Page 313 - Biosystems Engineering

290 Cha pte r Ni ne

where H = depth of liquid
b = radius of the liner surface of the liquid at the top of the
0
basket
g = gravitational force
The fluid pressure varies at different points along the wall. How-
ever, the centrifugal pressure on the wall of the basket is
P = 1 S ω ( r − x ) (9.6 )
2
2
2
c 2 1
where S is specific gravity of liquid and x is any distance from the
1
axis.
The stress on the wall of the basket is given by
F = r ( P + S T r , ω 2 ) (9.7)
T c 2
2
where P is fluid pressure (lb/ft ) and S is density of material of
c 2
which the basket is made.
9.3.3 Thickening
In thickening, the emphasis is on the solids as the end product of
separation—the compacting of suspended solids into dense slurry or
sludge in order to facilitate subsequent processing or disposal of solids.
Various settlers and thickeners are used in industrial operations,
such as Dorr thickener (batch as well as continuous settling charac-
teristics of the sludge) (Fig. 9.4). Laboratory tests are usually con-
ducted, and the data obtained can be processed to determine the
diameter of the thickener. Steps in the design are listed as follows:
1. Determination of compression point
2. Compression depth
3. Classifier area (from overflow data and zone tests )
4. Classifier depth

Total depth would comprise the clarification zone, the compres-
sion zone, and ineffective depth due to sloping of rakes and feed well.
Normally, mild steel, concrete, or wood is used. For corrosive solu-
tions, stainless, lead-lined, and rubber–covered steels may be used.
Considerations may be given to the following:

1. Floor space available
2. Conservation of heat
3. Amount of underflow solids
4. Periodic overloads and corrosive material