GB2517420A - A method of enhancing the filtration performance of filter felts - Google Patents

Abstract

A method of enhancing the filtration properties of PTFE felt The method includes the steps of providing a PTFE (polytetrafluoroethlyene) felt and subjecting the felt to one or more jets of water in order to further entangle the PTFE fibres and split the PTFE fibres into finer fibers. The felt may be a needled felt and the hydroentanglement step may involve the use of one or more nozzles arranged to direct the jets of water toward the felt. The felt may be manufactured by using the PTFE fibres on a supporting scrim and pushing a number of needles through the fibers and the scrim. The water jet(s) may be at a pressure of between 50 and 100 bar.

Description

A method of enhancing the filtration performance of filter felts

Technical Field of the Invention

The present invention relates to a method of enhancing the filtration performance of filter felts.

Background to the Invention

Bag filters incorporating industrial filter felts are used in heavy industries such as power stations, smelters, incinerators and grain handling where it is often necessary to filter by-products such as hot gases, dust and liquid as well as nuisance dusts. One form of felt material often used in such filtration applications is based on Polytetrafluoroethylene (PTFE) which has high chemical and thermal resistance. PTFE felt is traditionally manufactured using a needle punching technique which turns a number of PTFE fibres and a supporting woven scrim into a fabric.

The needle punching process involves the use of tens of thousands of fine needles which comprise a number of barbs along their respective lengths. With reference to Fig. 1, these needles are pushed through a collection of textile fibres and the supporting scrim that are sandwiched between two perforated plates (typically, several hundred punches (needle penetrations) are applied to every square centimetre of the textile). The textile fibres become caught in the needle barbs and are pushed through from one side of the scrim to the other. The needles are then retracted and the fibres remain in a tangled, intermingled state. The process is repeated from the other side of the fibres to form a sheet of non-woven fabric comprising many tangled fibres. These actions from opposite sides are repeated a large number of times.

A heat stabilisation stage is then performed by heating the fabric to temperatures in the region of 300°C to minimise the thermal shrinkage of the material in the end application. After the stabilisation stage, the product is cut to the correct width for the bag size required and the bags are then sewn together at the appropriate dimensions.

The filtration performance of the non-woven fabric is partly correlated to its ability to resist the flow of air through the fabric and this is affected to some extent by the fineness of the fibres used to produce the fabric. Textile fabrics with very fine fibres and, hence, a high resistance to airflow tend to be more effective filters than textile fabrics with relatively coarser fibres and lower resistance to airflow.

PTFE is a common fibre used to manufacture felts for use in filtration applications due to its high chemical and thermal resistance. However, a problem with PTFE fibres is that they are relatively coarse (typically 2 to 5 denier) and therefore produce a felt with relatively large air gaps through which particles that are intended to be trapped by the felt may pass. A second problem with PIFE is the fibre density is high for fibres which means that for a given felt weight there is less volume of actual filtering fibre to collect the filtrate.

It is an object of embodiments of the present invention to provide a method of enhancing the filtration performance of filter felts.

Summary of the Invention

According to an aspect of the present invention, there is provided a method of enhancing the filtration properties of PTFE felt comprising the steps of: providing a PIFE felt; and subjecting the felt to one or more jets of water in order to further entangle the PTFE fibres and split the PIFE fibres into finer fibres.

The PIFE felt may be made by providing a plurality of PTFE fibres on a supporting scrim and pushing a plurality of needles through the fibres and scrim, i.e. it has to be a needled felt.

Each jet of water may be projected from a respective nozzle. Each water jet may be projected at a pressure of between 50 bar and 500 bar, or between 100 bar and 400 bar.

Subjecting PTFE fibres to high pressure water jets causes them to preferentially split along their respective lengths. It is thought that this is because the structure of the fibres is akin to wood in that they have a "grain". The high pressure water is capable of inveigling its way into the grain and splitting the fibres along their longitudinal axes, thereby enhancing the filtration properties of the felt.

According to another aspect of the present invention, there is provided a method of manufacturing filter media comprising the steps of: providing a sheet of needled felt material; placing a plurality of loose fibres on the surface of the material; and subjecting the loose fibres to one or more jets of water.

The needled felt may comprise a scrim and one or more fibres that have been consolidated via the action of a plurality of needles. The loose fibres may be deliberately engineered to be split by the jets of water. The loose fibres may comprise separate components. The separate components may be smaller in cross section than the original loose fibres. The separate components may comprise two different materials. The two different materials may be nylon and polyester.

Detailed Description of the Invention

In order that the invention may be more clearly understood, an embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Fig. 1 shows an enlarged schematic view of a needle being inserted through a collection of fibres between two perforated plates; Fig. 2 shows an enlarged schematic view of a cross section of a needled PTFE felt; Fig. 3 shows a magnified view of a cross section of a needled PTFE felt; Fig. 4 shows a schematic representation of apparatus for subjecting textile to water jets and subsequently drying the textile; Fig. 5 shows a magnified view of the surface of a needled PTFE felt; Fig. 6 shows a magnified view of the felt shown in Fig. 5 after it has been subjected to water jets; and Fig. 7 shows a schematic view of apparatus used to test the filtration properties of filter media.

In order to manufacture a conventional needled PIFE felt, a plurality of white PTFE fibres is first conveyed between a series of rotating cylinders having a plurality of teeth which serve to comb the fibres into parallel arrays. This so called carding process distributes the fibres into a substantially uniform, thin, lightweight structure known as a web.

The web of PTFE fibres is then formed into a desired web structure using a cross layering process which laps the web back and forth onto a conveyor moving at a right angle to the cross-layering motion. The cross laid web is then placed onto a reinforcing PTFE scrim and passed through a needle loom where initial adhesion occurs between the fibres and the scrim. This "one sided" product may then be fed back through the machinery to attach fibres to the reverse side.

Both sides of the above product are subsequently needled to create a conventional loomstate (either single or double sided) felt. As shown in Fig. 2, the resulting needled felt comprises a central scrim layer C and two layers of PTFE felt A, B that have been attached to either side of the scrim by the needles passing therethrough. In single sided products, layer B is absent. Fig. 3 shows a magnified image of an actual needled PTFE felt. As can be seen, plugs of PTFE fibres have been pushed through the scrim from one side to the other by the needles to create the felt sheet.

With reference to Fig. 4, there is shown apparatus 1 for subjecting textile material such as felt to high pressure water jets. The apparatus 1 comprises rollers 5 which serve to convey sheets of material along designated paths through the apparatus 1. The apparatus 1 further comprises a central perforated cylinder 7 which is connected to a vacuum pump that serves to create a pressure drop across the cylinder wall so that any textile material 3 that is conveyed through the apparatus may be held against the cylinder wall. The vacuum pump further serves to draw away any water that penetrates the textile material 3. The rollers 5 are arranged to convey textile material along two alternate paths I, II around the central cylinder depending upon which side of the material 3 it is desired to treat.

A pair of injectors 9a, 9b is directed toward the wall of the central cylinder 7. Each injector 9a, 9b is connected to a water supply line lOa, lob which comprises a police filter 11 that serves to filter out any contaminants from the water conveyed along the supply line ba, lob. The supply line lOa, lob further comprises a high pressure water pump 12a, 12b that drives water along the supply line under pressure. The injectors 9a, 9b comprise metal plates having a plurality of holes or nozzles with diameters in the size range from approximately 0.06mm to 0. 18mm. Water from the supply line ba, lob is channelled to these tiny holes to produce very fine jets of high pressure water which may be directed at any material passing between the cylinder 7 and the nozzles. The apparatus further comprises a drying chamber 15 positioned adjacent the water jet chamber. The drying chamber 15 contains two rotating cylinders 17, 18 that convey the textile material 3 through the dryer 15 to enable the material 3 to be more fully dewatered.

In use, a pre formed needled PTFE felt sheet 3 is conveyed from right to left via rollers 5 which direct the sheet around the central perforated cylinder 7. As the felt 3 is conveyed around the central cylinder 7, one side of the needled felt 3 is subjected to a plurality of the very fine, high pressure water jets which are projected from the injectors 9a, 9b. The nozzles are arranged in equally spaced rows and are arranged to direct a curtain of water jets across the width of the felt.

The water]ets are projected at pressures of up to 500 bar and preferably in the range 100 bar to 400 bar.

Surprisingly, it has been found that when high pressure water jets are applied to an existing PTFE needled felt, the filtration properties of the felt are greatly enhanced. It is thought that this is because the action of the water jets causes the PTFE fibres to preferentially split along their respective lengths to produce an increased number of finer fibres. With reference to Figs 4 and 5, it can be seen that a number of the fibres originally having a width of approximately 0.1mm have been split into separate, relatively much finer fibres. This increased number of finer fibres reduces the size of the air pockets between the fibres and therefore enhances the ability of the felt 3 to collect dust in a gas stream.

When the felt 3 has been subjected to the water jets, the felt is dewatered by a vacuum slot and then subsequently passed through a dryer 15 to remove all residual moisture as it is conveyed around the two rotating cylinders 17, 18. The felt can then be heat stabilised at temperatures up to 300°C. The whole process from start to finish is carried out at a line speed of between lOm/min and 200 m/min and preferably at approximately 30 rn/mm.

Test Results Comparative tests have been conducted on test media with respect to cycle time conditioning, cycle time measuring, clean gas concentration and clean gas measuring in accordance with the internationally recognised standard for measuring filtration efficiency (ISO 11057). The tests are carried out using the equipment shown in Fig. 7 which comprises a substantially sealed rig 100 having a powder generator 101 at one end for generating dust particles that may be blown into the rig 100 using ambient air. Standard dust made from alumina monohydrate, otherwise known as Pural NF, is used to ensure every batch has the same particle size distribution. The test media 102 is positioned downstream of the powder generator 101 and supported in such a way that it traverses the path of the dust particles inserted into the rig 100. A gravimetric filter 103 is provided at the end of the rig 100 remote from the powder generator 101 and serves as an absolute filter that may collect any dust particles that may pass through the test filter media 102. A suction pump (not shown) is connected to the gravimetric filter 103 to suck dust laden air into the filter 103 in the direction denoted by arrow A. A pressure tank 104 capable of generating pressures of up to 6 bar is connected to the rig 100 and is provided to generate a cleaning pulse to regenerate the media.

The tests are carried out in four stages; a conditioning stage, an ageing stage, a stabilising stage and a measuring stage.

The conditioning stage involves forcing dust-laden air from the powder generator 101 through the test media 102 at a constant speed of approximately 2.0 mIs. Approximately 5g/m3 of dust is projected into the rig 100 from the powder generator 101 toward the test media 102 which is substantially circular and has a diameter of around 140mm. The tests are carried out at a stable temperature of approximately 25°C. As the dust is filtered from the gas stream it collects on the surface of the test media 102 and impairs the flow of air. Since the air is forced to flow at constant speed, the resistance to flow caused by the dust build up means that an increasing pressure differential across the sample is required. This pressure differential is monitored and when it reaches 1000 Pascal (Pa), a pulse of compressed air is introduced into the system to clean the dust cake off the test piece 102. This cycle is performed a total of 30 times and takes typically a few hours. The longer it takes for the pressure differential to rise to 1000 Pascal, the longer it takes for the medium to become clogged with dust. Consequently, a better filtration medium is characterised by a longer conditioning cycle time.

The ageing stage follows the conditioning stage and involves maintaining the airflow at a constant rate. Pulse cleaning is then performed every 5 seconds for a total of 10,000 times. This part of the test takes almost 14 hours.

The stabilising stage follows the ageing stage. Ten further cycles are performed as in the conditioning stage to return the sample to cleaning dependent on pressure as opposed to cleaning at a fixed time interval.

During the final measuring stage, the system is run for a minimum of 2 hours with pulse cleaning once again occurring when the differential pressure rises to 1000Pa. This is the main part of the test for obtaining performance data as stages 1, 2 and 3 are intended to simulate normal filter operation so that the measuring effectively occurs with a "weathered" sample.

Throughout the test all parameters are monitored and one gravimetric filter per stage is used to collect any dust leaking through the test media. The dust collected by this absolute filter is weighed and then divided by the total volume of air that passed through the filter material to obtain a dust concentration. The dust concentration is used to assess the actual filter efficiency. The closer the clean gas concentration value is to zero, the less dust has passed through the test filter.

Therefore, a low clean gas concentration value is desirable for filter media.

The equipment used for the test can measure the amount of dust bleeding through the filter, how quickly it blinds (chokes with dust), how the pressure across the filter increases and how often it needs to be cleaned. The comparative results are shown in Table 1 below which indicate that a felt that has been improved in accordance with the present invention is ten to twenty times more effective at reducing unwanted emissions than regular PTFE needled felt and almost as effective as ePTEE which is currently regarded as the best practice available for minimising emissions. The results also show that a felt that has been improved in accordance with the present invention has an increased cycle time which means that the filter builds up dirt more slowly and will therefore use less energy in maintaining air flow and less compressed air in cleaning when the filter is compared with standard needle felts.

Needled Felt Subjected Property Needled Felt ePTFE Membrane to Water Jets Cycle Time Conditioning (s) 457 565 484 Cycle Time Measuring (s) 152 170 457 Clean Gas Concentration 4.5 0.22 0.0 Conditioning mg/Am3 Clean Gas Concentration 11.7 0.9 0.0 Measuring mg/Am3 Table 1-Comparative filtration efficiency test results In a further development, 50 g/m2 of polyester/nylon bi-component fibres that are engineered to be split are placed on the surface of a sheet of 350 g/m2 polyester felt. In this embodiment, the composite fibres comprise sixteen separate alternating nylon and polyester components which are joined together along their respective lengths to form the unitary fibre that is approximately 14 microns in diameter. The composite fibres are engineered such that, when split, the individual components are smaller in cross section than the original fibre.

The combined felt sheet and fibre layer is conveyed through the apparatus shown in Fig. 4 around the central cylinder 7 where the fibre layer is subjected to high pressure water jets directed from the injectors 9a, 9b. The impact of the water jets on the fibre layer causes the fibres to split into their individual nylon and polyester components. Thus, each fibre is split into sixteen thinner fibres each having a diameter of approximately 4 to 5 microns. The force of the water distributes the fibres substantially uniformly and entangles them into the body of the supporting felt layer, thereby anchoring the top layer to be supporting layer. The splitting of the fibres increases the number per square metre of the fibres on the surface of the felt by a factor of approximately 10. This increased number of finer fibres per square metre gives rise to a filtration medium that is far superior to a standard 400 gIn2 polyester felt.

Property 400 g/m2 Standard 350 g/m2 Standard Polyester Felt polyester Felt with 50 g/m2 Surface Layer Cycle Time Conditioning (s) 464 579 Cycle Time Measuring (s) 192 292 Clean Gas Concentration 2.78 0.74 Conditioning mg/Am3 Clean Gas Concentration 0.52 0.05 Measuring mg/Am3 Table 2-Comparative filtration efficiency test results The above test was carried out on a 350 g/m2 felt with a 50 g/rn2 layer of splittable fibres that have been subjected to high pressure water jets and an equivalent weight 400 g/m2 polyester felt. The test results are shown in Table 2 which indicates that the filtration properties of the combined felt and splittable layer are far superior to those of the standard equivalent weight polyester weight. Whilst nylon/polyester bi-component fibres are used in this embodiment, it is envisaged that other splittable fibres may be used in the above method to produce an improved filtration medium.

It is of course to be understood that the above embodiments have been described by way of example only and that many variations are possible without departing from the scope of the invention.

Claims (12)

1. CLAIMS1. A method of enhancing the filtration properties of PIFE felt comprising the steps of: providing a PTFE felt; and subjecting the felt to one or more jets of water in order to further entangle the PIFE fibres and split the PIFE fibres into finer fibres.
2. 2. A method as claimed in claim 1, wherein the PTFE felt is made by providing a plurality of PTFE fibres on a supporting scrim and pushing a plurality of needles through the fibres and scrim.
3. 3. A method as claimed in claim 1 or claim 2, wherein each jet of water is projected from one or more nozzles.
4. 4. A method as claimed in any preceding claim, wherein each water jet is projected at a pressure of between 50 bar and 500 bar.
5. 5. A method as claimed in claim 4, wherein or each water jet is projected at a pressure of between 100 bar and 400 bar.
6. 6. A method of manufacturing filter media comprising the steps of: providing a sheet of needled felt material; placing a plurality of loose fibres on the surface of the material; and subjecting the loose fibres to one or more jets of water to entangle the loose fibres with the material and split the loose fibres into finer fibres.
7. 7. A method as claimed in claim 6, wherein the needled felt comprises a scrim and one or more fibres that have been attached to the scrim via the action of a plurality of needles.
8. 8. A method as claimed in claim 6 or claim 7, wherein the loose fibres are deliberately engineered to be split by the jets of water.
9. 9. A method as claimed in any of claims 6 to 8, wherein the loose fibres comprise separate components.
10. 10. A method as claimed in claim 9, wherein the separate components are smaller in cross section than the original loose fibres.
11. 11. A method as claimed in claim 9 or claim 10, wherein the separate components comprise two different materials.
12. 12. A method as claimed in claim 11, wherein the two different materials are nylon and polyester.