Note: Descriptions are shown in the official language in which they were submitted.
CA 02206~44 1997-0~
WO 96/15319 ' rC~ 95/00635
Method for dewatering in paper manufacture
The invention relates to a method for dewatering in paper manufacture
5 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
10 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
15 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-
20 tems, 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-
25 per machine with a demanding total construction as such. Further, itshould 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
30 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
35 centralized dewatering system is expensive both upon investment and
during operation, and it makes control and optimization of dewatering
very difficult.
CA 02206~44 1997-0~
WO g6/15319 PCT/F19S/00635
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-
5 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, qp), wherein
- the dewatering control system is arranged by
- selecting the desired rate (Vas) of removing a quantity of
water from said dewatering unit,
- the quantity (Vm) 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
Vas ~ Vm.
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
30 Iess 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
35 optimize dewatering in paper manufacture as a whole, with respect to
both the final result of the process itself and energy consumption.
CA 02206~44 1997-0
WO 96/15319 PCTtlil95/0063
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-
5 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 Vas,P constant curves (Vn+j) =
f(n, qp) = Emjn, where VaS = the rate of removing a quantity
of water, P = the permeability of the felt, Vn+j = 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, qp = the yield of the suction
pump in said dewatering unit, and Emjn = the minimum level
of energy consumption, and
Figure3 illustrates the overall implementation of the method in a
schematical view.
Particularly with reference to Fig. 1 it can be stated that the permeabil-
25 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
30 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
35 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
CA 02206~44 1997-0~
WO 96/15319 PCT/1il95/00635
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
5 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.
10 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, Vas = Vn. According to Fig. 1,
permeability will change during the time of use of the felt as a function
15 of time, i.e. P = f(t). For a certain permeability value P(t1), there is a
corresponding combination n~, qp1 which will satisfy the requirement
VaS = Vn at the minimum level of energy consumption Emjn. When the
permeability changes ~ P(t) (Vas ~ Vm = measured dewatering rate), a
new combination n2, qp2 is selected which will further satisfy the re-
20 quirement Vas = Vn at the minimum level of energy consumption Emjn.However, if Vas = Vn+1 (generally Vn+1), i.e. the desired rate of remov-
ing a quantity of water is changed, e.g. when the permeability is P(t2), a
change is made to the combination n3, qp3 which will satisfy the re-
quirement Vas = Vn+1 on the minimum level of energy consumption
25 Emjn. Consequently, the quantity of removed water Vm 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 Vm ~ Vas [Vn+j] or
e ~ o, then n1 ~ n2 = ~n and/or q~ ~ q2 = ~q, wherein n2 andtor q2 is
selected so that the requirement Emjn will be satisfied at the respective
30 dewatering rate Vn+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, qp with respect to energy consumption.
35 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.
CA 02206~44 1997-0~
WO 96/15319 PCr/1;195/0063S
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
5 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 wire3, which in the process of paper
10 manufacture pass a suction box4 or a certain group of suction boxes
for the purpose of dewatering. In case the dewatering unit 1 a 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
15 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
20 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 Vas on the desired rate of removing a quantity of water via a
line 15 to each dewatering unit 1a, 1b, 1c.. The comparing means 13
compares the values Vm and Vas, 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 unit7
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
CA 02206~44 1997-0~
WO 96/15319 PCT/rl75~ i3S
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
5 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
10 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
15 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 eitherto all dewatering units 1a, 1b, 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-
25 gards total dewatering and the division of its quantity between differentdewatering units, whereby each dewatering unit 1 a, 1 b, 1 c... 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
qp and rotational speed n) do not need to be used, and the simplest
30 systems can also be applied in a way that only one of the adjustments
(yield qp and rotational speed n) is in use.
