GB2094959A - Suspension heat exchanger - Google Patents

Abstract

A suspension heat exchanger for heat exchange between a granular material and a gas flow comprises a train of cyclones (3', 5', 7', 9') at least one of which is of the axial flow type having a gas inlet at one end and a gas outlet at the other end. The axis of the axial flow cyclone (3') is disposed vertically or obliquely in such a way that the gas inlet connected to a riser pipe (2') is above a gas outlet connected to a riser pipe (6'). This enables the heat exchanger to have a low structural height and low overall pressure drop compared with a conventional suspension heat exchanger. The heat exchanger may be used for preheating cement raw material by means of hot exit gas from a kiln (1') in which the cement raw material is to be burnt to clinker. <IMAGE>

Description

SPECIFICATION Suspension heat exchanger The invention relates to a suspension heat exchanger for heat exchange between a granular material and a gas, in which the granular material is suspended in the gas and subsequently separated therefrom. Such a suspension with subsequent precipitation can take place in various, successive stages, in which the material is suspended in a gas flow and separated in a cyclone, a kind of stepped countercurrent being used.

Such a heat exchanger consisting of one or more stages is equally suited for cooling hot granular material in a cold gas flow as for heating a granular material by means of a hot gas flow, e.g. for preheating cement raw material by means of a hot exit gas from the kiln in which the cement raw material is to be burnt to clinker.

It is known to build up a cement raw material preheater from a train of cyclones coupled in series, material separated from one cyclone being suspended in the inlet gas for the preceding cyclone as considered in the direction of the gas flow, the raw material being fed to the inlet gas for the last cyclone, whereas the preheated material from the material outlet of the first cyclone is passed to the kiln. From the last cyclone the now cooled exit gas is vented to the open air, if desired through a dedusting filter.

Usually, such a preheater comprises as many as four cyclones, but escalating energy prices have made it desirable to add at least one more cyclone in order to use as much as possible of the heat which may still be contained in the exit gas after the passage of the gas through the other cyclones.

One of the reasons why, up till now, the number of cyclones has been limited to four is that the addition of one further cyclone would cause an undesirable increase in the structural height of the preheater, and would increase the overall pressure drop with a consequent increased energy consumption of the smoke gas fan.

It is the object of the invention to devise a cyclone heat exchange having a low structural height and causing a low pressure drop, considering the number of cyclones.

This is achieved, according to the invention, by providing a suspension heat exchanger for heat exchange between a granular material and a gas flow and comprising a train of cyclones at least one of which is of the axial flow type having a gas inlet at one end and a gas outlet at the other, characterised in that the axis of the axial flow cyclone is disposed vertically or obliquely in such a way that the gas outlet is situated below the gas inlet.

If there are three cyclones coupled one after the other in series, the riser pipe emanating from the first cyclone and leading to the second one receives material from the third cyclone. The lower the point of emanation of that riser pipe, the lower the third cyclone may be situated. If then at least the first cyclone is of the axial flow type with the -gas outlet of the cyclone situated below its gas inlet, there is achieved the desirable low-lying point of emanation for the riser pipe to the second cyclone.

It is known that the pressure drop over an axial flow cyclone is smaller than over a conventional cyclone. Consequently, it has previously been suggested e.g. in GB 2 038 670 to build up a heat exchanger from axial flow cyclones with their axes horizontally disposed. This achieves a lower pressure drop than a conventional cyclone heat exchanger, but only a small reduction of the structural height, namely the reduction resulting from the generally smaller dimensions of the axial flow cyclones in comparison with conventional cyclones.

However, the material outlet hopper of the cyclone contributes considerably to the structural height of the cyclone stage and as the outlet hopper of an axial flow cyclone disposed with its axis horizontal has to span the entire cyclone length, the material outlet hopper of such a cyclone will be of approximately the same size as that of a corresponding conventional cyclone. This is because the length of the outlet hopper is determined by the necessity of its walls having to form an angle of inclination of approximately 650 relative to the horizontal in order to avoid accumulation of material.

If a axial flow cyclone is used with its axis vertical, the outlet hopper only needs to span the diameter of the cyclone. Even in an axial flow cyclone with its axis oblique, i.e. inclined to the horizontal, the material is entrained by the vertical flow down along the inclined cyclone housing to a material outlet situated at the gas outlet end, and the material outlet need only span a fragment of the cyclone length so that consequently only a short outlet hopper is required.

In a heat exchanger in accordance with the invention use is preferably made of an axial flow cyclone with an oblique axis having at one end a tangential gas inlet and at the other an axial gas outlet in the form of a central tube projecting through the end wall of the cyclone so as to form an annular dist collecting or precipitation chamber between the central tube and the cyclone shell, the chamber being at one end defined by the end wall of the cyclone and at the other end adjacent to the vorticai chamber of the cyclone being either undefined or defined by a plate mounted around the inner end of the central tube, the plate leaving between its periphery and the cyclone shell an annular slot through which dust can pass from the vortical chamber of the cyclone into the material precipitation chamber, which at its underside is provided with a material outlet hopper.

The end plate at the outlet end of the cyclone may be positioned at an inclination of at least 600 relatively to the horizontal, irrespective of the angle of inclination of the cyclone axis, i.e. in such a way that the end plate is not necessarily perpendicular to the cyclone axis, while the central tube preferably extends coaxially with the cyclone shell. Furthermore, the central outlet may, through a tangential development, continue in a tube extending perpendicularly to the cyclone axis.

In addition to the one or more cyclones of the axial flow type disposed with the or each gas outlet lower than the respective gas inlet, the heat exchanger may, in addition, comprise cyclones of conventional kind in which the gas inlet and gas outlet are situated approximately at the same height, and in which the pressure drop over the cyclone is larger than over an axial flow cyclone. It would for instance be expedient to use a conventional cyclone as the last cyclone through which the gas passes prior to being passed to a desdusting filter or vented into the open air. This is a consequence of conventional cyclones often having a higher degree of separation than existing axial flow cyclones.While this is of minor importance in stages in which material not separated is passed to a subsequent cyclone stage, material passed into the open air or to the filter results in loss of energy and material.

The invention will now be explained in more detail with reference to the accompanying drawings, in which: Figure 1 schematically shows a conventional preheater composed of cyclones in which the gas inlet and the gas outlet are situated at the same height; Figure 2 schematically shows a preheater composed according to the invention of cyclones in which the gas outlet is situated lower than the gas inlet; Figure 3 is an elevation of an axial flow cyclone with its axis obliquely disposed; Figure 4 is a section taken on the line IV-IV in Figure 3; Figure 5 is an elevation of another axial flow cyclone for use according to the invention; Figure 6 is a section taken on the line VI--VI in Figure 5; Figure 7 is a side elevation of the tangential inlet of an axial flow cyclone with a preseparator for suspended material;; Figure 8 is a front elevation of the tangential inlet shown in Figure 7; and, Figure 9 is a side elevation of an axial flow cyclone with a tangentially developed gas exit tube.

Figure 1 shows diagrammatically a preheater composed of conventional cyclones. Exit gas passes from the kiln 1 through a riser pipe 2 to a first cyclone 3. During the passage through the riser pipe 2 material is fed from a second cyclone 5 though a pipe 4. From the cyclone 3 the exit gas is passed through a tube 6 to the second cyclone 5, the fed material being preheated by the hot kiln gas during its passage through the riser pipe 2 and then being separated in the cyclone 3.

Material separated in a third cyclone 7 is passed through a pipe 8 into the tube 6. From the cyclone 5 the gas is passed to the third cyclone 7 through a tube 10 and hence through a tube 11 to a fourth cyclone 9 from which it is carried away through a tube 12 into open air or to a dedusting filter or, if desired, to another preheater stage.

From the cyclone 9 separated material is passed through a pipe 13 into the tube 10. Through a pipe 14 raw material is fed from a raw material store, or from subsequent preheater stages, if any, into the tube 11, and from the first cyclone 3 separated material passes through a pipe 1 5 down into the kiln.

In Figure 2, which, similarly, shows a diagrammatically a preheater composed of cyclones the gas inlet of each of which, according to the invention, is situated below the respective gas inlet, corresponding tubes and pipes are referred to by the same reference numbers as in Figure 1 but with dashes against them. The cyclones proper in Figure 2 are not shown in a way directly illustrating their shapes but only as square boxes, as indicated also in Figure 1 by the dotted line 1 6.

The cyclones 3' and 5' are like the cyclones 3 and 5 in Figure 1 disposed in such a way that pipes 4' and 15' with suitable inclination lead into the riser pipe 2' and the kiln 1' respectively. A pipe 8' carrying separated material from the cyclone 7' into the tube 6' opens into that tube near its middle. This means that the cylone 7' can be positioned so much lower, and the pipe 8', while retaining the necessary inclination, opens into the tube 6' immediately above the place where the tube leaves the cyclone 3'. Such a lowering will be limited by the cyclone 3' when the cyclone 7' is, as shown, situated immediately above the cyclone 3'. It is, however, possible to offset these two cyclones laterally in relation to each other so that the cyclone 7' can be positioned even lower partly alongside the cyclone 3'.

Similarly, it is possible to lower the cyclone 9' and the material supply 14' so that the total structural height of the preheater is considerably reduced, or in such a way that with the same structural height as previously it is possible to have more preheater stages. The cyclones may e.g. be disposed along a helix, the axis of which is also that of the riser pipe 2'. The structural height may be further reduced if the material outlet pipe from the individual cyclones leads from the periphery of the cyclone as indicated by the dotted line 8" in Figure 2.

According to the invention axial flow cyclones, disposed with their axes disposed vertically or obliquely are used as cyclones which meet the condition that the gas outlet must be situated as low as possible relatively to the gas inlet.

Figure 3 diagrammatically shows, partially broken away, such an oblique axial flow cyclone having at one end a tangential gas inlet 30 (Figure 4) imparting to the gas supplied with its content of suspended material a vortical movement within the cyclone shell 31. The gas leaves the cyclone through a gas outlet 32 leading from the other end of the cyclone and constituting within the cyclone a central tube which in the vicinity of its end positioned inside the cyclone is provided with an annular plate 34 for defining an annular material precipitation chamber 36.The outer diameter of the annular plate is somewhat smaller than the iriner diameter of the cyclone shell 31 so that a slot 35 ;s created through which material being flung against the cyclone shell 31 during the operation of the cyclone passes into the material precipitation chamber 36 to be passed on through an outer hopper 37, one side of which is constituted by the end plate of the cyclone, to a material outlet pipe 39. The annular plate 34 can be omitted so that the material precipitation chamber 36 opens into the vortical chamber of the cyclone over the full width between central tube and cyclone shell.

Figure 5 shows an alternative embodiment of an axial flow cyclone with an oblique axis. In this cyclone a material precipitation chamber 40 is formed by a material outlet hopper 42, connected to the cyclone, unsymmetrically in relation to a vertical plane through the cyclone axis, with slots 43 connecting the precipitation chamber and the cyclone, along adjacent generatrices at which the wa's of the outlet chamber 8 '4 connected to the cyclone shell 41. The axis of the gas outlet tube 44 emanating from the bottom of the cyclone coincides with that of the cyclone shell, while a tangential gas inlet 45 is provided at the opposite end of the cyclone.

The mode of operation of the cyclone shown in Figure 5 is best illustrated in connection with Figure 6. The tangential gas inlet makes the flow of air through the cyclone rotate in the direction of the arrow 46. By this rotation suspended material is flung outwardly against the cyclone shell 41 so that the material concentration at the shell is large.

A flow of air with large material concentration flows through the top one of the slots 43 into the precipitation chamber 40. As this flow of air, which is returned to the cyclone through the lowermost slot 43, on its way passes through an increasingly larger cross-section, the velocity of flow decreases and a good basis is created for precipitating the suspended material collected by the outlet hopper 42 and discharged through a material outlet 47. This type of cyclone can be used with very small inclinations to the horizontal.

Even though the outlet hopper in case of cyclones of this type has to span approximately the entire cyclone length this entails no considerable increase in the constructional height of the cyclone as the. cyclone so to speak is situated in the outlet hopper.

The degree of separation of an axial flow cyclone will, as mentioned, often be inferior to that of a corresponding conventional cyclone. To compensate for this disadvantage, the tangential inlet may be arranged as shown in Figures 7 and 8 illustrating a detail of the tangential inlet of an axial flow cyclone.

In Figure 7 a cyclone with a cyclone shell 71 is provided with a tangential gas inlet from a gas inlet tube 72. The shell is shown with its axis horizontal but would be used with its axis vertical or oblique.

As appears from Figure 8, the gas inlet tube 72 has an inner wall 73 which develops into the cyclone shell 71 while forming an edge 74, and an outer wail 75 which converges spirally towards the cyclone shell 71. At a place where the outer wall is still situated at a small distance from the cyclone shell, it develops into the outer wall of a material outlet duct 76 which is tangential to the cyclone shell. The inner wall of the duct over a short distance is constituted by the cyclone shell, which ends in an edge 78 opposite the place where the outer wall 75 of the gas inlet tube develops into the outer wall of the material outlet duct 76. An opening is thus formed in the cyclone shell, the opening being defined by the edges 74 and 78.

By this arrangement, part of the material suspended in the inlet gas will be separated as the centrifugal force makes it follow the outer wall 75 of the gas inlet tube 72, and subsequently passes into the material outlet duct through the slot formed between the edge 78 and the outer wall 75 of the gas inlet tube, while the gas with the remaining suspended material passes through the opening 77 into the cyclone for further separation of material.

Figure 9 shows a cyclone comprising a tangential gas inlet 91, a cyclone shell 92, a central gas exit tube 93 and a material outlet hopper 94. Through a tangential connection the central gas tube 93 is developed into a tube 95 which is turned 900 relatively to the gas exit tube 93. The tangential development is at the bottom provided with a material outlet hopper 96 through which material precipitated in the tube 95, acting as a riser pipe, can be discharged. The outlets of the two outlet hoppers 94 and 96 can be coupled together, if desired.

Claims (6)

1. A suspension heat exchanger for heat exchange between a granular material and a gas flow and comprising a train of cyclones at least one of which is of the axial flow type having a gas inlet at one end and a gas outlet at the other, characterized in that the axis of the axial flow cyclone is disposed vertically or obliquely in such a way that the gas outlet is situated below the gas inlet.

2. A heat exchanger according to claim 1, in which the axial flow cyclone has an oblique axis ,and has at one end a tangential gas inlet and at the other an axial gas outlet in the form of a central tube projecting through the end wall of the cyclone so as to form an annular dust collecting or precipitation chamber between the central tube and the cyclone shell, the chamber being at one end defined by the end wall of the cyclone and at the other end adjacent to the vortical chamber of the cyclone being either undefined or defined by a plate mounted around the inner end of the central tube, the plate leaving between its periphery and the cyclone shell an annular slot through which dust can pass from the vortical chamber of the cyclone into the material precipitation chamber, which at its underside is provided with a material outlet hopper.

3. A heat exchanger according to claim 1, in which the axial flow cyclone comprises a cylindrical shell having at one end a tangential gas inlet and at the other end a central gas outlet, and having material precipitation slots along two generatrices, the slots connecting the vortical chamber of the cyclone to a material precipitation chamber.

4. A heat exchanger according to claim 3, in which the material precipitation chamber is connected to the cyclone shell along generatrices forming the outer edges of the material precipitation slots, the generatrices being disposed unsymmetrically in relation to a vertical plane through the cyclone axis.

5. A heat exchanger according to any one of the preceding claims, in which the axial flow cyclone has at its one end a tangential gas inlet and a preseparatorforsuspended material in the form of a tangential material outlet duct, the outer wall of which constitutes a continuation of the outer wall of the tangential gas inlet.

6. A heat exchanger substantially as described with reference to any one of the examples illustrated in the accompanying drawings.