Apr 15, 2015
The Converflo headbox was developed to achieve a higher degree of fiber dispersion, which results in better sheet formation with more uniform distribution of fines through the sheet thickness.
A Converflo headbox is a hydraulic unit and uses no air cushion or rectifier rolls. Stock flows from a fan pump through the piping system and into a tapered header. A tube bank provides cross-machine distribution of the stock into the element area and out the slice.
The Converflo element consists of a series of thin converging channels formed by flexible plastic sheets called trailing members. Stock emerges from the tube bank and flows into these channels. The tube bank distributes flow into each channel uniformly across the machine. The channels keep the stock evenly dispersed while dampening large-scale turbulence created upstream. Final channel thickness determines the degree of turbulence and varies with the thickness and length of the sheets.
The original Beloit Converflo design includes an open stilling chamber between the tube bank and the element. Newer designs, such as the Concept IV-MH headbox, do not use a stilling chamber.
Converflo headboxes have compartments in the slice, apron, and support beam. Heated water may be circulated through these compartments to minimize the profile stability problems caused by starting up a cold headbox.
The Concept IV-MH Headbox uses the same basic components as the Converflo headbox, but adds innovations that improve its ability to deliver a uniform cross-directional stock flow to the slice.
In addition to the basic Converflo headbox components, the Concept IV-MH includes a parabolic inlet header, redesigned distributor tubes, redesigned Converflo sheets, and the capability of adding a Consistency Profiling System.
The Converflo dump valve is located in the inlet transition piece. This valve is a vacuum breaker that prevents damage to the Lexan Converflo sheets when the headbox is shut down.
When the fan pump stops, the stock flow reverses to drain. As the ends of the Converflo sheets settle down and touch, they form a seal. This results in the formation of a vacuum in the Converflo element area. A high vacuum can draw the sheets back, bending and damaging them.
The dump valve is interlocked to the fan pump relay. When the pump is on, the dump valve remains closed. When the pump is shut off, the valve opens and allows air to enter the piping system and headbox.
There may be temporary flow from the vacuum breaker that should be directed to the wire pit via a hose or pipe. The hose must allow air to enter the vacuum breaker quickly. The hose or pipe should be run to a funnel to allow air to enter.
For more information about maintaining headboxes contact your Valmet representative. Valmet's Paper Machine Maintenance School covers headboxes in detail.