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........................................................... Special Applications

Figure 8.9: High density screw pump (Reproduced with permission of Warren Pumps Inc.)

inlet to distribute the stock in a more uniform manner across the pipe.
This prevents the slugging effect as the stock enters the impeller. It also
agitates the stock in those cases where it tends to dewater rapidly. For
paper stock that has a high tendency to dewater, agitation in the suction
source is very important to the operation of the stock pump.

8.2.4.2 High density pumps
The High Density pumps are traditionally a positive displacement
design which often uses twin intermeshing pump screws driven by
external timing gears to maintain the efficient clearance between the
screws. See Figure 8.9.
8.2.4.3 The fan pump

As the final pump in the process, the performance of the Fan Pump is
particularly critical. It is a horizontal, double suction centrifugal pump
with an axially split casing which provides the optimum of efficiency
and reliability. It must also be able to provide a uniform pressure to the
headbox, as the quality of the paper can be severely affected by
excessive pulsations from the pump. Consequently, these pulsations
must be reduced to an absolute minimum.
Pressure pulsations can be initiated by each impeller vane as it passes the
cut-water of the volute. The pressure shocks created are about the same
as opening and closing a valve in rapid succession and can result in
unacceptable ripples in the sheet. Considerable strides have been made

=__;_____~i ................ Z ..................................................... ! ..... ~ ......................................................... ~ ....... 135 m