Page 165 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 143

& Ability to handle small liquid streams along with the & Clearances between the rotors and the casing are
gas from the process or precondensers. generally 0.1–0.5 mm (0.004–0.02 in.).
& No damage from liquid or small particulates in the & Back leakages across these clearances, which reduce
process fluid. pump capacity, increase with increase in DP between
& High efficiency. intake and exhaust.
& Inherently slow rotational speed (1800 rpm or less), & Therefore, these machines are limited for use across
which maximizes operating life. relatively small DP.
& Very small increase in gas outlet temperature. This is . What are the plus points of using dry vacuum pumps?
an advantage while handling flammable or temper- & Dry pumps are compact and energy efficient and do
ature-sensitive process fluids. not contribute to air pollution, a problem with oil-
& Minimal noise, vibration, and maintenance. Rebuild- sealed pumps, or water pollution, a problem with
ing is simple. steam jets and water-sealed liquid ring pumps.
& Can be fabricated from any castable metal. & Dry pumps are unique among vacuum pumps, be-
cause they do not require a working fluid to produce
. What are the disadvantages of liquid ring vacuum
vacuum, so nothing contacts the load being pumped.
pumps?
& Solvents or products aspirated from the process can
& Mixing of the evacuated gas with the sealant liquid.
be discharged to an aftercondenser. Contamination is
& Risk of cavitation, which requires that portion of the
not a concern and the condensate can be recycled
process load be noncondensable under the pump
directly to the process.
operating conditions.
& For these reasons, dry vacuum pumps have found
& High power requirement to form and maintain the
applications in semiconductor industry. The dry
liquid ring, resulting in larger motors than for other
pumps for the semiconductor industry are medium
types of pumps.
vacuum pumps.
& Achievable vacuum is limited by the vapor pressure
& Dry vacuum pumps are generally rotary lobe Roots
of sealant liquid at the operating temperature.
blower type. Roots blowers have limited application
& All liquid ring pumps must copewith cavitationwhen
as process vacuum pumps discharging against high
operating at low inlet pressures. Sealant liquid, seal-
pressure differentials to the atmosphere, but they are
ant temperature, impeller rpm, blade angle, and inlet
used extensively as vacuum boosters in the
pressure influence cavitation.
0.001–50 Torr range.
. What are the typical applications of liquid ring pumps?
& The potential for eliminating process contamination
& Used for vacuum distillation, vacuum drying, and
is the main reason for specifying dry vacuum pumps
evaporator service because the pump handles non- for fine chemicals and good manufacturing practices
condensables saturated with process vapors. plants.
. What is a dry vacuum pump? Name some dry vacuum . What is a rotary claw vacuum pump? Illustrate.
pumps used in industry. & Rotary claw: The geometric shape of this pump
& It is a positive displacement vacuum pump that
allows for a greater compression ratio across the
discharges continuously to atmospheric pressure and rotors at higher pressures.
in which the swept volume is free of lubricants or & Two claw rotors rotate in opposite directions of
sealing liquids.
rotation without touching, using timing gears to
& Rotary claw, rotary lobe, and rotary screw pumps are
synchronize the rotation.
examples of dry pumps in the CPI, particularly in & The gas enters through an inlet port opened to the gas
larger size pump applications.
during its rotation and fills the pump housing.
. What is the operating principle of dry vacuum pumps
& On the next rotation, the same trapped gas is com-
developed from rotary lobe Roots blower?
pressed and discharged as the discharge port opens.
& Rotary lobe Roots blower is a positive displacement
& Figure 5.32 illustrates rotary claw vacuum pump.
machine that normally operates as a dry compressor.
& A minimum of three stages in series is required to
& Two interlocking rotors on two parallel shafts syn-
achieve pressures comparable to those of an oil-
chronized by timing gears and rotating in opposite
sealed mechanical pump.
directions trap and transport gases.
& Some dry designs use two technologies in combina-
& Gears and bearings are oil lubricated, but are external
tion, for example, a rotary lobe as a booster for a claw
to the pump; the rotors run dry.
pump.