FI119249B - Method and apparatus for controlling the moisture content of paper - Google Patents

Description

119249

METHOD AND EQUIPMENT FOR ADJUSTING THE MOISTURE OF PAPER

The invention relates to a method for adjusting the moisture of paper during a papermaking process, wherein the paper web is dried in a drying section having at least one drying unit where drying is performed with a drying gas, measuring a proportional to the moisture of the paper web before drying the paper web.

The invention further relates to an apparatus for controlling the moisture of paper in a paper machine comprising a headbox, a mold, a dryer section, a reel and a measuring device for measuring a size proportional to the moisture of the paper web before drying, the dryer section having at least one drying unit the apparatus includes means for compensating the moisture disturbances 15 detected by the measuring device by means of a drying unit.

Typically, the moisture content of a paper web is adjusted by measuring the moisture profile after the drying section of the paper machine and correcting the feedback adjustment utilizing the moisture profile at some earlier stage. The humidity profile may be repaired, for example, by means of a steam box adapted, for example, to a press or a drying section. Of course, the moisture of the paper is also controlled by adjusting the drying section parameters. Typically, the drying section has a plurality of drying cylinders, most commonly controlled by controlling the pressure of the steam supplied to the drying cylinders.

: Y: It is still well-known to use a blower to dry paper: **. In blow-drying, the paper web is made to pass * ·. · * ·. around the so-called blow-up roll while blowing hot drying air towards the web. The blower roll is generally larger in diameter than the conventional drying cylinder and, for the blower members, is covered by a hot air hopper. The nozzle surface of the hood is at a certain distance from the surface of the roll, • · · *;]; * 30, whereby a drying zone is formed between the nozzle surface of the hood and the roll. As the paper * ··· 'web passes under the hood along the periphery of the blower roll, the hood is blown with hot drying air between the hood. Majority • * ·. ** ·. the air blown towards the paper web is returned, without discharge, to be heated and re-blown towards the web. In order to maintain the humidity of the blowing drying air at the desired level, some of the dehumidifying air returned from the track is removed as exhaust air and replaced with the required amount of fresh replacement air. Such an overflow system is disclosed, for example, in WO 99/32714 and Fl 1 982625. Typically, the overflow unit is controlled by controlling the air temperature and the air velocity. Changing the control variables of the blowing unit will naturally affect the moisture of the paper, but the control variables will also have very complex effects on other quantities of the blowing unit. Because of the difficult control of the adjustment, it has been extremely difficult to utilize the blowing units, for example, in the context of species changes. In practice, it has not been possible to use the blowing units to control the moisture of the paper web 10, but the blowing units have mainly been used to enhance the drainage of water at the beginning of the drying section.

It is an object of the present invention to provide a solution which provides very good moisture control of paper.

The method of the invention is characterized in that the faults are compensated for by adjusting the blowing speed of the drying unit, determining the effect of the blowing rate on the blowing temperature and compensating for the effect of the blowing rate change on the blowing temperature.

Further, the apparatus according to the invention is characterized in that the drying unit has means for adjusting its blowing speed and blowing temperature and that the means includes means for compensating for moisture defects in the paper by adjusting the blowing speed of the drying unit; effect on temperature.

It is an essential idea of the invention that the paper web is dried. in a drying section having at least one drying unit in which the drying takes place. drying gas and measuring the moisture of the paper web before drying.

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It is further essential that the moisture disturbances detected by the measurement compensate for,. dehumidifying unit by compensating for malfunctions by adjusting • · · 30 blowing unit blowing speed and determining the blowing unit blowing speed: ···: effect of blowing change on blowing temperature and compensating; : *: The effect of changing the blowing speed on the blowing temperature.

• · ·. * ·. The idea of a preferred embodiment is that the effect of a change in the blowing rate ./,* on the blowing temperature is compensated by • · * 35 switching. Another idea of the preferred embodiment is that the drying unit used to compensate for moisture disturbances is arranged after the middle of the drying section 3 119249. The idea of a third preferred embodiment is that the drying unit is an overflow unit. The idea of a fourth preferred embodiment is to measure the moisture of the paper web at several locations in the transverse direction of the paper web simultaneously using optical fibers 5.

An advantage of the invention is that the moisture disturbances on the dryer section can be compensated before the reel without the feedback humidity control changing the control. All in all, the moisture control of the paper is accurate and fast. By eliminating the cross-effect between the blowing speed and the heat-10 mode, the control of the drying unit is extremely accurate and fast.

The invention is explained in more detail in the accompanying drawings, in which Fig. 1 schematically shows a papermaking process; and Figure 5 schematically shows the dynamics of various control solutions.

Figure 1 schematically illustrates the papermaking process.

The paper is made on a paper machine having a headbox 1, from which the pulp is fed to former 2, where the pulp is formed into a fiber web 3. After the Former 2, the fiber web 3 is led to a drying section 4. Former 2 and dry: V; a press section 5 can be fitted between the drying section 4. After the drying section 4, the dry web is led to a roller 6. The paper machine may further include an adhesive press • ·. * · *. calendar, or coating units not shown for clarity. ···. Further, the operation of the papermaking machine is well known per se to the person skilled in the art and is therefore not further described herein.

. The paper machine further comprises a first measuring device 14 and a second measuring device 15. The first measuring device 14 measures the moisture of the paper web 3 before drying. The first measuring device 14 may be disposed, for example: between the press section 5 and the drying section 4, or then at the beginning of the drying section 4.

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The second measuring device 15 measures the moisture of the paper web after drying, „·. wherein the second measuring device 15 is disposed between the drying member 4 and the reel 6.

• · ·: 35 In addition to or instead of moisture, measuring devices 14 and 15 can measure, for example, • · 4 119249 the amount of water or basis weight of paper, or some other measure proportional to moisture.

The measuring devices 14 and 15 may be so designed as to have a gauge carriage moving back and forth across the web in a transverse direction relative to the web direction. On the other hand, there may also be a plurality of measuring sensors side by side, whereby the properties of the paper web are measured at several points in the transverse direction of the paper web simultaneously. With multiple adjacent gauges, the transverse profile of the paper web can be measured very quickly. Further, if there are a plurality of gauge sensors, they can be arranged to be movable back and forth a portion of the distance in the transverse direction of the paper so that the gauge sensors do not need to be adjacent but still the transverse profile of the paper web can be measured. The measuring devices 14 and 15 may include optical fibers, by means of which the measurement data from the paper web are further processed. In connection with the measuring devices 14 and 15, the paper web 3 may be provided with a free outlet, or the measuring devices 14 and 15 may be arranged to measure against the wire or roll, the paper web 3 being supported at the time of measurement.

Fig. 2 shows a part of the drying section 4. The drying section 4 has a number of conventional steam-heated drying cylinders 7. Some of the cylinders may be replaced by pressure rollers. In addition, the drying section 4 is provided with one or more inflating cylinders 8 with which the inflating nozzles 9 are arranged to transfer the hot air or gas or superheated steam to the fiber web to be dried. Blowing can be done either by "M * directly toward the paper web or by blowing through the wire. For clarity, the following figure does not: V: The fiber web, wires, auxiliary rolls, support structures and other similar dryer section 4 parts are known to those skilled in the art. · ♦ · With the help of the blowing units, the drying efficiency and drying speed of the drying section 4 can be significantly improved compared to, for example, the conventional drying section, in which the drying is based solely on steam-heated cylinders 7.

• Λ · 30 The blowing hood 9 blows, for example, hot air at high speed toward the paper web. The air temperature can be, for example: about 350 ° C and a speed of about 90 meters per second. When this hot air blew ···. ···. controlled by contact with a damp paper web to release drying air .. * · * thermal energy to the paper web. The water in the paper is then evaporated into the surrounding • 35 air. The cooling air, which has cooled to a temperature of about 250 ° C, is sucked back into the overflow hood as return air. The achievable evaporative power of 5 119249 depends on the blowing parameters, such as blowing air temperature, nozzle speed and air humidity level. Further, the evaporation efficiency also depends on the dry content of the paper web, the initial temperature and the composition of the paper web pulp, etc.

The hot drying air is guided through the blowing chamber 10 of the blowing hood 9 towards the paper web. The water evaporating from the paper web is sucked back with the return air into the hood, the hood exhaust air space 11. Most of the return air is recirculated from the exhaust air space by the recirculating air blower 12 back to the blowing chamber 10. The recirculating air blower 12 provides the required pressure. The amount of air produced by the circulating air blower 12, and thus also the blowing speed at the nozzle, is controlled by adjusting the engine speed of the circulating air blower 12.

After the convection air blower 12, the air is heated by an air heating unit comprising a burner chamber, a gas burner 13 and a flame guard. The burner-15 chamber is in principle part of the convection air duct. The blowing air temperature is controlled by changing the power of the gas burner 13. Temperature control also affects the blowing speed, since the density of the air changes as a function of temperature. Thus, if the rotational speed of the circulating air blower 12 is kept constant, the blowing speed increases as the blowing temperature rises. The power 20 of the gas burner 13 is controlled by adjusting the amount of gas supplied to the burner 13. In addition to or in place of the gas burner 13, the blowing air can also be heated by steam ··· through a heat exchanger.

"V The blowing chamber 10 distributes the blowing air evenly throughout: Y: the overflow area. Blowing air is blown through the small holes in the surface of the blowing chamber 10, i.e. the blowing nozzle. The blowing chamber 10 .1 ··. Still has larger outlets than the blowing nozzles. sucked into exhaust air space 11.

The spacing between the nozzle surface of the blowing chamber 10 and the paper web is typically, for example, about 25 mm. This distance directly affects the evaporation power of '·] · 1 30. If the distance is clearly greater than 25 mm, the drying performance will be reduced. On the other hand, if the distance is clearly less than this, problems may arise • in the event of a web break, for example, the paper web may collide with the blowing chamber. and this may cause a paper machine jam.

Typically about 20% of the return air is removed from the hood 10 to remove evaporated water from the paper 35 V web. The exhaust air is replaced by the combustion air supplied to the gas · 6 119249 burner 13 and the replacement air supplied directly to the exhaust air space 11.

Fig. 3 is a block diagram illustrating an adjustment solution according to the invention. In control and system engineering, the operation of the system can be described by 5 differential equations. In order to determine the function of the system in the desired situation, the differential equation must be solved. There are several possibilities for solving differential equations. One way to represent and solve differential equations is to use convolution integral transforms such as the Laplace transform. In this application, other convolutional integral transforms, such as Fourier and Z transforms, and by which the inventive solution can be described, are also understood as equivalent to Laplace transform 10. The mathematical representations of the Laplace transform and the inverse transform are as follows: oo i {/ (/)} = f (i) = je “f (t) dt, 0 00 o where L is the Laplace transform operator, L'1 is the Laplace transform. inverse transform operator, s is a Laplace transform of the $ -20 imaginary s -space, .. * · * t is a time variable,: Y: f (t) is a function of t and: * ·. · F (s) is a Laplace transform of with s.

Laplace transforms generally convert the time variable functions of the input and output 25 variables of the process, whereby the s-space describes both the time variable functions of the input and output variables and the frequency dependence of the input and output aV '. A typical simple transfer function is, for example! "·: J: x (j) l + ra · · 30 where G (s) is the process transfer function,: ·] Ä is the process gain," **: Td is the dead time of the process, 7 119249 process time constant, X (s) is the input quantity and Y (s) is the output quantity.

The inlet quantities may be, for example, the blowing rate 5 or the blowing temperature which affect the moisture of the paper being the output quantity.

The drying power of the blowing unit can be influenced by several control variables. However, the primary control variables for the blowing unit are the blowing temperature and blowing speed. 3 is a block diagram of the blowing temperature control circuit controlled by the temperature controller TIC and the transfer functions from the control circuit control to the blowing temperature GT and the blowing temperature to the paper humidity GM1. These transfer functions illustrate how the quantities interact with each other as a function of frequency.

The block diagram below shows the blowing speed control circuit, respectively, controlled by the blowing speed controller PIC, as well as the transfer functions from the control circuit control to the blowing speed Gv and the blowing speed to the paper moisture GM2. These transfer functions describe how different quantities interact with each other as a function of frequency.

The block diagram also shows the interference transfer function GD blowing-20 speed and blowing temperature. The interference transfer function G0 is caused by the blowing temperature and paper moisture when the blowing speed is changed.

Dynamically, the blowing temperature is considerably slower than: · Blowing speed. In addition, changes in the blowing speed have a rapid and disturbing effect on the blowing temperature and, at the same time, the moisture of the paper. As the • blowing speed increases, the blowing temperature decreases and vice versa. On the other hand, increasing the blowing speed causes the paper moisture to decrease first, but as the blowing temperature decreases, the **** paper moisture will eventually increase, if the temperature change is not compensated in any way. has no significant effect on the blowing rate * · *; * 30 In the process tests performed, it has been found that the time constants of the blowing speed and the blowing temperature are of quite different magnitudes. • The time constant refers to the time required to The time constant of the blowing rate in the above experiments was less than 10 a] * a 's and the time constant of the blowing temperature, respectively, was about five minutes Delays: 35 for both quantities were approximately equal in length The above time constants can be directly equated with paper moisture 8 119249 can be changed very much While changing the humidity of the paper can change the moisture of the paper much more slowly. When comparing blowing with, for example, cylinder drying, blowing is many times faster than cylinder drying and more accurate in drying paper.

Since there is a significant crossover effect between the blowing speed and the temperature, it is quite difficult to quickly adjust the blowing unit. The control can be improved and the effect of the interference caused by the cross-action can be significantly reduced by using the flow-on control of the overflow unit shown in Fig. 5, which is provided with a feed-in between the blowing speed Gm and the blowing temperature. Feedback can be defined as GFF = "Gd / GB ·

In this equation, the variables determined by process tests are used as variables.

All in all, the blowing speed and blowing temperature are controlled as follows: Blowing speed is controlled by feedback control A and blowing temperature is controlled by feedback control B. Setpoints for both controls can be set at a higher level, eg humidity control. 20 The setting values may also be local. For feedback, the blowing speed and blowing temperature are measured. The blowing velocity can be defined as a pre ->, t, · by the pressure sensor 16 in the hood shown in Fig. 4. The temperature information is again obtained from the temperature sensor 17, which can be placed, for example, as shown in Fig. 4 in the hood. In addition to the measured velocity and temperature, the quantities derived from the measured quantities, such as ···. * Air or energy in the air, can be used for control. Blowing Speed • ·

when changing, the TIC control of the blower temperature regulator is changed by the feed-back circuit GFF1 so that the blowing speed changes due to the blower speed change. . itching to the blowing temperature is eliminated. Feedback temperature control B

*> [· * 30 does not detect a change in temperature, that is, a change in the blowing speed does not cause a disturbance in the temperature. Because of the feedback temperature control *: · *. B operates on the basis of the measured blow temperature, this control does not change before. ···. as the blowing temperature changes from the setpoint. Thus, for example, if the blowing "speed is increased, the control of the blowing temperature control circuit i V 35 is respectively increased so that the blowing temperature does not change from the target value. Without the a" ** mentioned above, the feedrate between blowing speed and blowing temperature would not be increased. This is possible because changing the blowing speed quickly changes the blowing temperature in the opposite direction and therefore the moisture in the paper, so bypassing allows the blowing temperature to be kept at its target value even if the 5 blowing speeds are changed rapidly.

Figure 5 illustrates the dynamics of various humidity control solutions. The upper graph of Figure 5 shows the blowing rate, which is changed quite rapidly from a smaller value to a higher value. The middle graph shows the effect of the change in blowing rate 10 on the blowing temperature, and the lower graph shows how the moisture of the paper behaves. The dotted line in the middle and bottom graphs shows what happens if the control circuits are open. Thus, as the blowing speed increases, the blowing temperature decreases and the moisture of the paper first decreases due to the effect of the blowing speed, but then increases as the blowing temperature decreases.

If the blowing temperature is controlled by feedback control, the blowing temperature and paper humidity will behave as shown in the dotted lines in Figure 5. Thus, as the blowing speed increases, the blowing temperature first drops, but after a delay, the feedback corrects the blowing temperature to a previous level. At the same time, the moisture of the paper first decreases due to the increase of the blowing speed, and then the moisture of the paper increases due to the lowering of the blowing temperature. As the blowing temperature> av returns to its original value, the moisture in the paper will eventually decrease as well.

• ·: 25 In the invention again, as the blowing speed changes, the blowing temperature · ···! The age adjustment control is changed by means of a feed-in circuit, whereby the blowing temperature II! the temperature and humidity of the paper behave as a solid line.

“** Thanks to the feed-in circuit, the blowing temperature does not change significantly, although the blowing speed changes rapidly. As a result, the moisture in the paper is reduced in a controlled manner; Thus, the invention provides a quick, efficient and accurate control of the moisture of the paper. Figure 5 illustrates the situation. at a particular point of action, and the representation of FIG. den.

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Figure 3 also shows a second feed-in GFF2 for eliminating paper moisture defects prior to winding. The feed-in paper humidity control function works as follows. The moisture of the paper, or a quantity proportional to the moisture of the paper 10 119249, is measured by the measuring device 14 before the paper is dried, thereby obtaining information on the moisture profile of the paper before the paper is dried. The moisture disturbance in the web has a transfer function to the final moisture of the GDM paper. The moisture disturbance transfer function GDM can be easily determined, for example, by changing the parameters of the press section and keeping the drying section parameters constant during the test. Moisture disturbances are caused, for example, by variable retention, wet end disturbances, and changes in the wire section or press section. The purpose of the second feed-in GFF2 is to measure the moisture of the paper with the measuring device 14 and to correct the disturbances in the moisture of the paper 10 before the measuring device 15 following the drying section notices the change in the moisture of the paper. Measuring devices 14 and 15 are shown in Fig. 4. The measurement of paper moisture by measuring device 15 is traditionally used only in feedback control, where the control is changed only when a change in the adjustable quantity is disturbed at a very late stage. The presently controlled control again modifies the control according to certain dynamics because the interference can be measured and its effect on the variable to be controlled is known. Thus, the quantity to be measured is the moisture of the paper web 3 entering the dryer section 4, and the adjustable quantity is the final moisture of the paper, which can be quickly influenced at a top blasting speed of 20.

The humidity feedrate of the paper is calculated by the equation GfF2 = -G [»// (GC | _XGM2), ··· where

Gqm is the dynamics of the paper moisture interference, 25 G », is the dynamics between the blowing speed and the paper moisture, and • ··. ·· / Gcl is the dynamics of the closed blowing speed control loop.

"f The dynamics of the closed speed control loop GCL is influenced by the ***** fan speed control parameters and the dynamics between the fan control and the fan speed Gv. The closed system dynamics describes how v .: 30 control loop behaves when its setpoint is changed. In general, all of the above transfer functions and process models can be easily determined by process tests in a manner known per se.

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In Fig. 4, the drying section 4 shows five drying cylinders ···: 35 groups 18 and four overflow units 19. Since the travel time delay between the first moisture measurement unit 14 and the last overflow unit 11 119249 19 is quite long, at least several seconds can be the moisture disturbances to the part 4 are compensated before the reel 6 by changing the blowing speed of the last or, for example, the last two blowing units 19 without the feedback paper humidity control changing the blowing 5 speed setpoint. Thus, in the measurement after the drying section 4, the measuring device 15 does not notice any change in the moisture of the paper. Thus, the moisture disturbance is compensated by the rapid dynamics of the blowing rate and the sufficiently long running time delay by the drying section 4. Most preferably the moisture disturbances are compensated by the blowing unit 19 which is fitted after the middle of the drying section 4. The purpose of feed-in is to compensate for moisture interference in the final product so that the final paper moisture content remains within the target value. If feed-in is disabled, the remaining moisture in the paper changes. Only then is the blowing speed changed and the malfunction eliminated, since the feedback control adjusts the moisture to the difference base.

The drawings and the related description are intended only to illustrate the idea of the invention. The details of the invention may vary within the scope of the claims. Thus, the drying gas blown by the drying unit may be air or steam or some other drying gas. Further, the invention can be applied to drying units other than overhead units. Thus, the drying unit may be, for example, a Yankee drying unit, a flow-through drying unit or a fluid-drying unit. Further, the invention:] "· may also be applied in connection with pulp air drying units. Further ·) · The feed rate between blowing speed and blowing temperature may be accomplished by fuzzy logic or neural network, model-predictive complexity-25 or other multivariate control. Feedback can be applied to any of the above control methods, particularly preferred in the invention • ♦ ** ·· * is suitable for machine-controlled paper humidity Process models can vary according to operating points, eg models can be predefined for each operating point · ·· change.

• · * • · · · «· * ·« * φ * ·· * * ·· m • * · • * «• *

Claims (18)

A method for controlling moisture of paper during a papermaking process, in which method a paper web (3) is dried in a drying portion (4) comprising at least one drying unit, where drying is carried out with dry gas, one against the moisture of the paper web (3). Proportional quantity is measured before drying the paper web (3) and moisture disturbances detected in the measurement are compensated by means of the drying unit, characterized in that the disturbances are compensated by controlling the blowing speed of the drying unit, the effect of the blowing speed on the blowing temperature, and determining the blowing temperature. which is caused by a change in the blowing speed is compensated. 2. A method according to claim 1, characterized in that the effect on the blowing temperature caused by a change in the blowing speed is compensated by coupling. Method according to claim 2, characterized in that the coupling between the blowing speed and the blowing temperature is determined by the equation GFfi = -Gd / Gt, where Gffi is the coupling between the blowing speed and the blowing temperature, Gd is a transfer function for the blowing speed interference. ·: * And blowing temperature and Gt is a transfer function for controlling a control circuit to bias temperature. . ···! 25 4. A method according to any of the preceding claims, characterized in that the method. characterized in that in the compensation of the humidity disturbances detected in the measurement by means of the drying unit, the drying unit is controlled by means of coupling. The method according to claim 4, characterized in that in the compression of the humidity currents by coupling is determined by-; the coupling through the equation. Gff2 = -Gdm / (GclxGm2). "where tl I in V Gff2 is a coupling for the elimination of humidity disturbances in paper, Gqm is the dynamics of the paper moisture disturbances, Gm2 is the dynamic between the blow rate and the moisture of the paper and Gcl is the dynamics of the closed control circuit of the blow rate. 6. A method according to claim 3 or 5, characterized by the variables in the coupling equation, models determined by process tests are used. 7. A method according to claim 6, characterized in that the reduced models are updated when changing operating points. 10 8. A method according to any of the preceding claims, characterized in that the drying unit with which the detected humidity disturbances are compensated is arranged on the drying portion (4) on the latter half thereof. Process according to any of the preceding claims, characterized in that the drying gas is air. 15 10. A method according to any of the preceding claims, characterized in that the drying unit is a blowing unit. Method according to any of the preceding claims, characterized in that the moisture of the paper web (3) is measured simultaneously at several places in the transverse direction of the paper web (3). Apparatus for controlling the moisture of the paper in a paper machine, the paper machine comprising an inlet carriage (1), a molding (2), a drying section (4), a wheelchair (6) and a measuring device (14) for measuring of a relative moisture content of the paper web (3) before drying the paper web, • · V,! said drying portion (4) comprises at least one drying unit, wherein the drying of the paper web (3) is carried out with drying gas and which apparatus comprises the means for compensating the moisture disturbances detected by the measuring device (14) with · *. * · * . using the drying unit, characterized in that the drying unit has means for controlling its blowing speed and blowing temperature, and that the apparatus comprises ..... blowing rate - **; · the effect on blowing temperature and means for compensating for the effect on blowing temperature caused by a change in blowing speed ···. :: ten. • M Apparatus according to claim 12, characterized in that The temperature includes means for realizing coupling to compensate for the effect on the blowing temperature caused by a change in the blowing rate. Apparatus according to claim 12 or 13, characterized in that the apparatus comprises means for effecting coupling to compensate for the moisture disturbances detected by measurement by means of the drying unit. Apparatus according to any of claims 12-14, characterized in that the drying unit with which the detected humidity disturbances are arranged to be compensated is arranged on the drying portion (4) on the latter half thereof. 16. Apparatus according to any one of claims 12-15, characterized in that the drying gas is air. Apparatus according to any of claims 12-16, characterized in that the drying unit is a blowing unit. Apparatus according to any of claims 12-17, characterized in that the measuring device (14) is arranged to measure the transverse profile of the paper web (3) substantially simultaneously. • * * • i • · t · · I · * · • · · · · · ♦ • • • • • ♦ ♦ • »· •« ··· • ♦ • · • · · • · · · · · Φ ·· ♦ • · • I · • · ♦ ··· • · · · ··· • · · • «· ··« ··· • · ♦ ♦ · ·· 1 • · Φ • ♦ · • · • ·