Introduction
In the paper and paperboard manufacturing industry, the drying section is one of the most energy-intensive and technically critical section of the machine. For a multi-layer board machine, proper drying is essential to achieve the desired board strength, dimensional stability, surface properties, and final moisture content.
After the sheet leaves the press section with around 45–48% dryness, the drying section removes the remaining moisture through thermal evaporation until the board reaches approximately 92–94% dryness before winding or further processing.
Since the drying section consumes nearly 70–75% of the total energy used in paperboard production, understanding its concept, operation, and optimization is crucial for improving both product quality and mill efficiency.
Drying Concept of Paperboard Manufacturing:
Paperboard production involves removing a large amount of water from the fiber suspension. However, not all water can be removed mechanically.
Water removal takes place in three main stages of the board machine:
| Section | Water Removal Method | Typical Dryness |
| Forming Section | Gravity and vacuum drainage | 18–22% |
| Press Section | Mechanical pressing | 42–48% |
| Drying Section | Thermal evaporation | 93–95% |
The drying section uses steam-heated dryer cylinders and controlled ventilation systems to evaporate the remaining moisture from the sheet.
Two important physical processes occur during drying:
Heat Transfer: Heat moves from steam → dryer cylinder surface → paperboard sheet.
Mass Transfer: Water inside the sheet evaporates and is removed by air circulation.
“The efficiency of the drying process depends on how effectively these two processes are balanced.”
Working Principle of the Dryer Section:
The drying section operates mainly through contact drying using steam-heated rotating cylinders.
Basic Operating Process:
- High-pressure steam enters the dryer cylinder.
- Steam condenses on the inner surface of the cylinder shell.
- Heat transfers through the cylinder wall.
- The wet sheet comes into contact with the hot cylinder surface.
- Water in the sheet evaporates.
- Moist air is removed through the dryer hood ventilation system.
“This continuous heat transfer process gradually reduces the moisture content of the board as it passes through multiple dryer groups.”
Drying System divided different section:
The drying system of a multilayer board machine is typically divided into six main parts.
- Pre-Dryer Section:
The pre-dryer section is located immediately after the press section.
Purpose
- Increase sheet strength
- Remove the first portion of moisture
- Stabilize the sheet before the size press or coating section
Typical dryness increase: From 45% to about 85%
“The sheet passes over a series of dryer cylinders arranged in multiple groups with supporting dryer fabrics.”
2. Size Press Dryer Section:
In many board machines, a size press or film press is used to apply starch or chemical additives to improve surface strength and printing performance.
After the size press, the sheet moisture increases again. Therefore, an additional dryer section is required.
Purpose
- Dry applied starch or chemical solution
- Improve surface properties
Dryness typically increases from 60–65% to about 92%.
3. After Dryer Section
The after-dryer section performs the final drying before the sheet reaches the coating section.
Purpose
- Achieve final moisture level
- Improve dimensional stability
- Reduce curl and moisture variation
“Final dryness before coating normally reaches 92–93%.
4. Coating Dryer Section
In the production of coated grades such as folding boxboard (FBB) or coated back board (CBB), the sheet passes through one or more coating stations. Coating colour applied to the board contains water, which must be evaporated quickly and uniformly.
To achieve this, the coating dryer section often uses a combination of drying technologies, including
- Infrared dryers System
- Hot air dryers System
Purpose
- These systems provide rapid drying
- Rapid drying protecting the coating layer and preventing surface defects.
“After initial drying with these systems, steam-heated cylinders used to complete the drying process”
5. Curl Control and Conditioning
Curl control is particularly important in board machines, especially for multilayer structures where different layers may shrink differently during drying. Special conditioning dryers installed to adjust moisture distribution.
Purpose
- Used to control board curl and moisture profile.
“These systems help produce flat and stable board, which is essential for downstream processes like cutting and packaging.”
6. Cooling Cylinders
Before the board reaches the calendar and reel, it passes through cooling cylinders. Cooling water is running in these cylinders
Purpose
- Reduce the temperature of the sheet after the high-temperature drying process.
- Cooling stabilizes the board and prepares it for surface finishing operations.
Major Components of the Dryer Section:
Efficient drying requires several integrated systems working together.
- Dryer Cylinders
Dryer cylinders are the primary heat transfer equipment in the drying section.
They are typically made from high-grade cast iron to ensure good heat conductivity and mechanical strength.
Typical Specifications
| Parameter | Value |
| Cylinder diameter | 1500–1800 mm |
| Surface temperature | 70–140°C |
| Steam pressure | 3–5 bar |
“The sheet wraps partially around each cylinder, allowing heat to transfer into the wet board.”
2. Dryer Fabrics
Dryer fabrics, also called dryer screens, support the sheet and ensure stable sheet movement through the dryer section.
Functions of Dryer Fabrics
- Hold the sheet against the cylinder surface
- Improve heat transfer
- Maintain sheet stability
- Prevent sheet fluttering
“Fabric permeability typically ranges between 150 and 600 CFM, depending on machine speed and board grade.”
3. Pocket Ventilation System
The space between two dryer cylinders forms a pocket where moist air tends to accumulate.
Without proper ventilation, this humid air can reduce drying efficiency.
Pocket Ventilation Equipment
- Pocket ventilators
- Blow boxes
- High-velocity air nozzles
“These systems remove humid air and replace it with fresh heated air to improve evaporation.”
4. Dryer Hood System
The entire drying section is usually enclosed in a dryer hood to control the drying environment.
Functions of the Dryer Hood
- Maintain high air temperature
- Remove evaporated moisture
- Improve drying efficiency
- Recover heat energy
Typical hood operating conditions:
| Parameter | Typical Value |
| Air temperature | 80–120°C |
| Air humidity | Controlled |
| Air flow | High circulation |
“Proper hood balance between air supply and exhaust is essential for efficient drying.”
5. Steam and Condensate System
Steam provides the heat required for evaporation inside the dryer cylinders.
When steam condenses, it forms condensate water that must be removed continuously.
If condensate is not removed properly, several problems may occur:
- Reduced heat transfer
- Cylinder vibration
- Uneven drying
- Increased energy consumption
Common Condensate Removal Systems
- Rotary siphons
- Stationary siphons
- Turbulators
- Rotary steam joints
“Proper condensate management ensures consistent cylinder temperature and efficient heat transfer.”
Drying Zones in the Paperboard Sheet:
Drying occurs in two main stages.
- Constant Rate Drying Zone –
In the early part of drying:
- The sheet surface remains fully wet.
- Water evaporates rapidly.
- Most heat energy is used to convert water into vapor.
“The evaporation rate remains relatively constant.”
2.Falling Rate Drying Zon
Later in the process:
- Surface moisture decreases.
- Water inside fibers must migrate to the surface before evaporating.
- As a result, the drying rate gradually decreases.
“This stage requires more energy to remove bound water from fiber structures.”
Common Drying Problems in Board Machines:
- High Moisture at Reel
Possible causes include:
- Low steam pressure
- Poor condensate removal
- Insufficient hood ventilation
2. Sheet Fluttering
Sheet fluttering may occur due to:
- Poor pocket ventilation
- Low dryer fabric tension
- Excessive air turbulence
3. Curl in Paperboard
Curl often results from uneven drying between the top and bottom surfaces of the board.
Common causes include:
- Unequal steam pressure
- Uneven coating moisture
- Poor hood air balance
4. Sheet Breaks in the Dryer Section
Possible reasons include:
- Weak sheet coming from the press section
- High draw between dryer groups
- Contaminated dryer fabrics
4. Energy Consumption in the Dryer Section
The drying section consumes the largest portion of energy in paperboard manufacturing.
Typical energy distribution in a board machine:
| Section | Energy Use |
| Forming Section | 5% |
| Press Section | 10–15% |
| Drying Section | 70–75% |
| Other Systems | 10% |
“Improving drying efficiency can significantly reduce steam consumption and operating costs.”
Best Practices for Efficient Dryer Section Operation:
To achieve stable performance and optimal drying efficiency, mills should follow these best practices:
- Maintain proper steam pressure cascade
- Ensure effective condensate removal
- Keep dryer fabrics clean and properly tensioned
- Optimize pocket ventilation
- Maintain balanced hood air supply and exhaust
- Control draw between dryer groups
“Regular monitoring and preventive maintenance are essential to maintain stable dryer operation.”
Conclusion
The drying section is a critical part of any multi-layer board machine, transforming the wet sheet from the press section into a stable, dry paperboard ready for finishing and converting.
Efficient drying depends on the proper integration of steam heating systems, dryer cylinders, ventilation systems, and fabric support systems. By optimizing these factors, paper mills can achieve better board quality, improved machine efficiency, and reduced energy consumption.