CA1076861A - Method and apparatus for dewatering in a paper machine - Google Patents
Method and apparatus for dewatering in a paper machineInfo
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- CA1076861A CA1076861A CA288,563A CA288563A CA1076861A CA 1076861 A CA1076861 A CA 1076861A CA 288563 A CA288563 A CA 288563A CA 1076861 A CA1076861 A CA 1076861A
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- steam
- web
- roll
- nips
- suction roll
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Abstract
ABSTRACT OF THE DISCLOSURE:
Dewatering of a paper web is enhanced by steam treatment in the press section of a papermaking machine wherein the web is continously passed around a substantial sector of a suction roll between two nips formed with the suction roll by a water-receiving roll and a plain roll. A
steam box is mounted adjacent the web between the nips to subject the web to treatment steam in that sector. The steam can be saturated or superheated.
Dewatering of a paper web is enhanced by steam treatment in the press section of a papermaking machine wherein the web is continously passed around a substantial sector of a suction roll between two nips formed with the suction roll by a water-receiving roll and a plain roll. A
steam box is mounted adjacent the web between the nips to subject the web to treatment steam in that sector. The steam can be saturated or superheated.
Description
~ - - 107~816~
- The present invention relates to a process of and apparatus for enhancing the dewatering of a ~aper web in the press section o~ a paper machine.
The invention disclosed herein is intended to be an improvement to a system of thle type described in Applicant's Canadian Patent N 971.790~ This system, which is known and will be referred to herein as the "Sym-Press"* press section ("Sym-Press" is a trade mark), includes a closed transfer pres~
section which is a development which has been substantially responsible for recent increases in the operating speeds of paper machines. A reason for this i~ that, in addition to efficient dewatering, the Sy~n-Press press section has the further advantage that web breaks are virtually eliminated.
However, in the practice of further raising paper machine speeds, the free runs of the web following after the press section have now become a bottleneck, the free runs or open draws referred to carrying the web either from the press section to the drying section or through the first free interstices in the drying section~ Attempts have been made to avoid the web breaks at these points, and to avoid the consequent shutdowns of equipment by various techniques including by means of the expedients taught in Finnish patent 45,558, and United States Patent N 4,016,032. While these techniques have resulted in improvements, they have not completely eliminated web breaks occurring after the press section, which breaks have harmful effects on run ability of the paper machine.
Accordingly, a primary object of the present inven-tion is to provide a method and apparatus by which the running speed and reliability of a paper machine can be improved by enhancing - ~
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~076~361 the dewatering action in the press section so that the web leaves the press section having a higher dryness and higher strength than in previous devices.
It is appropriate to observe in this connection that the object of the invention is not so much to achieve in paper machines the optimum condition as regards heat economy, but rather to optimize the operation economy of the paper machine in view of its running efficiency and reliability in operation.
It is a well-known physical fact that the viscosity of water considerably decreases with increasing temperature of the water. This phenomenon has been utilized in paper machines in order to enhance the dewatering of the paper web. As examples of these prior art app oaches, reference is made to previously issued U.S. patents 2,907,690; 3,097,995; 3,560,333;
and 3,655,507. Of these, the first mentioned patent '690 dis-closes a normal paper machine press wherein a hot gas jet is directed from nozzles with small holes against the felt on the suction roll prior to the press nip, the invention being to lower the viscosity of the water residing in the felt and thereby to enhance the dewatering action.
The '995 patent also mentioned above discloses the use of steam supplying means in various parts of the paper machine and the means in question consists of a steam feeder box and a suction box opposed thereto. As regards the use of this particular design in the aid of enhanced dewatering in a press nip, the drawback to be observed is that the steam-supplying means has been placed rather a long distance before the con-ventional suction press nip, with the result that the web cools before it reaches the nip.
~` 30 In the '333 patent, there is disclosed means for the supplying of steam into the throat between the well known Yankee cylinder and the suction roll opposing it, the aim being ~ .
- The present invention relates to a process of and apparatus for enhancing the dewatering of a ~aper web in the press section o~ a paper machine.
The invention disclosed herein is intended to be an improvement to a system of thle type described in Applicant's Canadian Patent N 971.790~ This system, which is known and will be referred to herein as the "Sym-Press"* press section ("Sym-Press" is a trade mark), includes a closed transfer pres~
section which is a development which has been substantially responsible for recent increases in the operating speeds of paper machines. A reason for this i~ that, in addition to efficient dewatering, the Sy~n-Press press section has the further advantage that web breaks are virtually eliminated.
However, in the practice of further raising paper machine speeds, the free runs of the web following after the press section have now become a bottleneck, the free runs or open draws referred to carrying the web either from the press section to the drying section or through the first free interstices in the drying section~ Attempts have been made to avoid the web breaks at these points, and to avoid the consequent shutdowns of equipment by various techniques including by means of the expedients taught in Finnish patent 45,558, and United States Patent N 4,016,032. While these techniques have resulted in improvements, they have not completely eliminated web breaks occurring after the press section, which breaks have harmful effects on run ability of the paper machine.
Accordingly, a primary object of the present inven-tion is to provide a method and apparatus by which the running speed and reliability of a paper machine can be improved by enhancing - ~
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~076~361 the dewatering action in the press section so that the web leaves the press section having a higher dryness and higher strength than in previous devices.
It is appropriate to observe in this connection that the object of the invention is not so much to achieve in paper machines the optimum condition as regards heat economy, but rather to optimize the operation economy of the paper machine in view of its running efficiency and reliability in operation.
It is a well-known physical fact that the viscosity of water considerably decreases with increasing temperature of the water. This phenomenon has been utilized in paper machines in order to enhance the dewatering of the paper web. As examples of these prior art app oaches, reference is made to previously issued U.S. patents 2,907,690; 3,097,995; 3,560,333;
and 3,655,507. Of these, the first mentioned patent '690 dis-closes a normal paper machine press wherein a hot gas jet is directed from nozzles with small holes against the felt on the suction roll prior to the press nip, the invention being to lower the viscosity of the water residing in the felt and thereby to enhance the dewatering action.
The '995 patent also mentioned above discloses the use of steam supplying means in various parts of the paper machine and the means in question consists of a steam feeder box and a suction box opposed thereto. As regards the use of this particular design in the aid of enhanced dewatering in a press nip, the drawback to be observed is that the steam-supplying means has been placed rather a long distance before the con-ventional suction press nip, with the result that the web cools before it reaches the nip.
~` 30 In the '333 patent, there is disclosed means for the supplying of steam into the throat between the well known Yankee cylinder and the suction roll opposing it, the aim being ~ .
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1C~768~i1 to prevent the cooling of Yankee cylinder at the press nips.
In the '507 patent, a press nip with two felts is shown wherein pressurized steam-supplying means is provided inside one of the two rolls, the steam heating the felt for enhanced dewatering action in the subsequent nip.
The means for enhancement of dewatering which are based on the raising of the temperature of felt, web and/or roll have not gained very extensive application, particularly not in fast paper machines. This is partially due to the fact that it has not been possible, using the means of the prior art, to conduct to the points of supply a sufficiently high heat flow. This, again, is caused by the fact that, for example, to avoid destruction of the felt, the temperature of the treatment gas is limited and also because the available time is short because of the high web and felt speed, and -further on account of space consideration, the heat transfer surface is also quite restricted. An aim of the present invention is to provide a solution to the problems presented~
Briefly described, the invention includes a process of enhancing the dewatering of a paper web in the press section of a paper machine of the type having at least two press nips through which the web consecutively passes, the first one of -` the nips being formed between a water-receiving roll and a suction roll and a subsequent one of the nips, in the direction of web travel, being defined by the suction roll and a plain - roll, and wherein the web runs on the suction roll between the ~-~ nips, the improvement comprising the step of treating the web by exposing the outer surface thereof to hot steam between the two nips so that a significant proportion of the treatment steam condenses thereon, ther~by imparting the latent heat of the steam to the web.
`~ The invention also includes an apparatus for
1C~768~i1 to prevent the cooling of Yankee cylinder at the press nips.
In the '507 patent, a press nip with two felts is shown wherein pressurized steam-supplying means is provided inside one of the two rolls, the steam heating the felt for enhanced dewatering action in the subsequent nip.
The means for enhancement of dewatering which are based on the raising of the temperature of felt, web and/or roll have not gained very extensive application, particularly not in fast paper machines. This is partially due to the fact that it has not been possible, using the means of the prior art, to conduct to the points of supply a sufficiently high heat flow. This, again, is caused by the fact that, for example, to avoid destruction of the felt, the temperature of the treatment gas is limited and also because the available time is short because of the high web and felt speed, and -further on account of space consideration, the heat transfer surface is also quite restricted. An aim of the present invention is to provide a solution to the problems presented~
Briefly described, the invention includes a process of enhancing the dewatering of a paper web in the press section of a paper machine of the type having at least two press nips through which the web consecutively passes, the first one of -` the nips being formed between a water-receiving roll and a suction roll and a subsequent one of the nips, in the direction of web travel, being defined by the suction roll and a plain - roll, and wherein the web runs on the suction roll between the ~-~ nips, the improvement comprising the step of treating the web by exposing the outer surface thereof to hot steam between the two nips so that a significant proportion of the treatment steam condenses thereon, ther~by imparting the latent heat of the steam to the web.
`~ The invention also includes an apparatus for
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dewatering a paper web in the press section of a pap r machine of the type having a felted suction roll, a water-receiving roll forming a first nip with the suction roll and a plain roll forming a second nip with the suction roll, the web being passed around a sector of the suction roll successively between the first and second nips, comprising steam supply means mounted adjacent the web and means for conducting steam to the supply means, the supply means comprising at least one steam supply box extending across the entire width of the web adjacent the suction roll, the box having an interior space and an open side opening toward the suction roll and conforming to the curvature of the roll over a substantial sector thereof to define a flow path for treatment steam onto the sector.
Since according to the invention hot steam is used to raise the temperature of the web, the steam will in its condensation process release and impart to the web its consider-able latent heat, in the order of 2,270 kilojoules per kilogram (kJ/kg). When, furthermore, the point where the steam is supplied is located as taught by the invention, the supply surface can be made rather large and the steam can be induced ~ to penetrate immediately into the web which is being treated ; and also partly into the felt lying thereunder quite efficient-ly, also utilizing the vacuum zone at this part of the suction roll of the press. It i9 a further advantage of the invention - that, exactly at the point of supply specified in the inven- -tion, adequate space exists for expedient steam supply means.
It is also an advantage that when according to the invention the point of steam supply is placed after the first nip, then `~ the web has had time when entering that nip, which is advanta-geously formed between two felts and in which the web is efficiently dewatered in two directions, to attain a dry matter content (in the order of 20 to 35%) such that one need ; B
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not heat unnecessarily by the steam supply such water which .~ is even otherwise easy to remove from the web.
i. In order that the manner in which the foregoing and : .
I~ other-objects are attained in accordance with the invention .. . . .
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can be understood in detail, particularly advantageous embodiments thereof will be described with reference to the accompanying drawings, which form a part of this specification and wherein:
Fig. 1 is a schematic diagram of a paper machine including the press section wherein the method and apparatus of the present invention are appliecl;
Fig. 2 is an enlarged side elevation of the press section of the apparatus of Fig. 1, in partial section, showing the arrangement of the present invention;
Fig. 3 is a schematic plan view of the apparatus in accordance with the present invention;
Fig. 4 is a diagram illustrating the heat economy of the invention;
Fig. 5 is a graph illustrating the dryness of the paper web after the press section as a function of the quantity of steam supplied, at two different web speeds, the data illustrated therein having been recorded on a pilot paper - machine fitted with equipment in accordance with the invention;
~ Fig. 6 is a graph illustrating the viscosity of water . . .
20 as a function of temperature; ;
Fig. 7 is a graph illustrating the steam quantity X` required to raise the temperature of the web to a given value, ~ plotted against the dryness which the web has reached after `, the first nip; and ~;~ Fig. 8 is a graph illustrating the required steam quantity as a function of outgoing web temperatures.
The construction and operation of the paper machine ` will be understood by reference to Fig. 1 which shows a paper web W which is detached from the wire 52 between rolls 5 and ` 30 53 with the aid of a pickup roll 3 which operates within a felt loop 32 of its own. Web W remains adherent to felt 32 as a result of the effect of suction acting within roll 3. This .. . .. - . .,. -,, . --,~ . .~
~76861 suction zone is indicated by reference numeral 31. The web travels around roll 3 through an arc distance of about 150 after leaving the wire and then is engaged by a transfer suction roll 4 which operates within the loop of its own felt la. The suction zone of transfer roll 4 is indicated at 41. At the nip between rolls 3 and 4, the web is transferred from felt 32 to felt la.
After the web has been transferred onto felt la, this felt transports the web W to the first dewatering nip Nl of the press assembly, which nip is defined by the rolls 2a and 2b and the web is pressed between two felts la and lb. Roll 2a is a water-receiving roll or a recessed surface roll such as, for example, a grooved roll and roll 2b is a suction roll having either a continuous suction zone 21 or several suction compart-ments. Adherent to the surface of felt lb, the web passes onto the following dewatering nip ~2 which is defined by the juxtaposition of rolls 2b and 2c. Roll 2c is a plain (smooth surface) roll and has no felt, whereby the dewatering in nip -`~ ~2 takes place in the direction toward the suction roll 2b.
In the nip ~2' the web W is transferred from felt lb onto roll ; 2c which carries it into the following nip N3 of the press ~ system where the roll 2d is a recessed surface roll and is ., .
provided with a felt ld of its own.
~;~ From nip ~3, the web may be conventionally conducted - to the drying section. However, it is also possible to place additional nips against roll 2c. These may be either provided with felt, if desired to attain an enhanced dewatering effect, or without felt merely to obtain a smoothing effect, such as, for example, the roll 2e, which has been provided with a soft covering.
The felts of the system presented in the figures have been indicated with reference numerals la, lb, ld and 32, and .. . .
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the felt guiding rolls have been identified as 23 and 33, respectively. The suction zones of different rolls carry the reference numerals 21, 31, 41 and 51, and the cleaning doctor device of the roll 2c is denoted as 24.
In Fig. 1, the first two drying cylinders 81 of the drying section of the paper machine and the so-called lead-in cylinder 80 have been depicted. The breaks previously referred to which occur at the first open draws indicated at Dl, D2, D3, etc. of the web W are those the elimination of which is the object of the process and apparatus of the invention.
As shown in Figs. 1 and 2, a steam supply box 60 in accordance with the invention is mounted in association with the suction sector 21 of suction roll 2b in a four-roll, closed transfer, press section, the supply section of this box facing against the outer surface of web W traveling upon the felt lb over the suction zone 21, which surface is freely exposed at r this point. The details of the design of the steam supply box 60 are seen in Fig. 2. The supply surface of steam supply box 60 corresponding to the central angle ~ of the suction roll -lies within the holding section ~2 of the suction roll 2b. At the first nip Nl of the press, there is a dewatering suction sector ~1~ and at the second nip ~2~ there is another dewatering suction sector ~3. The shell 25 of the suction roll 2b iæ
perforated in a manner which is, in itself, previously known.
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In this particular suction roll, the sectors ~1 and ~3 are - connected to higher vacuum than the sector ~2 The steam supply box comprises a steam supply pipe 62 which lies within the supply box, the pipe being connected in the manner more clearly shown in Fig. 3 through a steam supply valve 76 to a steam distribution header 63 to which the steam arrives from equipment schematically represented by block 61, which equipment is a conventional component in a paper 1~768~1 machine. In connection with the pipe 62, control means 75 is provided to control actuating motor 77 of the steam flow control valve 76. In the manner schematically shown in Fig. 3, on the lateral margins of steam supply box 60, there is provided a suction means 74 which acts to prevent the harmful discharge of steam to the sides from the margins of box 60.
- As shown in Fig. 2, supply pipe 62 of the steam supply box 60 serves as the supporting structure for the box. Side beams and a shell plate 64 are attached to the steam supply pipes 62 by supporting members 73. The steam supply pipe 62 ~;`
` and the shell plate 64 define within themselves a steam supply space 70 into which the steam is supplied from pipe 62 through nozzle holes 67. In connection with nozzles holes 67, which are mutually longitudinally spaced along the entire length of pipe 62, there are steam flow control spindles 68 which can be adjusted by means of control screws 69. The steam supply holes 67 are suitably spaced at greater distances from each other near the end portions of the box in order to reduce steam leakage. Steam supply box 60 comprises an outer shell 65 and thermal insulation 66 between the inner and outer shells.
The supply surface of steam supply box 60 adjacent the free outer surface of web W comprises a curved plate :' ~
~; including plate portions 71 and 72. The curved plate can consist, for example, of a sintered material or a material which is perforated such that the steam can pass through it and act upon web W. It is possible under some circumstances to elimlnate the plate including portions 71 and 72, in which case the interior space 70 of the steam supply box can simply open toward the web. In order to dlstribute the steam as uniformly 1 as possible in the interior space of the supply box and thence onward, and in order to prevent destruction of the surface of web W, the steam supply apertures 67 open in the direction : . : .
- . ~ .
~768~1 opposite the supply plate portions 71 ancl 72, whereby the steam jets Fo have time to lose velocity and to become equalized in the space 70 before they discharge through the plate in the form of flows Fl onto web W.
Referring to the press part construction shown in Fig. 1, it is particularly important in the invention that the steam supply box 60 has been located, with reference to the direction of movement of the webl after the first nip Nl and before the second nip ~2 at the point where the web W upon felt lb passes over the holding sector ~2 f suction roll 2b. Most advantageously, the steam supply box is placed after such a nip wherein the dry matter content of the web is between 20 and 35%, preferably about 30%. The temperature of the wet web ` W is raised with the aid of the steam discharging from the steam supply box 60 by exposing it from the direction of its outer surface on a sector a of considerable magnitude of the suction roll lb to the effect of hot steam so that a remarkable and ~- significant portion of the treatment steam condenses expressly in the web W, thereby releasing its latent heat and, in this -~
manner, increasing the temperature of the web W and reducing . , the viscosity of the water, thereby enhancing the dewatering in the subsequent nip N2 and, if provided, also in the nips following ~2 Condensation of steam may also take place into -~
the felt lb, and part of the steam flow may pass through the felt lb into the suction chamber of roIl 2b.
Most advantageously, the treatment steam is substan-tially at atmospheric pressure in the in-terior space 70 of supply box 60, owing to the loss of pressure caused by the nozzle holes 67, so that the flow of steam into the web W and through it, and partly possibly also through the felt lb, being mainly accomplished as a result of the effect of the vacuum of the suction zone ~2 of the suction roll 2b.
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1~686~
In addition to the effects described, the invention causes as an extra effect that the felt loop during its continuous run through the steam treatment achieves an elevated temperature and passes through the preceding nip ~1' in this way enhancing the dewatering in the nip Nl.
According to the invention, the treatment steam is saturated steam, but it is also possible to use somewhat super- ;
heated steam although due consideration must be given to the thermal tolerance of the web W and above all of the felt lb thereunder. By using the apparatus of the invention, the steam can be discharged uniformly over an extensive area onto web W
and, since the steam is substantially at atmospheric pressure in the interior space 70 of the steam supply box 60, it is not ~ necessary to dimension the steam supply box like a pressure -~ vessel. Thus, the construction is relatively simply and inexpensive in this regard. -The method and apparatus of the invention are in no way confined to the details presented in the foregoing which are merely by way of illustration. For example, the steam supply merely by way of illustration. For example, the steam supply box 60 may be arranged to be adjustable as to its position, for example, by suspending it from articulated arms, not shown. By means of the suspension of the steam supply box 60 from articulated arms, the elastic suspension of box 69 may also be advantageously accomplished so that "lumps" of the web W cause no major damage. The spring arrangement may in this connection be realized~ for example, by utilizing air cylinders. It; is also possible to use a supply surface of the steam supply box 60 which is larger or smaller than that sho~
in the figure, and the extent of the surface which corresponds to an angle of the suction roll may be increased up to about 90. The lower limit of sector a is determined by the need of - - . . : . ., . . : .
107~
enhancement of the dewatering action. The invention may further-more be carried out so that, instead of using one steam supply box 60, two or more steam supply boxes mounted one after the other can be used, the interaction of these being adjusted so that the desired end result is obtained.
The provision of steam supply box 60 may also partly be employed to assist the keeping of the web W on the sector 2b of the suction box (in cooperation with the suction sector ~2) even at high speeds of the web W which speeds produce at this point rather high centrifugal forces which tend to lift the web off the surface of the roll 2b. In such a case, it may be desirable to use a pressure higher than atmospheric pressure in box 60 to assist in this function. `~
The general observation may be made that the size and placement of the steam supply box 60, or a plurality of such boxes, is determined, considering the dry matter content of web W, by space considerations, and according to the inven-tion one may advantageously use the web surface which remains free in the "Sym-Press" press section on the suction roll 2b at a point which is suitable in all respects.
It was observed above that the dewatering is enhanced through the fact that the web and the water present therein are heated such that the viscosity of the water decreases in the manner seen in the graph of Fig. 6. The effect of the method of the inven-tion may partly also be based on the cir-cumstance that when the steam ~ischarges into the comparatively cold web W, there occurs the so-called implosion phenomenon, a very abrupt collapse of steam bubbles, which on the basis of mechanical effects reduces the adhesion forces acting between the fibers of the web W and the water present therein, and which thereby facilitates the escape of the water from web W.
Thus, as the steam flows through the water-containing fiber * "Sym-Press" is a trade mark.
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network, it produces as it condenses in the water films between fibers, an effect breaking up these films and thus pxomoting ; the dewatering process.
The introduction of the procedure and apparatus of the invention is partly rendered possible by the felts which ;~ have only recently appeared on the market and which tolerate comparatively high temperatures.
In order to clarify the physical background of the invention, the heating of wet paper with the aid of steam will be considered in the following discussion with reference to Fig. 4.
Let us assume, to begin with, that the steam coming from the bo~ 60 all condenses in the web W, whereby the tempera ` ture of the web rises from Tl to T2. The heat losses to the environment are disregarded. The water quantity removed by the second nip N2 is denoted as ~V, the water removed at this point having the temperature T~ There are no fiber losses.
The heat balance equation may now be written:
m c T + lOO~kl c T + m i" = m c T + (100~ kl + m ) c T :
k k 1 lkl~ 2 k k 2 Lkl~ 2 (Symbols as in Fig. 4) where ck = specific heat capacity of the fiber c = specific heat capacity of water i" = enthalpy of the steam ~i~h ~T2 - Tl) Ck + 100 c/kl) mk (i" - c T2) Since the final, completed paper contains about 7% water, the steam consumption per kg of paper is found to be (mk = 0.93 mp):
m X = h = 0-93 (T2 - Tl) (ck + 100 c/kl) m (i" - c T2) Figs. 7 and 8 present, in the form of a set of curves, --~'7~861 the steam quantity X required for the treatment of the web W
according to the invention, in dependence on the moisture con-tent of the web when entering the treatment and the desired ultimate temperature T2.
Scrutiny of Figs. 7 and 8 reveals that the pressure Pe of the saturated steam introducecl in the box 60 is immaterial if only the steam supply pipe 62 and its supply holes 67 have been so dimensioned that the requisite flow from the pipe 62 - is obtained.
Example:
In view of establishing the effect of the procedure and apparatus of the invention, the experiments reported in the following were carried out on the pilot paper machine at Rautpohja of the company Valmet Oy. The press section employed in the experiements was substantially as presented in Fig. 1, except that the felt corresponding to the felt lb of the suction roll 2b also operated as pickup felt, that is, there was no felt corresponding to that indicated with reference numeral 32 in Fig. 1.
For the experiments, a box substantially as in Fig. 2 ;
was mounted adjacent to the suction zone of the suction roll 2b of the press section of the trial machine in the press between the first and second nips. Into this box saturated steam was ~;
conducted from a steam generator, and the following quantities were measured:
steam pressure pulp stock temperature temperature of the web water content of the fel-ts - dryness of the web.
The machine was run with speeds 10 and 13.5 m/s, and the most essential operating data were recorded. The paper stock consisted of conventional groundwood pulp.
As regards the results obtained in the tests, reference 1~768~;1 is made to curves A and B in Fig. 5 (corresponding to web speeds 10 and 13.5 m/s, respectively), which show the dryness, in percent, of the web W after the press section, as a func-tion of the quantity X of steam supplied according to the - invention.
The observation can be made as a summary of the test results that when steam is blown onto the web W by the aid of a box mounted on the suction roll of the press section, the dryness of the paper web after the press section increases markedly:
At speed v = 10 m/s k = 42.2% rising to 46.5%
At speed v = 13.5 m/s k = 40.8% rising to 45.3%
The water content of the felts also went down somewhat when steam was used. The felts got warmer, but during the comparatively short trial run, the heat was not observed to have caused any damage to the felts.
The maximum steam consumption was 0.29 and 0.26 kg per kg of paper at the speed of 10 and 13.5 mjs, respectively.
It may mentioned for comparison that in the drying section of a newsprint machine, the normal consumption is 1.66 kg steam per kg of paper. It is thus understood that application of the procedure of the invention requires a considerable amount of steàm, but in addition to the effect of improved runability of the paper machine, which is the object of the invention, the invention also results in a reduced steam consumption in the frying section because the web W is drier and warmer when it àrrives in the drying section. It is well known that the heat output of the first drying cylinders is spent in raising the temperature of the web, while no evaporation takes place.
Closer details of the experiments which were carried out are apparent from the tables following below. The results of measurement are stated in Table I. Table II specifies the ~. .. ~ . - : :.
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; felts which were used in the experiements and Table III gives the removed water auantities, calculated from the results.
TAsLE I
Running speed (m/s) 10 13.5 Loading (k~/M) (press pressures) First nip 60 60 Second nip 80 80 ` Third nip 90 90 i Degrees of vacuum (kPa) Wire suction roll 66.7 64.0 Pickup 26.7 28.0 Press suction roll, first s. 65.3 64.0 " " " second s. 21.3 22.7 " " " third s. 62.7 62.7 Steam supply overpressure (kPa) 0 300 0 100 300 Steam temperature (C.) - 143 - 120 143 Estima~ed steam quantity (g/s) - 102 - 53 102 -Base weight of paper (g/m ) 55.3 57.9 48.7 48.7 48.7 Dryness of paper, %
After the wire 19.5 19.2 18.3 16.9 16.9 After the second nip 35.8 39.0 34.2 35.4 36.9 After the third nip 42.4 46.5 40.8 42.6 45.3 Temperature of the paper (C.~
After the second nip 36.0 73.5 24.2 39.0 57.0 After the third nip 24.2 38.5 54.5 Water in the pickup felt (g/m2) 735 730 750 730 Water in the third felt (g/m2) (corresponds to felt ld in Fig. 1) 550 510 525 490 TABLE II
Pickup 1,084 g/m Lower 1,100 g/m Third felt 1,350 g/m (All these Celts are fully synthetic.) . ` ': i , `, . .~ , , , . .~.' ' ` ,'. ! '; ` ' ~076~
TABLE III
(Water removal quantities calculated from the results of measurement.) Speed (m/s) 10 13.5 ` Supply steam overpressure (kPa) O 300 0 100 300 Removed by third nip (g/m ) 21.8 22.3 21.4 2i.6 22.8 Removed by lst-2nd nip (g/m2) 120 142 115 140 145 Total 142 165 137 162 168 It is seen on scrutiny of Tables I and III that the dewatering is enhanced in the second nip, whereas in the third nip, the increase of water removal capacity is minor. Since the pickup felt is heated, the dewatering is also improved in the first nip, although the steam supply box has no direct effect of the first nip.
On studying the test results, it is appropriate to point out to begin with that fully exact conclusions and calculations were somewhat impeded by the difficulties encountered in stabilizing various parameters, such as, e.g., the stock temperature, and by inaccuracies of measurement, e.g., of the steam quantity measurements, as well as by the fact that the whole steam quantity does not pass through the web W, but part of it discharges through the sides of the box.
At all events, it is positively observable from the test results that the steam blowing considerably improves the dryness of the web, and that it simultaneously increases the ^
temperature of the web, which circumstances also contributes to higher drying capacity in the drying section, as was observed above.
The steam consumption of the box, per kg of paper, calculated from the estimates will be as follows:
1. v = 10 m/s Pe = 300 kPa x = 0.29 kg/kg 2. v = 13.5 mJs Pe = 300 kPa x = 0.26 kg/kg 1~7686~
3. v = 13.5 m/s Pe = 100 kPa x = 0.13 kg/kg If all steam were condensed in the paper, this would mean an increase of water content, per m2, in the different cases:
1. 17.1 g H20/m2 2. 12.7 g H20/m2 3. 6.3 g H20/m2 and this water mass adds to the amount that has to be removed in the second and third nips.
But it is noted that this complete condensation did not occur in the experiments because in that part of the steam spread into the environment, while part of its passes through ~
the web and the felt into the suction roll. ;;-Studies were also made concerning the changes in the quality characteristics of the p~per which might be observable in connection with the experiments just described. This was ;
done by analyzing specimens from consecutive runs with the steam supply disconnected in one run and connected in the other.
It appears that the steam treatment of the web improves to some degree the opacity of the finished paper. This is indicated by the following figures relating to newspring paper (57 g/m2, consisting of 76% ground wood and 24% chemical pulp): !
Steam Treated Normal Paper SCAN-OPACITY 94.2% 32.3%
The above increase in opacity can be regarded as statistically significant.
Various details of the invention are not confined to what has been presented in the foregoing for illustration only.
For instance, although in the foregoing first nip, etc., has been specifiecl, this does not exclude the possibility that there are nips prior to this particular nip, such as, e.g., a pickup nip or another nip carrying out a preliminary pressing 1CS76~
and dewatering operation.
While certain advantageous embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifica-tions can be made therein without departing from the scope of the invention as defined in the appended claims.
- 18 - ~:
~L~7~
dewatering a paper web in the press section of a pap r machine of the type having a felted suction roll, a water-receiving roll forming a first nip with the suction roll and a plain roll forming a second nip with the suction roll, the web being passed around a sector of the suction roll successively between the first and second nips, comprising steam supply means mounted adjacent the web and means for conducting steam to the supply means, the supply means comprising at least one steam supply box extending across the entire width of the web adjacent the suction roll, the box having an interior space and an open side opening toward the suction roll and conforming to the curvature of the roll over a substantial sector thereof to define a flow path for treatment steam onto the sector.
Since according to the invention hot steam is used to raise the temperature of the web, the steam will in its condensation process release and impart to the web its consider-able latent heat, in the order of 2,270 kilojoules per kilogram (kJ/kg). When, furthermore, the point where the steam is supplied is located as taught by the invention, the supply surface can be made rather large and the steam can be induced ~ to penetrate immediately into the web which is being treated ; and also partly into the felt lying thereunder quite efficient-ly, also utilizing the vacuum zone at this part of the suction roll of the press. It i9 a further advantage of the invention - that, exactly at the point of supply specified in the inven- -tion, adequate space exists for expedient steam supply means.
It is also an advantage that when according to the invention the point of steam supply is placed after the first nip, then `~ the web has had time when entering that nip, which is advanta-geously formed between two felts and in which the web is efficiently dewatered in two directions, to attain a dry matter content (in the order of 20 to 35%) such that one need ; B
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1~768~ ~
. . . . I
not heat unnecessarily by the steam supply such water which .~ is even otherwise easy to remove from the web.
i. In order that the manner in which the foregoing and : .
I~ other-objects are attained in accordance with the invention .. . . .
' .
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~768~;~
can be understood in detail, particularly advantageous embodiments thereof will be described with reference to the accompanying drawings, which form a part of this specification and wherein:
Fig. 1 is a schematic diagram of a paper machine including the press section wherein the method and apparatus of the present invention are appliecl;
Fig. 2 is an enlarged side elevation of the press section of the apparatus of Fig. 1, in partial section, showing the arrangement of the present invention;
Fig. 3 is a schematic plan view of the apparatus in accordance with the present invention;
Fig. 4 is a diagram illustrating the heat economy of the invention;
Fig. 5 is a graph illustrating the dryness of the paper web after the press section as a function of the quantity of steam supplied, at two different web speeds, the data illustrated therein having been recorded on a pilot paper - machine fitted with equipment in accordance with the invention;
~ Fig. 6 is a graph illustrating the viscosity of water . . .
20 as a function of temperature; ;
Fig. 7 is a graph illustrating the steam quantity X` required to raise the temperature of the web to a given value, ~ plotted against the dryness which the web has reached after `, the first nip; and ~;~ Fig. 8 is a graph illustrating the required steam quantity as a function of outgoing web temperatures.
The construction and operation of the paper machine ` will be understood by reference to Fig. 1 which shows a paper web W which is detached from the wire 52 between rolls 5 and ` 30 53 with the aid of a pickup roll 3 which operates within a felt loop 32 of its own. Web W remains adherent to felt 32 as a result of the effect of suction acting within roll 3. This .. . .. - . .,. -,, . --,~ . .~
~76861 suction zone is indicated by reference numeral 31. The web travels around roll 3 through an arc distance of about 150 after leaving the wire and then is engaged by a transfer suction roll 4 which operates within the loop of its own felt la. The suction zone of transfer roll 4 is indicated at 41. At the nip between rolls 3 and 4, the web is transferred from felt 32 to felt la.
After the web has been transferred onto felt la, this felt transports the web W to the first dewatering nip Nl of the press assembly, which nip is defined by the rolls 2a and 2b and the web is pressed between two felts la and lb. Roll 2a is a water-receiving roll or a recessed surface roll such as, for example, a grooved roll and roll 2b is a suction roll having either a continuous suction zone 21 or several suction compart-ments. Adherent to the surface of felt lb, the web passes onto the following dewatering nip ~2 which is defined by the juxtaposition of rolls 2b and 2c. Roll 2c is a plain (smooth surface) roll and has no felt, whereby the dewatering in nip -`~ ~2 takes place in the direction toward the suction roll 2b.
In the nip ~2' the web W is transferred from felt lb onto roll ; 2c which carries it into the following nip N3 of the press ~ system where the roll 2d is a recessed surface roll and is ., .
provided with a felt ld of its own.
~;~ From nip ~3, the web may be conventionally conducted - to the drying section. However, it is also possible to place additional nips against roll 2c. These may be either provided with felt, if desired to attain an enhanced dewatering effect, or without felt merely to obtain a smoothing effect, such as, for example, the roll 2e, which has been provided with a soft covering.
The felts of the system presented in the figures have been indicated with reference numerals la, lb, ld and 32, and .. . .
'' '; ~; '. ~ - ''' ' `-; ,' ;' -: ': . :
: , .: . . ;
~768~
the felt guiding rolls have been identified as 23 and 33, respectively. The suction zones of different rolls carry the reference numerals 21, 31, 41 and 51, and the cleaning doctor device of the roll 2c is denoted as 24.
In Fig. 1, the first two drying cylinders 81 of the drying section of the paper machine and the so-called lead-in cylinder 80 have been depicted. The breaks previously referred to which occur at the first open draws indicated at Dl, D2, D3, etc. of the web W are those the elimination of which is the object of the process and apparatus of the invention.
As shown in Figs. 1 and 2, a steam supply box 60 in accordance with the invention is mounted in association with the suction sector 21 of suction roll 2b in a four-roll, closed transfer, press section, the supply section of this box facing against the outer surface of web W traveling upon the felt lb over the suction zone 21, which surface is freely exposed at r this point. The details of the design of the steam supply box 60 are seen in Fig. 2. The supply surface of steam supply box 60 corresponding to the central angle ~ of the suction roll -lies within the holding section ~2 of the suction roll 2b. At the first nip Nl of the press, there is a dewatering suction sector ~1~ and at the second nip ~2~ there is another dewatering suction sector ~3. The shell 25 of the suction roll 2b iæ
perforated in a manner which is, in itself, previously known.
. ~
In this particular suction roll, the sectors ~1 and ~3 are - connected to higher vacuum than the sector ~2 The steam supply box comprises a steam supply pipe 62 which lies within the supply box, the pipe being connected in the manner more clearly shown in Fig. 3 through a steam supply valve 76 to a steam distribution header 63 to which the steam arrives from equipment schematically represented by block 61, which equipment is a conventional component in a paper 1~768~1 machine. In connection with the pipe 62, control means 75 is provided to control actuating motor 77 of the steam flow control valve 76. In the manner schematically shown in Fig. 3, on the lateral margins of steam supply box 60, there is provided a suction means 74 which acts to prevent the harmful discharge of steam to the sides from the margins of box 60.
- As shown in Fig. 2, supply pipe 62 of the steam supply box 60 serves as the supporting structure for the box. Side beams and a shell plate 64 are attached to the steam supply pipes 62 by supporting members 73. The steam supply pipe 62 ~;`
` and the shell plate 64 define within themselves a steam supply space 70 into which the steam is supplied from pipe 62 through nozzle holes 67. In connection with nozzles holes 67, which are mutually longitudinally spaced along the entire length of pipe 62, there are steam flow control spindles 68 which can be adjusted by means of control screws 69. The steam supply holes 67 are suitably spaced at greater distances from each other near the end portions of the box in order to reduce steam leakage. Steam supply box 60 comprises an outer shell 65 and thermal insulation 66 between the inner and outer shells.
The supply surface of steam supply box 60 adjacent the free outer surface of web W comprises a curved plate :' ~
~; including plate portions 71 and 72. The curved plate can consist, for example, of a sintered material or a material which is perforated such that the steam can pass through it and act upon web W. It is possible under some circumstances to elimlnate the plate including portions 71 and 72, in which case the interior space 70 of the steam supply box can simply open toward the web. In order to dlstribute the steam as uniformly 1 as possible in the interior space of the supply box and thence onward, and in order to prevent destruction of the surface of web W, the steam supply apertures 67 open in the direction : . : .
- . ~ .
~768~1 opposite the supply plate portions 71 ancl 72, whereby the steam jets Fo have time to lose velocity and to become equalized in the space 70 before they discharge through the plate in the form of flows Fl onto web W.
Referring to the press part construction shown in Fig. 1, it is particularly important in the invention that the steam supply box 60 has been located, with reference to the direction of movement of the webl after the first nip Nl and before the second nip ~2 at the point where the web W upon felt lb passes over the holding sector ~2 f suction roll 2b. Most advantageously, the steam supply box is placed after such a nip wherein the dry matter content of the web is between 20 and 35%, preferably about 30%. The temperature of the wet web ` W is raised with the aid of the steam discharging from the steam supply box 60 by exposing it from the direction of its outer surface on a sector a of considerable magnitude of the suction roll lb to the effect of hot steam so that a remarkable and ~- significant portion of the treatment steam condenses expressly in the web W, thereby releasing its latent heat and, in this -~
manner, increasing the temperature of the web W and reducing . , the viscosity of the water, thereby enhancing the dewatering in the subsequent nip N2 and, if provided, also in the nips following ~2 Condensation of steam may also take place into -~
the felt lb, and part of the steam flow may pass through the felt lb into the suction chamber of roIl 2b.
Most advantageously, the treatment steam is substan-tially at atmospheric pressure in the in-terior space 70 of supply box 60, owing to the loss of pressure caused by the nozzle holes 67, so that the flow of steam into the web W and through it, and partly possibly also through the felt lb, being mainly accomplished as a result of the effect of the vacuum of the suction zone ~2 of the suction roll 2b.
_ g _ .. .,. ,. .. .,, , . . . ., ~..... .
1~686~
In addition to the effects described, the invention causes as an extra effect that the felt loop during its continuous run through the steam treatment achieves an elevated temperature and passes through the preceding nip ~1' in this way enhancing the dewatering in the nip Nl.
According to the invention, the treatment steam is saturated steam, but it is also possible to use somewhat super- ;
heated steam although due consideration must be given to the thermal tolerance of the web W and above all of the felt lb thereunder. By using the apparatus of the invention, the steam can be discharged uniformly over an extensive area onto web W
and, since the steam is substantially at atmospheric pressure in the interior space 70 of the steam supply box 60, it is not ~ necessary to dimension the steam supply box like a pressure -~ vessel. Thus, the construction is relatively simply and inexpensive in this regard. -The method and apparatus of the invention are in no way confined to the details presented in the foregoing which are merely by way of illustration. For example, the steam supply merely by way of illustration. For example, the steam supply box 60 may be arranged to be adjustable as to its position, for example, by suspending it from articulated arms, not shown. By means of the suspension of the steam supply box 60 from articulated arms, the elastic suspension of box 69 may also be advantageously accomplished so that "lumps" of the web W cause no major damage. The spring arrangement may in this connection be realized~ for example, by utilizing air cylinders. It; is also possible to use a supply surface of the steam supply box 60 which is larger or smaller than that sho~
in the figure, and the extent of the surface which corresponds to an angle of the suction roll may be increased up to about 90. The lower limit of sector a is determined by the need of - - . . : . ., . . : .
107~
enhancement of the dewatering action. The invention may further-more be carried out so that, instead of using one steam supply box 60, two or more steam supply boxes mounted one after the other can be used, the interaction of these being adjusted so that the desired end result is obtained.
The provision of steam supply box 60 may also partly be employed to assist the keeping of the web W on the sector 2b of the suction box (in cooperation with the suction sector ~2) even at high speeds of the web W which speeds produce at this point rather high centrifugal forces which tend to lift the web off the surface of the roll 2b. In such a case, it may be desirable to use a pressure higher than atmospheric pressure in box 60 to assist in this function. `~
The general observation may be made that the size and placement of the steam supply box 60, or a plurality of such boxes, is determined, considering the dry matter content of web W, by space considerations, and according to the inven-tion one may advantageously use the web surface which remains free in the "Sym-Press" press section on the suction roll 2b at a point which is suitable in all respects.
It was observed above that the dewatering is enhanced through the fact that the web and the water present therein are heated such that the viscosity of the water decreases in the manner seen in the graph of Fig. 6. The effect of the method of the inven-tion may partly also be based on the cir-cumstance that when the steam ~ischarges into the comparatively cold web W, there occurs the so-called implosion phenomenon, a very abrupt collapse of steam bubbles, which on the basis of mechanical effects reduces the adhesion forces acting between the fibers of the web W and the water present therein, and which thereby facilitates the escape of the water from web W.
Thus, as the steam flows through the water-containing fiber * "Sym-Press" is a trade mark.
B
... ~ .. .. ~ . . . .
; ; . , `. - . .. .... . ..
~C~768~
network, it produces as it condenses in the water films between fibers, an effect breaking up these films and thus pxomoting ; the dewatering process.
The introduction of the procedure and apparatus of the invention is partly rendered possible by the felts which ;~ have only recently appeared on the market and which tolerate comparatively high temperatures.
In order to clarify the physical background of the invention, the heating of wet paper with the aid of steam will be considered in the following discussion with reference to Fig. 4.
Let us assume, to begin with, that the steam coming from the bo~ 60 all condenses in the web W, whereby the tempera ` ture of the web rises from Tl to T2. The heat losses to the environment are disregarded. The water quantity removed by the second nip N2 is denoted as ~V, the water removed at this point having the temperature T~ There are no fiber losses.
The heat balance equation may now be written:
m c T + lOO~kl c T + m i" = m c T + (100~ kl + m ) c T :
k k 1 lkl~ 2 k k 2 Lkl~ 2 (Symbols as in Fig. 4) where ck = specific heat capacity of the fiber c = specific heat capacity of water i" = enthalpy of the steam ~i~h ~T2 - Tl) Ck + 100 c/kl) mk (i" - c T2) Since the final, completed paper contains about 7% water, the steam consumption per kg of paper is found to be (mk = 0.93 mp):
m X = h = 0-93 (T2 - Tl) (ck + 100 c/kl) m (i" - c T2) Figs. 7 and 8 present, in the form of a set of curves, --~'7~861 the steam quantity X required for the treatment of the web W
according to the invention, in dependence on the moisture con-tent of the web when entering the treatment and the desired ultimate temperature T2.
Scrutiny of Figs. 7 and 8 reveals that the pressure Pe of the saturated steam introducecl in the box 60 is immaterial if only the steam supply pipe 62 and its supply holes 67 have been so dimensioned that the requisite flow from the pipe 62 - is obtained.
Example:
In view of establishing the effect of the procedure and apparatus of the invention, the experiments reported in the following were carried out on the pilot paper machine at Rautpohja of the company Valmet Oy. The press section employed in the experiements was substantially as presented in Fig. 1, except that the felt corresponding to the felt lb of the suction roll 2b also operated as pickup felt, that is, there was no felt corresponding to that indicated with reference numeral 32 in Fig. 1.
For the experiments, a box substantially as in Fig. 2 ;
was mounted adjacent to the suction zone of the suction roll 2b of the press section of the trial machine in the press between the first and second nips. Into this box saturated steam was ~;
conducted from a steam generator, and the following quantities were measured:
steam pressure pulp stock temperature temperature of the web water content of the fel-ts - dryness of the web.
The machine was run with speeds 10 and 13.5 m/s, and the most essential operating data were recorded. The paper stock consisted of conventional groundwood pulp.
As regards the results obtained in the tests, reference 1~768~;1 is made to curves A and B in Fig. 5 (corresponding to web speeds 10 and 13.5 m/s, respectively), which show the dryness, in percent, of the web W after the press section, as a func-tion of the quantity X of steam supplied according to the - invention.
The observation can be made as a summary of the test results that when steam is blown onto the web W by the aid of a box mounted on the suction roll of the press section, the dryness of the paper web after the press section increases markedly:
At speed v = 10 m/s k = 42.2% rising to 46.5%
At speed v = 13.5 m/s k = 40.8% rising to 45.3%
The water content of the felts also went down somewhat when steam was used. The felts got warmer, but during the comparatively short trial run, the heat was not observed to have caused any damage to the felts.
The maximum steam consumption was 0.29 and 0.26 kg per kg of paper at the speed of 10 and 13.5 mjs, respectively.
It may mentioned for comparison that in the drying section of a newsprint machine, the normal consumption is 1.66 kg steam per kg of paper. It is thus understood that application of the procedure of the invention requires a considerable amount of steàm, but in addition to the effect of improved runability of the paper machine, which is the object of the invention, the invention also results in a reduced steam consumption in the frying section because the web W is drier and warmer when it àrrives in the drying section. It is well known that the heat output of the first drying cylinders is spent in raising the temperature of the web, while no evaporation takes place.
Closer details of the experiments which were carried out are apparent from the tables following below. The results of measurement are stated in Table I. Table II specifies the ~. .. ~ . - : :.
~7686~
; felts which were used in the experiements and Table III gives the removed water auantities, calculated from the results.
TAsLE I
Running speed (m/s) 10 13.5 Loading (k~/M) (press pressures) First nip 60 60 Second nip 80 80 ` Third nip 90 90 i Degrees of vacuum (kPa) Wire suction roll 66.7 64.0 Pickup 26.7 28.0 Press suction roll, first s. 65.3 64.0 " " " second s. 21.3 22.7 " " " third s. 62.7 62.7 Steam supply overpressure (kPa) 0 300 0 100 300 Steam temperature (C.) - 143 - 120 143 Estima~ed steam quantity (g/s) - 102 - 53 102 -Base weight of paper (g/m ) 55.3 57.9 48.7 48.7 48.7 Dryness of paper, %
After the wire 19.5 19.2 18.3 16.9 16.9 After the second nip 35.8 39.0 34.2 35.4 36.9 After the third nip 42.4 46.5 40.8 42.6 45.3 Temperature of the paper (C.~
After the second nip 36.0 73.5 24.2 39.0 57.0 After the third nip 24.2 38.5 54.5 Water in the pickup felt (g/m2) 735 730 750 730 Water in the third felt (g/m2) (corresponds to felt ld in Fig. 1) 550 510 525 490 TABLE II
Pickup 1,084 g/m Lower 1,100 g/m Third felt 1,350 g/m (All these Celts are fully synthetic.) . ` ': i , `, . .~ , , , . .~.' ' ` ,'. ! '; ` ' ~076~
TABLE III
(Water removal quantities calculated from the results of measurement.) Speed (m/s) 10 13.5 ` Supply steam overpressure (kPa) O 300 0 100 300 Removed by third nip (g/m ) 21.8 22.3 21.4 2i.6 22.8 Removed by lst-2nd nip (g/m2) 120 142 115 140 145 Total 142 165 137 162 168 It is seen on scrutiny of Tables I and III that the dewatering is enhanced in the second nip, whereas in the third nip, the increase of water removal capacity is minor. Since the pickup felt is heated, the dewatering is also improved in the first nip, although the steam supply box has no direct effect of the first nip.
On studying the test results, it is appropriate to point out to begin with that fully exact conclusions and calculations were somewhat impeded by the difficulties encountered in stabilizing various parameters, such as, e.g., the stock temperature, and by inaccuracies of measurement, e.g., of the steam quantity measurements, as well as by the fact that the whole steam quantity does not pass through the web W, but part of it discharges through the sides of the box.
At all events, it is positively observable from the test results that the steam blowing considerably improves the dryness of the web, and that it simultaneously increases the ^
temperature of the web, which circumstances also contributes to higher drying capacity in the drying section, as was observed above.
The steam consumption of the box, per kg of paper, calculated from the estimates will be as follows:
1. v = 10 m/s Pe = 300 kPa x = 0.29 kg/kg 2. v = 13.5 mJs Pe = 300 kPa x = 0.26 kg/kg 1~7686~
3. v = 13.5 m/s Pe = 100 kPa x = 0.13 kg/kg If all steam were condensed in the paper, this would mean an increase of water content, per m2, in the different cases:
1. 17.1 g H20/m2 2. 12.7 g H20/m2 3. 6.3 g H20/m2 and this water mass adds to the amount that has to be removed in the second and third nips.
But it is noted that this complete condensation did not occur in the experiments because in that part of the steam spread into the environment, while part of its passes through ~
the web and the felt into the suction roll. ;;-Studies were also made concerning the changes in the quality characteristics of the p~per which might be observable in connection with the experiments just described. This was ;
done by analyzing specimens from consecutive runs with the steam supply disconnected in one run and connected in the other.
It appears that the steam treatment of the web improves to some degree the opacity of the finished paper. This is indicated by the following figures relating to newspring paper (57 g/m2, consisting of 76% ground wood and 24% chemical pulp): !
Steam Treated Normal Paper SCAN-OPACITY 94.2% 32.3%
The above increase in opacity can be regarded as statistically significant.
Various details of the invention are not confined to what has been presented in the foregoing for illustration only.
For instance, although in the foregoing first nip, etc., has been specifiecl, this does not exclude the possibility that there are nips prior to this particular nip, such as, e.g., a pickup nip or another nip carrying out a preliminary pressing 1CS76~
and dewatering operation.
While certain advantageous embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifica-tions can be made therein without departing from the scope of the invention as defined in the appended claims.
- 18 - ~:
Claims (16)
1. In a process of enhancing the dewatering of a paper web in the press section of a paper machine of the type having at least two press nips through which the web consecutively passes, the first one of the nips being formed between a water-receiving roll and a suction roll and a subsequent one of the nips, in the direction of web travel, being defined by said suction roll and a plain roll, and wherein said web runs on said suction roll between said nips, the improvement comprising the step of treating said web by exposing the outer surface thereof to hot steam between said two nips so that a significant proportion of the treatment steam condenses thereon, thereby imparting the latent heat of the steam to the web.
2. An improved process according to claim 1 wherein the treatment steam is brought into contact with the web sub-stantially at atmospheric pressure.
3. An improved process according to claim 1 wherein the web travels on a felt on the suction roll between the nips, and wherein the treatment steam flows through the web and at least partly through the felt primarily as a result of suction in a suction zone in the suction roll.
4. An improved process according to claim 3 wherein the web is subjected to steam so that when the steam condenses in the fiber network of the web, the water films existing between fibers are imploded.
5. An improved process according to claim 3 wherein the felt is also simultaneously steam heated so that dewatering in the first of said two nips is enchanced, the felt operating in both of the first and second nips.
6. An improved process according to claim 1 wherein the treatment steam is saturated steam.
7. An improved process according to claim 1 wherein at least part of the treatment steam is superheated steam.
8. An improved process according to claim 1 wherein the web is subjected to the treatment steam substantially immediately after a dewatering nip wherein the web has reached a 20 to 35% dry matter content.
9. A process according to claim 8 wherein the web has reached a 30% dry matter content.
10. A process according to claim 1 wherein the web is subjected to steam treatment such that the dry matter content thereof is increased by about 3 to 5% after the nip compared to normal pressing without steam treatment.
11. In an apparatus for dewatering a paper web in the press section of a paper machine of the type having a felted suction roll, a water-receiving roll forming a first nip with the suction roll and a plain roll forming a second nip with the suction roll, the web being passed around a sector of the suction roll successively between the first and second nips, the improvement comprising steam supply means mounted adjacent said web, and means for conducting steam to said supply means, said supply means comprising at least one steam supply box extending across the entire width of said web adjacent the suction roll, said box having an interior space and an open side opening toward said suction roll and conforming to the curvature of said roll over a substantial sector thereof to define a flow path for treatment steam onto said sector.
12. An apparatus according to claim 11 wherein said steam supply box comprises an inner pipe;
a shell portion: and means for supporting said shell portion on said inner pipe;
said shell portion comprising a concave wall adjacent said suction roll, said wall having means defining openings therethrough for the passage of steam, an inner wall defining, with said concave wall, a steam supply space, an outer wall spaced from said inner wall, and thermal insulating means between said inner and outer walls.
a shell portion: and means for supporting said shell portion on said inner pipe;
said shell portion comprising a concave wall adjacent said suction roll, said wall having means defining openings therethrough for the passage of steam, an inner wall defining, with said concave wall, a steam supply space, an outer wall spaced from said inner wall, and thermal insulating means between said inner and outer walls.
13. An apparatus according to claim 12 wherein said inner pipe includes means defining openings through a wall of said pipe for directing steam into said steam supply space in a direction away from the web.
14. An apparatus according to claim 13 wherein said supply box further comprises steam quantity adjustment means operatively associated with said openings through said wall of said pipe for controlling the flow of steam into said space, a plurality of said openings and associated adjustment means being disposed in side-by-side relationship along said pipe.
15. An apparatus according to claim 14 wherein said supply box further comprises suction means at opposite ends of said box adjacent the lateral margins of said web for preventing the espace of steam to the sides thereof.
16. In a process of enhancing the dewatering of a paper web in the press section of a paper machine of the type having at least two press nips through which the web consecutively passes, the first one of the nips being formed between a water-receiving roll and a suction roll and a subsequent one of the nips, in the direction of web travel, being defined by said suction roll and a third roll, and wherein said web runs on said suction roll between said nips, the improvement comprising the step of treating said web by exposing the outer surface thereof to hot steam between said two nips so that a significant proportion of the treatment steam condenses thereon, thereby imparting the latent heat of the steam to the web.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA288,563A CA1076861A (en) | 1977-10-12 | 1977-10-12 | Method and apparatus for dewatering in a paper machine |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA288,563A CA1076861A (en) | 1977-10-12 | 1977-10-12 | Method and apparatus for dewatering in a paper machine |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1076861A true CA1076861A (en) | 1980-05-06 |
Family
ID=4109748
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA288,563A Expired CA1076861A (en) | 1977-10-12 | 1977-10-12 | Method and apparatus for dewatering in a paper machine |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1076861A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5749158A (en) * | 1995-01-19 | 1998-05-12 | Voith Sulzer Papiermaschinen Gmbh | Apparatus for the application of steam onto a paper web |
-
1977
- 1977-10-12 CA CA288,563A patent/CA1076861A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5749158A (en) * | 1995-01-19 | 1998-05-12 | Voith Sulzer Papiermaschinen Gmbh | Apparatus for the application of steam onto a paper web |
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