Centrifugal Pumping

Microfluidic Lab-on-a-chip for Chemical/Biological Analysis and Discovery 

FIGURE 3.3 Cross-sectional images of liquid column in (a) hydrophilic channel and (b) hydrophobic PDMS channel. Water containing fluorescein was used as a sample liquid in confocal microscopic imaging [382]. Reprinted with permission from Springer Science and Business Media. 

Besides pumping, centripetal acceleration is created. A maximum fluid rotational velocity of up to 12 m/s, and a corresponding radial acceleration in excess of 106 g have been produced within a diamond-shaped microchamber (55 x 55 im). This notch chamber was constmcted along the side wall of an otherwise straight channel (30 pm wide, 30 pm deep) which was fabricated on a PDMS chip. This microstructure caused flow detachment at the opening of the notch, leading to recirculating flow of microvortex inside the notch [384]. 

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The author employs the Stuart centrifugal pump. No. 10, suoplied by Stuart Turner Ltd. 

Carbon nd Graphite. Carbon and graphite rendered impervious with 10—15% phenoHc, epoxy, or furan resia are among the most important materials for hydrochloric acid service up to 170°C. The most important appHcations of these materials for hydrochloric acid service are heat exchangers and centrifugal pumps. 

Example 3. A centrifugal pump moving a corrosive Hquid is known to have a time-to-failure that is well approximated by a normal distribution with a mean of 1400 h and a standard deviation of 120 h. A particular pump has been in operation for 1080 h. In order to plan maintenance activities the chances of the pump surviving the next 48 h must be deterrnined. 

The ease with which the separated products leave the bowl determines the richness of the fat. Fluid whole milk enters the separator under pressure from a positive displacement pump or centrifugal pump with flow control (Fig. 1). The fat (cream) is separated and moves toward the center of the bowl, while the skimmed milk passes to the outer space. There are two spouts or oudets, one for cream and one for skimmed milk. Cream leaves the center of the bowl with the percentage of fat ( 30 40%) controlled by the adjustment of a valve, called a cream or skim milk screw, that controls the flow of the product leaving the field of centrifugal force and thus affects the separation. 

Positive Pumps. Positive pumps employed by the food industry have a rotating cavity between two lobes, two gears that rotate in opposite directions, or a crescent or stationary cavity and a rotor. Rotary positive pumps operate at relatively low speed. Fluid enters the cavity by gravity flow or from a centrifugal pump. The positive pump also may use a reciprocating cavity, and may be a plunger or piston pump. These pumps are not truly positive with respect to displacement, but are used for metering product flow. 

Most centrifugal pumps have end-suction and top-discharge no22les, but both top-suction and top-discharge designs are not uncommon. 

Efforts to develop an artificial heart have resulted in a number of advancements in the assist area. The centrifugal pump for open-heart surgery, the product of such an effort, has frequently been used to support patients after heart surgery (post-cardiotomy), or as a bridge to life prior to transplant. 

The pump-mounting feet are located close to the centerline. As a result, the pump thermal growth is uniform on both sides of the centerline, leading to minimal distortions. The design is governed by the American Petroleum Institute (API) standard 610, "Centrifugal Pumps for General Refinery Service" (30). 

Disk Pumps. When pumping shear-sensitive or highly viscous fluids, it is desirable to reduce internal turbulence caused by the vanes. The disk pump design rehes on the centrifugal frictional effect of a vaneless disk. Whereas the efficiency of this pump is lower than that of similar centrifugal pumps having vanes, it is often the only solution to certain pumping appHcations. 

Positive Displacement Pumps. Positive displacement pumps foUow HI convention (see Fig. 1). As a rule, these pumps work against significantly higher pressures and lower flows than do kinetic, particularly centrifugal, pumps. Positive displacement pumps also operate at lower rotational speeds. There are many types of positive displacement pumps, for which designs are constantly being developed. Some of these are discussed herein. 

From the definition of specific speed (eqs. 9 and 10), it follows that reciprocating pumps operate at high pressures and low flow rates. Conversely, centrifugal pumps are appHed at lower pressures and higher flow rates. Many rotary pumps are selected for viscous Hquids having pressures equal to or less than, and capacities lower than, centrifugal pumps. However, these limits are relative and a gray area exists as some pump types cross boundaries into the domain of other types. 

NonmetalHc centrifugal pumps utilize polymers for all components coming into contact with the pumping media. Even the steel (qv) shaft is sleeved with plastic to isolate the shaft from the fluid. Mechanical seals are reversed so that the metal face does not come into contact with the corrosive... 

H. H. Anderson, Centrifugal Pumps, 3rd ed.. Trade Technical Press Ltd., London, 1980. 

Specification for Hori2ontal End Suction Centrifugal Pumps for Chemical Process," ASME B73.1M, Washington, D.C., 1991. 

D. J. MMethodfor Estimating the Net Positive Suction Head Required by Centrifugal Pumps, ASME 81-WA/EE-32, Washington, D.C., 1981. 

U. BoUeter and A. Erei, Proceedings of S eminar on Vibrations in Centrifugal Pumps, London, U.K., 1990. 

R. Hart, Best Practice Centrifugal Pumps. NPSH Definitions and Specifications, Du Pont Internal Specification, Wilmington, Del., May 18,1992. 

T. Dahl and L. Nelik, Computer Aided Design of a Centrifugal Pump Impeller, ASME 85-WA/EE-lO, Washington, D.C., 1985. 

Piping and pumps used for transfer of the butanols can be made of the same metal as tanks. Centrifugal pumps with explosion-proof electric motor drives are recommended (34). 

Impervious graphite centrifugal pumps, pipe fittings, and valves were developed because most chemical processes require the movement of Hquids. Graphite pipe and fittings in sizes ranging from 25 to 635 mm ID are used to convey corrosive fluids. 

The centrifugal pump directly driven by a variable-speed electric motor is the most commonly used hardware comoination for adjustable speed pumping. The motor is operated by an electronic-motor speed controller whose function is to generate the voltage or current waveform required by the motor to make the speed of the motor track the input command input signal from the process controller. 

Unhke the control valve, the centrifugal pump has poor or nonexistent shutoff capabihty. A flow check valve or an automated on/off valve may be reqiiired to achieve shutoff reqiiirements. This requirement may be met by automating an existing isolation valve in retrofit apphcations. 

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