GB2095578A - Grinding mill - Google Patents

Abstract

A mill e.g. an impact pulverizer in which ground material is transported out of a housing (7) thereof exclusively by flowing air, comprises an air inlet duct (10) disposed in the lower part of the casing (7). The inlet duct (10) has a width equal to that of a sieve (4) of the mill, is bounded by housing side walls and is disposed opposite to the outlet suction opening of an outlet (11) of equal width. <IMAGE>

Description

SPECIFICATION Grinding mill The present invention relates to a grinding mill, and has particular reference to a hammer mill for granulating grain and additives to granular or pulverized products. Such a hammer mill is generally utilized in the mixed fodder industry and in agriculture.

Hammer mills are known comprising a cylindrical grinding chamber, a rotor disposed concentrically within the grinding chamber and having rotatably mounted beaters, and a sieve surrounding the greater part of the grinding chamber and impact plates. Special feed devices are connected to the mill housing either rigidly or in vibration-insulated manner.

Access into the housing and the grinding chamber is effected through pivotal doors constituting a part of the housing. The throughput in hammer mills of the known designs is to a great extent dependent on the manner of feeding the product to be granulated into the grinding chamber, on the flow conditions that are established as the granulated product passes through the apertures of the sieve surrounding the grinding chamber, and on housing shape with regard to streamling for achieving optimum flow conditions in the housing, particularly in the region of charging the conveying air for ensuring a reliable transport of the product being ground.

For this purpose, housing variants having special openings and apertures in the lower part of the housing and in the pivotal doors have been developed. Arrangements are also known which, by adjusting a damper, make possible different flow cross-sections, the damper being flush with the casing wall.

The disadvantage of the housing designs in the known hammer mills where the transport is effected exclusively by flowing air consists in the fact that the form of entrainment by the air of the ground material is not optimum with respect to flow conditions. As a result, the throughput limit of the grinding chamber cannot be fully exploited and/or increased pressure losses occur in the region of the zone where the conveying air entrains the ground material. Such increased pressure losses reduce the quantity of air necessary for conveying and lead to random fluctuations in the conveying operations.

There is accordingly a need for a grinding mill in which the zone of entrainment by conveying air of ground material in a housing of the mill is such that optimum flow conditions can be obtained.

According to the present invention there is provided a grinding mill comprising a housing, grinding means arranged in the housing, a sieve to sieve material ground by the grinding means, outlet means to conduct ground material entrained by an air stream out of the housing, and an air inlet duct disposed in a lower region of the housing at a side thereof opposite to an outlet opening of the outlet means and bounded in part by wall means of the housing, the width of the inlet duct being substantially equal to the width of the outlet opening and to the width of the sieve measured in a corresponding direction.

Preferably, the inlet duct is adjustable by means of guide devices fixed to the inside of the housing, whereby the flow conditions in a grinding chamber of the housing and generally in the housing can be so adapted to the material to be ground and the throughput that damming-up of the ground material at a material inlet of the mill is avoided, the entrainment by the conveying air of the ground material takes place with such flow conditions that only small pressure losses occur, and the material does not emerge from the housing in the opposite direction to that of the conveying air.

In a preferred embodiment, the inlet opening situated in the lower part of the housing, which opening is of the same width as the sieve, is bounded by the lateral casing walls, is opposite to the outlet suction opening of equal width, and continues towards the centre of the housing as a duct, whereby the free cross-sectional area of the inlet opening and thus of the duct can be varied by height adjustment.

An embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawing, the single figure of which is a schematic sectional elevation of a hammer mill according to the said embodiment.

Referring now to the drawing, there is shown a hammer mill having a housing 7 enclosing a grinding chamber 2 and having an inlet 1 through which material to be ground and a portion of the air necessary for transporting the ground material can enter the grinding chamber. The chamber 2 is bounded by the housing side walls, by impact plates 5 and by a sieve 4. Rotatably arranged inside the grinding chamber 2 is a rotor 3, to which beaters 6 are fixed in groups. Ground material passes radially through the sieve holes out of the grinding chamber 2 and is transported away by the moving air (pneumatic conveying) to an outlet 11.

In order for large quantities of ground material to be conveyed, firstly large quantities of air and secondly high air speeds and thus high conveying speeds are necessary. It is advantageous for the grinding process for a portion of the conveying air to arrive in the housing 7 through the grinding chamber 2.

These primary quantities of air stabilize the infeed of the material to be ground into the grinding chamber 2, reduce the residence time in this chamber and accelerate the ground material after it has left the grinding chamber in the flow direction. The remaining quantity of air required for conveying of the ground material enters the housing 7 of the mill through the opening of a flow duct 10.

The cross-section of the flow duct 10 can be varied by a vertically adjustable shutter 9 and thereby the sucked in quantity of air can be varied. This intake suction cross-section can be varied between zero and a maximum value specific to each machine. The air quantity flowing through the flow duct 10 influences the vacuum in the housing 7, upon which in turn the quantity of air flowing through the sieve 4 is dependent. The vacuum in the housing 7 increases as the intake suction cross-section decreases. The total quantity of air, as a sum of the two partial quantities, must always be sufficiently large for satisfactory transporting away of the ground material to be ensured. Furthermore, by throttling the suction intake air in the case of products that are difficult to grind, the power consumption of the fan or blower used for creating the suction can be reduced.

A further advantage consists in the optimizing of the transporting of material away from the mill. In this way intake suction losses and flow pressure losses are kept low. By the form and arrangement of guide devices 8 attached to the housing and of the shutter 9, the airmaterial flow from the sieve 4 is so guided that only very small losses due to turbulence, surface tension and impact occur.

Moreover, the result is achieved that the angle between the active directions of the partial flows, that is air flow through flow duct 10 and air-material flow, is small and as a consequence large deflection accelerations are eliminated. The air flowing in through the flow duct 10 is stabilized and oriented and impinges at high speed on the air-material flow. In this way a premature build-up in the mill is avoided and escape of material particles out through the flow duct 10 is impeded.

Claims (4)

1. A grinding mill comprising a housing, grinding means arranged in the housing, a sieve to sieve material ground by the grinding means, outlet means to conduct ground material entrained by an air stream out of the housing, and an air inlet duct disposed in a lower region of the housing at a side thereof opposite to an outlet opening of the outlet means and bounded in part by wall means of the housing, the width of the inlet duct being substantially equal to the width of the outlet opening and to the width of the sieve measured in a corresponding direction.

2. A mill as claimed in claim 1, wherein the inlet duct extends inwardly of the housing towards a central vertical plane of the grinding means, means being provided to vary the height of the inlet duct thereby to correspondingly vary the flow cross-section of the duct.

3. A mill as claimed in claim 2, wherein the length of the inlet duct is equal to at least a quarter of the length of the housing measured in a corresponding direction.

4. A grinding mill substantially as hereinbefore described with reference to the accompanying drawing.