Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to tubular furnaces
utilizing radiation heat transmission.
Under world-wide shortage of crude petroleum in recent
years, coal is being noticed as substitution energy since coal
can be supplied relatively eas~ly. Establishment of techniques
to liquefy coal and produce fuels such as kerosence or light oil
and pe-trochemical raw material has been demanded.
Fig. 1 is a block diagram illust,rating the coal liquify-
1 0
ing process;
Figs. 2 and 3 show structure of a tubular furnace in
the prior art; Figs. 2(Al and 3(A) are front longitudinal section-
al views; Fig. 2~) is a transverse sectional view; Fig. 3(B~ is
a sectional side v:iew;
Fig. 4 shows structure of a tubular furnace according
to the present invention; (A~ is a front longitudinal sectional
view; (Bl is a sectional view taken along line A-A';
Fig. 5 is a segmentary view illustrating support struc-
ture of heating tube of the furnace;
Fig. 6 shows another embodiment of the invention (A) isa front longitudinal sectional view; (B~ is a transverse sectional
view; and (C), is a side cross-sectional view; and
Figs. 7 to 11 are ~arious graphs illustrating the con-
diti~ns (1) to ~5) according to the present invention,
A general coal llquefying process is shown in Fig.`l,
which comprises the steps of liquefying coal by adding a hydrogen
gas and the solvent into the coal, and removing impurities such
as sulfur content, nitrogen content, ash content from the lique-
fied coal. The solvent used in a coal liquefaction process (coal
~0
slurry) is that of consisting mainly of an aromatic compound
'~
: 172~
and/or an aliphatic compound, and the hydrogen gas is a hydrogen
rich gas consisting mainly of a hydrogen gas and containing a
light hydrocarbon gas and/or carbon monoxide or is simple sub-
stance of hydrogen gas. (These gas being hereinafter simply
referred to as "hydrogen gas".)
In such process, a coal slurry is obtained through mix-
ing said solvent in a pulverized coal, adding said hydrogen gas
thereto and then the slurry is heated at a preheating process of
Fig. 1. Therefore the coal slurry furnace used for preheating is
indispensable for coal liquefaction process, and a tubular furnace
utilizing radiation heat transfer is generally used therefor.
The tubular furnace is constituted principally of a
b~r~ rj
combustion chamber equipped with-a burnor-as a source for heat
generation and heating tubes 1-1/2~ - 6B (size of the tube defined
~IjS ~`
by, for example, ~H~ 3467/19781. The tube are arranged in the
com~ustion chamber and are heated with a high-temperature gas or
flame as radiation sources, so that the "fluid" within the tube
is heated,
In vario~ls coal li~uefying processes, although it is
dif~icult to specify a diameter of the pulverized coal due to
various limitations of hardness, reaction property of coal and
slurr~ transportation means used in the processes, it is general
that 70~ of coal particles has 200 Mesh (0.074 mm in diameter)
and the upper limit thereof is about 2 to 3 mm. Furthermore, in
general, a weight ratio between coal and solvent is selected in
the range of 25:75 to 75:25, and a ratio between the hydrogen gas
and coal is preferably 0.5 to20 Nm3 H2/Kg coal. However, these
ratios are suitably determined according to a property of products
to be obtained and a quality of coal to be processed. Incident-
3~ ally, part of the hydrogen gas may be heated in another preheating
if 29~ ~
1 process. Therefore, there is a possibility that slurry containinga small amount of the hydrogen gas may be heated. When the tubula
furnace is used as the coal slurry furnace, the following specif-
ications are required.
(1) A coal slurry is fluid of three phases, i.e. a yas
phase (mainly hydrogen gas and light hydrocarbon gas), a liquid
phase (solvent) and a solid phase (coal). The coal slurry is
pressurized at a high pressure of 50 - 300 Kg/cm2 G for reaction
with the hydrogen gas. When the fluid flows at a high rate and
180 U-shaped bends are used in the heating tubes, centrifugal
forces are applied to the bent portions and solid constituents
are liable to be separated from liquid constituents so that lnner
walls of the bent portions are abraded by the erosion and are
damaged, for example, to form a hole. Inversely when the fluid
flows slowly, solid constituents such as coal suspended in the
fluid are precipitated or settled, thereby allowing the heat
transmission tube to be clogged or plugged. This is remarkable
in the 180 U-shaped bends.
Accordingly, it is preferably that in the heating tubes
the fluid be held at a suitable flow rate and a usage of 1~0 U-
shaped bends causing the separation and deposit be avoided.
(2) Since a solvent including a large amount of
aromatic compounds and having high molecular weight is generally
used, heating must be uniformly performed. When a local over-
heating takes place, the coal slurry is decomposed and coked or
carb~nized. The product is deposited to inner walls of the heat-
ing tubes, causing the pluqging of the heating tubes while pre-
venting the fluid from flowing resulting in a further local over-
heating. Accordingly, heating of the heating tubes must be pe~-
formed as uniformly as possible.
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1 (3) Since the coal slurry within the heating tube
exists in a three-phase, i.e. gas phase, liquid phase and solid
phase, if there is a portion, of the tube, where the flow is
changed from a descending stream to an ascending stream and vice
versa, the tube is subjected to vibrations or fluctuations.
Accordingly, it is preferably that fluid within the
heating tube be held either in a descending flow or in an ascend-
ing flow.
Structures of con~entional heating furnaces are shown
in Figs. 2 and 3.
Referring now to Figs, 2(A) and (B), a heat insulating
material is lined on an inner wall of a cylindrical combustion
chamber 1, a heating tube 2 of circular spiral form is arranged
in a longitudinal direction along the inner wall of the combustion
chamber 1, and the heating tube 2 is heated from the center of the
furnaces using a burner 3 arranged at the center of a hearth.
The furnace of this type is advantageous in that 180
U-shaped bend causing erosion or clogging are not used. Further-
more~ the slurry flows in one direction from an upper portion to
a lower porition, thereby preventing the tube from vibrating
~owever, such a furnace has disadvantages in that it is difficult
t~ construct a furnace large in size on account of structural
limitations of the heating tube of circular spiral shape, Namely,
if the heating tube becomes larger than 8 m in turn diameter and
then 12 m in height, heat from the burner is not distributed
uniformly so as to cause a local overheating and construction
thereof is difficult also in vieW of the structure of the furnace.
Since the heating tube is heated at one side only using the burner
at the center, a radiation heat is not distributed uniformly and
the side facing the flame of the burner is heated strongly, so
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1 that the slurry is decomposed and coked or carbonized and the
product is deposited to the inner walls of the heating tube. As
a result, an effective flow path thereof is narrowed and a surface
temperature of the tube becomes extremely high.
Referring to Figs. 3(A) and (B~, along opposed inner
side walls of a combustion chamber 1' of box type is disposed a
serpentine heating tube 2' extending in a lateral direction. The
heating tube 2' is heated by burners 3' arranged in a line at the
center of a hearth.
In the tubular furnace of this type, since the heat
transmission tube 2' is not restricted comparatively in structure,
the furnace large in size is readily constructed. ~owever, such
furnace has disadvantages as described below.
Since the heating tube has 180 U-bend with a short
radius of curvature being twice as large as the tube inner dia-
meter or less for connecting linear portions, bend is damaged by
erosion or clogging. Also the heating tube is heated on one
side as in the furnace shown in Figs. 2 (A) and (Bl, thereby coking
the slurry resulting in local overheating or clogging.
If the heat transmission tube shown in Figs. 3(A) and
(B~ is modified in a serpentine form extending in the vertical
direction along opposed inner side walls, the flow cannot be held
in one direction hereby the heating tube is subjected to vibration.
In view of above noted defects inherent to the prior
art, the present invention provides as an object a tubular
furnace in which a heating tube of spiral and oval shape has lin-
ear portiors and curved portions each having a curvature radius
of five times as large as a tube inner diameter or more, the
heating tube is heated on both sides by radiation, whereby
specifications required as the tubular furnace are completely met
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~ :~729~2
1 and the fluid can be efficiently heated, the furnace having high
safety degree and excellent durability, and the furnace large in
size can readily be constructed.
The present invention will now be described referring
to Figs. 4 to 6.
In Figs. 4(A) and (B), a heating furnace body casing
10 is constructed on a foundation base 11, and a furnace wall 12
is made of heat-resistant material such as fire bricks, castable
refractories or mortars, and a combustion or burning chamber 13
is defined by the furnace wall 12. The body casing 10 is formed
in a box shape with transverse sections of a octagon being longer
in one direction than in the other, and the furnace wall 12 also
has a shape in conformity therewith, A heating tube 14 of oval
spiral shape in the longitudinal direction is arranged in a center
portion of the combustion chamber 13. The heating tube 14 is
composed of linear portions 14A and curved portlons 14B, each of
the curved portions having a center curvature radius r of five
times as large as a tube internal diameter h or more and prefer-
ably 7,5 times or more, In addition, the center curvature radius
used here means a curvature radius with respect to center of the
heating tube 14 and not outer curve or inner curve of the heating
tube 14.
The heating tube 14 of oval spiral shape is formed
along two lines of wound tubes so that fluid to be heated flows
through two path. The number of lines is determined correspond-
ing to that of required paths, for example, one line for one path
and three lines for three paths. A distance between the adjacent
tubes 14 of oval spiral shape is preferably pitch of 1.2 - 3 times
as large as the tube outer diameter and more preferably pitch of
twice,
; i 7 2 9 ~ ?
1 Upper and lower ends of the heating tube 14 are formed
respecti.vely as inlets 14a and outlets 14b for the fluid to be
heat.ed. The inlets 14a pass throush one side of the furnace wall
12 and are led out of the body casing 10, and the outlets 14b are
led out of the hearth 12A. A tube support 15 made of heat-resist-
ant cast steel is adapted to support the heating tube 14 in the
urnace. As best shown in Fig, 5, the tube support 15 is com-
posed of a support pole 15a extending in a vertical direction
inside of an inner circumference of the heating tube 14 and
support pawls l5b projecting outwardly in multiple stages from
the support pole 15a, so that a bottom of each tube the heat tran~-
m~ssion tube 14 arranged in multiple stages is supported by the
pawl 15b,
,~ ~t~f ~ / fs
Each of the tube support 15 has a lower end fixedly
held to the hearth 12A and an upper end held while passing
through a through-hole 12a of the furnace wall 12 at a ceiling
portion so that the~rmal expansion of the tube support 15 heated
at a high temperature within the furnace i.s allowed to escape in
the through-hole 12a, Floor burners 16 are used as a first heat
source to heat ins.ide circumferential surface of the heat trar.s-
mission tube 14. A plurality of the floor burners 16 are spaced
at a constant intervalti~ are arranged in a line at the center
portion of the hearth 12AJand confronted with the inside surface
c~76~ ar,,~
of the heating tube 14~arranged so as to yenerate flames upwardly.
W~ll burners 17 of linear shape are used as a second heat sou,rce
to heat an outside circumferential surface of the heating tube
14, A plurality of wall burners 17 are spaced at a suitab.le
interval in a circumferential direction at the bottom portion
inside the furnace wall 12.
Each of the wall burners 17 is composed of a burner
gun 17a bent upwardly along the furance wall 12 and a block 17b
5 172982
1 of burner tile bent in the same direction as the burner gun 17a.
The wall burner 17 generates flat flame upwards along inner sur-
face of the surface wall 12, thereby heating the furnace wall 12.
The outside circumferential surface of the heating tube 14 is
heated by radiation heat from the furnace wall 12. Wind-boxes
18 and 19 for two types of burners, i.e. the floor burner 16 and
the wall burners 17, serve as mufflers to reduce noise made by
the burners 16 and 17.
In this embodiment, there are installed four floor
burners 16 and twelve wall burners 17. The number of burners
may be set freely corresponding to the size of the furnace. In
order to prevent incomplete combustion or burning caused by inter-
ference of the flame between adjacent burners in this case, a
minimum burner center distance c between floor burners 16 and a
~minimum burner centers distance d between wall burners 17 prefer-
ably meet the following relationships.
c > k + 0.1 (m)
d > f + 0.1 (m)
wherein k is the maximum diameter ~m) of flame of floor burner 16;
and f is the maxirnum width (m) of block 17b of wall burner 17.
A stack 30 for discharging flue gas is installed at
the ceiling and a heat insulating material 3Qa is lined on an
inner surface of the funnel 30~ A damper 31 for adjusting draft
force of flue gas is disposed in the stack 30 and may be manually
set suitably.
With such a constitution, the fluid to be heated is
introduced in the inlets 14a of the heating tube 14 and is allowed
to flow from the upper portion to the lower portion through the
heating tube 14 and then flows to the outlets 14b. During the
above mentioned process it is heated by radiant heat txansmission
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; ~l 7 2 9 `~ '~
1 of the heating tube 14.
Of course, the heating tube 14 is slightly heated by
convectionfromthe high temperature flue gas flowing in the com-
bustion chamber 13.
If the center curvature radius r of the curved portion
14B of the heating tube 14 of spiral shape is five times, prefer-
ably, 7,5 times as large as the tube inner diameter h or more,
the curved portion 14B is not subjected to the damage such as
abrasion, and the heating tube 14 îs not clogged by the deposit
of solid constituents contained in slurry. In order to prevent
the local overheating in the vertical direction of the heating
tube 14 of spiral shape, it is found preferable from the follow-
ing studies that the center curvature radius r and distance a
from surface of the furnace wall 12 to the center of the heating
tube 14 should meet the following conditions,
It is preferable that center curvature radius r (unit:
m) meet the following conditions simultaneously.
r _ 5 x h (more preferably, r > 7.5 x h) (1
r _ 0.28 ~ g + 0.20 (2~
r _ 0,13 x b (31
wherein y is the firing rate per one floor burner 16 ~x 106 Kcal/
Hr); h is the tube inner diameter of heating tube 14 ~mi; and b
is the longitudinal length of spiral shape of heating tube 14,
i.e,, the center distance between uppermost and lowermost stages
of heat transmission tube 14 ~m~.
The above condition ~22 means the minimum distance to
separate the heating tube 14 from the floor burners 16, and the
condition ~3~ means the minimum value required to make the uni-
form distribution of heat absorption in the vertical direction
of the heating tube 14 of spiral shape by the floor burners 16,
; :i72~
1 It is preferable that the distance a (unit : m~ from
surface of the furnace wall 12 to the center of the heat trans-
mission tube 14 meet the following conditions.
I1 When the longitudinal length b of spiral shape of
the heating tube 14 is less than ll.2 ~:
0,848 c a (41
II~ When the longitudinal length b of spiral shape of
the heating tube 14 is equal to or greater than ll,2 m:
O.Q757 x b < a (5~
The above conditions (4~ and (5) means the minimum
value required to make uniform the distribution of heat absorp-
tion in the vertical direction of the heating tube 14 of spiral
shape by the wall burners 17.
The above described conditions Cl) to 15~ will now be
explained in more detail. Various studies have been made as to
the coal slurry furnace using four-inch inner diameter tubes or
pipes in accordance with the present invention.
Condition (ll: r ~ 5 x h (preferably, r 7,5 x hl
When the flow speed of the slurry is in the range of
l.2 to 3.2 m/s, a ratio ~ of an erosion rate of the linear portion
of tube to an erosion rate of a curvilinear or bent portion of
tube ~s shown in Fig. 7 where the ratio ~ is represented by the
expression ~ =
erosion rate of the bent portion (mm/day)
erosion rate of the straight portion (mm/day~
On the other hand, since at an inlet portion of the
heating tube coal perticles contained in the slurry fluid are not
yet soluted therein, unless the radius of curvature is suitably
selected, there would be caused a plugging or clogging in the-
bent portion. In case of the studies using the four-inch inner
diameter pipes, the flow speed at which the plugging will occur
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i `~72~2
1 is represented by Fig. 8 in which is the mean perticle degree
~mesh), and Vc is a minimum flow speed (m/s~ at which the plugging
does not occur.
As has been apparent from Figs, 7 and 8, there is no
fear that the erosion and plugging may occur significantly at the
bent portion in the range of r/h > 5, preferably r/h > 7.5. Thus,
it is understood that the bent portion may be designed in the
same manner as in the straight portion.
C dition 12): r - 0.28 x g + 0.20
If during the operation of the heating furnace, the
flame of burners impinges against the heating tube or pipes, the
heating tubes or pipes are remarkably locally heated to thereby
cause co~ing of fluid and damage of heating tubes, Therefore,
the heating tubes must keep a distance from the flame suitable.
It is experienced that if the following lower limit is exceeded,
the coking of fluid take place.
di > 0.107 x g + 0.113
where: di is the distance ~ml between the surface of
flame and the heating tube center,
The diameter of a flame produced by burners which
produce r~,latively thin flame compared with their capacity is
represented in Fig. 9 inwhich a first solid line on the left side
denotes a diameter of flame, a second solid line on the right side
denotes a center position of the heating tube nearest to the
burner defined by di and a dotted line shows a straight line in-
dicating the equation r = 0,28 x g + 0.20,
It will readily be understood that as far as the rela-
tionship r > 0.28 x g + 0.20, is gi~en, the heating tube may
be protected against the excessive burner heating at a safety
distance,
, 1729~
1 Condit_on (3): r - 0.13 x b
In general, difficulties have been experienced in
determininy a distribution of a thermal absorptivity, per unit
surface area q (kcal/m~ hr) of the heating tube because it re-
markably depends upon types of burner, configuration of flame,
con~iguration of furnace,.kind of fuel and the like. However, in
case where two heat absorbing planes composed of a row of heating
tubes are arranged to be confronted with each other in a vertical
and parallel relationship and the up-firing burners are disposed
at the center of the absorbing planes the relationship between
the geometrical relation of the two heat absorbing planes yb and
the non-uni~ormity of heat absorption max on the heating tubes
is shown in Fig. 1.0 by using natural gas as a fuel, in which
qav is the average heat absorption per unit surface area of
heating tube by the floor burner (kcal/m2hrl and qmax is the
maximum heat absorption per unit surface area of heating tube by
the floor burner ~kcal/m2hr). As is apparent from Fig. 10, at a
posi.tion represented by r = 0.13 x b, the relationship of qmax/qav
- 1,6 is establi.shed; namely, heat absorbing rat~ per unit surface
at the maximum heat absorbing portion of the heatiny tube is 1.6
times o~ the average thermal absorptive portion in heat ahsorption.
It is experienced that if non-uniformity in thermal absorption
mvre remarkable than this expression occurs, the portion in
which the maximum absorption takes place, is heated to
a high temperature and the liquid cokiny may be
caused.
Condi.tivns ~4) and ~5): 0~848 ~i a and 0,0757 x b - a
~i
In general, a wall used for heatiny the heati.ng
tube by radiation emitted from insulating materials or heat re-
sistant material such.as fire brick which are heated by the wall
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i ~729~2
1 burners is r~ferre~ to as a radiatio1- wall. l~1hen the distance
between the radiation wall and the heatiny tube is too short,
since the temperature of the radiation wall is not uniform, the
heatin~ tube is subjected to a non-uniform heating so that the
coal slurry is coked at the portion at which the temperature
is highest. Inversely, when the distance between the radiation
wall and the heating tube is too long, the tubular furnace
becomes uneconomically large in size, and the thermal efficiency
of the wall burners is markedly reduced. Therefore, the
suitable distance must be chosen.
The relationship in distance will now be explained
with reference to Fig. ll.
(a) In the case where the longitudinal length b of the
spiral shape of the heating tubes is equal to or greater
than ll.2: In Fig. ll, q' v is the average heat
absorption rate per unit surface area of the heating
tube by means of the wall burners (kcal/m2hr) and
q'max is the maximum heat absorption rate per
unit surface area of the heating tube by the wall
burners (kcal/m2hr).
As is apparent from Fig. ll, if the ratio a/b becomes
smaller than 0.0757, the ratio q'ma /q' exceeds l.6 thereby
making the local heating of the tube remarkable and causing a
coking in coal slurry. Therefore, the condition (5):
0.0757 _ a must be given.
- 13 -
^ ~729~
1 (b) In the case where the longitu~ al length b of the
spiral shape of the heating tubes is less than 11.2:
By substituting b in the condition (5) with 11.2,
the relationship of 0.848 _ a is given, which is the same as
condition (4). The condition (4) means that even if the value
of b is small, it is prohibited to be smaller than 0.848 m as
a minimum distance.
It is experienced that if the distance between the
center of the heating tube and the radiation wall is smaller than
- 13a -
i ~729~2
1 ~.848, the following disadvantages are noticed. (i) It is
difficult to construct the furnace. (ii~ It is difficult to
carry out the maintenance works in the furnace for inspection.
(iii) The flame of the wall burners will impinge against the
tube supports thereby causing the excessive heating. Therefore,
it is highly desired that the sufficient distance must be kept
with in the condition (4),
It is therefore necessary that if the value of b is
less than 11.2 m, the value of a be defined by the relationship
of 0.848 = a.
In the case that the size of the furnace is increased
resulting in increase of the height b while increasing the dis-
tance a in accordance with the above described condition (5) to
thereby increase the construction costs, the wall burners 27 may
be provided in a two-stage manner as shown in Fig~ 6(a) in the
vertical direction. In this case, a height of the spiral tube
to be heated by each one of the double-stage wall burners may be
reduced to halr the height of the single stage wall burners. Thus
the distance a may be also reduced in accordance with the con-
dition (5~ reducing the construction costs.
The above described conditions are generally used forthe tubular furnaces for three-phase fluids which are not limited
to the coal slurry of three-phases. Incidentally/ f~o~ the
~oregoing descri`ption the center curvature rad~us r in the con-
ditions (2) and (3) is expressed as a minimum necessary distance Q
(unit: meter~ from the center of the first heat source, i,e., floor
burner 16 to the heating tube 14.
When the tubular furnace in the embodiment shown in
Figs. 4A and B is used as, for example, a coal slurry heating
furnace in a coal liquefying plant, the features that the heat
transmission tube 14 of oval spiral shape has the curved portions
14B with center curvature radius in the longitudinal diréction
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~ 172982
1 being five times as large as the heat transmission tube inner
diameter or more and that the heat transmission tube 14 is heated
on both sides by the floor burners 16 and the wall burner~ 17
offers the following advantages.
(1) Since the 180 U-bend of short diameter being
twice as large as the tube inner diameter or less is not used but
the heat transmission tube of oval spiral shape having the center
curvature radius being five times as large as the tube inner dia-
meter or more is provided, the excessive erosion of fluid does
not occur at the curved portion which is the most critical portion
of heating tube. The curved portion is also free from clogging
by precipitation of solid material due to its curvedness.
(21 The heating tube is uniformly heated on inner and
outer circumferential surfaces of the loop-shaped tube arrange-
ment and the local overheating does not occur ~hile fluid is not
coked, and therefore tube clogging due to the coking is not
caused,
Fox example, when the same average heat flux (heat
absorption rate per unit area surface of the heating tube~ is
applied, if the heat transmission tube is arranged in pitch being
t~ice as large as the tube outer diameter, the maximum heat flux
produced at the side faced to the flame in double-side heating
becomes 1/1.5 in comparison with one-side heating.
(3~ The fluid always flow downwardly, namely flowing
is held in one direction, and therefore the heat transmission
tube is not subjected to vibration,
~ 4) If the linear portion of the heat transmission
tube is lengthened there~y e~tending the long diameter portion,
the furnace large in size can readily be constituted, Therefore,
there is no restriction in structure as in the conventional
heating tube of simple circular spiral shape, and uniform heating
is obtained in the furnace large in size by means af the double-
side heating.
l ~72g82
1 Accordingly, the furnace large in size can be construc-
ted without any restrictions~
Another embodiment of the present invention will now
be described referring to Figs, 6 (A~ and ~B).
This is an example of the large size heating furnace
structure of Fig. 4, The radiant section of the furnace is
divided into two chambers. Similarly to the furnace shown in
Fig. 4, each chamber 23 having heating tubes 24, Therefore, the
numbers of flower burners 26 and wall burners are increased. A
common heat recovery section 20A is proYided at the top of the two
radiant sections, and recovers heat from flue gas mainly by
convection. Linear portions of the heat transmission tube 24
are extended namely the long diameter direction is made long,
thereby forming the furnace large in size. As the furance becomes
large, the numbers of.the floor burners 26 and the wall burners
27 ~re increased. When the height of the heating tube 24 of spiral
shape is increased, a uniform heating is not effected which the
wall burners 17 which are mounted in single stage at the lower
portion of the furnace wall as shown in Eig, 4(A~, Therefore,
the arrangement in two lateral stages at the inner circumference
of the furnace wall is effected as shown in Fig, 6 (B~. In this
case, it is preferably that when the height b of spiral shape of
the heat transmission tube 2~ be b - 5.6 m, the number of lateral
stages of the wall burner 27 be one stage, and when b > 5.6 m,
the number of lateral stages be two stages. Of course, multiple
lateral stages more than double stages may be effected correspond-
ing to the height of the heating tube 24 of spiral shape, Effect
of installation of the heat recoYery section 2QA is that the
waste flue gas of high temperature i5 used to heat other process
fluids requiring heating, or if there is no suitable process
-16-
i 1729`~2
1 fluid, a waste heat boiler is installed to generate steam. ~eat
recovery is therefore effected and a thermal efficiency of the
heating furnace as a whole is improved,
Effect of installation of a plurality of the radiation
chambers 23, is as follows;
In the tubular heating furnaces designed in any manner,
the fluid is more or less coked within the heating tube.
Accordingly, in usual, decoking operation to introduce
stea~ and air in the heating tube and to remove coke adhered to
the inner wall of the tube is performed regularly. During the
decoklng operation the furnace cannot serve as a coal slurry
heater, When a plurality of the radiant chamber are installed as
in this embodiment, the furnace can consequently serve as a coal
slurry heater by means of alternate decoking operation between
the two radiant chambers, If plural furnaces having two radiant
chambers are each installed, decoking operation will be conducted
more easily.
~ s described above, a tubular furnace of the present
invention is constructed so that a heat transmission tube of
spiral shape i.s composed of linear portions and curved portions,
each.having the center curvature radius of five times as large
as the tube inner diameter or more, the heat transmission tube
is heated on both sides from the inner and outer circumferences,
whereby fluid can be heated uniformly and smoothly, and local
overheating in the heat transmission tube is prevented signif-
icantly, the elimination of use of 180 U-bend prevents damage
such as abrasion of inner wall of the curved position caused
by erosion or clogging of the tube due to the deposit of solid
constituents. contained in the slurry.
The flow of fluid is held in one direction either
downwardly or upwardly, whereby the heat transmission tube is
-17-
t 1729~2
1 prevented from vibrating. Tubular furnaces large in size can
readily be constructed employing the oval heating tube.
When the heating furnace according to the present in-
vention is used as coal slurry heating furnace in the preheating
process of the coal slurry composed of a three-phases, i.e.
vapour ~hydrogen gas), liquid ~solvent) and solid (coal) in the
coal liquefying processr technical problems such as erosion,
durability for coking and service life of heating tubes can be
improved.
1 0
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