GB1588611A

GB1588611A - Fluidised bed heat exchangers

Info

Publication number: GB1588611A
Application number: GB8114/78A
Authority: GB
Original assignee: Erk Eckrohrkessel GmbH
Current assignee: Erk Eckrohrkessel GmbH
Priority date: 1977-03-03
Filing date: 1978-03-01
Publication date: 1981-04-29
Also published as: NL7801388A; SE433401B; FR2382661A1; SE7802367L; PL107724B1; PL204987A1; FR2382661B1

Description

(54) IMPROVEMENTS RELATING TO FLHDISED BED HEAT EXCHANGERS (71) We, ERK ECKROHRKESSEL GmbH, a Company organised under the laws of the Federal Republic of Germany of Davoser Strasse 2b, 1000 Berlin 33, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a fluidised bed heat exchanger with a primary heat exchange means for removing from the fluidised bed the heat generated in a roasting or burning process, the fluidised bed containing chamber being limited on two opposite sides by walls consisting of tubes conveying a heat-exchange fluid.

In the operation of fluidised bed furnaces used, for example, for roasting pyrites, it is known to remove heat by means of heat-exchanger tubes partly immersed in the fluidised bed, a cooling medium flowing through the tubes. The tubes are often arranged to form a curtain or field of tubes and they can be either vertical or slightly sloping with respect to the horizontal, or horizontal (British Patent Speci fication 712 899).

Recently there has been increasing interest in fluidised bed furnaces, in which a combustible material (coal, oil, wastes or the like) is burnt in a layer of turbulent, inert material.

The process has the advantage of very high heat transfer rates in the combustion zone, with the result that even with comparatively low temperatures (in the region of 8000 C) in the combustion zone, for in the fluidised bed, heat is transferred at a high rate to heat-ex change devices positioned in these zones, the rate of heat transfer being higher than can be obtained from conventional radiation chambers.

In particular, experiments have shown that if the heat exchanger tubes are positioned slightly sloping relative to the horizontal, the rate of heat transfer is less than is obtained by using either horizontal or vertical tubes, the worst case being tubes at 450. It is thought that the heat transfer coefficient is influenced by the period of contact between the moving particles in the fluidised bed and the walls of the heat exchanger tubes, it having been found that the shorter the contact period, the higher is the heat transfer rate. If the tubes are horizontal, the particles move up and down on the peripheries of the tube. The period of contact is therefore short and heat transfer is good. On the other hand, if the tubes are vertical the contact period is longer, giving a lower heat transfer rate than is the case with horizontal tubes.

In a known fluidised bed heat exchanger with natural circulation of the cooling medium (compare the periodical "Works Engineering", July 1972, page 19), slightly inclined tubes are positioned in the fluidised bed, the tubes being welded at their ends to cylindrical walls which limit the fluidised bed at the sides. A disadvantage is that the tubes, whose ends are fixed in place, cannot expand individually and are subjected to compressive stresses, or apply thrust to the walls limiting the fluidised bed. A further disadvantage of the arrangement is that if a tube is damaged it is difficult, or almost impossible, to replace it.

These disadvantages can be remedied by using a pump to drive the cooling medium through the tubes immersed in the fluidised bed. This allows the tubes to be positioned as desired. In particular, they can be exactly horizontal and they can be installed quite unstressed. A heat exchanger of this kind is described in the periodical "The Steam and Heating Engineer", March 1971, pages 6 and 7 also April 1971, pages 28 and 29. But the known heat exchanger has the disadvantage that the pump consumes power and is costly to purchase (the regulations require a reserve pump). A further disadvantage is that the tubes at the walls have to be bend outwards, or otherwise specially constructed, to allow the tubes which are to be immersed in the fluidised bed to be led through.In other words, the walls limiting the fluidised bed must have openings and cannot be constructed as continuous or uninterrupted walls.

The problem tackled in the present invention is to provide a fluidised bed heat exchanger which is arranged so that natural circulation of the cooling medium can be used, and so that the tubes in the fluidised bed can be horizontal, the heat-absorbing medium nevertheless circulating reiably through them, the walls limiting the sides of the fluidised bed chamber being uninterrupted or continous, the arrangement allowing a damaged tube to be replaced in a simple manner.

According to the invention there is provided a heat exchanger comprising a chamber, means for creating a fluidised bed in a lower part thereof and primary heat exchange means for removal of heat generated in the fluidised bed contained with lower part of the chamber wherein vertical riser tubes each connected to a lower inlet distributor and an upper outlet collector are arranged to form at least two continuous opposite walls of the chamber, and wherein said primary heat exchange means comprises U-shaped tubes each connected to a single riser tube, each U-tube having two geneally horizontal arms arranged to penetrate into said lower part of the chamber and disposed vertically one above the other, the horizontal length of the U being greater than its height, whereby a heat exchange fluid introduced into the lower inlet distributor will circulate naturally to the upper outlet collector at least partially through said primary heat exchange means due to convection on heating.

The arrangement ensures that the riser tubes are heated over practically their whole lengths and most strongly in their lower regions. The heat-exchange fluid is driven powerfully upwards in them by natural convection, and also circulates reliably through the horizontal portions of the tubes immersed in the fluidised bed.

Advantageously, one or more of the following preferred features is adopted: 1) said primary heat exchange means comprises two or more U-tubes on any single riser tube, the U-tubes on each such riser tube being vertically aligned with one another.

2) there is at least one said riser tube which carries two or more such U-tubes which are arranged to nest U within U.

3) there is at least one riser tube connected to secondary heat exchange means vertically aligned above primary heat exchange means also carried by such riser.

The further heat-absorbing surfaces situated in the space above the fluidised bed produce extra convective drive produced by the wall tubes, which are heated over practically their entire lengths. This arrangement promotes even better circulation of the heat-absorbing medium.

Between the two connection points where the U-arms of the (innermost) U-tube of each heat-absorbing surface are connected to its riser tube, the latter preferably contains a constriction. This compels the heat-exchange fluid to flow through the U-tubes. If the constriction has an orifice for the through flow of fluid, the orifice should be situated near the heated side of the riser tube.

The tubes in the space above the vortex bed can also be U-tubes, but are preferably serpentine, because this provides a greater heat-exchange surface and consequently faster heat transfer. Between the two connection points where the arms of the (innermost) U-tube or serpentine tube of each heat-absorbing surface are connected to the riser tube, the latter preferably contains a further constriction plate, to compel the heat-exchange fluid to flow through these tubes. Again, if an orifice is present the orifice is better positioned near the heated side of the riser tube.

As the wall riser tubes themselves supply heatexchange fluid to the heat-exchange means, and also lead it away, it is not necessary to provide openings in the wall, that is to say, the wall is simple in construction and should be tight, for example finned tubes can be used, or closely positioned tubes with an external sheet covering.

Damaged tubes can be replaced quite easily, either by separating the wall riser tube from the lower inlet distributor and from the upper outlet collector, and then removing the wall riser tube together with its associated heat-exchange means, or by separating the portion of the wall tube which supports the heat-exchange means, and then welding a new portion in place.

The invention also makes it possible to modify the entire primary heat exchange means situated in the fluidised bed by changing the number of U-tubes connected in parallel to a wall riser tube. This is particularly useful in that the heat-absorbing surface can be adjusted to suit different operating conditions, such as heat load, type of fuel and the like.

A further advantage of the invention is that the heat-absorbing surfaces can be arranged in different ways to suit particular requirements by positioning opposite pairs of heat-absorbing surfaces either in the same vertical plane or in different planes, in which case they can if desired intermesh combwise over a certain portion of their lengths.

The heat-exchanging surfaces in the fluidised bed must be spaced a sufficient distance so as not to impede the movement of the fluidised bed. Consequently it is preferred that not all the wall riser tubes should support primary heat exchange means, these riser tubes nevertheless being connected to the same distributor and collector as the nearby wall tubes equipped with heat exchange means. To ensure an even flow of medium through all the wall riser tubes, those which are not connected to heat exchange means may also contain constrictions, to give them the same flow resistance.

Examples of the invention are represented in the drawing, in which: Figure 1 is a diagrammatic vertical longitudinal section through a first version of a fluidised bed heat exchanger according to the invention.

Figure 2 is a section taken in the plane II-II in Figure 1.

Figure 3 is a diagrammatic vertical longitudinal section through a second embodiment of fluidised bed heat exchanger according to the invention.

Figure 4 is a section taken in the plane IV-IV in Figure 3.

The fluidised bed heat exchanger shown in Figures 1 and 2 (in the present example this is a steam generator) has a rectangular furnace chamber 6 limited at the front by wall tubes 2, at the back by wall tubes 4 and at the two opposite sides of riser tubes 1'. The floor of the chamber 6 has a nozzle plate 19 through which air flows upwards under pressure into the furnace chamber 6 from a lower air chamber 20.

The nozzle plate 19 supports a layer of a loose, inert, spherical or granular material which is induced to flow, or is brought into turbulent motion so as to form a fluidized bed by the stream of air rising through the nozzle plate 19. A combustible substance is charged into the bed from above by means of one or more feed devices, which are not shown in the drawing.

The resultant heat is removed by a system of tubes 7a through which a cooling, or heatabsorbing, medium flows. These tubes are joined, at their inlet and outlet ends, to the sidewall riser tubes 1 The tubes 7a are bent U-shaped, several tubes 7a nesting together U within U to form a heat-absorbing surface 7, occupying a vertical plane, this surface having a height which is several times less than the length of the tube bundle. Between the two ends of the innermost U-shaped tube 7", where they are joined to the sidewall riser tube 1' the latter contains an orifice plate, or a closure plate, to constrict the riser tube and compel the cooling medium to flow through the tubes 7a.

The riser tubes 1', and the wall tubes 2 and 4 are connected to a lower distribution system consisting of longitudinal inlet distributor tubes 9 and transverse inlet distributor tubes 12, 14. At their upper ends the sidewall riser tubes 1' are connected to longitudinal outlet distributor tubes 10, whereas the front wall tubes 2 are connected to transverse outlet distributor tubes 13, and the back wall tubes 4 are connected to transverse outlet distributor tubes 15. Water is supplied to the inlet distri butor tubes throughdowncomer tubes 18 and return-flow dowcomer tubes 17. The steamand-water mixture produced in the heated riser tubes is conveyed by the system of outlet distributor tubes to a steam-separator drum 11, to which are also connected steam header tubes 16.

The heating gases produced by the combustion in the fluidised bed rise in the furnace chamber 6 and then, after passing over heatabsorbing surfaces in the upper portion of the chamber, flow away in the direction of the arrow 26 to -downstream filters, dust separators and the like.

The heat exchanger shown in Figures 3 and 4, which is also, as an example, a steam generator, has a fluidised bed fumace chamber of rectangular cross section and sidewall riser tubes 1, 1'. Near the front wall of the chamber there are front wall tubes 2 which curve inwards over a front furnace compartment 3, forming ceiling tubes here, and then rise vertically once more. Near the back wall of the furnace chamber there is a sheath or curtain of back wall tubes 4 which are bent to come over a back fumace compartment 5 as ceiling tubes, and then rise vertically once more.Between these two furnace compartments is a middle furnace compartment 6 whose upper portion, formed by the upper ends of the riser tubes, provides a comparatively narrow flue containing evaporator heating surfaces 7 and also further heating surfaces 8 (economizer, air preheater or the like).

Sidewell riser tubes 1, 1' have their lower inlets connected to longitudinal inlet distributor tubes 9, and their upper outlets connected to longitudinal outlet distributor tubes 10, 10' which are in turn connected to a steam separator drum 11 either directly (distributors 10) or indirectly through a collector tube 10, (distributor 10). The bent front wall tubes 2 are connected at their lower inlet ends to a transverse inlet distributor tube 12, itself connected to the longitudinal inlet distributor tubes 9, and at their upper ends to a transverse outlet distributor tube 13, which is itself connected to the longitudinal outlet distributor tubes 10.The bent back wall tubes 4 are connected at their lower and upper ends to transverse distributor tubes 14 and 15, themselves connected to the longitudinal distributor tubes and to the collector tube.

From the upper distributor tubes steam header tubes 16 take steam to the separator drum 11. Separated water by-passes the drum and returns downwards through unheated downcomers 17 to the lower distributor tubes, which also receive a supply of water through downcomer tubes 18 from the separator drum 11.

The common floor of the three furnace compartments 3,5,6 is in the form of a perforated nozzle plate 19, each compartment having, however, its own air chamber 20. The nozzle plate 19 supports a layer of inert material, which is fluidized by the air blown through it from below the air chambers 20. The upper surface 21 of the resulting fluidised bed is normally a little below the upper edges of the separator walls 22 extending between the furnace compartments.

Combustible material is fed by feeder devices (not shown) into the furnace compartments 3 and 6, whereas the back compartment 5 is a carbon burn-up cell receiving combustible material discharged from a separator which is shown at 23. The compartment 3 contains a superheater 24, which is partly immersed in the fluidised bed. In the furnace compartment 6 evaporator heating surfaces 7 and 7' are connected to the sidewall riser tubes 1', the heating surface 7' also being partly immersed in the fluidised bed.

As shown in Figure 4, each of the heating surfaces 7' consists of several tubes 7a bent into U-shape and connected at inlet and outlet to the riser 1'. Each group of tubes 7a connected to one riser 1' occupies a single vertical plane. At least those risers 1 which are connected to heating surfaces 7' are also connected to upper heating surfaces 7 in the form of tubes 7b bent serpentine and penetrating into the flue-portion of the furnace chamber 6 above the fluidised bed, each tube 7b having its inlet and outlet ends connected to the riser 1', the tubes 7b connected to each riser 1' occupy- ing a common vertical plane. Each riser 1' to which serpentine tubes 7b are connected has its own unheated downcomer 25.

In the heat exchangers described above, as examples, the heat-absorbing surfaces 7, 7' connected to opposite risers 1', at the two sides of the furnace chamber, are the same as each other and occupy the same vertical plane. But a better turbulent movement of the heat-transmitting particles in the fluidised bed, and a better heat transfer on the surfaces 7, 7' is sometimes obtained by staggering these surfaces in position, at the two sides of the furnace chamber, in which case the two surfaces 7 and 7' can be intercalated over a certain length. This increases heat transfer in the middle region of the furnace chamber to match the heat transfer near the chamber walls, where the riser also absorb heat.

WHAT WE CLAIM IS: 1. A heat exchanger comprising a chamber, means for creating a fluidised bed in a lower part thereof and primary heat exchange means for removal of heat generated in the fluidised bed contained in the lower part of the chamber wherein vertical riser tubes each connected to a lower inlet distributor and an upper outlet collector are arranged to form at least two continuous opposite walls of the chamber, and wherein said primary heat exchange means comprises U-shaped tubes each connected to a single riser tube, each U-tube having two generally horizontal arms arranged to penetrate into said lower part of the chamber and disposed vertically one above the other, the horizontal length of the U being greater than its height, whereby a heat exchange fluid introduced into the lower inlet distributor will circulate naturally to the upper outlet collector at least partially through said primary heat exchanger means due to convection on heating.

2. A heat exchanger according to Claim 1, wherein said primary heat exchange means comprises two or more U-tubes on any single riser tube, the U-tubes on each such riser tube being vertically aligned with one another.

3. A heat exchanger according to Claim 2, wherein there is at least one said riser tube which carries two or more such U-tubes which are arranged to nest U within U.

4. A heat exchanger according to any preceding claim, wherein there is at least one riser tube connected to secondary heat exchange means vertically aligned above primary heat exchange means also carried by such riser.

5. A heat exchanger according to Claim 4, wherein said secondary heat-exchange means also consist of tubes bent U-shaped through which fluid flowing in such riser tubes can circulate.

6. A heat exchanger according to Claim 4, wherein the secondary heat-exchange means consist of serpentine tubes through which fluid flowing in such riser tubes can circulate.

7. A heat exchanger according to any preceding claim, wherein there is at least one said U-tube whose riser tube contains a constriction between the points to which such U-tube is connected, to promote circulation of fluid through such U-tube or tubes.

8. A heat exchanger according to Claim 7, wherein between the points of connexion of each U-tube of said primary heat exchange means to its riser tube, such riser tube contains a said constriction.

9. A heat exchanger according to Claim 7 or 8 as dependent on Claim 5 or 6, wherein between the points of connexion of each tube of said secondary heat exchange means to its riser tube, such riser tube contains a said constriction.

10. A heat exchanger according to any preceding claim, wherein there are said heat exchange means which are substantially similar and which extend from each of said two opposite walls towards the centre of the chamber.

11. A heat exchanger according to Claim 10, wherein said heat exchange means extend in pairs, one of each pair from each wall, and the heat exchanges means of each pair occupy the same vertical plane.

12. A heat exchanger according to Claim 10, wherein the heat exchange means extend from said opposite walls in intercalated vertical planes.

13. A heat exchanger according to Claim 12, wherein the heat exchange means extending from said opposite walls are themselves intercalated at least over a portion of their horizontal lengths.

14. A heat exchanger according to any preceding claim, wherein some only of said tubes are connected to said primary heat exchange means.

15. A heat exchanger according to Claim 14 as dependent on claims 8 or 9, wherein those riser tubes which are not connected to heat exchange means contain constrictions to give them the same flow resistance as the tubes which are connected to said heat exchange means.

16. A heat exchanger substantially as herein described with reference to either Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. surfaces 7' consists of several tubes 7a bent into U-shape and connected at inlet and outlet to the riser 1'. Each group of tubes 7a connected to one riser 1' occupies a single vertical plane. At least those risers 1 which are connected to heating surfaces 7' are also connected to upper heating surfaces 7 in the form of tubes 7b bent serpentine and penetrating into the flue-portion of the furnace chamber 6 above the fluidised bed, each tube 7b having its inlet and outlet ends connected to the riser 1', the tubes 7b connected to each riser 1' occupy- ing a common vertical plane. Each riser 1' to which serpentine tubes 7b are connected has its own unheated downcomer 25. In the heat exchangers described above, as examples, the heat-absorbing surfaces 7, 7' connected to opposite risers 1', at the two sides of the furnace chamber, are the same as each other and occupy the same vertical plane. But a better turbulent movement of the heat-transmitting particles in the fluidised bed, and a better heat transfer on the surfaces 7, 7' is sometimes obtained by staggering these surfaces in position, at the two sides of the furnace chamber, in which case the two surfaces 7 and 7' can be intercalated over a certain length. This increases heat transfer in the middle region of the furnace chamber to match the heat transfer near the chamber walls, where the riser also absorb heat. WHAT WE CLAIM IS:

1. A heat exchanger comprising a chamber, means for creating a fluidised bed in a lower part thereof and primary heat exchange means for removal of heat generated in the fluidised bed contained in the lower part of the chamber wherein vertical riser tubes each connected to a lower inlet distributor and an upper outlet collector are arranged to form at least two continuous opposite walls of the chamber, and wherein said primary heat exchange means comprises U-shaped tubes each connected to a single riser tube, each U-tube having two generally horizontal arms arranged to penetrate into said lower part of the chamber and disposed vertically one above the other, the horizontal length of the U being greater than its height, whereby a heat exchange fluid introduced into the lower inlet distributor will circulate naturally to the upper outlet collector at least partially through said primary heat exchanger means due to convection on heating.