[0001] The present invention refers to a drying and desiccating system for plants for producing
web-like paper material, in particular but not exclusively, for plants for producing
rolls of toilet paper.
[0002] As it is known, in the production process of paper in general, and in particular
in the production of toilet paper, it is necessary to carry out an evaporative drying
step of the product being processed, in order to extract the excess water content.
The product that must be dried, normally made up of a paper pulp diluted with water,
is prepared and sent to the drying and desiccating system after a preliminary pressing
step. At the inlet of the drying and desiccating system, the sheet of paper being
processed contains a low content of solid part, equal to about 35%÷40% at a temperature
of around 35°C. In other words, after the pressing step, the sheet of paper being
processed can still contain up to 65% of water. The pressing step, therefore, is not
capable of eliminating all the water from the paper fibres, which must therefore be
removed through evaporation.
[0003] The finished product, in the specific case consisting of toilet paper, requires a
content of solid part that is much greater than the values mentioned above. It is
thus clear that in the evaporative drying step, it is necessary to extract most of
the residual water content from the sheet of paper, so as to obtain a continuous sheet
of paper that is sufficiently dry. After the evaporative drying and desiccating step,
the sheet of paper is stored in reels so as to be subsequently processed (so-called
"converting" step) and finally packed for delivery and for the final retail selling.
[0004] The evaporative drying and desiccating step of toilet paper is currently carried
out through a system provided with two separate devices, which act simultaneously,
however, upon the sheet of paper during such a drying and desiccating step.
[0005] A first drying device consists of a high yield hood, which provides for blowing hot
air, at a temperature of up to about 650°C in hoods of the latest generation, on the
sheet of paper being processed. The hot air is normally produced with a direct fire
burner.
[0006] High yield hoods also have the need of discharging the water vapour extracted from
the sheet of paper being processed into the atmosphere. The flow consisting of water
vapour and of the air transporting it is called in jargon "mist". This "mist" has
a high temperature, of about 400°C÷550°C in plants of the latest generation. Consequently,
the dispersion in the atmosphere of the "mist" substantially cuts down the thermal
efficiency of high yield hoods.
[0007] Simultaneously with the blowing, the sheet of paper being processed comes into contact
with a cylinder heated with steam having a diameter that can vary from about 1.5 m
to about 6 m. The cylinder consists of a container under pressure that contains process
steam up to about 10 bar G and more, inside it. The steam is normally produced by
conventional boilers, fire tubes, water tubes, or similar technologies, in turn equipped
with (methane, LPG, gas oil, etc.) fuel burners that have a maximum combustion efficiency
equal to about 90%.
[0008] In current drying and desiccating systems, all or part of the "mist" produced by
the high yield hood can be used to preheat the process air and the comburent air with
which the burners of the hood itself are fed. In some drying and desiccating systems,
like for example that described in the patent application for an industrial invention
N.
IT2007LU000003A, to the same applicant, the "mist" produced by the high yield hood can also be used
to produce low temperature and low pressure steam (about 3 bar G), which can be used
for feeding a steam box that is suitable for correcting the humidity cross-section.
In other solutions, like for example that described in the patent application for
the industrial invention N.
IT2009MI000364A, to the same applicant, the "mist" produced by the high yield hood can finally be
used to produce high pressure steam (up to 20 bar G), which can be directly reused
in the drying process thanks to the presence of a boiler that operates to aid the
main one.
[0009] However, the energy of the "mist" is not capable of generating all the steam necessary
for the drying process, but only 60%÷70% of such steam, therefore, it is unavoidably
necessary for there to be an external steam generator so as to provide for the remaining
amount. Moreover, such an external steam generator is necessary for the transient
steps of the drying process, such as the start up step of the entire plant for producing
the roll of paper (the so called "warm up" step) and/or the case in which the roll
of paper itself is broken.
[0010] In the document
US 4,615,122, a method for feeding steam and hot air to the desiccating cylinder in a drying and
desiccating system is described, in which the boiler is integrated with the hood,
potentially eliminating the external boiler. Such a solution has a configuration in
series between the boiler and the hood, with the problem of not being able to carry
out a separate regulation between the thermal conditions of the hood and those of
the boiler, since the hood and the boiler have the air flows in series and are regulated
by a single burner. It is thus possible to eliminate the external boiler, at the expense,
however, of the flexibility of regulation of the thermal conditions of the entire
drying and desiccating system and, consequently, at the expense of the production
flexibility of the desiccated paper.
[0011] The general purpose of the present invention, therefore, is that of making a drying
and desiccating system for plants for producing web-like paper material, in particular
but not exclusively for plants for producing rolls of toilet paper, that is capable
of overcoming the aforementioned problems of the systems made according to the prior
art.
[0012] In particular, one purpose of the present invention is that of making a drying and
desiccating system for plants for producing web-like paper material that has no external
steam generator whatsoever, so as to be completely independent both during the normal
drying step of the paper, and during the transient steps.
[0013] Another purpose of the invention is also that of making a drying and desiccating
system for plants for producing web-like paper material, in which both the thermal
conditions of the hood, and those of the boiler, can be regulated in a completely
independent manner, making it possible to regulate the temperature, operation pressures
and circulating air masses that are completely different for the two separate drying
devices (hood and boiler).
[0014] A further purpose of the invention is finally that of making a drying and desiccating
system for plants for producing web-like paper material, that is capable of reducing
both the energy consumption of the overall plant, and the pollutant emissions that
are dispersed into the atmosphere by the plant itself, since, as previously outlined,
the independent regulation of the hood and boiler allows the thermal conditions to
be configured in the most suitable way for the type of paper being processed.
[0015] These purposes according to the present invention are achieved by making a drying
and desiccating system for plants for producing web-like paper material, in particular
but not exclusively, for plants for producing rolls of toilet paper, as outlined in
claim 1.
[0016] Further characteristics of the invention are highlighted in the dependent claims,
which are an integrating part of the present description.
[0017] The characteristics and the advantages of a drying and desiccating system for plants
for producing web-like paper material according to the present invention shall become
clearer from the following description, given as an example and not for limiting purposes,
with reference to the attached schematic drawings, in which the single figure is a
schematic view of a preferred embodiment of such a system.
[0018] It should be made clear that, in the attached figure, identical reference numerals
indicate elements that are the same or equivalent.
[0019] With reference to figure 1, an evaporative drying and desiccating system, which can
be used inside a plant for producing web-like paper material is shown, made according
to the present invention. The system is wholly indicated with reference numeral 10.
[0020] The drying and desiccating system 10 firstly comprises a first drying device 12,
of the heated rotating cylinder type, also called "Yankee cylinder", on which the
web-like paper material to be desiccated (not shown) is dynamically wound, made up
of a sheet of paper having a high water content. The sheet of paper reaches the Yankee
cylinder 12 from the previous processing station, typically a pressing station, which
is well known in the state of the art and shall therefore not be described in detail
in the present description. Depending on how the pressing step of the sheet of paper
is carried out, however, the amount of water present in the sheet itself, can vary,
up to more than 60% by weight.
[0021] The Yankee cylinder 12 typically has a diameter that can vary from about 1.5 m to
about 6 m, whereas its width is equal to the width of the most common standard formats
of reels used in plants for processing paper. The Yankee cylinder 12 is made in the
form of a pressurised steam container, which contains a pipe system (not shown) inside
it, for the removal of the condensate.
[0022] The drying and desiccating system 10, therefore, comprises a second drying device
14, of the hood type, which at least partially covers the Yankee cylinder 12 and that
is capable of both blowing high temperature dry air on the sheet of paper wound on
said Yankee cylinder 12, and sucking the moist air, also at high temperature,and in
jargon called "mist", released from the sheet of paper itself during its desiccation
step. The dry air blown by the hood 14 typically has a temperature that can reach
about 700°C and more, whereas the moist air extracted from the sheet of paper can
have a temperature of up to 450°C and more.
[0023] The high temperature dry air, blown by the hood 14, is produced by a burner 16 that
uses a fuel (for example methane, LPG or gas oil) fed through a suitable feed pipe
18. Air, drawn from the atmosphere and conveyed, through a suitable inlet pipe 20
and by means of a fan 22, to the burner 16 itself, is used as a comburent. The flow
of combusted gas and air in outlet from the burner 16 forms the dry air for drying
and desiccating, which is sent in pressure to the hood 14 through one or more inlet
pipes 24.
[0024] The moist air at high temperature, extracted from the hood 14 from the sheet of paper
being processed, is on the other hand conveyed through one or more release pipes 26
and is at least partially sent, by means of a recirculation fan 28, to the burner
16 so that it can be heated and reused by the hood 14 itself for drying the sheet
of paper. The remaining part of moist air at high temperature extracted from the hood
14 is finally sent, through the release pipes 26, towards an air-air heat exchanger
30 (not always present), where it gives up part of its heat to the outside air to
be sent to the burner 16, and reintegration air, through the inlet pipe 20, and is
therefore expelled from the system 10 by means of an exhaust and recirculation fan
32.
[0025] Along the pipes 26 for releasing the moist air from the hood 14, at least one waste
heat boiler 34 is arranged, fed by such moist air or "mist" and that is able to generate
high pressure steam (between about 10 bar G and about 20 bar G) so as to in turn feed
a thermocompressor 36 and a reintegration or "make-up" valve 70, through a delivery
pipe 40, which deliver the steam to the Yankee cylinder 12. The steam produced by
the waste heat boiler 34 thus minimises, in the system 10, the use of a conventional
fuel boiler for feeding the Yankee cylinder 12.
[0026] According to the invention, along the pipes 26 for releasing the moist air from the
hood 14, upstream of the waste heat boiler 34, there is at least one auxiliary burner
58, which uses a fuel drawn through a first feed pipe 60 and that draws air, used
as a comburent, directly from the atmosphere. The comburent air is conveyed to the
auxiliary burner 58 through a second feed pipe 62, directly connected with the inlet
pipe 20 which brings dry air drawn from the atmosphere to the burner 16 of the hood
14, and a fan 66 for the comburent air.
[0027] The auxiliary burner 58 makes it possible to obtain an increase of the temperature
of the moist air, arriving from the hood 14 and from a recirculation and balancing
pipe system 72 and to be sent to the waste heat boiler 34, from a value of about 400°C÷420°C
up to about 600°C and more. Therefore, by introducing the auxiliary burner 58 before
the tube bundle of the waste heat boiler 34, it is possible for the drying and desiccating
system 10, according to the invention, to not have the conventional external boiler.
[0028] Moreover, by means of the pipes 72 and of one or more adjustment proportioning valves
38 there is a second recirculation and balancing circuit, in addition to that formed
by the release pipes 26, also for the air flow of the waste heat boiler 34. The regulation
of the flow of such recirculated air occurs through the adjustment proportioning valves
64 and 38 arranged on the release pipes 26 and on the recirculation and balancing
pipes 72, respectively, with the purpose of increasing the mass of air circulating
through the waste heat boiler 34 by an amount necessary so as to obtain the required
amount of steam, without modifying the process conditions of the hood 14 and of the
main burner 16. Consequently, the system 10 is provided with two separate groups for
feeding the hood 14 in cascade on the waste heat boiler 34. By using two separate
burners 16 and 58, two separate recirculation circuits 26 and 72 and a series of adjustment
proportioning valves 64 and 38, it is possible, in real time, to obtain the balancing
and the continuous and independent regulation of the air flows of the hood 14 and
of the waste heat boiler 34 and, therefore, of the relative heat and energy conditions.
[0029] Along the pipes 26 for releasing the moist air from the hood 14, upstream of the
auxiliary burner 58 and of the waste heat boiler 34, one or more gate valves can be
foreseen, not illustrated in the figure, that make it possible to by-pass the waste
heat boiler 34 and/or the auxiliary burner 58, for safety reasons.
[0030] The drying and desiccating system 10, therefore, comprises a first separator tank
42 that is able to collect the "flash steam" and the "blow-through steam", as well
as the hot condensates, coming from the Yankee cylinder 12 through a pipe 44. From
the first separator tank 42, the flash steam and the blow-through steam are reintroduced
into the thermocompressor 36 through a pipe 46, whereas the hot condensates are sent,
through a return pipe 48 and by means of a first circulation pump 50 for feeding the
waste heat boiler 34, to a second accumulator and separator tank 52.
[0031] A second circulation pump 54 pushes the condensates contained in the second accumulator
and separator tank 52, through a suitable pipe 56, in the tube bundle of the waste
heat boiler 34, that is thus capable of generating a biphasic fluid. Such a biphasic
fluid is sent to an upper cylinder body 68 of the waste heat boiler 34, that thus
operates as a water tube boiler: the liquid part of the biphasic fluid is reintroduced
into the accumulator and separator tank 52, operating as a lower cylinder body of
the waste heat boiler 34, whereas the part of steam of the biphasic fluid itself,
through the delivery pipe 40, feeds the thermocompressor 36 and the reintegration
or "make-up" valve 70.
[0032] As well as the components described thus far and illustrated in the figure, the drying
and desiccating system 10 can be equipped with all the devices for controlling the
level, the temperature and the pressure (not shown) that are necessary for it to operate
correctly, as well as a possible electronic programmable logic control system (PLC).
[0033] The waste heat boiler 34 is also equipped with all the foreseen safety equipment
(safety valve, manual resetting thermostat, etc.) that allow it to be approved in
compliance with the directive PED 97/23/EC relative to the safety of such devices.
[0034] It has thus been seen that the drying and desiccating system for plants for producing
web-like paper material, according to the present invention, achieves the purposes
previously highlighted. Thanks to the presence of the waste heat boiler, which uses
the energy of the exhaust "mist" of the drying devices, and of the auxiliary burner,
which in turn increases the energy of such a "mist", it is indeed possible to use
the steam produced in the drying and desiccating production process, thus leading
to a sensitive reduction of the incidence of specific energy per unit mass of finished
product. Moreover, there is also a reduction of CO
2 emissions in the overall production process of the roll of paper, since the auxiliary
burner, using "mists" at high temperature like the air in inlet to the combustion
chamber, optimises the combustion process.
[0035] The drying and desiccating system for plants for producing web-like paper material
of the present invention thus conceived can in any case undergo numerous modifications
and variants, all covered by the same inventive concept; moreover, all the details
can be replaced by technically equivalent elements. In practice, the materials used,
as well as the shapes and sizes, can be any according to the technical requirements.
[0036] The scope of protection of the invention is thus defined by the attached claims.
1. Evaporative drying and desiccating system (10) for a web-like paper material, the
system (10) comprising:
- at least one first drying device (12), of the pressurised rotating cylinder type
fed by steam, on which the web-like paper material to be desiccated is dynamically
wound, said cylinder (12) being internally provided with a pipe system for the removal
of the condensate,
- at least one second drying device (14), of the hood type, which at least partially
covers said cylinder (12) and that comprises at least one first burner (16) capable
of producing high temperature dry air, said hood (14) being both capable of blowing
said high temperature dry air onto the web-like paper material wound on said cylinder
(12), and of sucking the moist air released by said web-like paper material, and
- at least one waste heat boiler (34), fed by the moist air released by the web-like
paper material and disposed along the pipes (26) for releasing said moist air from
said hood (14), said waste heat boiler (34) being capable of generating high pressure
steam for feeding a thermocompressor (36) and a make-up valve (70) which deliver steam
to said cylinder (12),
characterised in that along the pipes (26) for releasing said moist air from said hood (14), upstream of
said waste heat boiler (34), there is also at least one auxiliary burner (58), capable
of increasing the temperature of said moist air coming from said hood (14) and to
be sent to said waste heat boiler (34).
2. System (10) according to claim 1, characterised in that said auxiliary burner (58) uses a fuel drawn through a first feed pipe (60) and draws
the air, used as a comburent, directly from the atmosphere through a second feed pipe
(62), directly connected with an inlet pipe (20) which brings dry air drawn from the
atmosphere to said hood (14), and through a fan (66) for comburent air.
3. System (10) according to claim 1 or 2, characterised in that said auxiliary burner (58) is capable of increasing the temperature of said moist
air coming from said hood (14) from a value of about 400°C÷420°C to a value of about
600°C and more.
4. System (10) according to any one of claims 1 to 3, characterised in that said waste heat boiler (34) is capable of generating steam at a pressure comprised
between about 10 bar G and about 20 bar G.
5. System (10) according to any one of the preceding claims, also comprising at least
one exhaust and recirculation fan (32) and a recirculation and balancing pipe system
(72) capable of forming a second recirculation and balancing circuit, in addition
to that formed by the pipes (26) for releasing the moist air from said hood (14),
for the air flows of said waste heat boiler (34) and of said auxiliary burner (58).
6. System (10) according to claim 5, characterised in that along the pipes (26) for releasing the moist air from said hood (14), upstream of
said auxiliary burner (58) and of said waste heat boiler (34), and along said recirculation
and balancing pipe system (72) one or more adjustment proportioning valves (64; 38)
are foreseen capable of sending amounts of moist air released by the web-like paper
material both into the atmosphere, and to said auxiliary burner (58), as well as being
capable of reintroducing it into the circuits of said hood (14) even after having
passed through said waste heat boiler (34), said one or more adjustment proportioning
valves (64; 38) regulating the flows continuously and allowing the system to be balanced
(10) in real time.
7. System (10) according to any one of the preceding claims, also comprising at least
one first separator tank (42) capable of receiving the steam and the hot condensates
coming from said cylinder (12), the steam coming from said cylinder (12) being reintroduced
in said thermocompressor (36) through a pipe (46).
8. System (10) according to claim 7, also comprising at least one second accumulator
and separator tank (52), said second accumulator and separator tank (52) receiving
the hot condensates coming from said first separator tank (42) through a return pipe
(48) and by means of at least one first circulation pump (50) for feeding said waste
heat boiler (34).
9. System (10) according to claim 8, also comprising at least one second circulation
pump (54) capable of sending to said waste heat boiler (34) the hot condensates contained
in said second accumulator and separator tank (52), said waste heat boiler (34) thus
being capable of generating a biphasic fluid.
10. System (10) according to claim 9, characterised in that said biphasic fluid is sent to an upper cylinder body (68) of said waste heat boiler
(34), the liquid part of said biphasic fluid being reintroduced into said second accumulator
and separator tank (52), operating as a lower cylinder body of said waste heat boiler
(34), and the steam part of said fluid being sent to a delivery pipe (40) which connects
said waste heat boiler (34) with said thermocompressor (36) and said make-up valve
(70).