HBKairknives

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HBKairknives

AIRKNIVES AND OTHER

CONTOUR COATING SYSTEMS

FOR PAPERBOARD

Herbert B. Kohler

Vice President

Kohler Coating Machinery Corporation

8817 Pleasantwood Avenue, NW

North Canton, Ohio 44720

KEYWORDS: Airknife coating, meteringequipment.

Airknives are metering devices used toreduce an excess of previously appliedcoating material on a moving web to auniform thickness.

There are two types of airknife operatingmethods. What we call "Air Brushing" is agentle jet of low velocity, impinging on avertical running coated substrate atapproximately a right angle, providing a

pressure dam which limits the amount ofcoating liquid passing by. Excess liquid runsback down the strip, under the influence ofgravity, to a pond or a drip off point. The jetdoes not remove any liquid from thesubstrate. For coating paper, this method haslargely fallen into disuse and will not bediscussed further.

We call "Airknifing" the method where a jet ofmoderate to high velocity impinges on acoated substrate supported by a roll, at an

angle on the order of 45 degrees opposed tosubstrate movement. The jet shears theliquid film, and removes the excess as liquid,spray, or mist. This excess is then collectedin a blow off containment system.

The theory in removal of excess coating by

the air doctor is called the filter cake theory.A coating is applied in excess to the sheet at

the applicator section. Water in the coatingimmediately begins to migrate at the interfaceof the wet coating and the paper web so thatthe coating at this point immediately becomessemi-dry or plastic. As the sheet of paperpasses under the airknife jet, the fluid coatingis removed from the sheet by the air doctorand is sheared at the point where the filtercake begins. There is a zone in the coatingcross section where the coating makes thetransition from the fluid to a semi-plasticcoating, and it is in this area that the air

doctor shearing takes place. The exact pointat which the shear takes place varies with theamount of energy that the air blast has. At avery high pressure or velocity of air from theair doctor, the air penetrates more into thefilter cake or plastic area, leaving less coatingon the sheet. It should be noted that thenormal air pressure used in the air doctor isfrom two to nine pounds; however, when aplastic non-absorbent web is coated by theair doctor system, pressure in the range oftwo to ten ounces is used in the air doctor.This would substantiate the filter cake theoryin that there is no filter cake formed in thiscase, and the air doctor is only shearing to adepth in the coating for which there is energyto penetrate it.1

Referring to Figure 1, shown on page 2, thereare four main components in an airknifecoating system.

1. The coating application system.

2. The airknife metering element.

3. The compressed air delivery system.

4. The coating exhaust and recovery system.


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COATING APPLICATION SYSTEM

Since air knifing was invented in the 1930's,many different applicator systems have comeand gone. Most methods used rolls, althougha number of die or fountain applicators havebeen tried.

Today, three systems predominate. Theseare shown in Figure 2, page 3. Roughly 70%of the applicator systems used today aresingle roll systems, 25% are two roll systemsand the remaining systems are three roll orother systems.

Single Roll

The single roll system is exactly that. It is asingle roll normally rotating in web direction

at a speed of 10 to 40% of the web speed,which picks up a crudely controlled amountof coating and applies it in excess to the webthat wraps the roll. Usually this wrap will befrom 3 to 4", and if well designed, it willmaintain a puddle of coating at the nip pointformed by the web as it touches the roll. Therun back of coating should be laminar innature and not a cascading effect in order to

get a good smooth application of coating tothe web and to prevent foam generationcaused by turbulent flow and cascading of thecoating off the roll. This system is the

simplest available and also the crudest one inoperation.2

It is used by the vast majority of airknifecoaters in operation, and is used almostexclusively at speeds below 250 meters perminute.

Two Roll

The two roll system provides an additionalmetering nip to reduce and level the amount

of coating applied to the web. It is theapplication unit of choice for moderately highspeed, wide machines. These would bemachines over 200 meters per minute inspeed and/or widths greater than 3.8 meters.

Three Roll

The three roll system was developed for highspeed air knifing (over 600 meters perminute) to further reduce and even thecoating applied to the web. The airknife isgenerally limited to pressures below 9.0 PSI(62.1 KPA), which limits the amount ofcoating which can be metered off at highspeeds. The three roll system extends theoperating range of the airknife, but its tighttolerances and complexity has limited it tonarrow width coaters (lessthan 3.8 meters wide).

Smoothing Roll

On many high speed systems it is a common


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NON-OPENING AIRKNIFE DUAL ROTATING AIRKNIFE

FIGURE 3

OPENCLOSED

OPENING AIRKNIFE

practice to use a smoothing roll on the coatedside of the web.

This roll is run reverse to web direction and isusually designed with as small a diameter asis structurally possible. Wrap angle is about5 degrees.

The primary purpose of this roll is to assist inthe removal of the film split pattern on theweb; however, the breaking of the foamgenerated from this process is also ofsignificant importance. The roll also serves toreduce the effect of dwell time from the pointof application to the point of action of theairknife metering device. Up to 90% of thecoating is re-distributed, preventing rapid de-watering and preventing the formation of afilter cake. The danger of long dwell

distances is water loss into the web to thedegree that the airknife becomes a poormetering device, due to its viscosity sensitivityand a need to run at higher air pressures.The re-distribution effect of the smoothing rollbefore the air jet keeps the coating insuspension longer, allowing better coatweight control and reducing the effect of thelong dwell distance.3

AIRKNIFE METERING ELEMENT

There have been numerous designs ofairknives over the last 60 years. The

technical differences of various geometriesand lip designs are beyond the scope of thisarticle.

All airknives fall into one of the threecategories shown in Figure 3, below.

Non-opening airknives were developed first.Opening style and dual airknives weredeveloped later to reduce cleaning time onpaper machines that could not stop the paperwhen making scrap.

AIR DELIVERY SYSTEM

The air delivery system of a coater consists offour basic elements:

1. The blower2. The aftercooler (if used)


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3. The piping4. Means of adjusting pressure and flow

Blowers

The blower for an airknife should always bea centrifugal type. These units deliver cleanair, free of water and oil, and completely freeof pulsation.

A blower should be sized according to theCFM required at the largest gap setting andhighest pressure at which you expect tooperate. Allowance should be made forpressure drop through the piping system andthe after cooler (if used).

CFM requirements vary with airknife length,airknife gap, and pressure requirements.The following table shows the requirementsper inch of opening at various airknife gapsfor a range of pressures, as well as theformulas from which they were calculated.

Blower Requirements for Airknives

Cubic feet per minute (CFM) of free air atstandard pressure of 14.7 lb. per square inchabsolute and 70 degrees F per inch at .040",.035", and .030" nozzle openings.

Table 1

CFM/INCH.040" .035" .030"

PSIG OPENING OPENING OPENING

1 5.9 5.1 4.4

2 8.3 7.3 6.23 10.1 8.9 7.64 11.6 10.2 8.85 12.9 11.5 9.8

6 14.2 12.6 10.87 15.3 13.6 11.68 16.6 14.5 12.49 17.3 15.4 13.210 18.6 16.2 13.9

Note:

CFM/Inch (.040" Opening)F = 5.86 * SQRT (P)

CFM/Inch (.035" Opening)

F = 5.133 * SQRT (P)

CFM/Inch (.030" Opening)F = 4.40 * SQRT (P)

Example: You have a 163" wide coater andyou wish to increase the blower size to allowyou to operate in a range from 3 to 5 psi at agap of .035". The system has an after coolerwith a maximum pressure drop of .5 psi andthe loss through the piping system is .5 psi orless. From Table 1 we read the valuecorresponding to .035" gap and 5 psi to be11.5 CFM per inch of length.

Therefore, at .035" gap and 5 psi, the airkniferequires 163 (11.5) = 1875 SCFM. Note: 1SCFM has the mass of 1 cubic foot of aircompressed from inlet conditions of 68degrees F and 14.7 psi absolute pressure.

Any variation from these conditions requiresthat modifications be made to the blowercalculations.

Since the system has an inherent pressuredrop of 1 psi, we require a machine capableof delivering 1875 SCFM at 6 psi. Thisrequires a 75 HP blower.

The After Cooler

The requirement for an after cooler depends


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largely on the amount of pressure boostprovided by the blower. Air temperatureincreases roughly 16 degrees Fahrenheit foreach pound of pressure increase. Generally,

any airknife system operating over 3.5 psi (24Kpa) could benefit from an after cooler.

Two methods of cooling the blower air aregenerally used; water cooled heat exchangersand water injection systems. Water cooledheat exchanger systems are used in mostinstallations today. They cool the air, withoutit contacting the cooling water. A moistureseparator is usually used at the exit of thecooler to remove any condensed moisture.

Water injection systems cool by injecting asmall amount of water mist into the airstream.The air is cooled by the massive heatabsorbed as the water evaporates.

The draw-back to this method is that anyimpurities in the water remain in theairstream. If the injection method is to beused, the preferred water source is chilledsteam condensate. A moisture separatorremoves any unevaporated water.

The main benefit to this system is that itdramatically reduces water evaporation by theair jet and reduces drying in the blowoff pan.Since the air is at almost 100% relativehumidity, it cannot absorb any more water.The air coming out of the heat exchanger haslow relative humidity and as a result canabsorb more water.

A properly designed injection system is lessexpensive initially, cheaper to operate andprovides more benefits than the heatexchanger system.

Piping

The general rule with piping systems is to

keep them as short as possible, with no sharpbends. To keep pressure loss to a minimum,piping from the blower should be at least 20%larger than the inside diameter of the blower

outlet, until it splits off to each end of theairknife. The tee should be the same size asthe main supply line, and made of hard pipe.

Flexible hose to each end of the airknifeshould be sized for flow equivalency.

Pressure and Flow Adjustments

Blowers can be throttled to adjust thedischarge pressure or inlet volume, or both, toany selected point within the operating

capability of the blower. This is usuallyaccomplished by installing and adjusting ablast gate or butterfly valve on the inlet ordischarge opening of the blower.

When the blower is throttled at the dischargeto a selected volume under standardperformance curve conditions, the blower willdeliver the full discharge pressure for the inletvolume as shown on the standardperformance curve. However, the throttlingdevice will supply sufficient resistance to airflow to provide the desired effective overalldischarge pressure beyond the throttlingdevice as required by the process. When thedischarge of a blower is throttled, anyeffective discharge pressure and inlet volumeabove the surge range can be attained, withinthe capability of the blower. However, the fullinput horsepower for the corresponding inletvolume shown on the standard performancecurve is required.

When the blower is throttled at the intake to aselected SCFM and discharge pressure, thethrottling device serves to create sufficientresistance to air flow to drop the absolutepressure at the blower inlet, so that thedesired discharge pressure above ambient isobtained.

The air volume (SCFM) entering and passing


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through the throttling device will, however,increase in volume because of the drop inpressure across the throttling device, so thatthe inlet volume to the blower (ICFM) exceeds

the SCFM. This condition lowers the inputhorsepower requirements. Therefore, it isrecommended that the throttling device beplaced on the inlet to the blower.

The third and most energy efficient method ofpressure and flow variation is by varying thespeed of the blower motor.

The volume entering a blower varies directlywith the speed in RPM, but the inputhorsepower varies with the cube of the speed

in RPM. Fairly large energy savings can bemade by this method.

Another advantage of variable speed controlis that it lowers the output temperature of theblower in comparison to the other twomethods.

The chart shown below in Figure 4 shows thefan volume output plotted versus input powerfor inlet throttling, outlet throttling, and twotypes of variable speed controls.

COATING EXHAUST AND RECOVERYSYSTEM

As with airknives, there have been manydifferent designs of exhaust and recoverysystems. Modern systems all share thefollowing characteristics.

1. Double wall stainless steel construction

for water circulation. Chilled watercirculates between the walls to chill anysurfaces that contact air or coating. Themoisture that condenses on these wallsprevents coating build up.

2. A vacuum exhauster system thatremoves 3 to 5 times the amount of airproduced by the airknife, from the pan.

3. Internal surfaces are smooth. Coating isseparated out of the air streamgradually, without sharp velocitytransitions that cause coating build up.Older designs used many internal bafflesthat removed coating by changing theair direction enough times to expend itsvelocity.

The major differences between modernsystems concern the basic separationphilosophies. Some systems use very largepans that expand the air within the pan untilits velocity will only allow it to carry veryminute coating particles.

Other designs use smaller pans that removemost of the liquid in the pan and separate the


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remaining coating out in a water cooledcyclone separator. Another system integratesthe cyclone principle within the pan itself.

Almost all modern systems pass theexhausted air through a light water mist toremove submicron particles. In many cases,this water is recycled as coating make upwater.

Airknife Set Up

One of the most misunderstood areas ofcoater operation is setting the airknifegeometry.

Referring to Figure 5, below, the followinginformation is required:

R = The radius of the backing roll.

D = The distance of the impingement pointbelow backing roll center line.

A = The angle below the horizontal centerline of a line passing through theimpingement point.

B = The angle above horizontal of the air jetcenter line. (This is not, repeat NOT,the metering angle.)

C = The angle between the center line of theair jet and a radial line passing throughthe impingement point. (This is themetering angle.)

L-R = The horizontal distance between thebacking roll and the top lip.

T-R = The distance along the air jet centerline from the tip of the lips to the roll.On most airknives this is almostimpossible to measure. T-R, the

metering angle C, and air pressure,control coat weight and appearance.

Step 1. Measure backing roll diameter,divide by 2 for roll radius R.

Step 2. Check distance D on both sidesof machine. Make sure bothsides are at the blow off panmanufacturer's recommendedheight and are equal. Toestablish impingement point onroll, check with air turned on.

Step 3. Calculate A, the angle of theimpingement point below centerline, by taking the inverse sin ofdistance D divided by the rollradius R.

Step 4. Find the center line of the air jetby taping a piece of string to thetop lip of the airknife. Measurethe angle from horizontal up tothe string with the air on.

Note: Most air knives have an air jetthat issues 5 to 7 degrees below theirmechanical center line. Measure andrecord angle B so that you know its


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relationship to the angle measurementdevice on the airknife. This anglemeasurement device usually reads outthe angle between the mechanical

center line of the airknife andhorizontal. Apply your correction factorto get angle B for any setting.

Step 5. Most airknives adjust lip to rolldistance along the horizontalaxis. The problem with thisdesign is that if you move theairknife in or out, it decreases orincreases, respectively, theairknife metering angle C byallowing the impingement point to

move up and down the roll.Whenever you make a change ofone parameter (angle or distance)with this design, you must correctthe other to avoid conflictingresults.

Some airknives move in and outalong the air jet center line whichallows either angle or distance tobe changed without affecting theother.

Most airknives operate with AngleC in a range of 52o plus or minus3 degrees. Calculate angle Bfrom angle C minus angle A. Seteven on both sides.

Step 6. Set l ip to roll distance L-Rbetween 3 to 4.5 mm (.120 to.180") as measured by droppingfeeler gages vertically between

the lip and the roll. Remember toadd the amount of your thickestpaper to this distance if you coatboard. Set equally on both sides.

Fine Tuning

Step 7. Of the two adjustments, angleand distance, you have the mostlatitude with distance. In general,

operate as far back as possible,using about 90% of your availablepressure at top machine speed.This will keep the lips as clean as

possible for the longest time andstill leave you some adjustmentfor process changes.

Step 8. The angle adjustment has themost effect on surface quality. Ingeneral, operating at too high anangle can cause vertical lines inmachine direction. Operating attoo low an angle can cause crossmachine lines that are 1/2 to 1"long. Both conditions gradually

fade away as you approach theoptimum angle.

Between these two extremes are a number ofconditions that can appear or disappear witha frustrating lack of consistency.

Mottle, orange peel, over-spray, and otherconditions are all affected to some extent byangle, distance, coating solids, coatingviscosity, sheet porosity, operating speed,sheet temperature, applicator roll speed,binder system and sheet tension.

If you make a change in angle or distance,make sure you correctly compensate theother adjustment to avoid masking the effectof the change.

The best trouble shooting procedure is tohave a regular monitoring program in place to

record the above variables for all gradesduring normal operation. Before adjusting theairknife, check to see if any other parameteris out of specification.

Contour Coating

The airknife is known as a contour coater. Bythat we mean that it applies a relativelyuniform coating thickness regardless of


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surface roughness. This has made itespecially useful for board grades that requireeven coverage rather than improvedsmoothness. It has also proven quite useful

in applications of coatings that cannot acceptmechanical contact or are themselvesabrasive or corrosive.

The airknife has had a longer operatinghistory than most coating methods. Its majormarket is now under attack. New rod coatersare coming on stream that are being used onthe same grades that airknives havetraditionally coated.

These new coaters use an air loaded rod

against a backing roll. The contour coating isprovided by the use of threaded rods thatmeter low viscosity, high solids coatingsvolumetricly. These coatings flow through thegrooves of the rod, and then over the surfaceof the sheet.

They form a slightly smoother contour coatingthan the airknife.

The major drawback with these systems has

been the wear of the threaded rods. Titaniumdioxide coatings (the major market forairknives) are extremely abrasive. Recentadvances in abrasion resistance coatingapplication systems have provided thesolution to this problem. Today, threaded rodsystems can replace almost any airknife withbenefits ranging from higher solids to betteruniformity and less off-quality paper.

Conclusion

The airknife is, and will remain, a valuablecoating tool for many years to come. Recentinroads to its core markets have dramaticallyreduced the number of new installations thatare made with airknives.

References:

1. Booth, George, "Coating Equipmentand Processes"

2. Booth, George, "Applicators for AirknifeCoating", 1985 Tappi Air CoatingSeminar, Sept. 1985.

3. Barton, A. K. "Airknife Coating ofNatural Kraft", 1985 Tappi AirknifeCoating Seminar, Sept. 1985.

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