tappi felt conditioning article

6
VOL. 80: NO. 7 TAPPI JOURNAL 103 PEER REVIEWED T HE EXTENT OF FELT CONDITION- ing varies by grade, machine speed, furnish (recycle or virgin fiber), paper machine age, and, obviously, papermaker knowledge and experience. Some- times, a little knowledge is danger- ous, and the frequently misused solu- tion to a problem,“more is better,” quite often is applied. Many mills have experienced felt conditioning problems and make changes in the process, such as more vacuum, more slot width, more showering, and even more uhle boxes. Sometimes the process is improved, but fre- quently there are negative results. There have been several press rebuilds which occurred partly due to poor application or lack of felt cleaning equipment. A discussion of the above items and variables, as well as some actual case studies, should lead to a better understanding of the entire felt conditioning process. FELT MOISTURE RATIO VS. PER- CENT MOISTURE Typically, felt moisture is surveyed with a “Scan-Pro,” a device which measures the amount of water in the felt as g/m 2 . This raw data is then reported in two ways: moisture ratio and percent moisture. A problem exists here because these values are often used synonymously, and they are not the same. Two simple equa- tions define these terms. Moisture ratio = Grams of water/Grams of felt (1) % moisture = Grams of water x 100/Grams of water + Grams of felt (2) Since both properties are unitless, one may not be aware when an error is made. For example, a felt with a moisture ratio of 0.40 has the same amount of water as the same felt with 28% moisture. Often the felt moisture studies are quickly looked at, and the papermaker will see 28 is less than 40 and not realize the felt moisture is equal. A worse situation exists when a felt is reported to be at, say, 32% moisture, and it is believed to be drier than one with a moisture ratio of 0.38. It is not. Typically, these two values will not be used in the same moisture study,but both will be used in studies reported by different felt vendors. Furthermore, regarding moisture studies, it is important to obtain moisture values before and after the uhle boxes. Even if a CD profile is not possible due to press geometry, an MD measurement should be taken. A moisture level measured in a felt only before a uhle box is about as practi- cal as trying to estimate reel moisture based on a sheet moisture measure- ment in the middle of the dryer sec- tion. UHLE BOX SLOT WIDTH VS. DWELL TIME Uhle boxes, or felt boxes and felt tubes, as they also are called, are the devices which remove water and contaminants from a wet or press felt. They appear simple and have no moving parts. However, a lot of good engineering goes into the correct design and application of uhle boxes. The first two variables in select- ing a uhle box are slot width and felt width. Slot width is proportional to machine speed and is responsible for dwell time over the vacuum zone. Dwell time is the time, usually expressed in milliseconds, that the felt is exposed to vacuum. A revision of TAPPI TIS 014-55, “Air Flow Requirements for Condi- tioning Press Felt at Suction Pipes,” has been underway for the last three or four years. The revised Technical Information Sheet, TIS 0404-27, was published in early 1996. Extensive research was conducted to predict moisture levels in modern press fab- rics. One of the most important vari- ables was found to be dwell time. ABSTRACT Felt conditioning is a very important part of the total papermaking process, but it is often misunderstood. There are many significant felt condi- tioning variables, including felt design, machine speed, vacuum level, vacuum factor, dwell time, uhle box and cover design, showering, vacuum system lay- out, and chemical cleaning. Often the cause/effect relationship of these vari- ables is not obvious.This results in inadequate sheet dewatering in the press, crushing and other quality defects, short felt life, and sheet mois- ture variations.This paper reviews the key factors in felt conditioning and presents actual case studies where measurable results have been achieved. Application: This paper would be useful in trou- bleshooting and optimizing press felt conditioning, resulting in improved felt performance. PRESSING A systems approach to felt conditioning DOUGLAS F. SWEET

Upload: soma-nathan

Post on 12-Dec-2015

82 views

Category:

Documents


12 download

DESCRIPTION

TAPPI Felt Conditioning Article

TRANSCRIPT

Page 1: TAPPI Felt Conditioning Article

VOL. 80: NO. 7 TAPPI JOURNAL 103

PEER REVIEWED

T HE EXTENT OF FELT CONDITION-ing varies by grade, machinespeed, furnish (recycle orvirgin fiber), paper machine

age, and, obviously, papermakerknowledge and experience. Some-times, a little knowledge is danger-ous, and the frequently misused solu-tion to a problem, “more is better,”quite often is applied. Many millshave experienced felt conditioningproblems and make changes in theprocess, such as more vacuum, moreslot width, more showering, andeven more uhle boxes. Sometimesthe process is improved, but fre-quently there are negative results.There have been several pressrebuilds which occurred partly dueto poor application or lack of feltcleaning equipment. A discussion ofthe above items and variables,as wellas some actual case studies, shouldlead to a better understanding of theentire felt conditioning process.

FELT MOISTURE RATIO VS. PER-CENT MOISTURE

Typically, felt moisture is surveyedwith a “Scan-Pro,” a device whichmeasures the amount of water in thefelt as g/m2. This raw data is thenreported in two ways: moisture ratioand percent moisture. A problemexists here because these values areoften used synonymously, and theyare not the same. Two simple equa-tions define these terms.

Moisture ratio = Grams ofwater/Grams of felt (1)

% moisture = Grams of water x100/Grams of water + Grams of felt

(2)

Since both properties are unitless,one may not be aware when an erroris made. For example, a felt with amoisture ratio of 0.40 has the sameamount of water as the same feltwith 28% moisture. Often the feltmoisture studies are quickly lookedat, and the papermaker will see 28 isless than 40 and not realize the feltmoisture is equal.

A worse situation exists when afelt is reported to be at, say, 32%moisture, and it is believed to bedrier than one with a moisture ratioof 0.38. It is not. Typically, these twovalues will not be used in the samemoisture study, but both will be usedin studies reported by different feltvendors.

Furthermore, regarding moisturestudies, it is important to obtainmoisture values before and after theuhle boxes.Even if a CD profile is notpossible due to press geometry, anMD measurement should be taken. Amoisture level measured in a felt onlybefore a uhle box is about as practi-cal as trying to estimate reel moisturebased on a sheet moisture measure-ment in the middle of the dryer sec-tion.

UHLE BOX SLOT WIDTH VS. DWELLTIME

Uhle boxes, or felt boxes and felttubes, as they also are called, are thedevices which remove water andcontaminants from a wet or press

felt. They appear simple and have nomoving parts. However, a lot of goodengineering goes into the correctdesign and application of uhleboxes.

The first two variables in select-ing a uhle box are slot width and feltwidth. Slot width is proportional tomachine speed and is responsible fordwell time over the vacuum zone.Dwell time is the time, usuallyexpressed in milliseconds, that thefelt is exposed to vacuum.

A revision of TAPPI TIS 014-55,“Air Flow Requirements for Condi-tioning Press Felt at Suction Pipes,”has been underway for the last threeor four years. The revised TechnicalInformation Sheet, TIS 0404-27, waspublished in early 1996. Extensiveresearch was conducted to predictmoisture levels in modern press fab-rics. One of the most important vari-ables was found to be dwell time.

ABSTRACTFelt conditioning is a very importantpart of the total papermakingprocess, but it is often misunderstood.There are many significant felt condi-tioning variables, including felt design,machine speed, vacuum level, vacuumfactor, dwell time, uhle box and coverdesign, showering, vacuum system lay-out, and chemical cleaning. Often thecause/effect relationship of these vari-ables is not obvious.This results ininadequate sheet dewatering in thepress, crushing and other qualitydefects, short felt life, and sheet mois-ture variations.This paper reviews thekey factors in felt conditioning andpresents actual case studies wheremeasurable results have beenachieved.

Application:

This paper would be useful in trou-bleshooting and optimizing press feltconditioning, resulting in improved feltperformance.

PRESSING

A systems approach to feltconditioningDOUGLAS F. SWEET

Page 2: TAPPI Felt Conditioning Article

104 TAPPI JOURNAL JULY 1997

PRESSING

A dwell time of 2–4 millisecondsyielded successful water removal (1,2). The formula for determining slotwidth is:

Slot width (mm) = Dwell (millisec-onds) x Machine speed (m/min)/60(3)

Slot (in.) = Dwell time (s) x Machinespeed (ft/min)/5 (4)

This formula is used to determinethe recommended total slot widthon each uhle box, not each slot. Forexample, a calculated slot width of25 mm (1 in.) would typically beapplied using a double slotted uhlebox with two 12.5 mm (0.5 in.)slots. Rearranging the equation tocalculate dwell time on an existinguhle box:

Dwell time (milliseconds) = Slot width(mm) x60/Machine speed (m/min)

(5)

Dwell time (s) = Slot width (in.) x5/Machine speed (ft/min) (6)

A dwell time significantly less than 2milliseconds can lead to incompletewater removal by the uhle box. Con-versely, a dwell time over 4 millisec-onds does not increase water removalenough to justify additional vacuumcapacity.

Slot widths vary typically between12 and 25 mm (0.5 and 1 in.). A slotless than 12 mm could plug or bridgeover. A slot width of 12 mm is usually

suitable for machine speeds up to1000 ft/min. A slot greater than 25mm could cause excessive felt wear.Usually, after the desired total slotwidth on a uhle box exceeds 25 mm,a double slotted box is used.

The research involved in the newTechnical Information Sheet showedthere was no meaningful differencein felt moisture exiting the uhle boxdue to single or double slots of thesame total width. A guide for deter-mining the total slot width, per uhlebox, based on a dwell time of 2 mil-liseconds, is shown in Table I.

UHLE BOX DESIGN AND VACUUMFACTORS

The next item after determining slotwidth and resulting area based onfelt width is to apply the vacuumfactor. TAPPI TIS 0502-01 recom-mends a minimum of 660 m3/min/m2

(15 ft3/min/in.2) open slot area (3).Furthermore, it is stated that factorshave ranged from 750 to 970m3/min/m2 (17 to 22 ft3/min/in.2),especially on felts heavier than 1375g/m2 (4.5 oz) and long nip or shoepresses.The uhle box vacuum capac-ity is determined by the formula:

Vacuum = slot area x vacuum factorcapacity (7)

Now that correct vacuum capacityhas been determined, an often for-gotten item is to properly size theuhle box diameter. On new orupgraded vacuum systems, much

effort is spent to minimize vacuumlosses within the vacuum systempiping. However, uhle box diametersare usually ignored. Since uhle boxesare conveying vacuum capacityacross the felt width while carryingliquids and solids, it makes sense to size them as is done with vacuum piping. Typically, 18–20 m/s(3500–4000 ft/min) are velocitiesused for vacuum piping with liquidwater (prior to a separator), and28–30 m/s (5500–6000 ft/min) arevelocities for vacuum piping withrelatively dry air (after a separator).Therefore, it is recommended to sizeuhle boxes using the wet air veloci-ties.For high vacuum capacity flows,it is necessary to increase thesevelocities further because of sizerestrictions within the press. Recom-mended uhle box diameters for vari-ous capacities are shown in Table II.

The quantity and placement ofuhle boxes is less of a science andmore of an applications experience.Theoretically,one uhle box per felt isadequate, assuming correct dwelltime and vacuum factor. However,two uhle boxes may be used on feltscarrying more water, where sheetfillers are used, on the fastestmachines, and for assuring goodmoisture profile on high quality,lightweight sheets. Machines operat-ing below 610 m/min (2000 ft/min)usually do not require two uhleboxes per felt. Also, on the driest feltpositions, only one uhle box isneeded (3rd or 4th presses).

Felt/machine speed, Total slot width,m/min (ft/min) mm (in.)

Below 300 (980) 12 (0.5)300-450 (100-1500) 15 (0.6)450-600 (1500-2000) 20 (0.8)600-750 (2000-2500) 25 (1.0)750-900 (2500-3000) 30 (1.2)900-1050 (3000-3500) 35 (1.4)1050-1400 (3500-4500) 40 (1.6)Above 1400 (4500) 46-50 (1.8-2.0)

Uhle box capacity, Uhle box diameter,m3/min (ft3min) mm (in.)

20 (700) 150 (6)34 (1200) 200 (8)54 (1900) 250 (10)79 (2800) 300 (12)110 (4000) 350 (14)160 (5600) 400 (16)230 (8000) 450 (18)290 (10000) 500 (20)

II. Recommended uhle box diametersI. Guide for determining slot width

Page 3: TAPPI Felt Conditioning Article

VOL. 80: NO. 7 TAPPI JOURNAL 105

Placement of uhle boxes can befrom horizontal to vertical felt runs.There have even been specially con-structed uhle boxes placed upsidedown in difficult press sections. Careshould be given to allow enoughspace to accommodate oscillatingneedle showers, chemical showers,and lube showers ahead of the uhlebox. Only a lube shower is requiredahead of the second of two uhleboxes.

Uhle box cover or wear stripmaterial and configuration also varywidely. High molecular weight poly-ethylene is most common, but it hasmany different formulations. Thesedifferences affect life and cost andshould be evaluated. Ceramic covermaterial has a relatively high initialcost, but it can be justified over time.

Ceramic wear strip/cover mater-ial is successfully used on uhleboxes. Its advantage is long life, espe-cially on higher speed machines. Beaware that not all uhle boxes willaccommodate ceramics due to struc-tural and space limitations or lack ofrigidity. The price of ceramic is ini-tially high and ranges from five to tentimes the price of polyethylene.Addi-tionally, there are different grades ofceramic with corresponding differ-ences in prices. The lowest costceramic is aluminum oxide; siliconnitride is the most expensive.Another advantage to using ceramicsis reduced or no pitch buildup onthe ceramic strips. This buildupvaries with mill and region.

The last major topic on uhle boxdesign is the design of the “slot”opening. Traditionally, a straight slot,multiple slots, or a herringbone pat-tern were the only choices available.With the development of seamedfelts, other designs have evolved tominimize seam wear. Some of thenew slot configurations have also ledto reduced felt wear in general andmay be worth consideration.

SHOWERINGFelt showering has experiencedextensive changes over the last tenyears. Some changes are due to theneed for cleaning modern press felts.Other new developments are due tonew equipment technology. Devel-opments of new designs and materi-als for press felts have resulted fromthe requirements of long nip andheavily loaded presses. These majorinnovations in pressing occurred inthe early 1980s, leading to significantinnovations in felt design and con-struction. These innovations spreadthrough most paper and boardgrades. Cleaning these modern feltsrequires modern showering tech-niques. Continuous, not intermittent,showering with a combination oflow pressure fan and lube showersplus high pressure, oscillating needleshowers must be applied properly.

A properly designed felt condi-tioning system will allow continuoususe of low- and high-pressure show-ers (4). A common belief is that auhle box only extracts water thatwas absorbed by the felt. Actually, ina well designed system, only one-third of the water removed by theuhle box comes from the sheet. Theother two-thirds of this water is fromlow and high pressure showers. Thispoint leads up to the fact that conta-minants in a felt cannot be removedonly by applying vacuum. Water isrequired to loosen and help conveydebris from the felt.

Fan showers are for evenly wet-ting the entire width of the felt. Theyoperate at relatively low pressures of3–4 bar (40–60 psig). Most oftenthese are placed directly ahead ofuhle boxes as lubrication showers.The shower must be designed to pro-vide complete and even coverage ofthe felt. Some mills are oscillatingthese showers to minimize effects ofplugged or poorly operating nozzles.It is important to have all fan nozzlesspraying uniformly. This also leads to

the use of showers with internalbrushes to aid in cleaning the noz-zles on the run.

High-pressure needle showersproduce a high energy spray whichis concentrated on a very small area.However,only a thin strip of clothingis cleaned as it moves past the noz-zle. It is not practical to have enoughneedle nozzles on the shower pipeto completely cover the felt. Thiswould also be prohibitive due topumping costs of high-pressurewater. The solution is to move theshower axially, back and forth acrossthe felt. This movement should be ata constant rate with little or no dwellat the end of the stroke.

The mechanisms used to producethe shower movement have includedpneumatic and hydraulic cylinders,gear motors with crank-arms and,more recently, self-contained, electro-mechanical shower oscillators. Theapparently simple, cylinder-drivenand crank-arm designs were limitedas they did not provide the uniformstroke rate and instant directionalchange for the shower.These oscillat-ing showers provide a random clean-ing of the felt and may completelybypass some areas. This non-uniformcleaning causes felt streaking and CDsheet moisture variations in theworst cases.

These self-contained, electro-mechanical shower oscillators wereintroduced in the early 1980s. Theyalso had the feature of speed control.This new feature achieved completecoverage cleaning of the felt by mov-ing the shower across the felt a dis-tance equal to the width cleaned bya single nozzle, with each full revolu-tion of the felt. This motion can becalculated by the equation:

S T/L = R (8)

where

Page 4: TAPPI Felt Conditioning Article

106 TAPPI JOURNAL JULY 1997

PRESSING

S = felt speed in m/min (ft/min)L = loop length of the clothing in m (ft)T = nozzle cleaning width in mm (in.)R = stroke rate in mm/min (in./min)

The stroke rate determined by thismethod is relatively much slowerthan that observed with previouslyused oscillators. The stroke ratecould vary from less than 6 mm/min(0.25 in./min) for a long, wet felt ona slow cylinder machine to 100 or150 mm/min (4 or 6 in./min) for atypical press felt on a high-speednewsprint machine. Many times thespeed control for the electro-mechanical oscillator will be inter-locked to paper machine speed con-trols so the machine speed for differ-ent grades can be tracked.

Shower stroke length is based onthe shower nozzle spacing. Mostnozzles for felt cleaning are placedon 150 mm (6 in.) centers. The rec-ommended stroke length is twotimes the nozzle spacing, i.e., 300mm (12 in.) stroke with nozzles on150 mm (6 in.) centers. The reasonfor this relationship between strokelength and nozzle spacing is to pro-vide 100% nozzle overlap in theevent a nozzle is plugged. Since nee-dle showers have relatively smallnozzle orifices, typically 1–1.5 mm(0.040–0.060 in.) diameter, the useof clean, filtered water is important.Additionally, as with fan showers,internal brush mechanisms are alsoused with needle showers. In somevery critical applications, a pipe-within-a-pipe configuration may beused to allow removal and mainte-nance of the shower and nozzles onthe run.

Usual pressures for needle show-

ers vary from 10 to 17 bar (150 to250 psig).Higher pressures can dam-age the felt, and lower pressures donot provide adequate cleaningenergy. Placement of these showersis also important. Cleaning modernpress felts requires needle showersto be located on the sheet side of thefelt. It is practically impossible toclean these multi-layer, heavy feltsfrom the inside as was done only 10or 15 years ago.The angle of the nee-dle spray with respect to the felt willvary with felt construction, showerdesign, and papermaker experience.There are many opinions on thismatter, so a new one will not beintroduced here.

The final and most importantpoint on shower placement is theproper distance from the shower tothe felt. The best cleaning isobtained at distances between 75and 150 mm (3 and 6 in.) from thefelt. Much greater distances willallow the needle stream to break upand form small droplets, producingsmall energy pulsations on the feltand even damaging the felt.

VACUUM SYSTEMThe vacuum system consists of pip-ing, separators, valves, separatorremoval pumps, seal/weir tanks, andthe vacuum pumps. An excess ofvacuum pumps will not replace apoorly designed system, undersizedpiping, or lack of separation equip-ment. This paper began with deter-mining the vacuum capacity and thesizing and selection of uhle boxes.This is the logical order of eventsleading up to the system design andvacuum pump selection.

One of the most importantpoints in the system design is tohave separate vacuum sources foruhle boxes on separate felts. Onevacuum pump per felt will not allowtwo or more felts of different porosi-ties to affect each other. There is noproblem with connecting two uhleboxes together on the same felt.

Next, the vacuum piping must besized (5). This is based on velocity, aswas done with the uhle box sizing.As stated earlier, vacuum pipingbefore a separator should be sizedwith a velocity of 18–20 m/s(3500–4000 ft/min). Piping after theseparator can be sized for 28–30 m/s(5500–6000 ft/min). The piping runsshould be horizontal or slopingdownward before a separator, neveruphill.After the separator,piping maybe routed as needed to reach the vac-uum pumps. Vacuum piping is mostoften stainless steel, but many sys-tems have used carbon steel betweenthe separators and vacuum pumps.

The vacuum separator is animportant element of this system.With the constant showering, thereare significant amounts of water,fiber, contaminants, and chemicals toremove before the vacuum pumps.Separator configuration may vary,but most designs employ a tangentialinlet. Since the operation of a separa-tor is primarily affected by the inter-nal velocity, more efficient separa-tion occurs at internal velocitiesaround 2.5–4 m/s (500–750 ft/min).Most separators are constructed ofstainless steel. The use of either oneor two separators on a felt with twouhle boxes is acceptable. The choiceis usually based on space availability.When two separators are used, thevacuum piping may be joined priorto the vacuum pump. However, thewater outlet piping should never bejoined. Each separator should haveits own barometric seal line or waterremoval pump.

Water removal from the separa-tors is also important. For most topfelt uhle boxes,a barometric seal linecan be used. Care must be exercisedin routing this piping to minimizeturns. Piping should be verticalwhen possible or never more than45° from vertical. Typical velocitiesfor sizing this piping are 1–2 m/s(4–6 ft/s). A minimum pipe diameterof 100 mm (4 in.) is recommended.

KEYWORDS Equations, equipment, felt conditioning,hydraulic equipment, moisture, mois-ture content, problem solving, showers.

Page 5: TAPPI Felt Conditioning Article

VOL. 80: NO. 7 TAPPI JOURNAL 107

The other end of the seal line isat the seal tank. Most modern instal-lations use multi-compartment sealtanks with one compartment perseparator and calibrated weirs forflow measurement. This allows sim-ple observation and assessment offelt water removal. The seal tankmust be located at a sufficient eleva-tion below the separator to over-come the effects of vacuum. This dis-tance must be the vertical distancemeasured between the bottom ofthe separator and the liquid level ofthe seal tank. The direct conversionfrom kilopascals to meters of wateris 3.4 kPa/m of water (1.0 in. Hg vac-uum per 1.14 ft of water). Since thisis a direct conversion,an additional 1m (3 ft) should be added to allow forfriction and as a safety factor. Alsoimportant is the volume of the sealtank. A simple rule is to size the sealtank volume for two times the vol-ume of the seal line piping.

The final and also very importantstep is to select the vacuum pump.Heavier felt designs on today’smachines require higher vacuum lev-els for dewatering. As discussed ear-lier, vacuum factors between 660and 880 m3/min/m2 (15 and 20ft3/min/in.2) will cause the uhle boxvacuum level to reach 60 kPa (17 in.Hg) and often 68 kPa (20 in. Hg).Therefore, it is important to select avacuum pump capable of a fairlyconstant capacity up to this maxi-mum vacuum level. Additionally, itmay be necessary to install a vacuumrelief valve to limit maximum vac-uum levels should the felt becomeexcessively filled and compacted.

CHEMICAL CLEANINGSome felts require more thanmechanical conditioning and clean-ing with showers and uhle boxes.Some paper and board grades havefurnishes and fillers that can adhereto the felt and may be difficult toremove unless chemical solvents ordetergents are used. An optimum felt

conditioning system utilizes bothmechanical and chemical means tokeep today’s modern, synthetic feltsopen and functioning.

Contaminants often found in theanalysis of old felts include solubleand insoluble material (6). The short-est list is the insolubles such as fiberand fines, clay, talc, titanium dioxide,and silica. The mechanical action ofshowers and uhle boxes can removemost of these. The soluble materialscome from the pulp mill furnish,recycled fiber supplies,and the brokesystem. They can be separated intothree groups: acid, alkaline, and sol-vent solubles. These groups includealuminum hydroxide, calcium car-bonate, wet- and dry-strength addi-tives, lignin, starch, size, fatty acids,glue, latex, oil, grease, and wax.

Removing these and other conta-minants with chemicals will requireexamination by the chemical sup-plier. Treating these conditions maybe through batch, intermittent, orcontinuous introduction of chemi-cals. In most cases, the chemicals areapplied to the felt through a station-ary fan shower. Depending on theapplication, the shower may belocated on the sheet or back side ofthe felt. For continuous or intermit-tent chemical cleaning, the shower isusually on the back side of the feltand positioned as close as possible tothe point after which the sheetleaves the felt surface. This providesthe most residence time to allow thechemicals to work before the uhlebox extracts them. Again, the chemi-cal supplier familiar with the mill’swet-end chemistry should beinvolved.

CASE STUDIESCase 1

The pick-up felt on a 7.1 m (280 in.)linerboard machine was operatingwith two 300 mm (12 in.) diameteruhle boxes with herringbone coversand a vacuum factor of 480m3/min/m2 (11 ft3/min/in.2). This low

vacuum factor prevented the vacuumlevel from reaching beyond about 40kPa (12 in. Hg). The vacuum pumpwas originally sized for two uhleboxes with double 16 mm (0.625 in.)slots and was capable of operating atup to 68 kPa (20 in.Hg).However,theslotted covers were replaced withherringbone covers when this posi-tion was changed to a seamed felt.The vacuum piping was properlysized and installed and included avacuum separator. The separatorflowed through a barometric sealline to a seal tank with a calibratedweir. Showering was good, with anelectromechanical oscillator drivingthe needle shower. A lube showerwas installed on each uhle box. Theprimary problem with the systemwas the excessive open area of theherringbone covers. In addition, theuhle boxes were undersized.

This system was improved withthe replacement of the uhle boxeswith new 350 mm (14 in.) diameterunits. Uhle boxes 400 mm (16 in.) indiameter were recommended, but350 mm (14 in.) units were the maximum size that would fit.More importantly, the covers weredesigned to have about half the openarea of the previous herringbonecovers. This yielded a vacuum factorof 880 m3/min/m2 (20 ft3/min/in.2).The notable item in this study wasthat there were no changes to thesystem other than new, larger uhleboxes with covers of less open area.

Results were significant andimmediate. Vacuum level at the uhleboxes started at 34 kPa (10 in.Hg) onnew felts and was at 50 kPa (15 in. Hg) within a week, due to initial compaction. Water removalincreased by about 380 L/min (100gal/min), as measured over the sealtank weir. The moisture ratio of thispickup felt dropped to 0.41, wherepreviously it was 0.55 to 0.60. Themeasurably drier felt absorbed morewater from the sheet and increasedsheet dryness from the press section.

Page 6: TAPPI Felt Conditioning Article

108 TAPPI JOURNAL JULY 1997

PRESSING

Steam pressure was reduced to thedryer at the same production rate.

Case 2Another linerboard machine, in thiscase a 100% recycled, 4.6 m (180 in.)wide machine, was converted from aconventional,two-bottom felt press toa tandem bottom felt. The conversionwas done to improve runnability andavoid a major press rebuild. This con-version led to a study of the existingfelt conditioning system. The systemwas inadequate, with limited shower-ing capability, undersized uhle boxesand low vacuum capacity.Vacuum fac-tors were below 440 m3/min/m2 (10ft3/min/in.2).All uhle boxes were con-nected on a common vacuum header.

The entire felt conditioning sys-tem was replaced, including vacuumpumps, separators, uhle boxes, andshowers. The system was designed toallow separate vacuum sources foruhle boxes on each felt.New vacuumfactors for the system were 790m3/min/m2 (18 ft3/min/in.2).

Within 36 h after startup, produc-tion rates increased by 31%.

Case 3A 5.6 m (220 in.) coated boardmachine was operating with poorfelt life, 28–30 days, and sheet defectscaused by high felt moisture. Feltshowering had to be intermittent

because the felts were already toowet. The felt conditioning systemwas studied, and several problemswere identified. These included acommon vacuum header for all uhleboxes, undersized uhle boxes, anexcessive slot open area resulting ina vacuum factor of less than 220m3/min/m2 (5 ft3/min/in.2), two uhleboxes per felt,no vacuum separators,inadequate showering, and under-sized vacuum piping. However, thevacuum pumps were in good condi-tion and were the correct size. Theywould be reused.

The uhle boxes were replacedwith just one per felt and slotted cov-ers yielding a vacuum factor of 660m3/min/m2 (15 ft3/min/in.2). Vacuumseparators with low NPSH removalpumps were installed, undersizedvacuum piping was replaced, and feltshowers were replaced with com-plete coverage designs. Again, thevacuum pumps were not changed.

Two new felts were installed, andimmediate results were observedshortly after startup. Machine speedincreased by 15 m/min (50 ft/min)with continuous showering, andmany sheet defects disappeared.Bothfelts were cut off after 40 days andwere determined to still have usablelife. Felt life is now 45–50 days.

CONCLUSIONA little theory and common senseapplied to most felt conditioning sys-tems can produce significant results.Often these are not expensive solu-tions, and payback is fast. Gatheringand interpreting data correctly is thekey to problem identification. Thesolutions are relatively easy. TJ

Sweet is regional sales director, Nash Pulp &Paper Div., Birmingham,AL.

Received for review June 12, 1996.Accepted Aug. 10, 1996.

LITERATURE CITED1. TIS 0404-27, “Air flow requirements

for conditioning press felts at suctionpipes.”

2. Bennett, J. and Tehan, J., TAPPI 1994Papermakers Conference Proceedings,TAPPI PRESS,Atlanta, p. 535.

3. TIS 0502-01,“Paper machine vacuumselection factors.”

4. Swett, G., “Mechanical felt condition-ing,” Nash-CVN Systems, 1989.

5. Sweet, D. F., TAPPI 1988 PapermakersConference Proceedings, TAPPI PRESS,Atlanta, p. 279.

6. Dickens, J. H.,TAPPI 1989 PapermakersConference Proceedings, TAPPI PRESS,Atlanta, p. 111.