Paper Mill | Process, Requirements, and it’s Types

Paper Mill

Pulp making and Paper making are the two main processes in a paper mill and the drives required are quite different for each and play a crucial role in turning raw materials into finished paper products. Both processes include different equipment and techniques, requiring specialized drives to meet their unique demands.

In this post, we will explore more about these processes in detail, and know how they work and the distinct requirements for them. This understanding is key to appreciating how paper is manufactured efficiently and effectively.

1. Pulp Making

Pulp is made in two ways-purely by mechanical means and by both mechanical and chemical processing. The former involves grinding logs of wood of about a meter length on large grind-stones. Grinders operate at almost constant speed and can be started under light load conditions. Hence, synchronous motors are considered as most suitable for grinder drives.

Since, they usually work with speeds of 200-300 rpm, geared drives are used, especially when the motors are installed in a separate room for protection from humid atmosphere. Usually, making pulp by purely mechanical means consumes more than fifty per cent of the total power requirement of a paper mill; hence, large-size grinders driven by 3000-4000 kW motors are, normally, considered as economical.

Pulp can also be made by cutting logs of wood into chips of several centimeters length and treating them with alkalies along with other raw materials like grass, rages etc. During the chemical treatment the material is continuously beaten. Woodchoppers have random load characteristics and their inertia is large, depending upon the size of the disc, on which the knives of the chopper are mounted. Beaters, usually are required to start with a large load.

2. Paper Making

The machine that makes paper in a paper mill has to perform the job of forming sheets, removing water from sheets, drying of sheets, pressing of sheets, and reeling up of sheets. Therefore, the paper is made in the following five sections:

• (i) Couch section (wire section),
• (ii) Press section,
• (iii) Dryer section,
• (iv) Calendar section, and
• (v) Reel section.

The schematic layout of the different sections that make up the paper is given below. The paper pulp suspension with a moisture content of 98 percent to 99 percent is transferred uniformly to the wire. Most of the quantity of water drips through the wire mesh and the rest is removed by suction. At the end of the wire section, the moisture content would have reduced to about 80 percent.

In the press section, which follows the wire section, sheet is pressed between woollen felts so as to squeeze out water from the wet sheet and the web leaves the press section with moisture content of 65 to 60 per cent. In dryer section, the sheet is further dried by passing it over and under the heat cylinders until the desired dryness, usually 6 per cent of moisture content, is obtained. In calender section, sheet is subjected to pressure and friction so that a compact and smooth surface of sheet results. In the reel section, the sheet is wound up on a mandrel.

Requirements of Paper Machine Drive

1. Must maintain a constant speed for efficiency.
2. Speed should be adjustable up to a 10:1 range for versatility.
3. A sag take-up system is needed to introduce the web into the calendar manually.
4. Tension variations in the calender and reel sections require an overriding tension control system.
5. Maintenance of wire needs speeds of 10–25 m/min, controlled with a button.
6. Slow speeds of 10–25 m/min are required for setting up felts, wires and heating dryer cylinders.
7. Smooth and quick starts are important to prevent high starting currents.
8. The control system should be flexible as it has to handle different tasks easily.

Types of Drives Used for Paper Machine

There are two types of drives employed for making paper form:

1. Pulp-line shaft drive and
2. Sectional drive.

1. Pulp-Line Shaft Drives

In this form of drive, the various sections of the paper machine are driven from a line shaft running the full length of paper machine. Cone pulleys and belt-combinations drive the various section of the machine from line shaft through right angled gear reductions.

Normally, electric motors are employed to drive the transmission shaft. Both AC and DC drives can be used practically loss-less speed control. In AC drive, only the AC commutator motor with shunt characteristic is of use for obtaining an economic speed control system. Its advantage is in the possibility of connecting the motor directly to a three-phase supply, thereby eliminating an AC-DC converter, which would be required if a DC drive were used.

However, the speed of an AC commutator motor depends on load and hence, its use as a paper machine drive with stringent requirements of constant speed is no longer advisable. Also, its speed range (usually of the order 1: 3) as well as the power required greatly affects the size of the motor. The open loop speed control of the AC commutator motor is sluggish in comparison with a DC drive, as speed is varied by adjusting an induction regulator and shifting the brush rocker.

2. Sectional Drives

In sectional drive each section of the paper machine has its own electrical motor. All the motors are operated from a common supply bus. By varying bus bar voltage, the speed of the paper machine can be controlled. By adjusting the filed excitation of any motor, it is possible to vary the speed of that particular motor with respect to other motors.

Comparison Between Line Shaft Drive and Sectional Drive

1. Cost and Maintenance

• Line shaft drives are cheaper in terms of electrical equipment but need extra mechanical parts, which increases costs and maintenance efforts.

2. Draw (Speed Difference)

• In line shaft drives, the draw is adjusted by shifting belts on cone pulleys, but belt slippage affects accuracy, and no automatic correction is possible.
• Feedback control systems are used by Sectional drives to automatically correct draw variations by adjusting speed references.

3. Repeatability of Settings

• Precise potentiometers are used by Sectional drives to, allowing, allowing draw settings to be accurately recorded and reused for consistent paper quality.
• Line shaft drives lack this precision, making repeatability difficult.

4. Loop Removal

• Sectional drives can adjust speeds slightly to remove loops without disturbing draw settings.
• Line shaft drives require changing draw settings, often leading to overcorrection and difficulty in maintaining proper tension.

5. Rate of Loop Removal

• Sectional drives can maintain a constant rate of loop removal at any speed.
• Line shaft drives require varying belt adjustments depending on speed, with more adjustment needed at lower speeds.

6. Independent Operation of Sections

• Sectional drives allow each section to operate independently. For example, the wire section can run at crawl speed for cleaning, while the dryers warm up at a higher speed.
• Line shaft systems force all sections to run at the same speed.

7. Crawl Function

• Sectional drives can put a section on crawl speed for troubleshooting or checking after minor issues.
• Line shaft drives cannot offer this flexibility.

8. Inch Reverse

• Sectional drives allow reverse inching, helpful for clearing paper jams in the dryer section.
• Line shaft systems lack this feature.

9. Safety and Interlocks

• Sectional drives have safety interlocks for each section, preventing startup unless safety conditions are met and tripping only the faulty section during issues.
• Line shaft drives lack section-specific safety, increasing the risk of accidents due to human error.

10. Overloading Protection

• Sectional drives detect and stop overloaded sections to prevent damage.
• Line shaft drives cannot isolate overloaded sections and there is a risk of machine breakdown.

Frequently Asked Questions (FAQs)