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PAKISTAN COUNCIL OF SCIENTIFIC & INDUSTRIAL
RESEARCH LABORATORIES COMPLEX, KARACHI
OFF: UNIVERSITY ROAD, KARACHI-75280.
RACKS &
SCREENS...

screen is a device with openings for removing bigger
suspended or floating matter in sewage which would
otherwise damage equipment or interfere with
satisfactory operation of treatment units.

Figure Definition sketch for types of screens used in wastewater
treatment

Design Consideration
Velocity
 The velocity of flow ahead of and through the screen
varies and affects its operation.
 The lower the velocity through the screen, the greater is
the amount of screenings that would be removed from
sewage.
 However, the lower the velocity, the greater would be the
amount of solids deposited in the channel.

 Hence, the design velocity should be such as to permit
100% removal of material of certain size without undue
depositions.
 Velocities of 0.6 to 1.2 mps through the open area for
the peak flows have been used satisfactorily.
 Further, the velocity at low flows in the approach
channel should not be less than 0.3 mps to avoid
deposition of solids.

Head loss
 Head loss varies with the quantity and nature of
screenings allowed to accumulate between cleanings.
 Head loss through screens mainly depends on:
 Size and amount of solids in waste water
 Clear openings between bar
 Method of cleaning and its frequency
 Velocity of flow through the screens

 The head loss through clean flat bar screens is
calculated from the following formula:
h = 0.0729 (V2 - v2)
where, h = head loss in m
V = velocity through the screen in mps
v = velocity before the screen in mps

Another formula often used to determine the head loss through a bar
rack is Kirschmer's equation:
where h = head loss, m
b = bar shape factor (2.42 for sharp edge rectangular bar, 1.83 for
rectangular bar with semicircle upstream, 1.79 for circular bar and
1.67 for rectangular bar with both u/s and d/s face as semicircular).
W = maximum width of bar u/s of flow, m
b = minimum clear spacing between bars, m
hv = velocity head of flow approaching rack, m = v2/2g
q = angle of inclination of rack with horizontal

The head loss through fine screen is given by
where, h = head loss, m
Q = discharge, m3/s
C = coefficient of discharge (typical value 0.6)
A = effective submerged open area, m2

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Rack & Screen

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