Page 166 - Fluid mechanics, heat transfer, and mass transfer

144 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

FIGURE 5.32 Rotary claw pump.

. Describe theworking of rotary screw pumps for vacuum & Thesepumpsdonotrequireaworkingfluidtoproduce
service? vacuum, so nothing contacts the load being pumped.
& Two long helical rotors in parallel rotate in opposite & Noncontact design facilitated by timing gears.
directions without touching, synchronized by helical & High rotational speed reduces the ratio of gas slip to
timing gears. displacement, increases net pumping capacity and
& Gas flow moves axially along the screw without any reduces ultimate pressure.
internal compression from suction to discharge. & No contamination of evacuated gas.
& Pockets of gas are trapped within the convolutions of & Can discharge to the atmosphere.
the rotors and the casing and transported to the & Multiple staging provides inlet pressures below
discharge.
1 mmHg abs while discharging to atmosphere.
& Compression occurs at the discharge port, where the
& May discharge gases at high temperatures; in some
trapped gas must be discharged against atmospheric
situations, as high as 300 C or more. Liquid in ring
pressure.
pumps acts as a cooling and flushing medium for
& Each convolution of the rotor acts similarly to a stage cleaning the pump internals of process material.
in series with the one behind it.
Disadvantages
& At least three convoluted gas pockets in the rotor are
& Dry pumps have difficulty in handling slugs of
required to achieve acceptable vacuum levels.
liquids. Knockout pots are generally required. A
& The rotary screw type vacuum pump can give
liquid ring pump is the only type of vacuum pump
extremely high vacuum levels in the range of
that can safely handle slugs of liquids.
10–100 mm (low absolute pressures).
& Cannot handle particulates. The tight clearances
& These pumpsmovegasalongthelengthofthescrews.
make some of the dry pumps sensitive to particulate
Gas compression and mechanical compression does
buildup.
not occur until the final half-turn of the screws.
& Inlet filters and knockout pots, along with discharge
. What are the advantages and the disadvantages of dry
silencers are normally required for a complete op-
vacuum pumps?
erating system.
Advantages & Eliminating a liquid within the pumping chamber
& Rugged rotor design, constructed of sturdy cast or also eliminates a method of sealing between the
ductile iron without any flimsy rotating components. pump clearances.
Due to lack of condensation, pump can be fabricated & Due to the high operating temperature, some process
of standard, inexpensive cast iron. gases may have a tendency to polymerize.
& Dry pumps are compact and energy efficient, and do & Internal clearances are to be kept tight, as tight as
not contribute to air pollution, a problem with oil- 0.1 mm to reduce gas slippage, allowing for thermal
sealed pumps, or water pollution, a problem with expansion of the rotors.
steam jet ejectors and water-sealed liquid ring & May require an inert gas purge for cooling or to
pumps. protect the bearings and seals from the process gas.