GB2149688A

GB2149688A - Hammer mill

Info

Publication number: GB2149688A
Application number: GB08330783A
Authority: GB
Inventors: Geoffrey Thomas King
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-11-18
Filing date: 1983-11-18
Publication date: 1985-06-19
Also published as: GB8330783D0

Abstract

In order to provide for accurate positioning of a grinding liner 23 with inner housing 11 in relation to the hammers 22, it is preferred that such a liner should be mounted on circumferentially spaced points of the housing. This has the secondary advantage of leaving free spaces 32 outside the grinding liner 23 so that secondary air can be mixed with the primary air before the ground material is feed to an integral classifier 26, 27, 28. The secondary air cools both the grinding liner and the primary air, and its flow may be controlled by vanes or fins on the outside of the grinding liner 23. <IMAGE>

Description

SPECIFICATION Hammer mill This invention relates to hammer mills, and more particularly to hammer mills with integral classifiers.

An example of such a hammer mill is shown in published patent application GB - A 2111856. As described in that specification, hammers run close to a serrated peripheral wall of a casing to define a milling zone. On a iaboratory scale machine, it is possible to achieve manufacturing tolerances sufficiently close to obtain an accurate gap between the hammers and the casing wall even with a fabricated casing. However when operating on a large scale machine such accuracy of fabricated casing itself becomes difficult to achieve without inordinate expense.

One way of overcoming this is to use a grinding liner which is mounted within the casing. It has been found that if a free space is left outside the liner, then the grinding linear is cooled by airflow over the outside of the liner.

In accordance with a first aspect of the invention, there is provided a hammer mill comprising a rotor mounted in a casing and carrying hammers running on a path close to a grinding liner mounted in the casing, in which the grinding liner is mounted onto the casing at accurately located circumferentially spaced points to define free spaces outside the grinding liner.

The mounting points may be defined by ribs cast into the casing and machined to provide accurate location of the grinding liner.

Particularly when using a fabricated casing, it is advantageous if the mounting points are radially adjustable to ensure coaxility of the liner and the path of the hammers.

The mill preferably includes an integral air classifier comprising discs rotating coaxially, and preferably at the same speed as the rotor, to define classifying passages through which the ground material is carried by an air stream.

The feed material is also preferably fed into a grinding zone between the path of the hammers and the grinding liner by a primary air stream, and a stream of secondary air is fed via the free spaces outside the grinding liner to mix with the primary air stream before passing to the classifying passages.

In accordance with a second aspect of the invention there is provided a hammer mill including an integral air classifier, in which feed material including an integral air classifier, in which feed material to be ground is arranged to be fed into a grinding zone between a grinding liner and the path of rotary hammers by a primary air stream, and means is included for introducing a secondary air stream round the outside of the grinding liner to mix with the primary air stream upstream of the classifier.

The outside of the grinding liner may be provided with vanes to direct the flow of the secondary air, and the vanes may be angled to induce circumferential swirl.

The use of secondary air enables the requisite air flow to be achieved in the classifier without putting unnecessary design constraints on the mill itself, and also enables the ground material in the primary air to be cooled immediately on exit from the mill. Also the flow of secondary air round the outside of the grinding liner, which may be fairly thin, gives rise to useful cooling effects on the liner. The vanes also assist in cooling by acting as fins.

The invention will be further described with reference to accompanying drawings in which: Figure 1 is a diagrammatic section illustrating one form of the invention; and Figure 2 is a section of the part of a casing and liner to illustrate some details.

Turning first to Figure 1, an outer stationary casing, which may be fabricated or cast, is illustrated at 11 and is shown as being provided with a first inlet 12 for airborne feed material, i.e. a stream of primary air with suspended unground material. A second inlet 13 is for secondary air, and the casing 11 also has an outlet 14 for ground material and further outlet 15 through which coarse or rejected material may be rejected and drawn off by means of a screw conveyor 16 or the like. The arrows indicate that the coarse or reject material may either be discarded or recycled to the inlet 12.

A drive shaft 17 carries a rotor 18 which is generally in the form of a cylinder. A wear plate 19 is provided on the rotor adjacent the inlet 12 to minimise wear on the rotor and to provide a part which can be discarded when worn without the whole rotor needing to be replaced. Discs 21 mounted on the rotor carry between them a plurality of hammers 22 which run adjacent a grinding liner 23 of annular form mounted within the housing 11 in accurate relationship to the path of the hammer 22 to define a grinding zone 24. The grinding liner 23 carries an annular orifice plate 25 which directs the flow of primary air and ground material from the grinding zone and also controls the residence time of the material. The primary air then conveys the ground material radially outwardly towards the classifier.Secondary air entering via the inlet 13 passes along the outside of the grinding liner and mixes with the primary air and ground material as the two streams enter the classifier.

The classifier itself consists of discs 26 and 27 mounted on the rotor for rapid rotation so as to define between them radial passages 28 within which the ground particles suspended in the air stream are subjected to both centrifugal and centripetal force. The coarse particles are caused to reverse and flow out to the periphery of the casing and discharge via the outlet 15, while the finer material is carried through the passages 28 by means of the air stream and emerges via the interior of the rotor, which is perforated, to the outlet 14.

It will be seen that the discs 27 are of tapered form to even out the air velocity within the passages. In this connection, reference may be made to published patent application GB-A2110962.

It will be appreciated that in order to provide the necessary grinding action, the spacing between the hammers 22 and the inner space of the grinding liner 23 must be accurately controlled, and to this end, the grinding liner 23 may be mounted in the casing 11 by means of cast-in ribs 31 which are machined into coaxility with the shaft 17. In Figure 2, there is shown half the casing, which itself is constituted by a casting, and half of the grinding liner. It will be seen that this arrangement leaves free spaces 32 outside the grinding liner, and it is through these spaces that the secondary air will flow. The secondary air will have a cooling effect on the liner 23, particularly if the latter is fairly thin in nature, and this effect may be enhanced by the addition of vanes 33, which act as cooling fins and which also direct the flow of the air within the spaces 32.For instance, the vanes may be parallel with the axis of the shaft 17, or they may be angled, either helically or otherwise, to induce some swirl in a circumferential direction into the air.

It will be understood that the ribs 31 are not necessarily continuous for the whole length of the free space 32 but may allow communication between adjacent parts of that free space.

It will also be appreciated that the internal face of the grinding liner is likely to be serrated.

Various modifications may be made within the scope of the invention. For instance the classifier discs 26 and 27 are shown as rotating solidly with the rotor 18 which carries the discs 21 and the hammers 22. It may be preferable to have independent control of the speed of rotation of the classified discs and hammers, and for this purpose an arrangement of coaxial telescopically related shafts could be used to provide the different speeds of rotation.

Further, it is to be noted that although the casing is shown as being continuous over the whole length of the grinding liner 23, some parts of the casing could be omitted, and this would provide freer access to the outside of the grinding liner for the secondary air.

The drawing of Figure 1 illustrates a form of casing which could well be fabricated and in order to provide the necessary accuracy of locating the grinding liner, spaced location points would be provided. In the case of a fabricated casing, these points would normally be radially, and possibly also axially and in circumferentially adjustable.

Claims

1. A hammer mill comprising a rotor mounted in a casing and carrying hammers running on a path close to a grinding liner mounted in the casing, in which the grinding liner is mounted onto a casing at accurately located circumferentially spaced points to define free spaces outside the grinding liner.

2. A hammer mill as claimed in claim 1, in which the mounting points are adjustable.

3. A hammer mill as claimed in claim 1, in which the spaced mounting points are defined by ribs cast into the casing and machined to provide accurate location of the grinding liner.

4. A hammer mill as claimed in claims 1, 2 or 3, comprising an integral air classifier comprising discs rotating coaxially with the rotor to define rotary classifying passages through which the ground material is carried by an air stream.

5. A hammer mill as claimed in claim 4, in which the discs are mounted for rotation at the same speed as the hammers.

6. A hammer mill as claimed in claims 4 or 5, in which the feed material to be ground is fed into a grinding zone between the path of the hammers and the grinding liner by a primary air stream, and an inlet for secondary air is provided so that secondary air is fed via the free spaces outside the grinding liner to mix with the primary air stream before passing into the classifying passages.

7. A hammer mill as claimed in claim 6, in which the outside of the grinding liner is provided with vanes to control the flow of the secondary air.

8. A hammer mill as claimed in claim 7, in which the vanes are angled to induce circumferential swirl in the secondary air.

9. A hammer mill including an integral air classifier, in which feed material to be ground is arranged to be fed into a grinding zone between a grinding liner and the path of rotary hammers by a primary air stream and means is included for introducing a secondary air stream round the outside of the grinding liner to mix the primary air stream upstream of the classifier.

10. A hammer mill substantially as hereinbefore described with reference to the accompanying drawings.

10. A hammer mill as claimed in claim 9, in which the outside of the grinding liner is provided with vanes to control the flow of the secondary air.

11. A hammer mill as claimed in claim 10, in which the vanes are angled to induce circumferential swirl in the secondary air.

12. A hammer mill substantially as hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed, and have the following effect: (b) New or textually amended claims have been filed as follows: CLAIMS

1. A hammer mill including an integral air classifier, in which feed material to be ground is arranged to be fed into a grinding zone between a grinding liner and the path of rotary hammers by a primary air stream and to pass axially therethrough, and comprising means for introducing a secondary air stream into an annular space and outside the grinding liner to mix with the primary air stream downstream of the said annular space and upstream of the classifier.

2. A hammer mill as claimed in claim 1, in which the outside of the grinding liner is provided with vanes to control the flow of the secondary air.

3. A hammer mill as claimed in claim 2, in which the vanes are angled to induce circumferential swirl in the secondary air.

4. A hammer mill as claimed in claim 1, 2 and 3, in which the grinding liner is mounted onto a casing at accurately located circumferentially spaced points to define free spaces outside the grinding liner for the flow of the secondary air stream.

5. A hammer mill as claimed in claim 4, in which the mounting points are adjustable.

6. A hammer mill as claimed in claim 4, in which the spaced mounting points are defined by ribs cast into the casing and machined to provide accurate location of the grinding liner.

7. A hammer mill as claimed in any of the preceding claims, in which discs constituting the classifier are mounted for rotation at the same speed as the hammers.

8. A hammer mill as claimed in any of the preceding claims, in which an inlet for secondary air is provided so that secondary air is fed via the free spaces outside the grinding liner to mix with the primary air stream before passing into the classifier.

9. A hammer mill as claimed in any of the preceding claims, in which the grinding liner is associated with an orifice plate downstream of the grinding zone to control the resistance time in the grinding zone and direct the flow of primary air and ground material towards the classifier.