Paper Machine Drying

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Key Questions

The following key questions are answered in this module:

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.

Sample Video Transcript

Below is a transcript of the video sample provided for this module:

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.