TECH AGIT Agitation Equipment

8/3/2019 TECH AGIT Agitation Equipment

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673331 Food Processing Machinery Kanthamoon W. 1

Agitation equipment

Liquid are most often agitated in some kind of tank or vessel. The top of the vessel may be

open to the air, or it may be closed. The proportions of the tank vary widely, depending on thenature of the agitation problem. A standardized design such as that shown in Fig. 9-1.

Figure.9-1 Typical agitation process vessel.

An impeller is mounted on an overhung shaft; the shaft is driven by a motor, sometimes directly

connected to the shaft but more often connected to it through a speed-reducing gearbox. Accessories

such as inlet and outlet lines, coils, jacket, and wells for thermometers or other temperatures-

measuring devices are usually included.

The impeller creates a flow pattern in the system, causing the liquid to circulate through the

vessel and return eventually to the impeller

Impellers are divided into two classes

1. Axial-flow impellers which generate currents parallel with the axis of theimpeller shaft.

2. Radial-flow impellers which generate currents in a tangential or radialdirection.

The three types of impellers are propellers, paddles, and turbines, other special impellers

are also useful in certain situations, but the three main types solve perhaps 95% of all liquid-agitation problems


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Propellers

A propeller is an axial-flow, high-speed impeller for liquids of low viscosity. Small

propeller turn at full motor speed, either 1,150 or 1,750 rpm; larger ones turn at 400 to 800 rpm. The

flow currents leaving the impeller continue through the liquid in a given direction until deflected by

the floor or wall of the vessel. The highly turbulent swirling column of liquid leaving the impeller

entrains stagnant liquid as it move along, probably considerably more than an equivalent column

from a stationary nozzle would. The propeller blades vigorously cut or shear the liquid. Because of

the persistence of the flow currents propeller agitators are effective in very large vessels.

A revolving propeller traces out a helix in the liquid, and if there were no slip between

liquid and propeller, one full revolution would move the liquid longitudinally a fixed distance

depending on the angle of inclination of the propeller blades. The ratio of this distance to the propeller diameter is known as the pitch of the propeller. A propeller with a pitch of 1.0 is said to

have square pitch.

Various propeller designs are illustrated in Fig. 9-2. Standard three-bladed marine

propellers with square pitch are most common; four-bladed, toothed, and other designs are

employed for special purposes.

Figure.9-2 Mixing propeller: (a) standard three-blade; (b) weedless; (c) guarded.

Propellers rarely exceed 18 in. in diameter regardless of the size of the vessel. In a deep

tank two or more propellers may be mounted on the same shaft, usually directing the liquid in the

same direction. Sometimes two propellers work in opposite directions, or in push-pull, to create a

zone of especially high turbulence between them.

Impellers

Propellers Paddles Turbines


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Paddles

For the simpler problems an effective agitator consists of a flat paddle turning on a vertical

shaft. Two-bladed and four-bladed paddles are common. Sometimes the blades are pitched; more

often they are vertical.

Paddles turn at show to moderate speeds in the center of a vessel; they push the liquid radial

and tangentially with almost no vertical motion at impeller unless the blades are pitched. In deep

tanks several paddles are mounted one above the other on the same shaft. In some designs the

blades conform to the shape of a dished or hemi-spherical vessel so that they scrape the surface. A

paddle of this kind is known anchor agitator. Anchors are useful for preventing deposits on a heat-

transfer surface, as in a jacketed process vessel, but they are poor mixers.

Industrial paddle agitators turn at speeds between 20 and 150 rpm. The total length of a

paddle impeller is typically 50 to 80% of the inside diameter of the vessel. The width of the blade is

0ne-sixth to one-tenth its length. At very slow speeds a paddle gives mild agitation in an unbaffled

vessel. At higher speeds baffles become necessary. Otherwise the liquid is swirled around the vessel

at high speed but with little mixing

TurbinesSome of the many designs of turbine are shown in Fig. 9-3. Most of them reasonable

multibladed paddle agitators with short blades, turning at high speeds one a shaft mounted centrally

in the vessel. The blades may be straight or curved, pitched, or vertical. The impeller may be open,

semi enclosed, or shrouded. The diameter of the impeller is smaller than paddles, ranging from 30 to

50% of the diameter of the vessel.

Figure 9-3: (a) open straight blade; (b) bladed disk; (c) vertical curved blade; (d) shrouded curved

blade with diffuser ring


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Turbines are effective over a very wide range of viscosities. In low-viscosity liquids

turbines generate strong currents which persist throughout the vessel, seeking out and destroying

stagnant pockets. Near the impeller is a zone of rapid currents, high turbulence, and intense shear.

The principal currents are radial and tangential. The tangential components induce vortex and

swirling, which must be stopped by baffles or by a diffuser ring if the impeller is to be most

effective

Flow patter n in agitated vessels

The type of flow in agitated vessel depends on the type of impeller, the characteristics of

the fluid, and the size and proportions of the tank, baffles, and agitator.The velocity of the fluid at any point in tank has three components, and the overall flow

pattern in the tank depends on the variations in these three velocity components from point to point.

The first velocity component is radial and acts in a direction perpendicular to the shaft of

the impeller.

The second component is longitudinal and acts in a direction parallel with the shaft.

The third component is tangential, or rotational, and acts in a direction tangent to circular

path around the shaft.

In the usual case of a vertical shaft, the radial and longitudinal components are useful and

provide the flow necessary for the mixing action. When the shaft is vertical and centrally located in

the tank, the tangential component is generally disadvantageous. The tangential flow follow a

circular path around the shaft, create a vortex at the surface of the liquid, as shown in Fig. 9-4., and

tends to perpetuate, by a laminar flow circulation, stratification at the various levels without

accomplishing longitudinal flow between levels. If solid particles present, circulatory currents tend

to throw the particles to the outside by centrifugal force from where they move downward and to the

center of tank at the bottom. In unbaffled vessel circulatory flow is induced by all types of

impellers, whether axial flow or radial flow. At high impeller speeds the vortex may be so deep that

it reaches the impeller and gas from above the liquid is drawn down into the charges. Generally this

is undesirable.


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Figure 9-4 Vortex formation and circulation pattern in an agitated tank

Prevention of swirling

Circulatory flow and swirling can be prevented by any of three methods. In a small tank, the

impeller can be mounted off center, as shown in Fig. 9-5. In a larger tank, the agitator may be

mounted in the side of the tank, with the shaft in a horizontal plane but at an angle with a radius, as

shown in Figure 9-6

Figure 9-5 Off-center impeller


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Figure 9-6 Side-entering

Propeller

In a large tank with vertical agitators, the preferable method of reducing swirling is to install baffles,

which impede rotational flow without interfering with radial or longitudinal flow. A simple and

effective baffling is attained by installing vertical strips perpendicular to the wall of the tank, as

shown in Fig 9-7

Figure 9-7 Flow pattern

In a baffled tank with aCentrally mounted propeller

agitator

For turbines, the width of the baffle need be no more than one-twelfth the tank diameter; for

propellers, no more than one-eighteenth the tank diameter, with side-entering, inclined, or off-center

propellers baffles are no need. Once the swirling is stopped, the specific flow pattern in the vessel

depends on the type of impeller.

Propeller agitators drive the liquid straight down to the bottom of the tank, where the

stream spreads radially in all directions toward the wall, flows upward along the wall, and return to

the section of the propeller from the top. This pattern is shown Fig 9-7. Propellers are used when

strong vertical currents are desired; e.g. when heavy solid particles are to be kept in suspension.

They are not ordinarily used when the viscosity of the liquid is greater than about 50 P


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Paddle agitators give good radial flow in the immediate plane of the impeller blades but are

poor in developing vertical currents. This is the main limitation of paddle agitators. As a result

paddles are ineffective in suspending solids.

Turbine impellers drive the liquid radially against the wall, where the stream divides, one

portion flowing downward to the bottom and back to the center of the impeller from below, and the

other flowing upward toward the surface and back to the impeller from above, as shown in Fig 9-4.

Turbines are especially effective in developing radial currents, but they also induce vertical flows,

especially when baffled. They are excellent in mixing liquids having about the same specific

gravity.

Flow number

A turbine or propeller agitator is a pump impeller operating without a casing and with undirected

inlet and output flows. Consider the flat-bladed turbine impeller shown in Fig 9-11.

Figure 9-11 Velocity vectors

at tip of turbine impeller blade.

u2 is the velocity of the blade tips, V u2 and Vr2 are the tangential velocities of the liquid leaving the

blade tips respectively; V 2 is the total liquid velocity at the same point.

Assume that the tangential liquid velocity is some fraction k of the blade-tip velocity, or

Vu2 = k u2 = n Dk a (9-1)

Since u 2 = n Da .

The volumetric flow rate through the impeller

q = Vr2A p (9-2)

Ap is taken to be area of the cylinder swept out by the tips of the impeller blades, or

A p = W Da (9-3)

wherea

D = impeller diameter


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W = width of blades

From the geometry of Fig.9-11

Vr2 = (u2-Vu2) tan 2 (9-4)

Substituting for V u2 from (9-1) gives

Vr2 = ( ) 2tan1 k n Da (9-5)The volumetric flow rate, from Eqs (9-2) to (9-4) is therefore

q = 222 tan)1( k nW Da (9-6)

For geometrically similar impellers W is proportional toa

D , and hence, for given values of k and

2

q

3

anD (If may be shown that for a given impeller, k and 2 are related by the equation

k k

=

12

tan 2 (9-7))

The ratio of these two quantities is called the flow number N Q,

NQ 3anD

q(9-8)

Equation (9-6) to (9-8) show that if 2 is fixed, N Q is constant. In general angle 2 , the angle at

which liquid leaves the impeller, is note equal to the angle of the tip of the blades. For marine

propellers, 2 may be considered constant; for turbines it is a function of the relative size of the

impeller and the tank. For design of baffled agitated vessels the following values are recommended.

For marine propellers: N Q = 0.5

For six-bladed turbines with a DW =0.2

NQ = 0.93a

t

D

D

where t D is the vessel diameter.Power consumption

The power for mixing was measured electrically, though accurate results were difficult to obtain

because of the difficultly of assessing the power used in the belt drive and gears. Some of their

results are shown in Fig 9-12 From curve 1, it is seen that the time of stirring fell off quite steadily

with an increase in speed. The addition of four simple baffles (25 mm x 100 mm positioned 50 mm

from the wall), reduced the time for stirring but increased the power requirement (curve 2). Curve 3

and 4 show the change in power consumption at various speeds with and without baffles.


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Figure 9-12 Power and time of mixing

as a function of speed for paddle agitator

Power correlations

To estimate the power required to rotate a given impeller at a given speed, empirical correlations of

power (or power number) with the other variables of the system are needed. The form of such

correlations can be found by dimensional analysis, given the important measurements of the tank

and impeller, the distance of the impeller from the tank floor, the liquid depth, and the dimensions

of the baffles if they are used.

The various linear measurements can all be converted to dimensionless ratios, called shape factors, by dividing each of them by one of their number which is arbitrarily chosen as a basis. The diameter

of the impeller D is suitable choice for this base measurement, and the shape factors are calculated

by dividing each of the remaining measurements by the magnitude of D. The impeller diameter D is

then also taken as the measure of the size of the equipment and used as a variable in the analysis,

just as the diameter of the pipe was in the dimensional analysis of the friction in pipes. Two mixers

of the same geometrical proportions throughout but of different sizes will have identical shape

factors but differ in the magnitude of D. Devices meeting this requirement are said to possess

geometrical similarity.

D H

DW

Dh

D

W

D

Z

D

D B AT ;;;;; ; must be the same in two systems.

If the boundary conditions are fixed, then one variable such as power P can be expressed in term of

a number of other independent variables:

( ) ,,,,, gg DnP c= Application of the method of dimensional analysis gives the result

=

g DnnD

Dn

Pg c22

53,


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By taking account of the shape factors

= D H

D Z

D D

g DnnD

Dn

Pg AT c,.......,,,,

22

53

53 DnPg c is the power number N p.

2nDis the Reynold number N Re.

g

Dn2

is the Froude number

NFr .

N p = (NRe,NFr , D H

DW

Dh

D

W

D

Z

D

D B AT ;;;;; )

Mixer s for pastes and plastic masses Mixers described in this section are change-can mixers; kneaders, dispersers, and masticators;

muller and pan mixers.

Change-can mixers

These devices blend viscous liquids or light paste, as in food processing or paint manufacture. In the

pony mixer consist of several vertical blades held on a rotating head and positioned near the wall of

the can. The blades are slightly twisted. The can rests on a turntable driven in a direction opposite to

that of the agitator, so that during operation all the liquid or paste in the can is brought to the blades

to be mixed. When mixing is complete, the agitator head is raised, lifting the blades out of the can;

the blades are wiped clean; and the can is replaced with another containing a new batch.

In the beater mixer in Fig 9-13b the can is stationary. The agitator has a planetary motion so

that it repeatedly visits all parts of the vessel. Beaters are shaped to pass with close clearance over

the side and bottom of the mixing vessel.

Figure 9-13 Double-motion paste mixer: (a) pony mixer; (b) beater mixer


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Kneaders, dispersers, and masticators

Kneading is a method of mixing used with deformable or plastic solids. It involves squashing the

mass flat, folding it over on itself, and squashing it once more. Most kneading machines also tear

the mass apart and shear it between a moving blade and a stationary surface.

A two-arm kneader handles suspensions, pastes, and light plastic masses. In all these

machines the mixing is done by two heavy blades on parallel horizontal shafts turning in a short

trough with a saddle-shaped bottom. The blades turn toward each other at the top, drawing the mass

downward over the point of the saddle, then shearing it between the blades and the wall of the

trough. The circles of rotation of the blades are usually tangential, so that the blades may turn at

different speeds in any desired ratio. A small two-arm kneader with tangential blades is sketched in

Fig 9-14.

Figure 9-14 Two-arm kneader.

Figure 9-15 kneader and disperser blades: (a) sigma blade: (b) double-naben blade;(c) disperer blade.

Muller mixers

It gives a distinctly different mixing action from that of other machines. Mulling is a smearing or

rubbing action to that in a mortar and pestle. Fig9-16. In this particular design of muller the pan is

stationary and the central vertical shaft is driven, causing the muller wheels to roll in a circular path

over a layer of solids on the pan floor. In another design the axis of the wheels is held stationary and


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the pan is rotated; in still another wheel are not centered in the pan but are offset, and both the pan

and the wheels are driven. Mullers are good mixers for batches of heavy solids and paste.

Figure 9-16 Muller mixer

Mixers for dry powders

Many of the machines described in this section can blend solids when they are dry and free-flowing

as well as when they are dump, pasty, rubbery, or plastic.

Ribbon blenders

A ribbon blender consists of a horizontal trough containing a central shaft and a helical

ribbon agitator. Atypical ribbon mixer is shown in Fig. 9-17. Two counteracting ribbons are

mounted on the same shaft, one moving the solid slowly in one direction, the other moving it

quickly in the other. The ribbons maybe continuous or interrupted. Mixing results from the

turbulence induced by the counteracting agitators, not from mere motion of the solids through the

trough. The trough is open or lightly covered for light duty, closed and heavy-walled for operation

under pressure or vacuum. Ribbon blenders are effective mixers for thin pastes and for powders that

do not flow readily. The power they require is moderate.

Figure 9-17 Ribbon mixer

Tumbling mixer

Many materials are mixed by tumbling them in a partly filled container rotating about a

horizontal axis. Tumbling barrels effectively mix suspensions of dense solids in liquids and heavy


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dry powers. In Fig 9-18. The double-cone mixer shown at (a) is a popular mixer for free-flowing dry

powders. A batch is charged into the body of the machine from above until it is 50 to 60 percent

full. The ends of the container are closed and the solids tumbled for 5 to 20 min. The machine is

stopped; mixed material is dropped out the bottom of the container into a conveyer or bin. The twin-

shell blender shown at (b) is made from two cylinders joined to form a V and rotated about a

horizontal axis. Like a double-cone blender, it may contain internal sprays for introducing small

amounts of liquid into the mix or mechanically driven devices for breaking up agglomerates of

solids. Twin-shell blenders are more effective in some blending operations than double-cone

blenders. Tumbling mixers are made in a wide range of sizes and materials of construction. They

draw a little less power than ribbon blenders.

Figure 9-18 Tumbler mixers : (a) double-cone mixer; (b) twin-shell blender

Internal screw mixers

Free-flowing grains and other light solids are often mixed in a vertical tank containing a

helical conveyor which elevates and circulates the material. In the type shown in Fig. 9-19 the

double-motion helix orbits about the central axis of a conical vessel, visiting all parts of the mix.

Figure 9-19 Internal screw mixer (orbiting type)

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TECH AGIT Agitation Equipment