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Oscillating Screens

o Oscillating Screens are characterized by a relatively low

speed (300 or 400 oscillations per minute) in a plane

essentially parallel to the screen..

o It is confined in general to the finer openings below 6 mm.

o It is the cheapest form of screen on the market and is used

for batch screening.

Reciprocating Screens

o Reciprocating screens are driven by an eccentric under the

screen at the feed end.

o The motion varies from gyratory (about 2 in. in diameter) at

the feed end to reciprocating motion at the discharge end.

o These screens are usually inclined about 5 degrees, giving

the screen a motion normal to the cloths of about 1/10 in.

o This type of screen is popular and widely used for screening dry chemicals down to

about 300 mesh.

Revolving Screens (Trommels)

o In a trommel screen, feed enters through a

conveyor or hopper into the higher end of a

slightly inclined rotating cylindrical screen.

o The tumbling action of the trommel mixes the

material as it flows through the cylinder, so

every particle contacts the screen surface.

The undersize falls through the screen, while the oversize remains and is discharged at

the lower end of the cylinder.

ADVANTAGES DISADVANTAGES

low headroom low power requirement

high cost of maintenance of the

screen and the supporting structure

owing to vibration

low capacity compared with inclined

high-speed vibrating screens

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Screen. In selecting the screen for the equipment, one must take considerations the screen

type, mesh size, and open area of the screen.

Screen type. Screen can be synthetic (either nylon or polyester) or wire (stainless steel,

magnetic stainless steel, carbon steel, or plated or coated carbon steel).

Mesh size.Choosing mesh size is critical in selecting a screen because the right mesh size

allows an efficient screening rate while ensuring that oversize particles and contaminants are

effectively removed from the on-size particles. Mesh size is the number of openings in the

screen, in each direction, from center to center of parallel filaments or wires per linear inch.

Open area. The greater the open area, the greater the sifting capacity.

MATERIAL BALANCES OVER SCREEN

Simple material balances can be written over a screen which are useful in calculating the

ratios of feed, oversize and underflow from the screen analyses of the three streams and

knowledge of the desired cut diameter.

Let F, D and B be the mass flow rates of the feed, overflow and underflow, respectively

and xF, xD and xBbe the mass fraction of material A in these three streams. The mass fractions

of material B in the feed, overflow and underflow are 1-xF, 1- xD and 1-xB.

Since the total material fed to the screen must leave it either as underflow or as overflow,

F = D + B (Eq. 1)

The material A in the feed must also leave in these two streams and

FxF= DxD+ BxB (Eq. 2)

Elimination of B from Eq. 1 and Eq. 2 gives

(Eq. 3)

Elimination of D gives

(Eq. 4

EFFECTIVENESS OF SCREENS

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The effectiveness of a screen (often called the screen efficiency) is a measure of the

success of a screen in closely separating materials A and B.

A common measure of screen effectiveness is the ratio of oversize material A that is

actually in the overflow to the amount of A entering with the feed. These quantities are Fx Fand

DxD, respectively. Thus

(Eq. 5)

Where EAis the screen effectiveness based on the oversize. Similarly, an effectiveness E B

based on the undersize materials is given by

(Eq. 5)

A combined overall effectiveness can be defined as the product of the two individual ratios,

denoted by E,

(Eq. 6)

Also, by substituting Eq. 3 and Eq. 4 to Eq. 6 gives

(Eq. 7)

Factors Effect ing the Eff ic iency of th e Screens

Rate of Feeding

Particle Size

Moisture

Worn or damaged Screens

Binding (Clogging) of Screens

Electro Static Charge

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Questions

1. The measure of the success of a screen in closely separating materials A and B.

a. Screen Capacity

b. Screen Analysis

c. Size Ratio

d. Screen Efficiency

Answer: D. Screen Efficiency it defines how much material A was separated from B

2. The following are the factors that affects screen efficiency, EXCEPT

a. Moisture

b. Rate of Feeding

c. Particle Size

d. None of the above

Answer: D. Screen Efficiency it defines how much material A was separated from B.

3. The following are the factors that affects screen efficiency, EXCEPT

a. Moisture

b. Rate of Feeding

c. Particle Size

d. None of the above

Answer: D. None of the above all of them affects how well will be the screening process is.

4. Reciprocating Screens has

a. High Cost Maintenance

b. Low Cost Maintenance

c. No Cost Maintenance

d. None of the above

Answer: A. High Cost Maintenance it is because of the motion that varies gyratory.

5. Revolving Screens has

a. High Efficiency

b. Low Efficiency

c. No Efficiency

d. None of the above

Answer: B. Low Efficiency It has low capacity when it comes to fine solids.

Problem Solving

A quartz mixture having the screen analysis shown in Table 30.1 is screened through a

standard 8-mesh screen. The cumulative screen analysis of overflow and underflow are given in

Table 30.1. Calculate the mass ratios of the overflow and underflow to feed and the overall

effectiveness of the screen.

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Solution:

The cut-point diameter is the mesh size of the screen, which from Table 30.1 is 2.362 mm. Also

from Table 30.1, for this screen,

xF = 0.15 xD =0.43 xB = 0

From Eq. 3, the ratio of overflow to feed is

= 1.1628

From Eq. 4, the ratio of underflow to feed is

= 0.6512

The overall effectiveness, from Eq. 7, is

=0.7661

REFERENCES:

Ansery, A. Particle Size Measurement Date Retrieved: July 6, 2014, from:

http://teacher.buet.ac.bd/ashfaqansery/Particle%20Technology%20(ChE309)/lecture2.pdf

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McCabe, W. L. (1993). Unit Operations of Chemical Engin eering. 5th ed. New York:

McGraw Hill Inc.

Walas, S. M. (1990). Chemical Process Equipment: Select ion and Design. USA:

Butterworth-Heinemann

Brown, G. G. (1950). Unit Operations. New York: John Wiley & Sons, Inc.