Page 156 - Fluid mechanics, heat transfer, and mass transfer

134 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

pumps are the exception to this, due to their very tight & Ideally, a metering pump should be capable of han-
clearances. They maintain flow, with low slip). dling a wide range of liquids, including those that are
& There are also some exceptions to this rule. Gear toxic, corrosive, hazardous, volatile, and abrasive.
pumps are sometimes made with pressure loaded & Should be capable of generating sufficient pressure to
end plates to minimize lateral clearance to the permit injection of liquids into processes.
thickness of the liquid film. However, their radial & Reciprocating simplex plunger pumps with variable
clearances (between gears and case) cannot be speed drive or a stroke-adjusting mechanism are
made too tight in order to avoid metal-to-metal provided to vary flow as per requirement.
contact, unless differential pressures are low. Screw & Designs include the following:
pumps are available for low-viscosity liquids: two-
➢ Packed plunger: Reciprocating plunger or pis-
screw designs, supported externally, and driven by
ton moves fluid through inlet and outlet check
the timing gears, can have very tight radial clear-
valves.
ances and maintain low slip even at reasonably
➢ Diaphragm: Uses a flexible diaphragm to move
high pressures. The shafts must be oversized in
fluid through inlet and outlet check valves.
such cases; otherwise, the long span of the rotor
➢ Gear pumps: Use the spacing between gear sets to
between bearings would cause sagging deflections
move fluid.
and potential contact between the rotors and the
casing. ➢ Peristaltic tubing pumps: Use rollers to move fluid
through flexible tubing.
. What are the considerations involved in pumps for food
applications? ➢ Rotating/reciprocating: Valveless pump uses only
one moving part, a rotating reciprocating piston,
& Specific standards and specifications govern pumps
both to move fluid and to accomplish valving
for the food industry.
functions.
& Allowable materials of construction include stainless
. What are the applications of metering pumps?
steel, certain plastics, and certain grades of carbon.
Another specification covers many design issues & Metering pumps are used in virtually every segment
such as self-cleaning capabilities, absence of internal of industry to inject, transfer, dispense, or proportion
crevices where bacteria may spread, and seals and fluids. The chemical process industries have by far
seal chamber dimensions. the greatest diversity of applications.
& Lobe pumps are often used in these applications. & In addition to a broad range of industrial applications,
Lobes do not contact (in theory) and are driven by the metering pumps and dispensers are used extensively
external timing gears. Absence of contact prevents in laboratory, analytical instrumentation, and auto-
any contaminant passing through the pump. mated medical diagnostic equipment:
& Shafts are robust, as is the rest of the pump, to ensure ➢ Industrial applications cover industries such as
low deflections of rotors, resistance to piping loads, chemical, petroleum, electronics, water and
and so on. wastewater treatment, food processing, agricul-
ture, metal finishing, aerospace, automotive and
. What is a proportioning/metering pump?
mining.
& Proportionating/metering pump is a controlled vol-
➢ Analytical instrumentation includes chemical ana-
ume, positive displacement pump, which will accu-
lyzers and environmental monitors.
rately transfer a predetermined volume of fluid (liq-
uid, gas, or slurry) at a specified amount or rate into a ➢ Medical applications are in the areas of diagnostic
process or system. systems, dialysis, disposable component assem-
bly, and so on.
& These pumps are capable of both continuous flow
metering and dispensing. . What are the considerations involved in the selection of
metering pumps?
& Flow rates are predetermined accurately and are
repeatable within 1% (within the specified range & Volume: Flow rate or volume per dispense.
and turndown ratio). & Pressure: Operating pressure, system pressure, and
& Adjustablevolume control is typically inherent in the differential pressure (pressure across the inlet and the
design. discharge ports).
& Traditionally, they were reciprocating (back and & Accuracy: Accuracy and precision (repeatability).
forth action) designs although rotary configurations & Temperature: Process temperature, ambient temper-
(gear and peristaltic) are increasingly used for meter- ature, and nonoperating temperature (component
ing applications. sterilization).