GB1569185A

GB1569185A - Dust extraction devices

Info

Publication number: GB1569185A
Application number: GB45255/76A
Authority: GB
Original assignee: THOMAS Ltd GWYN
Current assignee: THOMAS Ltd GWYN
Priority date: 1977-10-20
Filing date: 1977-10-20
Publication date: 1980-06-11

Description

(54) IMPROVEMENTS RELATING TO DUST EXTRACTION DEVICES (71) We, GWYN THOMAS LIMITED a British Company of 15, Highfield Road, Edgbaston, Birmingham, Bl5,West Midlands, do hereby declare the invention for which we pray that a Patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: The invention is concerned with improvements relating to dust extraction devices, particularly of the kind (hereinafter referred to as devices of the kind specified) in which dust or otherwise solid particulate material is entrained upon a stream of air, and removed by passing the stream of air through a filter medium.

Devices of the kind specified (also known as gas-solid filtration devices) are conventionally used in situations (usually industrial) where a large quantity of dust is created (such as in a shot-blasting or grinding operation) and/or where it is required that the environment be effectively dust-free.

In industrial processes, it is important that the efficiency of solid removal, often known as particulate collection efficiency, be as high as possible. Fabric filters, properly designed, applied and maintained, have the ability to operate in many cases with collection efficiences in excess of 99.5%. Hence the use of fabric filtration has increased considerably over the past twenty years, particularly when compared with the use of cyclones and wet collectors which, in a great majority of cases, present a significantly lower standard of collection efficiency.

In situations where the filtered gas (which is usually air) is being discharged to atmosphere, then the need to reduce atmospheric pollution to the minimum level feasible in terms of technology and economics is obvious, and in many (and increasing) cases is a statutory obligation.

However, when the dust is non-hazardous and non-toxic, it may be desired that the air from the extraction device be discharged back into the working area instead of being discharged to the outside atmosphere. In these circumstances, considerations of hygiene, health and safety call for the highest degree of filtration of the returned air, usually at a level in excess of that which is achieved where the air is discharged to the outside atmosphere.

One of the characteristics of fabric filters is the need periodically to clean the filter elements by removing the collected dust which has been deposited on the upstream side of the filter medium. Dust in this condition is normally known as filter-cake, and various methods of removing the dust are used, one of the more common procedures being to agitate or shake the filter medium for a period not usually longer than two minutes, thereby causing the cake to be dislodged from the fabric to fall into a filter hopper below. Since it is necessary that the cake freed from the surface of the medium should be enabled to fall without difficulty into the filter hopper, it is normal practice to agitate the filter medium when there is no air flow in the normal working direction, since this would (a) tend to return the collected dust to the filter medium, or prevent it from leaving the surface of the filter medium, and (b) cause the filter elements to be relatively rigid and so impede the shaking action required for the cleaning purpose.

For these reasons, it is practice to clean the filter medium by shaking the filter element under conditions of no air flow. However, this process necessarily disturbs the filter medium, and tends to cause fine dust to work through the fabric to rest on, or near, the downstream side. Thus, when filtration of C'-v.st laden air or gas through the shaked filter elements is resumed, it is common to find that dust which has worked into and through, or practically through, the filter medium by the shaking action, is carried by the air flow to the point of air discharge in such quantities as to be easily observable for a brief period of time, usually up to two minutes, until the disturbed particles have been cleared from the system. It is also typical that the finer and drier the dust being filtered, and the higher the air velocity through the filter medium, the greater and more noticeable is the resulting dust carryover immediately after cleaning.

In cases where the filtered air is returned to the working area, this carryover may reach such levels as to be unacceptable.

It has been attempted to overcome or reduce this problem by reducing the air velocity through the filtration medium, to reduce the tendency of particles being carried into the filter medium by air flow, and/or by the provision of a secondary static filter. However, whereas static filters may minimise the observable effects which take place immediately after a shakingfrestart operation, other than in the first few months of operation they do not significantly reduce the overall quantity of dust discharge from the extraction device, without being replaced. Thus, it has heretofore been accepted that it is prudent to discharge the filtered air to atmosphere. This approach, whilst avoiding the risks outlined, can be expensive and inconvenient in at least two ways.

Firstly, the dust collector when sited inside the factory must be connected to the outside atmosphere by ducting, which limits easy moving of the collector should the machine, apparatus or process served be relocated, and in any event the ducting may be expensive and inconvenient when, for example, the collector is located in a crane-swept bay or is geographically remote from outside atmosphere through its location in the building.

Secondly, where and when it is necessary to heat the working atmosphere within the building for reasons of comfort, or to maintain a statutory minimum working temperature, the discharge of warm air to the outside atmosphere requires its replacement by fresh air which will need warming, and this imposes an additional load on the factory heating system, with associated increase in cost.

In addition, the provision of a secondary filter itself involves considerable additional cost in capital, running and maintenance aspects; such secondary filters require periodic inspection and replacement, and.impose an additional resistance to air flow through the extraction device, with associated higher power cost.

The invention provides a dust extraction device comprising an inlet chamber; an outlet chamber; an inlet duct extending to the inlet chamber and through which in use dust-laden air is delivered to the inlet chamber; a filtration medium, through which dust-laden air may flow from the inlet chamber to the outlet chamber; an outlet duct extending from the device; a fan which is operative to cause air to flow from the inlet chamber through the filtration medium to the outlet chamber; a bypass duct through which air may flow from the outlet chamber to the inlet chamber; and control mechanism movable between a first condition in which the fan causes air to flow from the outlet chamber via the by-pass duct into the inlet chamber to be refiltered by the filtration medium and a second condition in which the fan causes air to flow from the outlet chamber through the outlet duct from the device.

This invention is particularly suitable where the dust extraction device comprises means operative periodically to dislodge dust from the upstream sides of the filtration medium.

Such means may comprises mechanism periodically operative to agitate the filtration medium, and/or periodically operative to causes air flow from the outlet chamber through the filtration medium to the inlet chamber. Advantageously the filtration medium comprises a plurality of filtration elements, advantageously textile fabric filtration elements.

Preferably the control mechanism is operative automatically, in consequence of startup of the device (including restart of the device subsequent to operation of any agitating of the filtration medium) initially to adopt its first condition and subsequent to the elapse of a predetermined period of time to adopt its second condition. However, the control mechanism may be operated manually.

By the use of the device set out above, any dust normally carried over upon start or restart of the device will be returned to the upstream side of the filter medium, where it will be substantially retained. When substantially all the dust normally carried over is so retained by resumption of airflow, the airflow from the filtration elements may be discharged back into the working area again, and the extraction device operated in the normal way.

In addition, whereas the invention is primarily intended for use in dust extraction devices utilising fabric filters (e.g. woven, felted, natural or synthetic) this invention can be utilised in any extraction device where the filtration takes place over a separate definable section of filter element, functioning in a manner similar to that of fabric filters.

There will now be given a detailed description, to be read with reference to the accompanying drawing, of a preferred embodiment of this invention, which has been selected to illustrate this invention by way of example.

The accompanying drawing is a schematic perspective view of the preferred embodiment of this invention.

The preferred embodiment of this invention is a floor-mounted dust extraction device comprising a housing 10, in a central part of which is located a filtration section, provided by a number of tubular fabric filtration elements 12.

Located beneath the elements 12 is an inlet chamber 14 into which an inlet duct 16 opens: located above the elements 12 is an outlet cham ber 18 over which is mounted impeller means in the form of a centrifugal fan 20. An exhaust port of the fan 20 extends to an outlet duct 22, from which air delivered by fan 20 may flow from the device.

In the use of the device, the fan is operated to draw air from the inlet chamber 14 through the elements 12 into the outlet chamber 18, and to expel such air from the device through the outlet duct 22. Dust-laden air is delivered to the device through the inlet duct, and as the air is drawn through the filtration elements 12, such dust collects on the upstream side of said elements and relatively clean air is expelled from the device.

The elements 12 are supported by elongate frames 24, connected to an eccentric of a motor 26. After a period of continued operation of the device, the fan 20 is switched off, and the motor 26 is switched on, vibrating the frames 24, and hence the elements 12, to cause collected dust, in the form of cake, to fall from the upstream surfaces of the elements 12 and into a bin 28 located beneath a receiving hopper 11. Periodically, the bin 28 is removed from the device, to be emptied.

During agitation of the elements 12 by frames 24, some of the dust deposited on the upstream surfaces of said elements will tend to work through the fabric of the elements to or towards the downstream surfaces thereof.

Thus, upon starting of the fan 20, or restart thereof subsequent to a shaking operation, such dust as may have worked its way through towards the downstream surfaces of the elements may be carried from the elements on the stream of air, and be delivered through the outlet duct 22 from the device.

To prevent this from happening, or at least minimise the effects, the device comprises a by-pass duct 30 which extends between the outlet duct 22 and the inlet chamber 14. At the juncture between the by-pass duct 30 and the outlet duct 22, there is provided valve mechanism in the form of a flap valve 32, said valve being mounted for pivotal movement between a first position (shown in dotted lines in the drawing) in which the outlet duct 22 is closed, and a second position (shown in full lines in the drawing) in which the by-pass duct 30 is closed.

Thus, upon startup of the device, or restart thereof subsequent to a shaking operation, the valve 32 is caused to adopt its first position.

Initially, air drawn through the elements 12, and which is likely to contain small quantities of dust, is caused to flow from the outlet chamber 18 through the by-pass duct 30 and back into the inlet chamber 14, and is refiltered.

After the elapse of a short period of time (preferably one to two minutes) the valve 32 may be moved to its second position, where upon air drawn by the fan 20 through the elements 12 will be caused to be delivered from the machine, and may be issued into the working area.

Thus, any dust which may have worked its way through the elements will be returned to the upstream surfaces of the elements, where it is filtered from the air in the normal way.

Advantageously, the valve 32forms part of automatic control mechanism of the device (not shown) which, upon startup of the device, including restart of the device subsequent to operation of any operation to dislodge dust from the upstream side of the filtration elements, initially adopts a first condition in which the valve 32 is in its first position, said control mechanism being operative automatically subsequent to the elapse of a predetermined period of time to adopt a second condition, in which the valve 32 is in its second position.

In practice, it has been found that switching of the control device automatically from its first to its second condition, after the elapse of a period of time subsequent to startup in the order of two minutes or less is sufficient to reduce the quantity of dust emitted from the outlet duct back to an acceptable quantity.

However, this period may vary, depending upon the degree of purity of air required, and upon the type of filtration operation being carried out (including the type of filtration medium, and material being filtered).

It has been found that, with the valve 32 in its first position, there is little or no air movement within the inlet duct 16 a short distance away from the inlet chamber 14. Once steady state conditions have been reached, the quantity of air drawn by the fan 20 through the elements 12 will be equal to the quantity of air delivered through the by-pass duct 30 to the upstream surfaces of the elements: thus, there is no pressure variation within the inlet duct 16 a short distance from the inlet chamber.

It will be appreciated that the device described above, and illustrated in the drawings, has been selected for the purposes of illustrating the principles of this invention only, and that for practical purposes, the general construc tion and arrangement may and will vary. Thus, in place of the tubular elements 12, filtration elements in the form of envelopes may be used.

Furthermore, in place of the agitating mechanism above described, cake may be freed from the upstream surfaces of the filtration elements by the use of reverse airflow. However, in any such construction in which, upon start or restart of the extraction device, air having a high dust concentration is inially issued from the device, the principles hereinabove defined may be used to advantage.

WHAT WE CLAIM IS: 1. A dust extraction device comprising an inlet chamber; an outlet chamber; an inlet duct extending to the inlet chamber and through which the use dust-laden air is delivered to the inlet chamber;a filtration medium, through which dust-laden air may flow from the inlet

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. in the form of a centrifugal fan 20. An exhaust port of the fan 20 extends to an outlet duct 22, from which air delivered by fan 20 may flow from the device. In the use of the device, the fan is operated to draw air from the inlet chamber 14 through the elements 12 into the outlet chamber 18, and to expel such air from the device through the outlet duct 22. Dust-laden air is delivered to the device through the inlet duct, and as the air is drawn through the filtration elements 12, such dust collects on the upstream side of said elements and relatively clean air is expelled from the device. The elements 12 are supported by elongate frames 24, connected to an eccentric of a motor 26. After a period of continued operation of the device, the fan 20 is switched off, and the motor 26 is switched on, vibrating the frames 24, and hence the elements 12, to cause collected dust, in the form of cake, to fall from the upstream surfaces of the elements 12 and into a bin 28 located beneath a receiving hopper 11. Periodically, the bin 28 is removed from the device, to be emptied. During agitation of the elements 12 by frames 24, some of the dust deposited on the upstream surfaces of said elements will tend to work through the fabric of the elements to or towards the downstream surfaces thereof. Thus, upon starting of the fan 20, or restart thereof subsequent to a shaking operation, such dust as may have worked its way through towards the downstream surfaces of the elements may be carried from the elements on the stream of air, and be delivered through the outlet duct 22 from the device. To prevent this from happening, or at least minimise the effects, the device comprises a by-pass duct 30 which extends between the outlet duct 22 and the inlet chamber 14. At the juncture between the by-pass duct 30 and the outlet duct 22, there is provided valve mechanism in the form of a flap valve 32, said valve being mounted for pivotal movement between a first position (shown in dotted lines in the drawing) in which the outlet duct 22 is closed, and a second position (shown in full lines in the drawing) in which the by-pass duct 30 is closed. Thus, upon startup of the device, or restart thereof subsequent to a shaking operation, the valve 32 is caused to adopt its first position. Initially, air drawn through the elements 12, and which is likely to contain small quantities of dust, is caused to flow from the outlet chamber 18 through the by-pass duct 30 and back into the inlet chamber 14, and is refiltered. After the elapse of a short period of time (preferably one to two minutes) the valve 32 may be moved to its second position, where upon air drawn by the fan 20 through the elements 12 will be caused to be delivered from the machine, and may be issued into the working area. Thus, any dust which may have worked its way through the elements will be returned to the upstream surfaces of the elements, where it is filtered from the air in the normal way. Advantageously, the valve 32forms part of automatic control mechanism of the device (not shown) which, upon startup of the device, including restart of the device subsequent to operation of any operation to dislodge dust from the upstream side of the filtration elements, initially adopts a first condition in which the valve 32 is in its first position, said control mechanism being operative automatically subsequent to the elapse of a predetermined period of time to adopt a second condition, in which the valve 32 is in its second position. In practice, it has been found that switching of the control device automatically from its first to its second condition, after the elapse of a period of time subsequent to startup in the order of two minutes or less is sufficient to reduce the quantity of dust emitted from the outlet duct back to an acceptable quantity. However, this period may vary, depending upon the degree of purity of air required, and upon the type of filtration operation being carried out (including the type of filtration medium, and material being filtered). It has been found that, with the valve 32 in its first position, there is little or no air movement within the inlet duct 16 a short distance away from the inlet chamber 14. Once steady state conditions have been reached, the quantity of air drawn by the fan 20 through the elements 12 will be equal to the quantity of air delivered through the by-pass duct 30 to the upstream surfaces of the elements: thus, there is no pressure variation within the inlet duct 16 a short distance from the inlet chamber. It will be appreciated that the device described above, and illustrated in the drawings, has been selected for the purposes of illustrating the principles of this invention only, and that for practical purposes, the general construc tion and arrangement may and will vary. Thus, in place of the tubular elements 12, filtration elements in the form of envelopes may be used. Furthermore, in place of the agitating mechanism above described, cake may be freed from the upstream surfaces of the filtration elements by the use of reverse airflow. However, in any such construction in which, upon start or restart of the extraction device, air having a high dust concentration is inially issued from the device, the principles hereinabove defined may be used to advantage. WHAT WE CLAIM IS:

1. A dust extraction device comprising an inlet chamber; an outlet chamber; an inlet duct extending to the inlet chamber and through which the use dust-laden air is delivered to the inlet chamber;a filtration medium, through which dust-laden air may flow from the inlet

chamber to the outlet chamber; an outlet duct extending from the device; a fan which is operative to cause air to flow from the inlet chamber through the filtration medium to the outlet chamber; a by-pass duct through which air may flow from the outlet chamber to the inlet chamber; and control mechanism movable between a first condition in which the fan causes air to flow from the outlet chamber via the by-pass duct to the inlet chamber to be refiltered by the filtration medium, and a second condition in which the fan causes air to flow from the outlet chamber through the outlet duct from the device.

2. A device according to Claim 1 comprising dust dislodgement means operative periodically to dislodge dust from the upstream side of the filtration medium.

3. A device according to Claim 2 wherein said dust dislodgement means comprises mechanism periodically operative to agitate the filtration medium.

4. A device according to one of Claims 2 and 3 wherein said dust dislodgement is periodically operative to causes air flow from the outlet chamber through the filtration medium to the inlet chamber.

5. A device according to any one of the preceding claims wherein the filtration medium comprises a plurality of filtration elements.

6. A device according to any one of the preceding claims wherein the filtration medium comprises textile fabric.

7. A device according to any one of the preceding claims wherein the control mechanism is operative automatically, in consequence of start start-up of the device (including re-start of the device subsequent to operation of any agitating of the filtration medium) initially to adopt its first condition, and subsequent to the elapse of a predetermined period of time to adopt its second condition.

8. A device according to any one of Claims 1 - 6 wherein the control mechanism is operative manually.

9. A device according to any one of the preceding claims wherein the fan is operative to draw air from the outlet chamber, and the control mechanism comprises a valve which when the control mechanism is in its first condition adopts a position to cause air drawn by the fan from the outlet chamber to flow through the by-pass duct into the inlet chamber, and which when the control mechanism is in its second condition adopts a position to cause air drawn by the fan from the outlet chamber to flow through the outlet duct from the device.

10. A dust extraction device constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.