Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a heat exchanger, for
exchanging heat between hot gases and water, and more
particularly relates to a boiler for generating steam.
Boilers having tubes are classified into two
types, dependent on whether the water flows within the
tubes, or is outside the tubes. In a water tube boiler, the
water flows within the tubes, with hot combustion gases
flowing around the tubes.
At the present time, there are a wide variety of
water tube boilers available. These include numerous
different features, dependent upon the desired capacity of
the boiler and other characteristics.
One earlier boiler design is disclosed in U.S.
patent 4,355,602. This boiler includes a plurality of tubes
which are bent and extend between upper and lower manifolds.
The tubes are bent to define a number of superposed
chambers. The combustion gases are produced by a burner at
the bottom of the boiler located in a combustion chamber and
then must flow successively through the various chambers in
a sinuous path. Baffles can be provided in the chambers to
make the path even more sinuous, with the intention of
promoting heat transfer between the gas and the water
flowing through the pipes. The intention is to provide a
boiler design that is simple to construct, assemble and
operate. It is also intended to have a high efficiency, and
be capable of handling varying loads.
However, one serious drawback with the design of
the boiler of this U.S. patent is in its ability to monitor
the water level in the boiler and supply additional water as
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required. Indeed, the patent is largely silent as to how
this aspect of the boiler would be tackled.
Where it is configured as a boiler, downcomers are
provided in known manner, for return of water from an upper
steam drum to the bottom of the boiler. The water can then
return up through the pipes to be further heated. This
arrangement has the further advantage that there is a
considerable and rapid circulation of water and a
water/steam mixture up through the pipes and then down
through the downcomers. This improves the heat transfer
between the pipes and the water/steam within them.
Whilst the rapid circulation promotes heat
transfer, it creates problems for monitoring the water level
within the boiler. A simple proposal is to provide a
connection to a downcomer, which is then connected to
appropriate water level sensors and pumps etc.
Conventionally, such a connection would be connected to both
a pump for delivering fresh water to the boiler and to a
level sensing switch. The level sensing switch activates the
pump when a certain lower water level is reached, and also
acts as a low water cutoff (LWCO) switch. The low water
cutoff is actuated when a second, lower water level is
reached. The low water cutoff switch closes off the burner
of the boiler. The intention is that if the pump fails to
maintain a sufficient water level in the boiler, then the
low water cutoff operates, as a safety measure to prevent
burnout of any tubes etc.
However, the high velocity of the water through
the downcomer, and its irregular turbulent flow, causes the
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pressure at any pipe take off port located on the downcomer
to fluctuate considerably. Consequently, the effective water
level sensed by the low water cutoff switch and by the pump
varies. This causes inaccurate and incorrect operation of
both the pump and the low water cutoff switch.
What is required is an arrangement, which enables
a continuous, accurate monitoring of the water level in a
boiler, irrespective of any irregularities and pressure
fluctuations caused in the flow in downcomers and pipes.
One possibility for monitoring the water level
would be to make a connection to the steam drum, where water
and steam are separated. However, as the water level in the
steam drum is relatively low, the connection would have to
be taken from the lowermost part of the steam drum. If, in
known manner, a pipe was connected horizontally to such a
connection with the pump and LWCO switch located above the
pipe, this would give very little room for the placement of
the pump and LWCO switch. In practice, there may be as
little as half an inch of spare vertical height, in which to
adjust the position of the LWCO switch and the pump. Bearing
in mind the sort of conditions in which such a boiler is
installed, such tolerances are not generally acceptable. It
is generally necessary to provide a greater range of
tolerance, for installation of individual components.
Further, it is by no means certain that a
connection to the steam drum would given the desired uniform
water level reading. The water flow in the steam drum is
subjected to the action of the incoming water from the
tubes, and consequently pressure surges can occur.
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In accordance with the present invention, there is
provided a tube assembly for a boiler comprising a lower,
water drum, an upper drum, a plurality of tubes extending
between the lower and upper drums for heating water therein,
a downcomer pipe extending between the lower and upper drums
for a return flow of water, which downcomer pipe includes a
section of large cross-section adjacent the upper drum, and
an inlet for a level sensing conduit opening into said
section to enable water level within the tube assembly to be
sensed without any substantial fluctuations in pressure due
to f]ow within the downcomer.
The inlet into the section of large diameter can
further be covered by a protective plate, to prevent
fluctuations in the fluid flow causing fluctuations in the
sensed water level. By this means, the LWCO switch and the
pump can accurately sense the water level, and operate
accordingly.
The present invention also provides a boiler
including a tube assembly as just defined. The boiler
includes a housing and a burner assembly for generating a
flow of hot combustion gas.
For a better understanding of the present
invention and to show more clearly how it may be carried
into effect, reference will now be made, by way of example,
to the accompanying drawings, which show a preferred
embodiment of the present invention and in which:
E'igure 1 is a side view of a boiler incorporating
a tube assembly of the present invention;
Figu~e 2 is a sectional view of the tube assembly
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along line 2-2 of Figure l;
Figure 3 is a sectional view of the tube assembly
along line 3-3 of Figure l;
Figure 4 is a sectional view of the tube assembly
along line 4-4 of Figure l;
Figure 5 is a plan view of the tube assembly of
the boiler;
Figure 6 is a horizontal view with an upper part
of the tube assembly, on a larger scale;
Figure 7 shows, on an enlarged scale, a side view
of an alternative drop leg configuration;
Figure 8 shows a horizontal section through the
drop leg of Figure 7 along line 7-7; and
Figure 9 shows a front view of a protective shield
of the drop leg of Figure 7.
A tube assembly as a whole is denoted by the
reference 10, and is shown in Figure 1 within a housing 12
(shown in outline). A burner 14, which can be of known
design, is shown schematically within the housing 12. The
tube assembly 10, housing 12 and burner 14 together form a
boiler for generating steam.
The tube assembly 10 has a lower, water drum 16
and an upper drum 18, which serves as a water/steam
separator. The upper drum 18 has an outlet nozzle 19. A
plurality of tubes 20 extend between the drums 16, 18. In
this case, there are thirty-seven tubes 20 on either side of
the boiler. In general, for a large part of their lengths,
the tubes 20 are close enough toge~her to form a continuous
wall or partition, as explained in detail below.
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The tubes 20 comprise first tubes 20, 21 and 22 on
one side of the boiler, and second tubes 24, 25 and 26 on
the other side of the boiler.
The first tubes comprise three distinct groups of
tubes, namely first front tubes 21, first central tubes 22
and first rear tubes 24. There are four first front tubes 21
and three first rear tubes 23, with the remaining thirty
tubes being central first tubes 22. Similarly, there are
four second front tubes 24 and three second rear tubes 26,
with the remaining thirty second tubes being central tubes
25.
Generally, all the tubes 20 have upper and lower
end portions which extend generally laterally or
horizontally out from the drums 16, 18. For the lower drum
16, the lateral end sections are given the references 30,
32. It can be seen that the lateral end sections 30 extend
out horizontally, whilst the lateral end sections 32 extend
upwardly at a slight angle. As shown in Figure 1, the
lateral end sections 30, 32 alternate along the tubes 20.
The purpose of this is to ensure that the openings in the
lower drum 16 are well spaced from one another, to enable
the lower drum 16 to be formed of relatively thin material
whilst meeting appropriate codes for steam boilers.
Similarly, at the top, upper lateral end sections 34, 36 are
provided. Again, the end sections 34 are generally
horizontal. The other upper end sections 36 incline
downwardly from the upper drum 18, corresponding to the
upward inclination of the end sections 32. This again
staggers the openings necessary in the drum 16 for the tubes
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20. As shown in Figure 1, each pipe 20 is provided either
with lateral end sections 30, 34 or lateral end sections 32,
36.
Referring back to the individual groups of tubes,
as shown in Figure 2, the first front tubes 21 include a
straight central portion 40 extending between the lateral
end sections. The second, front tubes 24 have vertical
portions 42, 44 which are continuous with two transverse
portions 46, 48. A central vertical portion 50 extends
between the other ends of the transverse portions 46, 48. It
will be seen that the transverse portion 46 slopes upwardly
slightly towards the central vertical portion 50, whilst the
other transverse portion 48 slopes upwardly away from the
central vertical portion 50. This should minimize the
possibility of vapour locks occuring.
Referring to Figure 3, it can be seen that the
first and second central tubes 22, 25 have, in some ways,
similar configurations. They both include transverse
portions. The first central tube 22 has a long, lower
vertical portion 52 which continues into a transverse
portion 54. The tube 22 then turns through almost 180 and
continues back along a second transverse portion 56. A short
upper vertical portion 58 then connects the transverse
portion 56 to the respective end portion 34 or 36.
Similarly, each second central tube 25 comprises a
lower vertical portion 60, which is shorter than the lower
vertical portion 52. This continues into lower and upper
transverse portions 62, 64. An upper vertical portion 66
connects the top transverse portion 64 to the respective end
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portion 34 or 36.
The junction between the first transverse portions
54, 56 forms a bend 55 which abuts the upper vertical
portion 66 of the second tubes. Similarly, the second
transverse portions 62, 64 form a bend 63 which abuts the
long lower vertical portion 52 of the first tube.
Referring to Figure 4, there is shown the first
rear tubes 23 and the second rear tubes 26. The first rear
tubes 23 each have a lower vertical portion 68, which
continues into a lower transverse portion 70. From the other
end of the transverse portion 70, a short central vertical
portion 72 continues into an upper transverse portion 74. A
short upper vertlcal portion 76 is provided extending to the
lateral end portion 34 or 36.
Each of the second rear tubes 26, with the
exception of one of them, simply comprises a central
vertical portion 78 extending between the respective lateral
end portions 30, 34 or 32, 36. One rear central tube 26 has
a lower end section 80 that rises vertically from the lower
drum 16. It is connected by an inclined portion &2 to a
vertical portion designated by the reference 84, but not
directly visible in Figure 4. This tube acts as a vent tube,
to prevent steam pockets occurring in the lower drum 16. The
central vertical portions 72 again abut the second rear
tubes 26.
Many of the transverse portions are at the same
height and form continuous surfaces, so as to define a
number of chambers. Thus, the lower transverse portions 46
of the front tubes are at the same height as the lower
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transverse portions 62 of the second central tubes. The
upper transverse portions 64 of the second central tubes are
at the same height as the lower transverse portions 70 of
the first rear tubes 23. The lower transverse portions 54 of
the first central tubes 22 are at the same height as the
upper transverse portions 48 of the second front tubes.
Also, the upper transverse portions 56 of the first central
tubes are at the same height as the upper transverse
portions 74 of the rear tubes. As shown, all the transverse
portions are inclined slightly, so as to slope upwardly in
the direction of fluid flow. This is with the intention of
preventing vapour locks.
Consequently, a number of chambers are formed,
causing the hot gases to flow in a sinuous path upwards from
the burner 14. These chambers are given consecutive
reference numerals 91 through 97 in the figures, indicating
the direction of flow of combustion gas.
Thus, hot gas from the burner 14 flows upwardly to
an inlet chamber 91 beneath the transverse portions 70. It
can then flow forwardly through a lower centeral chamber 92
in the centre of the tube assembly 10, until it reaches a
front chamber 93 having approximately twice the height of
the chamber 92. The hot gas can then rise and return
rearwardly back into an intermediate central chamber 94.
This in turn leads into a rear chamber 95, having similar
dimensions to the front chamber 93. The gas can then rise
again and enter an upper central chamber 96. I'he gas returns
through the chamber 96 until it reaches an open outlet
chamber 97 at the front. The gas can then pass to an exhaust
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stack. This sinuous gas path ensures there is ~ood heat
transferred to water flowing in the tubes 20.
In known manner, the tube assembly 10 is provided
with a front, downcomer pipe 100 and a rear downcomer pipe
102. In the configuration of the downcomer pipes 100, 102
and the tubes 20 is such as to promote rapid circulation of
water, thereby to further increase the heat transfer from
the hot gas to the water. Consequently, the velocity of
water and water/steam within the tube assembly 10 is high,
making it difficult to obtain a uniform and reliable
measurement of the water level within the tube assembly 10.
The rear downcomer pipe 102 includes upper and
lower inclined sections 106, 108 and a central section 110.
This shape enables the housing to include an access opening.
This can include flanges 112 for field erection. Further,
the rear downcomer pipe 102 can include an inlet 114 and a
chemical feed 116. As shown in Eigure 4, the upper drum 18
can include a continuous blowdown opening 118, and (Figure
2) the lower or mud drum 16 can include a blowdown opening
119 .
In accordance with the present invention, the
front downcomer pipe 100 is provided with an enlarged
section 120, which is of larger diameter or cross-section
than the rest of the pipe 100. This enlarged section 120
extends vertically down below the upper drum 18. A lower
section 122 of the downcomer pipe 100 extends horizontally
out from the enlarged section 120. The lower section 122
then turns through a right angle and extends vertically
downwards beside the burner 14. At the lower end, the lower
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section 122 turns through another right angle before
entering the lower water drum 16 horizontally. As for -the
rear downcomer pipe 102, optional flanges 124 can be
provided to facilitate erection in the field.
Reference will now be made to Figure 6 which shows
a detail of the upper drum 18 and the enlarged section 120,
together with associated pipe work for measuring the water
level. On one side of the section 120, towards the bottom
thereof, there is an inlet 130, connected to a level sensing
conduit 132. The conduit 132 extends horizontally. A
protective shield 134 is provided within the section 120.
The conduit 132 extends horizontally and is joined
by threaded cross members 136 to first, second and third
vertical pipes 141, 142 and 143. Additionally, threaded
unions 138 are provided in the conduit 132.
At the top of the drum 18, a further horizontal
conduit 146 is joined to the drum 18 by a short vertical
pipe 148 and a further threaded cross member 136. Further
cross members 136 connect the horizontal conduit 146 to the
upper ends of the vertical pipes 141, 142 and 143.
In the first vertical pipe 141, there is a pump
150 which is activated when the water level falls to a first
level indicated at 152.
In the second vertical pipe 142, there is a
control switch 154 which serves as an LWCO switch and a
control switch for the pump 150.
The levels at which the swit-ches 154 actuate are
indicated by the lines 152, 155 respectively.
The third vertical pipe 143 simply contains a
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union 158. At its upper end, it is connected by a cross
mcmber 136 to an auxiliary LWCO control switch 160.
Reference will now be made to Figures 7-9, which
show an alternative for the section of large cross-section
or drop leg configuration, including details of a protective
shield design. For simplicity, like components are given the
same reference numerals as in the earlier figures, and the
description of these components is not repeated.
In Figure 7, a drop leg of enlarged section is
denoted by the reference 170. As before, the drop leg 170 is
connected to a lower section 122, including flanges 124. As
shown, the lower section 122 joins the drop leg 170
approximately half way along its height. A nipple 172 is
provided at the bottom of the drop leg 170.
With the drop leg 170, there is a protective
shield 174 provided in front of the inlet 130. As shown, the
inlet 130 and shield 174 are slightly above the junction
with the lower section 122 and 2" below the top of the drop
leg 170. Details of the shield 174 are shown in Figures 8
and 9.
The shield 174 is formed from an angle section
member 176. This is closed at the top by a generally
triangular plate 178, welded or otherwise secured to the
angle section member 176 and the side wall of the drop leg
170. The bottom of the angle section member 176 is open, as
indicated at 180 in Figure 9. At the top of each side of the
member 176, there is a circular opening 182.
The shield 174 is configured so that, in use,
water flowing down through the downcomer 100 does not
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interfere with a pressure or water level measurement taken
through inlet 130. Thus, the openings 182 and open bottom
180 enable the water pressure to be sensed at the inlet 130,
whilst preventing any extreme fluctuations caused by
turbulence, water velocity etc. The open bottom 180 and
openings 182 permit a certain amount of restricted flow past
the inlet 130.
It has been found that, for the protective shield
to give a proper shielding effect, the drop leg 170 needs to
have a certain size in relation to the pipe forming the
lower section 122. In general, it has been found that the
ratio between the internal cross sectional area of the drop
leg 170 to the internal cross sectional area of the lower
section 122 should be 5:1 at least. For example, the drop
leg 170 can be formed from 8" standard E.R.W. pipe, whilst
the lower section 122 is formed from 4" E.R.W. pipe.
In use, with the burner 14 operating, water in the
tubes 120 is heated, so as to generate steam. The resultant
water and steam mixture rises rapidly through the tubes 20
to the upper drum 18, which acts as a water/steam separator.
It is estimated that, under appropriate operating
conditions, the quantities of water and steam in the tubes
are approximately equal. The lower density of steam
consequently causes the mixture to rise rapidly.
Simultaneously, water separated out in the upper drum 18
flows down through the downcomers 100, 102, to replenish the
water in the lower water drum 16. This water from the drum
16 is then drawn up through the pipes 20. This ensures that
there is a vigorous circulation within the tube assembly 10,
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giving good heat transfer.
The provision of the protective shield 134 or 174
ensures that the level of water in the drop leg 120, 170 can
be accurately monitored, without being affected by pressure
fluctuations caused by the vigorous flow.
The water level at the inlet 130 is sensed by the
sensing switch 154. If the water level falls below the pump
operating level 152, then the pump 150 is actuated, to
replenish the water in the tube assembly 10. Typically, the
pump operating level 152 could be equivalent to a level of
4" of water within the upper drum 18.
If the water level falls still further, for
example due to a pump malfunction, then when it reaches the
low water cutoff level 155, the switch 154 causes the burner
14 to be turned off. This should ensure that no damage to
the tubes 20 occurs, due to heating in the absence of
sufficient water.
As a further backup, and in accordance with
various codes for boiler design, the auxiliary LWCO switch
160 is provided, with separate circuitry etc. This ensures
that, if the switch 154 malfunctions, the boiler is still
shut down.
The switch 154 could be any suitable float switch.
For example, it could be a McDonnell 157 series pump
control, manufactured by McDonnell ~ Miller ITT Fluid
Handling Division. Alternatively, it could be an Optigain
switch manufactured by Optigain Limited of West Hill,
Ontario.
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