CA1311119C - Method of drying a paper board or paper web - Google Patents
Method of drying a paper board or paper webInfo
- Publication number
- CA1311119C CA1311119C CA000588089A CA588089A CA1311119C CA 1311119 C CA1311119 C CA 1311119C CA 000588089 A CA000588089 A CA 000588089A CA 588089 A CA588089 A CA 588089A CA 1311119 C CA1311119 C CA 1311119C
- Authority
- CA
- Canada
- Prior art keywords
- web
- band
- drying
- pressure
- bands
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
- 238000001035 drying Methods 0.000 title claims abstract description 78
- 238000000034 method Methods 0.000 title claims abstract description 20
- 239000000123 paper Substances 0.000 title claims abstract description 7
- 239000011087 paperboard Substances 0.000 title claims abstract description 7
- 230000006835 compression Effects 0.000 claims abstract description 24
- 238000007906 compression Methods 0.000 claims abstract description 24
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 24
- 238000001816 cooling Methods 0.000 claims abstract description 19
- 238000010438 heat treatment Methods 0.000 claims abstract description 16
- 238000001704 evaporation Methods 0.000 claims description 6
- 239000011148 porous material Substances 0.000 claims description 5
- 239000007788 liquid Substances 0.000 claims 2
- 239000000203 mixture Substances 0.000 claims 2
- 239000002184 metal Substances 0.000 abstract description 36
- 238000009833 condensation Methods 0.000 abstract description 4
- 230000005494 condensation Effects 0.000 abstract description 4
- 239000000835 fiber Substances 0.000 abstract 1
- 239000000498 cooling water Substances 0.000 description 11
- 239000003570 air Substances 0.000 description 9
- 229920000136 polysorbate Polymers 0.000 description 6
- 239000007789 gas Substances 0.000 description 3
- 230000000875 corresponding effect Effects 0.000 description 2
- 230000001419 dependent effect Effects 0.000 description 2
- 230000008020 evaporation Effects 0.000 description 2
- 230000033001 locomotion Effects 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 229920006395 saturated elastomer Polymers 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 241001052209 Cylinder Species 0.000 description 1
- 206010013786 Dry skin Diseases 0.000 description 1
- 208000036366 Sensation of pressure Diseases 0.000 description 1
- 230000004075 alteration Effects 0.000 description 1
- 239000012080 ambient air Substances 0.000 description 1
- 238000009835 boiling Methods 0.000 description 1
- 239000002826 coolant Substances 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000010291 electrical method Methods 0.000 description 1
- 239000003546 flue gas Substances 0.000 description 1
- 230000004907 flux Effects 0.000 description 1
- 239000002803 fossil fuel Substances 0.000 description 1
- 238000005457 optimization Methods 0.000 description 1
- 230000002093 peripheral effect Effects 0.000 description 1
- 238000003825 pressing Methods 0.000 description 1
- 230000000630 rising effect Effects 0.000 description 1
Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
- D21F5/00—Dryer section of machines for making continuous webs of paper
- D21F5/004—Drying webs by contact with heated surfaces or materials
Abstract
Abstract A method of drying a fibre web such as a paper or paper board web, wherein air is first removed from the web and a dryer felt, whereafter they are led together between two parallel metal bands moving at the same speed as the web and the felt. For drying the web, the band on the side of the web is heated by means of a heating device before the band reaches the drying zone and, correspondingly, the band on the side of the dryer felt is cooled before the band reaches the drying zone and comes into contact with the dryer felt. In the method, for adjusting the mechanical 2 compression exerted perpendicularly on the surface of the web, the temperature of the cooling band is adjusted so that the difference between the condensation pressure of the steam condensed on the surface of the band on the side of the dryer felt and the pressure acting on the outer sur-face of the bands is substantially equal to the desired mechanical Z compression. Further, the drying rate of the web is adjusted by adjusting the temperature of the heated band by means of the heating device in such a way that a desired amount of water can be evaporated from the web within the drying zone. Since the pressure loss created by the steam flowing through the dryer felt is substantially insignificant with respect to the condensing pressure of the steam condensing on the surface of the band, and the temperature difference between the heated band and the cooled band affects substantially this pressure loss only, these adjustments do not substantially affect each other and they can be carried out in desired order and adjusted suitably in view of the properties of the web to be dried.
Description
:~ 3 ~
A method of drying a paper board or paper web The invention relates to a me~hod of drying a paper board or paper web by leading the web together with at least one dryer felt between two moving, air-tight bands having a good thermal conductivity and being substantially parallel with each other over a distance, the bands enclosing the web therebe~ween over the entire width thereof, thus forming a drying zone, wherein the web and each dryer felt are exposed to an air removal treatment before being led between the bands, whereby air is removed out of their pores to the greatest possible degree; a substantially equal pressure is arranged to act on the out.er surface of each band; the band making contact with the web is exposed to a heat treatment at least prior to the dry-ing zone for evaporating water from the web, and the band making contact with the dryer fel~ is exposed to a cooling treatment for condensing the water evaporat-ing from the web into the dryer felt; and the dryer felt is separated from the dried web after the bands and the condensed water is removed therefrom.
Finnish Patents 54514, 51537 and 69141, among others, describe the drying of a moving web in a spacé
which is defined between a heated and a cooled surface and w~ich is as free of aix as possible.
Finnish Patent 54514 discloses a solution in which the surface positioned against the web is heated to about 100C, and the surface positioned against the dry~r felt is cooled to a relatively low temperature typically below 40C. Finnish Patent 61537, in turnr discloses a solution in which the surface positioned against the web is heated to a high temperature typi-cally about 180C, and the cooling surface is kept at a temperature typically above 100C, whereby the sur-faces have to be pressed against each other with ahigh pressure t~pically above O.3 MPa in order to prev~nt the boiling of the water contained in the web and in the dryer felt.
The above solutions are intended ~o be applied in such a way that both the heated and the cooled sur-face are metal bands moving in the direction of move-ment of the web at the same speed as the web, whereby a web to be dried and a dryer felt r wire or some other mat permeable to steam are led between the bands in such a way that the web to be dried will be posi-tioned against the heated band and the dryer felt or the like against the cooled band so that the o-ther side of the dryer fel-t or the like will be positioned against the ~eb to be dried. in the embodiments disclosed, a fixed box containing pressurized satu-rated steam is positioned outside the heated metal band. The box is open towards the mo~ing metal band in such a way that the steam is in direct contact with the metal band or a condensa~e forming thereon~ the edges of the box being sealed off again~t the surface of the band. A box similar ~o that described above is positio~ed outside the cooled metal band, water being fed into the box approxim~tely at the pres~ure of the steam contained in the box positioned outside the h~ated band but at ~ temperature lower than that of the steam.
In addition to the embodiment described above, Finnish Patent 61537 further discloses an application in which the hot sur~ace consists of the outer surface of an internally heated metal cylinder, and the cooled surface of a metal band extending almost around the cylinder in parallel with the surface thereof, whereby the web and the dryer felt are led between the cyl-inder and the band moving at an e~ual rate.
~ 3 ~
In the abo~e-described applications, heat is transferred to the hot metal surface within the drying zone itself through the band, that is, through the me-tal layer one surface of which consists of said hot metal surface. Heat is transferred into the metal layer from a source of heat positioned behind the other surface of the metal layer, when saturated steam condenses and delivers its latent heat into the metal band. In the solutions disclosed, no appreciable amounts of heat are transferred into the metal band outside the drying zone when the band moves within the area of the steam box. In practice, -the metal band is endless~ so that heat is dissipated -from the metal band outside the press section into -the rolls intended for the rotation of the band and into the ambient air.
As the hot surface within the drying zone consists of the ou~er surface of the metal cylinder, heat is transferred into the metal casing from the source of heat positioned within the cylinder typically over the whole peripheral 7ength of ~he casing. However, ~his is disadvantageous in that due to the thickness of the cylinder casing the amount of heat which is trans ferred from within the casing to the outer surface thereof is considerably smaller than the amount of heat trans~erred through a thin metal band, wherefore very high temperatures are required in the cylinder in order that the heat flux required for the drying of the web could be transferred to the surface of the cylinder. This, in turn, requires that the steam should be ver~ hot, which is economically disadvanta-geous. Consequently, the use of a metal band as a hot a surface is economically more advantageous.
Attention has been paid to these matters in the solution disclosed in Finnish Patent 69141, where the outer surface of a conventional thick-walled drying ~ 3 ~
cylinder acts as a cooled surface in the drying process while a relakively thin metal band acts as ~he hot metal surface. For a major part the metal band is heated outside ~he drying ~one formed by the cylinder and the band before the band reaches the drying zone.
In this solution, the metal band is not actually heated within the drying zone but the heat required in the metal band for the evaporation of the moisture contained in the web is for a major part derived from the heat energy transferred to the metal band during the preheating step. Thereby the average temperature of the metal band drops within the drying zone in pro-portion as the drying proceeds and the band moves on within the drying zone together with the web.
In the above patenks, it is shown by examples that in practice a preheating rising the temperature of the metal band by a few tens of degrees provides sufficiently energy for the evaporation of a high amount of moisture from the web without having to heat the metal band within the drying zone.
All the above applications have the drawback that during the drying process the drying of the web is dependent on the pressure of the cooling water, the temperature of the cooling water, and the mechani-cal compression exerted on the web perpendicularly to the surface of the web, i.~., a Z compression, which is due to the followiny ma-tters.
After havîng penetrated part of the web and the wire or felt, the steam evaporated from the web is condensed on to the cooled band or on to the conden-sate formed thereon at a saturation pressure corre-sponding to the local temperature. The local tempera-ture, in turn, is determined by the temperature of the cooling water. The temperature of the cooling water is affected by the temperature difference caused by the ~ 3 ~
heat transfer from the co~ling water to vne side of the cooled band and further therefrom to the other side of the cooled band. Further, the pressure of the steam at the wire or felt making contact with the sur-face of the web is determined by the above-mentioned condensation pressure which is affected, though to a small degree, by the pressure loss oCCurr.ing when ~he steam passes fxom the interspace between the web and the wire to the condensation surface. In accordance with the force balance, the mechanical Z compression to be exerted on the surface of the web is equal to the difference between the pressure exerted on the outside of the hot and the cold band and the pressure of the steam in the interspace between the web and the wire.
In the publications mentioned above, the disad-van-tageous dependance between the temperature and the drying rate typical of o-ther drying processes has been eliminated by adjusting -th~ temperature of the cooling surface to such a high value that the drying rate i5 decreased to a desir~d level. However~ there still re-mains the restric-tive factor that the temperature of the heating steam and correspondingly the pressure ~hereof as well as the temperature of the cooling water and the mechanical Z compression exerted on the web are dependent upon each other. For the adjustabil~
ity of the drying process, this dependence is very disadvantageous and causes unnecessary expenses and impairs -the operational optimization of the drying process in the production of a paper board and paper web.
The object of the invention is to providP a method by means of which the restrictive factors and drawbacks of the drying procedures described above are avoided and by means of which each one of the three important process parametres, i.e., the temperature of the web, the local drying rate, and the mechanical Z
compression exerted on the web, can be adjusted to a desired level separately in each drying zone, whereby the length of the dryinq zones can be varied within limits as wide as possible and the drying process can be optimized in view of both the quality of the web and the production process as a whole. The method ac-cording to the invention is characterized in that for adjusting the mechanical compression exerted on the web to a desired value, the pressure acting on the outer surface of the bands and the temperature of the cooling band are adjusted so that the difference be-tween the pressure acting on the outer surface of the bands and the saturation pressure of the steam de-termined by the temperature of the cooling band is substantially equal to the desired mechanical com-pression force; and for adjusting the drying rate of the web to a desired value the temperature of the band exposed to the heat treatment and heating-the web is adjusted before it reaches the drying zone so that i-t corresponds to the desired drying rate.
The basic idea of the invention is that the magnitude of ~he mechanical Z compression to be exerted on the web within the drying zone is de-termined, and the external pressure acting on the out-side of the bands pressing them towards each other and the temperature of the band to be cooled are adjusted in such a way that the pressure difference between the pressure acting on the bands and -the pressure of the condensing steam is essentially equal to the desired mechanical Z compression; on the other hand, the tem-perature of the band to be heated is adjusted to a suitable value on the basis of the desired drying rate. According to the invention, either one of these ~ 3 ~
adjustments can be effected Eirst, because the temperature difference between the bands does not afrect the Z compres-sion to any greater degree in view of the pressure adjus-tment.
~ ccording to a still further broad aspec-t of the present invention, there is provided a rnethod of drying a paper board or a paper web by leading the web -together with at least one dryer felt be-tween -two movlng, air-tight bands having a good thermal conductivity and being substantially parallel with each o-ther over a dis-tance. The bands enclose the web therebetween over the en-tire wid-th thereo~ to form a drying zone having a drying rate. The method comprises exposing a web and a dryer fel-t to an air removal treatment whereby air is removed out of pores -to the greatest possible degree. The web and the dryer felt are passed be-tween -two moving, air--tight bands which are substantially parallel to each other. An outer surface of each of the bands is subjected to substantially equal pressure. One of the bands which makes contact with the web is heated prior to entry into the drying zone to a temperature for evaporating wa-ter from the web. The other of the bands which makes con-tact with the dryer felt is cooled to a predetermined -temperature before it reaches the drying zone for condensing the water evaporated from the web into the dryer felt. The dryer felt is separated from -the web after passing through -the bands and condensed water is removed therefrom. The mechanical compression exerted on the web is adjusted to a prede-termined value by adjusting the pressure acting on the outer surface of the bands and the temperature of the cooling band -to produce a difference be-tween the pressure acting on the outer surface of -the bands and the satura-tion pressure of s-team condensed in drying the web determined by the temperature of -the cooling band which is subs-tantially equal to -the desired mechanical compression force. The drying rate of the web is adjusted to a desired value by adjus-ting the temperature of the band exposed to the heating which heats -the web beEore it reaches the dry:ing zone so -tha-t it corresponds to the desired drying rate.
~ 3 ~
- 7a -The invention will be described in more detail with reEerence to the attached ~igure, which shows an apparatus suitable for reali~ing one application of the method according to the invention.
The dryer comprises a heated metal band 1 and a cooled metal band 2 which move in parallel with each other khrough a drying zone. A steam box 3 is pro-vided for the heated metal band and a water box 4 for the cooled metal band. The heated band 1 moves around its turning rolls Sa and 5b ~nd the cooled band 2 around its turning rolls 6a and 6b. A web 7 to be dried is pas~ed into the drying zone so as to make contact with the lower surface of the heated metal band 1; in this embodiment, this lower sur-f~ce acts as the hot surfac~ within the drying zo~e. A dryer felt 8 permeabl~ to ~as and formed by one ~r more ~elt and/or wires positioned one upon another is corre-spondingly pa~ed b~tween ~he bands 1 and 2 in such a way that one surface of the dryer felt 8 makes con-tact with the cooled metal band 2 ~hile the other s~r-face thereof makes contact with that surface of the web 7 to be dried whi.ch faces away from the heated me-tal band. Both the web 7 and the dryer felt 8 are passed through an air r~moval unit 9 before being led between the bands. In the air removal unit, air is removed from the pores of -the web 7 and the dryer felt 8 to the greatest possible degree e~g. by applying therethrough saturated steam at a suitable temperature so that the air molecules are pushed out oE the pores and replaced with water molecules contained in the steam. For heating the heated band, a heating unit 10 ~ 3 ~
is provided on bo~h sides of the heated band 1 at a point where it mo~es alone, i.e., between the rolls ~a and 5b outside the drying ~one ormed between the rolls 5a and 5b and 6a and 6b, respectively. By means of khe heating unit the temperature of the band 1 is adjusted to a temperature corresponding to the desired drying rate by app]ying hot medium 11 through -the heating unit lO. The medium may consist of steam, flue gases from fossil fuels or some other gas heated to a suikable temperakure. It is obvious that the in-vention also comprises an application in which the band 1 is heated by some electrical method wi-thin some other portion of the band than the drying zone and an application in which heat is transferred to the band from a stationary surface in contact -therewith, such as one or both of the turning rolls 5a and 5b.
In the heating unit 10 the band is heated to a desired temperature, whereby the heated band section moves around the -turning roll Sa to the entry end o~
the drying zone and makes con~act with the transported web 7. Steam 12 at a suitable temperature and pressure is introduced into the steam box 3 pro-vided for the heated band. Correspondingly, cooling water 13 at a desired temperature and at a pressure substantially equal to the pressure of the steam l2 is introduced into the wat~r box 4. However, the pressure of the cooling water is preferably slightly higher than th~t of the steam 12 for supporting the bands 1 and 2 as well as the web 7 and the dryer felt 8. Part of -the steam 12 may escape past edge seals 14a and 14b provided between the band 1 and the box 3 and past seals (not shown) provided on the sides of the box 3 so as to be pressed against the band either in the Porm oP steam or condensate. When the rest oP the steam is condensed on the upper surface o~ the band 1, it is gathered and removed from the box 3 by means of suitable devices not shown. From the box 4, part of the water leaks between seals 15a and 15b and between side seals providecl on the water box 4 against the band 2, while the remaining warmed cooling water 16 is re~oved through one end of the box 4.
In the method, the pressure of the steam 12 and that of the water 13 are adjusted so that a prede-termined external pressure acts on the bands, which pressure tends to compress the web 7 perpendicularly to the surface thereof in a direction Z. To achieve the desired mechanical Z compression, the feeding tem-perature of the cooling water 13 is now adjusted to a value such that the pressure of the steam condensing on the surace of the band 2 on the side of the dryer felt 8 settles on the desired level so that the dif-ference between the external pressure acting on the outside of the bands 1 and 2 and the condensation pressure of the condensing steam is equal to the compression to be exerted on the web 7. In a case taken as an example, the steam contained in the steam box 3 is at a temperature of about 100C and at a pressure only slightly abo~e the atmospheric pressure, and the temperature of the water contained in the water box ~ is about 30C. In this case, the steam evaporated from the web 7 is condensed on the upper surface of the cooled hand 2 typically at about 80C
and the condensing pressure of the steam would be 47.3 kPa. Since the pressure loss from -the interspace between the web 7 and the dryer felt 8 to the con-densing surface of the band 2 would be about 230 Pa, the steam pressure in the interspace between the dryer felt 8 and web 7 woulcl be about 47.6 kPa and the mechanical z compression exerted on the web would be abou-t 101.3 kPa - 47.6 kPa = 53.7 kPa. The pressure value 101.3 kPa is -~ha maynitude of the external pres-sure exerted on the web. Supposing that the metal band 1 to be heated would reach the drying zone at 150C
and the web 7 would consist of newsprint with a gram-mage of 45 g/m2 and a dry matter content of 45~, the drying rate would, however, be typically about 200 kg/~s x m2).
In -the case described above, it would be very easy to alter the Z compression exerted on the web without substantially affecting the drying xate. The only alteration required would be to change the tem perature of the cooling water. With a cooling water temperature of 40C, for instance, the situation would be as follows. The temperature of the steam condensing on the band 2 would be about 90C and, corresponding-ly, -the condensing pressure of the steam about 70.1 kPa. The pressure loss from the interspace be-tween the web 7 and the dryer f21t 8 to the condensing surface of the band 2 would now be about 160 Pa and the pressure of the steam in the interspace between the web 7 and the dryer felt about 70.3 kPa. The Z
compression exerted on the web 7 would be 101.3 kPa-70.3 kPa = 31.0 kPa. The drying rate, however, would remain substantially the same as above, that is, at about 200 kg/~s x m2).
However, it is obvious that the drying method according to the inv~ntion also offers an opportunity to use compressed steam in the steam box 3 while the band 1 is preheated in a prehea~er 10. According to the invention, it would thereby be possible to adjust the mechanical Z compression exerted on the web 7 to any leve]. within the range from 0 to Pml wherein Pm is slightly lower than the saturation pressure of the steam created at the lowest temperature of the band 1 occurring within the drying zone. Accordin~ to the in~
~ 3 ~ 9 vention it is thus possible to separately adjust the Z
compression undergone by the web 7 to a desired level while the drying rate can be kept even at a very high value or, if required by the properties of the web~ at a very low value. As it is sufficient for the drying process that the band 1 to be heated is heated before it reaches the drying zone, a substance such as steam or gas at a suitable temperature can be introduced into the box 3, whereby the funct.ion of the substance is not to heat the band 1 but merely to act as a medium for transm.itting pressure to the surface of the band 1. Correspondingly, the cooling of the cooled band 2 can be effected before it reaches the drying zone by means of a separate cooling device 18 into which cooling medium 19 is introduced for cooling the band to a sui-table temperature. The cooling device 18 is sealed on the surface of the band 2 by means of seals 20a and ~Ob positioned on both sides thereo and correspondingly by side seals (not shown) extending in the direction o-f movement of the band. When the band 2 is cooled by the cooling device 18, water at a suit-able pressure and temperature can be introduced into the water box 4, the function of the water being main-ly to transmit p.ressure to the outer surface of the band 2.
Only one embodiment o-f the method according to the invention and one way of applying it have been de-scribed above. However, the invention is by no means restricted to the embodiment disclosed, but it can be varied freely within the scope of the claims. I~ both the heated and the cooled surface are heated and cooled, respectively, before they reach the drying zone/ it is possible to apply atmospheric pressure, for instance, within the drying zone, whereby in certain cases the steam box 3 can be omitted, and air or some other gas can he introduced in place of water into the box 4 m~rely for suppor~iny the bands and the web and dryer felt positioned therebetwaen, provided that the apparatus is horizontally positioned. When the atmospheric pressure acts on the bands 1 and 2, the magnitude of the mechanical Z compression acting on the web 7 can be adjusted simply by varying the temperature of the band 2 by means of the cooling de-vice 18. Correspondingly, the drying rate can be simply adjusted by means of the heating device 10 of the band 1., and the whole process can be controlled easily and simply.
A method of drying a paper board or paper web The invention relates to a me~hod of drying a paper board or paper web by leading the web together with at least one dryer felt between two moving, air-tight bands having a good thermal conductivity and being substantially parallel with each other over a distance, the bands enclosing the web therebe~ween over the entire width thereof, thus forming a drying zone, wherein the web and each dryer felt are exposed to an air removal treatment before being led between the bands, whereby air is removed out of their pores to the greatest possible degree; a substantially equal pressure is arranged to act on the out.er surface of each band; the band making contact with the web is exposed to a heat treatment at least prior to the dry-ing zone for evaporating water from the web, and the band making contact with the dryer fel~ is exposed to a cooling treatment for condensing the water evaporat-ing from the web into the dryer felt; and the dryer felt is separated from the dried web after the bands and the condensed water is removed therefrom.
Finnish Patents 54514, 51537 and 69141, among others, describe the drying of a moving web in a spacé
which is defined between a heated and a cooled surface and w~ich is as free of aix as possible.
Finnish Patent 54514 discloses a solution in which the surface positioned against the web is heated to about 100C, and the surface positioned against the dry~r felt is cooled to a relatively low temperature typically below 40C. Finnish Patent 61537, in turnr discloses a solution in which the surface positioned against the web is heated to a high temperature typi-cally about 180C, and the cooling surface is kept at a temperature typically above 100C, whereby the sur-faces have to be pressed against each other with ahigh pressure t~pically above O.3 MPa in order to prev~nt the boiling of the water contained in the web and in the dryer felt.
The above solutions are intended ~o be applied in such a way that both the heated and the cooled sur-face are metal bands moving in the direction of move-ment of the web at the same speed as the web, whereby a web to be dried and a dryer felt r wire or some other mat permeable to steam are led between the bands in such a way that the web to be dried will be posi-tioned against the heated band and the dryer felt or the like against the cooled band so that the o-ther side of the dryer fel-t or the like will be positioned against the ~eb to be dried. in the embodiments disclosed, a fixed box containing pressurized satu-rated steam is positioned outside the heated metal band. The box is open towards the mo~ing metal band in such a way that the steam is in direct contact with the metal band or a condensa~e forming thereon~ the edges of the box being sealed off again~t the surface of the band. A box similar ~o that described above is positio~ed outside the cooled metal band, water being fed into the box approxim~tely at the pres~ure of the steam contained in the box positioned outside the h~ated band but at ~ temperature lower than that of the steam.
In addition to the embodiment described above, Finnish Patent 61537 further discloses an application in which the hot sur~ace consists of the outer surface of an internally heated metal cylinder, and the cooled surface of a metal band extending almost around the cylinder in parallel with the surface thereof, whereby the web and the dryer felt are led between the cyl-inder and the band moving at an e~ual rate.
~ 3 ~
In the abo~e-described applications, heat is transferred to the hot metal surface within the drying zone itself through the band, that is, through the me-tal layer one surface of which consists of said hot metal surface. Heat is transferred into the metal layer from a source of heat positioned behind the other surface of the metal layer, when saturated steam condenses and delivers its latent heat into the metal band. In the solutions disclosed, no appreciable amounts of heat are transferred into the metal band outside the drying zone when the band moves within the area of the steam box. In practice, -the metal band is endless~ so that heat is dissipated -from the metal band outside the press section into -the rolls intended for the rotation of the band and into the ambient air.
As the hot surface within the drying zone consists of the ou~er surface of the metal cylinder, heat is transferred into the metal casing from the source of heat positioned within the cylinder typically over the whole peripheral 7ength of ~he casing. However, ~his is disadvantageous in that due to the thickness of the cylinder casing the amount of heat which is trans ferred from within the casing to the outer surface thereof is considerably smaller than the amount of heat trans~erred through a thin metal band, wherefore very high temperatures are required in the cylinder in order that the heat flux required for the drying of the web could be transferred to the surface of the cylinder. This, in turn, requires that the steam should be ver~ hot, which is economically disadvanta-geous. Consequently, the use of a metal band as a hot a surface is economically more advantageous.
Attention has been paid to these matters in the solution disclosed in Finnish Patent 69141, where the outer surface of a conventional thick-walled drying ~ 3 ~
cylinder acts as a cooled surface in the drying process while a relakively thin metal band acts as ~he hot metal surface. For a major part the metal band is heated outside ~he drying ~one formed by the cylinder and the band before the band reaches the drying zone.
In this solution, the metal band is not actually heated within the drying zone but the heat required in the metal band for the evaporation of the moisture contained in the web is for a major part derived from the heat energy transferred to the metal band during the preheating step. Thereby the average temperature of the metal band drops within the drying zone in pro-portion as the drying proceeds and the band moves on within the drying zone together with the web.
In the above patenks, it is shown by examples that in practice a preheating rising the temperature of the metal band by a few tens of degrees provides sufficiently energy for the evaporation of a high amount of moisture from the web without having to heat the metal band within the drying zone.
All the above applications have the drawback that during the drying process the drying of the web is dependent on the pressure of the cooling water, the temperature of the cooling water, and the mechani-cal compression exerted on the web perpendicularly to the surface of the web, i.~., a Z compression, which is due to the followiny ma-tters.
After havîng penetrated part of the web and the wire or felt, the steam evaporated from the web is condensed on to the cooled band or on to the conden-sate formed thereon at a saturation pressure corre-sponding to the local temperature. The local tempera-ture, in turn, is determined by the temperature of the cooling water. The temperature of the cooling water is affected by the temperature difference caused by the ~ 3 ~
heat transfer from the co~ling water to vne side of the cooled band and further therefrom to the other side of the cooled band. Further, the pressure of the steam at the wire or felt making contact with the sur-face of the web is determined by the above-mentioned condensation pressure which is affected, though to a small degree, by the pressure loss oCCurr.ing when ~he steam passes fxom the interspace between the web and the wire to the condensation surface. In accordance with the force balance, the mechanical Z compression to be exerted on the surface of the web is equal to the difference between the pressure exerted on the outside of the hot and the cold band and the pressure of the steam in the interspace between the web and the wire.
In the publications mentioned above, the disad-van-tageous dependance between the temperature and the drying rate typical of o-ther drying processes has been eliminated by adjusting -th~ temperature of the cooling surface to such a high value that the drying rate i5 decreased to a desir~d level. However~ there still re-mains the restric-tive factor that the temperature of the heating steam and correspondingly the pressure ~hereof as well as the temperature of the cooling water and the mechanical Z compression exerted on the web are dependent upon each other. For the adjustabil~
ity of the drying process, this dependence is very disadvantageous and causes unnecessary expenses and impairs -the operational optimization of the drying process in the production of a paper board and paper web.
The object of the invention is to providP a method by means of which the restrictive factors and drawbacks of the drying procedures described above are avoided and by means of which each one of the three important process parametres, i.e., the temperature of the web, the local drying rate, and the mechanical Z
compression exerted on the web, can be adjusted to a desired level separately in each drying zone, whereby the length of the dryinq zones can be varied within limits as wide as possible and the drying process can be optimized in view of both the quality of the web and the production process as a whole. The method ac-cording to the invention is characterized in that for adjusting the mechanical compression exerted on the web to a desired value, the pressure acting on the outer surface of the bands and the temperature of the cooling band are adjusted so that the difference be-tween the pressure acting on the outer surface of the bands and the saturation pressure of the steam de-termined by the temperature of the cooling band is substantially equal to the desired mechanical com-pression force; and for adjusting the drying rate of the web to a desired value the temperature of the band exposed to the heat treatment and heating-the web is adjusted before it reaches the drying zone so that i-t corresponds to the desired drying rate.
The basic idea of the invention is that the magnitude of ~he mechanical Z compression to be exerted on the web within the drying zone is de-termined, and the external pressure acting on the out-side of the bands pressing them towards each other and the temperature of the band to be cooled are adjusted in such a way that the pressure difference between the pressure acting on the bands and -the pressure of the condensing steam is essentially equal to the desired mechanical Z compression; on the other hand, the tem-perature of the band to be heated is adjusted to a suitable value on the basis of the desired drying rate. According to the invention, either one of these ~ 3 ~
adjustments can be effected Eirst, because the temperature difference between the bands does not afrect the Z compres-sion to any greater degree in view of the pressure adjus-tment.
~ ccording to a still further broad aspec-t of the present invention, there is provided a rnethod of drying a paper board or a paper web by leading the web -together with at least one dryer felt be-tween -two movlng, air-tight bands having a good thermal conductivity and being substantially parallel with each o-ther over a dis-tance. The bands enclose the web therebetween over the en-tire wid-th thereo~ to form a drying zone having a drying rate. The method comprises exposing a web and a dryer fel-t to an air removal treatment whereby air is removed out of pores -to the greatest possible degree. The web and the dryer felt are passed be-tween -two moving, air--tight bands which are substantially parallel to each other. An outer surface of each of the bands is subjected to substantially equal pressure. One of the bands which makes contact with the web is heated prior to entry into the drying zone to a temperature for evaporating wa-ter from the web. The other of the bands which makes con-tact with the dryer felt is cooled to a predetermined -temperature before it reaches the drying zone for condensing the water evaporated from the web into the dryer felt. The dryer felt is separated from -the web after passing through -the bands and condensed water is removed therefrom. The mechanical compression exerted on the web is adjusted to a prede-termined value by adjusting the pressure acting on the outer surface of the bands and the temperature of the cooling band -to produce a difference be-tween the pressure acting on the outer surface of -the bands and the satura-tion pressure of s-team condensed in drying the web determined by the temperature of -the cooling band which is subs-tantially equal to -the desired mechanical compression force. The drying rate of the web is adjusted to a desired value by adjus-ting the temperature of the band exposed to the heating which heats -the web beEore it reaches the dry:ing zone so -tha-t it corresponds to the desired drying rate.
~ 3 ~
- 7a -The invention will be described in more detail with reEerence to the attached ~igure, which shows an apparatus suitable for reali~ing one application of the method according to the invention.
The dryer comprises a heated metal band 1 and a cooled metal band 2 which move in parallel with each other khrough a drying zone. A steam box 3 is pro-vided for the heated metal band and a water box 4 for the cooled metal band. The heated band 1 moves around its turning rolls Sa and 5b ~nd the cooled band 2 around its turning rolls 6a and 6b. A web 7 to be dried is pas~ed into the drying zone so as to make contact with the lower surface of the heated metal band 1; in this embodiment, this lower sur-f~ce acts as the hot surfac~ within the drying zo~e. A dryer felt 8 permeabl~ to ~as and formed by one ~r more ~elt and/or wires positioned one upon another is corre-spondingly pa~ed b~tween ~he bands 1 and 2 in such a way that one surface of the dryer felt 8 makes con-tact with the cooled metal band 2 ~hile the other s~r-face thereof makes contact with that surface of the web 7 to be dried whi.ch faces away from the heated me-tal band. Both the web 7 and the dryer felt 8 are passed through an air r~moval unit 9 before being led between the bands. In the air removal unit, air is removed from the pores of -the web 7 and the dryer felt 8 to the greatest possible degree e~g. by applying therethrough saturated steam at a suitable temperature so that the air molecules are pushed out oE the pores and replaced with water molecules contained in the steam. For heating the heated band, a heating unit 10 ~ 3 ~
is provided on bo~h sides of the heated band 1 at a point where it mo~es alone, i.e., between the rolls ~a and 5b outside the drying ~one ormed between the rolls 5a and 5b and 6a and 6b, respectively. By means of khe heating unit the temperature of the band 1 is adjusted to a temperature corresponding to the desired drying rate by app]ying hot medium 11 through -the heating unit lO. The medium may consist of steam, flue gases from fossil fuels or some other gas heated to a suikable temperakure. It is obvious that the in-vention also comprises an application in which the band 1 is heated by some electrical method wi-thin some other portion of the band than the drying zone and an application in which heat is transferred to the band from a stationary surface in contact -therewith, such as one or both of the turning rolls 5a and 5b.
In the heating unit 10 the band is heated to a desired temperature, whereby the heated band section moves around the -turning roll Sa to the entry end o~
the drying zone and makes con~act with the transported web 7. Steam 12 at a suitable temperature and pressure is introduced into the steam box 3 pro-vided for the heated band. Correspondingly, cooling water 13 at a desired temperature and at a pressure substantially equal to the pressure of the steam l2 is introduced into the wat~r box 4. However, the pressure of the cooling water is preferably slightly higher than th~t of the steam 12 for supporting the bands 1 and 2 as well as the web 7 and the dryer felt 8. Part of -the steam 12 may escape past edge seals 14a and 14b provided between the band 1 and the box 3 and past seals (not shown) provided on the sides of the box 3 so as to be pressed against the band either in the Porm oP steam or condensate. When the rest oP the steam is condensed on the upper surface o~ the band 1, it is gathered and removed from the box 3 by means of suitable devices not shown. From the box 4, part of the water leaks between seals 15a and 15b and between side seals providecl on the water box 4 against the band 2, while the remaining warmed cooling water 16 is re~oved through one end of the box 4.
In the method, the pressure of the steam 12 and that of the water 13 are adjusted so that a prede-termined external pressure acts on the bands, which pressure tends to compress the web 7 perpendicularly to the surface thereof in a direction Z. To achieve the desired mechanical Z compression, the feeding tem-perature of the cooling water 13 is now adjusted to a value such that the pressure of the steam condensing on the surace of the band 2 on the side of the dryer felt 8 settles on the desired level so that the dif-ference between the external pressure acting on the outside of the bands 1 and 2 and the condensation pressure of the condensing steam is equal to the compression to be exerted on the web 7. In a case taken as an example, the steam contained in the steam box 3 is at a temperature of about 100C and at a pressure only slightly abo~e the atmospheric pressure, and the temperature of the water contained in the water box ~ is about 30C. In this case, the steam evaporated from the web 7 is condensed on the upper surface of the cooled hand 2 typically at about 80C
and the condensing pressure of the steam would be 47.3 kPa. Since the pressure loss from -the interspace between the web 7 and the dryer felt 8 to the con-densing surface of the band 2 would be about 230 Pa, the steam pressure in the interspace between the dryer felt 8 and web 7 woulcl be about 47.6 kPa and the mechanical z compression exerted on the web would be abou-t 101.3 kPa - 47.6 kPa = 53.7 kPa. The pressure value 101.3 kPa is -~ha maynitude of the external pres-sure exerted on the web. Supposing that the metal band 1 to be heated would reach the drying zone at 150C
and the web 7 would consist of newsprint with a gram-mage of 45 g/m2 and a dry matter content of 45~, the drying rate would, however, be typically about 200 kg/~s x m2).
In -the case described above, it would be very easy to alter the Z compression exerted on the web without substantially affecting the drying xate. The only alteration required would be to change the tem perature of the cooling water. With a cooling water temperature of 40C, for instance, the situation would be as follows. The temperature of the steam condensing on the band 2 would be about 90C and, corresponding-ly, -the condensing pressure of the steam about 70.1 kPa. The pressure loss from the interspace be-tween the web 7 and the dryer f21t 8 to the condensing surface of the band 2 would now be about 160 Pa and the pressure of the steam in the interspace between the web 7 and the dryer felt about 70.3 kPa. The Z
compression exerted on the web 7 would be 101.3 kPa-70.3 kPa = 31.0 kPa. The drying rate, however, would remain substantially the same as above, that is, at about 200 kg/~s x m2).
However, it is obvious that the drying method according to the inv~ntion also offers an opportunity to use compressed steam in the steam box 3 while the band 1 is preheated in a prehea~er 10. According to the invention, it would thereby be possible to adjust the mechanical Z compression exerted on the web 7 to any leve]. within the range from 0 to Pml wherein Pm is slightly lower than the saturation pressure of the steam created at the lowest temperature of the band 1 occurring within the drying zone. Accordin~ to the in~
~ 3 ~ 9 vention it is thus possible to separately adjust the Z
compression undergone by the web 7 to a desired level while the drying rate can be kept even at a very high value or, if required by the properties of the web~ at a very low value. As it is sufficient for the drying process that the band 1 to be heated is heated before it reaches the drying zone, a substance such as steam or gas at a suitable temperature can be introduced into the box 3, whereby the funct.ion of the substance is not to heat the band 1 but merely to act as a medium for transm.itting pressure to the surface of the band 1. Correspondingly, the cooling of the cooled band 2 can be effected before it reaches the drying zone by means of a separate cooling device 18 into which cooling medium 19 is introduced for cooling the band to a sui-table temperature. The cooling device 18 is sealed on the surface of the band 2 by means of seals 20a and ~Ob positioned on both sides thereo and correspondingly by side seals (not shown) extending in the direction o-f movement of the band. When the band 2 is cooled by the cooling device 18, water at a suit-able pressure and temperature can be introduced into the water box 4, the function of the water being main-ly to transmit p.ressure to the outer surface of the band 2.
Only one embodiment o-f the method according to the invention and one way of applying it have been de-scribed above. However, the invention is by no means restricted to the embodiment disclosed, but it can be varied freely within the scope of the claims. I~ both the heated and the cooled surface are heated and cooled, respectively, before they reach the drying zone/ it is possible to apply atmospheric pressure, for instance, within the drying zone, whereby in certain cases the steam box 3 can be omitted, and air or some other gas can he introduced in place of water into the box 4 m~rely for suppor~iny the bands and the web and dryer felt positioned therebetwaen, provided that the apparatus is horizontally positioned. When the atmospheric pressure acts on the bands 1 and 2, the magnitude of the mechanical Z compression acting on the web 7 can be adjusted simply by varying the temperature of the band 2 by means of the cooling de-vice 18. Correspondingly, the drying rate can be simply adjusted by means of the heating device 10 of the band 1., and the whole process can be controlled easily and simply.
Claims (7)
1. A method of drying a paper board or paper web by leading the web together with at least one dryer felt between two moving, air-tight bands having a good thermal conductivity and being substantially parallel with each other over a distance, the bands enclosing the web therebetween over the entire width thereof to form a drying zone having a drying rate, the method comprising the steps of:
exposing a web and a dryer felt to an air removal treatment whereby air is removed out of pores to the greatest possible degree;
passing said web and dryer felt between two moving, air-tight bands which are substantially parallel to each other;
subjecting an outer surface of each of said bands to substantially equal pressure;
heating the one of said bands which makes contact with said web prior to entry into said drying zone to a temperature for evaporating water from said web;
cooling the other of said bands which makes contact with said dryer felt to a predetermined temperature before it reaches said drying zone for condensing said water evapo-rated from said web into said dryer felt;
separating said dryer felt from said web after passing through said bands and removing condensed water therefrom;
adjusting the mechanical compression exerted on the web to a predetermined value by adjusting the pressure acting on the outer surface of the bands and the temperature of the cooling band to produce a difference between the pressure acting on the outer surface of the bands and the saturation pressure of steam condensed in drying said web determined by the temperature of the cooling band which is substantially equal to the desired mechanical compression force; and adjusting the drying rate of the web to a desired value by adjusting the temperature of the band exposed to said heating which heats said web before it reaches said drying zone so that it corresponds to the desired drying rate.
exposing a web and a dryer felt to an air removal treatment whereby air is removed out of pores to the greatest possible degree;
passing said web and dryer felt between two moving, air-tight bands which are substantially parallel to each other;
subjecting an outer surface of each of said bands to substantially equal pressure;
heating the one of said bands which makes contact with said web prior to entry into said drying zone to a temperature for evaporating water from said web;
cooling the other of said bands which makes contact with said dryer felt to a predetermined temperature before it reaches said drying zone for condensing said water evapo-rated from said web into said dryer felt;
separating said dryer felt from said web after passing through said bands and removing condensed water therefrom;
adjusting the mechanical compression exerted on the web to a predetermined value by adjusting the pressure acting on the outer surface of the bands and the temperature of the cooling band to produce a difference between the pressure acting on the outer surface of the bands and the saturation pressure of steam condensed in drying said web determined by the temperature of the cooling band which is substantially equal to the desired mechanical compression force; and adjusting the drying rate of the web to a desired value by adjusting the temperature of the band exposed to said heating which heats said web before it reaches said drying zone so that it corresponds to the desired drying rate.
2. A method according to claim 1 wherein said subject-ing step applies atmospheric pressure on the outer surface of said bands.
3. A method according to claim 2 wherein said pressure is transmitted to said outer surface of said band heating said web by means of a gas or a gas mixture.
4. A method according to claim 2 wherein said pressure is transmitted to said outer surface of said band heating said web by means of steam.
5. A method according to claim 2 wherein said pressure is transmitted to said surface of said band cooling said dryer felt by means of a gas or a gas mixture.
6. A method according to claim 2 wherein said pressure is transmitted to said outer surface of said band cooling said dryer felt by means of a liquid.
7. A method according to claim 6 wherein said liquid is water.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FI880407A FI78755C (en) | 1988-01-29 | 1988-01-29 | FOERFARANDE FOER TORKNING AV EN KARTONG- ELLER PAPPERSBANA. |
| FI880407 | 1988-01-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1311119C true CA1311119C (en) | 1992-12-08 |
Family
ID=8525816
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000588089A Expired - Lifetime CA1311119C (en) | 1988-01-29 | 1989-01-12 | Method of drying a paper board or paper web |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US4899461A (en) |
| EP (1) | EP0326348B1 (en) |
| AT (1) | ATE69842T1 (en) |
| CA (1) | CA1311119C (en) |
| DE (1) | DE68900453D1 (en) |
| FI (1) | FI78755C (en) |
Families Citing this family (20)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FI92736C (en) * | 1993-02-01 | 1994-12-27 | Tampella Oy Valmet | Dryer for drying fiber web |
| US5291666A (en) * | 1993-04-23 | 1994-03-08 | International Paper Company | Apparatus for drying roll material |
| FI97485C (en) * | 1995-02-14 | 1996-12-27 | Valmet Corp | Drying device for drying a fiber web and drying part in a paper machine |
| US5637195A (en) * | 1995-03-23 | 1997-06-10 | Westvaco Corporation | Method to reduce off-taste and/or odor from hygienic paper packages |
| FI98080C (en) * | 1995-11-17 | 1997-04-10 | Valmet Corp | Drying device for drying the fibrous web |
| US5619806A (en) * | 1996-02-26 | 1997-04-15 | Warren; David W. | Drying of fiber webs |
| DE19614887A1 (en) * | 1996-04-16 | 1997-10-23 | Voith Sulzer Papiermasch Gmbh | Machine for the production of a material web |
| US6139686A (en) * | 1997-06-06 | 2000-10-31 | The Procter & Gamble Company | Process and apparatus for making foreshortened cellulsic structure |
| US5935381A (en) * | 1997-06-06 | 1999-08-10 | The Procter & Gamble Company | Differential density cellulosic structure and process for making same |
| US5938893A (en) * | 1997-08-15 | 1999-08-17 | The Procter & Gamble Company | Fibrous structure and process for making same |
| US6221798B1 (en) | 1997-07-24 | 2001-04-24 | International Paper Company | Method for producing laminated webs |
| FI104275B (en) * | 1998-02-18 | 1999-12-15 | Valmet Corp | Method and apparatus for drying a fibrous web |
| US6280573B1 (en) | 1998-08-12 | 2001-08-28 | Kimberly-Clark Worldwide, Inc. | Leakage control system for treatment of moving webs |
| US6318727B1 (en) | 1999-11-05 | 2001-11-20 | Kimberly-Clark Worldwide, Inc. | Apparatus for maintaining a fluid seal with a moving substrate |
| US6610173B1 (en) | 2000-11-03 | 2003-08-26 | Kimberly-Clark Worldwide, Inc. | Three-dimensional tissue and methods for making the same |
| US6701637B2 (en) | 2001-04-20 | 2004-03-09 | Kimberly-Clark Worldwide, Inc. | Systems for tissue dried with metal bands |
| US9121606B2 (en) * | 2008-02-19 | 2015-09-01 | Srivats Srinivasachar | Method of manufacturing carbon-rich product and co-products |
| CA2716190A1 (en) * | 2008-02-19 | 2009-08-27 | Srivats Srinivasachar | Method of manufacturing carbon-rich product and co-products |
| FI8397U1 (en) * | 2009-05-27 | 2009-08-14 | Metso Paper Inc | Device for making fiber web |
| US9589817B2 (en) * | 2011-04-15 | 2017-03-07 | Illinois Tool Works Inc. | Dryer |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1502040A (en) * | 1975-12-30 | 1978-02-22 | Tampella Oy Ab | Method of drying a cardboard or a paper web and drying device for applying this method |
| FI61537C (en) * | 1981-02-19 | 1982-08-10 | Tampella Oy Ab | REFERENCE TO A CONTAINER WITHOUT CONTAINER TORKNING AV EN PAPPERS- ELLER LIKNANDE POROES BANA |
| FI66041C (en) * | 1982-04-06 | 1984-08-10 | Tampella Oy Ab | FOERFARANDE FOER TORKNING AV EN POROES BANA I EN LAONGZONSPRESS |
| US4506475A (en) * | 1983-05-10 | 1985-03-26 | Elliott James J | Hanger for potted plants |
| FI69141C (en) * | 1984-10-09 | 1985-12-10 | Tampella Oy Ab | OVER ANCHORING FOER TORKNING AV EN PAPPERSBANA ELLER LIKNANDE |
-
1988
- 1988-01-29 FI FI880407A patent/FI78755C/en not_active IP Right Cessation
-
1989
- 1989-01-12 US US07/296,506 patent/US4899461A/en not_active Expired - Lifetime
- 1989-01-12 CA CA000588089A patent/CA1311119C/en not_active Expired - Lifetime
- 1989-01-25 AT AT89300685T patent/ATE69842T1/en not_active IP Right Cessation
- 1989-01-25 EP EP89300685A patent/EP0326348B1/en not_active Expired - Lifetime
- 1989-01-25 DE DE8989300685T patent/DE68900453D1/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| ATE69842T1 (en) | 1991-12-15 |
| EP0326348A1 (en) | 1989-08-02 |
| US4899461A (en) | 1990-02-13 |
| FI78755C (en) | 1989-09-11 |
| FI78755B (en) | 1989-05-31 |
| EP0326348B1 (en) | 1991-11-27 |
| FI880407A0 (en) | 1988-01-29 |
| DE68900453D1 (en) | 1992-01-09 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKLA | Lapsed |