iZ73~3~6 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 ~ s , .; ' :~ ' ' -lZ73856 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 , , ' :, , 12'~38St~

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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: . . -, -,. . . -- . , -- . ' ., --127;~856 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 lZ73856 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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, 127;~hS6 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 12~;~856 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 , " ' "~- ' ' ' -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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lZ~38S6 g 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 12~î38S~

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 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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1~3856 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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~Z7;~856 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, - - . , - , :
,,,, ',' ~ ' 1'~73856 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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