CHAPTER 21 Mechanical Design Of Mixing Equipment

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CHAPTER 21Mechanical Designof Mixing EquipmentD. S. DICKEYMixTech, Inc.J. B. FASANOChemineer, Inc.21-1INTRODUCTIONMixing equipment must be designed for mechanical and process operation. Although mixer design begins with a focus on process requirements, the mechanicaldesign is essential for successful operation. Usually, a competent manufacturer ofmixing equipment will take responsibility for the mechanical design. However,process conditions, such as impeller operation near a liquid surface, can imposesevere mechanical loads. Similarly, the process environment will influence theselection of a motor enclosure. In many ways the process requirements can havea direct impact on the mechanical design. In other ways, such as the naturalfrequency of a mixer shaft, appropriate mechanical design must be determinedby the equipment designer. Whatever the reason, knowledge of the mechanicalrequirements for a mixer will help guide the engineer toward a design that willmeet both process and mechanical criteria.The purpose of this chapter is to provide practical information about themechanical design of mixing equipment. Therefore, descriptions, equations, andnomenclature will be given in both U.S. engineering units and metric units.Descriptions and equations using U.S. engineering units will follow commonindustrial practices used in the United States with design information for materials measured in inches and motors specified in horsepower. Descriptions usingHandbook of Industrial Mixing: Science and Practice, Edited by Edward L. Paul,Victor A. Atiemo-Obeng, and Suzanne M. KrestaISBN 0-471-26919-0 Copyright 2004 John Wiley & Sons, Inc.1247

1248MECHANICAL DESIGN OF MIXING EQUIPMENTmetric units will reference materials commonly measured in millimeters (mm),while equations will do calculations in meters (m).Metric units in equations will follow SI metric practice. To avoid confusion,values in the text that are also used in equations will use standard SI units even ifmore reasonable numeric values are possible with prefixes. Units for variables inU.S. engineering units (U.S. Eng.) are shown in brackets []. Units for variablesin metric units (Metric) will be shown in braces {}. The nomenclature list showsboth U.S. engineering units and metric units used in the equations. Care mustbe taken to use the correct units, since several equations contain dimensionalconstants. Results can be incorrect if the wrong units are used.21-2MECHANICAL FEATURES AND COMPONENTS OF MIXERSBecause of the diversity of fluid mixing applications and variety of vessels,many different styles of mixers are used in industrial applications. Mixer sizesinclude small fractional-horsepower portable mixers to huge 1000 hp plus mixers.Although normally viewed as a single piece of equipment, like a pump, the typicalmixer is composed of several individual components, such as a motor, gearreducer, seal, shaft, impellers, and tank, which is often designed and purchasedseparately. Although highly customized for many applications, most mixers area combination of standard components, sometimes with modifications, and oftenwith unique characteristics, such as shaft length.Generalizations, especially for mixers, can misrepresent individual situations,but some features are common to the largest number of mixers built worldwide.The most common motive force for a mixer is an electric motor, so a knowledgeof standard motor characteristics is useful. Most mixers operate at or below typical motor speeds, so some type of speed reduction is common. Speed reductioncan be accomplished with several different types of gears, usually in enclosedhousings, or with belts and sheaves. Besides speed reduction, antifriction bearings are found in all types of rotating equipment. Some type of seal aroundthe rotating shaft is required for closed-tank operation and the type depends ondegree of seal required, operating pressure, and operating temperature.The shaft for a mixer, especially a large one, involves significant mechanicaldesign, partly because of the myriad of shaft lengths, impeller sizes, and operatingspeeds, and partly because both strength and rigidity are necessary for a successfuldesign. The combination of custom process and mechanical design necessary formixers is unique for chemical process equipment. Mechanical design does notend with the shaft, since strength and practical issues remain for the impeller.Another part of mixer design is the tank in which the mixer is used, sincetank dimensions influence mixer features, especially shaft length. Conversely, amixer requires tank features, such as baffles, support strength, and other tankinternals. Materials of construction, although most commonly metal alloys formixers, depend on process chemistry and operational requirements.Other mechanical features can be important in special-purpose mixers, such ashigh-shear mixers, dry-solids mixers, and static mixers. Without revealing trade

MECHANICAL FEATURES AND COMPONENTS OF MIXERS1249secrets or emphasizing proprietary technology, elements of the same mechanicaldesign considerations apply to special-purpose mixers. The primary mechanicalemphasis in this chapter is on equipment discussed elsewhere in this book.Each key element of the mechanical characteristics of mixers will be coveredin this section. Although not comprehensive with respect to each topic, the equipment and design requirements discussed should cover most of the mixer typesand applications. Even with the diversity of mixing equipment, features such asmotors and materials of construction are mechanical considerations, common toall types of mixers.21-2.1Impeller-Type Mixing EquipmentImpeller-type mixing equipment represents the largest category of general purposemixing equipment for fluid processing applications. From the process view ofimpeller-type equipment, an impeller, usually composed of blades mounted to acentral hub and rotated by a drive shaft, pushes and moves the material to bemixed. The mixing action and the process results are primarily a result of thismaterial, usually fluid, motion. The mechanical design of impeller-type mixingequipment is responsible for the process by which some form of energy, such aselectricity, is converted into fluid motion. That fluid motion is ultimately dissipatedas heat, hopefully after the process objectives are accomplished.To present an organized understanding of mixing equipment, some commonterminology is used to describe typical characteristics. Each category of equipment has some loosely defined limits, often with overlap to other categories,depending on features provided by different manufacturers of the equipment.21-2.1.1 Portable Mixers. Portable mixers may or may not be truly “portable,” depending on size and mounting. However, the term portable mixer mostoften refers to mixers with 14 hp to 3 hp drives mounted with either a clampor a bolted-swivel mount. Smaller mixers are usually considered laboratory orpilot-plant equipment and are not often used in industrial production processes.Most portable mixers operate at either motor speed, such as 1800 rpm (30 rps)or 1200 rpm (20 rps) with 60 Hz power, or with a single-reduction gear drive(approximately a 5 : 1 speed reduction) for 350 rpm (5.83 rps). Although detailsof impeller types vary, axial flow impellers, such as marine propellers or threeblade hydrofoil impellers, are used most often. A typical direct-drive portablemixer is shown in Figure 21-1 and a gear drive portable in Figure 21-2.21-2.1.2 Top-Entering Mixers. The designation top-entering mixers hasbecome accepted as a more restrictive term than the name would imply. Topentering mixers are usually considered the equivalent of portable mixers withflange mountings, or perhaps larger mixers but with light-duty gear drives andmotors less than 10 hp (7460 W). This designation is less of a true definitionthan an accepted industry practice used to describe basic mixer products.By this definition, top-entering mixers have flange or pedestal mounts, compared with the clamp or swivel-plate mounts used on portables. Most top-entering

1250MECHANICAL DESIGN OF MIXING EQUIPMENTFigure 21-1Direct-drive portable mixer. (Courtesy of Lightnin.)mixers are mounted on the vertical centerline of a tank with baffles, but maybe off-center or off-center, angle mounted. Longer shafts and larger impellerscause more severe loads on top-entering mixers than portable mixers. A typicaltop-entering mixer is shown in Figure 21-3. Most top-entering mixers have anaxial flow impeller, such as a hydrofoil impeller or sometimes a marine propeller.Typical seals for top-entering mixers are basic stuffing boxes or single, mechanical seals. For reasons of mechanical strength, sealing pressures are typically30 psig (207 000 Pa) or less. For reasons of cost, single dry-running mechanical seals are common. More detail about different types of seals is given inSection 21-5.21-2.1.3 Turbine Mixers. Turbine mixer is another industry designation thattypically refers to more robust mixer designs that may have a variety of impellerand seal types and may have motors from 1 hp (746 W) to 1000 hp (746 000 W)or larger. The various sizes for turbine mixers are depicted in Figure 21-4. Turbine

MECHANICAL FEATURES AND COMPONENTS OF MIXERS1251Figure 21-2 Gear-drive portable mixer. (Courtesy of Lightnin.)mixers are usually mounted vertically on the centerline of a cylindrical tank orrectangular basin or chest. The broader designation of turbine mixers may includetop-entering mixers. Turbine mixer drives may be used with high viscosity, closeclearance impellers. Although none of these mixer designations are absolute andsome equipment falls outside common or convenient terminology, knowing typical terminology can be helpful to understand the capabilities and limitations ofdifferent equipment.Because of the broad use and versatile characteristics of turbine mixers, typicalcomponents are described at the beginning of this chapter. Essentially all turbinemixers have a motor, speed reducer, shaft, and impeller(s). Seals are used whencontainment is required. In this chapter we discuss motor and speed-reducer characteristics that commonly apply to turbine-style mixers. The shaft and impellerdesign characteristics are also typical for turbine mixers. A subset of these component characteristics and design procedures apply generally to other mixers.

1252MECHANICAL DESIGN OF MIXING EQUIPMENTFigure 21-3 Top-entering mixer with mechanical seal. (Courtesy of Lightnin.)Obviously large, custom motors would never be applied to a portable mixer, butexplosion-proof motors would.21-2.1.4 Side-Entering Mixers. Side-entering mixers are what the nameimplies, mixers that enter the tank or vessel from the side. For such mixersto mix the tank contents, they must be mounted below the liquid level. Consequently, they are most often mounted near the bottom to assure blending of thetank contents even at a low liquid level. The major disadvantage to side-enteringmixers is a submerged shaft seal, which must operate in the process fluid. Processfluids may be lubricants, such as petroleum products, or abrasives, such as paperpulp and slurries. Many lubricant products require a positive seal, while abrasive products cause wear problems. The advantages of side-entering mixers areeconomic ones: lower initial cost, no mounting support on top of the tank, andsimple speed reduction because of higher operating speeds than those of mostturbine mixers. Many side-entering mixers use belt-drive, speed reductions, and

MECHANICAL FEATURES AND COMPONENTS OF MIXERS1253Figure 21-4 Different-sized turbine mixers and drives. (Courtesy of Chemineer.)Figure 21-5 Side-entering mixer with pillow-block bearings. (Courtesy of Chemineer.)pillow-block bearings. A typical side-entering mixer is shown in Figure 21-5.Both belt drives and bearing types are discussed later in this chapter.21-2.1.5 Bottom-Entering Mixers. Bottom-entering mixers are usually thesame basic drive arrangement as a turbine mixer, but mounted on the bottom of the tank. A bottom-entering mounting is shown in Figure 21-6. Mostbottom-entering mixers have the disadvantage of a submerged seal without the

1254MECHANICAL DESIGN OF MIXING EQUIPMENTFigure 21-6 Bottom-entering mixer.pp. 89–94.)(Chemical Engineering, August 2,1976,cost advantages of side-entering mixers. Bottom-entering mixers are used whenprocess requirements or tank geometry makes top or side mounting impractical.21-2.2Other Types of MixersAlthough portable, top-entering, or turbine mixers account for the largest numberof mixers built for the process industries, other common mixer categories withunique features are also important.21-2.2.1 High Viscosity Mixers. While turbine mixers can handle low tomoderate viscosities, high viscosity fluids [100 000 cP (100 Pa · s) and greater]usually require some type of close-clearance impeller design. The diameter ofa typical turbine-style impeller is less than 70% of the tank diameter. Closeclearance impellers for high viscosity applications are 85 to 95% of the tankdiameter. Some close-clearance impellers even have flexible scrapers, which areeffectively 100% of the tank diameter.Important mechanical features of high viscosity mixers are the low speed andhigh torque required to rotate large impellers in viscous fluids. Equally important,but more subtle, are requirements for the tank to have a very round cross-section.The tank must be round so that the clearance between the impeller and the wallremain nearly constant, and the shaft must be centered for the same reason.Shaft and impeller designs are primarily for strength and based on the hydraulicforces caused by viscous drag. Although high viscosity impellers can take manyforms, two of the more common varieties are the helical-ribbon (Figure 21-7)and anchor-style (Figure 21-8) impellers.21-2.2.2 High-Shear Mixers. High-shear mixers have many features opposite to those of high viscosity mixers. Typical high-shear mixers have smallimpellers, 10 to 20% of the tank diameter, and operate at high speeds, 1000rpm (16.7 rps) to 3600 rpm (60 rps). To operate at high speeds, without requiring excessive power, high-shear impellers usually have small blades. The blades

MECHANICAL FEATURES AND COMPONENTS OF MIXERS1255Figure 21-7 Helical-ribbon impeller. (Courtesy of Chemineer.)Figure 21-8 Anchor impeller. (Courtesy of Lightnin.)may appear as teeth on the edge of a disk or slots and holes in a rotating cylinder.A typical high-shear disk impeller is shown in Figure 21-9. The slotted-cylinderdesign is generally used for both a rotating and stationary element, called arotor–stator design, as shown in Figure 21-10. Some high-shear mixing devicesare used in-line, like pumps with high-shear blades inside a small housing,through which liquid flows or is pumped. Viscous fluids must be pumped throughmost in-line mixers. Such in-line style mixers or homogenizers still require somemechanical design, although with less emphasis on a long shaft support and moreemphasis on tight tolerances.A few high-shear mixing devices use impinging or interacting hydrodynamicflow to accomplish dispersion and mixing. These mixers operate more like

1256MECHANICAL DESIGN OF MIXING EQUIPMENTFigure 21-9High-shear impeller. (Courtesy of INDCO.)Figure 21-10 Rotor–stator high-shear impeller. (Courtesy of IKA Works.)static mixers, with the mixing power provided by an external pump, often ahigh-pressure positive-displacement pump.21-2.2.3 Double-Motion Mixers. As the name implies, double-motion mixers have a combination of mixer motions. Many double-motion mixers are acombination of a high viscosity, close-clearance mixer and high-shear mixer.The high viscosity part of the mixer provides bulk motion of the fluid(s), especially near the tank walls, and the high-shear mixer creates dispersion, often oftwo phases, either two liquids or a liquid and solids.The double motion comes from two shafts with at least two impellers operatingin the same tank. Other double-motion mixers have coaxial shafts with a closeclearance impeller and turbine impeller(s) operating at different speeds. Somemixers have shafts that move relative to the vessel, as in planetary motion mixers,

MECHANICAL FEATURES AND COMPONENTS OF MIXERS1257where intermeshing impellers rotate on their own axis and move around theaxis of the tank. Double-motion mixers provide a diversity of mixing actionsselected to handle difficult or changing batch mixing requirements. The cost ofmore complicated equipment is offset by the ability to handle a wider range ofmixing needs.21-2.2.4 Dry-Solids Mixers. Dry-solids mixers are normally applied to flowable powdered materials. The action of the mixers can be categorized as (1) tumblemixers; (2) convective mixers, which use a ribbon, paddles, or blades to movematerial; (3) high-shear mixers, which create a crushing action like a mortar andpestle; (4) fluidized mixers, as in fluidized beds; and (5) hopper mixers, which usedischarge and recirculating flow to cause mixing (Harnby et al., 1992). Althougheach type of dry solids mixer uses different equipment to accomplish the mixingaction, the design methods discussed in this chapter for motors, drives, and evenseals may apply.21-2.2.5 Static Mixers. The mechanical design of static mixers, also calledmotionless mixers, is unique compared with other types of mixing equipment.Most other mixers involve some type of rotating equipment. Static mixers haveno moving parts, and therefore design methods resemble those of piping andpressure vessels. The mixing elements of a static mixer can take many forms,but the most common is the twisted element style, shown in Figure 21-11. Mostelements are merely inserted and fixed into a section of pipe, although someare designed to be removable for cleaning and others are sealed to the wallof the pipe. Design of the elements themselves is largely proprietary, althoughthe pipe sections in which the elements fit are designed to piping standards fordimensions and end connections. Most static mixers are housed in the same sizeor one-size-larger pipe than the adjacent runs of piping and are the same materialand schedule (wall thickness).21-2.2.6 Other Mixers. A variety of other devices and methods can be usedas mixers. Flow devices, such as jets and nozzles, can be used as mixers. RisingFigure 21-11 Kenics static mixer. (Courtesy of Chemineer.)

1258MECHANICAL DESIGN OF MIXING EQUIPMENTgas bubbles from injected air will cause mixing. Pulses of liquid or gas can createinteresting flow patterns. It is beyond the scope of this book to provide mechanicaldesign characteristics for such a diversity of equipment. However, within thescope of the equipment and methods described in this chapter, elements of manymixers and mixing systems can be designed or selected with an understandingof the basic requirements.21-3MOTORSMotors are an essential part of most mixers, since a rotating shaft with an impelleris common. Electric motors are without doubt the primary source of rotatingpower for mixers. Air and hydraulic motors are used for some applications,especially where a combination of variable speed and explosion-proof performance are needed. Diesel engines are used occasionally where electric power isunavailable or unreliable.21-3.1Electric MotorsElectric motors take almost as many different forms as mixers. Motors can beclassified by size, power source, enclosure, and even application. An essentialpart of any electric motor is the nameplate. Without the information found on anameplate, most motors look like a cylindrical or rectangular housing with wiresleading in and a rotating shaft coming out. Understanding the information on amotor nameplate will help identify an existing motor or specify a new motor.Although some information is unique to individual manufacturers, much of theinformation is essential for proper operation and application of a motor.Some or all of the following information can be found on a typical motornameplate: Catalog number: specific to the manufacturer.Model number: specific to the manufacturer.Phase: single, three, or direct current.Type: classification depends on the manufacturer.NEMA (National Electrical Manufacturers Association) electrical design: B,C, and D are most common and represent torque characteristics of the motor.Duty: most motors are rated for continuous operation, especially for mixers.However, motors for 15, 30, or 60 min duty are available.Frequency (Hz): electric frequency in cycles per second.Speed (rpm): revolutions per minute of shaft at full load.Voltage: single or multiple voltages, depending on winding(s).Amperage (FLA): full-load motor current.Power (hp): horsepower at rated full-load speed.Frame size: standard designation of dimensions.

MOTORS1259 Maximum ambient temperature (max. amb.) in Celsius (centigrade): usually40 C [104 F]. Insulation class: standard insulation classes are B, F, and H, which establishthe maximum safe operating temperature for the motor. Enclosure: indicates how the motor is protected and sealed from the surroundings. Service factor: a measure of continuous overload capacity.A comprehensive description of manufacturer-specific information, such ascatalog number, model number, type, and so on, can be found in the company’scatalog. Many catalogs have a section of engineering data that may have moreextensive tables of dimensions, enclosure features, and design calculations. Somemanufactures even have separate technical data books (Leeson, 1994).Because electric motors are used for an enormous range of applications andmanufactured to many unique specifications. The full range of motor featurescannot be covered in this book. The features most common for industrial mixerapplications will be emphasized.21-3.1.1 Phase. Alternating current can be categorized as either single- orthree-phase power. Single-phase power has a complete cycle of voltage from analternating maximum positive value to a maximum negative value and back to themaximum positive value. Most household and office power in the United Statesis single phase. Three-phase power, commonly found in industrial environments,is carried by three conductors with three voltage cycles starting out of phase withone another. With three-phase power the voltage between two conductors nevergoes to zero, resulting in a smoother, more nearly constant voltage differentialacross motor windings. Most applications with motors 3 hp (2200 W) and largeruse three-phase power.21-3.1.2 Type. Motor type depends on the manufacturer, power, and application. The most common motor type used on a mixer for single-phase power is acapacitor start motor. Capacitor start motors can be designed for both moderate(175% or less) and high (300% of full load) starting torque. Torque is the twistingforce (moment of force), created by the motor and applied to the rotating shaft.Moderate torque is adequate for most mixer applications since impeller power isproportional to speed cubed in turbulent conditions, thus keeping starting torqueslow. Capacitor start motors use a start capacitor and a start switch. The startswitch takes the capacitor and start winding out the electric circuit when themotor reaches approximately 75% of full-load speed. Split-phase motors can beused in light-duty applications, because of moderate to low (100 to 125%) starting torque and high starting current. Split-phase motors have no capacitor, onlya start switch to drop out the start winding.Three-phase motors have a high starting torque, high efficiency, and low current requirement. The torque characteristics are described by NEMA electricaldesign, which is discussed in the next section. Three-phase motors do not use acapacitor, switch, or relay for starting.

1260MECHANICAL DESIGN OF MIXING EQUIPMENTOther types of motors that may be encountered in mixer applications are gearmotors, pony motors, and brake motors. Gear motors are composed of an electricmotor with an attached gear reducer. Spur, helical, or worm gears can be usedin single or multiple reductions to achieve a wide range of output speeds. Motorpower, output speed, and output torque are all essential design variables.A pony motor is a small gear motor used to turn a larger motor at slow speedand to provide additional starting torque. A pony motor or variable speed drivemay be used to slowly start a mixer that could be embedded in settled solids.Pony motors are rarely used on mixers today because of available variable speeddrives. Care must be exercised to match the output torque rating of the ponymotor with the input torque rating for the mixer drive.Brake motors have a fail-safe, stop-and-hold, spring-set brake on the backof the motor. When power fails, the brake sets and holds the motor and load.This feature is rarely needed on a mixer since the mixed fluid usually acts as itsown brake.21-3.1.3 NEMA Electrical Design. Three-phase motors are classified byelectrical design type, B, C, or D, defined by NEMA. Design B motors providenormal (100 to 200%) starting torque at normal starting current and are suitable for most mixer applications. Design C motors provide high (200 to 250%)starting torque at normal starting current and may be used for special mixerapplications, provided that the drive and shaft are not overloaded during startup.Design D motors have high (275%) starting torque with high slip at low startingcurrent and are rarely used on mixers.21-3.1.4 Duty. All motors used for mixer applications should be rated forcontinuous duty, since even batch runs may take more than the anticipated timeshould problems develop.21-3.1.5 Frequency. The frequency of alternating current is measured inHertz (cycles per second). Sixty-cycle (60 Hz) current is used throughout NorthAmerica. Fifty-cycle (50 Hz) current is used in Europe and in many countriesin Asia. The frequency of the current supplied affects the operating speed of analternating current (AC) motor.21-3.1.6 Speed. A typical AC motor is designed to operate within 2 to 3%of the synchronous speed. Synchronous speed depends on the number of polesin the winding:U.S. Eng.N [rpm] 120MetricN {rps} 2fpfp(21-1)

MOTORS1261where N is rotational speed [rpm] {rps}, f is frequency [Hz (cycles/s)] {Hz},and p is the number of poles in the motor rotor. Typical motor speeds usedfor mixers with 60-cycle (60 Hz) power are 1800 rpm (30 rps) and 1200 rpm(20 rps), which correspond to four- and six-pole windings. Additional speedsoccasionally encountered with mixers are 3600 rpm (60 rps) and 900 rpm (15rps). Corresponding speeds for 50 cycle (50 Hz) power are 1500 rpm (25 rps),1000 rpm (16.7 rps), 3000 rpm (50 rps), and 750 rpm (12.5 rps). Whether amixer is designed to operate with 60 or 50 Hz power makes a major differencein the appropriate speed reduction for a mixer, since impeller power is a strongfunction of operating speed.Multispeed motors can be built by using different connections to a singlewinding or with multiple windings. All single-winding two-speed motors have a2 : 1 speed ratio, such as 1800/900 rpm (30/15 rps). Multiple winding motors canhave two speeds, such as 1800/1200 rpm (30/20 rps). Multispeed electric motorshave a large effect on mixer applications, because a 2 : 1-speed motor typicallyhas an 8 : 1 effect on impeller power for turbulent conditions. Even a 3 : 2-speedmotor has a 3.4 : 1 power effect for turbulent conditions. Multispeed motors canbe applied when a viscosity change results in increased impeller power. However,motors are usually constant torque, so a 2 : 1 speed motor delivers only half themaximum power at the low speed. Multispeed motors have largely been replacedby variable speed (variable frequency) drives, because of the large power changewith mixer speed.21-3.1.7 Voltage. Like phase, voltage depends on the electrical supply to thelocation of the motor. Typical voltages in the United States are 125 and 230 Vfor single-phase power and 230 and 460 V for three-phase power. In Canada,575 V, three-phase power is available in many industrial environments. Otherlow voltages, such as 200 and 208 V, can be found in certain facilities. Highervoltages, such as 2300 and 4160 V, are available in specific situations and maybe needed for large motors. The higher the voltage, the lower the amperage andtherefore the smaller the wire size and switching or starter capacity required fora given motor power.21-3.1.8 Amperage. Amperage describes how much current is required to runa motor. A motor nameplate typically shows full-load amperage (FLA), which isthe amount of current required when the motor is loaded to the rated power. Poweror wattage of a motor is theoretically the product of voltage times amperage.However, motors are sized based on mechanical output. Because no motor is100% efficient, the inefficiency is added to the theoretical power and reflectedin the amperage required to operate a motor. Minimum efficiency standards formotors are established by the government to avoid unnecessary waste of energy.Motor manufacturers can offer higher-efficiency motors, which will waste lessenergy and therefore run cooler. High-efficiency motors are usually required whenused with variable speed drives, such as variable frequency invertors, because ofreduced cooling and efficiency at lower speeds.

1262MECHANICAL DESIGN OF MIXING EQUIPMENT21-3.1.9 Power. Power, in horsepower or kilowatts, is the primary criterionused to establish motor size. Commercially available motors, like those mostoften used on mixers in the United States, come in standard sizes, such as (inhorsepower) 0.25, 0.33, 0.5, 0.75, 1, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60,75, 100, 125, 150, 200, and 250. Larger motors are nonstandard but typicallyfollow similar increments of nominal power. Motors for international use arerated in kilowatts of power and r