FLOATER DRYER AND PROCEDURE FOR ENHANCING ITS OPERATION

Abstract: A floater dryer for drying a web-shaped (W), moving material, in particular a paper or cardboard web. The dryer comprises a plurality of radiations/air blowing units (10) which are provided on one side of the web (W) or on both sides (10A, 10B). Said units (10) have been shaped to be blow boxes having on their side facing the web (W) a contact-free carrier surface (21R, 27) and into conjunction with which is blown, through a nozzle aperture (20a, 20b) opening on the leading and/or trailing edge of said carrier surface, an air jet (Ful), or air jets (Fua, Fub). The jets will have a substantially large component parallel to said carrier surface. In conjunction with the radiation/air blowing units (10) are provided radiation elements (30) from which into the treatment interval (P1, P2, P-) is directed radiation (SO) through a window (27). The window at the same time serves as carrier surface for the air support of the web (W). The air flows (F3, F4) from the blow box are also conducted to serve as cooling air for the radiation elements (30) and for components in conjunction therewith. On the side opposite to the carrier surface (27, 21R) of the radiation/air blowing units (10) may be provided a mirror arrangement (32, 33) which returns radiation that has passed through the web (W), back to the web (W).

Description: Floater dryer and procedure for enhancing its operation The object of the present invention is a floater dryer for drying a web-like material, particularly a paper or cardboard web, said dryer comprising a plurality of radiation/air blowing units which are provided on one or both sides of the web, -and said units being de¬ signed to be blow boxes of which the side facing the web is a contaσt- free carrier surface and in conjunction with which is blown an air jet, or air jets, through a nozzle aperture opening onto the leading or trailing edge of said carrier surface, and which jets will have a cσrriponent of substantial magnitude parallel to the plane of said carrier surface. In addition, the invention concerns a procedure for enhancing the drying of web-like moving material wherein combined radiation and air drying is applied, this being implemented with -combined radiation/air blowing units, through their nozzle slit or slits an air blow jet or air blow jets being directed into a treatment interval, and the web that is being dried being supported with their aid without contact. In prior art so-called floater dryers are known in which a paper web, a cardboard web or equivalent is dried without contact. Floater dryers .are for instance used in paper coating apparatus after a roll or brush applicator to support without contact and to dry the web which is wet owing to the coating substance. Various drying and supporting air nozzles and arrays thereof are applied in floater dryers. Said blow nozzles may be classified by two groups: over¬ pressure nozzles and subatmospheriσ pressure nozzles, both kinds being applicable in the floater dryer and procedure of the invention. The craπmonest floater dryers of prior art in present use are exclus- ively based on air blowings. Partly for this reason, the floater dryers became rather bulky because the distance ever which the floater dryer is active has to be rather long so that high enough drying effect might be achieved. In part, these drawbacks are due to the fact that in air drying the penetration depth of drying is rather minimal. In prior art various types of dryers are known which are based on the effect of radiation, above all of infrared radiation. Using infrared radiation affords the advantage that the radiation has a fairly high penetration depth, which increases with decreasing wave- length. Application of infrared dryers in drying a paper web has been hampered, among other t_hings, by fire hazard because infrared radiators attain rather high temperatures, e.g. 2000 °C if it is desired to achieve a drying radiation with sufficiently short wave¬ length. Reg-arding the state of art, reference is furthermore made to DE OS 2351280, which discloses a certain kind of α__ιibination of floater dryer and infrared dryer operating with over-pressure nozzles. In the patent application just cited is disclosed a one-sided floater dryer cσ prising consecutive nozzle boxes spaced in relation to each other. These boxes have nozzle slits on their marginal parts, through said slits air jets being directed against the web thereabσve, speci- fically at right angles, these jets when they meet the web being deflected outward at the nozzle box. Between said nozzles infrared radiators have been disposed which fill the interval between nozzles. As far as the present applicant is aware, said dryer has not came into any widespread use, at least, which is believed to be due to the C-rαimstance that it has not been understood, neither structurally nor in the energy economy respect, in said nozzle design to combine air and radiation drying in an advantageous way. The structure is moreover one-sided, and it __equires rather much space in the direction of travel of the web if one wishes to attain high enough drying power, for instance in paper after-treatment installations. A drawback with major effect encumbering said DE-OS, and other infra dryers of prior art, is that in them the space between the infra- dryer and the web being dried is not ventilated, with the consequence that the humid air in said space absorbs radiation and this lowers the efficiency. In infra-dryers of prior art, moisture transfers from the surface of the web that is being dried to the air virtually only by effect of free convection, and this lowers the evaporating power. The main objective of the present invention is to avoid the drawbacks outlined in the foregoing. The aim of the invention is to provide a novel floater dryer -cαrtibining air and radiation drying which is more advantageous than any earlier design in the st__ucture of the dryer installation as well as its energy economy, and a procedure for enhancing the operation of a floater dryer. The object of the invention is also to provide a combination of air and radiation dryer which presents a lower risk of fire compared with floater air dryers of prior art. An additional object of the invention is to provide a combination of air and radiation dryer in which the contact-free floater dryer can be made shorter and more compact than before. Hereby irachine hall space will be saved, and better energy economy will be promoted. In order to achieve the aims presented, and others which will become apparent later on, the floater dryer of the invention is mainly characterized in that in conjunction with said radiatioiVair blowing units have been disposed radiation elements from which radiation is directed into the treatment interval through a window, said window at the same time having been arranged, on its part, to serve as carrier -surface in the air support of the web, and that the air lows of said blow box have in part at least been conducted to serve as cooling air for the radiation elements and for -components adjacent to them. e proc-edure of the invention for enhancing the operation of a floater dryer is mainly ch__raσterized in that the drying radiation is directed on the web in said air supporting and air drying interval, that the treatment interval is ventilated with said air blowing, or blowings, and the air boundary layer in conjunction with the web is broken up in order to enhance the drying effect of radiation, and that the treatment air is used to ventilate and cool the components and spaces in conjunction with the radiation elements. By a ccmbining a floater dryer and a radiation dryer in the way taught by the invention the following advantages, among others, are gained over radiation dryers of prior art. Evaporation from the web is made more efficient even" if the power output of the radiation source were reduced to seme extent. The degree of efficiency of the drying process can be increased because the important interval between the infra- radiator and the web is ventilated. The dryer can be built to be more enclosed than before and larger drying units can be used than at present. Moreover, the dryer can be thermally lagged with greater efficiency than before. In the invention the supporting and drying air blowing is used towards lowering the terrpe__ature of the radiator structure, e.g. of the quartz glass or equivalent, whereby the fire hazard will be less. Compared with floater dryers of prior art exclusively b-ased on drying and supporting blow jets the following advantages, among others, are gained. Higher evaporation capacity than before can be irplemented, and control of the drying profile in the transversal direction is feasible more advantageously than before. Control of drying pαw-ar becomes possible, if desired separately for each unit and/or radiation element, whereby good profile management is achieved. since in the invention the drying energy is advantageously supplied predominantly in the form of infra-radiation, the air apparatus and duct -system, which used to require much space, can be substantially reduced and thereby smaller apparatus dimensions become possible even though the apparatus unit size can be increased from what it was before. It is possible in the dryer of the invention to use for nozzle struc¬ tures either over-pressure nozzles or subat_to.sph.sric pressure nozzles, which -are substantially similar to the well-known FLOAT or FOIL nozzles. In the invention, the protective glass of the infra-lamps advantageously serves as carrier surface. In the invention, the supporting and drying air is advantageously used to cool the holders of the d_r_fra-lamps and other -components in the vicinity, and at the same time the air itself is warmed up and the dry air which has been warmed up in this manner is conducted with the aid of slit nozzles against the web in such manner that the blowing has a component of sub-stantial magnitude parallelling the web. In the following, the invention is described in detail, referring to certain eirfoodiment examples of the invention, presented in the figures of the drawing attached, to the details of which the invention is in no way narrowly confined. Fig. 1 presents in schematic elevational view a -combined radiation and air dryer according to the invention. Fig. 2 presents, on a larger scale, in vertical section the design of the combined radiation and air dryer unit applied in the floater dryer of Fig. 1. Fig. 3 presents the subatmospheric pressure nozzle of the invention and its geometry. As shown in Fig. 1, the web , for instance a paper or cardboard web coated on both sides in a roll or doctor coater, is being dried and at the same time treated without contact with a radiation/air dryer according to the invention. In the dryer, the path of the web W is gently wavy. The dryer comprises a plurality of elongated radiation blowing units 10A above the web, extending in the cross-web direction, and similar units 10B below the web. The topside and underside units 10 have been disposed in j tercalation, halfway in the intervals L ' between the opposed units 10. The units 10 operate with over-pressure, producing -an over-pressure region P j _ in the space between their carrier surface 27 and the web . The upper .and lower units 10 may be identical. In the following is described the design and operation of the unit 10, referring to Fig. 2. The unit 10 comprises a nozzle box of symmetrical design with refer- ence to its central plane K- . The nozzle box comprises an end wall 11 having an aperture lie for inco irig air Fη_. The box has vertical outer walls 12a and 12b and vertical walls 13a and 13b. The walls 12a,13a, respectively 12b,13b, define between themselves side spaces 15a and 15b, which continue on the side facing the web , forming nozzle apertures 20a,20b, which are confined on the outside by the chamfered marginal parts 16a,16b of the outer walls. The nozzle apertures 20a,20b are confined on the inside by L-shaped corner parts 21a,21b. of which the outer surface 21R is rounded with radius R, starting at the nozzle apertures 20a,20b. Between the walls 13a and 13b is provided a mirror wall 19 reflecting, the radiation S 0 , with a thermal lagging 18 thereabσve. Below the .wall 19, wall portions 24a and 24b resistant to high t_eιιpera.ture have been provided, on their outer sides infrared radiator elements (1-arnps) 30 being fixed in holders 29a and 29b, there being a plurality of -such radiators in succession in the transversal direction of the web . The radiation space 31 of the radiator elements 39 towards the web is confined, on the side facing the web, by a quartz glass window 27, which lias been mounted in grooves 28a and 28b of the L-parts 21a and 21b. Electricit_y is carried to the infra-radiators 30 by means of leads 26a and 26b, whic have been fixed on connection strips 25a and 25b on the outer sides of the walls 24a and 24b. A mirror 33 on the opposite side of the web , in conjunction with which a thermal lagging 32 has been provided, cooperates with the unit 30. In the following the operation of the floater dryer and the steps of the procedure of the invention shall be described with reference to Figs 1 and 2. The dicing/supporting air is introduced through the apertures lie of the units 10 in the space 17, whence it is distrib- uted through apertures 14a and 14b as a flow F2 into the side spaces 15a and 15b, whence cooling air flows F 3 for the infra-radiators 30 are conducted into the space 23 through apertures 22a and 22b in the walls 24a and 24b. Moreover, flows F4 are conducted from the spaces 15a and 15b through slits, apertures or equivalent at the top end of the walls 21a and 21b into the radiations space 31, to serve as cooling air for the radiator elements 30 and for -components adjacent to them. Said cooling air is discharged e.g. into an air recircu- lation or through the grooves 28a and 28b of the radiation window, or through other apertures, into the space Pτ_. In the same manner described above, the drying -and supporting air can be efficiently utilized also towards cooling the infrared radi¬ ators and components in their vicinity, and the air which has thus been warmed up can be efficiently utilized in web drying -and supporting. Through the nozzle slits 20a and 20b, air jets Fua and Fub are di- rected against each other into the space Pη_, where a drying effect is exerted on the web , in addition, by the radiation S 0 from the infrared radiators 30, this radiation entering through the window 27. The window 27 contributes to fanning the carrier surface of the air nozzles 20a,20b, The air jets Fua and Fub are not directed at right angles against the web : they are specifically directed under a suitable angle a against each other. The magnitude of the angle a is between 40 and 70 degrees as a rule. The curved outer surface 2R of the L-shaped walls 21a and 21b serve as curved Coanda surfaces of the nozzles 20a and 20b, which "draw" the flaws Fua and Fub towards each other and onto the side of the drying interval Pτ_. The flows Fua and Fub create in the drying interval P ] _ an over¬ pressure region which keeps the web at an appropriate distance from the carrier surface of the air nozzles. At the same time, a-ccording to the p__oc-edure of the invention the flows Fua and Fub efficiently break up the boundary air layer in conjunction with the web and promote the effect of the radiation drying SQ on the web W. Furthermore, the flows Fua and Fub ventilate the drying intervals Pτ_ and thereby reduce the harmful absorption of infra-radiation Sg in the drying interval. Part of the radiation S 0 passes through the web , -and this radiation is returned to constitute the radiation S 2 drying the web W in the drying interval P 2 . Thus, one achieves with the blow effects Fua and Fub of the invention, in addition to normal air support and air drying effect, enhancement of the radiation drying effect of the infra-radiation elements 30, which have even st__uctur_ιlly been advantageously integrated in the units 10. The width s of the nozzle apertures 20a,20b is usually on the order of s = l-2mm. The gap e-_ of drying interval Pτ_ is usually on the order of eτ_ = 5-40 mm, and the gap e 2 of the drying space P 2 between , the irtirror 33 and the web, usually on the order e 2 = 5-40 mm. It is essentiaal in the invention that the blow effects Fua and Fub are not directed at right angles against the web but instead under a given angle a towards the carrier surface 27, whereby said blow effects produce the above-described radiation drying-boosting effect, in addition to the effects known in the art. It is advantageous if the blow nozzles 20a and 20b are so oriented that the blow jets do have a certain component perpendicular against the web W, because thanks to this -component, combined with the other factors, the bound¬ ary air layer present in conjunction with the web can be successfully broken up. m Fig. 3, the length of the mirror 33 has been denoted with L Q . This length L- Q is substantially equal to the length in the direction of the web of the treatment interval Pι,P 2 « ~--~ ratio of said length LQ to the distance L between units 10 is IQ/L = 0.3-0.7, preferably about 0.5. Fig. 3 presents a schematic cross section of the subatmospheric pressure nozzle of the invention, which has only on one margin of its unit 10c a nozzle aperture 20a, from which a blow jet Ful is blown out under the angle a with reference to the web . This blowing produces in the air/radiation treatment interval P- a .subatmospheric pressure region, which in a manner known in itself in the .art supports and stabilizes the web . In the radiation interval P- the radiation drying effect Sg is exerted with infrared elements 30. The infrared window 27 has been fixed between L-shaped holders 21a and 21c, in grooves 28a and 28c in the latter. The infra-elements 30 are mounted between holders 29a and 29c, these holders being affixed to the walls 24a and 24c. The unit 10c has another end wall 12c without nozzle aperture in its conjunction, and the flow Fu2 blown into the treatment interval P- discharges as a flow Fu2 at the wall 12c. In other parts the design is like that of Fig. 2. The floater dryers of the invention are either one-sided or two- sided but most appropriately, and to the greatest efficiency, they are two-sided -and it is to advantage to use in them the mirror el¬ ements 33 described in connection with Fig. 2, by which the infra- radiation that has passes through the web is returned to dry the web . It is advantageous in the floater dryer of the invention if the major part of the drying energy is directed against the web specifi- cally in the form of radiation Sg, whereby the air apparatus can be made small in size and the efficiency can be improved. For example, 70 to 90% of the total drying energy are rradiation energy and the remainder are energy introduced together with the drying and .support¬ ing air. When, as taught by the invention, a plurality of radiation elements 30 lying in parallel in the cross-web direction are used, the moisture profile of the web W in the transversal direction can be advan¬ tageously controlled by making adjustable the electric power which is fed each radiation element 30 or to different groups of -such elements. It becomes possible, in this way, to control the moisture profile even very accurately, and steeply; this is further assisted by the fact that the greater part of the drying energy is directed against the web W specifically in the form of radiant energy. More¬ over, the overall level of drying can be controlled by controlling the power level of the elements 30. These controls are faster and more accurate, and implementable more simply, compared with the alternative that said --controls would be effected in the way of prior art, i.e., by -oontrOlling the air quantities or the state of the drying air. The latter modes of control are particularly awkward in profile control, and they lead to complex apparatus designs. In the present invention also that important advantage can be realized that the quantity and velocity of the blow air may be selected speci¬ fically in view of web support and stabilizing, so that a maximally trouble-free and stable passage through the dryer is achieved; this is obvious since the drying effect proper can be adjust-ed and con¬ trolled by controlling or setting the power of the radiation Sg. In the following are stated the claims, various details being -allowed to vary within the inventive idea thereby defined and to deviate from that which has been presented in the foregoing by way of example only.

Claims: Claims 1. A floater dryer for drying a web-shaped (W) , moving material, in particular a paper or cardboard web, said dryer cαitprising a plurality of radiation/air blowing units (10) which are provided on one side of the web (W) or on both sides (10A,10B) and which units (10) have been shaped to be blow boxes having on their side facing the web (W) a contact-free carrier surface (21R,27) and into conjunction with which is blown, through a nozzle aperture (20a,20b) opening on the leading and/or trailing edge of said carrier surface, an air jet (Ful) , or air jets (Fua,Fub) , which will have a substantially large ■component parallel to said carrier surface, characterized in that in conjunction with said radiation/air blowing units (10) have been disposed radiation elements (30) from which into the treatment inter- val (P]_,P 2 ,P-) is directed radiation (S 0 ) through a window (27), said window being arranged at the same time, in part, to serve as carrier surface for the air support of the web (W) , and that the air flows (F3,F ) from said blow box are at least in part conducted to serve as cooling air for the radiation elements (30) and for cσrtpo- nents in conjunction therewith. 2. Floater dryer according to claim 1, diaracterized in that on the side opposite to the carrier surface (27,21R) of the radiation/air blowing units (10) is provided a mirror arrar_gement (32,33) which lias been arranged to return radiation that has passed through the web (W) , back to exert its effect on the web (W) . 3. Floater dryer according to claim 1 or 2, ciiaracterized in that the dryer has been arranged to act two-sidedly on the web in that it comprises at a given spacing (L) air/radiation units (10A,10B) on both sides of the web (W) (Fig. 1) . 4. Floater dryer according to any one of claims 1-3, -characterized in that in the air/radiation units (10) nozzle slits (20a,20b) have been provided on both sides of the carrier surface (27,21R) so that in conjunction with the carrier surface an over-pressure region is ' formed which serves as treatment interval (P ] _) , and that said opposing nozzles have been arranged to blow air jets (Fua,Fub) under a given angle (a) against each other (Fig. 2) . 5. Floater dryer according to claim 4, characterized in that the glass window constituting the radiation window (27) is mounted in two, preferably L-shaped, parts (21a,21b), the window (27) being fixed in grooves (28a,28b) in their outer margin, that the outer surface (2IR) of said L-shaped parts (21a,21b) or equivalent is curved with a given radius (R) , and that said curved outer surface (21R) constitutes a Coanda surface in conjunction with the nozzle apertures (20a,20b) , by said Coanda surface the air jets (Fua,Fub) being deflected towards the treatment interval (Pτ_) . 6. Floater dryer according to any one of claims 1-5, characterized in that the interval air flows (F 3 ,F 4 ) in the radiation blow box have been conducted into contiguity with the spaces (23,31) of the radiation elements through apertures (22a,22b) or equivalent, to cool said spaces, their walls and coitponents in conjunction therewith. 7. Floater dryer a-ccording to any one of claims 1-3, characterized in that on only one side of the carrier surface (20a) of the air/ _ radiation units (10c) there is a nozzle aperture (20c) from which an air jet (Ful) can be directed into an air drying .and air support interval (P-) with subatmo-spheriσ pressure (Fig. 3) . 8. Procedure for enhancing the drying of a web-shaped, moving ma¬ terial, in said procedure being employed combined radiation and air drying, implemented with combined radiation/air blowing units (10) , i rσugh a nozzle slit (20a), or nozzle slits (20a,20b), an air jet (Ful) , or air jets (Fua,Fub) , being directed into a treatment interval (Pτ_,P-) by which the web that is being dried is supported without contact, c___aracte__ized in that the drying radiation (Sg) is directed on the web (W) in said air sι_pporting and air drying interval (P 1 ,P 2 ;P-), that with said air jet (Ful), or air jets (Fua,Fub), said treatment interval (Pη_;P-) is ventilated and the air boundary ' layer in conjunction with the web (W) is broken up for enhancement of the drying effect elicited by the radiation (S 0 ) , and that with the treatment air components and spaces in conjunction with the radiation elements are ventilated and cooled. 9. Procedure according to claim 8, characterized in that the radiation window is in part utilized as air carrier .surface in said drying interval (Pχ;P-) . ιo. Procedure according to claim 8 or 9, c__ιarac±erized in that the p__Ocedure is applied either in treatment intervals (P T J carrying over-pressure or in treatment intervals (P-) carrying subatmo-spheric pressure. ιι. Procedure according to any one of claims 8-10, -characterized in that the drying radiation (S j ) which has passed through the web (W) being dried in the prccedure and being supported without contact is returned into the web (W) with the aid of a mirror arrangement (33, 32). 0 12. Procedure according to any one of claims 8-11, c-haracterized in that the proportion of radiant energy used in the procedure is 70 to .90% and that of the drying air, 30 to 10% of the total drying energy. 5 13. Procedure according to any one of claims 8-12, diaracterized in that the radiation power of different radiation elements (30) , or groups of radiation elements, lying in parallel in the direction across the web (W) is separately controlled in order to control the moisture profile of the web (W) that is being dried. 0 5

Inventors: KORPELA MATTI, KORPELA, MATTI
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1987
Main classification: D21F5
IPC codes: B05C9, B05D3, D21F5, F26B3, F26B13