[0001] The present invention relates to a method and arrangement for stabilising operation
of an impingement dryer in a paper or board machine according to the preambles of
the enclosed independent claims.
[0002] Impingement dryer in a paper machine or the like uses burners for heating the air
which is used for drying the fibrous web. The burner chamber may be integrated inside
the hood of the impingement dryer in order to save space and minimise heat losses.
Ideally the flame of the burner head should burn steadily in the burner chamber and
be surrounded by the combustion gases. Ideally the air to be heated should flow from
the air inlet of the burner chamber past the flame, almost parallel to it and without
disturbing the flame. The hottest combustion gases should be directed to the pressure
manifolds of the impingement dryer, from which manifolds the heated air is distributed
to the drying members, such as drying nozzles.
[0003] In practice, however, the space inside the hood of the impingement dryer is limited.
Therefore air is often brought towards the burner head and the flame in an angle,
sometimes even at straight angle to burner head and the flame. The result is that
the air entering to the burner chamber pushes the flame towards the wall of the burner
chamber, which may become overheated when the extremely hot flame comes into a contact
with it. The flame temperature may typically be, for example, around 1500 °C at the
core of the flame. This kind of temperature is too high for normal heat-resistant
steel qualities, which easily results in damages in the burner chamber wall. If the
burner chamber wall is damaged, the hood of the impingement dryer is damaged and/or
the outer surface of the hood is locally overheated. This leads to risks for other
equipment and for operating personnel of the paper machine. It has not been possible
to solve these problems by selecting materials with better heat-resistance for the
burner chamber wall constructions, as the heat is also transferred by conduction through
the wall material to the structures of the impingement dryer, which leads to local
overheating and damage. A solution, where an insulation layer is arranged around the
burner chamber walls, has been in use. However, it is difficult to cover the whole
surface of the burner chamber, and the insulation layer reduces the already scarce
space available inside the impingement dryer hood. Furthermore, the insulation layer
is typically attached to the dryer structures by using fasteners or supports made
of metal, which easily lead heat to unwanted locations. Also, if an interstice is
accidentally left or formed between the individual slabs of the insulation layer,
the heat leakage from the interstice forms a serious local overheating problem.
[0004] Impingement dryers with integrated burner heads may also suffer from flame instability
and flame control problems, which are due to the incoming air disturbing the burner
chamber conditions.
[0005] An object of the present invention is to minimise or even completely eliminate the
problems existing in the prior art.
[0006] One object of the present invention is to stabilise the function of the burner chamber
of an impingement dryer and to reduce the damages and downtime due to the overheating.
[0007] These objects are attained with the invention having the characteristics presented
below in the characterising parts of the independent claims.
[0008] Some preferred embodiments of the present invention are presented in the dependent
claims.
[0009] The embodiments mentioned in this text relate, where applicable, to the impingement
dryer as well as to the method according to the invention, even if this is not always
separately mentioned.
[0010] Typical method according to the present invention for stabilising operation of an
impingement dryer in a paper or board machine or the like, comprises
- arranging air to be heated to enter a burner chamber of the impingement dryer through
an air inlet opening of the burner chamber, the air entering in an angle in relation
to at least one burner head in the burner chamber, the burner head having a flame
opening in its first end,
- heating air in the burner chamber by bringing the air into a contact with a flame,
- transferring heated air from the burner chamber to blowing members, such as blowing
nozzles, of the impingement dryer,
- arranging between the air inlet opening of the burner chamber and the flame opening
of the at least one burner head, in the direction of the air flow, a guiding member
comprising at least one aperture, and
- dividing the air flow, which enters through the air inlet opening of the burner chamber
so that a first air flow travels through the at least one aperture of the guiding
member and a second air flow travels through a gap formed between the guiding member
and the burner head.
[0011] Typical arrangement according to the present invention for stabilising flame conditions
in a burner chamber of an impingement dryer in a paper machine, board machine or the
like, comprises
- a burner chamber, delimited by burner chamber walls, and comprising an air inlet opening
for entry of air to be heated,
- an air inlet channel connected to the air inlet opening of the burner chamber,
- at least one burner head comprising
- a flame opening for a flame at a first end of the burner head,
- at least one feed connection at an opposite second end of the burner head,
- a burner axis extending from the first end to the second end of the burner head,
whereby the burner head is arranged in the burner chamber in vicinity of the air inlet
opening, the burner axis being in an angle in relation to the gas inlet channel. The
arrangement comprises further a guiding member, which comprises at least one aperture,
is arranged between the air inlet opening of the burner chamber and the flame opening
of the at least one burner head, the guiding member covering,
[0012] in the flow direction, the flame opening, whereby a gap is formed between the burner
head and the guiding member.
[0013] Now it has been surprisingly found out that the flame of a burner head can be easily
stabilized if a guiding member with apertures, preferably a guiding plate, is arranged
between the air inlet opening of the burner chamber and the flame opening of the burner
head. The guiding member divides the air flow entering the burner chamber into a first
air flow and a second air flow, the first air flow travelling through the apertures
of the guiding member and a second air flow travelling chamber through a gap, which
is formed between the guiding member and the burner head. In this manner the air flow
entering the burner chamber does not collide with full force with the flame, but the
air flow is directed further away from the flame opening as well as at least partially
aligned with the flame direction. It has been observed that in the impingement dryer
comprising an arrangement according to the present invention the flame is more stable,
and it does get into contact with chamber walls as much, if at all, as in conventional
impingement dryers.
[0014] According to one preferred embodiment of the invention the guiding member is preferably
a guiding plate, which comprises at least one aperture, preferably a plurality of
adjacent apertures. Guiding member may be made of heat-resistant steel. The guiding
member comprises preferably a plurality of apertures, which have a height of 100 -
500 mm, preferably 150 - 250 mm, and width of 300 - 2500 mm, preferably 500 - 1500
mm. The distance between two adjacent apertures is typically 10 - 200 mm, preferably
20 - 120 mm. The guiding member may be curved or otherwise formed in an appropriate
way. Usually the guiding member extends through the whole burner chamber, from its
first side wall to the second side wall.
[0015] The guiding member is arranged between the air inlet opening of the burner chamber
and the flame opening(s), so that a gap is formed between burner head and the guiding
member. Typically the height of the gap is 10 - 70 mm, preferably 20 - 50 mm. Furthermore,
the guiding member is arranged between the air inlet opening and the flame opening
of the burner head so that a straight flow from the air inlet opening to the flame
opening is prevented. One purpose of the guiding member is to divide the air flow,
and to guide the first part of the air flow away from the area around the flame opening
so that that the first air flow comes into contact with the flame and the surrounding
combustion gases first in the main part of the burner chamber. At the same time, another
purpose of the guiding member is to guide the second part of the air flow to the burner
chamber in a direction, which is parallel to the flame, and thus reduce the flame
disturbance.
[0016] The first and second air flows are selected appropriately, depending on the design
of the burner chamber, burner head(s) and flame temperatures. Generally it may be
concluded that if the second air flow is too small the guiding member may be overheated
by the flame. On the other hand, if the second air flow is too large the pressure
loss of the arrangement increases and the second air flow may even start to disturb
the flame. According to one embodiment of the invention the proportion of the first
air flow to the second air flow is 80:20 - 97:3, preferably 85:15 - 95:5.
[0017] The arrangement according to present invention is especially suitable for use in
impingement dryers, where the air flow entering the burner chamber is introduced into
the chamber from the side of the burner, i.e. the flow direction of the entering or
incoming air flow is not parallel with the direction of an undisturbed burner flame.
In other words, there is an angular relationship between the incoming air flow and
the undisturbed flame. Typically the angle between the incoming air flow and the undisturbed
flame is in the range of 70 - 100 degrees, typically about 90 degrees. In other words,
the angle between the length axis of the air inlet channel and the burner head axis
is in the range of 70 - 100 degrees, typically about 90 degrees.
[0018] In this application the term "impingement dryer" describes a drying device located
at the drying section of a paper machine or the like. Typical impingement dryer comprises
a burner chamber, which is integrated inside the hood of the impingement dryer. In
the burner chamber the air is heated to a temperature 200 - 500 °C by bringing it
into a contact with a flame(s) of the burner head(s). The heated air is led from the
burner chamber through pressure manifolds to blowing members, such as blowing nozzles,
which blow the air with high velocity towards a web to be dried. The drying of the
web occurs without contact, i.e. the impingement dryer is a contactless drying device.
[0019] The burner chamber walls are typically made of heat resistant steel. The burner chamber
comprises an air inlet opening for the incoming air which is to be heated and outlet
opening(s) for leading the heated air out of the burner chamber to the pressure manifolds.
At least one burner head is arranged in the burner chamber near the vicinity of the
air inlet opening. Preferably the burner chamber comprises a plurality of adjacent
burner heads, each with a flame opening. The number of burner heads depends on the
width of the burner chamber. Normally there is 1 - 6 adjacent, preferably 2 - 5, burner
heads in one burner chamber. Distance between two adjacent burner heads may be 200
- 2000 mm, preferably 400 - 1200 mm.
[0020] The burner head comprises a first end and a second end, and a burner head axis extending
from the first end to the second end of the burner. The burner head axis is usually
arranged in an angle in relation to the air inlet channel. The first end of the burner
head faces the main part of the burner chamber and comprises a flame opening. The
main part of the burner chamber is here understood as that part of the burner chamber,
which extends from the flame opening of the burner head to the outlet opening(s) of
the burner chamber. The second end of the burner head is connected to at least one
feed connection for feeding the appropriate fuel and combustion air to the burner
head and the flame. Typically the temperature of the flame is 800 - 2500 °C, more
typically 1000 - 1500 °C and natural gas, liquefied natural gas, liquefied petroleum
gas, diesel oil or kerosene may be used as fuel.
[0021] According to one preferred embodiment of the present invention the total cross-sectional
area of the feed connection(s) is smaller than the cross-sectional area of the burner
head. In case the burner head is connected to several feed connections, their cross-sectional
areas are summed together in order to obtain the total cross-sectional area. The total
cross-sectional area of the feed connection(s) is normally compared to the cross-sectional
area of the burner head at the flame opening. The narrow feed connection(s) enable
that air may also flow behind the burner from the first side surface of the burner
head to the second side surface of the burner head. Thus it is possible to form a
flow channel between the second end of the burner head and a burner chamber end wall,
i.e. behind the burner head, as well as between the burner head and a burner chamber
wall, and divide the air flow, which enters through the air inlet opening of the burner
chamber, so that a third air flow travels through the flow channel. The third air
flow thus enters the burner chamber between the burner head and the burner chamber
wall, on the opposite side of the burner head than the second air flow. The volume
of the third air flow may be 10 - 50 %, preferably 20 - 45 %, more preferably 30 -
45 %, of the total volume of the air flow, which enters the burner chamber through
the air inlet opening.
[0022] According to one embodiment of the invention a division plate may be arranged in
the flow channel between the burner chamber wall and the burner head in order to divide
the third air flow further into a cooling flow and a heated flow, which passes the
flame. The division plate is typically made of heat-resistant steel. In this manner
it is possible to use the air flow nearest to the burner chamber wall for cooling
the burner chamber wall and the air flow nearest to the flame for transfer of thermal
energy, while minimising the flame disturbance. Thus, the local overheating may be
avoided and the thermal energy is better transferred to the air flow, which is to
be heated. Typically the third air flow is divided so that the proportion of the cooling
flow to the heated flow may be 5 - 50 %, preferably 10 - 35 %. An appropriate arrangement
of the division plate provides sufficient flow between the division plate and the
burner chamber wall. If the distance between the burner chamber wall and the division
plate is too small, the cooling air flow may not effectively inhibit heat conduction
to the burner chamber wall. The division plate is typically arranged at a first distance
of 5 - 50 mm, preferably 10 - 30 mm from the burner chamber wall, and at a second
distance of 50 - 300 mm, preferably 80 - 200 mm from the burner head.
[0023] At least a part of the air flow entering through the air inlet opening of the burner
chamber may be recirculated from the space between the blowing members, such as blowing
nozzles, and the web. The impingement dryer may thus comprise means for removing humid
air from the space between the blowing members and the web as well as means for circulating
this air back to the air inlet opening of the impingement dryer. The temperature of
the circulated air may be typically 100 - 350 °C, more typically approximately 250
°C. Use of circulated air reduces the need for heating of the air in the burner chamber
and saves thus energy.
[0024] According to one embodiment of the invention at least a part of the air flow entering
through the air inlet opening of the burner chamber is preheated air, which taken
from outside of the impingement dryer, for example from machine room or from outdoors.
For example, machine room air may be preheated to a temperature of 100 - 250 °C, typically
150 - 200 °C by using available heat recovery means, such as heat recovery means of
the drying section of a paper mill or heat recovery means of the gas heated dryer(s)
itself. The preheated air may also be used as combustion air which is fed to the burner
head(s) through feeding connection(s). Thus the waste heat of the drying section of
a paper machine may used for preheating the air flow and/or the combustion air.
[0025] According one embodiment of the invention air from the preheated air is mixed before
the burner chamber with the air recirculated from the space between the blowing members
and the web. In other words, machine room air and/or outdoor air may be added to and
mixed as makeup air to circulated air before the air flow enters the burner chamber.
Typically the amount of makeup air from outside the impingement dryer hood is 3 -
20 weight-%, preferably 5 - 15 weight-%.
[0026] The invention is described in more detail below with reference to the enclosed schematic
drawings, in which
- Figure 1
- shows a schematic view of an impingement dryer,
- Figure 2
- shows a burner chamber arrangement of an impingement dryer according to the present
invention.
[0027] Figure 1 shows a schematic view of an impingement dryer. The impingement dryer 1
comprises a hood 2 and a number of blowing nozzles 3, 3', 3", with which heated air
is blown against the web to be dried (not shown). Inside the hood 2 have been arranged
a burner chamber 4, in which the drying air is heated to an elevated temperature,
as well as pressure manifolds 5 for transferring the heated air from the burner chamber
4 to the blowing nozzles 3, 3', 3". The air in the impingement dryer is circulated
by using a circulating fan 6. The fan 6 causes the air to flow through an air inlet
channel 10 to the burner chamber 4 through an air inlet opening 11. The air flowing
through the burner chamber 4 from the air inlet opening 11 to the inlets of the pressure
manifolds 5 is heated to a desired temperature when it is flowing past a flame 9.The
air flows inside the impingement dryer are described in Figure 1 by using arrows.
[0028] To the first end 4' of the burner chamber 4 is arranged a burner head 7. The burner
head 7 comprises a flame opening 8 at the first end of the burner head 7. In the second
end of the burner head 7 there is feed connections 12 for feeding the fuel, such as
natural gas, as well as combustion air to the burner 7.
[0029] The flame 9 is presented in Figure 1 as undisturbed, under optimal conditions. However,
it is easily conceivable from Figure 1 that in a conventional impingement dryer there
is a risk that the air flow entering the burner chamber via the air inlet opening
11 disturbs the flame 9 and "pushes" it towards the wall of the burner chamber 4.
[0030] Figure 2 shows a burner chamber arrangement of an impingement dryer according to
the present invention. A burner head 7 is arranged in the first end 4' of the burner
chamber 4. In the vicinity of the burner head 7, perpendicularly to the burner axis
71, an air inlet channel 10 is connected to the burner chamber 4 through an air inlet
opening 11. A guiding member 13 is arranged between the flame opening 8 and the air
inlet opening 11. The guiding member 13 is formed to divide the air flow coming from
the air inlet channel 10 and entering the burning chamber 4 through the air inlet
opening 11 into a first air flow 14 and to a second air flow 15. The air flows 14,
15 are designated with arrows. The first air flow 14 enters the main burner chamber
4" through apertures (not shown) of the guiding member 13. In this manner the entry
of the first air flow 14 in to the main burner chamber 4" is displaced in relation
to the flame opening 8, and shifted towards the centre of the burner chamber 4. In
other words, the disturbances to the flame originating from the first air flow 14
are minimised, even totally eliminated. The second air flow 15 enters the main burner
chamber 4" through a gap 16, which is formed between the burner head 7 and the guiding
member 13. In this manner the second air flow 15 flows parallel to the flame past
the flame opening 8. In other words, the second air flow 15 minimises, even totally
eliminates the disturbances around the flame (not shown). At the same time the second
air flow 15 cools the guiding member 13 and reduces the risk that the guiding member
13 is overheated by the flame.
[0031] A feed connection 17 for feeding fuel and combustion air to the burner head 7 is
arranged to the second end of the burner head 7. It is possible to arrange the cross-sectional
area of the feed connection 17 smaller than the cross-sectional area of the flame
part 7A of the burner head 7. In this manner it is possible, if so required, to form
a flow channel 18 between the second end of the burner head 7 and a burner chamber
end wall 19'. The flow channel 18 continues between the burner head 7 and a burner
chamber wall 19 in the burner chamber 4. Thus a part of the air flow, which enters
the burner chamber 4 through the air inlet opening 11, forms a third air flow 20,
depicted with arrows.
[0032] Further, it is possible, but optional, to arrange a division plate 21 to the flow
channel 18, to the space between the burner chamber wall 19 and the burner head 7.
This division plate 21 divides the third air flow 20 to a heated flow 20' and to a
cooling flow 20". The heated flow 20' is guided past the burner head 7 so that the
flow 20' is heated by the flame. The cooling flow 20" is guided past the burner chamber
wall 19 so that it cools and flushes the wall 19. In this manner, the risk for wall
overheating is reduced. The division plate 21 may be arranged perpendicularly in relation
to the burner axis 71 or it may be curved, bent or L-shaped, preferably curved, bent
or L-shaped.
[0033] Even if the invention was described with reference to what at present seems to be
the most practical and preferred embodiments, it is appreciated that the invention
shall not be limited to the embodiments described above, but the invention is intended
to cover also different modifications and equivalent technical solutions within the
scope of the enclosed claims.
[0034] The invention relates to a method and arrangement for stabilising operation of an
impingement dryer (1) in a paper or board machine or the like. The method comprises
arranging air to be heated to enter a burner chamber (4) of the impingement (1) dryer
through an air inlet opening (11) of the burner chamber (4), the air entering in an
angle in relation to at least one burner head (7) in the burner chamber (4), the burner
head (7) having a flame opening (8) in its first end, heating air in the burner chamber
(4) by bringing the air into a contact with a flame (9), and transferring heated air
from the burner chamber (4) to blowing members of the impingement dryer (1). Between
the air inlet opening (11) of the burner chamber (4) and the flame opening (8) of
the at least one burner head (7) is arranged, in the direction of the air flow, a
guiding member (13) comprising at least one aperture, and the air flow, which enters
through the air inlet opening (11) of the burner chamber (4) is divided so that a
first air flow (14) travels through the at least one aperture of the guiding member
(13) and a second air flow (15) travels through a gap (16) formed between the guiding
member (13) and the burner head (7).
1. Method for stabilising operation of an impingement dryer (1) in a paper or board machine
or the like, the method comprising
- arranging air to be heated to enter a burner chamber (4) of the impingement (1)
dryer through an air inlet opening (11) of the burner chamber (4), the air entering
in an angle in relation to at least one burner head (7) in the burner chamber (4),
the burner head (7) having a flame opening (8) in its first end,
- heating air in the burner chamber (4) by bringing the air into a contact with a
flame (9),
- transferring heated air from the burner chamber (4) to blowing members of the impingement
dryer (1),
characterised in
- arranging between the air inlet opening (11) of the burner chamber (4) and the flame
opening (8) of the at least one burner head (7), in the direction of the air flow,
a guiding member (13) comprising at least one aperture, and
- dividing the air flow, which enters through the air inlet opening (11) of the burner
chamber (4) so that a first air flow (14) travels through the at least one aperture
of the guiding member (13) and a second air flow (15) travels through a gap (16) formed
between the guiding member (13) and the burner head (7).
2. Method according to claim 1, characterised in that proportion of the first air flow (15) to the second air flow (14) is 80:20 - 97:3,
preferably 85:15 - 95:5.
3. Method according to claim 1 or 2,
characterised in
- forming a flow channel (18) between the second end of the burner head (7) and a
burner chamber end wall (19') as well as between the burner head (7) and a burner
chamber wall (19), and
- dividing the air flow, which enters through the air inlet opening (11) of the burner
chamber (4), so that a third air flow (20) travels through the flow channel (18).
4. Method according to claim 3, characterised in that volume of the third air flow (20) is 10 - 50 %, preferably 20 - 45 %, more preferably
30 - 45 %, of the total volume of the air flow, which enters through the air inlet
opening (11).
5. Method according to claim 3, characterised in arranging a division plate (21) to the flow channel (18) between the burner chamber
wall (19, 19') and the burner head (7) in order to divide the third air flow (20)
further into a cooling flow (20") and a heated flow (20').
6. Method according to claim 5, characterised in that the proportion of the cooling flow (20') to the heated flow (20") is 5 - 30 %, preferably
10- 50 %.
7. Method according to any of preceding claims 1 - 6, characterised in that at least a part of the air flow entering through the air inlet opening (11) of the
burner chamber (4) is recirculated from the space between the blowing members and
a web.
8. Method according to any of preceding claims 1 - 7, characterised in that a part of the air entering through the air inlet opening (11) of the burner chamber
(4) is preheatead air, which taken from outside of the impingement dryer (1).
9. An arrangement for stabilising flame conditions in a burner chamber (4) of an impingement
dryer (1) in a paper machine, board machine or the like, the arrangement comprising
- a burner chamber (4), delimited by burner chamber walls (19, 19'), and comprising
an air inlet opening (11) for entry of air to be heated,
- an air inlet channel (10) connected to the air inlet opening (11) of the burner
chamber (4),
- at least one burner head (7), comprising
- a flame opening (8) for a flame (9) at a first end of the burner head (7), and
- at least one feed connection (17) at an opposite second end of the burner head (7),
- a burner axis (71) extending from the first end to the second end of the burner
head (7),
whereby the burner head (7) is arranged in the burner chamber (4) in vicinity of the
air inlet opening (11), the burner axis (71) being in an angle in relation to the
gas inlet channel (10),
characterised in that a guiding member (13), which comprises at least one aperture, is arranged between
the air inlet opening (11) of the burner chamber (4) and the flame opening (8) of
the at least one burner head (7), the guiding member (13) covering, in the flow direction,
the flame opening (8), whereby a gap (16) is formed between the burner head (7) and
the guiding member (13).
10. Arrangement according to claim 9, characterised in that the height of the gap (16) is 10 - 70 mm, preferably 20 - 50 mm.
11. Arrangement according to claim 9 or 10, characterised in that the guiding member (13) comprises a plurality of apertures, which have a height of
100 - 500 mm, preferably 150 - 200 mm, and width of 300 - 2500 mm, preferably 500
- 1500 mm.
12. Arrangement according to claim 9, 10 or 11, characterised in that the arrangement comprises a plurality of adjacent burner heads (7), each with a flame
opening (8).
13. Arrangement according to claim 12, characterised in that the distance between two adjacent burner heads (7) is 200 - 2000 mm, preferably 400
- 1200 mm.
14. Arrangement according to any of preceding claims 9 - 13, characterised in that the total cross-sectional area of the feed connection(s) (17) is smaller than the
cross-sectional area of the the burner head (7).
15. Arrangement according to claim 14, characterised in that a division plate (21) is arranged between the burner head (7) and the burner chamber
wall (19), at a first distance of 5 - 50 mm, preferably 10-30 mm from the burner chamber
wall (19), and at a second distance of 50 - 300 mm, preferably 80 - 200 mm from the
burner head (7).