FR2556014A1 - Hood with a differentiable drying flow for correcting the moisture profile of a papermaking sheet, and corresponding correction process - Google Patents

Abstract

The invention relates to a hood 1 divided into a number of compartments 2, 3, 4, themselves comprising spaces for blowing hot air 2a, 2b, 2c, and suction spaces for recycling 2A, 2B etc. The blown air is modulated in temperature by batteries of electrical resistors R1, R2, R3. Obtaining a very high evaporation power without disadvantages.

Description

Differentiable drying flow hood for correction of a paper sheet's hunidite profile, and corresponding correction method.

 The invention relates to the field of paper machines, and more specifically the part of the machine where the sheet is dried.

 We know that a paper sheet contains a very high proportion of water at the exit of the canvas.

The leaf then passes into a dry area where the required proportion of water is removed.

 This operation leads to a sheet having a moisture profile, in the cross direction, relatively irregular.

 The invention relates to a device for correcting this profile, that is to say, to mitigate its irregularities, because such irregularities are of course very detrimental to the subsequent behavior of the sheet.

These problems have been known for a long time and there are several devices to make these corrections,
A first known system is one where the sheet is dried and re-wetted in a regular pattern by water spraying, either directly on the sheet or on the cylinder felt, modulated rinsing means. This system of over-drying rehumidification is obviously very energy consuming, does not correct the cause of the irregularity of profile found in the paper (which makes the phenomenon worsens, with adverse consequences at the machine level itself, for example fouling of the felt), and induces a risk of "curl" or "rolling due to the irregular rewetting of the paper sheet at the end of drying.

 A second known system is to blow hot air on the sheet by means of a hood by modulating the flow of air blown.

Such a system can only work - and is only used on - known as 'sANKEE'.

These special machines are equipped with a cylinder of very large diameter called "cylinder
YANKEE "These machines are either monocylindrical or multi-cylindrical, it being understood that in either case the greater part of the drying is carried out on a very large cylinder and the still wet sheet is plated and literally glued to this cylinder by a press.

 This characteristic makes it possible to use relatively high blowing powers (and thus achieve a reasonable evaporation power) without the sheet coming off the cylinder and "bat".

 The disadvantage of known devices for blowing hot air is that to obtain the desired result, one must use a very high blowing power.

 On a machine containing several drying cylinders, such a blow causes the leaf to beat at the places where the sheet passes from one cylinder to the other, which is obviously rhédibitoire.

This is the reason why such systems are only about satisfactory on single-cylinder machines where the sheet is mechanically plated on the cylinder.

 Systems are also known where the correction is carried out by infrared radiation.

 These systems are suitable for multicylindrical dryer machines, and use batteries of lamps, tubes, refractory bricks preferably emitting IR radiation whose wavelength and temperature function, which conditions the penetrating power and the power of the radiation. . These modulated power systems have significant limitations. In the first place, the applicable power per unit area is limited because of the risk of fire. Secondly, the evaporation capacity is limited among other factors because of the time required for the migration of water from the heart of the sheet to the surface.

 The result is that the evaporation power of the system of this type known under the name "IMPACT" or "INFRO" can not exceed forty kilos of water / m2 of surface covered by the device.

 With a higher evaporation power, there is a loss of surface resistance and a beginning of thermal degradation which results in scorching of the surface of the sheet. This phenomenon is of course rhédibitoire for the use of paper.

 On the other hand, the paper coming out of this system has been heated in places and there are problems at the time of the passage of the sheet on the "finishing" systems such as the bar located at the top of the machine.

 In addition, such a system is no longer suitable for so-called "heavy" papers (as soon as the grammage exceeds 90-100 g / m2). In particular, for such papers, the application of a suitable evaporation power is accompanied by a burning of the surface and a heart-damaging moisture residue to the paper itself.

 We still know a system working with electromagnets according to the principle of FOUCAULT currents. This system is currently only applicable to YANKEE dryers. The electromagnets are placed under the cylinder; it is therefore necessary, on the one hand, to have sufficient space and, on the other hand, a cylinder driven by a motor powerful enough to withstand the braking induced by the electromagnets. Such a system is therefore absolutely not suitable for a normal dryer with several small cylinders.

 Despite their inconsistencies, some of these systems are used, for lack of more powerful devices.

 There is therefore a need quite marked for a device having at least the advantages of known systems without the disadvantages.

 The invention achieves this result by a device allowing a very high evaporation power while remaining very flexible in use and leading to a correction efficiency at least equal to that of the best prior systems.

 Other features and advantages of the invention will be better understood on reading the description which follows and with reference to the accompanying drawing, in which the single figure shows an embodiment of the invention. the references have the following meanings: hood 2,3,4: compartments 2a, 2b, 2c, 3a,...: air blowing volumes cbaud 2A, 2B, 3A, ...: aspiration volumes of blown air, intended for recycling.

 5: Papermaking sheet, cross direction.

6: lower edge of compartments 7: fan
R1.R2.R3: modulated power electric resistors
by thyristor
Vl.V2.V3: Blown air flow modulation valves
with manual remote control V'1.V'2.V'3: recycled flow modulation valves
V5 manual remote control: Exhaust air control valve
V6: Air flow control valve new T1.T2.T3. : Thyristor regulation boxes S1.S2.S3. : Temperature probes
The means of the invention therefore consists in blowing hot air on the sheet, this hot air being channeled by a number of compartments distributed in the cross-machine direction of the sheet, the temperature of the hot air being itself modulated according to the compartments according to the moisture profile of the sheet in the cross direction, the blown air being largely recycled by means of suction volumes made within said compartments.

 According to a completely preferred variant, the air is heated by shielded or non-energized electrical resistances by thyristors which can be controlled either by a system regulating the applied power (so-called current or intensity limiting system " or by a regulation coupled with a probe measuring the temperature of the gas flow.

 Inside a compartment, the resistances will be supplied in parallel so that, in the event of a break in one of the resistors, the other one (s) may still function and the defective resistor may be replaced during a shutdown. normal maintenance. This results in a fast and precise modulation of temperatures.

 The recirculation of the blown air does not only serve to greatly improve the thermal balance of the system.

 In fact, the recirculation of the air makes it possible especially to avoid the flapping of the sheet even if the blowing power is high.

 In the prior art, attempts to recycle hot air had, on the contrary, failed, the various complex parameters of such a system could never be controlled, which led to a phenomenon of aspiration of the leaf in some places.

 The modulation of the blown air temperatures also contributes to the fact that the flapping of the sheet is avoided, in that this modulation makes it possible for the first time to limit the blown flow without impairing the efficiency.

 Thus, this system according to the invention operates thanks to the combination of the hot air blowing modulated in temperature in the cross direction and the suction of this blown air, the overall result being the absence of flapping of the sheet , a very good profile correction efficiency and a remarkable evaporation power applicable.

 This last characteristic, which should not be confused with the correction efficiency, is capital from the industrial point of view, because it determines inter alia the speed of scrolling of the sheet, and therefore the productivity of the machine.

 According to a preferred embodiment, the vertical walls of the compartments have, up to the lower edge (6) of each compartment, and in the machine direction (that is to say perpendicular to the plane of the single figure) a profile not rectilinear but sawtooth, or having corrugations on either side of the vertical plane of the wall, or a similar profile, so as to avoid the delimitation of a solution of continuity in the moisture profile between two areas of the sheet next to two adjacent compartments. On the contrary, a non-rectilinear profile will favor the establishment of a temperature gradient around the border of two such zones.

 In another preferred embodiment, the compartments will form an angle in relation to the direction of the sheet. This angle will be calculated so that the area corresponding to the vertical wall separating two compartments covers the entire width of the compartment in a developed length of the hood.

 Also preferably, the vertical walls of the compartments will include thermal insulation to prevent heat transfer from one compartment to another.

 The following example illustrates the invention without limiting its scope.

Example
Speed: 400 m / min
Paper: 110 g / m2
Moisture profile: maximum: 4.46
minimal: 1.85
average: 3.05%
Case No. 1
The machine is maintained at a constant speed and the maximum humidity remains at 4.46Z.

Quantity of water evaporated by the hood: per hour and per meter of mach width - in the zone with humidity masimale: 68.9 kg per meter width of machine - in the zone with minimum humidity: O kg per meter width of machine
On average: 31.56 kg per meter of machine width
To maintain the humidity at 4.46, the amount of steam introduced into the dryer will be reduced.

 On the other hand, for one kilogram of paper paper, the amount of fibrous and other materials will be reduced from 969.5g per kg to 955.4g / kg, a saving of materials of 1.45%.

Case No.

 If the paper machine is limited by the drying capacity, the use of the corrective hood allows an increase in speed. the evaporative capacity of the hood is at least 60 kg of water per square meter per hour, which, for a hood having a useful developed length of 2320m corresponding to its installation above a dryer of 1.5 m diameter, gives an evaporative capacity of 132 kg per meter of machine width and for a single hood.

 The possible production increase by using the maximum capacity of the hood on the wettest area of the sheet can reach (for example if the hood is applied after a gluing press from which the sheet of paper comes out with 31 g of water per 100 g of dry paper) 187 from initial production.

Initial capacity:
400 x 60 (0.31-0.0305) x 0.110 = 738, kg i
Additional capacity = 132 kg / hour
Total capacity = 870 kg / h / m
(870/738) - 1 = 0.179 = 18z.

The following table gives a comparison of the main prior systems and the system according to the invention.
It can be seen that the invention provides a much higher efficiency; - does not change the temperature of the sheet, - allows a remarkable correction of the moisture profile.

 As an indication, an electrical power of 160-200 kw can blow 1 air at a temperature of 100-300 ° C and achieve the desired results.

 It should be noted that the invention also makes it possible to operate by eliminating "spikes" of damp or by limiting drying in the driest places, and thus to function differently according to the periods of the year at "expensive" or good electricity. market".

 The invention thus makes it possible to effectively correct the moisture profile of the sheet in the cross direction, and provides a speed gain of at least 2 to 4% which results in a productivity gain of 1.8 to 3, About 6% (for a 90v output).

 Moreover, the invention makes it possible to tolerate in the sheet the presence of approximately 0.6 of additional water, which results in a flatter paper on the rail, by significant savings in fibers constituting the leaf as well as energy and waste.

 This recycling also greatly improves the energy balance.

 It is also noted that these results are obtained without the operation of the device is accompanied by the disadvantages and risks inherent in prior techniques: flapping of the sheet, risk of fire, burning of the surface, rolling, etc ...

mentioned above.

 Finally, the fact of performing the correction on a very wet sheet (because the device is not placed at the end of drying as for example IR systems) provides a much better performance of the posterior dryer: no risk of sticking of the leaf on the felts, linting, cleanliness of the dryer, no "fuzz" (white caused to print by the elimination of small elements of the paper).

 The invention, by a combination of original means which, despite blowing and recycling, avoids the phenomenon of beat, brings a cascade of improvements all important in the industrial manufacturing, and leads to a product of very superior quality .

Board
Paper production of 2500 kg / h / meter of largo

<tb><SEP> Technique <SEP> IR <SEP> Blow <SEP> Invention
<tb><SEP> l <SEP> hot air <SEP>
<tb><SEP> to <SEP> debit <SEP> modulated
<tb><SEP> Yield (+)
<tb><SEP> net
<tb><SEP> - <SEP> average <SEP>#<SEP> 20% <SEP> 16% <SEP> @ <SEP> 36
<tb><SEP> - <SEP> maximum <SEP> 27% <SEP> 22% <SEP>46,;;<September>
<tb><SEP> Temperature
<tb><SEP> of <SEP> the <SEP> sheet
<tb><SEP> exit
<tb><SEP> dry <SEP> modified <SEP> no <SEP> modified <SEP> | <SEP> no <SEP> changed <SEP>
<tb><SEP> Quantity
<tb><SEP> maximum
<tb><SEP> water <SEP> evapo
<tb><SEP> rable
<tb><SEP> kg / h / m <SEP> of
<tb><SEP> width <SEP> 58 <SEP> 132
<tb><SEP> Quantity <SEP> of water
<tb><SEP> evaporable
<tb><SEP> without <SEP> effects
<tb> secondary <SEP>
<tb> harmful <SEP> (+++)
<tb><SEP> kg / h / m <SEP> of <SEP> wide <SEP> 30
<tb><SEP> Correction
<tb> maximum
<tb><SEP> Applicable <SEP> (@) <SEP><SEP> 1.2% <SEP> 0.5%
<Tb>
(+) quantity of water actually evaporated / theory
(++) with air at 275 C
(+++) resulting for example from the migration of coloran @@ @@ / c @@@@ s
sizing materials.

 (@) given the average actual yield, the inaccurate actual moisture measurements by existing paper machine systems, side effects etc ...

Claims (6)

REVENDTCATIONS 1. A machine for drying a paper machine, for correcting the wet profile of the fenille, characterized in that it is arranged above and in the cross-machine direction of the sheet, has several compartments (2,3 4) separated dividing this hood in the cross direction, each compartment being itself divided into volumes (2a, ..) of air blowing and volumes (2A, ...) of air suction open to their lower part close to the surface of the sheet and respectively connected, at their upper part, to means for blowing hot air or air suction, and in that each compartment is supplied with hot air by a circuit comprising an independent means of modulating the tempera-ture of the hot air as a function of the degree of humidity of the portion of the sheet facing the lower suverture of said compartment. 2. Hood according to claim 1, characterized in that the modulating means consists of shielded or non-energized electrical resistances by thyristors which can be controlled either by a system controlling the applied power or by a control coupled with a probe measuring the temperature of the gas stream. 3. Hood according to claim 1 of 2, characterized in that the blowing and suction circuits comprise airflow modulation valves. 4. Hood according to any one of claims 1 to 3, characterized in that the vertical walls defining the compartments have a profile, in the machine direction, sawtooth or the like, at least in their lower part, to avoid the delimitation of a solution of continuity of humidity on the sheet, or in that, for the same purpose, the vertical walls are slightly offset from the machine direction. 5. sotte according to any one of claims 1 to 4, characterized in that the vertical walls defining the compartments are provided with a thermal insulator. 6. A method for correcting the cross-directional moisture profile of a paper sheet, with a high efficiency and a high evaporation power, characterized in that the sheet of air is blown at the same time. the flow rate and the temperature are modulated according to the cross direction, and in that the majority of the blown air is sucked for recycling.