NZ250447A - Pressure transducer for air filter uses load cell - Google Patents

Description

250447 No.: 250447 Date: 14/12/93 NEW ZEALAND PATENTS ACT, 1953 COMPLETE SPECIFICATION pate ■i: J 1 3 Dec 'CE 1994 IVCD TRESSURE TRANSDUCER" We, Filtercorp International limited, a company duly incorporated under the laws of New Zealand, of 22-24 Baypark Place, Beachhaven, Auckland, New Zealand, 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:- 250447 This invention relates to pressure transducers and in particular to a transducer for measuring gaseous pressure differential across a chamber wall. The pressure transducer described has particular application in air cleaning systems.

A common form of air cleaning system forces dust-laden air through filter bags fabricated from a material such that dust intended to be removed builds up on one surface of each bag. A multiplicity of bags may be used together in parallel and suspended from apertures in a dividing wall or cell plate within a purpose-designed vessel. Such a system is known as a "baghouse".

It is known to monitor the pressure differential across the filter bags to provide an indication of the state of the filter bags and thus initiate manual or automatic bag cleaning. The pressure differential arises due to frictional and kinetic energy losses through each bag. Conventional systems use a water manometer with sensing tubes connected into the vessel either side of the cell plate. Electronic pressure transducers have also been proposed but, as with the U-tube manometer, require coupling hoses and sensing ports open to the interior of the cell.

These conventional pressure indicators suffer from the disadvantage that the transducer connection on the dusty air side of the cell fills with contaminants to cause erroneous pressure differential readings. Clogged manometer ports either give a false indication of dust build-up on the filter bags, or cause early indications of filter problems to be missed.

It is therefore an object of the present invention to provide a gaseous pressure transducer which will at least go some way towards overcoming the abovementioned disadvantage or which will at least provide the industry with a useful choice.

Accordingly, in one aspect the invention may broadly be said to consist in a pressure transducer foi measuring gaseous pressure differential across a wall having an electrical output which changes as a function of said pressure differential comprising: a diaphragm adapted to be sealably mounted over an aperture in said wall, a load cell having first and second load points, said first load point of said load cell being fixed to said diaphragm, and coupling means which rigidly connect the said second load point of said load cell to said wall, the output nodes of said load cell forming the output nodes of the transducer. 250447 In a second aspect the invention may broadly be said to consist in an air cleaning system comprising a vessel having a dividing wall which forms two chambers within said vessel, said dividing wall including one or more apertures therethrough, one or more dust filter bags sealably suspended from a corresponding dividing wall aperture and a pressure transducer for measuring the pressure differential across said dividing wall to provide an electrical output which changes as a function of the pressure differential, said pressure transducer in turn comprising: a diaphragm adapted to be sealably mounted over an aperture in said wall, a load cell having first and second load points, said first load point of said load cell being fixed to said diaphragm, and coupling means which rigidly connect the said second load point of said load cell to said wall, the output nodes of said load cell forming the output nodes of the transducer. The preferred form of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a diagramatic representation of a dust extraction system and associated pressure differential monitoring and Figure 2 is a cross-section of a pressure transducer according to the present invention, and Figure 3 is a cross-section of a pressure transducer according to a further embodiment of the present invention.

Figure 1 shows a baghouse dust extraction system for dust-ladeft air with which the pressure transducer of the present invention is used. A cell or vessel 1 receives dust-laden air through port 2 and discharges clean air through port 3. Dust entrained within the incoming air is filtered out by being passed through a number of filter bags which share the total air flow. The filter bags are suspended from a cell plate 5 which divides cell 1 into a dusty air side 6 and a clean air side 7. Filter bags 4 are suspended from corresponding apertures 8 uniformly dispersed across cell plate 5. Air passes through the filter bags leaving dust on the exterior surface of the bags and the clean air passes up through apertures 8 to be extracted through port 3.

A pressure transducer 9 monitors the pressure differential across cell plate 5 and then across the parallel arrangement of filter bags. As the dust on the filter bags 4 250447 builds up, so too does the pressure differential across cell plate 5. This is monitored by pressure transducer 9 and a signal from the pressure transducer drives a digital display 10 which indicates pressure drop and may also drive a chart recorder 11 and/or an alarm 12. The analogue output 13 from the pressure transducer may be used to control a filter bag cleaning system such as pulsed compressed air jets directed into the filter bags from the top of cell 1. Preferably the compressed air jets are activated when the pressure differential across the cell plate 5, as indicated by the pressure transducer 9, reaches a preset level The construction of pressure transducer 9 is shown in Figures 2 and 3. It is shown mounted in an aperture 14 in cell plate 5. The primary sensing element of the transducer is a load cell 15 which, when provided with a suitable electrical input signal, provides an electrical output via conductors 16 representative of the force applied to the load points of the load cell. With reference to Figure 3, conductors 16 may be terminated at a printed circuit board 24 in an enclosure 23. With reference to Figures 2 and 3, the load cell is mounted rigidly to cell plate 5 through load point 17 which is fixed to a perforated cover 18. Cover 18, with perforations 26, extends beyond the periphery of aperture 14 and is bolted or otherwise fixed to the cell plate across aperture 14.

With reference to Figure 3, a mounting ring 25 complete with drilled and tapped holes may be used to provide easy installation without nuts, in particular for food grade applications where threads are not permitted on the product side of the dust collector.

With reference to Figures 2 and 3, £ flexible diaphragm 19, which may be made for example from rubber or a plastics material, is fixed to transducer cover 18 about its periphery 20 and in sealing engagement with the rim of aperture 14. Diaphragm 19 thus is subject to the same pressure differential as is cell plate 5. This is because cover 18 is perforated, allowing the upper surface of diaphragm 19 to be subject to the same pressure as applies to the clean air side of cell plate 5.

Diaphragm 19 comprises a conical portion 21 and a base portion 22. Base portion 22 is fixed to the second load point 23 of load cell 15. Thus a force F applied to diaphragm 19 in the direction of the arrow will be transmitted to load cell load point 23 and as load point 17 is fixed, load cell 15 will provide an electrical output proportional to force F. In the application described, force F will be produced by the 0 44 7 pressure differential between the dusty air and clean air sides of cell plate 5. The output of load cell 15 will therefore be proportional to the pressure differential across the filter bags 4.

A cantilever type load cell has been shown, but other configurations could be used to provide similar results. It is preferred that the load cell be physically small since there are advantages in providing a pressure transducer which is of relatively small total size. The load cell should have a capacity less than 5 kg in order to provide sufficient accuracy at the pressure differentials normally experienced in the application described. An example of a suitable load cell is a Tanaka model C2G1 series load cell manufactured by EMC It will be appreciated that this load cell is given by way of example only.

The present transducer has the advantage that no tubes or ports are required which can be contaminated. The diaphragm protects the transducer from dust as a natural consequence of its structure and function and the perforations in cover 18 may be provided with filters to prevent what little dust may enter the clean air plenum from entering the interior of the transducer. The only connections required between the transducer and the external measuring system are the electrical conductors from the load cell. 25044 7

Claims (27)

CLAIMS: WHAT 11 WE CLAIM IS

1. A pressure transducer for measuring gaseous pressure differential across a wall having an electrical output which changes as a function of said pressure differential comprising: a diaphragm adapted to be sealably mounted over an aperture in said wall, a load cell having first and second load points, said first load point of said load cell being fixed to said diaphragm, and coupling means which rigidly connect the said second load point of said load cell to said wall, the output nodes of said load cell forming the output nodes of the transducer.

2. A pressure transducer as claimed in claim 1 including a perforated plate, adapted to be fixable over an aperture in said wall, said plate adapted to be rigidly connected to said wall, said load cell disposed between said diaphragm and said plate, and said second load point is rigidly connected to said plate.

3. A pressure transducer as claimed in either claim 1 or claim 2 wherein said diaphragm is a frusto-conical section, the periphery of said diaphragm adapted to be sealably fixed around an aperture in said wall

4. A pressure transducer as claimed in claim 3 wherein said periphery of said diaphragm includes a flanged portion, said flanged portion sealable around an aperture in said wall.

5. A pressure transducer as claimed in claim 4 wherein said flanged portion is adapted to be disposed between said perforated plate and said wall, and said transducer includes fixing means, adapted to fix said plate to said wall, said plate and said wall sealably securing said flanged portion of said diaphragm therebetween.

6. A pressure transducer as claimed in claim 5 wherein said fixing means comprise a plurality of nuts and bolts, said bolts adapted to pass through said plate and said wall, and said nuts, when tightened on said passed-through bolts, draw together said plate and said wall. -7- 250447

7. A pressure transducer as claimed in claim 5 wherein said fixing means comprise a plurality of bolts and a mounting ring, said bolts adapted to pass through said plate and said wall, said mounting ring having receiving holes to receive said bolts, said mounting ring being adapted to be disposed on the diaphragm side of said wall, and said bolts when tightened in said receiving holes draw together said plate and said mounting ring.

8. A pressure transducer as claimed in any one of claims 1 to 7 wherein said load cell is a cantilever load cell.

9. A pressure transducer as claimed in any one of claims 1 to 8 wherein said load cell has a capacity less than 5 kg.

10. A pressure transducer as claimed in claim 2 or any one of claims 3 to 9, when dependent on claim 2, wherein the perforations of said perforated plate are provided with dust filters.

11. An air cleaning system comprising a vessel having a dividing wall which forms two chambers within said vessel, said dividing wall including one or more dust bag apertures therethrough, one or more dust filter bags sealably suspended from a corresponding dividing wall dust bag aperture, a sensing aperture in said wall, and a pressure transducer for measuring the pressure differential across said dividing wall to provide an electrical output which changes as a function of the pressure differential, said pressure transducer in turn comprising: a diaphragm sealably mounted over said sensing aperture in said wall, a load cell having first and second load points, said first load point of said load cell being fixed to said diaphragm, and coupling means which rigidly connect the said second load point of said load cell to said wall, the output nodes of said load cell forming the output nodes of the transducer. -8- 250447

12. An air cleaning system as claimed in claim 11 wherein said pressure transducer includes a perforated plate, fixed over said sensing aperture in said wall, said plate rigidly connected to said wall, said load cell disposed between said diaphragm and said plate, and s^id second load point is rigidly connected to said plate.

13. An air cleaning system as claimed in either claim 11 or claim 12 wherein said diaphragm is a frusto-conical section, and the periphery of said diaphragm is sealably fixed around said sensing aperture in said wall.

14. An air cleaning system as claimed in any one of claims 11 to 13 wherein said periphery of said diaphragm includes a flanged portion, and said flanged portion is sealed around said sensing aperture in said wall

15. An air cleaning system as claimed in claim 14 wherein said flanged portion is disposed between said perforated plate and said wall, and said transducer includes fixing means, fixing said plate to said wall, said plate and said wall sealably securing said flanged portion of said diaphragm therebetween.

16. An air cleaning system as claimed in claim 15 wherein said fixing means comprise a plurality of nuts and bolts, said bolts passing through said plate and said wall, and said nuts, when tightened on said passed-through bolts, drawing together said plate and said wall.

17. An air cleaning system as claimed in claim 15 wherein said fixing means comprise a plurality of bolts and a mounting ring, said bolts passing through said plate and said wall, said mounting ring having receiving holes to receive said bolts, said mounting ring being disposed on the diaphragm side of said wall, and said bolts, tightened in said receiving holes, draw together said plate and said mounting ring.

18. An air cleaning system as claimed in any one of claims 11 to 17 wherein said load cell is a cantilever load cell. -9- 250447

19. An air cleaning system as claimed in any one of claims 11 to 18 wherein said load cell has a capacity less than 5 kg.

20. An air cleaning system as claimed in any of claims 12 to 19, when dependent on claim 12, wherein the perforations of said perforated plate are provided with dust filters.

21. An air cleaning system as claimed in any one of claims 11 to 20 wherein the output of said load cell drives a digital display.

22. An air cleaning system as claimed in any one of claims 11 to 21 wherein the output of said load cell drives a chart recorder.

23. An air cleaning system as claimed in any one of claims 11 to 22 wherein the output of said load cell, upon satisfying a preset condition, causes activation of an alarm.

24. An air cleaning system as claimed in any one of claims 11 to 23 wherein said air cleaning system includes filter bag cleaning means, and the output of said load cell controls the operation of said filter bag cleaning means.

25. An air cleaning system as claimed in claim 24 wherein said filter bag cleaning means comprises pulsed compressed air jets, and the output of said load cell, upon satisfying a preset condition, causes activation of said pulsed compressed air jets.

26. A pressure transducer substantially as herein described with reference to and as illustrated by the accompanying drawings.

27. An air cleaning system substantially as herein described with reference to and as illustrated by the accompanying drawings.