Highlights of Second Session

1
Highlights of Second
Session Different laws for power requirement
Crushing Efficiency Factors Influencing Choice
of Size Reduction Equipment Crushing Equipments
(Jaw Crusher, Gyratory Crusher)
2
Hammer Mill In
a hammer mill, swinging hammerheads are
attached to a rotor that rotates at high speed
inside a hardened casing. The principle is
illustrated in Figure
3
(No Transcript)
4
The material is crushed and pulverized between
the hammers and the casing and remains in the
mill until it is fine enough to pass through a
screen which forms the bottom of the casing.
Materials Handled In Hammer Mills 1) brittle
2) fibrous materials
5
BALL MILL A cylindrical or
conical shell rotating about a horizontal axis,
partially filled with a grinding medium such as
natural flint pebbles, ceramic pellets or
metallic balls. The material to be ground is
added so that it is slightly more than fills the
voids between the pellets. The shell is rotated
at a speed which will cause the pellets to
cascade, thus reducing particle sizes by
impact.
6
Continued.
7

• The optimum speed of rotation is about 75 of the
  
• critical speed, which is defined as the speed
  which
• causes the steel balls to centrifuge.
• Application of Ball Mill
• Talcum Powder
• 2) Dry powder in food processing

8
Cutting Machines Slicing- Rotary or
reciprocating cutting knives are set to cut
material to the desired thickness, perhaps as
product is conveyed on a belt. Sometimes the
food is held against the blade by centrifugal
force, or sometimes the food may be held in a
carriage as it travels across the blade an
alternative for firmer fruits such as apples is
forcing the fruit through a tube with stationary
knife edges arranged radially along the length
of the tube
9
Dicing This often involves the product passing
through two sets of rotary knives at right
angles an alternative is forcing the product
through a grid, then cutting it off at the
required length
10
Chopping A bowl chopper is commonly used for
comminuting meats, harder fruits etc. A slow
speed rotating bowl moves the ingredients
beneath a set of high speed blades, achieving
chopping and mixing Pulping brush or paddle
finishers use brushes or paddles rotating
inside a stationary screen
11
SIEVING Sieving is probably the easiest and most
popular method for size analysis and separation
of the components within powders. A sieve is an
open container which has uniform square
openings in the base. The screen aperture is
defined as the space between the individual
wires of a wire mesh screen, and the mesh
number is the number of wires per linear inch.
12
(No Transcript)
13
(No Transcript)
14
Given the following sieve analysis
plot a cumulative sieve analysis and estimate the
weight fraction of particles of sizes between
0.300 and 0.350 mm and 0.350 and 0.400 mm.
15
(No Transcript)
16
Given the following sieve analysis
between 300 and 350 mm as 13and   350 and 400 mm
as 9.
17
Screening Rate Rates of
throughput of sieves are dependent upon a number
of factors          Nature and the shape of
the particles, Frequency and the amplitude of
the shaking, Methods used to prevent sticking
or bridging of particles in the apertures of
the sieve and Tension and physical nature of
the sieve material.
18
SETTLING
Definition
In settling, two immiscible liquids, or a liquid
and a solid, Gas and a solid differing in
density, are separated by allowing them to come
to equilibrium under the action of gravity, the
heavier material falling with respect to the
lighter.
19
Why do we need settling?

• Settling can be used to separate any particle
  from any fluid, whether it is a liquid from a
  gas, or a solid from a liquid.
• The removal of solids from liquid sewage wastes
• Getting crystals from what they precipitated from
• Settling solid food particles from a liquid food
• Settling the slurry from a soybean leaching
  process.

20
TYPES OF SETTLING

• Free settling of discrete particles
• 2. Zone (hindered) settling

21
FREE SETTLING
Definition Free settling implies that
the particles are settling Independently
of oneanother, i.e., a low concentration of
particles in a large volume of fluid.
particles settle independently of each other
Conti
22
HINDERED SETTLING
Definition If the settling is carried out with
high concentrations of solids to liquid so that
the particles are so close together that
collision between the particles is practically
continuous and the relative fall of particles
involves repeated pushing apart of the lighter
by the heavier particles it is called hindered
settling.
particles interfere with each other
23
APPLICATIONS

• primary settling - mainly free
• secondary settling
• -clarification - mainly zone
• -thickening - mainly compressive
• -gravity thickening of sludge - mainly
  compressive
• -water treatment (coagulation and
  flocculation)
• mainly flocculent

24
FORCES ACTING ON THE PARTICLE

• The total amount of force exerted on a particle
  can be
• broken down into four categories.
• Force due to acceleration
• Buoyancy force
• Gravitational force
• Drag force

Conti
25
Gravitational force Gravitational force acting
Downwards due to acceleration due to gravity

F mg

26
Buoyancy force
Buoyancy is the upward force exerted on an
object immersed in a fluid.
ARCHIMEDES PRINCIPLE Archimedes
Principle of buoyancy states that the
upward force on an object in a fluid is equal
to the weight of the fluid that is displaced.

27
DRAG FORCE
Definition The force in the direction of flow
exerted by the fluid on the solid is called as
drag force. Two Types Of Drag 1) Wall drag
2) Form Drag
28
The most common method of mathematically modeling
the drag force is the equation,
Fd1 CdA?v2
2 Fd Drag
Force. Cd Drag Coefficient A Cross-sectional
Area perpendicular to the flow ? Density of the
medium v Velocity of the body relative to the
medium
29

• Drag Coefficient
• The direction of the drag force is always
  opposite the direction of
• the body's velocity.
• The drag coefficient Cd is not constant. Cd
  depends upon the
• velocity of the body, viscosity of the medium,
  the shape of the body,
• and the roughness of the body's surface.

30
The Reynolds number has been found to be a useful
dimensionless number that can characterize the
drag coefficient's dependence upon the
velocity.
ReL ? v
? Re Reynolds number.L
Characteristic length of the body along the
direction of flow. ? Dynamic Viscosity of the
medium. ? Density of the medium.v Velocity
of the body relative to the medium.
31
Drag Coefficient
Relt100 CD24/Re
Regt1000 CD0.44
32
TERMINAL VELOCITY
Definition Under a constant force, (force of
gravity, Centrifugal force) particles in a
liquid accelerate for a time and thereafter move
at a uniform velocity. This maximum velocity
which they reach is called their terminal
velocity. The terminal velocity depends upon
the size, density and shape of the particles, and
upon the properties of the fluid
33
END OF SESSION