Flotation Dryers: Expanding Applications for Air Foils

William R. Henry Advance Systems, Inc., Green Bay, Wisconsin, USA

Abstract Air foils are single-sided flotation bars that can both heat and transport a moving web. A continuous and smooth air curtain over the surface of the foil provides an effect that is much like a “non-contacting idler roll”. They are particularly well-suited for drying applications that may include low web tension, controlled drying rates, light or heavy substrates, and/or prevention of wrinkles or curling. In the inverted position, an air foil can lift and carry a moving web without making contact with the web. With this degree of versatility, the air foil is often preferred for dryers and ovens that might otherwise use single and double sided slot nozzles, slot nozzles over idler rolls and/or impingement air bars. Introduction Flotation dryers using air foils have been successfully integrated in continuous web applications for more than five decades. However, the more recent trends toward thinner and light-weight substrates, coatings of low solids content, and fast-drying solvents have combined to create new and tougher performance requirements for flotation dryers and a corresponding resurgence of the air foil as a preferential flotation device. Traditional two-sided flotation air bars used for heating both sides of the web have previously been widely applied yet their versatility is somewhat limited. The single-sided feature of the air foil bar removes many of the air bar restrictions and therefore is emerging as a more desirable means of transporting, drying and curing some of the most challenging substrates and coatings.

Figure 1: Web flotation with a single-side air foils

Figure 2: 4-zone air foil dryer with top retraction

Mechanics of the Air Foil The air foil produced by Advance Systems, Inc. (US Patent No. 5,370,289) directs a stream of air across its surface to form a continuous air bearing that will support a moving (or static) web. The required operating characteristics of an air foil are that the heated air be evenly distributed along the entire bar, that the air be smooth (no turbulence), and that a slightly positive pressure be formed above the foil.

The air foil flotation bar consists of the air distribution chamber that contains a series of internal pressure drops that cause the heated air to evenly distribute in the cross-machine direction before exiting through the single slot on the top of the foil; an angled platform attached to the foil serves transition the air stream from a high to lower velocity without generating air turbulence or web flutter. In operation, the air stream across the top of the air foil first employs a Coanda Effect by which a moving fluid stream will tend to follow a curve surface. The air exiting the air distribution chamber is directed across the curved opening to form a continuous and parallel flow of air on the surface of the foil. The curved shaped that is formed by the air chamber and the dissipation platform or “wing” then creates a downward push of the web explained by the Bernoulli Principle. This principle of physics states that as the velocity of a fluid increases, the pressure exerted by that fluid decreases. For an air foil, the high velocity air between the web and the foil creates low pressure under the web relative to the slow moving air (and higher pressure) above the web. Consequently, the higher air pressure above the web pushes the web down and against the compressed air stream along the surface of the air foil. When the foils are arranged in a series, the web is supported by the air stream moving across each air foil. At each air foil, the web is first pushed up by the air exiting the air foil slot under the web and is then pushed down by the relatively higher pressure above the web. These two forces form an undulating pattern of the web as it moves in the machine direction.

Figure 3: Flotation air stream for Air Foil

Figure 4: Web flotation over air foils

coated web point of lowest pressure

air foil

dissipation platform

air distribution chamber

heated air

Like an impingement air bar, the air foil is non-contacting (the gap between the air foil and the web will be approximately 4-8 mm). Air bars float the web by directing heated air perpendicular to the web surface. As such, there must always be an opposing air bar to create an equal and opposite force in order to hold the web in position. It is this high air turbulence or “impingement” that creates a high coefficient of heat transfer into the web and coating.

Air foils have a lower heat transfer coefficient because the flotation air is not directed at the web but is parallel to the web surface. However, since this parallel and curved movement of air also serves to hold the web in position, it is not necessary to have any air directed on the opposite side of the web except as needed for drying or curing. This is one of the key differences in the performance characteristics of air bars versus air foils. Impingement air bars also require a certain amount of web tension (usually greater than 0.1 kN/m) to prevent the web from billowing away from each air bar. The air foil does not require an opposing force on the opposite side of the web to hold the web in place and consequently, requires very little web tension for operational stability. Performance Characteristics of an Air Foil The geometry and air flow characteristics of the air foil create unique qualities that are well-suited for a growing trend of products that utilize delicate webs that may be prone to wrinkles, be of light weight, and/or possess a low tensile strength. In addition, the processing of very fast or very slow drying solvents may benefit from an air foil dryer configuration. These benefits may be best explained by comparing the performance characteristics of air foils to that of slot nozzles and/or impingement air bars.

• Single-sided flotation - Air foils are usually mounted beneath the moving web and use heated air to float the web. Since there is no contact with the web, air foils may be used with single sided coatings (top or bottom), two-sided coatings, or saturated webs. No apparatus above the web is required to steer or hold the web in place. The spacing between air foils in the machine direction may vary from 0.3 to 2 meters. Essentially, bottom-mounted air foils will support and transport a web much like a series of idler rolls; with the notable excepts that air foils have no moving parts (i.e. bearings, drives, belts), do not contact the web, and are able to transfer heat energy into the web to aid in the drying and/or curing of the coating(s).

Figure 6: Picture of a web floating on multiple air foils

Figure 5: Opposing impingement air bars

• Various top-side drying techniques are possible - Since the use of air foils does not require a specific downward force of air on the top of the web (as with air bars), mechanisms that produce various air flow characteristics can be used for drying the top-side of the web. Hole bars above the web can emit a gentle “flood” of air that is necessary for fast drying solvents. When even lower drying rates are needed, a co-flow or counter-flow of air parallel to the web may be used. For dryers that may be used for both water-based (needing forceful drying) and solvent-based (needing gentle drying) coatings, the force of drying can be varied with the combined use of a variable frequency fan (to modulate air velocity) and slot nozzles that can be positioned at varying distances above the web (to modulate air impingement) . For instance, for water-based coatings, slot nozzles might be positioned 3 cm above the web with an emitting air velocity of 1,500 m/min; for a solvent-based coating the drying rate can be reduced by using the same slot nozzles elevated to 25 cm above the web with an emitting air velocity set at 300 m/min. This variety and range of drying configurations above the web would not be possible with impingement air bars.

• Independent of web tension - Impingement air bars located on opposite sides of a moving web exert alternating vertical forces on the web. The process line tension prevents the web from excessive pass line deviation; see Figure 5. The greater the force or impingement exerted by the air bars, the greater the tension required to hold the web in place. The tension range for impingement air bars is typically between 0.1 kN/m and 0.5 kN/m. Air foils do not exert a vertical force on a moving web, in fact as discussed previously, the curved shape of the air foil creates a positive pressure above the web that holds the web down and on top of the air parallel air stream. As such, the amount of machine direction web tension that is needed to hold web on centerline is less than 0.02 kN/m. This characteristic can be of significant advantage for webs of low tensile strength or webs that may easily deform under elevated drying temperatures.

• Prevents wrinkling and curling - The positive force above the web that is caused by the flow of air

moving over the curved surface of the air foil acts across the entire width of the web. Essentially, that pressure pushes all parts of the web down to the air stream. Any undulations in the web (i.e. MD corrugations, wrinkles, downward edge curl or upward edge curl) will be pressed down and flat against the supporting air stream. The effect is much like that of an idler or bowed roll that is used to smooth out a moving web. While the effect is the same, it is all done without contacting the web! As such, air foils can be used with two-sided coated or saturated webs to smooth out the web and prevent curling. Because idler rolls often require some degree of wrap to be effective, it is not uncommon to see roll support dryers either arranged in an arched configuration requiring considerable overall height or utilizing a roll drive system to ensure the rollers move at the same

Figure 7: Retractable slot nozzles over air foils enable user to change the “drying power” of a dryer and

thereby vary and control solvent evaporation rates

speed as the web. Since air foils create the same effect without contacting the web, neither arched designs nor roller drive systems are needed. In fact, there are no moving parts in an air foil at all.

• Inverted mounting will hold web from top side - The movement of air across the air foil surface creates a positive pressure on the opposite surface of the web regardless of the orientation of the air foil bar. Thus, if an air foil is inverted (positioned above the web instead of below the web) the web will be pushed up and held against the air foil’s air stream. In the inverted position, a series of air foils can then be used to “carry” a web from the top surface without actually contacting the web. This transporting mechanism can be useful in many situations whether heated air is used or not. In combination with air turns, it is particularly useful in multi-pass dryers in which a coated or saturated web must change direction while being dried; all without contacting the surface of the web.

Figure 12: 3-Pass non-contacting air foil & air turn dryer for coated one-side, coated two-side, or saturated webs. All air is heated.

Figure 8: NO air flow: corrugations in machine direction

Figure 9: WITH air flow: corrugations are smoothed out at each air foil

Figure 10: NO air flow: downward edge curl

Figure 11: WITH air flow: flotation wave form serves to remove edge curl at each air foil

Web width = 0.25 m

Examples of unique drying applications for air foils Very-thin foil: One of the most persistent difficulties of drying an adhesive coating on thin aluminum foil (0.01 mm and less) is the possibility of creating wrinkles. Once a wrinkle is formed, it can not be removed and the product is unusable. Heretofore, most coating operations used rollers (bowed or arched) to ensure that the web remained flat as it was dried. But the natural web hold-down affect caused by the positive air pressure above the air foil, now enables producers to run 0.01 mm aluminum foil wrinkle-free, without rollers, and without contact of any kind during the drying process (see Figure 13).

Low-tensile films: Low tensile films, such as low density polyethylene, are subject to distortion especially at the elevated temperatures that occur in a drying or curing process. Impingement air bars are somewhat limited in their application to low-tensile webs because they require a web tension (usually 0.1 to .05 kN/m) that can cause web stretching or bagging at elevated drying temperatures. An alternative transport method meant to minimize web distortion is the use of direct-drive or tendency-driven rollers to isolate and minimize the tension in the dryer, but bottom-side contact with the web limits this configuration to top coated webs. However, web contact and the mechanics, controls and costs associated with a roller drive system can all be eliminated with the use of air foils. Air foils do not require a web tension over 0.01kN/m and therefore can operate in a tension range that will not deform a sensitive web. Figure 14 shows the stable flotation of a LDPE film at a web tension of less than 0.014 kN/m. Fast-drying solvents: The evaporation of solvents of high volatility (i.e. acetone, MEK, toluene, alcohols) must be controlled (slowly released) to avoid defects resulting from “skinning over”, blistering, bubbling, etc. In some cases, rapid evaporation may also cause unusually high solvent concentration levels that can quickly exceed flammability limits. Air bars generate high air velocities on both sides of the web (hence, high drying rates) so they are rarely used with fast-drying solvents. But the fact that air foils float the web from only one side (usually the bottom and uncoated side), the amount of drying air directed on the top coated side can be modulated as needed. For instance, the lowest drying rates are achieved by directing a volume of air parallel to, and in the same direction as, the moving web. As such, there is very little impingement or interaction of the air with the solvents. The rate of evaporation is regulated by varying the temperature, volume and speed of this “co-flow” air stream. Another means of establishing a controlled slow drying rate is the use of “hole bars” positioned above the web and air foils. The drying air flows from many small holes distributed over the entire area of the bar to create a slow flood of air with a low impingement force on the surface of the web. At the same time, the hot air of the air foils serves to pump heat energy into the web and coating from the underside. This underside air helps to heat and move the solvent up and through the coating and thereby preventing the coating from forming a “skin”. The skin is a premature formation of the coating caused when the solvent preferentially evaporates from the top of the coating surface. In heavy coatings, this top-down formation of the coating can seal the surface before the solvent beneath it is able to fully evaporate. When the solvent finally does vaporize, the

Figure 13: Air foil dryer for adhesive coating on thin foil

Figure 14: Air foil flotation of LDPE film having web tension of 0.014 kN/m

Pull weight = 3.4 N

expanding and trapped vapor causes a blister, bubble or other surface defect on the coating. The bottom-side heating of the air foils serves to counter this effect by simultaneously heating the wet coating layer or saturant from the underside and produce an even evaporation rate throughout the wet coating layer.

Webs that are subject to curling or baggy edges: Curled edges can occur with many types of substrates. It is especially prevalent in processes in which a wet surface coating is not applied to the full width of the web. In that situation it is the differential drying rate between the uncoated and coated portions of the web that results in the curled edge. In severe cases, the edge folds under (or over) the web and will then contact or snag on nearby dryer components. The result is a deposition of coating inside the dryer or worse; a sheet break. A baggy edge that might have been caused by an improperly wound roll may also result in fold-overs or coating deposits. Once again, it is the natural positive pressure exerted on the web by an air foil that serves to counteract curled or baggy edges. The picture below shows how a curled edge is picked up and made flat as it passes over each air foil.... much like an idler roll would straighten a curled edge.

Multi-pass dryers: Where floor space is limited or total required drying/curing length is quite long, a dryer may be configured with multiple passes within the same dryer enclosure. A common configuration for horizontal multiple is to use rollers to turn and support the web. However, when the coated web is turned and a top side coating is then on the bottom side, the idler rolls become problematic. Air turns and air foils provide a different means of transporting the web; a means that does not include any physical contact with the web. As discussed previously, the air foils on the under side of a moving web perform much like non-contacting idler rolls. Web weights up to 1,000 g/m2 have been successfully supported with air foils. To conserve air volume,

Figure 15: Co-flow dryer with air foils

Figure 16: Low impingement hole bar

Figure 17: Dryer with hole bars over air foils

Figure 18: Web handling of edge curl or baggy edges with air foils

the spacing between air foils can be up 2.5 meters. Air foils also provide the unique capability being mounted above the web to lift and transport the web without contact. “Top-mounted” air foils are especially useful when a web is saturated or coated on the bottom side for which contacting idler rolls would be inappropriate and the air flow of bottom-mounted air foils might disturb the coating surface. In the picture below, top-mounted air foils are using hot air to both dry and transport the web. The web has a weight of 280 g/m2 and the space between each air foil is 1.8 meters. Note that in the inset picture, the gap between the web and the air foil surface is approximately 0.5 cm.

Summary When it is desirable for a flotation dryer to have maximum drying power (transfer of heat energy into the web/coating), then the impingement air bar is still the best choice. But in many other process applications where other criteria may be of equal or greater importance, the air foil flotation dryer may be uniquely appropriate. Air foils have the ability to float and dry a web of low tensile strength or a web under low line tension. They may be used to perform the same function as idler rolls yet unlike idler rolls, air foils will also heat the web, do not have moving parts, and will not scratch or otherwise contact the web. A variety of air delivery methods (i.e. hole bars, slot nozzles, or co-flow air) can be used in combination with air foils to handle fast drying solvents or coatings that are sensitive to strong air turbulence. Air foils are uniquely able to dry and transport a moving web from above the web without contact. With a growing trend toward substrates of light weight or low caliper, of fast-drying organic solvents, and of coatings that may be of low solids, multi-layers or two-sided, it is likely that dryers and ovens using air foil flotation methods will continue to experience expanding applications. Presented: April 20, 2010 TAPPI PLACE Conference, 2010 Albuquerque, NM

Figure 19: Air foils mounted above the web will lift and carry the without contacting the web

Expanding Applications for Air FoilsFlotation Dryers

Presented by:William R. HenryPresidentAdvance Systems, Inc.

• Flotation Dryers with Air Foils

• Physics of Air Foils

• Performance Characteristics ApplicationsSingle-sided flotation thin film, replaces idler rolls

Variable top-side drying techniques solvents/water

Requires little web tension low tensile webs

Counters wrinkles, curling adhesives, C2S

Can be top-mounted multi-pass dryers

Expanding Applications for Air FoilsExpanding Applications for Air Foils

Flotation Dryers with Air FoilsFlotation Dryers with Air Foils

Flotation Dryers with Air FoilsFlotation Dryers with Air Foils

Physics of Air FoilsPhysics of Air Foils

Physics of Air FoilsPhysics of Air Foils

coated web point of lowest pressure

air foil

dissipation platform

supplementalair

air distribution chamber

heated air

Physics of Air FoilsPhysics of Air Foils

Physics of Air FoilsPhysics of Air Foils

• Single-sided flotation

Performance CharacteristicsPerformance Characteristics

web direction

air foils

single-sided flotationPerformance CharacteristicsPerformance Characteristics

Adhesive coated aluminum foil = 0.01 mm

at 300 mpm

Performance CharacteristicsPerformance Characteristicssingle-sided flotation

• Single-sided flotation

• Variable top-side drying techniques

Performance CharacteristicsPerformance Characteristics

web tension

web tension

Flotation Air BarsFlotation Air Bars

Air Foils

Variable top-side drying techniques

Variable height for variable

drying “power”hole barsslot nozzles

Performance CharacteristicsPerformance Characteristics

web tension

web tension

Flotation Air BarsFlotation Air Bars

Air Foils

co- or counter-flow airslot nozzles

Variable height for variable

drying “power”

Air foils = “non-contacting idler rolls”

Performance CharacteristicsPerformance CharacteristicsVariable top-side drying techniques

Variable top-side drying techniquesPerformance CharacteristicsPerformance Characteristics

• Single-sided flotation

• Variable top-side drying techniques

• Requires little web tension

Performance CharacteristicsPerformance Characteristics

Air bars > 130 N/m

Tendency/direct drives > 30 N/m

Arched rollers > 180 N/m

Air Foils > 20 N/m

Requires little web tensionPerformance CharacteristicsPerformance Characteristics

Requires little web tensionPerformance CharacteristicsPerformance Characteristics

Requires little web tensionPerformance CharacteristicsPerformance Characteristics

Requires little web tensionPerformance CharacteristicsPerformance Characteristics

• Single-sided flotation

• Variable top-side drying techniques

• Requires little web tension

• Counters wrinkling, curling

Performance CharacteristicsPerformance Characteristics

Counters wrinkling, curlingPerformance CharacteristicsPerformance Characteristics

Counters wrinkling, curlingPerformance CharacteristicsPerformance Characteristics

Counters wrinkling, curlingPerformance CharacteristicsPerformance Characteristics

• Single-sided flotation

• Variable top-side drying techniques

• Requires little web tension

• Counters wrinkling, curling

• Can be top-mounted

Performance CharacteristicsPerformance Characteristics

“Bottom – Mount”

“Top – Mount”

Can be top-mountedPerformance CharacteristicsPerformance Characteristics

Can be top-mountedPerformance CharacteristicsPerformance Characteristics

Can be top-mountedPerformance CharacteristicsPerformance Characteristics

• Airfoils act as “non-contacting idler rolls“

• Airfoils provide ability to vary the type of top-side drying

• Airfoils do not require or exert significant web tension

• Airfoils counter wrinkling and curling

• Airfoils = most versatile means of flotation drying or curing

ConclusionsConclusions

Please remember to turn in your evaluation sheet...

Thank you

Presented by:

William R. HenryAdvance Systems, Inc.