GB2189270A - Press section of papermaking machine - Google Patents

Abstract

A press section 10 of a papermaking machine is provided with a steam box 40 and compartmentalised suction box 50 for treatment of a wet paper web 12 prior to its passage through the nip between press rolls 30,32, each compartment being associated with a control valve 56n for controlling the vacuum level therein. The valves may be manually controlled or automatically controlled by computer 110 according to the moisture level of the web detected by sensors 112n. Both the steam box and suction box extend across the width of the wet paper web. Improved uniformity of moisture level across the paper web is apparently achieved. <IMAGE>

Description

SPECIFICATION Vacuum apparatusfor press section of papermakíng and s,milarrnachrnes Background of the Invention This invention relates to papermaking machines and othersimilarmachines in which a mixture of solids and liquids is deposited on a porous moving web and gradually deliquified to produce a matrix of solids intheform of a continuous orsubstantially continuous web. For convenience herein, all such machines will be referred to as papermaking machines, andthe products of all such machineswill be referred to as paper.The particular area of application ofthe present invention is the press section of paper-making machines, i.e., the section in which the partially dry paper web is typically disposed on afeltweb and passed between rollers to mechanically express some ofthe remaining liquid from the paper web.

In atypical papermaking mach ine, the mixture of solids and liquids is initially deposited on a continuously moving foraminous wire screen or mesh known as a Fourdrinierwire. After partial deliquification ofthe Fourdrinierwire,the paper web is transferred from the wire to a continuously moving felt web for conveyance into the press section ofthe machine. In the press section, the paperweb (usually together with the felt web) is passed through the nip of at least one pair of press rollers to mechanically express additional liquid from the paper web. After the press section, the paper web is transferred to a dryer section ofthe machine where heat is used drive off still more liquid, leaving the paper web in its final, relatively dry state.

It is generally advantageous to remove as much liquid as possible priortothe dryer section in orderto increase the overall energy-efficiency of the process. It has previously been proposed to apply steam to the paper web in the press section to heat the paperweb and thereby lower the viscosity ofthe liquid in the web.This tendsto increasethe amount of liquid that is removed from the paper web by the press section rollers. It has also been previously proposed to apply vacuum to the press section felt web below the steam source to increase the effectiveness of the steam source in heating the web (See Dickens et al. U.S.

patent 3,097,994, column 6, lines 10-12). The Dickens etal. patent also apparently recognizesthatthe addition of vacuum may dewaterthe web somewhat and contribute to steam penetration of the web (see column 6, lines 43-50).

A problem with such prior proposals for using steam and vacuum in the press section of a papermaking machine is that the resulting apparatus is generally ineffective to prevent nonuniformity in the paper web moisture level across the machine (i.e., perpen dicular to the axis of travet of the paperweb) Far example, it is not atalt uncommonforthe moisture level ofthe paper webto vary considerably (e.g., by 510X) when comparing the dryness across the paper machine Such nonuniformity is highly undesirable dueto customercomplaints related to the printability, runabiiity, strength, etc., ofthe resulting paper, and aFso because it may require the paper maker to incur extra expense in over-drying certain sections ofthe paper web so that no portion ofthe web exceeds maximum moisture content specifications. Prior proposals have proven ineffective to prevent moisture level nonuniformity ofthe type described above.

It is therefore an object of this invention to provide apparatus for use in the press section of a papermaking machine which both enhances liquid removal in the press section and significantly reduces or substantially eliminates moisture level nonuniformity across the paper web.

It is another object ofthe invention to improve the efficiency, and particularlythe energy efficiency, of papermaking machines by (1) reducing the amount of steam which must be applied by the press section steam shower, (2) recovering and reusing heat contained in any uncondensed steam and condensate associated with the press section steam shower, (3) reducing the amount of heatthat must be applied in the dryer section as a result of more effective moisture removal in the press section, and (4) improving the cleaning and therefore the performance ofthe press section felt web as a result of use ofthe hot press section steam shower condensate as felt shower water.

Summary ofthe Invention These and other objects of the invention are accomplished in accordance with the principles of the invention by applying steam andvacuumtothe respective opposite surfaces of the paper web in the press section of a papermaking machine, the vacuum being applied in such awaythatthe level of vacuum in each of a plurality of zones which are spaced across the width ofthe web can be individualiy adjusted to promote uniformity in the paper web moisture level across the web exiting from the press section. In a preferred embodiment, the vacuum is applied by a vacuum box having a plurality of comparmentswhich are spaced across the width ofthe machine.Each compartment is connected to a vacuum header by a conduit having a valve for controlling the strength or level ofthevacuum in the associated compartment.

The valves may be controlled manually or automatically, e.g., by an appropriately programmed, conventional, general-purpose digital computer. The computer may be driven in part by sensors which monitor the paper web moisture level in a plurality of zones spaced across the web at an appropriate location which is typically downstream from the vacuum box. Each zone corresponds to a respective one ofthe vacuum box compartments, andthe valvefor each compart- ment iscontrolled at least in part by the output signal of the moisture level sensorforthe zone associated with that compartment.

The vacuum source for the vacuum box is preferably atype of pump (e.g., a conventional liquid ring pump) which can directly accept and substantially condense any steam pulled through the paper web into the vacuum box. (alternatively a dry4ype steam compressor can be employed where elaborate condensate separators are installed ahead ofthe compressorso asto allow compression ofthe unused steam for re-injection into a suitable steam headerfor reuse, e.g., in the dryer section, other steam shower, steam trim heater, etc.).The condensate and heated excess pumping liquid discharged from the pump can be used at any other point in the papermaking process where hot, clean or relatively clean water is required (e.g., as hotshowerwaterfor reconditioning the press section felt). hot showerspray can also be augmented by a vacuum box connected to a second conventional liquid ring vacuum pump. The heated excess pumping liquid discharged from the second pump can also be used as hotshowerwaterorthe like.

The hot but relatively dirty water separated by separators from both vacuum pump intake lines can also be used elsewhere in the papermaking machine or can be subjected to heat exchange to recover additional heatforotherpurposes.

Furtherfeatures ofthe invention, its nature and variousadvantageswill be more apparentfrom the accompanying drawings and the following detailed description ofthe invention.

Briefdescription ofthe Drawings FIG. is is a schematic elevational view of the press section of a papermaking machine constructed in accordance with the principles ofthis invention.

FIG. 2 is a partial schematic plan viewofthe apparatus of FIG. 1.

Detailed description ofthe Preferred Embodiments As shown in FIG. 1,the paperweb 12 passes through the press section 10 of a papermaking machine from left to right. An "endless " felt web 20 moves at the same velocity in the clockwise direction. Felt web 20 supports paper web 1 2from the vicinity of roller 22 least through the nip between press rollers 30 and 32.

Felt web 20 is supported and driven by rollers 22,24, 26,28, and 30. Rollers 30 and 32 express liquid from paper web 12 in the conventional manner, but the effectiveness ofthese rollers is enhanced bythe practice ofthis invention. Paperweb 12may enter press section 10 from the Fourdrinierwire section (not shown) ofthe papermaking machine, and may exit from press section 10 to the dryer section (also not shown) ofthe machine.

Steam isappliedto paperweb 12 by a steam box 40 disposed immediately above the upper free surface of the paper web between roller 22 and press rollers 30 and 32. Any suitable steam box construction may be used as shown,for example, in Dupasquier U.S.

patent 2,838,982, Dickens eta I. U.S. patent 3,097,994, Dupasquier U.S. patent 3,726,757, Speidel U.S. patent 3,945,881,Throp U.S. patent 4,330,947, Wells U.S.

patent 4,331,510, Shelor U.S. patent Re. 31,065, or Wells U.S. patent 4,545,857.

Directly opposite steam box40 and belowfeltweb 20 isvacuum box 50. As shown in FIG. vacuum box 50 is divided into a plurality of separate vacuum compartments or chambers 52a-n, each ofwhich is capable of applying a different level of suction to the adjacent portion offeltweb 20. Chambers 52 are spaced out across the width ofthe paper web 12 and are preferably collectively exhaustive ofthe width of that web. Each of chambers 52a-n is connected to vacuum header 60 via a respective one of conduits 54a-n. Each ofconduits 56a-n includes respective one of flow control valves 56a-n.Adjustment of each valve 56 controls the vacuum level in the associated chamber 52. Accordingly, the position of each valve 56 controls the amount of suction applied to the associated portion of feist web 20 and, via the felt web, to paper web 12.

Vacuum header 60 is connected to separator70 by vacuum main 62. Separator70 separates the liquid and gaseous components ofthefluid flowing from vacuum main 62. The liquid component flows down drop leg conduit 72 to liquid reservoir74. (If lack of height prevents use of a barometric leg, a liquid removal pump may be employed instead.) Because the liquid from vacuum header 60 typicallycontains some suspended solids from paper web 12, the liquid in reservoir 74 is relatively "dirty". This liquid is also relatively hot as a result of its contact with steam from steam box40, and because it may include some condensed steam. Accordingly, reservoir 74 may provide a source of hot, dirty liquid usable elsewhere in the papermaking machine.Alternatively or in addition, the liquid in reservoir 74 may be supplied to heat recovery apparatus for recovery of some ofthe heatfrom that liquid for use elsewhere in the process (e.g.,forgeneration of steam via a heat pump). Pump 76 and conduit 78 convey liquid from reservoir74to whatever apparatus 120 is provided for use of that liquid.

The gaseous component from separator 70 flows via conduit 80 to the intake port of vacuum pump 82, which is the ultimate source ofthevacuum in vacuum box50.Vacuum pump 82 is preferablya pumpofthe type that can directly accept steam. For example, in the depicted embodiment, pump 82 is a conventional liquid ring pumpsuch as is shown, for example, in Bissell petal. U.S patent4,498,444. Excess pumping liquid and condensed steam discharged from pump 82 is conveyed to liquid reservoir 86 via conduit 84.

The liquid in reservoir 86 is relatively clean. It is also relatively hot as a result of the absorption of a portion ofthe heat of compression in pump 82 and the condenstation ofsteam in pump 82. Accordingly, the hot, relatively clean liquid in reservoir 86 can be used anywhere in the papermaking machinethatsuch liquid is needed. In the depicted embodiment, liquid from reservoir 86 is used as hotshowerwaterfor reconditioning felt web 20. In particular, liquid from reservoir 86 is conveyed by pump 88 and conduit 90 to shower head 92 which sprays the liquid on felt web 20 to recondition the felt in the conventional manner.

Shower head 92 is augmented by vacuum box 94 which is supplied with vacuum by another pump 102.

Pump 102 is preferably simiiarto pump 82. The liquid and gaseous components ofthe fluid from vacuum box 94 are separated from one another by separator 96. The liquid component flows down drop leg conduit 98to reservoir74. (Once again, if lack of height prevents use of a barometric leg, a liquid removal pump may be employed instead.) The gaseous componentflowstothe intake port of pump 102via conduit 100. Hot excess pumping liquid from pump 102 is discharged to reservoir 86 via conduit 104.

Although a particular number of suction chambers 52 isshown in the drawings, itwill be understoodthat any number of such chambers can beemployedto achieve the desired number of regions across paper web 12 in which the applied suction can be individuallycontrolled. For example, chambers 52thatare 6-12 inches wide may be employed in a papermaking machine 330 inches wide. The chambers 52 are not necessarily all ofthe same width. For example, narrnwer chambers 52 may be used nearthe center of the paper web where moisture content entering the press section is frequently highest.It will be understood that increased vacuum in a particularvacuum chamber increases the subsequent deliquifying effect of press rollers 30 and 32 on the associated portion of pape web 12 by increasing the penetration of the steam in that portion of the web. Sufficiently high suction will pull some of substantially all of the steam applied to the associated portion ofthe paper web completely through the web. Some direct deliquifying ofthe paperweb, augmented by the associated steam, may also occur as a result ofthe applied vacuum.

Because vacuum box 50 is made up of a number of individually controlled vacuum chambers, the amount of suction applied to each associated portion of the width of paperweb 12 can be adjusted to compensate for, and thereby reduce or substantially eliminate, any nonuniformity in the moisture content in the web transverse to the direction of travel oftheweb,i.e., across the machine.For example, because the central portion of paper web 12 istypicallywetterthan the marginal edge portions, it will generally be desirable to set valve 56to provide higher vacuum in the suction chambers 52 nearthe centerofweb 12.This will increase the heating effect ofthe steam from steam box 40 nearthe center of web 12, thereby enhancing the deliquifying effect of press rollers 30 and 32 on that portion of the web. It will also increase the direct deliquifying effect ofthe suction chambers nearthecenteroftheweb.Aswill nowbeapparentto those skilled in the art, elements 52 and 56 can be constructed and controlled to provide any desired vacuum profile across the width of the machine.

Valves 56 may be manually controlled. Alternatively, valves 56 may be automatically controlled by appropriate control circuitry which may include a suitably programmed digital computer 110. Control by computer 110 may be based on monitoring the moisture level in paperweb 12 downstream of vacuum box 50. For example, in the depicted embodiment, a plurality of conventional moisture level senscrsii2a-n arespacedacrosspaperweb 12a-n a 12 downstream from press rollers 30 and 32. Each of sensors 112 detects the moisture level in the adjacent portion of paper web 12 corresponding to the portion ofthewebserved bya respective one ofvacuum chambers 52.Accordingly, when one of sensors 112 detects a moisture level in excess of a predetermined maximum threshold level, computer 110 responds by controlling the associated valve 56 to increase the vacuum in the associated suction chamber 52. Conversely, when any one of sensors 112 detects a moisture level below a predetermined minimum threshold level, computer 110 responds by controlling the associated valve 56 to decrease the vacuum level in the associated vacuum chamber 52. In this way computer 110 can be used to dynamically control valves 56to achieve and maintain the desired moisture level and/orthe desired moisture level profile acrossthe machime, within the limits attainable by the disclosed apparatus.

In addition to the ability to achieve a desired moisture level profile across paper web 12 (Generally a uniform moisture level profile across the web is preferred), the apparatus ofthis invention has a number of other advantages. The use ofvacuum box 50 in conjunction with steam box 40 increases the effectiveness ofthe applied steam by improving steam penetration of paper web 12. Accordingly, substantially less steam must be applied by steam box 40to achieve a predetermined deliquifying effect than would otherwise be required. In addition, vacuum box 50 reduces ambient moisture levels by pulling excess steam through paperweb 12 and felt web 20 and conveying it away from the paper forming line. This has many beneficial effects such as reducing the tendency of the steam to corrode the papermaking machine.For optimum steam scavenging,vacuum box 50 is preferably at least as extensive in all directions in plan view as steam box 40 that little or no steam can escape from eiements 40 and 50. It should also be noted that vacuum box 50 has a direct deliquifying effecton paperweb 12. In addition, much ofthe unused heat associated with the steam that passes through webs 12 and 20 is recovered and reused, first in the hot shower water applied to felt web 20 by shower head 92, and then wherever the liquidfrom reservoir74 is used. The use of vacuum box 50 in advance ofthe nip or press rollers 30 and 32 also helps prevent entrapment of air between paper web 12 and felt web 20. Such trapped air can be forced through the paperweb by press rollers 30 and 32, thereby causing defects in the finished paper.

It isto be understood that the embodiments shown and described herein are merely illustrative of the principles ofthe invention and that various modificatins can be made by those skilled intheartwithout departing from the scope and spirit ofthe invention.

For example, although computer control of valves 56 is shown in the drawings, twill be understood that valves 56 could alternatively be manually controlled as explained above. Similarly, although the liquid from reservoir 86 is shown being used as hot shower water for reconditioning felt web 20, it will be understood that any other suitable use can be made of this liquid.

Claims (16)

1. Presssection apparatusforusein partially deliquifying the paper web in a papermaking machine comprising: a moving felt web for supporting the paper web as it passesthroughthe press section; first means disposed adjacent the free surface ofthe paper web at a location in the press section where the paper web is supported by the felt web, the first means applying steam substantially across the width ofthe paper web transverse to the axis of motion of he felt web;; second means disposed adjacent the free surface of the felt web opposite the first means for applying suction to the felt web to pull steam from the first means into and through the paper and felt webs and to pull liquid from the paper web through the felt web, the second means including a plurality of separate suction compartments, each of which applies suction to a respective one of a plurality of laterally adjacent portions ofthefeltweb, the laterally adjacent portions being collectively exhaustive of the width ofthe paper web; and third means for individually controlling the strength ofthesuction applied by each suction compartmentto permit control ofthe amountofsteam pulled into and through each associated portion ofthewidth of the paper web and to permit control ofthe amount of liquid pulled from each associated portion ofthe paperweb.

2. The apparatus defined in claim 1 further comprising: fourth means for separating the gaseous and liquid components ofthe fluid pulled into the second means; fifth means for compressing the gaseous compo- nent of said fluid to at least partially condense the steam pulled into the second means to produce hot water; and sixth means for reusing the hot water in the papermaking machine.

3. The apparatus defined in claim 2 wherein the felt web is a recirculating closed loop, only a portion of the length of which is in contact with the paperweb at any one time, and wherein the sixth means comprises: seventh means for spraying the hot water on a portion ofthe felt web which is not in contactwith the paper web for reconditioning the feltweb for subse- quent reuse in contact with the paper web.

4. Theapparatusdefined in claim 2furthercom- prising: eighth means for reusing the liquid component of said fluid in the papermaking machine.

5. The apparatus defined in claim 4wherein the eighth means comprises: heat exchange means for recovering at least a portion of the heat from the liquid component of said fluid for reuse in the papermaking machine.

6. The apparatus defined in claim 3 further com- prising: ninth means disposed adjacent the felt web on the side ofthefeltweb oppositethe seventh means for applying suction to the felt web to pull the hot water from the seventh means through the felt web.

7. The apparatus defined in claim 6 further comprising: tenth means for separating the gaseous and liquid components ofthefluid pulled into the ninth means; eleventh meansforcompressing the gaseous com ponent ofthefluid pulled into the ninth means; and twelfth means for reusing any hot water produced bytheeleventh meansinthepapermaking machine.

8. The apparatus defined in claim 7 wherein the twelfth means supplies any hot water produced by the eleventh means to the seventh means.

9. The apparatus defined in claim 7 further com prising: thirteenth means for reusing the liquid component produced by the tenth means in the papermaking machine.

10. The apparatus defined in claim 9 wherein the thirteenth means comprises: heat exchange means for recovering at least a portion of the heatfromthe liquid component produced by the tenth means for reuse in the papermaking machine.

11. The apparatus defined in claim 1 further comprising: a first liquid ring pump for supplying suction to the second means.

12. Theapparatusdefined in claim 6further comprising: asecond liquid ring pumpforsupplyingsuctionto the ninth means.

13. The apparatus defined in claim 1 wherein the third means comprises: a plurality ofvalves, each ofwhich is associated with a respective one of the suction compartments for controlling the strength ofthe suction in the associated compartment.

14. Theapparatusdefined in claim 13further comprising: fourteenth means for monitoring the moisture contentofthepaperwebin a plurality of laterally adjacent zones across the width ofthe paper web, each zone being aligned with a respective one ofthe suction compartments parallel to the axis of motion of the felt web; and fifteenth means responsive to the fourteenth means for controlling each valve in accordance with the moisture content in the paper web zone associated with the suction compartment controlled by that valve.

15. The apparatus defined in claim 13further comprising: a source of suction; and a plurality of suction conduits, each ofwhich is connected between the source of suction and a respective one ofthe suction compartments, each of the valves being disposed in a respective one ofthe suction conduits for controlling the strength ofthe suction in the associated compartment.

16. The apparatus defined in claim 1 wherein the second means applies suction to an area ofthe felt web which is at least substantially co-extensive with the area of the paperwebto which thefirstmeans applies steam.