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Based on: Industry Standards and Best Practices

Languages: English

Sample Transcript

On a paper or board machine, the drying process involves both heat transfer and mass transfer. Heat transfer is the movement of thermal energy due to a temperature difference. In multi-cylinder drying systems, the driving force for heat transfer is the temperature difference between the sheet and the dryer cans. This process is called contact drying, because the sheet directly contacts the dryers. Heat transfer occurs when the steam condenses inside the dryer can and gives up its latent heat to the inside surfaces. The heat energy from the condensing steam is then transferred by conduction through the dryer cans' cast iron shell to the sheet. Because the steam inside the dryer is saturated, the steam temperature is directly related to the steam pressure.

Paper Machine Drying

Training Time: 42 minutes

On conventional paper and board machines, the energy required to dry the sheet to the desired moisture target at the reel is supplied by a series of rotating, steam-heated, dryer cylinders. These cylinders, or "cans," are grouped into multiple drive sections, which go progressively faster in order to adjust for machine direction stretch and to maintain sheet tension. This module will provide a general overview of the drying section of a paper machine.

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In multi-cylinder drying systems, the driving force for heat transfer is the temperature difference between the sheet and the dryer cans

Drying rate (the rate at which water can be removed from the sheet) increases during sheet heating, but then decreases as sheet moisture decreases

The water vapor resulting from water in the sheet evaporating must be removed from the areas around the sheet

Learning Objectives

  • Describe the design and operation of a typical multi- cylinder dryer section
  • List the objectives of paper machine drying
  • Define the terms "latent heat," "boiling point," "evaporation," "condensation," "superheated," and "blow-through steam"
  • Identify, describe, and state the purpose of dryer syphons, dryer fabrics, pocket ventilation systems, hood makeup air and exhaust systems, and blow boxes
  • Describe heat transfer and mass transfer during sheet drying
  • Describe the three phases of drying - sheet heating, constant rate, and falling rate
  • List the factors that affect sheet drying
  • Identify and describe single-tiered and double-tiered dryer sections
  • Explain the purpose of blow-through steam
  • Describe the two ways blow-through steam can be recovered for reuse
  • Identify and describe safety hazards and safety guidelines for multi-cylinder dryer sections
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Paper Machine Drying FAQs

What is the purpose of dryer fabrics on multi-cylinder machines?
Dryer fabrics improve heat transfer and help control cross-machine direction shrinkage and sheet distortions by holding the sheet tightly against the can surfaces. They also drive dryer cans on some machines.

What is the purpose of dryer hoods?
Originally, hoods were used to improve machine room conditions. They have evolved and their current purpose is to improve control of air flows, temperatures, and humidites around the sheet as it dries.

What are single-tiered dryer sections?
Single-tiered dryer sections have one level of heated dryer cans, plus one level of rolls. They typically utilize a serpentine-shaped "unirun" fabric that supports the sheet through the entire section.

Why are single-tiered dryer sections often followed by double-tiered sections?
Because only one side of the sheet contacts the dryers in single-tiered sections, they are often followed by double-tiered sections to make the two sides of the sheet more equal in appearance.

What is the purpose of pocket ventilation systems?
Pocket ventilation, or PV, systems introduce hot dry air across the machine in dryer pockets to keep the air around the sheet from becoming saturated and improve drying. This helps ensure a flat moisture profile at the reel by maintaining uniform humidities across the machine.

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