The paper machine headbox is an equipment that supply very low consistency stock into the forming section uniformly. The formation and uniformity of the final paper product depends on the dispersion of the fiber and fillers. The furnish delivery system varies on the construction of the headbox. It has a significant effect on paper qualities. All the headbox are not suitable for all grade of paper and machine speed. A well constructed Headbox can provide high quality board and paper and higher productivity. So the design should be properly. Now a days Hydraulic type headboxes are commonly used in the pulp and paper industry. The headbox is also known as flowbox.
Paper machine headbox types
Normally the headbox is categorized into three types such as open type, air pad type and hydraulic type. Among them the hydraulic headbox is designed for a high speed modern paper machine (above 300 m/min) whereas open headbox is used in low speed paper machine. In hydraulic and Air pad headbox , the discharge velocity from the slice depends directly on the feeding pump. For this reason they commonly known as pressurized headbox. The hydraulic headbox has no air pad, it is fully enclosed.
In case of air pad headbox a pond level is maintained. Above the level it contains pressurized air. The stock level is known as head, which is controlled by air pressure. The head determines the slice jet speed. It contains two or three Rectifier roll.
Moreover multi-layer hydraulic headbox is available which is designed for making tissue paper and lightweight containerboard. Each layer has different furnish compositions and delivery system.
Headbox jet velocity calculation
The headbox jet velocity is very important factor for manufacturing high quality paper. You should accurately maintain the jet-to-wire velocity ratio; it should be in the range of 0.97 to 1.03. The headbox jet velocity can be calculated from the following Bernoulli’s equation:
V2 = 2gh
‘V’ is the jet velocity (m/s)
‘g’ is the acceleration constant, due to gravity (9.81 m/s2)
and ‘h’ is the Height of stock in the headbox. (m)
The headbox has many functions among them the following three functions are very important.
- It spread the stock uniformly along the width of the forming fabrics.
- It provides the stock at constant velocity according to the paper machine speed.
- Create control turbulence and uniform suspension to eliminate fiber gathering.
If the headbox does not perform these functions correctly then the quality of the paper will be greatly hampered.
You will get many benefits from the well constructed headbox. It can improve formation and paper quality, reduce number of breaks and downtime, fewer pinholes, dirt spots, improve runnability, increase machine speed, stabilized paper caliper, improve water removal, and reduce chemical cost. All these properties are achievable, if the CD basis weight profile, main fiber direction, uniform formation, flatness, etc. are perfect.
Rectifier roll or evenor roll
This roll are used in paper machine headbox to control both the even out flow irregularities and to create turbulence to keep the fiber dispersed up to the slice opening. It is also known as hollow perforated roll or holey roll. Normally the hole diameter is about 2 to 4 cm.
Paper machine headbox slice
The paper machine headbox slice is a full width nozzle with a completely adjustable opening to give the desired rate of flow. The slice contains two lips; top lip and bottom lip. Among them the top lip is adjustable; you can up or down them with hand operating or motor driven. On the other hand the bottom lip is fixed. The distance between the two lips are called slice opening. This slice opening controls the stock jet velocity which is vital for paper formation.