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Based on: Industry Standards and Best Practices

Languages: English

Sample Transcript

If the stock jet speed is greater than the wire speed, the jet-to-wire ratio is greater than one. In this case, the fibers will tend to align in the cross machine direction increasing the sheet strength in that direction. Sheet formation and sheet appearance are both impacted by the jet-to-wire ratio. When the jet speed and wire speed are almost equal, when the ratio is close to one, the fibers will be well distributed and randomly oriented in the sheet. Somewhere around a ratio of one, the machine direction and cross machine direction strengths will be about equal, creating a square sheet. Formation and appearance are both positively affected by the squaring process.

Hydraulic Headboxes - Stock Jet Geometry for Fourdriniers

Training Time: 26 minutes

A hydraulic headbox is designed to deliver a uniform, low consistency stock stream to the Fourdrinier wire. It is important to monitor the geometry of the stock jet because it greatly impacts the appearance and performance of the sheet. Only by understanding the variables that affect the stock jet, and how the jet geometry affects sheet properties, can one begin to make adjustments to the headbox to optimize sheet quality.

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When the jet-to-wire ratio is less than 1, the fibers will be "dragged" by the wire in the machine direction

The jet-to-wire ratio is equal to 1 when the stock jet speed and wire speed are equal

The L/b ratio is affected by horizontal slice adjustments and it greatly impacts the stock jet angle and landing point

Learning Objectives

  • Describe the purpose of the headbox in the papermaking process
  • Define the terms "hydraulic," "jet impingement angle," "jet landing point," "sheet formation," "jet-to-wire ratio," and "rush/drag"
  • Describe the impact of stock jet geometry and sheet formation on the appearance and strength properties of the sheet
  • Identify and describe common headbox adjustments and settings that affect the stock jet geometry
  • Describe the effects of the slice opening and fan pump speed on stock jet geometry
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Hydraulic Headboxes - Stock Jet Geometry for Fourdriniers FAQs

What is the jet impingement angle?
On a Fourdrinier machine, it is the angle between the stock jet and the wire as the jet strikes the wire

Why is the stock jet geometry so important?
Stock jet geometry is important because it affects sheet formation, and sheet formation affects both the appearance and the performance of the sheet

What is the difference between "pressure forming" and "velocity forming"?
Pressure forming occurs with very high jet angles, where the stock jet is forced down into the wire. Velocity forming occurs with low jet angles, and the stock jet lands more gently on the wire.

What is the "jet-to-wire ratio"?
It is the ratio of the stock jet speed and the wire speed, and it greatly affects the orientation of the fibers in the sheet

What is the "L/b ratio" and why is it important?
It is the ratio of the horizontal spacing to the vertical spacing between the slice lip and apron lip. It is important because it affects the jet landing point and the jet impingement angle.

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