Feed Consistency

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.