FIELD OF THE INVENTION

The present invention relates to an apparatus for continuous gravimetric metering of pourable material, such as a fuel material, preferably comprising coal, comprising:a housing comprising a first and a second mutually spaced wall, a charging port through said first wall, a discharging port through one of said walls displaced from said charging port and at essentially same radius as said charging port with respect to a predetermined vertical axis, a primary air port through the other wall opposite to said discharging port, said primary air port having a primary outlet opening and being connected to a compressed air supply;a rotor mounted in and sealed to said housing for rotation about said predetermined vertical axis including a plurality of pockets extending therethrough at spaced locations separated by a rotor partition wall having a thickness t, said pockets having a pocket opening front edge as seen in the rotational direction of the rotor and said pockets forming a first ring of pockets along the same radius as said charging and discharging ports, a circular measuring path being defined between the charging port and the discharging port, the pockets receiving material via the charging port and material being emitted through said discharging port during the rotation of the rotor;drive means configured for rotating said rotor in a rotational direction;speed indication and control means configured for controlling the rotational speed of said rotor;load-measuring means configured for measuring the weight of the material traveling along the measuring path.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,528,848 discloses an apparatus of the above type. Through the charging port material is charged to the pockets of rotating rotor. The rotation of the rotor moves material along the measuring path from a position opposite the charging port to a position opposite the discharging port. At the discharging port the material is discharged through the discharging port by means of compressed air ejected through the primary outlet opening of the primary outlet port arranged opposite the discharging port. The rotational speed of the rotor varies based on the desired amount of feed material to be discharged through the discharging port and delivered to a burner per time unit (feed rate). For low feed rates the rotational speed of the rotor will be low. The fast extraction of the material from the pockets by means of the compressed air results for low feed rates in a situation, where one pocket is emptied before the next pocket has moved into the primary outlet opening of the primary air port as seen in a horizontal projection. This causes pulsations of the flow of material out through the discharging outlet port and accordingly pulsations in the feed of material to the burner (alternating phases of air with material and solely air).

SUMMARY OF THE INVENTION

The object of the present invention is to solve and/or at least remedy the above problem and provide a less fluctuating or more uniform flow of material out through the discharging port for low feed rates.

The above object is obtained by an apparatus of the above stated type being characterized in that the other wall of the housing comprises a secondary air port having one or more secondary outlet openings including a first secondary outlet opening and being connected to a compressed air supply, the secondary outlet opening(s) of the secondary air port having an secondary outlet opening front edge and a total outlet area being substantially smaller than that of the primary outlet opening and being displaced from the primary outlet opening of the primary air port in opposite direction to the rotational direction of the rotor and being arranged opposite to said discharging port.

The compressed air supply which is connected to the secondary outlet opening can be the same compressed air supply as the compressed air supply which is connected to the primary outlet opening. Alternatively, the primary and the secondary outlet openings can be connected to different compressed air supplies.

By the phrase “a secondary air port having one or more secondary outlet openings including a first secondary outlet opening” is to be understood that the first secondary outlet opening of the secondary air port can comprise a single outlet opening. However, the first secondary outlet opening can also comprise a number of secondary outlet openings. The number of secondary outlet openings can be arranged on the same radial line, i.e. on the same angle of rotation in front of the primary outlet opening of the primary air port, or on different angles of rotation in front of the primary outlet opening. In the latter case the secondary outlet openings can be arranged on essentially the same radius with respect to the predetermined vertical axis.

By means of the present invention it is obtained that a portion of the amount of the material in a pocket is discharged by a relatively small amount of compressed air emitted through the first secondary outlet opening when the pocket passes the first secondary outlet opening and the remainder material in said pocket discharged by a larger amount of compressed air emitted through the primary outlet opening when the pocket passes the primary opening. The relatively small amount of compressed air emitted through the first secondary outlet opening is selected to cause pneumatic material extraction from the pocket, but is on the other hand so low that the extraction occurs slowly. The larger amount of compressed air emitted through the primary outlet opening is selected to ensure that the remainder material in the pocket is extracted from the pocket, i.e. all material is removed from the pocket. As a result, material is at least essentially constantly extracted through the discharging port and the pulsation and the flow of material out through the discharging port at least reduced.

According to the invention the total outlet area of the secondary outlet opening(s) can be 0.1% to 25%, such as 0.1% to 20%, 0.5% to 15%, 1% to 15%, 1% to 10% 1.5% to 10%, 1.5% to 7%, 2% to 7% or 3% to 5% of the outlet area of the primary outlet opening.

The radial length of the secondary outlet opening can be less than 30% of the radial length of the primary outlet opening, such as less than 25%, 20%, 15%, 10%, 5%, 2.5% of the radial length of the primary outlet opening

According to the invention the front edge of the secondary outlet opening can be displaced at least a rotational angle Xminof the rotor from the primary outlet opening, Xminbeing defined as ¼ or ⅓×(360°/number of pockets in a pocket ring)

Additionally, according to the invention the secondary outlet opening can be displaced less than an angle of rotation Xmaxof the rotor from the primary outlet opening, Xmaxbeing defined as 5×(360°/number of pockets in a pocket ring).

Thereby, when the secondary outlet opening is displaced an angle of rotation corresponding to at least (360°/number of pockets in a pocket ring), material is first ejected from a given pocket by air emitted thereto through the secondary outlet opening and at least an angle of rotation of least (360°/number of pockets in a pocket ring). Later, the remainder material in the given pocket will be ejected from that pocket by air emitted through the primary outlet opening. However, by means of air emitted through the primary outlet opening material will be ejected from a pocket arranged in front of the given pocket during the rotation of the rotor corresponding to at least (360°/number of pockets in a pocket ring), whereby material is at least essentially constantly extracted through the discharging outlet port and/or pulsations of the flow of material out through the discharging port are reduced.

The rotor may also have more than two, such as three, four or five, co-axially arranged rings of pockets.

According to the invention, the displacement of the secondary outlet opening in respect to the primary outlet opening and the thickness t of the rotor partition walls is dimensioned so that when the secondary outlet opening is completely covered by a partition wall, the pocket opening front edge of a pocket opening is within the primary outlet opening as seen in a vertical projection.

It is thereby obtained that material is essentially constantly extracted and/or pulsations of the flow of material out through the discharging port are reduced.

Further, according to the invention the first secondary outlet opening of the secondary air port can be oblong and extend towards the primary outlet opening.

In principle the secondary outlet opening could extend up to and into the primary outlet opening at a connection point to form a connection between the first secondary outlet opening of the secondary air port and the primary outlet opening of the primary air port.

However, at the connection point the connection between the secondary and primary outlet openings should be so narrow that essentially no compressed air is emitted therethrough and the said openings essentially separated in respect to the emission of air.

Additionally, according to the invention the primary air port and the secondary air port can be configured as a single air port being provided with two separate and mutually spaced outlet openings in the form of the primary outlet opening and the first secondary outlet opening.

Further, according to the invention the primary air port and the secondary air port can be separate air ports, such as air ports formed by separate tubes or hoses, the air ports being optionally separated by a common port partition wall.

The connection line between the primary and secondary air port, respectively, and the compressed air supply can be configured for and/or provided with means configured for providing a predetermined flow of and pressure of compressed air emitted through the primary and the secondary outlet openings.

The means configured for providing a predetermined flow and pressure of compressed air emitted through the primary and the secondary outlet openings can comprise adjustable valve means.

According to the invention the first wall can be an upper wall and the second wall can be a lower wall.

Further, according to the invention the discharging port can be provided in the first wall, i.e. the upper wall.

However, the discharging port can also be provided in the second wall, i.e. the lower wall.

The pockets can essentially have the shape of a sector of an annulus and be separated by radially extending pocket partition walls.

Further, the apparatus can comprise a second ring of pockets being arranged co-axially with the rotor axis and inwardly of the first ring of rotor pockets.

The second ring of pockets can have essentially the shape of a sector of an annulus and being mutually separated by pocket partition walls, preferably being radially extending partitioning walls and preferably being circumferentially spaced apart from the rotor pockets of the first rotor by for instance approximately ½×(360°/number of pockets in the ring of pockets), whereby a partition wall of the second ring of pockets is arranged essentially midway between two partition walls of a pocket of the first ring of pockets.

The drive means for the rotor can be an infinitely variable drive motor.

Sealing elements means can advantageously be provided between the first and second walls, respectively, and the rotor, especially between the rotor and the housing in the area of the discharging port.

