Reference Manual

17−3
Figure 17-5. Hydraulic Headbox
Drawing is from TAPPI’s Making Pulp and
Paper Series and is used with permission.
turbulence inside the headbox and to break up the
fiber flocs so fibers are well distributed. This type
is good for slower paper machines and specialty
machines with a wide range of flow requirements.
D Hydraulic — These devices are completely
filled with stock. Because no air cushion exists
many hydraulic headboxes use a pressure
attenuator at the headbox manifold to reduce any
pressure variations. Since hydraulic headboxes
have no rectifier rolls, they are designed to
deflocculate the stock by changes in velocity as
the stock passes through tube bundles or across
flat sheets. The discharge velocity from the slice
depends directly on the feeding pump pressure.
This type of headbox can be found on most new
paper machines (figure 17-5).
D Dilution Control — This type uses dilution
or consistency control across the width of the
headbox to correct errors in basis weight (figure
17-6). By opening a particular dilution valve, the
computer will add dilute white water to the inside
of the headbox in precisely the right point to dilute
a heavy streak. If the sheet is too light, the
computer removed some of the dilute water by
closing the valve. There can be as many as one
hundred or more dilution valves located along the
Figure 17-6. Dilution Control Headbox
Drawing is from TAPPI’s Making Pulp and
Paper Series and is used with permission.
back of the headbox, all of which are individually
controlled.
Each type of headbox contains a headbox slice.
The slice is a full-width orifice or nozzle with a
completely adjustable opening to give the desired
flowrate. The jet of stock emerging from a typical
headbox slice contracts in thickness and deflects
downward as a result of slice geometry. The jet
thickness, together with the jet velocity,
determines the volumetric discharge rate from the
headbox.
Every slice has a top lip and an apron (bottom lip),
both constructed of suitable alloy materials to
resist corrosion. The top lip is adjustable up or
down as a unit (main slice) and also in local areas
by the use of individual micro-adjusters. These
small adjustment rods are attached to the slice lip
and, with manual adjustments or the help of a
computer system, can improve the distribution of
the stock and allow for a more uniform basis
weight across the width of the paper machine. The
rectifier and hydraulic headboxes utilize adjustable
slices in the same way.
Forming Wire
The stock leaving the headbox slice, typically
between 0.5 - 1% solids, is deposited onto a
synthetic forming fabric (or wire). Water
immediately begins to drain through the forming