Sieving efficiency

Sieving process

The sieving efficiency (also sieving quality ), a term used in process engineering , describes the quality of a separation process using a sieve . The efficiency decreases with the throughput related to the sieve area and depends on many other factors (see below). ${\displaystyle \eta }$

There are several definitions of sieve efficiency, one of which is:

${\displaystyle {\begin{alignedat}{2}\eta &={\frac {F_{d}}{F_{a}}}&&\cdot 100\,\%\\&={\frac {f_{a}-f_{g}}{(1-f_{g})f_{a}}}&&\cdot 100\,\%\\&\leq 1&&\cdot 100\,\%\end{alignedat}}}$

With

• f = mass fraction of the sieve fines in%, that is the mass F of the sieve fines per total mass M:
  • Index _a = in the task
  • Index _d = in the passage (fine material)
  • Index _g = in the overflow (coarse material).

If there is no damage to the sieve bottom, f _{d is} usually 100%:

${\displaystyle f_{d}=1\Leftrightarrow F_{d}=M_{d}}$

d. H. Only fine material is conveyed through the sieve, but no coarse material (see Fig.); therefore the sieving efficiency is only calculated for the coarse material when designing a machine.

In this case, the mass fraction of the fine screen in the overflow can be at most as large as in the task:

${\displaystyle f_{g}\leq f_{a},}$

namely if the sieving efficiency is zero:

${\displaystyle \eta =0\Rightarrow f_{g}=f_{a},}$

and minimally zero, namely when the sieving efficiency is 100%:

${\displaystyle \eta =1\Rightarrow f_{g}=0.}$

Influencing factors

• Throughput of the screening machine
• Type and shape of the screen lining
• Machine code number K _V
• Moisture of the screenings.

Derivation

When deriving the above The following mass balances are helpful in formulas :

• for the total mass: ${\displaystyle M_{a}=M_{d}+M_{g}}$
• for the fine screen: ${\displaystyle F_{a}=F_{d}+F_{g}}$
• The combination of the two balances above results in the following for the non-screen fine: (usually yellow, see above). ${\displaystyle M_{a}-F_{a}=M_{g}-F_{g}}$ ${\displaystyle M_{d}-F_{d}=0,}$

literature

• Paul Schmidt, Rolf Körber, Matthias Coppers: Sieving and sieving machines: Basics and application