Dryer Felt Design
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 or cans that are grouped together into drive sections. Each drive section is equipped with one or two dryer felts to hold the sheet tightly against the heated dryer cans. This improves heat transfer to maximize drying, and also helps control cross-machine direction shrinkage and other sheet distortions. On many machines, dryer felts also drive some of the cans. This course covers the design and construction of dryer felts.
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Learning Objectives
• List the functions of a dryer felt • Identify appropriate machine locations for single-tier and double-tier felt sections • List the typical properties desired of a dryer felt • Differentiate between woven, spiral, and needled dryer fabrics • Describe the impact of dryer felt permeability and surface roughness on the product and on the drying process • List common materials of construction for modern dryer fabrics • Describe the different yarn types used in dryer felts • Explain why dryer felts are heat set by the manufacturer • Identify and describe dryer felt edge treatments • Identify safety hazards and safety guidelines related to dryer felts
Specs
| Course Level | Intermediate |
| Languages | English, French, Polish |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
What is a “unirun” felt run?
“Unirun” felts have a serpentine shape to support and transport the sheet around all of the cans in a section, which is especially important for the wetter, weaker sheet in early dryer sections
Why are there two names: dryer felts and dryer fabrics?
They were originally called “dryer felts” because they were constructed of wool and cotton, and more closely resembled felted fabrics. Today, they are more often called “dryer fabrics” because faster machine speeds require more permeable fabrics.
How are the dryer fabrics used in early dryer sections different from the fabrics used in later dryer sections?
Generally speaking, dryer fabrics with lower permeabilities are used in earlier felt sections to “pump less air” and improve stability and support for the wetter, weaker sheet.
Why are surface characteristics important for dryer fabrics?
Surface characteristics are important because a rough surface can mark the sheet. It can also carry along more “boundary layer air,” which can lead to sheet flutter and even sheet breaks.
Why are dryer fabrics heat set by the manufacturer before they are delivered?
“Heat set” means that the fabrics are subjected to high temperatures for a period of time. This improves their mechanical stability during use.
Sample Video Transcript
Surface roughness or the amount of variation in the fabric surface is important because a rough surface can mark the surface of the sheet, especially if the sheet is wet. This negatively affects sheet appearance and can impact other sheet properties. Perhaps more importantly, especially on fast machines, a rough surface tends to carry more boundary layer air along with it. In unorun felt sections, as the sheet and felt approach a roll or can, this air can become trapped between the converging surfaces and cause the sheet to lift off of the fabric. In sections with open draws between the cans, this can cause sheet flutter in the pockets.
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