Environmental Engineering FOURTH EDITION

278 ENVIRONMENTAL ENGINEERING

Tll.:l:y, E ,

0.1 91,1

II lFjl

0.01

0.01 0.1 1.0 10 100

Particle size (x) cm

Figure 14-4. Rosin-Rder particle size distribution equation. In this sample
n = 1.0 and xc = 1.6.

Table 14-1. Rosin-Rammler Exponents for Typical Shredder Installation’

Washington, DC

Wilmington, DE

Charleston, SC

St. Louis, MO

Pompano Beach, FL

0.689

0.629

0.823

0.995

0.587

2.77

4.16

4.03

1.61

0.67

=From Stratton andAlter (1978).

If Y = 0.90, or 90%, Eq. (14.2) reduces to

(14.3)

where xgg is the size at which 90% of the particles pass. Ordinarily, n = 1 .O, in which
case xgg = 2.3~~. Table 14-1 gives some typical values of x, and n.
In addition to particle size reduction, shredder performance must be measured
in terms of power use, expressed as specific energy (lulowatt hourdton of refuse
processed). Figure 14-5 illustrates how solids feed rate, shredder speed, moisture
content, and size of particles in the feed are interrelated.
Asemi-empirical equation often used to estimate the power requirements for shred-
ders was developed by Bond (Bond 1952). The specific energy Wrequired to reduce a
unit weight of material 80% finer than some diameter LF to a product 80% finer than
some diameter Lp, where both LF and Lp are in micrometers (pm), is expressed as

(14.4)

where Wi is the Bond work index, a factor that is a function of the material processed

for a given shredder and a function of shredder efficiency for a given material. W has

the dimension of kilowatt hours/ton (kwldton) if Lp and LF are in micrometers. The

factor 10 also corrects for the dimensions. Typical values of the Bond work index for

MSW and other specific materials are given in Table 14-2.

W= 10Wi[---] 1 1

45 dG3