PROPELLER-TYPE MIXERS E. S. BISSELL Mixing Equipment Company, Ino., Rochester, N. Y.
DISCUSSION of any type of mixer should begin with the mixing vessel. Not only do shape and proportion of the vessel have a direct effect on mixing action, but the material of construction must be considered (electrolysis, ability to support weight, etc.). It is also true that many of the most severe use requirements hinge upon the method of attaching the mixing' unit to the tank. A cas;al survey ,f the entire field of mixing equipment shows that the tendency of a large number of manufacturers is toward building complete mixers-that is, the containing vessel as well as the agitator proper. This trend becomes more obvious when high-viscosity mixers are considered. On the other hand, propeller-type mixers lend themselves particularly for adaptation to any reasonably well-shaped vessel, thus permitting the engineer to design for capacity,
space requirements, multiple usage, etc., with comparative freedom. This adaptability also permits the use of tanks already available. I n addition, the inherent characteristics of this type of mixer enable its removal without leaving residual parts in the mixing vessel to interfere with other uses.
Effect of Containing Vessel on Efficiency The shape of the tank does, however, affect the efficiency, and the following facts should be kept in mind: Round tanks are better than square tanks, and square tanks are better than rectangular tanks. The shape of the bottom is also a factor: A flat bottom is better than a dished-in bottom; a dished-out bottom is better than a flat one; and a conical bottom requires critical location. The proportion of depth of tank (or liquid level) to tank diameter is also important. For top-to-bottom turnover, the depth should a t least equal the diameter. Depth greater than diameter, however, is practicable even t o ratios of 5 to 1. As the depth-to-diameter ratio decreases, velocity of upcurrents gradually approaches zero, a t which point solids cannot be kept in suspension if their density is greater than the liquid. Heating coils, baffles, and other tank fixtures act as velocity traps, and their presence requires a larger mixing unit to secure a given degree of agitation. After considering the shape of a tank, its size and whether it is open or closed are the important factors: For open tanks up to 8 feet deep, use portable or rim-attachable models. For open tanks up to 12 feet deep, use
p e r m a n e n t m o u n t i n g with step bearing. For closed tanks, flat-topped (atmospheric pressure only) : Up to 8 feet deep, use angle flange mounting. Up to 12 feet deep, use angle flange mounting with step bearing.
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(Left) P O R T A B L EATTACHABLETJNIT D I R E C TD R I V E(1725 OR 1125
WITH
R. P. M.)
(Center) ANGLE FLANGEADAPTER FOR T O P V E S S E L S , W H I C HB O L T B
FLAT
DIRECTLY TO COVER
(Right) G E A R - D R I V E T R I P o D - S T Y LHI MIXER F O R P R E S S U R EVESSELS,A T TACHABLE TO FLANUED NOZZLE 493
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INDUSTRIAL AND ENGINEERING CHEMISTRY
For closed tanks, dished-head (atmospheric pressure only) : Up to 8 feet deep, use companion flange mounting. Up to 12 feet deep, use companion flange mounting with step bearing. For closed tanks, dished-head (pressure or vacuum) : Up to 8 feet deep, use tripod mounting and flange adaptation. Up to 12 feet deep, use tripod mounting and step bearing.
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Considerable discrepancy can be experienced in measuring viscosity, since few laboratories take the necessary precaution of using standard liquids of known viscosity for calibration purposes. A more simple precaution is to use handbook values for several commonly available materials and check these values with the instrument at hand. Errors of 600 per cent have been noted with certain instruments commonly in use. Other mistakes occur from an erroneous conception of viscosity. Figures obtained from crude flow cups are occasionally referred t o as absolute viscosity. Propeller-type mixers work well with thixotropic materials such as paint. The Gardner mobilometer provides a simple means for determining the existence of thixotropy in a material and gives a usable figure for the degree. Thixotropic materials may show a high viscosity under static condition, 20
L
15
H O R S E POWER
FIGURE1. EFFECTOF SPEED INCREASE ON POWER
Y
P B
I O
K
c
For tanks with a greater depth than 10 or 12 feet, or where on head room is available, or where the top of the vessel must be kept clear, use side (or bottom) entering units. Tanks holding up to 450,000 gallons are thus successfully agitated.
:z 5
L 15
20
G A L L O N S PER REVOLUTION
Material to Be Mixed The upper range of viscosity of the material to be mixed with available forms of propellers is apparently around 3600 centipoises. This range includes over 90 per cent of all applications. Above this point, power requirements increase
FIGURE2. GALLONSDISPLACED PER REVOLUTION FOR PROPELLERS OF DIFFERENT DIAMETER
yet flow easily. Other materials, such as cooked starch, exhibit inverted viscosity, and many of them cannot be agitated with anything but a slow, scraper type of agitator. With these materials, the more work, the less flow. Emulsions of simple type (asphalt emulsions) can be put together with propeller-type mixers. No propeller-type mixer, however, should be considered as a substitute for a homogenizer or colloid mill. By using perforated or sawtoothed propellers at a high speed, the action is increased as compared with standard equipment.
Theory of Propeller-Type Mixing
UNRETOUCHED PHOTO(STRIAE) SHOWING COMPLEX CURRENTS CREATED BY PROPELLER-TYPE AGITATORS rapidly and the advantage begins to lie with complete mixing units. The power curve is, however, for all practical purposes a straight-line function and can be directly related to viscosity in centipoises for a given set of conditions.
True mixing requires that dispersion of the different materials end with uniform relationship throughout the mix, whether large or small samples are involved. This can be accomplished only by setting up currents within the tank in such a way that differentials in direction and velocity occur in a multiplicity of loci. It is along the margins of these streams that actual mixing takes place, as well as directly a t the propeller. This phenomenon has been studied by the stria method, and new developments can be expected as a result. One fact has been wel1,proved: The existence of swirl or vortex indicates poor mixing since there is no interchange of position between particles under such conditions. The off-center principle of agitation does away entirely with swirl or vortex and thus secures uniformity in a shorter time than other methods. Since no baffles are required, power requirements are usually lower. Unfortunately, no criteria exist for measuring turbulence. Primary measurements with various types of flowmeters, differential pressure gages, surface speed indicators, etc., have not been found to be applicable. Secondary criteria, such as suspension of graded solids, rate of dissolving, etc.,
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INDUSTRIAL AND ENGINEERlNG CHEMISTRY
offer great difficulties in standardization due to container shape, effect, etc. Tertiary criteria offer some hope but have been only tentatively investigated. Bleaching action of fuller’s earth on vegetable oils is dependent upon degree of agitation. Time required to bleach to a given color would provide B reference point. The lack of criteria will apparently remain for some time to plague the honest manufacturer and the thoughtful engineer. Performance and side-by-side comparison must still be utilized. A real field for research exists, and any manufacturer should be glad t o lend equipment for that purpose. It is barely possible that some new unit such as a “galowatt” (watt gallon) can be derived from power requirements per gallon agitated. There is some evidence that such a relationship can be established. I n any case other than suspensions of solids, the time element necessarily enters into the equation. For simple blending operations, astonishingly small units are satisfactory, where time need not be a factor. All other factors in the technology of propeller-type mixing have been satisfactorily worked out, as shown by Figures 1 to 4. These and
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motor for any standard current and to meet any operating conditions. I n addition, the motor should be of standard make, and the arrangement of internal parts, fan, brushes, etc., should be accessible for maintenance or service without dismantling the mixing unit. Motors furnished for vertical
-I Lu D
0 U
0 . 4
I
I
I
I
I
z
3
I 4
H P. R E Q U I R E D 4 0 0 RPM. OPERATING
IN W A T E R
FIGURE4. POWERINCREASE FOR PROPELLERS OF DIFFERENT DIAMETER
I000 2000 3000 V i s c o s i t y Centiposes
4000
FIGURE 3. TYPICAL CURVEOF-POWER INCREASE IN VISCOSITY
related facts enable t h e m a n u f a c t u r e r who knows how to apply them t o furnish a mixer that will properly agitate the material a t hand, without being badly underor overpowered. It is assumed that the manufacturer of mixing e q u i p m e n t will provide facilities for testing customers’ samples and thus do away with guesswork or off-the-shelf selling.
Use Requirements .After the proper size propeller, speed, and horsepower have been selected in relation to tank capacity and viscosity, the mechanical details or user’s requirements must be met. The method of attaching the propeller to the tank has already been discussed. Three other groups of requirements are discussed in the following paragraphs. MOTORS. The motor is fundamentally i m p o r t a n t . The user has the right to expect t h e m a n u f a c t u r e r to furnish a
(Above) SIDE-ENTERINQ UNITWITH VEEBELTDRIVE;(Below) NEWSTYLE SIDE-ENTERING UNITRECENTLY DEVELOPED, SHOWING MECHANICAL SEPARATION OF BEARINGS FROM STUFFING Box
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TABLEI. CASEHISTORIES OF PROPELLER Case I Material
Fish oil
Tank size, ft. Tank capacity, gal. Reason for agitation Unit installed
20 diam., 31 deep 72,000 Blending Side-entering AOC unit
Horsepower Construction material Motor
7.5 Steel, cast iron Semienclosed
Approx. cost of complete installation
$500
Case I11 Acid and fine solids (2 Ib./gal.)
Case I1 Paint (20 lb./gal.)
......... 600 Tinting, coloring Special side-entering unit with special bulkhead 3 Steel, cast iron Explosion-proof
$600 (including vessel)
TABLE11. MIXINGPROBLEM WORKSHEET Please quote on mixing equipment based on following:
PRODUCTS Materials to be mixed.. . . . . .. .......... Viscosity .................. ravity . . . . . . . . . . . . . . . . . . . (If unknown compare to water, motor oil, sirup, molasses) (underline) What result is desired (underline)? 1. A blend of liquids. 2. Dissolving materials. 3. Putting solids in suspension. 4. Forming an emulsion. 5. Stirring while cooking, etc. 6.. .................................. Are abrasives present?. . . . . . . . . . . . . . . ..What kind?. How is mixing operation carried on a t present? At what temperature?. ........................................ What degree of agitation is desired-violent, medium, mild (underline)? I s operation to be in open tank, closed tank-airtight, under pressure, vacuum (underline)?
.............
TANK,KETTLE,VAT, ETC. (WRITEIN YOURTERM) Dimensions of tank: Diameter. . . . . . . . . . . . . . ,Height. . . . . . . . . . . .
. .Thickness a t rim (including flange) -flat, dish, cone (underline) If container is irregular in shape or contains heating coils, baffles, or other members, draw rough sketch and show location (use back of this sheet). Size of batch usually mixed, depth to which tank is filled and dis from t o p to liquid level. Gallo Depth fromtop . . . . . . . . . . . . . . . . ............ TANK,KETTLE,VAT, VESSEL,DIQESTER
If tank is closed (airtight, pressure, vacuum) and not over 10 feet deep, enclose print or accurate sketch of tank showing available openings, position, dimensions, bolt circle of flange. Give preference as to how mixer enters tank, through side or top. If no opening exists in top, we furnish prints showing proper location of flange nozzle to be welded in. If tank is over 10 feet deep i t will be necessary t o enter through the side of tank. Send sketch showing wall thickness, manner of construction, material, exact inside and outside diameter, and location of exterior structural parts which might interfere with installation. WORKINQCONDITIONS Will mixer be used indoors or outdoors?. . . . . . . . . . . . . . . . Are conditions dry, steamy, dusty?. ................... Are products to be mixed flammable or explosive?. ...... What current is available (voltage and frequency)?. . . . . . . . . . . . . . . If possible, specify type of motor you prefer-splash-proof, totally enclosed, explosion-proof. Name ....................... Title ............................ Company. . . . . . . . . . . . . . . . . . . . Street ....................... City ................State.........
mixing
25 high, 10 diam. 14,000
......... .........
15 Special alloy Special insulation
Case IV Solvent
6.5 diam., 7.25 deep
......... .........
Top-entering tripod unit; pressure involved 0.5
.........
Special cycle, explosionproof * $800 (including special features $275 of construction)
operation should be designed for that purpose. They should not be horizontal motors converted to that position by the use of a bracket. Motor frames should be available in different materials where operating conditions are unusually severe. MATERIALS OF CONSTRUCTTON. Materials, including all exposed parts of the mixer, should be variable to meet operating conditions. Obviously, an aluminum-frame motor might be recommended in order to save weight in one instance but could not be recommended in an alkali plant. Similarly, i t is desirable to furnish housings, clamps, bearing members, tubes and chucks, shafts and propellers of the various corrosion-resisting alloys. Shaft and propeller materials most frequently called for are Monel metal and 18-8 stainless steel. Shaft and propellers are frequently covered with either lead or rubber for severe acid conditions. Hard unmachinable alloys are not usable for shafts or other parts requiring machining. No material has yet been adapted that can withstand hydrochloric acid (over 140O F.), although work continues in this field. MECHANICAL PERFORMANCE. The usual requirements of good workmanship and sound engineering are included in this group. A reliable mixer is more than a motor, a shaft, and a propeller. It is a power tool with problems of machine design all its own, including torque, bearing alignment, shaft strength, etc. The simple criteria applied to any machine or mechanical equipment should be rigidly applied here. One should be just as critical of long unsupported shafts in a mixer as in any other device. Improperly spaced bearings are as bad here as in a pump or an engine.
Future Trends in Mixing Two trends are now apparent: Larger tanks are being used for plant operations, and propeller-type mixers lend themselves particularly well to large tank work. Continuous mixing will develop as fast as metering and controls are available. The blending of complex mixtures in small chambers with continuous inflow and outflow is entirely practicable in use in several widely different applications. Each job must be individually worked out. The applications in Table I indicate the low cost, variety of conditions, and wide use of propeller-type agitators. Table I1 shows a form of questionnaire sent by the mixer manufacturer. R E C ~ I V EOctober D 26, 1937.
