Abstract: This invention relates to a method and an apparatus for heat treatment of a powder material comprising a device for post-combustion of the combustible constituent remaining in the powder material which contains, or is mixed with, a constituent apt to cause an exothermic reaction either with a gas or with another constituent, proceeding from a precalciner ensuring a primary combustion of the said constituent, so as to achieve the complete combustion of the said constituent.

Abstract: In a combustion apparatus, gas for combustion is fed in a combustion chamber receiving a heating medium such as sand to continuously blow the heating medium upwardly to thereby circulate the heating medium in the combustion chamber.

Abstract: A plant for combustion of a particulate fuel in a fluidized bed in a combustion chamber has the fuel supplied into the bottom of the combustion chamber by means of a pneumatic transportation system via nozzles. Fuel for the plurality of nozzles is transported through a duct to a distributor and from there via individual conduits to the nozzles of the combustion chamber. These conduits include flow-inhibiting, pressure-drop generating means, by means of which a pressure drop is generated between the distributor and the combustion chamber. These pressure-drop inducing means influence the velocity of flow and hence the transport capacity of the transport gas so that a desired, normally uniform, distribution of fuel to the bed is secured.

Abstract: A vertical conduit closed at one end by a head wall and connected at its opposite end to a fluidization gas delivery means is provided with side blowing nozzles longitudinally spaced in planes. A shutter is movable inside the conduit so that the bed in which the conduit is disposed in use can be progressively brought into a state of fluidization by supplying fluidization gas to the nozzles successively and cumulatively starting from the top. The invention obviates the need for the introduction via the bottom of the fluidized bed of compressed air at a high pressure when starting up the fluidization of the bed.

Abstract: Pulverized mineral material is calcined by suspending it in a current of gas constituted by a combustive gas and a fuel whose combustion furnishes the necessary calories for the calcination. To enable solid fuels rich in ash, substantially inert and difficult to condition to be used, the fuel is gasified in a fluidized bed hearth by blowing air from below through the bed whereby a gaseous fuel is disengaged from the bed and flows upwardly with the very finest particles of the solid fuel suspended therein. This is mixed with air and the mineral material to calcine the same.

Abstract: Improved dehydration section construction for pulse jet combustion apparatus for the drying of particulate materials that includes vibratable finger like sections at the downstream end thereof to minimize particle accumulationthereon.

Abstract: A batch-operated furnace apparatus for manufacturing powdered silicon nitride comprises a reaction furnace having a reaction chamber, an upper portion, a bottom portion and a side portion, a feed pipe placed at the upper portion of the reaction furnace for feeding starting materials in a fine or coarse particle form, an exhaust pipe placed at the upper portion of the reaction furnace for removing gasses from the reaction chamber, a gas distributor plate positioned at the bottom portion of the reaction chamber for introducing nitrogen gas into the reaction chamber so as to form a fluidized bed at the bottom portion of the reaction chamber, a heater for heating the starting materials of the fluidized bed within the reaction chamber, a discharge port formed in the gas distributor plate, a discharge pipe joined to the discharge port of the gas distributor plate, a valve seat formed at the discharge port, a discharge valve means having the valve which engages the valve seat of the discharge port, and a guide tube

Abstract: Pulse combustion apparatus for the drying of particulate material having the combustion chamber and the exhaust system components that direct the exhaust gas into a settling and collection chamber disposed on a common longitudinal axis and wherein the volume of back flow gas flowing through an adjacent augmenter duct is utilized to control the dew point within the settling and collection chamber.

Abstract: An apparatus for thermally and pneumatically treating granular solids to remove organic and inorganic chemical additives which are bonded to the granular solids or in admixture with them, to provide a purified granular solid product which is suitable for reuse, for example foundry sand for use in high strength molded cores, or for other productive uses, such as landfill. Granular solid feed material is preheated in a dilute phase zone of a fluidized bed, organic chemical additives are thermally oxidized in a dense phase zone of the fluidized bed, and remaining inorganic chemical additives are separated and removed from the granular solids in a contiguous pneumatic impaction zone. The purified granular solids are removed from the pneumatic impaction zone and organic and inorganic materials are elutriated from the fluidized bed and removed from the head space.

Abstract: An apparatus for calcining a granular or powdered mineral, more specifically hydrated calcium sulphate, comprising a vessel the side wall or walls of which slope inwardly toward a bottom of small cross-sectional area, as compared with an upper region of the vessel, and provided with a downwardly extending heating tube connectable in an upper portion to a gas source and an opening in a lower portion into the interior of the vessel adjacent to the bottom thereof, the arrangement being such that, in use, hot gases issuing from the lower portion of the heating tube will mix and heat substantially all the contents of the vessel.

Abstract: A fluidized-bed assembly for the treatment of metallic and nonmetallic objects utilizes a replaceable nozzle-plate which assembly can be received within the chamber and includes an inwardly inclined side wall which is provided with nozzles in addition to those of the central portion of the nozzle-plate. The assembly can be fed with gas by pipes from above or from below.

Abstract: Gypsum is calcined by contacting it with the hot gases spewing out of a combustion tube submerged in the mass of gypsum. The formation of dead-burned anhydrite is reduced by surrounding the combustion tube with a second tube called a draft tube. Gypsum passing through the annulus between the tubes is heated by the hot gases and is ejected from the annulus to prevent repeated excursions into the hot gas zone at the muzzle of the combustion tube.

Abstract: Improved dehydration section construction for pulse jet combustion apparatus for the drying of particulate materials that includes a plurality of exhaust flow disturbing rings on the confining walls thereof.

Abstract: A fluidized bed reactor (10) wherein a sulfur-containing particulate fuel is combusted or gasified in a fluidized bed (30) of particulate material, which includes a sulfur oxide absorbent, is provided with a solids cooling apparatus (40) adapted to receive particulate material draining from the bed and cool same via indirect heat transfer to a cooling fluid.

Abstract: There is here disclosed a calcining apparatus for a powdery material comprising a first preheating series composed of a calcining kiln and preheaters at a plurality of stages for heating the powdery material with an exhaust gas from the calcining kiln, a second preheating series composed of a calcining chamber disposed on an exhaust gas pipe extending from a rotary kiln and other preheaters at a plurality of stages for heating the powdery material with an exhaust gas from a rotary kiln, a gas carrying pipe for connecting the first and second preheating series in the middle thereof, and a flow path through which the powdery material fed to first and second preheating series passes is provided with a powdery material carrying pipe for connecting both series in front of second preheaters from the bottom in both series, and is further provided with a combining duct for combining powdery materials from both the series in the rear of the second preheaters from the bottom in both series, the flow path for the powder

Abstract: Heat treating apparatus in which a diffuser mounted beneath a bed of sand in a heating chamber passes gas through the sand to fluidize it. In one form of the invention, the gas rises through slits coined in a metal plate that supports the sand. In another embodiment of the invention, the gas rises through ceramic fiber mesh which uniformly fluidizes the sand.

Abstract: A valve for the pneumatic distribution of fluidizable material comprising a main compartment with a fluidization grid and with an overflow pipe supplied with fluidizable material via a pipeline. The main compartment is put into communication by means of a passage with several auxiliary compartments with a fluidization grid and with an overflow pipe, the fluidized material leaving via any one of the overflow pipes, as desired.

Abstract: A method and apparatus for the heat treatment of fine-grained material wherein the degree of efficiency of the last cyclone of a multi-cyclone preheater preceding a filter is reduced in the finest grain size range and at least a proportion of the filter dust deposited in the filter is passed directly to the calciner. In this way material is prevented from caking and flowing in batches in the apparatus.

Abstract: In a two-stage combustion method first and second circulating fluidizing bed systems are utilized. The first bed system is operated under reducing (sub-stoichiometric) conditions so as to minimize NO.sub.x formation. Solids in the flue gases are separated and returned to the first bed system, while the flue gases are subjected to afterburning in the second bed system. Oxygen containing gas is supplied to the second bed system to effect afterburning, and additionally sulfur absorbing and/or NO.sub.x formation decreasing agents are supplied to the second system. When burning coal, the first system is operated at a temperature of about 700.degree.-1000.degree. C., while the second system is operated at about 600.degree.-950.degree. C. The flue gases may be cooled, just prior to effecting separation of the solids, in one or both of the bed systems.

Abstract: A nozzle assembly is provided for use in apparatus for treatment of particulate material wherein the particulate material is conveyed through a treatment zone in fluidized condition, said fluidization being accomplished by a high velocity flow of gas directed against an imperforate member provided by the conveyor which gas flow is discharged from an elongated slot defined by the nozzle assembly and whose lengthwise direction extends in the direction that the particles are conveyed.

Abstract: Apparatus for cooking-dehydration and sterilization-drying of organic wastes, comprises a chamber (1) heated by a double walled enclosure (2) that communicates with fire tubes (3), the enclosure (2) receiving hot gases from a firebox (4). The hot gases pass through conduits (5) to diffusion nozzles (6) immersed in the products (7) to be dried. Some of the combustion gases are recycled by a ventilator (8) via a conduit (9) and a cyclone (10) to the firebox (4). The ventilator (8) draws in fresh air through a preheating heat exchanger (11) that is heated by the combustion gases which are then vented.

Abstract: The present invention concerns an ignition device for igniting fuel within a heating boiler having a combustion section operating according to the fluidized bed principle. The ignition device comprises a jacket-like body portion with a surface facing away from the fluidized bed of particles such as sand being furnished with a heating member, and the surface of the jacket-like body portion facing the fluidized bed of particles having heat transfer elements. The heat transfer elements may be rib-shaped, while a lagging or equivalent structure may encircle the ignition device, to form an intermediate space therebetween through which combustion gas is arranged to flow into the jacket-like body portion. A member for pre-heating the combustion gas flowing through this intermediate space, may also be provided, which is in turn cooled by the inflowing combustion gas.

Abstract: Apparatus for supplying gas to a fluidization grid which has two separate supply circuits, one for compressed air and one for fluidizing air, the orifices of each circuit ending at the pyramids of the grid. The orifices of each circuit, are arranged in groups, and are connected by pipes to a stator cylinder in which a rotatable rotor tube extends. A motor is arranged to rotate the rotor tube to a number of predetermined positions. The rotor tube is connected to a supply pipework and is pierced with passage openings which are arranged upon rotation of the rotor tube to correspond to the pipes.

Abstract: In a method and an apparatus for burning away organic components in raw phosphate, the burning taking place in two stages carried out in two reactors each having its separate air supply. A first suspension transport reactor (3) and a second reactor (5) are coupled in series via a separator (4), to which reactors combustion air (9), (11), preheated raw material (14), (15) and, if desired, fuel (8) are supplied separately. The reaction temperature in both reactors preferably lies in the range 700.degree.-850.degree. C. The majority of the organic components are burnt in the first stage which operates with a suspension having a density of 1-50 kg/m.sup.3. The remaining part of the organic components is burnt in the second stage which operates with a suspension density of 75-1600 kg/m.sup.

Abstract: A fluidized bed furnace includes a furnace housing having an outer shell. A furnace base and an outer top plate are secured to the respective lower and upper ends of the housing. A vertical retort has an opened upper end and an opened lower end. The retort is arranged in an opening formed in the outer top plate and extends downwardly into the center of the housing. Heat insulating material is disposed between the outer shell and the vertical retort. A retort base assembly is used to close the lower end of the retort. An upper support device is formed integrally with the retort at its upper end for supporting the vertical retort on top of the outer top plate so as to permit downward growth only during thermal expansion. A lower support device is interposed between the lower surface of the retort base assembly and the upper surface of the furnace base which supports substantially all the weight of the retort, the weight of the load of fluidizable media and the weight of a load of material to be heat treated.

Abstract: The combustion, cooling or other treatment of solids with the aid of gas may take place on a grate carrying or conveying said solids while the gas is passed through openings provided in the surface of the grate. In order to retain said solids entirely above said grate surface, to cool the grate surface sufficiently and to distribute the gas evenly in the solids to be treated, thin slots, inclined in the direction of transport, curved in the manner of a siphone and maintaining a high resistance to gas penetration are provided in grate plates composed to form a grate. The slots are formed between elements of such grate plates which can be manufactured by casting. The design of these grate plates avoids the necessity of handling dribblings passing said grates. The grate plates can be aerated through grate beams carrying them.

Abstract: An apparatus for the thermal removal of lacquer and like coating materials from metal and ceramic objects comprises a fluidized-bed retort which is partly heated by a burner in a burner chamber formed in a unitary housing with the retort. Combustibles in the retort gases are recycled to an afterburner chamber for combustion to provide additional heating for the retort. The bottom of the retort is surrounded by an annular gap through which the afterburner gases pass and then to an outlet connected to the afterburner chamber by a valve.

Abstract: An apparatus for the thermal removal of lacquer and like coating materials from metal and ceramic objects comprises a fluidized-bed retort which is partly heated by a burner in a burner chamber. Combustibles in the retort gases are recycled to an afterburner chamber for combustion to provide additional heating for the retort. The bottom of the retort is provided with a layer of refractory heat-storage material which serves as a thermal buffer leveling the heating of the interior of the retort.

Abstract: The apparatus consists of a cylindrical drying chamber, which is provided with an upwardly conical bottom. The fluidization and drying medium is supplied to the chamber through a substantially circularly extending slit between the conical bottom and the wall of the drying chamber from an annular distributor for the fluidization and drying medium. A stirrer is placed coaxially in the chamber, the blades of said stirrer being parallel to the conical bottom. Preferably, the blades of the stirrer are positioned at a small distance from the conical bottom. The cylindrical drying chamber and the annular distributor may have a common wall, which is provided with a heat insulating layer on the side facing the distributor.

Abstract: An improved fluidizing apparatus includes a gas distribution plate which is supported by support means located below the level of the gas distribution plate. The support means comprises beam members extending across the interior of the vessel and a number of rods standing upright on the beam members, with the gas distribution plate placed on the uppermost ends of the rods. The support means can also include annular members made integral with the beam members and/or a projection or projections projecting inward from the inner wall surface of the vessel. A number of rods can stand upright on the annular members.

Abstract: A ring-gap nozzle for distributing a fluid medium over a plate, for instance over a bottom plate in a fluidized bed dryer, having two dish-shaped pressed parts suiting to each other complementarily being assembled in facing relationship and nested into each other and forming a ring gap through which the medium, deflected by 90.degree. or more, flows to the bottom plate, preventing both the settling of the product to be treated in the fluidized bed dryer on the plate as well as the reflux of the product backwardly into the nozzle when the flow of medium is cut off. This type of nozzle is simple to manufacture and does close the passage without any complicated manipulation automatically.

Abstract: According to the present invention, white cement clinker particles are not burned in a rotary kiln and the burned clinker particles are not immersed in the water to be cooled. White cement clinker raw materials are burned in a fluidized bed burning furnace with a fluid bed or a spouted bed and the clinker particles thus burned are reduced with a reducing gas. The clinker particles thus reduced are first cooled with the water which is sprayed over the clinker particles and then further cooled with the air. The white cement clinker raw material particles fed into the fluidized bed burning furnace are burned into clinker particles while forming a fluidized bed and then the clinker particles thus burned drop through a jet injection pipe into a reduction chamber disposed below the burning furnace and are reduced. The clinker particles thus reduced are rapidly cooled to 600.degree.-700.degree. C.

Abstract: The air dispersion plate at a lower part of a fluidized bed combustion furnace is provided with a large number of air injection nozzles uniformly distributed therethroughout and made of a porous refractory material having good air permeability and high heat and wear resistances. All of the fluidizing air supplied into the wind box below the dispersion plate is injected through these nozzles into the combustion chamber to fluidize the fluidizable material, which, after combustion is stopped, cannot flow through the nozzles into the wind box to cause heat damage and contamination therein and in the air supply pipe.

Abstract: A nozzle plate construction for underjet coke ovens for distributing and metering combustion-supporting air supplied upwardly to regenerators through a sole flue extending parallel to the chamber axis. The sole flue is closed at the top by plate elements having apertures therein. In accordance with the invention, the various plate elements are in the form of troughs having plane base plates each formed with one longitudinal gap therein. Associated with each longitudinal gap is a metering element so mounted at its ends so as to be adjustable in its distance from the plane of the base plate. In this manner, the air flowing upwardly into the regenerator sections can be accurately controlled and uniformly distributed. At the same time, the nozzle plate construction of the invention is light in weight while being very stable and easy to assemble.

Abstract: A method and apparatus for treating granular materials which require to be heated to drive off volatile constituents and which require to be cooled, for example foundry sand containing an organic binder. The method comprises the step of maintaining a mass of the material at a treatment temperature lying in the range 250.degree.-400.degree. C. for between four and thirty hours. The mass may be initially heated by virtue of a manufacturing process in which the mass has been previously used. Alternatively, the mass may be initially heated by a pre-heating step such as heating in a fluidized bed. Alternatively, the mass may be heated by being placed in heat transfer relationship with a second mass of the material which has been previously heated, for example, in a fluidized bed. Apparatus for performing the method is also disclosed.

Abstract: For processing matter in a turbulent mass of particulate material, a substantially annular processing region (14; 104; 206) is provided and is preferably in the form of a substantially annular processing chamber having a radially inner wall which includes a waist (38; 122). A flow of fluid and matter to be processed are admitted to the processing region through one or more inlets (28, 46; 108, 124; 214, 220) with the flow of fluid being directed generally circumferentially into the processing region. In the processing region, matter to be processed is embedded in a compact turbulent band of particulate material for processing. Once processing is complete, the processed matter is withdrawn from the processing region, preferably by entrainment in an exhaust flow of the fluid.

Abstract: Raw phosphate material is calcined to remove pollutants, such as organic compounds and sulphides, by heat treatment in two suspension calciners (3,5) arranged in series. Fuel supplied at burners (8) is burnt in the calciners with air, supplied in parallel through pipes (9) from a cooler (7) for the treated material. The material is preheated in a preheater, for instance a pipe (13) and cyclone (2) by exit gases from the calciner before entering the calciners and, after leaving the calciners, is separated from the gas streams in separators (4,6).

Abstract: To prevent the tube to a pressure measuring means from being blocked by solid material mobile in a pressure container, a small part of the clean pressure medium from a source of such medium is injected into the tube so that the tube is continuously purged with clean pressure medium. By locating the pressure medium input at a suitable position along the tube relative to the pressure measuring means, the desired purging of the tube is obtained without appreciably increasing the pressure in the tube, and thus minimizing possible errors in measurement of the pressure. If, for example, pressure medium is passed into a pressurized combustion chamber through a surrounding pressurized container, and the tube to the pressure-measuring means passes through said surrounding pressurized container, the clean pressure medium can be bled into the tube through a hole bored in the side of the tube within the pressurized container.

Abstract: A method of heat-treating particles by contacting within a contacting receptacle particles fed from the top portion of the receptacle with a heated gas introduced from its bottom portion, which comprises(1) moving the particles sequentially from an upper to a lower moving layer portion through a multiplicity of moving layer portions formed by partitioning said contacting receptacle with a plurality of funnel-shaped supporting plates, said movement being effected via porous plates provided in the individual moving layer portions to thereby prevent blocking of the particles,(2) meanwhile causing the heated gas to rise sequentially from the lowermost moving layer portion to upper moving layers to contact it with the particles in each of the moving layer portions, the heated gas being supplied through a gas pipe leading from the gas layer of each moving layer portion to a gas dispersing plate provided in the adjoining upper moving layer portion to disperse the heated gas introduced in each adjoining upper moving

Abstract: A chamber for the treatment of particulate matter in a fluidized bed, e.g. at elevated temperatures, comprises an apertured bottom for the admission of carrier gas into the overlying space. The latter is divided by upstanding partitions into at least one medial compartment and at least two additional compartments adjoining the at least one medial compartment at the opposite sides of the latter. The apparatus further comprises a duct of substantially triangular cross-section with a horizontal base and an upwardly sloping side walls located in the at least one medial compartment, with the side walls of the duct extending in directions parallel to the partitions, and the base of the duct being imperforate and resting on the wall of the chamber and preventing the entry of the carrier gas into the at least one medial compartment from below; and a nozzle arrangement in each of the upwardly sloping side walls of the duct.

Abstract: A support structure for a distribution grid to be mounted in a reaction vessel wherein the reaction vessel is formed with an inwardly-extending annular shoulder portion to which is mounted, such as by welding, a ring-member whereby the distribution grid is mounted on such support structure.

Abstract: A method of operating an adiabatic fluidized bed reactor, and an adiabatic fluidized bed reactor, including the step of providing pressurized air to an adiabatic fluidized bed reactor both through openings located in a support surface and through openings located in the reactor walls having outlets below the surface of the bed of granular material. The structure of the adiabatic fluidized bed reactor includes support surface air distribution nozzles extending through a support surface, reactor wall air distribution openings extending through the reactor walls having outlets below the surface of the bed of granular material, and separate control valves for controlling the flow of pressurized air to the support surface air distribution nozzles and to the reactor wall air distribution openings. The method and apparatus of the present invention result in higher reactor hydrodynamic turndown ratios than can be achieved with conventional single-bed reactors. For example, a hydrodynamic turndown ratio of 6.

Abstract: A fluidized bed combustor is adapted to deprive a combustion gas issuing therefrom of dust by bringing the combustion gas into contact with a filter medium which is a fluidizing solid discharged in conjunction with slagging ash out of the combustor.

Abstract: In all refractory gas distributor is used to distribute fluidization gas to a fluidized bed reactor. The distributor slabs contain cast-in gas passages which distribute the gas to uniformly distributed grid holes to the surface. The slabs are continuously supported by a bed of refractory filling the bottom head of said reactor vessel and pressure equalization passages are provided to place the lower surface of the grid plate in fluid communication with the upper surface of the grid plate.

Abstract: A fluidizing feed apparatus (20) for supplying particulate material to a fluidized bed furnace (12). A housing (22) defines a chamber which is divided by a perforated distribution plate (24) into a gas plenum (26) beneath the plate and a particulate fluidizing plenum (28) above the plate. Conveying gas is fed under pressure to the gas plenum to pass upwardly therefrom through the perforated distribution plate. The upwardly directed conveying gas fluidizes the particulate material so as to establish a discrete bed (60) of fluidized particulate material superadjacent the distribution plate (24) and a splash zone (70) thereabove. Transport conduits (14) open into the splash zone (70) at a selected distance above the surface (65) of the discrete bed (60) to receive particulate material and conveying gas therefrom for transport to the fluidized bed (16) of the furnace (12).

Abstract: An apparatus for the calcination of fine grained material, particularly material used to produce cement clinker employing at least two separate, essentially parallel preheater lines, one of which is treated with exhaust gases from a clinker-forming zone and the other is equipped with a calcination zone which itself is fed by hot combustion air from the clinker cooler. In the area of the preheater line which is attached to the exhaust gas conduit of the clinker-forming zone, a divider device is arranged for dividing the stream of material. A first conduit directs a portion of the material into the exhaust gas conduit of the clinker-forming zone and a second conduit directs material into the calcination zone. The calcination zone is in the form of a vertical reaction duct having a downwardly bent pipe at its upper end. There is also provided a conduit which connects the vertical reaction duct with the exhaust means for hot gases in the clinker-forming zone.

Abstract: A pulse combustor apparatus including a combustor tube having open ends and containing at least one combustion zone where combustion of a fuel occurs and heat is released, resulting in the formation of a standing acoustic mode with nodes and anti-nodes in the tube. The combustion zone is located approximately half the distance between the acoustic pressure node and anti-node where the acoustic pressure oscillation lags the acoustic velocity oscillation by approximately one-quarter wavelength of the oscillation. Furthermore, the combustion air must flow towards the acoustic pressure anti-node. The fundamental principle relating to the occurrence of the pulsations in the combustor is the interaction between the combustion processes and both the non-zero acoustic velocity and acoustic pressure oscillations at the designated combustion zone location.

Abstract: A precalciner for cement raw meal is divided into upper and lower parts. The upper part is shaped into a cylinder, while the lower part is constituted by upper and lower inverted conical portions. The upper inverted concial portion has a cone angle smaller than that of the lower inverted conical portion and is connected to the lower end of the upper part through a throttle device.

Abstract: A nozzle for a fluidized bed firing system in which a pipe for transporting coal/air mixture has an end surrounded by a hood. This pipe extends into the fluidized bed, and an axial exit opening is located between the hood and the pipe. The pipe, moreover, is provided with that axial exit opening, while the hood is spaced from this exit opening. A deflecting face, moreover, is located in a vicinity opposite to the exit opening. A downwardly-pointing baffle is attached underneath a lower edge of the hood, to the pipe so as to be adjustable in height. The deflecting face may be in the form of a cone with a tip pointing towards the exit opening of the pipe. The hood may be provided, furthermore, with a hemispherical cap.

Abstract: To cut building and operating costs, a heat exchange apparatus for thermally treating a pulverulent material by a countercurrent flow of gas comprises a series of superposed successive separators, each separator having a cylindrical body with a horizontal axis and the cylindrical body having two ports at respective ends and another port at a mid-point thereof. Gas ducts connect the successive separators in series so that the smoke gas flows from a furnace in a current through the successive separators in a first direction. Respective gas ducts lead to and from the ports of respective separators tangentially with respect to the cylindrical body thereof.