Читайте также: |
|
The feed consistency determines the amount of liquor that has to pass the screen
at a given pulp production capacity. Pressure screens can operate at feed consistencies
up to 4% or 5%. The latter figure represents hardwood pulp, which generally
allows higher feed consistencies than softwood pulp. The limiting factor
defining the feed consistency ceiling is reject thickening. A screen’s pulp capacity
increases with rising feed consistency, until a point is reached when it rapidly
decreases due to blinding caused by excessive reject thickening.
A screening system with a high feed consistency is more compact and requires
less electrical energy than a low-consistency system due to the reduced amounts
of liquor pumped around. It is, however, also more demanding to control because
it operates closer to the critical point of reject thickening. At higher consistency,
blinding is favored not only by the increasing population of fibers but also by a
reduced backflush through the screen apertures. It has been shown that the intensity
of the pressure pulse goes down considerably with increasing pulp feed consistency
[13].
The passage ratio decreases as feed consistency goes up [6]. While different
opinions exist about the effects of feed consistency on screening efficiency, it is
likely that the latter is not significantly affected by the feed consistency [15]. However,
a more dilute feed is clearly improving the fractionation efficiency [19].
Several designs of modern washing equipment require feed consistencies between
3% and 4%. If such a piece of equipment is installed downstream of the
screen, only higher-end feed consistencies can provide the needed levels of accept
consistency. If the accept consistency is not critical, a good compromise between
screening efficiency, operability and power consumption for standard screening
applications may be found in the feed consistency range of 2.5% to 3.5%, with softwood
furnish at the lower end and hardwood furnish at the higher end of the range.
Temperature
The operating temperature affects the behavior of both liquor and solids. On the
one hand, the pulp fibers become more flexible at higher temperatures (see also
Section 6.3.3.1), while on the other hand the liquor viscosity decreases with higher
temperatures, improving the turbulence in the screening zone. Both of these
effects cause the screen capacity to rise [12].
Дата добавления: 2015-10-21; просмотров: 91 | Нарушение авторских прав
<== предыдущая страница | | | следующая страница ==> |
Screen Basket | | | Rotor Tip Velocity |