Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to the drying of material such as
paper web and to the control of heating apparatus. More particularly, the
invention relates to a method for drying material such as paper web in drying
apparatus which includes a high frequency dryer.
Capacitance high frequency heating apparatus are known in the art
and the use of such apparatus for drying bands of dielectric material such as
paper and the like has been found to be quite satisfactory. Such an apparatus
is disclosed in United States Patent No. 3>681,558, for example.
It is an object of the present invention to improve the drying of
material such as paper web and to do so at reduced energy consumption.
It is also an object of the present invention to improve the
operation of heating apparatus which includes a high frequency dryer and to
reduce the energy consumption of the apparatus.
It is another object of the present invention to improve the
operation of heating apparatus which includes a high frequency dryer to
provide dried material having an improved moisture profile.
It is still another object of the present invention to improve the
operation of heating apparatus which includes a first drying section and a
high frequency dryer.
These and other objects of the present invention are achieved
according to the invention by matching the drying which takes place in a first
drying section of a heating apparatus to the drying which takes place in a
high frequency dryer. More particularly, the long-term mean value of the
evaporation power or evaporation heating of the high frequency dryer is main-
tained at a predetermined value while the evaporation power or evaporation
heating of the first drying section is controlled. In other words, control
of the evaporation in the first drying section permits the evaporation load
in the high frequency dryer to be maintained constant. Thus, the evaporation
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heating supplied to the high frequency dryer is maintained at a substantially
constant and predetermined mean long-term value while the evaporation heating
provided to the first drying section is varied. Selection of the evaporation
power provided to the first drying section, i.e., control of the first drying
section, and selection of the predetermined constant mean evaporation power
supplied to the high frequency dryer are made in accordance with the final
mean moisture of the material that is desired. Maintaining the high frequency
dryer at a long-term constant me~an evaporation power reduces the overall
energy consumption of the heating apparatus.
According to one aspect of the invention, the long-term evaporation
power of the high frequency dryer is maintained at the predetermined constant
mean value by variation of the electrode field strength while the mean anode
current is maintained at a substantially constant and predetermined long-term
value, the mean anode current being indicative of the mean evaporation power.
In accordance with another aspect of the invention, the long-term
evaporation power of the high frequency dryer is maintained constant, while
the evaporation power is varied over the short-term to minimize moisture
deviations in the dried material from the final mean moisture value desired.
This is accomplished in accordance with the invention by maintaining the
electrode field strength of the high frequency dryer constant and by short-
term variation of the anode current thereof. Short-term variation of the
evaporation power to the high frequency dryer while maintaining the mean
long-term value of the evaporation power provided to the high frequency dryer
constant, will still provide the desired final mean moisture content of the
material.
In accordance with another aspect of the invention, it has been
found that equalization of the deviations of the moisture content from the
final mean moisture content depend upon the amount of moisture evaporated by
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the high frequency dryer as well as the desired mean final moisture content
itself. In accordance with this aspect of the invention, the ratio of the
deviations from the mean moisture content of the material before it enters
the high frequency dryer and after it emerges from the high frequency dryer
are used as a measure of the equali~ation.
A method according to the invention for drying material such as
paper web to obtain a predetermined substantially constant mean moisture
content in the dried material comprises the steps of: drying the material
in a first drying section, thereafter further drying the material in a second
capacitance high frequency dryer, maintaining the evaporation heating provided
to the high frequency dryer at a predetermined, substantially constant, mean
long-term value by varying the electrode field strength of the high frequency
dryer, and varying the mean evaporation heating provided to the first drying
section in accordance with the moisture content of the dried material after
it emerges rom the high frequency dryer.
The moisture content of the material is measured ater it emerges
from the first drying section and after it emerges from the high frequency
dryer, and the evaporation heating provided to the first drying section is
varied in accordance with the difference in the mean moisture contents of the
material before it enters the high frequency dryer and after it emerges from
the high frequency dryer. Preferably the evaporation heating provided to the
first drying section is varied to maintain this difference in the moisture
contents substantially constant.
According to one aspect, mentioned above, of the invention, the
evaporation heating provided to the high frequency dryer is varied over a
short term by varying the anode current thereof to minimize deviations of
the moisture content in the dried material from the predetermined mesn moisture
content and thereby equali~e the moisture profile of the dried material.
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In accordance with another aspect of the invention, the sub-
stantially constant value of the difference referred to above in the mean
moisture contents of the material is selectet in accordance with a ratio
of the mean deviation of the moisture content from the mean moisture content
before the material enters the high frequency dryer and after it emerges
from the high frequency dryer.
In a disclosed embodiment, the values of the measured moisture
content of the material and the desired predetermined final mean moisture
content of the dried material are supplied to a computer which includes a
program memory and which is programmed to compute the mean moisture values
and the mean deviations therefrom. The computer is further programmed to
select an optimum ratio of the mean deviation of the moisture content from
the mean moisture content of the material before the material enters the
high frequency dryer and after it emerges from the high frequency dryer, and
to control the first drying section and the high frequency dryer in accordanc0
with this optimum ratio and the computed and predetermined moisture content
values.
In accordance with the preferred embodiment of the invention, the
anode current of the high frequency dryer is used as an indication ofl and
as an equivalent for, the evaporation load of the high frequency dryer. As
mentioned, the evaporation load of the high frequency dryer can be held at a
predetermined value by maintaining the mean value of the anode current
constant. This can be done by changing the electrode field strength.
These and other aspects of the invention will be more apparent from
the following description of the preferred embodiment when considered with
the accompanying drawings.
The present invention is illustrated by way of example and not
limitation in the figures of the accompanying drawings in which:
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Figure 1 is a block diagram of a heating apparatus including a first
drying section and a high frequency dryer and the control system therefor, and
illustrates the method according to the invention, and
Figure 2 is a plot of a final mean moisture content UE o~ the
material in percent as the abscissa axis and the difference ~ UHF in mean
moisture contents before and after the high frequency dryer as the ordinate
axis for different equali~ation factors F and field strengths ~ r ~ and
illustrates the relationship between the equalization of drying in the high
frequency dryer and the evaporation load of the high frequency dryer.
Referring now more particularly to the drawings, a preferred
embodiment of the method according to the invention may be carried out by
the apparatus shown in block form in Figure 1 according to the relationships
shown in Figure 2.
In Figure 1, the apparatus is used to dry paper web 1 which is
advanced and is traveling in the direction of the arrow 2. The paper web is
first supplied to a conventional drying section 3 and is dried to a large
extent therein on drying cylinders (not shown). The paper web 1 is advanced
to a capacitance high frequency dryer 4 for further drying and equalization
of the moisture profile. The high frequency dryer 4 can be of the type
described in United States Patent No. 3,681,558 mentioned above.
The moisture content of the paper web is measured after it emerges
from the first drying section 3 and before it enters the high frequency dryer
4, and after it emerges from the high frequency dryer 4. The moisture contents
are measured by a traversing measuring apparatus 7 disposed between the drying
section 3 and the high frequency dryer 4 and by another traversing measuring
apparatus 5 disposed at the output of the high frequency dryer 4.
In Figure 1, UE designates the final mean moisture content in
percent of the web after it emerges from the high frequency dryer and ~ UE
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designates the mean deviation in percent of the moisture content of the
paper web from the mean moisture content UE after the web emerges from the
high frequency dryer. Ua designates the mean moisture content in percent of
the paper web after it emerges from the drying section 3 and ~ Ua designates
the mean deviation in percent of the moisture content of the paper web from
the mean moisture content after the web emerges from the drying section 3.
Nz designates the evaporation power of drying section 3 and NH the evaporation
power of the high frequency dryer 4. The mean moisture content UE and the
mean deviation therefrom a UE of the paper web after it emerges from the high
frequency dryer are computed from the values of the moisture content measured
by traversing apparatus 5 and the mean moisture content UA and the mean
deviation therefrom ~ Ua of the paper web after it emerges from the drying
section 3 are computed from the values of the moisture content measured by
traversing apparatus 7.
The evaporation power Nz of the drying section 3 is controlled by
control 31 and the evaporation power NH of the high frequency dryer 4 is
controlled by a control 41.
In accordance with the invention, to obtain a predetermined desired
final mean moisture content of the paper web, the evaporation load NH of the
high frequency dryer 4 is maintained substantially constant by control 41
while the evaporation load Nz of the drying section 3 is varied by control
31 in accordance with the mean value of the moisture content UE of the paper
web after it emerges from the high frequency dryer.
The field strength (or the electrode voltage) of the high frequency
dryer is regulated so that the mean value of the anode current of the high
frequency dryer is kept constant. In this manner, the mean evaporation load
NH of the high frequency dryer 4 is maintained substantially constant. The
evaporation load Nz of the drying section 3 is meanwhile varied in accordance
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with the difference in the mean moisture content UA of the paper web before
it enters the high frequency dryer and the mea~ moisture content UE of the
paper web after it emerges from the high frequency dryer. The difference
UA - UE in percent is designated A UHF and is maintained substantially
constant by variation of the evaporation load Nz.
In order to equalize the moisture profile of the paper web as it
emerges from the hi~h frequency dryer 4, the evaporation load NH provided to
the high frequency dryer 4 is varied over the short term by variation of the
anode current thereof. This tends to minimize deviations of the moisture
content in the dried paper web and thereby equalizes the moisture profile of
the dried paper web. Short-term variation of the evaporation load NH in the
high frequency dryer while maintaining the long-term evaporation load NH
substantially constant will provide the desired final mean moisture content
UE in the dried paper web.
The long-term time period for forming the mean values of the
moisture content can be, for example, about two minutes. In this manner,
the control 41, in conjunction with five times the requlating time of the
control 31, operates to provide an optimum equalization factor for drying.
As mentioned, short-term variations of the moisture content are equalized by
the high frequency dryer 4 at a constant field strength.
In accordance with the invention, a relationship has been found
between the final moisture content UE at the exit of the high frequency dryer
4, the mean deviation ~ UE therefrom, the mean moisture content UA at the
entrance of the high ~requency dryer and the mean deviation ~ Ua therefrom.
; This relationship is shown in Figure 2 in which the final mean moisture
content UE in percent isplotted against the difference ~ UHF in percent of
the mean moisture content UA less the mean moisture content UE. The equali-
zation factor F is defined as the ratio of UA
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As illustrated in Figure 2, it was found that theequali~ation factor
F depends on the moisture difference ~ UHF, i.e. on the evaporation load NH
of the high frequency dryer 4 as well as the mean final moisture UE. The
relationship, which has been confirmed by measurement results, between the
equalization factor F and moisture difference ~ UHF is illustrated for
different percentage final moisture contents UE. The relationship between
the equalization factor F and the final moisture contents UE and the
difference ~ UHF and the relative field strength ~ r is also illustrated
in Figure 2. The relative field strength is equal to E J where E is
E max
the instantaneous field strength and EmaX is the maximum field strength in
the high frequency dryer.
Referring now again to Figure 1, a computer 6 receives the measured
moisture contents of the paper web from the traversing apparatus 5 and 7 as
the web emerges from the drying section 3 and from the high frequency dryer
4, respectively. The computer calculates from these measurements the
moisture pro~ile, i.e., the average final moisture UE in percent, the mean
deviation ~ UE therefrom in percent, the mean moisture UA of the paper web
in percent after the web emerges from the drying section 3 and the mean
deviation ~ Ua therefrom in percent. The computer 6 is progra~med and includes
a program memory 61 which enables the computer to select the evaporation loads
Nz and NH of the drying section 3 and high frequency dryer 4, respectively.
The computer 6 is operative to vaTy the evaporation load Nz and select the
constant long-term load NH of the high frequency dryer in accordance with the
measured moisture contents to provide the final moisture content and moisture
profile in conjunction with the program memory 61.
Advantageously, the relationship of the equalization Factor F, the
field strength ~ r' the final moisture content UE and the moisture content
difference ~ UHF is entered into the computer 6 via the program and the program
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memory 61~ Thus, upon selection of the desired final moisture content, the
computer 6 in accordance with the measured values of the moisture contents
and the program and program memory 61, selects the optimum equalization factor
F and the evaporation loads Nz, NH.
To obtain an optimum and defined equalization factor F, it was
found that the moisture content difference ~ UHF should be maintained at a
selected constant value rather than permitting the value of ~ UHF to vary.
Referring to Figure 2 again, if ~ UHF were not held constant, then a reduction
of the equalization factor F from 2.75 (operating point A) to 2.00 (operating
point B) would result in, for a desired final moisture of 7%, a reduction of
the value of ~ UHF from about 7 to about 3. By maintaining the moisture con-
tent difference ~ UHF substantially constant at a predetermined value, the
long-term mean of the evaporation load NH may be maintained constant, as
mentioned above, thd the desired final moisture content of the paper web may
be obtained by varying the evaporation load Nz of the drying section 3. Thus,
the field strength of the high frequency dryer 4 may be varied to move the
operating point in the direction of (a). Deviations of the moisture content
of the paper web as it emerges from the high frequency dryer 4 can be leveled
out by maintaining the field str0ngth of the high frequency dryer constant
over the short term and by varying the anode current thereof. Suitable power
reserves are maintained for this purpose. Variation of the evaporation load
NH of the high frequency dryer 4 moves the operating point in the direction
(b)-
As mentioned above, the computer is programmed and the computermemory 61 includes the information set forth in Figure 2. Thus, upon
selecting the desired final moisture content, the computer controls operation
of the drying section 3 and the high frequency dryer 4 to provide the optimum
equalization factoT F, ~he constant long-term evaporation load NH of high
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frequency dryer 4, the control of the evaporation load Nz of the drying
section 3, and the short-term variation of the evaporation load of the high
frequency dryer 4.
In accordance with the procedure described above, equalization of
the moisture profile of the paper web may be obtained with an optimum
distribution of the heating power provided to the drying section 3 and the
high frequency dryer 4. The drying capability of the dryer is thereby
increased and energy is saved.
The advantages of the present invention as well as certain changes
and modifications of the disclosed embodiments thereof will be readily
apparent to those skilled in the art. It is the applicant's intention to
cover by his claims all those changes and modifications which could be made
to the embodiments of the invention herein chosen for the purposes of the
disclosure without departing from the spirit and scope of the invention.
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