Page 154 - Fluid mechanics, heat transfer, and mass transfer

132 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

the idler. The idler gear rotates on a stationary pin and since piping modifications to the existing setups
operates inside the rotor gear. As the two gears come are very costly.
out of mesh, they create voids into which the liquid ➢ Bidirectional: By simply reversing the direction
flows. of motor rotation, many PD pumps will pump in
& When the gears come back into mesh, there will be reverse, which can be advantageous in many
areductionin voidspaces and liquid is forced processes. Centrifugal pumps can pump in only
out of the discharge port. A crescent is formed one direction. In some installations, two centrif-
between the two gears that functions as a seal ugal pumps are used: one for loading, and another
between the suction and the discharge by trapping for unloading, which doubles the piping runs,
the liquid carried between the teeth of the rotor and valves, and auxiliaries. A single rotary pump
the idler. woulddothe same job.(Note: Relief valves
& Internal gear pumps are effective with viscous li- should be installed in both directions in such
quids, but do not perform well for liquids containing cases.)
solid particles. ➢ Flow maintainability: PD pumps produce almost
constant flow, regardless of properties of the fluid
External Gear Pumps: and conditions (viscosity, pressure, and tempera-
& External gear pumps have a similar pumping action ture). For centrifugal pumps, a change in fluid
to internal gear pumps in that two gears come intoand properties and external conditions would result in
out of mesh to give rise to flow. The external gear a definite change in performance.
pumps have two identical gears rotating against each ➢ Metering capability: PD pumps can be used as
other. Each gear is supported by a shaft with bearings convenient and simple metering devices. Centrif-
on both sides of each gear. ugal pumps have no such capabilities.
& External gear pumps work well in high-pressure ➢ Inventory reduction: Since PD pumps can pump
applications, such as hydraulics, but are not effective a wide variety of fluids in an extreme range of
in applications requiring critical suction conditions. viscosities, the same pump parts inventory is
. What are the advantages of rotary pumps? required for a wide range of applications through-
out the plant. Centrifugal pumps require a greater
& The advantages of rotary pumps are the following:
➢ Efficiency: 20–50% (depending on pressure) multitude of sizes for different applications, which
results in increased inventory of parts.
higher efficiency for most typically pumped
liquids, with high viscosities. . What are the salient features of a rotary screw pump?
➢ Viscous liquids handling: Above approximately & Rotary screw pump makes use of two long helical
300–1000 SSU, a centrifugal pump simply cannot rotors in parallel, which rotate in opposite directions.
be used, as viscous drag reduces efficiency to The rotors do not touch each other.
nearly zero. Positive displacement pumps continue & Helical timing gears are used to synchronize the
to pump at high efficiency, with no problems. rotation.
➢ Pressure versatility: Practically unaffected by & When used for moving gases as a vacuum pump, it
pressure, within a wide range. moves the gas axially along the screw without any
➢ Self-priming: A typical ANSI-dimensioned cen- compression from suction to discharge.
trifugal pump that is commonly found in many ➢ Pockets of gas are trapped within the convolutions
chemical plants cannot lift liquid. A standard of the rotors and the casing and transported to the
positive displacement pump, such as a gear pump, discharge.
can easily lift liquid in the range of 0.3–7 m. ➢ At least three convoluted gas pockets in the
➢ Centrifugal pumps can be made self-priming, by rotor are required to achieve acceptable vacuum
adding a priming fluid chamber in the inlet, how- levels.
ever, this adds expense. ➢ Modern designs use a variable pitch (number of
➢ Inlet piping: Centrifugal pumps are extremely convolutions per unit length), which essentially
sensitive to inlet piping details. Improper piping consists of two individual short rotors, each with a
may cause an increased NPSH R , cavitation, high different pitch, connected in series.
vibrations and possible damage to seals and ➢ The gas at the inlet is first transported by the lower
bearings. PD pumps are less sensitive to inlet pitch portion of the rotors and then by the higher
piping and can be a real solution for many pitch portion, which results in internal compres-
difficult installations with space constraints, sion of the trapped gas.