Patent Publication Number: US-2011048343-A1

Title: Steam boiler equipped with cooling device

Description:
TECHNICAL AREA 
     The present invention concerns a steam boiler with a circulation system for boiler water that preferably has natural circulation. 
     BACKGROUND TO THE INVENTION 
     In steam boilers in which combustible material is combusted in a furnace, hot flue gases are formed that are conducted out of the steam boiler while the hot flue gases emit thermal energy to the boiler water circulating in a circulation system for the boiler. A prior art method of transferring thermal energy from hot flue gases to boiler water in a small steam boiler of an older type involves using a Field&#39;s steam boiler system from the late 19 th  century in this steam boiler. To increase the heating area, vertical tubes (Field&#39;s tubes), closed at the lower ends, are used in this. They are suspended from the furnace roof. Inside these tubes are inner circulation tubes which are open at both ends. When the boiler is in operation, heat applied to the outer gap between the concentrically arranged tubes will achieve a steam/water mixture that has a lower density than the water in the inner tube. This creates natural circulation in these tubes which effectively increases the heat-absorbing area in the boiler. Heat is thus supplied to the stationary water volume above the sheet metal roof of the furnace from these Field&#39;s tubes. Despite early implementation, this application of Field&#39;s tubes has only been used to extend the heating area of a stationary, static volume of liquid to which heat is to be supplied. These Field&#39;s tubes are freely exposed in the furnace. 
     Regardless of how the heat is transferred to the boiler water, the flue gases, on their way out of the steam boiler, are conducted through ducts in which parts of the boiler are subject to extreme stress. If, for example, the steam boiler contains a cyclone for the separation of particles from the hot flue gases, the cyclone normally has an outlet tube which is extremely exposed to the hot flue gases. To allow it to last longer, it has been proposed, for example in U.S. Pat. No. 4,913,711, that the outlet tube can be cooled with cooling water fed using a pump to piping that leads to the outlet tube. A major disadvantage in this case is that a pump is required to cool this component. Another disadvantage is that the connection tube passes through the cyclone volume, thus disturbing the cyclone effect. This connection tube must be well protected against erosion. 
     There are also examples of cooled outlet tubes embodied by vertical water-bearing tubes next to each other. These tubes receive their water from below. One disadvantage of this is that these supply tubes are long and subject to severe erosion and may disturb the operation of the cyclone. 
     It is easy to see that, in a steam boiler, not only the outlet tube of a cyclone may be subject to stress on account of heat but that the high temperature may also constitute a problem for the material in the walls and other components in the entire steam boiler. The aim of the present invention is to offer an improved steam boiler in which various exposed parts of the steam boiler can become more durable by being cooled effectively. 
     DESCRIPTION OF THE INVENTION 
     The present invention concerns a steam boiler with a circulation system for boiler water that preferably has natural circulation. The circulation system comprises water pipes arranged in such a way that, when the steam boiler is in operation, they can circulate boiler water through the pipes in a circuit in which water passes from the steam dome in the outer down tube to the furnace and convection parts and in which water and steam pass from the furnace and convection parts up to a steam dome in which steam is separated from the circuit. 
     The steam boiler comprises a device designed to cool a selected, exposed part of the steam boiler. The cooled device comprises an outer tube and an inner tube placed in the outer tube. The outer tube has an upper end with an open connection to one of the water pipes for boiler water and a sealed lower end. The inner tube has an upper open end connected to a water pipe for boiler water and a lower open end at the outer tube&#39;s sealed lower end. In some embodiments, the inner tube&#39;s upper open end is located at the outer tube&#39;s upper end and they are connected to the same water pipe or flow for boiler water. The cooled device extends at least partially in a vertical direction downwards from the point at which the outer tube is connected to a water pipe. 
     The steam boiler may comprise an element that is exposed to hot gases when the steam boiler is used. The element that is exposed to hot gases when the steam boiler is used may consist of a wall in the steam boiler or a holder for the heat exchanger in the path of the flue gas or the cooled device may be arranged in or in fact constitute this wall or holder. 
     The element that is exposed to hot gases when the steam boiler is used may also consist of, for example, a thermocouple or a through tube that extends through the pipe for boiler water and through the inner tube of the cooled device and, at its very end, connects to the outer tube of the cooled device. 
     The steam boiler may comprise a cyclone for separation of particles from hot flue gases, in which connection the cyclone has an outlet tube for the hot gases. In such case, the cooled device may be arranged to create a cooled outlet tube. 
     The cyclone&#39;s outlet tube may be constructed of or comprise a number of cooled devices, each of which cooled devices comprises an outer tube and an inner tube arranged in the outer tube, which outer tube, at an upper end, has an open connection to one of the pipes for boiler water and a sealed lower end and the inner tube has an upper end at the outer tube&#39;s upper end and a lower open end at the outer tube&#39;s sealed lower end and the cooled device may extend at least partially in a vertical direction downwards from the point at which the outer tube is connected to the water pipe. 
     In some embodiments, at least some of the pipes for boiler water may be inside the cyclone&#39;s walls and extend in a vertical direction from a lower part of the cyclone to a higher part of the cyclone. Some of the pipes for boiler water that extend inside the cyclone&#39;s walls may, in such case, be connected to cooled devices provided with an outer and an inner tube. 
     According to one embodiment, the steam boiler may have walls that form a flue gas duct in which one or more elements are suspended by means of a holder, which holder comprises a cooled device in accordance with the present invention, i.e. a cooled device with an outer tube and an inner tube arranged in the outer tube, which outer tube, at an upper end, has an open connection to one of the water pipes for boiler water and a sealed lower end, in which connection the inner tube has an upper open end at the outer tube&#39;s upper end and a lower open end at the outer tube&#39;s sealed lower end and the cooled device extends at least partially in a vertical direction downwards from the point at which the outer tube is connected to the water pipe. 
     According to another embodiment, a flue gas duct formed from the walls of the steam boiler may comprise a separating partition wall that separates two parts of the flue gas duct from each other. In such case, the partition wall may be at least partially constructed of cooled devices in accordance with the present invention, i.e. cooled devices comprising an outer tube and an inner tube arranged in the outer tube, which outer tube, at an upper end, has an open connection to one of the water pipes for boiler water and a sealed lower end, in which connection the inner tube has an upper open end at the outer tube&#39;s upper end and a lower open end at the outer tube&#39;s sealed lower end and the cooled device extends at least partially in a vertical direction downwards from the point at which the outer tube is connected to the water pipe. 
     According to another embodiment, the cooled device is used as a dust-separating beam in a flue gas duct formed from the walls of the steam boiler. In such case, a number of cooled devices may be placed next to each other in the flue gas duct. When the cooled device is used as a dust-separating beam, the cooled device may be provided with outer rails that extend along the cooled devices. 
     The inner tube&#39;s upper end may be funnel-shaped. 
     The inner tube may, at its upper end, have a mouth that is at a slant to the inner tube&#39;s longitudinal axis. 
     The connection to the water pipe for boiler water may be in an area in which the lower side of the pipe for boiler water has been expanded to meet the outer tube. 
     At the upper end of the outer tube a lid may be arranged to cover part of the gap area between the outer and the inner tubes. 
     At its upper end, the inner tube may be inclined towards the wall of the outer tube. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows schematically a section of a steam boiler. 
         FIG. 2  shows in perspective and in section a cyclone for a steam boiler. 
         FIG. 3  shows a cooled device in accordance with the present invention. 
         FIG. 4  shows in perspective the construction of an outlet in a cyclone provided with cooled devices. 
         FIG. 5  is a side view of the upper part of the cyclone outlet shown in  FIG. 4 . 
         FIG. 6  is a top view of part of the cyclone outlet shown in  FIG. 4 . 
         FIG. 7  shows a section of a part in the lower part of  FIG. 4 . 
         FIG. 8  shows in section part of a cyclone with a cooled outlet tube. 
         FIG. 9  is a schematic side view of a sand seal between the cyclone and the furnace in a steam boiler. 
         FIG. 10  is an enlargement of part of  FIG. 9  showing another embodiment of the cooled device. 
         FIG. 11  shows a section of an embodiment of the cooled device used in the invention. 
         FIG. 12  is a schematic side view of another embodiment. 
         FIGS. 13   a  and  13   b  show another embodiment seen in a side view and a top view. 
         FIG. 14  is a schematic side view of an embodiment in which one or more cooled devices are used as holders for one or more elements suspended in a flue gas duct. 
         FIG. 15  shows schematically how one or more cooled devices are used as a wall that supports elements. 
         FIG. 16  shows schematically how a partition wall may be arranged in a flue gas duct. 
         FIG. 17  shows in section a side view of an embodiment of the invention. 
         FIG. 18  shows a variant of the embodiment shown in  FIG. 17 . 
         FIGS. 19-30  show side views in section of various embodiments of the cooled device itself. 
         FIGS. 31   a - 34   b  show sections of further embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a steam boiler  1  that comprises a furnace  6  in which combustion takes place. The steam boiler  1  may, for example, be a CFB boiler (Circulating Fluidised Bed boiler) in which the combustion air is supplied at the base. The steam boiler  1  has a circulation system  2  for boiler water. The circulation system  2  comprises water pipes  3 ,  4  arranged in such a way that, when the steam boiler  1  is in operation, they can circulate boiler water through the pipes  3 ,  4  in a circuit in which water and steam pass from the furnace  6  up to a steam dome  5  in which steam is separated from the circuit and water runs back towards the furnace  6 . 
     In  FIG. 1 , the reference number  3  indicates water pipes in which water and steam rise towards the steam dome  5 , while the reference number  4  designates water pipes in which water runs down from the steam dome  5  towards the area of the furnace  6 . The downward water pipes  4  may appropriately be arranged separate from the furnace  6 , for example on the outside of the steam boiler  1 , so that no heat is supplied to water running from the steam dome on its way down. Down in the area of the furnace  6 , the water may then be used to absorb thermal energy and transfer the thermal energy to the steam dome. The circulation of the boiler water in the pipes is driven by the strong heat generated in the furnace  6 . Low-density water mixed with steam rises in the pipes while water with a higher density from the steam dome  5  runs down. 
       FIG. 1  and  FIG. 2  also show that the steam boiler  1  comprises a cyclone  7  that is used to separate ash and sand from the hot flue gases formed in the furnace  6 . In the cyclone  7 , the hot flue gases can pass out through the outlet tube  11  while sand and ash fall towards the lower part  9  of the cyclone  7 . Combustible material that has not been fully combusted is also separated and can return to the furnace  6  via the lower part  9  of the cyclone  7 . The hot flue gases in the cyclone  7  cause major stress, in particular to the outlet tube  11 . Such outlet tubes  11  are normally made of fireproof sheet metal. However, the outlet tube is sensitive to hot gas corrosion, in particular in waste-fired boilers. It is also difficult to cope with expansion close to the suspension. The consequence is reduced durability, in some cases only 2-3 years. In addition to major costs for each replacement, the downtime itself also entails costs as a result of non-production.  FIG. 1  and  FIG. 2  also show how water pipes  3  for boiler water run inside the cyclone  7 &#39;s walls  8  and from there to the steam dome  5 . 
     Other parts of the steam boiler  1  are also exposed to stress by the hot gases. 
     A cooled device  12  will now be explained with reference to  FIG. 3 . The cooled device  12  is shown in  FIG. 3  connected to a pipe  3  for flowing boiler water. The cooled device  12  comprises an outer tube  13  and an inner tube  14  arranged in the outer tube  13 . The outer tube  13  has, at an upper end  15 , an open connection to the water pipe  3  for boiler water and a sealed lower end  16 . The inner tube  14  has an upper open end  17  that is also connected to a water pipe  3  and a lower open end  18  at the outer tube&#39;s sealed lower end  16 . In the embodiment in accordance with  FIG. 3 , the inner tube  14 &#39;s lower end  18  is arranged at the outer tube  13 &#39;s sealed lower end  16  and both the outer tube  13  and the inner tube  14  are connected to the same water pipe  3  for boiler water. However, embodiments are conceivable in which the outer tube  13  and the inner tube  14  are connected to different pipes  3  for boiler water. The cooled device  12  extends at least partially in a vertical direction downwards from the point at which the outer tube  13  is connected to a water pipe  3 .  FIG. 1  shows how the water pipe  3  for boiler water runs horizontally. It is suitable for the cooled device  12  to be connected at a point where the water pipe  3  is horizontal but the water pipe  3  could also be inclined upwards in the direction of flow. The cooled device  12  works as follows. When the cooled device  12  extends downwards into an area with hot flue gases, water that is located in the gap between the outer tube  13  and the inner tube  14  will be mixed with steam and have a lower density than the water located inside the inner tube  14 . The steam/water mixture therefore rises in the gap between the outer tube  13  and the inner tube  14 . The water located inside the inner tube  14  has a higher density and will fall instead. Part of the boiler water that flows in pipe  3  will then be sucked down in the inner tube  14  and will subsequently return upwards, absorbing thermal energy from the area around the cooled device  12 . The boiler water in the water pipe  3  flows from left to right in the figure, as shown by the arrows. 
     The normal water flow rate in the water pipe  3  is in the range 0.3 to 1.5 m/s. The water flow rate established in the downward tube  14  is 0.5 to 2.5 m/s and in the outer gap between tubes  13  and  14  it is 0.2 to 1.0 m/s. 
     In accordance with the present invention, a cooled device  12  is used that works according to the principle shown in  FIG. 3  to cool a selected, exposed part of the steam boiler  1 . 
     With reference to  FIGS. 1-2  and  4 - 8 , the steam boiler may comprise a cyclone  7  to separate solid particles such as ash, sand or fuel that was not combusted in the furnace from flue gases. Water pipes  3  pass the cyclone on their way up to the steam dome  5 .  FIG. 2  shows how water pipes  3  for boiler water extend in a vertical direction from a lower part  9  of the cyclone  7  to a higher part  10  of the cyclone  7 . In accordance with one embodiment, the cyclone  7 &#39;s outlet tube  11  may comprise at least one cooled device  12 . As explained earlier in connection with  FIG. 3 , the cooled device  12  comprises an outer tube  13  and an inner tube  14  arranged in the outer tube  13 . The outer tube  13  has, at an upper end  15 , an open connection to a water pipe  3 . In this case, the cooled device  12  may use the water in water pipe  3  to cool the outlet tube  11 . 
     In an embodiment best shown in  FIGS. 4-7 , the outlet tube  11  consists of a number of the cooled devices  12 , each of which comprises an outer tube  13  and an inner tube  14  arranged in the outer tube  13 . 
     The water pipe  3  that is used is suitably one of the water pipes  3  that run inside the walls  8  of the cyclone  7 . In practice, a number of such water pipes  3  pass through the cyclone  7  and meet higher up in the system.  FIGS. 4 and 5  show how some water pipes  3   a  continue directly upwards when they reach the area of the outlet tube  11 , while other water pipes  3   b  proceed to the cooled devices  12  that have an outer tube  13  and an inner tube  14 , as shown in  FIG. 3 . In  FIGS. 4 and 5 , the inflow  3  in is established to the pipes  3   a ,  3   b  towards the tube ends that are in a horizontal plane, oriented in towards the centre of the cyclone, and the outflow  3  out is established in the tube ends that are vertically oriented in the figure. As a suggestion, every other water pipe  3   b  may proceed to the cooled devices  12 . In this case, the cooled devices  12  may be arranged side by side so that together they form an outlet tube  11  for hot flue gases, as shown in  FIG. 4 . How this embodiment works is shown best in  FIG. 8 . As shown in  FIG. 8 , the boiler water rises in water pipes  3  that run inside the walls of the cyclone  7 . When the water reaches the area of the outlet tube  11 , some of the boiler water will descend in the cooled devices  12  that form the outlet tube  11 . This boiler water will move downwards in the inner tubes  14  of the cooled devices while the steam/water mixture will, on account of its lower density, move upwards in the outer gap between tube  13  and tube  14 . 
       FIG. 8  also indicates how the cooled devices  12  are connected to each other, at their lower ends, via an annular tube  33 . 
     The cooled device  12  may thus, in itself, form durable parts of the steam boiler  1  exposed to heat. Alternatively, the cooled device  12  may also be used to cool an element that, when the steam boiler  1  is used, is exposed to hot flue gases (directly or indirectly). Such an embodiment will now be explained with reference to  FIG. 9  and  FIG. 10 .  FIG. 9  shows a sand seal located in the lower part of the cyclone  7 . The sand seal comprises a wall  19  that, when the steam boiler  1  is in operation, is exposed to major stress via strong heat. As shown in  FIG. 10 , a cooled device  12  in accordance with the present invention, is arranged inside the wall  19  in the part marked XIII in  FIG. 9 . The cooled device  12  in the sand seal&#39;s wall  19  is constructed in the same way as the cooled device  12  in  FIG. 3  and it works in the same way. The cooled device  12  is connected to a pipe  3  for boiler water that rises inside the cyclone  7 &#39;s walls  8  and part of this boiler water is therefore used to cool a wall  19  in the sand seal of the cyclone  7 . After the cooled device  12 , the water pipe  3  continues vertically upwards while the pipe up to the cooled device runs horizontally. 
     Another embodiment is shown in  FIG. 11 . In the embodiment shown in  FIG. 11 , a through tube  21  has been passed through the outer wall of the water pipe  3  and through the inner tube  14  of the cooled device  12 . At its lower end, the through tube  21  connects to the outer tube  13  of the cooled device  12 . The through tube  21  consists of an element that may be used to add an additive to the steam boiler or to suck out flue gases for sampling. Without the cooled device  12 , the hot flue gases would act on this element unimpeded. The cooled device  12  can now cool the through tube and contribute to increasing its service life. 
     Another embodiment will now be explained with reference to  FIG. 12  and  FIG. 27 . In embodiments with an inner through tube  21 , a thermocouple  20  may be introduced via the through tube  21 . In such case, the thermocouple is kept cooled using the cooled device  12 . 
     Another embodiment will now be explained with reference to  FIGS. 13   a  and  13   b .  FIG. 13   a  shows how a number of dust separation beams were created by a number of cooled devices  12  being suspended as cooled separation beams in a flue gas duct. The cooled devices  12  may suitably be suspended in the flue gas duct  23  so that they form a thin row as shown in  FIG. 13   b . The cooled devices  12  in  FIG. 13   a  are designed according to the same principle as shown in  FIG. 3  with an outer tube  13  and an inner tube  14 . A water pipe  3  for boiler water in which water flow is established is connected to the cooled devices in  FIG. 13   a . As shown in  FIG. 13   b , the cooled devices used as separation beams may be provided with outer rails  28  that extend along the cooling devices  12 . The outer rails  28  contribute to intercepting particles, for example ash particles, which then fall. 
     Another embodiment is shown in  FIG. 14 .  FIG. 14  shows how an element  24  in a flue gas duct  23  may be suspended in the flue gas duct  23  in a cooled device  12  which functions as a holder for this element. The element  24  may consist, for example, of a tube structure such as part of a superheater or economiser. The holder for the tube structure  24  is then cooled and is better able to resist heat stress. 
     Another embodiment is shown in  FIG. 15 . A separately fired superheater  35  is supported here by a wall created from cooled devices  12 , each of which comprises an outer and an inner tube in accordance with the principle shown in  FIG. 3 . Of course, here too the cooled devices  12  are connected to a water pipe  3  for boiler water in which a water flow is established. 
       FIG. 16  shows an embodiment in which a number of cooled devices  12  form a partition wall in a flue gas duct, which partition wall separates one part of the flue gas duct  23  from another part. The partition wall here has an increased service life due to its ability to resist heat. Of course, these cooled devices  12  also have an outer and an inner tube in accordance with the principle shown in  FIG. 3  and they are connected to a pipe  3  for boiler water. 
       FIG. 17  shows schematically how a number of cooled devices  12  may be connected to a water pipe  3 , for example to form a row of separation beams in accordance with  FIGS. 13   a  and  13   b  or a partition wall in accordance with  FIG. 16 . 
       FIG. 18  shows a variant of the arrangement shown in  FIG. 17 .  FIG. 17  shows how the inner tube  14  in which water runs down is connected to a water pipe  3  other than the outer tube  13  in which a water/steam mixture with lower density moves upwards. The boiler water flows here in a first water pipe  3 , descends via the inner tube  14 , rises in the gap between tube  13  and tube  14  in one or more cooled devices  12  and flows out in a second water pipe  3 . 
     A few more embodiments will now be explained with reference to  FIGS. 19-21 . 
     In conventional Field&#39;s boilers, the cooling tubes are designed to make it easier for the boiler water to be sucked down in the inner tube  14 . Thus the inner tube  14  may, at its upper end  17 , be shaped as a funnel  29  to reduce the pressure drop in this part and, to a certain extent, guide the rising steam/water mixture away from the inlet of the inner tube. This embodiment may also be applied when the cooled devices  12  are connected to a water pipe with boiler water in which a water flow is established. 
     In the embodiment in accordance with  FIG. 20 , the effect of the water flow is utilised in the tube  3 . The inner tube  14  has here, at its upper end, been given a mouth that is at a slant to the tube&#39;s longitudinal axis. This is so that the water that, in  FIG. 20 , flows from left to right, finds it easier to flow into the inner tube  14  and establish a static pressure against the inlet of the inner tube. 
     In the embodiment in accordance with  FIG. 21 , the embodiment is adapted to an established water flow in the pipe  3  which may contain some steam. The cooled device  12 &#39;s connection to the water pipe  3  has here been arranged in an area  31  of the water pipe  3  in which the lower side of the water pipe  3  has been expanded to meet the outer tube  13 . This lower position is advantageous if the water pipe  3  contains a lot of steam. 
     Another aspect will now be explained with reference to  FIGS. 22-25 , in which the effect of the water flow in the tube  3  is used. In connection with a cooling device  12  that is connected to a water pipe  3 , it is desirable for steam formed in the boiler water that has already passed through the outer tube  13  of the cooled device  12  not to be sucked back down in the inner tube  14 . 
       FIG. 22  shows how a lid has been arranged at the upper end  15  of the outer tube to cover part of the gap area between the outer tube  13  and the inner tube  14 . When the flow in the water pipe  3  goes from left to right in  FIG. 22 , the lid  32  prevents steam in the outer tube  13  from entering the water pipe  3  in this area. Steam goes instead out to the right in the figure, i.e. the steam continues in the water pipe. In the inner tube  14 , boiler water that comes from the left in the figure under increased pressure via the inlet is instead sucked down. The lid also produces an ejector effect on the steam/water mixture that flows out from the outer tube as the flow rate increases in this area. 
       FIG. 23  shows a similar embodiment but the lid  32  is extended here. The idea here is that the water flow passing with a certain increased ejector effect will take the steam/water mixture leaving the cooled device  12  with it. 
     The embodiment in accordance with  FIG. 24  shows how the inner tube  14 , at its upper end, is inclined towards the wall of the outer tube  13 . This also results in a reduced risk of steam being sucked into the inner tube  14 . 
     The embodiment in accordance with  FIG. 25  shows how the embodiment in accordance with  FIG. 24  may be combined with the embodiment in accordance with  FIG. 21 . 
       FIG. 26  shows an embodiment that, in principle, is similar to the embodiment in accordance with  FIG. 21 . In addition, a through tube  21  has been added that can be used to suck out samples from the steam boiler or to add an additive. 
       FIG. 27  shows an embodiment that is similar to that in  FIG. 26  but in which the through tube  21  has also been utilised to introduce a thermocouple  20 . 
       FIG. 28  shows an embodiment that is similar to that in accordance with  FIG. 27  but here a drainage tube  36  has also been added that can be used to drain the cooled device  12 . 
       FIG. 29  shows an embodiment in which the inner tube  14  is arranged at the inner wall of the outer tube  13 . This may also reduce the risk of steam being sucked down in the inner tube  14 . The inner tube  14  may have a rear edge  37  at the side of the inner tube  14  that is downstream in the water pipe  3 &#39;s direction of flow. The rear edge  37  that sticks up on the downstream side also contributes to reducing the risk of steam being sucked down in the inner tube  14 . 
     In accordance with the embodiment shown in  FIG. 30 , the inner tube  14  has been split in two in its upper part. The reference number  38  refers to the holder for the inner tube  14 . 
       FIGS. 31   a  and  31   b  show an embodiment in which the cooled devices  12  are arranged connected to a joint upper box  39  located at an angle in relation to the water pipe  3  for boiler water. The cooled devices  12  jointly form a wall that may, for example, be a partition wall. The cooled devices  12  may be connected to each other via the connection pieces  40 , which may consist, for example, of welded-on sheet metal or flat steel. 
       FIGS. 32   a  and  32   b  show an embodiment equivalent to that in accordance with  FIG. 31   a  but in which the lower ends of the cooled devices  12  are also connected via a joint lower box  34  that also constitutes a joint lower closed end for the outer tubes  13 . 
       FIGS. 33   a  and  33   b  show an embodiment that is, in principle, similar to that in  FIG. 32   a . However, these figures also show a drainage tube  36  that has been passed down through an inner tube  14  in a cooling device  12 . 
       FIGS. 34   a  and  34   b  show an embodiment that differs from the embodiment in accordance with  FIG. 33   a  in that the drainage tube  36  does not run through an inner tube  14 . Instead one of the outer tubes in a row of the cooled devices only contains a drainage tube. The flow of water down in the cooled devices  12  takes place here in the inner tubes  14  in the other cooled devices. 
     The embodiments shown in the figures may be used in steam boilers of different types and, in principle, everywhere there is a heated space or a part that is exposed to heating and where there is also a flow of water in a pipe that passes the area or the part that needs to be cooled. 
     The invention offers a simple method for cooling parts that are exposed to strong heat. When the invention is applied to the outlet tube in a cyclone, it offers the advantage that the existing flow of boiler water can be used and no separate pipe is required into the cyclone from outside with the additional complications this entails. In addition, no separate pump for the flow of cooling water is required.