METHOD FOR DEWATERING IN PAPER MANUFACTURE

Abstract: For applying a method according to the invention, the dewatering system is formed in a decentralized manner, wherein a separate dewatering unit (1a, 1b, 1c...) is formed for suction boxes (4) with the same negative pressure level. Further, a dewatering strategy is made on the basis of the yield of the suction pump (18) of the respective dewatering unit (1a, 1b, 1c...) and/or on the adjustment of the rotational speed of the electric motor (19) of the suction pump. When applying the method, the desired rate of removing a quantity of water (Vas) is selected in the respective dewatering unit and the quantity (Vm) of water separated is measured in the respective dewatering unit. The yield of the suction pump (18) of the dewatering unit (1a, 1b, 1c...) and/or the rotational speed of the electric motor (19) of the suction pump is adjusted on the basis of the dewatering strategy, if Vas NOTEQUAL Vm.

Description: Method for dewatering in paper manufacture The invention relates to a method for dewatering in paper manufacture according to the preamble of claim 1. Dewatering in paper manufacture is conducted by a suction system, whereby a certain quantity of water is removed in the wire and press section of a paper machine or the like mechanically by means of a pressure difference between the negative pressure generated by the water separation system and on the other hand the higher pressure prevalent in the environment, and by an air flow caused by the same. Using a dewatering system based on negative pressure, it is also pos¬ sible to maintain the running order of the press felts, and further, the dewatering system can be used also to facilitate the transfer of the pa¬ per web from one part of the paper machine to another. Dewatering systems of prior art, such as the dewatering system dis¬ closed in the publication GB-2 129 026, are so-called centralized sys- terns, where the whole unit generating a negative pressure in the de¬ watering system is connected to suction boxes or the like, usually being provided with a water separation unit. It is difficult to construct a centralized dewatering system, because the pipe system required in it must be designed as a separate unit of its own in connection with a pa- per machine with a demanding total construction as such. Further, it should be noted that a centralized dewatering system involves a risk for the process of paper manufacture, because breakdowns and possible failures in the unit generating a negative pressure in the centralized de¬ watering system may in the worst case stop the whole process of paper manufacture. Particularly the implementation of the control system in a way to allow options for controlling individual suction boxes or groups of suction boxes for optimizing the process of paper manufacture is very difficult to arrange with a centralized dewatering system, or at least it is not advantageous in view of energy consumption. Consequently, a centralized dewatering system is expensive both upon investment and during operation, and it makes control and optimization of dewatering very difficult. It is an aim of this invention to present an improved system for dewater¬ ing in the manufacture of paper. Using the method, most of the prob¬ lems of prior art can be eliminated and the standard of prior art can be thus improved. The method according to the invention is primarily char- acterized in that a separate dewatering unit is arranged for each suction box or the like and/or for certain groups of suction boxes with the same negative pressure level, that - a dewatering strategy is formed on the basis of the yield of the suction pump and/or on the adjustment of the rotational speed of the electric motor of the suction pump in the re¬ spective dewatering unit V = f(n, q p ), wherein the dewatering control system is arranged by selecting the desired rate (V as ) of removing a quantity of water from said dewatering unit, - the quantity (V m ) of water removed from said dewatering unit is measured, and the yield of the suction pump and/or the rotational speed of the electric motor of the suction pump in said dewatering unit is adjusted on the basis of the dewatering strategy, if V as ≠ V m . Thus the above presented method gives the following main advan¬ tages: Firstly, the investment costs of the paper machine are reduced, because the pipe systems and other constructions are considerably less complex than in conventional embodiments. Secondly, by selecting the optimal number of dewatering units e.g. according to compatible suction levels and driving outputs and further by selecting the quantity of water to be removed in the dewatering unit according to the requirements of the process of paper manufacture, it is possible to optimize dewatering in paper manufacture as a whole, with respect to both the final result of the process itself and energy consumption. Some advantageous embodiments of the method according to the in¬ vention are presented in the following dependent claims on the method. The invention will be described in more detail in the following descrip- tion, with reference to the embodiment shown in the appended draw¬ ings. In the drawings, Figure 1 shows the change in the permeability of the felt during its use in a schematic manner in a coordinate system P = f(t), where P = permeability and t = time, Figure 2 shows in a coordinate system V as ,P constant curves (V n±i ) = f(n, q p ) = E min , where V as = the rate of removing a quantity of water, P = the permeability of the felt, V n±i = the selected rate of removing a quantity of water in said dewatering unit in a certain situation, n = the rotational speed of the electric motor in the dewatering unit, q p = the yield of the suction pump in said dewatering unit, and E min = the minimum level of energy consumption, and Figure 3 illustrates the overall implementation of the method in a schematical view. Particularly with reference to Fig. 1 it can be stated that the permeabil- ity of the felt is an important factor in dewatering. As known, the perme¬ ability of the felt drops decisively after an exchange as quickly as in a few hours (in practice 2 to 3 days) of operation and is subsequently re¬ duced less drastically during the service life of the felt (in practice 4 to 5 weeks). Thus in view of dewatering, in the control process of paper manufacture, the change in the permeability of the felt must be taken into account as well as other criteria for dewatering in the process of paper manufacture. At a certain point in the process of paper manu¬ facture, a certain quantity of water is to be removed from the paper web in the wire and press section, and naturally this must be adjustable in the process of paper manufacture for achieving desired paper quality properties. Some noteworthy provisions in this respect include the fact that the paper web may be clogged up if the dewatering or suction effect is too great, or the felt may be dried too much, as well as the fact that the paper web may thus adhere to the felt. Dewatering of the paper web must thus be controlled, whereby for maintaining a certain rate of removing a quantity of water constant, adjustments should be made substantially continuously, primarily because of the change in the permeability of the felt. In addition to these adjustments, also the rate of removing a quantity of water in a certain dewatering unit must be adjusted precisely due to requirements in the process of paper manufacture. For controlling the overall process presented above, this invention is based on a decentralized dewatering system, one dewatering unit being illustrated in Fig. 2 with respect to the dewatering strategy. At a certain moment of time in said dewatering unit, V as = V n . According to Fig. 1 , permeability will change during the time of use of the felt as a function of time, i.e. P = f(t). For a certain permeability value P(t-,), there is a corresponding combination r-i , q p1 which will satisfy the requirement v as = v n at the minimum level of energy consumption E min . When the permeability changes → P(t) (V as ≠ V m = measured dewatering rate), a new combination n 2 , q P 2 is selected which will further satisfy the re- quirement V as = V n at the minimum level of energy consumption E min . However, if V as = V n+1 (generally V n+1 ), i.e. the desired rate of remov¬ ing a quantity of water is changed, e.g. when the permeability is P(t 2 ), a change is made to the combination n 3 , q p3 which will satisfy the re¬ quirement V as = V n+1 on the minimum level of energy consumption E min . Consequently, the quantity of removed water V m is continuously measured in the dewatering unit, and adjustments are made, if neces¬ sary, in a manner described above, i.e. in the order, if V m ≠ V as [V n±i ] or e ≠ o, then n 1 -> n 2 = Δn and/or q-j → q 2 = Δq, wherein n 2 and/or q 2 is selected so that the requirement E min will be satisfied at the respective dewatering rate V n±j , in other words, at a certain required dewatering rate, a change in the permeability of the felt or in another factor of the rate of removing the required quantity of water will cause an optimal new combination n, q p with respect to energy consumption. Particularly qp indicates in this context an adjustment in the yield of the suction pump, which may be conducted as an adjustment by a throttler or guide blade and/or a diffusor. Figure 3 illustrates the overall implementation of the method in a sche¬ matic view. The dewatering system comprises several dewatering units 1a, 1b, 1c, etc. placed successively in connection with the wire and/or press section of a paper machine. In Fig. 3, the dewatering units are defined by dotted lines. In the following, the foremost dewatering unit 1a of Fig. 3 will be described. The schematic view of the dewatering unit 1a illustrates a paper web 2 and a felt or wire 3, which in the process of paper manufacture pass a suction box 4 or a certain group of suction boxes for the purpose of dewatering. In case the dewatering unit 1a comprises two or more suction boxes, their group is selected so that these suction boxes have essentially the same level of negative pressure. Suction boxes 4 belonging to the same dewatering unit 1 are connected to a water separation apparatus 5 via a connection line 6. The water sepa¬ ration apparatus 5 is, in turn, connected on one hand by a connection line 8 to the suction unit 7 of the dewatering unit 1a and on the other hand by a connection line 10 to a water collection system 9. In the water separation apparatus 5, the quantity of the water separated in the water separation apparatus is continuously measured by a measuring device 11 (shown schematically in Fig. 1), the measuring result being conveyed via a line 12 to a comparing means 13. The dewatering sys¬ tem comprises also a central control unit 14, which sends a setting value V as on the desired rate of removing a quantity of water via a line 15 to each dewatering unit 1a, 1 b, 1c... The comparing means 13 compares the values V m and V as , and the difference e is a control signal to a regulator 16 equipped with the control strategy for the respective dewatering unit 1a (Fig. 2), on the basis of which the yield of a suction pump 18 and the rotating speed of an electric motor 19 are adjusted via lines 20 and 21 to a suction unit 7 operating as the apparatus for producing a negative pressure. The suction unit 7 comprises control means, such as chokes or the like, or means for achieving blade control. These means are illustrated schematically with the reference numeral 22. Air heated by compression in the suction pump 18 is conveyed via a line 23 to a heat exchanger 24, where the air emits part of its thermal capacity e.g. to the operation or process water of the paper machine. In an alternative or in addition to the said operation, the line 23 can also be connected inside the hood of the paper machine. It is advantageous to design the suction unit 7, at least the suction pump 18 and the electric motor 19, as an integrated unit on the same shaft. It is also advantageous that the rotational speed of the common shaft of the suction pump 18 and the electric motor 19 in the suction unit is in the so-called high-speed range, i.e. higher than 25 000 revolu¬ tions per minute. Using high-speed technique is particularly important for application of the method, because an integrated unit applying high¬ speed technique is compact, wherein it can be easily placed in connection with the overall construction of a paper machine in each de¬ watering unit 1a, 1b, 1c... Further, an integrated suction unit 7 is light in weight, so that it is easy to handle during reparation and maintenance operations. Moreover, a suction unit 7 applying the high-speed technique does not vibrate and has a low noise level. Also the possibility in embodiments applying the high-speed technique to use contact-free bearings, particularly magnetic bearings, reduces costs of investments and maintenance. The dewatering system 9, the heat exchanger 24, as well as the central control unit 14 are common either to all dewatering units 1a, 1 b, 1c... or to some of them. The central control unit 14 forms an overall strategy for dewatering between different dewatering units 1a, 1b, 1c..., as re- gards total dewatering and the division of its quantity between different dewatering units, whereby each dewatering unit 1a, 1 b, 1c... carries out a strategy of its own on the basis of continuous measurements. It is obvious that at least in some control situations, both possibilities (yield q p and rotational speed n) do not need to be used, and the simplest systems can also be applied in a way that only one of the adjustments (yield q p and rotational speed n) is in use.

Claims: Claims: 1. A method for dewatering in paper manufacture, particularly in connection with the wire and/or press section of a paper machine, wherein a paper web (2) is moved on the support of a felt (3) of the like, and water is absorbed by a suction effect from the paper web (2) to the felt (3) or the like, and water is removed by a suction effect from the felt (3) or the like, wherein several suction boxes (4) or the like, being part of a de¬ watering system, are arranged on the travel path of the felt (3) or the like, wherein - a negative pressure is produced in the suction box (4) or the like by an apparatus (7) for developing a negative pressure, for directing a suction effect through a suction opening sys¬ tem in the suction box (4) or the like at least to the felt (3) or the like, and wherein - the water absorbed from the felt (3) or the like is separated into the dewatering system by a water separation system being part of the dewatering system in connection with the paper machine and comprising several water separation units, characterized in that for applying the method, the dewatering system is decentralized in a manner that a separate dewatering unit (1a, 1 b, 1c.) is arranged for each suction box or the like and/or for certain groups of suc¬ tion boxes with the same negative pressure level, that a dewatering strategy (V) is formed on the basis of the yield (q p ) of the suction pump (18) and/or on the adjustment of the rotational speed (n) of the electric motor (19) of the suction pump in the respective dewatering unit (1a, 1b, 1c.) V = f(n, q p ), wherein the dewatering control system is arranged by selecting the desired rate (V as ) of removing a quantity of water from said dewatering unit, - measuring the quantity (V m ) of water removed from said dewatering unit (1a, 1b, 1c.), and adjusting the yield of the suction pump (18) and/or the rotational speed of the electric motor (19) of the suction pump in said dewatering unit (1a, 1 b, 1c.) on the basis of the dewatering strategy, if V as ≠ V m . 2. A method according to claim 1 , characterized in that the suction pump (18) and the electric motor (19) of the apparatus (7) for developing a negative pressure are formed as an integrated unit on the same shaft. 3. A method according to claim 1 or 2, characterized in that the rotational speed of the common shaft of the suction pump (18) and the electric motor (19) in the dewatering unit (1a, 1b, 1c.) is selected in the so-called high-speed range, i.e. higher than 25 000 revolutions per minute. 4. A method according to any of the claims 1 — 3, character¬ ized in that the apparatus (7) for developing a negative pressure in the dewatering unit (1a, 1 b, 1c.) is placed in connection with said suc¬ tion box (4) or group of suction boxes. 5. A method according to claim 1, characterized in that the yield of the suction pump (18) is adjusted by adjusting the choke, dif- fusor and/or guide blades. 6. A method according to claim 1 , characterized in that the air coming from the suction pump (18) is conveyed to the heat ex¬ changer (24), where the air emits part of its thermal capacity into the operation or process water (25) of the paper machine and/or the air is blown into the hood of the paper machine. 7. A method according to claim 1 , characterized in that a dewatering strategy is formed on the basis of the minimum consump¬ tion of energy in a manner that for each permeability value of the felt (3) or the like and for each rate of removing a quantity of water (V as ), a value is selected: as = f(n, q p ) = E min , where E mjn = the minimum level of energy consumption with the values V as , P, n^ q^, wherein V as = the rate of removing a quantity of water, P = the permeability of the felt or the like, n- | = the rotational speed of the electric motor in the dewatering unit, and q p1 = the yield of the suction pump satisfying the condition mirv 8. A method according to claim 1 , characterized in that the quantity of the water (V m ) separated in said dewatering unit (1a, 1 b, 1c.) is continuously measured in the water separation apparatus (5) by a measuring device (11). 9. A method according to claim 1 , characterized in that the determination of the desired rate of removing a quantity of water (V as ) of dewatering units (1a, 1 b, 1c.) of the decentralized dewatering sys¬ tem is conducted by the central control unit (14), where the overall strategy is determined for dewatering and the rate of removing the quantity of water (V as ) required for carrying out this overall strategy is • determined for each dewatering unit (1a, 1 b, 1c.). 10. A method according to claim 1 , characterized in that the water collection system (9) is formed in an integrated manner, wherein the water discharged from the decentralized dewatering system is col- lected into connection lines (10) in the water collection system (9).

Inventors: SAEILY KARI, SAEILY, KARI
Applicants: HIGH SPEED TECH LTD OY, SAEILY KARI, HIGH SPEED TECH OY LTD., SAEILY, KARI
Application year: 1995
Main classification: D21F1
IPC codes: D21F1