacuum refrigeration systems produce expansion occurs across a converging–diverging
chilled water in the range of 415 C for cooling nozzle (Figure 5.32).

process heat exchangers, air conditioning and many & This results in supersonic velocityoff the motivefluid
other applications where chilled water is required. nozzle, typically in the range of Mach 3–4.
Flash cooling and concentration of liquid solutions
& This creates a low-pressure zone for pulling the gases
are other applications for the steam vacuum refrig-
from the process equipment into the ejector. High-
eration system.
velocity motive fluid entrains and mixes with the
& Steam jet ejectors can be designed to pump liquids
suction gas.
and even finely divided solids for pneumatic con- & The mixture passes through the diffuser (venturi), in
veying systems. In the latter service, they are used to
the converging section of which kinetic energy (ve-
transfer fluidized catalyst.
locity) is converted to pressure as cross-sectional
& Liquid jet ejectors can be used for applications
flow area is reduced. During the mixing, much of
involving agitation, mixing or metering liquid solu- the kinetic energy of the motive steam is converted
tions. Water operated ejectors are used for fume into heat energy. At the throat section,a shock wave is
scrubbing in which largevolumes of gases and vapors established, boosting pressure energy at the expense
can be entrained with good water economy. In these of kinetic energy across the shock wave.
applications, obnoxious, corrosive, or poisonous & Flow across the shock wave goes from supersonic
fumes can be sucked out of an enclosed area by
ahead of the shock wave, to sonic at the shock wave
means of a simple water jet ejector.
and subsonic after the shock wave.
& Ejectors have found many applications for compres-
& In the diverging section, velocity is further reduced
sing fluids to pressures above atmospheric.
and converted into pressure.
& Steam jet thermocompressors are a very efficient
& The recompression into pressure is achieved with an
means of boosting low-pressure waste steam to a
efficiency of about 80%.
pressure where it can serve a useful purpose. High-
& Motive pressure, temperature, and quality of the
pressure motive steam that might otherwise be
motive steam are critical variables for proper ejector
throttled for use in an evaporator or heat exchanger
performance.
can be used to actuate a steam jet thermocompres-
& Calculation of a required motive nozzle throat diam-
sor. The thermocompressor can serves two pur-
eter is based on the necessary amount of motive
poses, namely, throttling the high-pressure steam
steam, its pressure, and specific volume.
to the desired pressure and compressing low-pres-
sure waste steam to a pressure where it is again & Motive steam quality is important because moisture
useful. droplets affect the amount of steam passing through
the nozzle. High-velocity liquid droplets also erode
& Steam jet air compressors are useful for supplying
ejector internals, reducing performance.
compressed air in an explosion hazardous area where
electrical equipment would have to be of explosion . What are the components that may be present in the
proof construction and relatively expensive. vapor load to an ejector?
& Compressed air at 140 kPag (20 psig) for pneumatic & Air, noncondensable gases, condensable vapors, and
controls is a typical application for steam jet air steam from distillation overheads and water vapor.
compressors. & Air leakage, which is common to subatmospheric
& Handling slurries and granular solids. systems, is usually the basic component of the load to
an ejector and the quantities of water vapor and/or
& Heating liquids by direct contact.
condensablevapor are normally directly proportional
. What is the principle of operation of an ejector? What is
to the air load.
its general name?
& Entrainment.
& An ejector uses momentum of one fluid to move
. Give approximate estimates for leakage of air and
another.
noncondensable and condensable vapors into ejectors
& Its general name is jet pump.
from process systems, to be used in ejector selection and
& The operating principle involves conversion of pres-
design.
sure energy of the motive fluid, generally steam, into
& For very tight, small process systems, an air leakage
kinetic energy (velocity). Pressure of the steam used
of 1–2.5 kg/h may be taken.
as motive fluid is in the range of 7–14 barg.
& For moderately tight, small process systems (about
& The motive steam expands adiabatically through the 3
15 m volume), air leakage may be of the order of
steam nozzle into the suction chamber. This adiabatic
4–5 kg/h may be a reasonable estimate.