che 154 lecture 3b screening equipment

8/11/2019 Che 154 Lecture 3b Screening Equipment

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Asst. Prof. Jewel A. CapunitanDepartment of Chemical Engineering

College of Engineering and Agro-Industrial TechnologyUniversity of the Philippines Los Baos

ChE 154 - Transfer Operations II1stsem. 2014-2015

SCREENING EQUIPMENT

and CAPACITY


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TYPES OF SCREENING EQUIPMENT

GRIZZLY SCREENS VIBRATING SCREENS

OSCILLATING SCREENS

RECIPROCATING SCREENS TROMMELS or REVOLVING SCREENS

REELS

MECHANICAL SHAKING SCREENS

GYRATORY SCREENS


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GRIZZLY SCREENS

- consists of a set of parallel bars (made of manganese steel)

separated by spacers at ends

- usual cross section of bars: trapezoidal with wide base upward

to prevent choking

- Feed size range: for large sizes particularly 1 inch and over


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Stationary Grizzly

-simplest, least expensive

-slope from horizontal : 20- 50

-for materials 0.05m ( 2 inches)

and coarser

-scalping or rough screening of

dry materials

-not satisfactory for moist andsticky materials

GRIZZLY SCREENS


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Flat Grizzlies

-parallel bars are in horizontal

plane

-used to retain occasional pieces

too large to retain for the

following plant equipment

GRIZZLY SCREENS

Vibrating Grizzly

-bar grizzlies mounted on

eccentrics so that the entire

assembly is given a back-and-

forth movement


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VIBRATING SCREENS

- used when large capacity and high efficiency are desired

- advantages: accuracy of sizing, increased capacity per unit

area, low maintenance cost per ton of material handled, saving

in installation space and weight

- fine sizing


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VIBRATING SCREENS

- mechanically vibrated screens: vibration is generated

mechanically, transmitted from high speed eccentrics to thecasing of the unit screen

- electrically vibrated screens: vibration to the casing or to the

screens comes from heavy duty solenoid or electromagnet

- mesh 325 to mesh 4; light, fine, dry materials and metal


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OSCILLATING SCREENS

- characterized by low speed oscillations ( 5 to 7 oscillations per

minute) in a plane essentially parallel to the screen cloth


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OSCILLATING SCREENS

- 0.013 m (0.5 inch ) to 60 mesh; sometimes used for light free

flowing materials at 200-300 mesh


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RECIPROCATING SCREENS

- motion varies from gyratory (about 2 inches in diameter) at

the feed end to a reciprocating motion at the discharge end

- usually inclined about 5 degrees, giving the screen a motion

normal to the cloth of about 1/10 inch

- further vibration is caused by balls bouncing against the lower

surface of the screen cloth


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RECIPROCATING SCREENS

- for fine separations even down to 300 mesh

- used to handle dry, light, or bulky materials, light metal

powders, powdered foods, and granular materials

- not for heavy tonnages of material like rock or gravel


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TROMMELS or REVOLVING SCREENS

- consist of a rotating cylinder of perforated sheet metal or wire

screen

- open at one or both ends, and the axis of the cylinder is

horizontal or inclined (3/4 inch to 3 inches per foot of length)

so that the material is advanced by the rotation of the cylinder


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- usually about 3-4 ft in diameter, 5-8 ft in length and driven at

relatively low speeds of 15 to 20 rpm

- low capacity and efficiency

- best suited for inch to 2 inches


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- used to screen stringy bark and compost materials for use as

landscaping and soil amendment products

- used in resource recovery to segregate solid waste in various

forms including MSW (garbage), compost, incinerator ash and

construction debris, or in mining and aggregate operations for

scrubbing rock and sand.


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REELS

- revolving screens driven at relatively high speed

- generally 24 to 40 inches in diameter and 5 to 8 ft long

- speed of rotation : 100-200rpm (above critical speed for a

trommel)

- used in the flour milling industry and for other light, dry, non-abrasive materials


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MECHANICAL SHAKING SCREENS

- consist of a rectangular frame

which holds wire cloth orperforated plate and is slightly

inclined and suspended by loose

rods or cables or supported from a

base frame by flexible flat springs.

- the screens are given reciprocating

motion

- low headroom and power

requirement

- high cost of maintenance and low

capacity (compared to inclined

high-speed vibrating screens)


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GYRATORY SCREENS

- boxlike machines either round or square with a series of

screen cloth nested atop one another with oscillatory motionin circular orbit

- have an auxiliary vibration caused by balls bouncing against

the lower surface of the screen

- size : 0.6 to 1.5m ( 24 to 60 inches)


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TYPES OF SCREENS


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SELECTION OF SCREENING EQUIPMENT

1. Full description of material(s) involved

- Physical characteristics (hardness, shape, diameter)

- Flow characteristic (free flowing, sluggish or sticky)

- Moisture content

- Temperature

2. Normal and maximum total rate of feed to the screen

3. Compete sieve analysis of the feed and the desired product

4. Separation(s) and the purpose of screening


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SELECTION OF SCREENING EQUIPMENT

5. Dry or Wet Screening

6. Others

-ways of delivering feed to the screen

-previous screening experience with the material

-operating hours per day

-power availability

-space limitations


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PARAMETERS TO BE CONSIDERED DURING

A SCREENING OPERATION

1. METHOD OF FEED Feeding should be done properly to obtain maximum capacity

and effectiveness

Feed should be spread evenly over the whole surface area of

the screen

Flow should be parallel to the longitudinal axis of the screen

surface

Low velocity


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2. SCREENING SURFACE3. ANGLE OF SLOPE

Need to use optimum slope( to ensure that the screen can still

remove the fines while retaining the tails)

slope,rate of travel, bedthickness

4. DIRECT ROTATION

High effectiveness if material flow is against the screen

rotation

High capacity if material flow is same as screen rotation


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5. VIBRATION AMPLITUDE AND FREQUENCY Should minimize blinding of the screen surface

Depends on the size and weight of material being handled and

type of screening surface

6. NOISE AND SAFETY

Sources of noise:

- Impact of feed on the screen surface (reduced by rubber

bearings)

- Driving mechanism (reduced by enclosure in a box or

addition of rubber linings)


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SCREEN CAPACITY

measured by mass of material that can befed per unit time to a unit area of the screen

function of the feed rate

actual screen capacities:

0.05- 0.2 ton/hr-ft2-mm for grizzlies

0.20.8 ton/hr-ft2-mm for vibrating

screen


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SCREEN CAPACITY

Type of ScreenCapacity Range

(Tons/ft2area mm aperture 24 hr)

Grizzlies 16

Stationary Screens 15

Vibrating Screens 520

Shaking and Oscillating

Screens 28

Trommels 0.32

Table 3 The approximate capacity of screens for dense

materials such as ores (p. 16, Brown)


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SCREEN CAPACITY

Relationship between Efficiency and Capacity

Capacity Efficiency: opposing factors!

* The over-all chance of passage of a given undersize particle

is a function of:

- # of contacts between particle and screen surface

- probability of passage during a single contact


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SCREEN CAPACITY

Greatest chance of passing a particles occurs if:

it struck the surface perpendicularly

it were so oriented that its minimum dimensions were parallel

to the screen surface

it were unimpeded by other particles

it did not stick to, or wedge into the screen surface


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SCREEN CAPACITY

Effect of mesh size on capacity of screens

Probability of passage depends on:

fraction of the total surface represented by openings

ratio of the diameter of the particle to the width of an opening

in the screen

number of contacts between the particle and the screen

surface

* If these factors are constant: average # of particles through a

single screen opening in unit time is nearly constant.


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SCREEN CAPACITY

*The maximum capacity of a given screen is roughly

proportional to the screen opening.

Rule of thumb:

Capacity (mass per unit time, divided by mesh size) = constant

for any specified conditions of the operationCoarse screens with 0.25-4 in. opening:

1 to 8 tons/hr-ft2

Screens with 0.05-0.25 in. opening:

0.1 to 1.0 tons/hr-ft2


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SCREEN CAPACITY

Factors affecting capacity of screens:

interference of the bed of particles with the motion of others

blinding

cohesion of particles to each other

adhesion of particles to screen surface

oblique direction of approach of particles to the surface

moisture content


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SCREEN CAPACITY

Through flow Method of Matthews

Eqn. 19-7, pp. 19-23, Perry:

A = 0.4Ct / ( Cu x Foa x Fs)

where:

A = screen area

Ct= through flow rate

Cu= unit capacity (Fig. 19-21, Perry)

Foa= open area factor (Fig. 19-22, Perry)

Fs= slotted opening factor (Table. 19-7, Perry)


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SCREEN CAPACITY


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SCREEN CAPACITY


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SCREEN CAPACITY

Sample problem:

Crushed stone is fed to a screen having an area of 2.2 ft2and

square opening of mesh 4 (Tyler Standard). Determine the

capacity of the screen in tons/hr.

che 154 lecture 3b screening equipment

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