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PMC IMPELLER SELECTION FOR PROCESS RESULTS

For most applications, one of the patented, axial-flow Philadelphia Hydrofoil impellers will be thebest selection. For various special mixer duties, other impeller types are suitable.

In addition to impeller type based on efficiency, power number is important. Lower power numberimpellers must be larger in diameter than higher power numbers for a given motor horsepowerand impeller speed. Although low power number impellers are generally the most efficient, ifimpeller diameter exceeds 40-50% of vessel diameter, pumping capacity falls off sharply. In suchcases, usually for high-power, high-intensity mixing requirements, a higher power numberimpeller must be selected, in order to reduce impeller diameter. Finally, flow rate and directionmust be considered. For most applications, axial flow will produce the best mixing action and willresult in the highest impeller efficiency. However, occasionally radial flow is desirable, to break upplug flow, to provide high wall velocity, or to provide high shear for dispersion applications.

PHILADELPHIA MIXERS CAN HELP

Our superior staff of factory application engineers and field sales engineers can analyze any mixing requirements and make the best selection.

Sometimes a computer simulation or laboratory test is required. The Philadelphia MixersLaboratory in Palmyra, Pennsylvania is equipped with a state-of-the-art Dantec, fiber optics laser-Doppler fluid velocity measurement system, capable of a complete flow analysis of anyimpeller or combination of impellers.Further, Philadelphia Mixers specializes in small-scale testing of customerís samples, with guaranteed scale-up or results to full size. In 1968, we ran tests on a rapd-settling coal slurry in 50 gallon size and scaled it up to an8 million gallon vessel! Our scale-upcalculations were perfect, producingidentical results in 8 million gallons to those observed in our laboratory.

Regardless of the complexity of theproblem, Philadelphia Mixers has theright impeller for the job. A descriptionof our standard arsenal of impellersfollows, with application informationabout each.

IMPELLER COMBINATIONS

Multiple impellers can have interactive flow patternsthat can decrease their efficiency. Dual axial or anaxial/radial combination can produce interference flowpatterns as shown:

POWER NUMBER

The power number, Np, is defined as:

Where, P = impeller power (hp)Sg = specific gravity of fluidN = impeller speed (rpm)D = impeller diameter (ft.)

= viscosity (cp)

Power number tends to be constant for a givenimpeller for Reynoldís number in excess of 10,000.It is this constant value that is referred to as theimpellerís power number. However, power numberincreases at low Reynoldís number (small vesselsand/or high viscosity):

INTERFERNACE FLOW

COLINEAR FLOW

AXIAL FLOWImpeller Designs

LS HYDROFOIL

The Philadelphia LS (low shear) axial-flow hydrofoil isprotected by U.S. Patent No. 5,297,938. It is themost flow-efficient impeller available for low viscositymixing (up to 2,500 cp). It is the impeller of choice formost liquid blending and solids suspension applica-tions. The 3LS39 has a 0.30 power numbr and is themost-efficient version. The 4LS45 with a 0.55 powernumber is useful when the 3LS39 would have toolarge an impeller diameter.

PBT

The axial-flow Pitched Blade Turbine is more flow-effi-cient than radial-flow impellers, and produces morefluid shear than high-efficiency hydrofoil impellers. It isan economical choice when both flow velocity andfluid shear are desirable and/or when use of a lower-power number hydrofoil would result in too large animpeller diameter (e.g. high-intensity liquid blending).The 4PBT45 has a 1.62 power number. The three-blade 3PBT39 has a 1.0 power number. The 4PBT32has a 1.10 power number. The side-bladed 320 vari-ety is more efficient than 450 blade angle for high vis-cosity and solids suspension applications.

HS HYDROFOIL

The Philadelphia HS (high solidity) axial-flow hydrofoilis protected by U.S. Patent No. 5,326,226. It is characterized by a wide blade, and is the mostflow-efficient impeller available for high viscosity mix-ing (2,500 cp ñ 100,000 cp) and for high head applica-tions (e.g. gas dispersion and draft tube). The HS isthe impeller of choice for gas dispersion where bottomflow velocities are also required (e.g. simultaneousgas dispersion and solids suspension). The MHSblade is a slightly narrower version, suitable for lowergas rates and lower viscosities.

COUNTERFLOW IMPELLER

Some shear-thinning high viscosity fluids are bestmixed by large-diameter dual-action impellers. ThePhiladelphia Counterflow impeller has been found tobe the most efficient device for some of these veryspecial fluids, generally with a viscosity of 25,000 cp ñ75,000 cp, and a shear-thinning index of 0.1 ñ 0.5.

Laboratory testing isrecommended to determine if theCounterflow impeller isbeneficial to a specificfluid.

RADIAL FLOWImpeller Designs

CBTThe Curved Blade Turbine is the most flow-efficientradial flow turbine. It is useful for flow velocity sensi-tive applications where radial flow is desirable tobreak up plug flow (e.g. multi-stage alumina digester)or where high wall velocity is needed (e.g. heat trans-fer). The 4CBT90 has a 2.52 power number.

SMITH TERBINE

The vertical Flat Blade Turbine can be the impeller of choice when very high torque is desirable for blending at the expense of efficiency, or when highshear is required for other than gas dispersion (e.g.liquid-liquid emulsions or high-intensity solids scrub-bing). A radial flow impeller, the 4FBT90 has a 3.60power number.

FBT

The radial-flow Smith Turbine is similar to theRushton, except curved pipe-sections replace the ver-tical flat blades, resulting in a lower power number andincreased gas dispersion efficiency. The 6DS90 Smithturbine has the highest efficiency for gas dispersion. Ithas an ungassed power number of 3.6.

RUSHTON TURBINE

The radial flow Rushton Turbine, with four or six disc-mounted vertical flat blades, is the traditional impellerfor gas dispersion, but has largely been supplanted bythe HS and Smith Turbine for same. The 6DR90Rushton Turbine has a 4.75 power number, the high-est of any common impeller. As such, it is useful forvery high intensity mixing where lower-power numberimpellers would be too large.

Ask us about ProTech Servicesavailable exclusively

from Philadelphia Mixers

Philadelphia Mixers’ Sales Officesand Representatives

• Consulting and Mixing Design Services

• Complete Scale-Up and Testing Services

• On-Site Start-Up

• On-Site Employee Training Programs

• Process Technology Seminars

• Extended Product Warranties

• Drive Exchange Program

• Inspect and Repair Program

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