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Nova; 13', 1962
E. c. MILLER
3,063,431.
STEAM GENERATING UNIT
Filed May 31,v 1961
2 Sheets-Sheet 1
FIG. I.
EARLE 6‘. MILLER
INVENTOR.
BY;
' ATTORNEY
Nbw I13‘, 139625
E‘; c:. MILLER‘
33,063,431?
STEAM; GENERAHNG'; UNIT“
Filed? May’ 31 , 1963;.
2} Sheets-Sheet; 2,‘
/7
I22
INVENTOR
EARLE 0. M/L'L‘ER'
BYwMJ/i MI’. .4 I‘!
.
ATTORNEY
ice
3,063,431‘
Patented Nov. 13, 19527
2
3,063,431
STEAM GENERATING UNIT
Earle C. Miller, Worcester, Mass, assignor to Riley
Stoker Corporation, Worcester, Mass, a corporation of
Massachusetts
FIG. 3 is a schematic view of the unit to show its op
eration at low load, and
FIG. 4 is a similar schematic view of the unit taken
during high load operation.
In the specification which follows, the expressions
“longitudinal,” “transverse” refer to those directions as
applied to a steam generating unit in the ordinary prac
tice in that art and in general refer to the ?ow of gas
Fiied May 31, 1961, Ser. No. 113,936
8 Cim'ms. (£1. 122-473)
This invention relates to a steam generating unit and
through the unit.
more particularly to apparatus arranged to generate steam 10
Referring to FIG. 1 ?rst, wherein are best shown the
and to regulate the temperature of superheated steam
general features of the invention, the steam generating
which is produced therein.
There have been many methods suggested in the past
for maintaining the temperature of the superheated
steam in a steam generating unit at a ?xed value, irrespec
tive of changes in load on the unit. Among these meth
ods is that of lay-passing portions of the gas away from
sections of superheater heat exchange surface.
Also,
unit, indicated generally by the reference numeral 11, is
shown as consisting of a furnace 12 and a boiler 13
mounted on a supporting structure 14. The furnace
15 2 consists of a forward wall 15 and a rearward wall
16 which, with side walls 17, de?ne a vertically-elon
gated combustion chamber 18. Underlying the combus
tion chamber is a slag basin 19 formed of refractory
there is the method in which the temperature of the
gases passing over convective superheaters is regulated
by regulating the residence time of the gases in the main
combustion chamber by the use of tilting burners. There
is the method of superheat control by gas recirculation
21 located in its lower portion directly overlying the
slag basin 19 and provided on its downwardly-directed
the furnace is moved to various positions between the
ward wall 16, and the side walls 17. These water-wall
material.
The forward wall 15 is provided with a nose
surface with a burner 22.
A similar nose 23 is pro
vided in the rearward wall 16 and a burner 24 is mounted
in which gases from the back passes are returned in regu
on the downwardly-directed surface of that nose.
lated amounts to the main combustion chamber to in 25
The boiler 13 consists of an upper steam-and-water
crease or decrease the mass How of gases over convection
rum 25 joined by downcomer tubes 26 to a lower drum
superheater units. Of course, there is also the old method
27, both drums extending transversely of the unit. Large
of desuperheaing the superheated steam by injecting a
downcomer pipes 28 extend from the lower drum 27
spray of water into it. Another method of controlling
to a header 29 extending around the periphery of the
superheat is described in the patent to Miller No. 30 slag basin 19. Water-wall tubes 31 extend upwardly
2,947,289, in which the ?ame rising vertically through
from the header 29 along the forward wall 15, the rear
front wall and the rear wall in order to regulate super
heat. There are some boiler conditions under which this
tubes are connected at .the upper part of the furnace to
Every other water-wall
last method of controlling superheat does not give a 35 tube on the back wall 16 is formed with a forward loop
sufficient range of superheat temperature change. While
32. as will be described more fully hereinafter. These
it is true that this method can be combined with desuper~
tubes serve to de?ne an upper pass 34 between its for
eating methods and with by-pass methods of superheat
wardly projecting portions and the roof 35 of the fur
control to give sufficient range, these last two methods
nace. A refractory wall 36 extends downwardly from
40
affect the overall e?iciency of the boiler by discarding
the steam-and-water drum 25 and serves to divide the
heat. These de?ciencies in the prior art have been obvi
rearward portion of the unit into back passes 37 and
ated by the present invention in a novel manner.
the steam-and-water drum 25.
38.
A duct 39 leads from the back pass 38 to a dust
It is, therefore, an outstanding object of the present
collector 41 which, in turn, is connected through a ro
invention to provide a steam generating unit having ap
tary regenerative air heater to an induced draft fan, a
paratus for the control of superheat in which the heat 45 breaching and a stack, all not shown but usual in the art.
discarded from the cycle at certain loads is reduced to
A force draft fan (not shown) is connected through
a minimum value.
the air heater to a duct 46 having branch ducts 47 and
Another object of the invention is the provision of an
48 leading to the burners 24 and 22, respectively.‘ A
apparatus for the control of superheat whose action is not
duct 49 leads from the duct 46 to an attrition~type pul
50
detrimental to the overall operation of the steam generat
verizer (not shown). Pipes 53 lead from the output
ing unit.
of the pulverizer to the burners 22 and 24.
Another object of the invention is the provision of an
As is evident in FIG. 1, the steam-and-water drum 25
apparatus for the control of superheat which makes use
is provided with a separator of the usual type and tubes
of the flame-positioning method and in which the range
55 54 lead from the upper part of the steam-and-water drum
of control has been greatly increased.
to a header 55 arranged beside the forward wall 15 of
A still further object of the present invention is the
the furnace. Below the header 55 is arranged ‘another
provision of an apparatus for the control of superheat in
header 56 of a similar type. Joiningthese headers and
which the advantages of the gas recirculation method
lying within the furnace‘ against the forward wall 15 are
of superheat control are obtained without the necessity
60 radiant superheater tubes 57. Tubes 58 lead from the
for expensive ducting and controls.
header 56 to a header 59 which is connected through a
With these and other objects in view, as will be ap
convection superheater 61 lying in the pass 37 to a con
parent to those skilled in the art, the invention resides
vection superheater 62 lying in the upper pass 34. The
in the combination of parts set forth in the speci?cation
output of the superheater 62 is connected to superheater
and covered by the claims appended hereto.
The character of the invention, however, may be best 65 platens 63 hanging downwardly from the roof 35 into a
restricted passage 33 ‘between the forward wall 15 and
understood by reference to one of its structural forms,
the foremost portion of the tube loops 32. The output of
as illustrated by the accompanying drawings in which:
FIG. 1 is a vertical sectional view of a steam generat
the platen 63 is connected to a superheated steam header
64 which, in turn, is connected by a high pressure pipe
70 65 to the turbine (not shown).
The output of the high pressure section of the turbine
FIG. 2 is a horizontal sectional view of the unit taken
on the line II—II of FIG. 1;
(not: shown) is connected by means of a pipe 66 to a
ing unit embodying the principles of the present inven
tion;
3,063,431
3
d
midpoint between the front and rear wall of the furnace
to provide a vertical portion 118. Each tube is then bent
to follow the bottom surface of the roof 35 to provide a
roof portion 119. Mounted on the tubes 32 are refrac
header 67 arranged adjacent the rearward wall 16 of the
furnace. A similar header 68 is mounted above the
header 67 and the two are joined by a radiant reheater
69 which lies closely'adjacent the inner surface of the
tory blocks 121 forming a ba?ie 7122. This ba?ie extends
partly along the forward parts of the inclined portions
117 of the loops 32 and partly along the lower parts of
the vertical portions 113. The rearward edge of the battle
rearward wall 16. The header 68 is connected by a pipe
71 ‘to an input header 72 connected to one end of a con
vection reheater 73 lying in the upper pass 34'between
the superheater 62 and the superheater platen 63. The
is located somewhat forwardly of the rear wall 16 to
output of the reheater 73 is connected to a reheater steam
header 74 which, in turn, is connected by means of a 10 provide a passage 123 therebetween. The forward edge
of the battle is in general alignment with the vertical por
high pressure pipe 75 to a low pressure section of the
tion 118 of the tubes 32 and between the'forwa'rd edge
turbine (not shown). The burners 22 and 24 are simi
of the baffle and the front wall is located the passage 33
lar to those described in Patent No. 2,947,289. For in
leading to the upper pass 34. The convection 'superheater
stance, in the case of the burner 22 the air duct 48 is
connected to a housing 76. Centrally of the housing is 15 62 is located entirely above the ba?ie 122 and has no
substantial portions which are rearwardly' of the rear
located a continuous spark igniter (not shown) and a
edge of the baliie. At the same time, the convection re
gun 78 for introducing pulverized coal into the burner.
heater 73 is located above the baf?e at the forward por
The conduit 53 leading from the pulverizer 51 is con
tion of it and none of the reheater extends forwardly of
nected to the gun and in the central part of the gun is a
the forward edge of the ‘baffle or forwardly of the screen
gas gun to insure ignition. The water-wall tubes 31 are
provided by the vertical portions 113 of the tubes 32.
bent rearwardly in the vicinity of the burner 22, to form
The superheater platens 63 lie entirely within the passage
passages for the flow of ‘fuel and air. At the mouth of
33 forwardly of the forward edge of the ba?ie and of the
theburner below the gun 78 are located pivoted vanes
vertical portions 118. The refractory blocks 121 ?t
83 ‘which are connected through a mechanical linkage to
tan actuating rod 84. In the upper part of the burner are 25 tightly ‘around the tubes and are bonded together by ce
ment in the usual manner to provide a gas-tight baffle
located pivoted vanes 85 which are connected through a
construction.
mechanical linkage to an actuating rod 86. The burner
The operation of the apparatus will now be readily
24 is similarly provided with a fuel gun 87, lower vanes
understood in View of the above description. The fur~
88 whose angularity is adjustable by means of an actuat
ing rod 89, and upper pivoted vanes 91 whose angularity 30 nace 12 and the‘boiler '13 operate in the usual manner.
Because of the restricted nature of the bottom of the
is adjustable by means of an actuating rod 69, and upper
combustion chamber 18 below the level of the noses 21
pivoted vanes 91 whose angularity is adjustable by means
and 23, it is very hot and quite complete combustion
takes place in this lower high-temperature cell. The
the piston rod of a hydraulic linear actuator 96, while 35 gases passing upwardly in the restricted zone between
of an actuating rod 92.
'
W The ‘actuating rod 86 for the vanes 85 is connected to
the noses 21 and 23 form a ?ame which ‘may be manipu
the actuating ‘rod d2 of the vanes 91 is connected to a
similar actuator 97. The actuator 96 is connected by
conduits 98 and 99 to a controller 101 and the linear
actuator .97 is connected to the controller by means of
V conduits 102 and 103.
lated so as to flow close to the front wall 15, close to
the rear wall 16, or any position intermediate of the walls.
In the sense of “?ame,” the applicant means the line of
The. air duct 48 leading to the 40 greatest mass'?ow and highest gas temperature which is
usually indicated in a furnace as a ?ame; this flame may
burner '22 is provided with a control damper 104 which is
connected for pivotal action through a mechanical linkage
to the piston rod of a linear actuator 105 whose piston is
movable under the control of conduits 186 and 197 by
or may not be luminous, depending upon various factors
such as the nature of the fuel and‘ the completeness of
combustion. The ?ame may be manipulated in a manner
which it is connected to the controller 1&1. In a similar 45 described in the above-identi?ed patent of Miller by ad
manner the duct 47 leading .to the burner 24 is provided
with a pivoted damper Hi8 which is connected-through
a linkage mechanism to the piston rod of a linear actuator
109. This actuator is connected to the controller 101 by
means of conduits 111 and 112. The superheated steam
header 64 has mounted therein a temperature-indicating
device 113 of the usual type, and this device is connected
through a line .114 to the controller 1111,. A similar
justing the angles of the vanes in the burners through
the linear actuators $6 and 97 or by adjusting the ‘air
flow through the burners by means of the actuators 105
and 109 which manipulate the dampers 104 and 108.
50
'
Referring now to FIGS. 3 and 4, it is evident that the
position of the ?ame 124 is manipulated by changing
the angularity of the vanes in the burners. In FIG.V3,
which shows the relationships at low load, the ?ame is
shown as rising close to the front wall 15. The ?ame
temperature-measuring device 115 resides in the reheated 55 124‘passe's directly along the front wall 15 and'through
steam ‘header 74 and is connected by a line 116 to the
controller 101. “The controller 1511 is of the usual type
used in temperature control applications; it consists of an
apparatus which is well known in the art for converting
the passage ‘33 between the front edge of the ba?ie 122
and the front wall. From there, the gas ?ow is directed
under the roof 35 through the upper pass 34. In the
process of doing this, these gases pass over the'super
electrical signals in the lines 114 and 116 into hydraulic 60 heater platens 63, the convection reheater 73, and the
convection superheater 62, and eventually pass through
?ow throughthe'lines 106,107, 98, 99, ‘111, 112, 102,
partof‘the'present invention in its detailed form, it is
the’ gas olftake 125 leading from the furnace into the
backpasses. Because the ?ame extends upwardly along
the front part of the furnace,’ a zone'of reduced pressure
adequately to describe the present invention.
place from the upper pass 34 downwardly through the
and 103 leading to the various hydraulic linear actuators
associated with the apparatus; since the controller is not
not felt'that a speci?c description thereof is not necessary 65 is created under the ba?le 122 and a flow of gases takes
"
'
1 -As has been stated, each of the alternate tubes in the
rear'water-wall tubes of the wall 16 of the furnace have’
been bent forwardly to form a loop 32 which resides
above the combustion chamber. Each tube is bent at an
angle of approximately 45° to the vertical to provide a'
portion 7117 extending forwardly from a position'w'ell
above the lower drum 27 of the boiler. Each tube is then
pass 123 between the back Wall 16 and the rearward edge
of the'ba?ie 122. Now, it is well known that uncorrected
superheat tends to be low atlow loads because of the.
rising. character of a convection superheat curve. In the
present case, the ?ame 124 andthe greatest mass of
gases passes by the most direct path from the burners to’
the convection sections of superheater. This means that
they reach these superheaters rat the highest temperature
bent vertically at a position somewhat forwardly of'the 75 because there has been the least possibility of heat radia
5
3,063,431,
I
6
tion to the walls of the furnace to cool the gases. This
gives a tendency to a higher superheat temperature.
Furthermore, the ?ame is closer to radiant superheater
sections 57 and the superheater platens 63 which absorb
elongated combustion chamber having walls, fuel-burn
close adjustment of superheat temperature.
side walls covered with water-wall tubes, fuel-burning
ing apparatus located at the bottom of the combustion
chamber to produce a ?ame, a gas off-take opening into
the upper part of one of the walls, a ba?le extending
considerable heat by radiation from the combustion 5 across the combustion chamber in a position spaced away
chamber. Most importantly, however, the gases which
from the 'wall opposite the said one of the walls to pro
?ow from the upper pass 34- downwardly through the
vide a passage located between the Wall opposite the said
pass 123 to mix with the gases from the burners to pass
one of the Walls and the baf?e and extending over the
upwardly once more through the forward pass 33 con
battle, said ba?'le including means providing an opening
stitute a form of gas recirculation which increases the 10 located in the ba?le immediately adjacent the said one of
mass ?ow of gases over the convection superheater sur
the walls, a convection superheater located in the passage,
faces, thus raising the temperature of steam in the super
and a ?ame-positioning means associated with the fuel
heaters. This recirculation of gases takes place auto
burning apparatus causing the ?ame to be located at a
matically without the use of ducting, dampers, or com
position from adjacent the said one of the walls at high
plicated controls.
15 load to cause a ?ow of gas directly through the said
Referring to FIG. 4, which shows the relationships
opening to the said passage to adjacent the said wall
which exist at high load, the ?ame 124 is shown as be
opposite at low load to cause a recirculation of gas from
ing positioned close to the rear wall 16, so that a large
the passage to the underside of the ba?le through the
percentage of the gases have a tendency to go directly
said opening to aid in maintaining the temperature of
through the rear pass 123 and through the gas oiftake 20 superheated steam at a predetermined value.
125 to the backpass 37. Some of the gases, of course,
3. A steam generating unit, comprising a vertically
will go upwardly through the forward pass 33, across
elongated combustion chamber having front, rear, and
the upper pass 34, and then through the gas offtake 125,
side walls, fuel-burning apparatus located at the bottom
but generally speaking, the greatest volume of gases will
of the combustion chamber to produce a ?ame, a gas off
go through the pass 123. This has a tendency to lower 25 take opening into the upper part of the rear wall, a ba?le
the temperature of superheat. First of all, there is no
extending across the combustion chamber in a position
recirculation downwardly through the pass 123 as was
spaced away from the front wall to provide a passage
provided at low load. Secondly, those gases which pass
located between the front wall and the battle and located
over the convection superheaters 62 and 63 have reached
over the baffle, said bat?e including means providing
these areas by a longer path since they are forced to ?rst 30 an opening located in the baf?e immediately adjacent the
go toward the rear wall of the furnace and then move for
rear wall, a convection superheater located in the pas
wardly to the furnace so that their residence time in the
sage, and means associated with the fuel-burning ap
furnace is longer (and their temperature when they reach
paratus to position the ?ame at a desired position be
the convection superheater is lower). Furthermore, the
tween the front wall and the rear wall, the ?ame-position
?ow of gases over the convection units is smaller in 35 ing means causing the v?ame to be positioned adjacent
amount; that is to say, the mass ?ow over the convection
the front wall at low load to cause a recirculation of
superheaters is less. Also, with the ?ame at the rear
gas from the passage to the underside of the baffle through
of the furnace, the radiation to the radiant superheater
the said opening and adjacent the rear wall at high load
57 and the radiant parts of the superheater platen 63 is
to cause a flow of gas directly through the said opening
less. All of these factors have a tendency to lower super 40 to the said passage to aid in maintaining the tempera
heat. At intermediate loads, of course, the movement
ture of superheated steam at a predetermined value.
of the ?ame to positions between the front and rear walls
4. A steam generating unit, comprising a vertically
will produce variations of these factors and will permit
elongated combustion chamber having front, rear, and
It is obvious that minor changes may be made oin the 45 apparatus located at the bottom of the combustion cham
form and construction of the invention without departing
ber to produce a ?ame, a gas off-take opening into the
from the material spirit thereof. It is not, however, de
upper part of the rear wall, alternate tubes of the rear
sired to con?ne the invention to the exact form herein
wall extending forwardly from a position adjacent the
shown and described, but it is desired to include all such
gas off-take and then extending upwardly, a baffle mount—
as properly come within the scope claimed.
ed on the saidalternate tubes and extending across the
The invention having been thus described, what is
combustion chamber in a position spaced away from the
claimed as new and desired to secure by Letters Patent, is:
front wall to provide a passage located between the front
1. A steam generating unit, comprising a combustion
wall and the baffle and extending over the baffle, said
chamber having front, rear, and side walls, fuel-burning
baf?e including means providing an opening located in
apparatus located at the bottom of the combustion cham 55 the ba?le immediately adjacent the rear wall, a convec
her to produce a mass flow of hot products of combus
tion superheater located in the passage, and means as
tion, a gas off-take opening into the upper part of the
sociated with the fuel-burning apparatus to position the
rear wall, a ba?le extending across the combustion cham
?ame at a desired location between the front wall and
ber in a position spaced away from the front wall to
the rear wall, the ?ame-positioning means causing the
provide a passage located between the front wall and 60 ?ame to be positioned adjacent the front wall at low
the ba?ie and extending over the baffle, said ba?le in—
load to cause a recirculation of gas from the passage to
cluding means providing an opening located in the ba?le
the underside of the baf?e through the said opening and
immediately adjacent the rear Wall, a convection super
adjacent the rear wall at high load to cause a ?ow of
heater located in the passage, and positioning means as
gas directly through the said opening to the said passage
sociated with the fuel-burning apparatus to locate the line 65 to aid in maintaining the temperature of super-heated
of greatest mass ?ow at a desired position between the
steam at ‘a predetermined value.
front wall and the rear wall, the positioning means caus
5.A steam generating unit, comprising a vertically
ing the line of greatest mass ?ow to be located adjacent
elongated combustion chamber having front, rear, and
the front wall at low load to cause a recirculation of gas
side walls and a roof, fuel-burning apparatus located- at
from the passage to the underside of the baffle through 70 the bottom of the combustion chamber to produce a
the said opening and adjacent the rear wall at high load
?ame, a gas off-take opening into the upper part of the
to cause a ?ow of gas directly through the said opening
rear wall, a ba?le extending across the combustion cham
to the said passage to aid in maintaining the temperature
ber in a position spaced away from the front wall to pro
of superheated steam at a predetermined value.
vide a passage located between the front wall and the
2. A steam generating unit, comprising a vertically 75 ba?ie and extending between the ba?le and the roof, said
3,063,431‘
0,
r
7
,
baffle including ‘means providing an opening located in
the ba?ie immediately adjacent the ‘rear wall, a convec
tion superheater located in the passage, superheater
platens hanging downwardly between the ba?le and the
front wall into 'the‘combustion chamber, and ‘means asso
ciated with the fuel-burning apparatus to position the
r
0
wall extending forwardly and ‘upwardly from a ‘position
adjacent the gas oif-taketojfortm‘an inclined portion and
extending vertically toprovide a vertical portion, a baf?e
mountedjon the adjoining ‘partsjof the inclined and ver
tical portions of the said alternate tubes ‘and ‘extending
across the combustion chamber to provide a passage
located between the front wall and the front edge of the
ba?le and extending between the upper surface of the
?ame to be positioned adjacent the front wall at low
‘baffle and the roof wall, said baf?e including means pro
load to cause a recirculation of gas from the passage to 10 viding an opening located in the baf?e immediately ‘adja
the underside of‘the ba?le through the said opening and
cent the rear wall, a convection superheater located in
?ame at a desired position between the front wall and
the rear wall, the ?ame-positioning means causing the
adjacent the rear Wall at high load to cause a ?ow of gas
the passage, superheater platens hanging downwardly
directly through the said opening to the said passage
between the baffle and the front wall below the lower
most portion of the baffle into the combustion chamber,
to aid in maintaining the temperature of superheated
steam at a predetermined value.
6. A steam generating unit, comprising a vertically
and flame-positioning means associated with the fuel
burning apparatus to cause the ?ame to be located in a
elongated combustion chamber having front, rear, side,
position from adjacent the front wall at low load to cause
and roof walls covered with water-wall tubes, fuel-burn
ing apparatus located at the bottom of the combustion
chamber to produce a ?ame, a gas off~take opening into
the upper part of the rear wall, alternate tubes of the
rear wall extending forwardly from a position adjacent
the gas take-oil and then extending upwardly, a ba?le
a recirculation of gas from the passage to the underside
mounted on the said alternate tubes and extending across
the combustion chamber in a position spaced away from
the front wall to provide a passage located between the
front wall and the baffle and extending between the ba?ie
and the roof wall, said bai?e including means providing
an opening located in the baffle and immediately adja
cent the rear wall, a convection superheater located in
the passage, superheater platens hanging downwardly be
tween the ba?le and the front wall into the combustion
chamber, and means associated with the fuel~burning
apparatus to position the ?ame at a desired location be
tween the front wall and the rear wall, the flame-position 35
of the baffle through the said ‘opening to adjacent the
rear wall at high load to cause a ?ow of gas directly
through the said opening to ‘the said passage to aid in
maintaining the temperature of superheated steam at a
predetermined Value.
8. A steam generating unit, comprising a vertically
elongated combustion chamber having walls, fuel-burning
apparatus located at the bottom of the combustion cham
ber to produce a ?ame, a gas off-take opening into
upper part of the furnace, a ba?le extending across
combustion chamber in a position spaced away from
of the walls to provide a passage located between
the
the
one
the
said one of the walls and the bafde, said ba?le including
means providing an opening located in the baffle imme
diately adjacent a wall opposite the said one of the walls,
a convection superheater located in the passage, and flame
positioning means associated with the fuel-burning ap
,ing means causing the flame to be located adjacent the
paratus causing the ?ame to be located at a position from
front wall at low load to cause a recirculation of gas from
adjacent the said one of the walls at low load to cause a
the passage to the underside of the battle through the said
recirculation of gas, from the passage to the underside of
the ba?le through the said opening to adjacent the said
opening and adjacent the rear wall at high load to cause
a ?ow of gas directly through the said opening to the 40 opposite wall at high load to cause a ?ow of gas directly
said passage to aid in maintaining the temperature of
through the said opening to "the said passage to aid in
superheated steam at a predetermined value.
maintaining the temperature of superheated steam at a
7. A steam generating unit, comprising a vertically
elongated combustion chamber having front, rear, side,
and roof walls covered with water-wall tubes, fuel-burn— 45
ing apparatus located at the bottom of the combustion
chamber to produce a ?ame, a radiant superheater lo
cated adjacent the front wall, a radiant reheater located
adjacent the rear wall, a gas off-take opening into the
upper part of the rear wall, alternate tubes of the rear 50
predetermined value.
'
References ?ited in the ?le of this patent
UNITED STATES PATENTS
2,271,643
Jacobus ____; _______ __ Feb. 3, 1942
2,699,762
Koch
2,947,289
Miller ______________ __ Aug. 2, 1960
_______ _,_ ____ __ Jan.
18,
1955
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