22. Typical Yankee Application Setup Quick Disconnect For Pressure Equalization of Home Made Shower Bars- Not Needed for Professionally Engineered Shower Bars Quick Disconnect (or Flush Line) Double Coverage Shower Bar Static Mixer Fresh Water Source Crepe aid Release aid Filter
23. Creping Trial: Overall Trial Plan Initial feed rates need to be determined for each trial individually; double click on the chart at left to determine crepe aid flow
1. Increase sheet bulk: debonding the sheet increases its bulk. 2. Increase sheet softness: sheet softness is a function of of bulk, limpness, and smoothness. Creping increases bulk and makes the sheet more limp (weaker) because of fiber debonding). 3. Increase sheet absorbency: debonding the sheet opens up the sheet structure, making it easier for water to enter the sheet. 4. Increase sheet stretch: debonding (up to a point) increases stretch by giving the sheet more “give”. However, too much debonding will decrease stretch because stretch is measured as the amount of sheet elongation at sheet failure. The sheet will fail sooner if it is too weak.
Percent crepe is the oldest method of measuring sheet stretch. It is still used to give an approximation of what is going on at the creping blade. Percent crepe is also important because most tissue products are sold by area, not by weight. Therefore, the more a sheet is creped, the less final product will be produced per tonne of base paper.
1. If the sheet is not held against the dryer by some adhesive force, it will release before it contacts the creping blade- you will be making “machine glaze” paper (wax paper, butcher wrap, burger wrap) instead of tissue. 2. The sheet still needs to be pulled off the dryer properly; otherwise it will tend to break at the creping blade. 3. Dry creped sheets (about 90% sheet solids) typically lose 50-70% of their non-creped strength. Wet creped sheets lose much less, but tend to shrink in the after dryer section and are much less soft. Wet creping is typically used for commercial and industrial towel grades.
1. The Yankee coating prevents the creping and cleaning blades from coming into direct contact with the dryer surface, which will tend to damage the dryer. 2. The way the sheet is held against the dryer during creping is a major factor in how the sheet is creped. 3. Having a good, even coating on the Yankee helps produce an evenly built roll at the reel. Sheets that are unevenly creped tend to have soft and “tight” sections. The tight sections are much less bulky than the soft sections, creating low spots in the reel (corrugations). An extreme case of dryer streaking will cause carbon marking of the sheet.
Some information on the meaning of the various angles: 1. Cutting angle: Lower cutting angles are harder on the sheet; higher cutting angles are easier on the sheet. 2. Blade bevel angle: Beveling the creping blade reduces the “shelf” on the creping blade, producing a finer crepe. However, beveled blades are easier on the sheet and do not “work” it as much. Therefore, beveling the blade increases sheet strength and stretch, while reducing bulk and softness. 3. Blade angle: Lower blade angles produce a finer crepe; higher blade angles are more likely to cut the sheet and remove more coating from the dryer. 4. Take off angle: A take off angle that is too high will tend to pull the sheet off the Yankee too easily, interfering with good creping. One that is too high will tend to cause the creping blade to pick the sheet, causing holes and breaks.
1. Cross linking: In this stage, carrier water is evaporated and the polymer will crosslink with itself, creating a continuous film on the dryer surface. 2. Glass transition: In this stage, the polymer film begins to melt, becoming tacky and flowable. The glass transition temperature of the particular polymer is important, as it will determine what type of polymer is suitable for a particular situation. For instance, crepe aids that are suitable for “through air dryers” are generally not suitable for Yankee dryers because they have a higher glass transition temperature and wouldn’t become tacky enough in conventional creping. 3. Rewetting: In this stage, the polymer film picks up water from the sheet, becoming hydrated (it was originally dehydrated during the cross linking phase). It is important for the polymer to be able to accept water from the sheet and have intimate contact with the sheet. 4. Setting: In this stage, the polymer dehydrates again, forming a strong film that holds the sheet against the dryer surface. Good contact with the dryer surface aids even, faster drying of the sheet and allows for good sheet creping. 5. Doctoring: This is where the coating is partially removed from the dryer surface by the doctor blade (and cleaning blade, if one is used). 6. Cured: On this part of the dryer, only the hard, fully cured coating is present. This hard coating protects the dryer surface from damage.
1. The type of metallized coating on the dryer impacts how easily a coating is formed. Cast iron and molybdenum (“moly”) coat easily, while stainless steel coats less readily. 2. The inorganic substrate helps the organic coating stick to the dryer surface. Usually the inorganic portion of the coating is provided by the hardness in the water (stock and spray boom). However, sometimes mills that have very soft water add MAP or TSP to promote a good inorganic layer on the Yankee. 3. The cured organic coating is what protects the Yankee surface from the creping and cleaning blades. 4. The soft organic coating is where the creping blade “rides”. It is removed every revolution of the Yankee, so it must be reapplied continuously. The soft organic coating is modified by the release oil that is fed in conjunction with the crepe aid. The release oil interferes with the integrity of this portion of the coating, keeping it softer than the cured portion.
1. Furnish: hemicellulose is a compound that is found in wood. It is a natural adhesive, provided some level of coating on the Yankee. The more hemicellulose that is left in the stock, the heavier the natural coating on the dryer. Hemicellulose can be washed out of the stock: high yield, unwashed furnish has much more hemicellulose than lower yield, washed furnishes. Recycle fiber contains little hemicellulose. 2. Dryer surface: a. cast iron and moly coated dryers develop a coating more easily than a stainless steel coated dryer. b. A worn spot on the dryer will provide poor contact with the sheet, leaving it wetter- that part of the dryer will not develop a coating as easily as the rest of the surface. 3. Blades: a. The more heavily the cleaning blade is loaded, the more coating is removed each revolution of the dryer. Cleaning blades are generally used to even out the coating left on the dryer b. It is important to oscillate doctor and cleaning blades. Otherwise, they will tend to damage the dryer surface, leaving narrow, worn bands on the surface. c. Beveled crepe blades are gentler on the sheet than square blades. They are good for preserving sheet strength at the expense of bulk and softness development. 4. Dryer surface temperature: a portion of the dryer with poor condensate removal will be cooler than the rest of the dryer, and will develop a relatively poor coating. 5. Uneven moisture profiles are one of the most common causes of uneven coating development. It is very important to minimize moisture profile problems. 6. Too much water at the pressure roll nip makes it harder for the crepe aid to crosslink and form a good coating on the dryer.
This one should be pretty much self-explanatory.
1. It is very important to avoid damaging the Yankee. Once the Yankee surface is damaged, it must be reground before the defect will go away. You may develop a permanent wet streak in the sheet by damaging the Yankee. 2. It is important to make salable paper. It is difficult to sell paper that is full of holes or has black streaks in it. 3. Of course, it is also important to make paper that conforms to specifications.
1. If you can see yourself in the Yankee, you need more coating. 2. If you have an uneven coating on the Yankee, you have two basic choices: a. Have too much coating on the high spots, and the right amount of coating on the low spots. You will be prone to sheet holes and breaks. b. Have the right amount of coating on the high spots, and not enough on the low spots. You will be prone to sheet corrugation, carbon marking, and Yankee damage.
1. Dry end holes generally exhibit one of the following two characteristics: a. The hole is very “clean” around the edges- there is virtually no fiber going into the hole from the edges. b. The hole has a flap on the trailing edge. If there are relatively long fibers going into the hole from the edges, the hole is from the wet end. Wet end holes also may exhibit a disruption of the sheet around the hole: a ring of fibers (wire deposit) or crushing (press section sheet disruption). 2. Carbon marks come from direct metal to metal contact between the creping blades and the dryer surface. You will probably smell “burning metal”- the odor is very distinctive.
1. In general, to increase sheet strength: a. Improve fiber to fiber bonding (formation, fiber fibrillation, wet pressing) b. Improve sheet formation c. Minimize sheet debonding during creping d. Use a “stronger” furnish, especially well-refined softwood
1. Increasing % crepe increases the stretchiness of paper. 2. Remember that stretch is measured at the point of failure- if the sheet is very weak, it will fail early and not provide a good stretch value. Reducing the intensity of creping will leave the sheet stronger, and may give the sheet a higher stretch value.
1. In general, bulk is developed by preventing good fiber to fiber bonding. Therefore, do the opposite of what you would normally do to increase strength. However, you should still avoid calendering, as it compresses the sheet.
1. Softness is a combination of “bulk softness” and sheet smoothness. Bulk softness is developed by maximizing bulk and minimizing sheet bonding. Sheet smoothness is developed by maximizing formation and calendering. Note that calendering will reduce bulk while improving smoothness, so it is usually not done, or is done lightly, when making consumer grade tissue (bath or facial tissue).
1. Here are the three basic strategies. Among them, the felt-supported sheet is the least often seen in my experience).
1. All you need is a couple of feed pumps to feed the products to the fan pump suction.
1. An atomizing shower (expensive and prone to plugging) is common for this type of application.
1. It is critical to have a good, double coverage spray bar to give even coverage across the dryer. 2. Use the smallest volume nozzles possible given the cleanliness of the fresh water used as carrier water. It is best to heat the shower water to around 60 o C. It is important to have a filter in line after the static mixer to catch gels that might be present .
This setup is generally preferable to using a run tank for the coating/release mixture. Run tanks are prone to deposition; they also introduce lag time when making feed rate adjustments.
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