WO1990012920A1

WO1990012920A1 - Application of superheated steam

Info

Publication number: WO1990012920A1
Application number: PCT/CA1990/000111
Authority: WO
Inventors: Robert James Harry Mason
Original assignee: Macmillan Bloedel Limited
Priority date: 1989-04-20
Filing date: 1990-04-05
Publication date: 1990-11-01
Also published as: EP0469005A1; US4945654A; JPH04504739A; AU5423890A; CN1020934C; CN1050580A; CA1319286C

Abstract

Superheated steam is applied directly to the surface (211) of a web of paper or the like to significantly increase at least the surface temperature of the web impinged by the superheated steam thereby to significantly heat the paper. The web, after steam treatment, is immediately passed into a nip (230). The steam applicator (200) is maintained at a temperature above 100°C to ensure that condensation problems are substantially avoided and most if not all the steam condensed in the web. An applicator (200) for superheated steam comprises a heated chamber (204) having a steam inlet (202) and apertures through which steam is directed from the chamber toward a travelling web (210), the chamber is heated to a temperature to ensure that the steam leaving the chamber through the apertures (212) has the desired degree of super heat, i.e. moisture to heat content to raise the temperature of the web without undue amount of condenstation forming on the equipment.

Description

APPLICATION OF SUPERHEATED STEAM

Field of the Invention

Present invention relates to a steam applicator for applying superheated steam to a web such as a web of paper and to a method of surface treating a web of paper by applying steam from an applicator maintained at a temperature of more than 100°C to increase the temperature of the web significantly while avoiding significant condensation problems.

Background of the Invention

Calendering paper by supercalendering, hard nip calendering or soft nip calendering has been used in the paper industry for many years and the operations of such calenders have been studied and numerous reports written.

In Pulp and Paper Canada, Volume 75, No. 11, November 1974 in a paper by Kerekes and Pye entitled "Newsprint Calendering: An Experimental Comparison of Temperature and Loading Effects", an experimental program is reported wherein nip loading, number of nips and temperature of the rolls were discussed particularly in reaction to hard roll calendering. The conclusions reached indicate that heating the rolls can reduce the number of nips or pressure necessary to obtain a selected density and that higher temperature rolls improve the gloss.

Canadian Patents 1,011,585 issued June 7, 1977 to Anderson is one of the earlier discussions of what appears to be a moisture and temperature gradient calendering system wherein moisture is applied to the sheet and then the sheet is drawn over a hot platen.

The concept of moisture gradient calendering was discussed in British Board Industries Fed., London, 1978, Vol. 2, pages 641-669 in an article entitled "The Effect of Moisture and Moisture Gradients on the Calendering of Paper" by Lyne. In this paper the concept of plastic flow and softening of lignin by moisture and temperature is discussed and the effect of these parameters on the surface of a web under conditions of conventional roll temperature, high roll temperatures and a combination of high roll temperatures and moisture application immediately before the calender nip is examined.

In an article entitled 'Temperature-gradient Calendering" by Crotogino published in the Tappi Journal/October 1982, pages 97-101, the

SUBSTITUTE SHEET effect temperature gradient calendering of paper is described. The surface of the paper web is heated but the heat does not have time to penetrate to the inside of the paper web which remains relatively cool as the web passes through the calender nip. This results in development of improved surface properties without significant compaction of the middle of the sheet thereby to reduce strength loss by the calendering operation.

Dunfield et al, in an article entitled "Gravure Printability of Steam-Treated Machine Calendered Newsprint" published in the Journal of Pulp and Paper Science: Vol. 12, No. 2 March 1986, describes, among other things, the application of steam in a steam shower before the nip of a calender and indicates that improved results can be obtained by moisture gradient calendering even without added heat.

U.S. Patents 4,624,744 issued November 25, 1986 and 4,749,445 issued June 7, 1988 to Vreeland describe the effects of temperature gradient calendering using a soft backing roll.

In a paper given at the 1988 Annual Meeting of the Canadian Pulp and Paper Association, Technical Section, January 28-29, 1988, entitled "The Effect of Calender Steam Treatment and Roll Temperature on Newsprint Properties" by Keller, the effect of temperature gradient calendering with steam treatment before the high temperature nip was further examined and it was found that the steam treatment before the high temperature nip produced the highest MD Tensile and CD Tear strength values for a given roughness. Also the use of a high roll temperature increases gloss for a given density. It was further noted that the application of steam slightly reduced the beneficial effect of the high calender temperature on gloss and reported that the addition of steam even when calendering to higher densities did not develop the same gloss as was developed using high temperature rolls only.

In a paper entitled "The Effects of Z-Direction Moisture and Temperature Gradients in the Calendering of Newsprint" by Gratton et al, published in Journal of Pulp and Paper Science, Volume 14, No. 4, July 1988, pages J82-J90, the effect of both temperature and moisture gradient calendering on the surface of paper were examined and it was concluded that the application of moisture to the surface of the paper when using a high temperature calender roll was not significantly different compared with calendering with a roll at the same temperature without added moisture.

SUBSTITUTE SHEET A paper entitled "Effects of Moisture and Temperature on Paper Properties with Implications for Hot Calendering", Kalender Seminar Konigsbronn 1988-03-25 by Black, further discusses moisture content and temperature in the calendering nip and concludes that one of the major advantages of hot calendering is rapid solidification on the exit side of the hot nip thereby reducing springback indicating that a high moisture content is only an advantage when sufficient water is evaporated in the nip to achieve a rapid solidification. This paper also suggests that it might be advantageous from a brightness point of view to limit the temperature of the surface to less than about 150°C.

Miscellaneous reports MR 109 by the Pulp and Paper Research Institute of Canada, in a paper entitled "Hard-Nip and Soft-Nip Calendering of Uncoated Groundwood Papers", March 1987 by Crotogino et al, a review of the established, new emerging calendering techniques is provided and it is stated that steam showers may be used to help reduce bulk and improve the surface properties of a paper.

In the Black paper above discussed, it has been suggested that too high a roll temperature in the calender may be detrimental to the gloss characteristics of the treated paper. It will be apparent from the above that the concepts of temperature and moisture gradient calendering have been well investigated and the basic conclusion reached by the experts is that temperature gradient or high temperature calendering is beneficial, that moisture gradient calendering is also beneficial but when both moisture and temperature gradient calendering are used, gloss is substantially the same or possibly reduced compared with that obtained if only temperature gradient calendering were being carried out.

In a paper entitled "Calender Steam Showers - An Effective New Way of Hot Calendering" by Hilden et al, Tappi Journal, Vol. 70, July 1987, describes a new design of steam shower for applying steam in the calender stack. This shower is more fully described, it is believed, in the October, 1988 Tappi Journal entitled "Practical Aspects of Calender Steam Showers" by Vyse et al, pages 87-90.

In a more detailed article entitled Calender Steam Showers - A New Effective Means of Hot Calendering by Hilden and Sawley published in Pulp and Paper Canada, Volume 88, No.12 (1987) T452-T455 the steam

SUBSTITUTE SHEET application of steam to paper in the calenders is described. Similar equipment and processes are described in the Finishing and Converting Conference 1986, TAPPI Proceedings pages 95-100 by Hilden and Sawley and the May 14-16, 1987 Spring Conference of the Canadian Pulp and Paper Association Preprints pages 1-6 inclusive. All of the three publications are recited since there are minor differences in what is disclosed in each publication. For example, in the Pulp and Paper reference, high energy transfer is referred to and a steam iron effect which includes contact between the sheet and the steam applicator is described whereas in the other papers no contact is referred to. In the latest publication, of which applicant is aware and which it is believed relates to the same steam shower device, entitled Controlling Paper Smoothness using Calender Steam Showers by Vyse and Sawley presented in 1989 at the Annual Meeting of the Canadian Pulp and Paper Association, (see pages A205-A209 in the preprints for the meeting), further information is provided on the steam pressures and temperatures used in the system and steam showers are stated to be close to, i.e. close clearance from the sheet (page A206, Column 1, line 6).

In the later publication the steam pressure supplied to the steam shower is defined as high as 8 psig at temperatures of 115-125°C. In the earlier papers, the highest temperature that could be reached on the paper was 100°C and this was determined by the temperature of the sheet approaching the steam shower. No absolute value for the sheet temperature is described in the latest paper however the plotted temperature increase appears to reach about 20° in Figure 3 but is defined as 23° minimum in Table 1.

In all cases the increase in smoothness obtainable when practicing the method of these papers appears to be up to about 10 points (Sheffield smoothness) improvement which is barely significant.

In all of the above devices wherein steam is applied to the web while the steam supphed to the appHcator or shower may contain a minor degree of superheat, by the time it is cooled by the applicator itself and reaches the paper, the steam likely is about saturated, i.e. a temperature of probably no more than 100°C.

One of the problems in utilizing current technology is condensation problem of the steam not contained within the intestees of the

BSTITUTE SHEET webs. Also, the use of saturated steam Hmits the maximum temperature to which the surface of the sheet may be raised to about 100°C.

The use of superheated steam in paper making is not new. The effect of the application of super heated steam during drying is discussed in a paper entitled "Effect of Superheated Steam Drying on Paper Properties" by David et al, was presented to the Annual Meeting of the Canadian Pulp and Paper Association, Technical Section, January 28-29, 1988, see pages B233- B237 of the preprints. In this paper, superheated steam is appUed to a handsheet to dry the handsheet and it was found that some of the physical and optical properties of the resultant dry paper were significantly better than those that would be obtained with normal air drying, however no practical way of drying using superheated steam is suggested.

A Brief Description of the Present Invention It is an object of the present invention to provide an applicator for applying a steam in superheated steam form to the surface of a moving web, particularly an appHcator for impinging superheated steam directly onto the surface of a travelling web.

It is a further object of the present invention to provide a method of applying superheated steam directly to the surface of a moving web while reducing significantly the amount of condensate formed in the immediate vicinity of the appHcator.

It is a further object of the present invention to reduce the amount of condensation formed from steam from the applicator other than that used to heat the web and faciHtate surface finishing of paper web in a calender.

Broadly the present invention relates to a method and apparatus for applying steam to a surface of a web comprising deHvering steam in superheated condition from an appHcator located immediately adjacent to a surface of said web by said steam, maintaining the temperature of said appHcator at above 100°C and impinging said superheated steam directly against said surface of the web thereby to heat at least said surface of said web without significant condensation of said steam other than on said web.

Preferably the system of the present invention will be used in conjunction with a calender using either hard rolls or a combination of soft and hard roUs and the superheated steam will preferably be applied to the surface

SUBSTITUTE SHEET of the web to first be contacted by the hard roll.

Broadly the present invention also relates to a method comprising treating the surface of a travelHng web by applying steam in superheated steam form and by directing steam from an applicator maintained at a temperature of at least 110°C to apply superheated steam directly on to a surface of said web as it travels past an appHcator to heat at least said surface of said web to modify the characteristics of at least the surface of the web and passing said web through a treatment zone while the characteristics of said surface are substantially in their modified form to faciHtate treatment of said web in said treatment zone. The treatment zone will normally take the form of a nip to reform the web to reduce the caliper of said web while modifying the surface characteristics of the web.

The present invention also relates to an appHcator for applying a fluid in a gaseous state to a travelHng surface of a web, said applicator comprising a chamber, means for introducing steam into said chamber, heater means for heating said chamber to transfer heat to said steam in said chamber, outlet means from said chamber, means for moving a web past said outlet means, said outlet means being directed toward a surface of said web to be contacted by said steam, said web moving across said outlet means in close proximity thereto so that said steam issuing from said outlet impinges directly on said surface without significant cooHng of said superheated steam between said outlet means and said surface.

Preferably said close proximity is sufficiently close that said web contacts a surface of said appHcator, said surface of said applicator being heated to and maintained at a temperature above 100°C and transferring heat to said web.

Preferably said treatment zone will comprise a calender nip.

Brief Description of the Drawings Further features, objects and advantages wiU be evident from the foUowing detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which.

Figure 1 is a side elevation of a conventional calender stack on a paper machine. Figure 2 fllustrates one location for application of superheated

SUBSTITUTE SHEET steam in accordance with the present invention.

Figure 3 iUustrates another location for appHcation of superheated steam on a conventional calender.

Figure 4 shows a conventional supercalender but with superheated steam appHcators of the present invention located at selected locations in the stack.

Figure 5 is a schematic iUustration of a hard nip calender incorporating the superheated steam appHcators of the present invention.

Figure 6 is similar to figure 5 but showing a soft nip calender incorporating the present invention.

Figure 7 is a side elevation view of an appHcator constructed in accordance with the present invention and divided into segments extending transverse of the web, i.e. broken into a pluraUty of compartments each extending only a portion of the width of the web. Figure 8 is a schematic section through an appHcator constructed in accordance with the present invention.

Figure 9 is a view of the outlets from the applicator in one embodiment of the present invention.

Figure 10 shows a modification of the present invention provided with a bowed surface.

Figure 11 is a section on the line 11-11 of the applicator of Figure 10.

Description of the Preferred Embodiments The present invention wiH be described primarily with respect to calenders but it wiH be apparent that it may be appHed in many other appHcations particularly for treating or making paper.

Figure 1 shows a typical, calender stack 10 composed of a pluraHty of roUs arranged in the stack. Some of the roUs, such as the roUs 12, 14 and 16 may be heated and an intermediate roll such as the roll 18 may be a variable crown roU to ensure more uniform appHcation of pressure. Similarly the bottom roU 20 may be a swimming roll to help maintain the uniformity of nip pressures along the axial length of the nips. Not all of the rolls need to be driven.

GeneraUy only one roU such as the roll 16 is driven. Superheated steam from applicator 22 which is maintained at a

SUBSTITUTE SHEET temperature above 100°C is directed against the surface of the web of paper 24. The appHcators 22 may be positioned in any suitable location in the calendar for example as indicated in Figure 2 to apply superheated steam to the adjacent exposed face of the paper web 24 to preheat this surface immediately before it enters the nip and contacts the roU 12. In many cases the roU 12 may be a heated roU. The effect of the appHcation of superheated steam to the surface of the web 24 wfll be described below.

Figure 3 shows an arrangement wherein the paper web is stripped from one of the rolls of the calender, in this case roU 16, is passed over a turning roU 28 and is heated by superheated steam directed thereagainst from the appHcator 22A positioned within the loop of the paper 24. As described above, the appHcator is maintained at a temperature above 100°C, preferably above 110°C, and superheated steam is appHed to the surface of the web 24 immediately before contact with the roU 16. The higher the appHcator temperature the more heat that may be transferred to the web, thus it is preferred to operate with appHcator temperature higher than 120°C but not so high as to damage the web.

GeneraUy in most applications on the paper machine superheated steam may be appHed in the same location as saturated steam is normaUy appHed to heat and add moisture to the web but with the applicator maintained at above 100°C and superheated steam being applied the appHcator can be made smaH so it can apply steam close to a nip and apply the amount of energy required.

Figure 4 is a view similar to figure 1 but illustrates a typical supercalender having an unwind stand 30, calender stack 32 and a rewind stand 34. The calender stack is normaUy formed by alternating soft roUs 36 and intervening hard roUs 38 that generaUy wiU be heated. Superheated steam is appHed by the appHcators 22 in the iUustrated arrangement first to one surface of the web 40 and then to the other surface of the web 40 as indicated by the appHcators 22B and 22C respectively. It wiU be noted that in each case the superheated steam is appHed to the surface of the web 40 that is about to contact a hard roU 38. As in the previous arrangements the appHcator 22 is maintained at a temperature of above 100°C.

In Figure 5 a temperature gradient on-line calender utilizing a pair of hard roUs forming each nip is iUustrated. As shown the rolls 42 and 44

SUBSTITUTE SHEET are heated by magnetic induction heaters 46 and 48 (other heating means such as internal oil heating may be used). The rolls 42 and 44 form nips with soHd or hard roUs 50 and 58 respectively. The paper web travels from the first nip formed between the roUs 42 and 50 to the second nip formed between the roUs 44 and 58. Immediately prior to the first nip when practicing the present invention a superheated steam appHcator 22D is provided to direct superheated steam onto the surface of the web 54 on the side of the web that wiU contact the heated roU 42. A second appHcator 22E is positioned to direct superheated steam onto the bottom or opposite surface of the web 54 to heat this surface immediately before the web 54 traverses the second nip. The side of the web 54 heated by the appHcator 22E is the side that contacts the heated roll 44.

While rolls 42 and 44 that contact the side of the web against which superheated steam is appHed have been shown as heated such heating may not be necessary depending on the condition of the surface of the web when it contacts these roUs. The appHcation of superheated steam permits obtaining a web surface temperature significantly higher than those attained when saturated steam was applied and the effect of heating the rolls is reduced. When the temperature is raised sufficiently high by the application of superheated steam (and heated applicator), it may be desirable to cool these roUs to freeze the surface of the web before it leaves the nip.

The arrangement iUustrated in figure 6 is essentially the same as figure 5 and like reference numerals have been used to Ulustrate like parts. The difference between the embodiment of Figure 6 and that in Figure 5 is in Figure 6 rolls 50 and 58 are intended to be soft rolls or alternatively cooled roUs depending on the appHcation to which the process is to be applied. Again it wiU noted that the appHcators 22D and 22E apply superheated steam to the surface of the web to be contacted by the hard or forming roll as this is the surface to be modified (smoothed). The rolls 42 and 44 as above described need not be heated. The particular structure of the appHcators i.e. 22, 22A 22B, 22C,

22D, 22E, 22F, wiU be described in detaU hereinbelow with reference to Figure 8 and 9, however generally each is provided with at least one row of apertures extending substantially the full width of the paper sheet to apply superheated steam over the full width of the paper sheet. If desired, these apertures may be replaced by a narrow slot. Means will be provided to maintain the

SUBSTITUTE SHEET appHcator at a temperature above 100°C. Normally as will be described below this wiU comprise a heater, but if the incoming steam is sufficiently superheated the incoming steam may be used to maintain the appHcator above 100°C.

The appHcators 22 obviously wUl extend substantially the full width of the web such as the webs 24, 40 and 54 and each wiU be maintained at a temperature above 100°C and wiU have the capabiHty of applying superheated steam to the web. If cross machine profiling is desired, i.e. different rates or temperatures of steam appHcation in different areas spread transversely of the direction of travel of the web are desired, the appHcator wiU be divided into a pluraHty of isolated side by side compartments, such as the appHcator iUustrated in figure 7 and indicated at 22F. In this arrangement there are six different compartments indicated at 60, 62, 64, 66, 68, and 70 divided into groups which in this case are composed of pairs of adjacent compartments such as compartments 60 and 62; 64 and 66; and compartments 68 and 70. Each of these groups of adjacent compartments are fed with steam from the main header 80 via branch ducting 82, there being one duct 82 for each pair of compartments. The flow through each of the branch lines or ducts 82 is controUed by a valve 84. These valves 84 may each be individually controUable if desired. If desired each compartment may be isolated from the others and separate ducting 82 and values 84 provided for each compartment.

Each of the compartments 60, 62, 64, 66, 68, and 70 preferably is provided with its own heater as iUustrated at 86, 88, 90, 92, 94, and 96 respectively and each of the compartments may be provided with its own independent temperature control 87 (only two shown but one may be provided for each heater 86-96). Temperature may also be regulated by providing two steam headers at different temperatures and controUably blending steam from each heater into each compartment to control the temperature of the incoming steam as desired to a temperature between the temperatures of the steam in the two heaters. In some cases where incremental control across the web is not needed, a single compartment with a single heater may be used to extend the fuU width to be treated of the web.

A cross section through a particular compartment is iUustrated in Figure 8. Steam is directed into the appHcator 200 (which may be any one of the appHcators 22 to 22F inclusive) via the inlet 202 and enters a compartment 204. A central partition 205 causes the steam to flow along one side and then

SUBSTITUTE SHEET along the other side of the compartment when passing from the inlet 202 to the outlet orifices 212. Heating coils 206 are provided on at least one side of the compartment 204, in the Ulustrated arrangement a heating coil or the like 206 and/or 208 (i.e. heaters 86-96 inclusive) is provided one on each side of the compartment 204. Any suitable type of heater, provided it will develop the required heat and temperature for transfer to the steam, may be used as heater 206 and/or 208 and thus as heaters 86-96, for example, induction heaters heated by steam, hot oU or other hot fluids may be used. The steam itself may be used to heat the appHcator if the incoming steam is sufficiently superheated that even after losing heat to the appHcator the steam contacting the web stiU has the required degree of superheat to heat the web surface to the desired degree and maintain the applicator at a temperature of over 100°C. AppHcant prefers to use an electrical heater and to transfer heat to the steam and to the web contacting the appHcator. Within the chamber 204 the temperature of the steam entering through inlet 202 is adjusted to insure the steam is superheated and has the desired degree of superheat when leaving the applicator to contact the web as wUl be described hereinbelow. Preferably the steam will be heated to a temperature higher than the incoming steam thereby to superheat or further superheat the steam immediately prior to appHcation to the surface of the web 210.

The paper temperature approaching the appHcator will normaUy be less than about 90°C and in those appHcations where the web actually contacts the appHcator heat is transferred between the appHcator and the web. Under these conditions it is important that either the degree of superheat of the steam entering the appHcator be sufficient to compensate for the heat transfer between the web and appHcator to ensure that the steam leaving the appHcator has the required degree of superheat or sufficient heat be added via the heating coUs or the like to the appHcator again to ensure that the temperature of the superheated steam appHed to the web is sufficiently high.

Even when there is no contact between the web and the applicator it is very important to insure that the temperature of the applicator is above 100°C or significant condensation problems may be encountered because of the compact design. The temperature of the appHcator 22 should be maintained at least sufficiently high that the steam issuing therefrom remains superheated until

SUBSTITUTE SHEET it contacts the web.

The superheated steam in the chamber 204 issues through apertures such as the circular aperture 212 and directly impinges on the adjacent surface 211 of the web 210. GeneraUy, as iUustrated in Figure 9, the appHcator 200 (i.e. 22-

22F) wfll be provided with a pluraHty of apertures 212 arranged in a row extending substantiaUy the fuU width of the appHcator which normaUy wiU extend the full width of the web to be treated. It has been found that when utilizing the present invention and applying superheated steam, the size of the apertures that can be employed may be smaUer than the normal aperture size used in the appHcation of saturated steam, however the precise size as indicated by the diameter D and spacing as indicated by the dimension S are not critical and in fact aU of the orifices 112 could be interconnected to form a slot. However, care must be taken to ensure that the total area of the outlet does not apply too much steam or too Httle back pressure on the steam or the velocity of the steam exiting the appHcator may be too low for some applications. In a particular example as wiU be discussed below, apertures of 1/32 of an inch in diameter at 1/lOth of an inch spacing, i.e. D= 1/32 of an inch and S= 1/10 of an inch were found to be satisfactory. Clearly these dimensions may be changed.

The use of a heated appHcator permits the feed or incoming steam to be at a low pressure and not be significantly superheated if at all and the steam to be superheated immediately before appHcation to the web. This is simpler than supplying steam with a degree of superheat to compensate for cooling of the steam by the appHcator and stiU be sufficiently superheated when the steam contacts the web, i.e. preferably with a steam temperature at least 110°C. In some cases depending on the appHcation to which the superheated steam is to be appHed, appHcator temperatures maintained as high as practical generaUy higher than about 125°C. The distance X between the appHcator and the web (see Figure

7) generaUy should not exceed about 1 inch and even at a spacing of 1 inch the superheated steam expands and loses much of its heat before it contacts the paper and wiU not be as effective in reducing the amount of condensate formed around the equipment. Thus it is preferred to maintain the distance X small generaUy less than 1/8 inch and in most cases, particularly when applying

SUBSTITUTE SHEET superheated steam to a paper web for calendering, equal 0, i.e. the appHcator surface 220 (see Figures 8 and 9) will contact the paper web and wiU deflect the web, for example a deflection of about 1/4 inch when positioned in the middle of a 2 foot span has been found satisfactory. It wiU be apparent that the temperature of the surface 220 of appHcator is relatively high and heat will be transferred from the appHcator surface to the web to contribute to the heating of the web. The heat transfer from the appHcator to the web will depend on the speed of the web and degree of contact between the web surface and the heated appHcator and the temperature of the web and appHcator. The distance Y between the appHcator orifices or outlets 112 and the nip 230 between the roUs such as those iUustrated at 232 and 234 (which are representative of the calender nips above described) simply determines time (dependent on web speed) for the steam to transfer heat into the web and then for the surface to cool. Heat penetration is desirable to ensure the surface of the sheet is at elevated temperature to the required depth. It is preferred to position applicators 22 relatively close to the nip and Y generally will be no greater than 2 feet and preferably will be less than 6 inches, and most preferably less than about two inches. It is not clear exactly what the maximum spacing or distance y that may be tolerated is since the surface of the web has been observed to cool rapidly, yet the web seems to retain the characteristics necessary to facUitate calendering.

It wiU be apparent that the speed of the web has a bearing on the maximum length Y that can be tolerated and still have the web in desired condition when entering the nip since the faster the sheet is travelHng the less time to travel distance Y and the less time for cooling. Heat transfer to the web is extremely rapid and cooling is more time dependent.

The roU 232 may, if desired, be heated and may be equivalent to the roUs 12, 38, 42 or 44 described above while the roU 234 which combines with the roU 232 to form the nip 230 may be equivalent to any one of the roUs 14, 36, 50, or 58, i.e. may be any suitable hard or soft roU. It wiU be apparent that the surface 214 heated by the jets issuing from the outlets 212 passes directly into contact with the roU 232 in the nip 230.

In a calendering operation for calendering paper the amount and temperature of the superheated steam appHed coupled with the heat transfer between the applicator and web wUl be sufficient to heat at least the surface of

SUBSTITUTE SHEET the web at least at the point of contact by the steam sufficiently high to modify the characteristics of at least the surface of the web to facUitate the calendering operation and produce the desired surface finish to the web.

It is possible to darken the surface of a paper web by overheating and this should be avoided thus generaUy the surface of the web should not exceed a temperature where darkening of the particular web being treated occurs.

The effectiveness of heating the appHcator was determined by measuring the steam temperatures 1/2 inch away from the outlet apertures of 0 the appHcator with and without the heater activated. When the heater was activated the appHcator block temperature was maintained at about 250°C.

Example 1

Table 1 5 Steam Temperature at X= 1/2 inch

AppHcator

Back Applicator Block AppHcator

Pressure (psi) Heated to 250°C Block Not he.

°C °C nυ

1 204 70

2 171 72

3 160 78

4 144 80 5 5 132 80

As shown in Table 1 with the appHcator heated the steam temperature measured at 1/2 inch from the appHcator surface is considerably 0 higher than when the appHcator is not heated. Also it wiU be noted that as the flow of steam through the device increased (pressure increased), the temperature of the steam decreased, i.e. the heating capabflity of this version of the appHcator was not sufficient to maintain the high temperature at the high flow rate used. When there was no heating of the appHcator the temperature 5 of the steam sHghtly increased as the flow increased but the temperature remained low in comparison with that obtained when the block was heated. Operation without heating the appHcator produces a condensation problem. If the incoming steam to the appHcator were sufficiently superheated this steam could be used to heat the appHcator to above 100°C so that the steam 0 contacting the web would be sufficiently superheated to pre-soften the surface

SUBSTITUTE S of the web.

In this example, the heating chamber of the heated applicator was about 5 inches by 1/2 inch wide and the appHcator was 30 inches long and provided with 6 steam inlets over the 30 inches, i.e. a header with 6 branch pipes was used so that each section or compartment of the appHcator was 5 inches in the cross machine direction.

The outlet from the heated appHcator was composed of a straight row of orifices about 1/32 of an inch in diameter spaced 1/10 inches apart.

Example 2

The temperature of the superheated steam likely to contact the web was measured using an appHcator with two rows of orifices while varying the steam flow rates. The block temperature was maintained in the range of about 225-250°C (block temperature is to a degree dependent on the temperature and flow rate of the incoming steam).

The temperature of the steam was measured at different spacings from the appHcator, i.e. distance X was varied from 3/64th of an inch to one inch.

It is apparent from Table 2 that at the lower flow rates the temperature of the steam is relatively high at short distances of X, i.e. up to about over 200°C whereas at the large spacing, i.e. X = 1 inch, the temperature of the steam was about 120° at .02 psi pressure and reduced to 77° at the higher flow rates of 0.16 psi. It is apparent that the shorter the spacing of the temperature probe from the applicator the higher the temperature and that up to spacings of 5/8th of an inch the steam temperature, regardless of the steam flow under the conditions examined, did not reduce below 100°C or become non-superheated (this occurred at 3/4 inch spacing and a back pressure between .04 and .09 psi and at 1 inch spacing somewhere between 0.02 and 0.03 psi back pressure). It wiU be apparent that under most conditions examined the steam temperature appHed to the web wiU exceed 115°C and that the flow combined with the heat transfer from the appHcator itself wUl ensure that the surface of the web is significantly heated.

SUBSTITUTE SHEET Table 2

Inches Spacing from Applicator Surface

X 3/64" 1/8" 1/4" 1/2" 3/4" 1"

Internal Steam Block Steam Block Steam Block Steam Block Steam Block Steam Block Chamber Temp. Temp. Temp. Temp. Temp. Temp. Temp. Temp. Temp. Temp. Temp. Temp. Pressure

°C

PSI

0.00 200 243 146 252 88 243 65 248 59 255 52 244

.01 216 233 197 238 170 238 143 239 139 242 137 247

.02 220 234 211 237 201 236 156 234 132 243 120 246

.03 216 232 206 234 204 233 143 228 122 239 90 236

.04 215 230 204 232 203 232 130 229 108 235 86 233

.09 215 230 204 231 204 231 126 229 99 231 81 231

.12 215 230 205 230 204 230 125 230 95 229 80 231

.13 214 229 204 230 204 230 123 232 91 227 79 230

.15 213 227 204 228 204 230 125 234 94 228 77 231

.16 213 228 202 228 204 229 119 229 91 228 76 231

.16 213 228 202 227 203 230 114 227 91 229 79 230

.16 212 227 202 228 203 229 113 225 90 229 77 231

At the end of the experiment the heater for the block was turned off and the steam was appHed at fuU flow rate, i.e. 0.16 psi back pressure and the rate of cooling of the appHcator was measured. At the commencement of the measuring the temperature was about 230°C After 3 minutes it had dropped 10 degrees. After 5 minutes it had dropped about 30 degrees. After 8 minutes over 50 degrees to a temperature of about 180° and after 10 minutes was down to about 165° for a total temperature drop over 10 minutes of almost 70°C thereby clearly indicating that the heater was contributing significantly to the temperature of the steam.

It wiU be apparent that with a different heater or more effective heater, the actual heat transferred to the steam in the applicator may be changed.

Example 3

A series of tests were run using a steam back pressure of 0.12 psi and the steam temperature was measured 1/16 inch from the appHcator block as the appHcator block was heated. Table 3 shows the results obtained.

SUBSTITUTE SHEET Table 3

AppHcator X= 1/16 inch BlockTemp., °C Steam Temp., °C

100 100.1 105 100.6 110 102.5 115 111.4 120 113.0 125 119.0 130 125.8 135 129.0 140 132.8 145 137.6 150 141.7 155 146.2 160 149.7 165 153.6 170 157.4 175 162.0 180 165.7 185 169.6

190 177.4

195 180.6

200 185.7

205 193.4 210 195.3 215 199.0 220 202.0 225 210.0 230 214.0 235 218.0

StabUized at approx. 236°C (block)

For the heater used the highest block temperature reached was about 236°C.

It wiU be apparent that with the block at at least 100°C the steam issuing therefrom is stiU sHghtly superheated at the 1/16 inch spacing. At an applicator block temperature of 110°C the steam had about 2.5° superheat at the 1/16 inch spacing. Obviously if the steam temperature could be measured closer to the block it would be even higher.

Example 4

As above indicated in the discussion of the prior art, temperature has a very pronounced effect on gloss and moisture content has more effect on roughness, by adjusting

SUBSTITUTE SHEET appHcator block temperature and back pressure, i.e. flow, one could obtain any reasonable desired ratio of temperature to moisture content in the steam contacting the paper so that for any given paper the device can be fine tuned to optimize the properties of the paper. In a comparison the present invention with saturated steam appHcation, paper treated in a conventional supercalendering operation using a calender arrangement having 11 nips and 5 steam showers (3 showers appHed to one side and 2 to the other) was compared with essentiaUy the same paper treated in a supercalender having 10 nips and 2 appHcators of the present invention operating with a spacing X = 0 and a block temperature of 160°. Table 4 compares the results obtained at the same calender loading. The nip roU temperatures used were about 80 to 90°C.

Table 4

Change in

Density - Gardiner Parker Porosity grm/cm Gloss 75% Roughness ml/min

% (Units)

5 Conventional +.02 +3% ■15 Steam Showers 11 Nips

2 AppHcators of Present Invention + .13 + 15% -42 10 nips

The above tests were on a supercalender, however similar relative improvements are obtainable using soft calendering or hard nip calendering.

WhUe the above description is related primarily to calendering, it wiU be understood that the appHcator may be used in a variety of different appHcations, for example, in the press section of the paper machines it is possible to use the present invention to apply superheated steam, i.e. more BTU's per pound of moisture added to the paper and practice impulse drying. It wiU also be apparent that because the temperature of the steam issuing from the appHcator is controUed by the heat added in the appHcation by providing suitable heat transfer means the desired degree of heat (temperature) to moisture content can be adjusted within limits immediately before appHcation to a web by simply changing the degree of superheating. If more moisture is required more steam is appHed and thereby the ratio of temperature to moisture may be reduced or alternatively

SUBSTITUTE SHEET more temperature may be appHed to less steam to increase this ratio as desired for the particular appHcation. This also permits cross machine moisture profiling.

Similarly in the above description the single appHcator has been used, obviously a pluraHty of appHcators side by side might be used to increase the amount of superheated steam directed at the web or one applicator with sufficient size heaters and retention time but having more than one row of apertures may be used.

The size of the appHcator may be made very small in comparison to conventional steam showers or appHcators, thus it may be appHed in locations when a conventional steam appHcator does not fit. Also because it can apply superheated steam, i.e. is maintained at a temperature above 100°C preferably above 105°C or 110°C it can be used to apply heat and moisture to webs travelling thereby in other equipment than paper making equipment for example in corrugators for making corrugated paper board. Also instead of smoothing and improving printabiUty the nip roUs might apply a pattern to the web for example to obtain a sheen or matte finish or other pattern. As shown in Figures 10 and 11, the applicator 200A is simUar to the applicator 200, however the main surface 220A through which the apertures 212A are formed is bowed, i.e. is convex in shape. The paper web 212 passes over the bowed surface 220A under tension and thus conforms with the bow to the surface. In the Ulustrated arrangement the surface 220A is raised in the centre and lower in the edges where the web leaves the surface, which tends to expand the web laterally, i.e. make the web wider by tending to expand lateraUy as iUustrated by the arrows 300 and 302, in the same manner as a web is expanded by bowed roll. If desired the orientation of the curved surface to the web could be arranged to make the web tend to contract laterally by applying the web to the surface 220A at a location where the surface 220A is longer than the length of the surface 220A at the location where the web is separated from the surface.

AppHcation steam delivered to the applicator 212A via spaced pipes 306 leading from a steam header 304, issues from the apertures 212A (equivalent to the apertures 212 of the Figure 8 and 9 embodiments) and heats the web 211 in the same manner as the steam from the jets 212. In the Figure 10 and 11 embodiment, the web is forced against the appHcator surface 220A and thus the appHcation steam issuing from jets 212A also acts as a lubricating film or fluid bearing to facUitate transport of the web over the surface 220A.

In the Figure 10 and 11 arrangement the appHcator 212A has an indirect fluid heat exchanger. The indirect heater is formed by a plurality of passages 310 through the

SUBSTITUTE SHEET appHcator 200A. A heated fluid such as steam is introduced into one end of the indirect heat exchanger as indicated at 308, passed through the passages 310 to transfer heat to the appHcator (and appHcation steam) and is removed via the line 312.

The heating fluid in the passages 310 may be any suitable fluid at the required temperature and capable of transferring the required heat to the appHcator and appHcation steam. Such fluids include, for example, high temperature steam, high temperature oU, etc. If steam is used as the heating fluid at least some of the steam in line 312 could be fed as appHcation steam into header 304.

Having described the invention modification wiU evident to those skiUed in the art without departing from the spirit of the invention as defined in the appended claims.

SUBSTITUTE SHEET

Claims

1. A method of applying steam to a surface of a web comprising delivering steam in superheated condition from an appHcator located immediately adjacent to a surface of said web to be impinged by said steam, maintaining said appHcator at a temperature above 100°C and impinging said steam on said surface without significant condensation of said steam other than on said web.

2. A method as defined in claim 1 wherein said steam appHcator is maintained at a temperature of at least 110°C.

3. A method as defined in claim 1 wherein heat is transferred to said steam in said appHcator immediately before said steam leaves said applicator for application to said surface.

4. A method as defined in claim 3 wherein said appHcator is maintained at a temperature of at least 110°C.

5. A method of calendering of paper comprising applying superheated steam directly from an appHcator to a surface of a web maintaining said appHcator at a temperature of above 100°C so that said superheated steam contacts said web and heats at least said surface of said web and immediately passing said web whUe at elevated temperature produced by the appHcation of superheated steam into a nip formed by a pair of roUs one of which contacts said surface of said web.

6. A method as defined in claim 5 wherein said appHcator maintained at a temperature of above 110°C.

7. A method as defined in claim 5 wherein heat is transferred to said steam in said appHcator.

8. A method as defined in claim 7 wherein said appHcator is maintained at a temperature of above 110°C.

9. A method as defined in claim 5 wherein said web is positioned so that the distance between said surface of said web and a surface of said appHcator from which said superheated steam is discharged is less than 1/2 inch.

10. A method as defined in claim 8 wherein said web is positioned so that the distance between said surface of said web and a surface of said appHcator from which said superheated steam is discharged is less than 1/2 inch.

11. A method as defined in claim 9 wherein said distance is zero and wherein said surface of said web and said surface of said appHcator from which said superheated steam

SUBSTITUTE SHEET is discharged are in contact and heat is transferred from said appHcator to said web.

12. A method as defined in claim 10 wherein said distance is zero and wherein said surface of said web and said surface of said appHcator from which said superheated steam is discharged are in contact and heat is transferred from said appHcator to said web.

13. A method as defined in claim 5 wherein appHcator is maintained at a temperature above 125°C.

14. A method as defined in claim 7 wherein appHcator is maintained at a temperature above 125°G

15. A method as defined in claim 11 wherein appHcator is maintained at a temperature above 125°C.

16. A method as defined in claim 12 wherein said appHcator is maintained at a temperature above 125°C.

17. An appHcator for applying superheated steam against a travelling surface of a web, said appHcator comprising a chamber means for introducing said steam into said chamber, heater means for heating said chamber and increasing the temperature of said steam, outlet means from said chamber, means for moving said web past said outlet means, said outlet means opening directly toward a surface and web to be contacted by said steam, means for relatively positioning said web and said outlet means in close proximity to ensure that said steam at elevated temperature issuing from said outlet impinges directly on said surface of said web without significant cooling of said steam between said chamber and said web.

18. An appHcator as defined in claim 17 wherein said means for relatively positioning positions said web in contact with a surface of said applicator through which said outlet means opens as said web traverses said applicator.

19. An appHcator as defined in claim 18 further comprising a calender nip into which said web passes after appHcation of said steam said surface of said web.

20. An appHcator as defined in claim 17 wherein said heater means comprises an indirect fluid heated heat exchanger in said appHcator.

21. An appHcator as defined in claim 18 wherein said surface is a bowed surface.

SUBSTITUTE SHEET