1. GOVERNMENT ENGINEERING COLLEGE, VALSAD
DEPARTMENT OF CHEMICAL ENGINEERING SUBJECT: MECHANICAL OPERATION ( ) TOPIC NAME: AGITATION

2. PREPARED BY: SUBMITTED TO: PROF. ARUN PATEL
SR. NO.
ENROLLMENT NO.
NAME 1.
MEHTA VAIBHAV 2.
MENON ABHIMANYU 3.
MISTRY KARTIK 4.
MITTHA SHUBHAM 5.
MODI ANIRUDDH

3. INTRODUCTION
Though often confused, agitation and mixing are not synonymous.
Agitation refers to the induced motion of a material in a specified way, usually in a circulatory pattern inside some sort of container.
Mixing is the random distribution, into and through one another, of two or more initially separate phases.
A single homogenous material, such as a tankful or cold water, can be agitated, but it cannot be mixed until some other material is added to it.

4. Purposes of Agitation
Liquids are agitated for a number of purposes. These purposes include
Suspending solid particles.
Blending miscible liquids, for example, methyl alcohol and water.
Dispersing a gas through the liquid in the form of small bubbles.
Dispersing a second liquid, immiscible with the first, to form an emulsion or it suspension of fine drops.
Promoting heat transfer between the liquid and a coil or jacket.

5. AGITATED VESSELS
A standardized design such as that shown in Figure, however, is applicable in many situations.
The proportions of the tank vary widely, depending on the nature of the agitation problem.

6. Construction
Liquids are most often agitated in some kind of tank or vessel, usually cylindrical in form and with a vertical axis.
The top of the vessel may be open to the air; more usually it is closed.
The tank bottom is rounded, not flat, to eliminate sharp corners or regions into which fluid currents would not penetrate.
The liquid depth is approximately equal to the diameter of the tank.

7. An impeller is mounted on an overhung shaft, that is, a shaft supported from above.
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, jackets and wells for thermometers or other temperature measuring devices are usually included.
The impeller causes the liquid to circulate through the vessel and eventually return to the impeller. Baffles are often included to reduce tangential motion.

8. Impellers Impeller agitators are divided into two clauses.
Those that generate currents parallel with the axis of the impeller shaft are called axial-flow impellers.
Those that generate currents in a radial or tangential direction are called radial-flow impellers.
The three main types of impeller for low-to moderate-viscosity liquids are propellers, turbines and high-efficiency impellers.
For very viscous liquids, the most widely used impellers are helical impellers and anchor agitators.

9. Propellers
A propeller is an axial-flow, high-speed impeller for liquids of low viscosity.
Small propellers turn at full motor speed, either or rpm; larger ones turn at 400 to 800 rpm.
The direction of rotation is usually chosen to force the liquid downward, and the flow currents leaving the impeller continue until deflected by the floor of the vessel.
The highly turbulent swirling column of liquid leaving the impeller entrains stagnant liquid as it moves along, and the propeller blades vigorously cut or shear the liquid.

10. A revolving propeller traces out a helix in the fluid, 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.
A typical propeller is illustrated in Figure (a). Standard three-blade marine propellers with square pitch are most common; four-blade, toothed, and other designs are sometimes employed for special purposes.
In a deep tank two or more propellers may be mounted on the same shaft, usually directing the liquid in the same direction.

11. Impellers for the liquids of moderate viscosity:
Three-Blade Marine Propeller
Simple straight-blade Turbine
Disk Turbine
Concave-blade CD-6 Impeller
Pitched –blade Turbine

12. Turbines
Four types of turbine impeller are illustrated in Figure. The simple straight-blade turbine shown in Figure (b) pushes the liquid radially and tangentially with almost no vertical motion at the impeller.
The currents it generates travel outward to the vessel wall and then flow either upward or downward. Such impellers are sometimes called paddles.
In process vessels they typically turn at 20 to 150 r/min.

13. The disk turbine, with multiple straight blades mounted on a horizontal disk as shown in Figure (c), creates zones of high shear rate; it is especially useful for dispersing a gas in a liquid because at moderate speeds the gas is forced to flow radially to the blade tips, where it is dispersed by the high shear.
Also widely used for gas dispersion is the concave-blade CD-6 disk turbine shown in Figure (d).
A pitched-blade turbine as shown in Figure (e) is used when good overall circulation is important, because it provides some axial flow in addition to the radial flow.

14. High-efficiency impellers
Variations of the pitched-blade turbine have been developed to provide more uniform axial flow and better mixing, as well as to reduce the power required for a given flow rate.
The high-efficiency impeller HE-3 has three slanted blades that are crimped to decrease the blade angle near the tip.
The A310 fluid-foil impeller uses airfoil-shaped blades which taper so that they are narrower at the tip than at their base.
These impellers are widely used to mix low or moderate viscosity liquids, but they are not recommended for very viscous liquids or for dispersing gases.

15. Flow patterns
The way a liquid moves in an agitated vessel depends on many things; the type of impeller; the characteristics of the liquid, especially its viscosity; and the size and proportions of the tank, baffles and impeller.

16. The liquid velocity at any point in the tank has three components:
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 a circular path around the shaft.
In the usual case of a vertical shaft, the radial and tangential components are in a horizontal 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 follows a circular path around the shaft and creates a vortex in the liquid, as shown in Figure for a flat-blade turbine.

17. “Standard” Turbine Design
The designer of an agitated vessel has an unusually large number of choices to make as to type and location of the impeller, the proportions of the vessel, the number and proportions of the baffles, and so forth.

19. Power Consumption