The paper machine headbox is an equipment that supply low consistancy pulp (stock) uniformly into the forming section. The formation and uniformity if the final paper product depends on the dispersion of fibre and fillers. Machine headbox had major effect on paper qualities. There are different types of headboxes are present, all are not suitable for for all grades and types of paper and machine speed. A well constructed headbox will give good quality paper. So the design headbox should be proper. The headbox is also called as "Stuffbox" or "Flowbox".
Paper machine headbox types:
In general the Headbox is catogarised into 3 major types, as Open type, Air padded type, and Hydraulic type.
Hydraulic type of headbox is designed for high speed paper machines (speed above 350 m/min). Whereas open type open type headbox is designed for low speed paper machine. In Hydraulic and Air padded headedbox, the discharge velocity from the slice depends on the feeding pump. For this reason they are generally called as "Pressurised Headbox". The hydraulic headbox has no air pad, it is fully closed.
Incase of air padded headbox pond level is maintained. Above the pond level it containes pressuried air. To maintain this air pressure we use headbox blowers. The stock level is known as Head, which is controlled by air pressure. The head determines the slice jet velocity or speed. These type of headboxes contains two or three "Holey rolls or rectifier rolls", and a "headbox shower".
Multi layered headbox is available for making Tissue paper and light weight container board. Foe each layer it has different furnish compositions and delivary system.
Headbox jet velocity calculation:
Jet velocity is the very important factor in the manufacturing of high quality paper. We have to maintain jet-to-wire velocity ratio; 0.97 to 1.03. The headbox jet velocity can be calculated from Bernoulli's equation:
V2=2gh
Headbox Functions:
The headbox serves many functions among them the following three functions are very important in the paper production process. If the headbox doesn't perform these three functions acceptably, the quality of the paper produced will be greatly diminished.
1. Spread the low consistancy stock uniformly along the width of the wire.
2. Accelerate the stock and maintain constant velocity according to the paper machine operating speed.
3. Create control turbulance and suspend the pulp fibres throughout the out going slurry to eliminate fibre gathering.
Parts of Headbox:
The Headbox generally contains following parts:
1. Diffuser
2. Compressor line
3. Holly rolls
4. Rotating shower
5. Bottom lip
6. Top lip
Holey roll or Rectifier roll or evenor roll:
These rolls are genreally 2 to 3 present in headbox. These rolls are used for controlling both even outflow irregularities and to create turbulance to keep the fibre dispersed up to the slice opening. Normally hole diameter is 3 to 4 cm.
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| PAPER MACHINE HEADBOX |
Rotating shower:
This shower is used for continuous headbox cleaning purpose and also consistency maintenance.
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 bottom lip is adjustable; we can up or down or move sides also by hand operated or motor driven. On the otherhand top lip is fixed. The distance between the top lips are called Slice opening. The slice opening controls the stock jet velocity which is very important for the paper formation.
The slice spindle contains a gearbox, connecting rod, Spindle, and hook which lifts bottom lip.