FILTRATION APPARATUS

Abstract: A filtration apparatus for removal of vaporised liquid contaminants from a hot gas including a housing having a gas inlet and a gas outlet, the housing defining a gas flow path between said gas inlet and outlet, cooling means located downstream of the inlet for cooling said gas flow to a temperature sufficiently low to achieve condensation of said vaporised contaminants, coalescing means located downstream of the cooling means and arranged such that the gas flow passes therethrough, and filter means located downstream of the coalescing means and arranged such that the gas flow passes therethrough, the coalescing means serving to cause the condensed vapour to coalesce and form droplets of a size which are sufficiently large to be removed from the gas flow by said filter means.

Description: FILTRATION APPARATUS The present invention relates to a filtration apparatus. It is a general aim of the present invention to provide a filtration apparatus of relatively simple design which is capable of removing contaminants from a flow of hot gas, in particular, a hot gas flow which is subsequently vented to atmosphere. The contaminants may be solids in particulate form, such as dirt or ash, and/ or solids/liquids in vapour form. These contaminants may include water, substances such as mineral oils, organic compounds such as paraffins, etc. In certain industrial processes ovens are used which create hot waste gases. These waste gases tend to contain contaminants and so cannot be recycled into the oven to save the heat requirement of the oven. Accordingly such ovens consume large quantities of energy; a large proportion of which is not utilised since it is exhausted to atmosphere. It is desirable therefore to provide a filtration apparatus capable of removing contaminants from the hot gas flow to enable the gas to be safely exhausted to atmosphere without causing pollution of the environment and also at the same time extracting heat from the gas flow to reduce energy costs. Whilst a wide variety of filtration apparatus are known for removing solid particulate material from hot dry gases great difficulties are encountered in successfully removing vapourised liquid contaminants, in particular oils and waxes. According to one aspect of the present invention there is provided a filtration apparatus for removal of vaporised liquid contaminants from a hot gas including a housing having a gas inlet and a gas outlet, the housing defining a gas flow path between said gas inlet and outlet, cooling means located downstream of the inlet for cooling said gas flow to a temperature sufficiently low to achieve condensation of said vaporised contaminants, coalescing means located downstream of the cooling means and arranged such that the gas flow passes therethrough, and filter means located downstream of the coalescing means and arranged such that the gas flow passes therethrough, the coalescing means serving to cause the condensed vapour to coalesce and form droplets of a size which are sufficiently large to be removed from the gas flow by said filter means. Preferably additional filter means are provided located between the inlet and cooling means and arranged to permit the hot gas and vaporised contaminants to pass therethrough whilst filtering out of the gas flow gas borne solid particulate contaminants. The cooling means preferably comprise a heat exchanger which is used to recover heat from the hot gas for use at a remote location. The cooling medium used in the heat exchanger may be a liquid, for example water, or a gas, for example air. Various aspects of the present invention are hereinafter described with reference to the accompanying drawings, in which :- Figure 1 is a schematic illustration showing an apparatus according to the present invention in situ Figure 2 is a longitudinal section through an embodiment according to the present invention Figure 3 is a front view of the embodiment shown in Figure 2 Figure 4 is a schematic perspective view, partly broken away, of the apparatus shown in Figures 2 and 3 Figure 5 is a schematic part plan view of the apparatus shown in Figure 4 Figure 6 is a non detailed schematic plan view of the filter elements shown in Figure 5 Figure 7 is a sectional view taken along line VII- VII in Figure 6 Referring initially to Figure 1 there is shown a stenter oven 10 which produces a hot gas exhaust 11. Carried in the hot gas exhaust are solid particles such as lint, ash etc and vaporised liquid contaminants. The liquid contaminants vary depending upon the material being processed in the stenter oven and usually comprise a mixture of oils, waxes and water. The exhaust 11 is fed to a filter apparatus 20 according to the present invention and the gas flow is cooled and filtered to remove the solid and vaporised liquid contaminants. The filtered gas 21 (in this case air) is then exhausted to atmosphere. The heat recovered from the hot gas is circulated via a circulation system 23 to a remote location 24 for use of the recovered heat. A specific example of a filtration apparatus 30 according to the present invention is illustrated in Figures 2 and 3. Apparatus 30 includes a housing 31 which defines a tubular passageway 32 having at one end an inlet 33 and having at the opposite end an outlet 34. As seen in Figure 3 the tubular passageway is generally square or rectangular in cross-section. An inlet cowling 33a and an outlet cowling 34a (Figure 4) are attached to the inlet 33 and outlet 34 respectively for providing a connection between the filter apparatus and a conduit along which the gas flow passes from the stenter to atmosphere. A filter 36 is located immediately downstream of the inlet 33 and serves to remove solid contaminants from the gas flow passing along the passageway. In the present example, hot exhaust gases from a stenter oven carry particles of ash and lint. Preferably the filter 36 comprises an array of separate filter elements 37 which collectively extend across the entire cross-section of the passageway 32. Each filter element 37 includes a wad of filter material 38 which is supported in a rigid frame 39. The filter material is chosen such that the hot gas and vapour contaminants contained therein pass therethrough but that the solid particles are filtered out. Preferably the filter material is chosen so as to have a progressive filtering effect in the direction of flow of the gas along the passageway ie its filtering effect progressively increases in the direction of flow of the gas. In this way the filter material does not become clogged so quickly. A suitable filter material is a glass fibre mat, in which the density of the mat increases across the depth of the mat; a suitable example is a CONFIGLASS/oil strip filter media pad. A series of cross-members 83 and uprights 84 are provided to provide a framework which acts as a support for the filter elements 37. Preferably the cross-members 83 include upper and lower channels 85,86 into which frames 39 slidingly fit. Accordingly each filter element 37 is removably mounted on the cross-members so as to enable them to be readily replaced. Depending on the temperature of the gas flowing through filter 36 it is possible for some of the liquid contaminants to condense on the filter 36. Such contaminants, for instance water, drain toward the bottom of the passageway 32. Located downstream of the filter 36 is a coalescing assembly 40. A cooling chamber 43 is provided between the coalescing assembly 40 and filter 36 in which cooling means 41 (shown in Figure 4) are located. The cooling means serves to cool the gas emerging from the filter 36 prior to its entry into the coalescing assembly 40. The main function of the cooling means is to reduce the temperature of the incoming gas down to a level at which condensation of the vaporised contaminants may occur. This has the effect of causing a physical change in the vaporised contaminants; namely a large proportion of the vaporised contaminants change from a vaporised form into a mist form ie. very fine droplets of liquid are created. A large proportion of the very fine droplets will coalesce on relatively cool surfaces and fall from the gas flow under gravity. To remove such liquid from the cooling chamber 43 the floor of chamber 43 is provided with drain openings (not shown) which permit the liquid to drain into a drain channels 46 located beneath housing 31. The remainder of the very fine droplets remain gas- borne and are carried by the gas flow into the coalescing assembly 40. The main purpose of the coalescing assembly 40 is to cause the gas borne fine droplets to coalesce and thereby become larger to enable them to be filtered out of the gas flow. In the present example, the coalescing assembly 40 comprises an array of coalescing elements 45 which collectively extend across the cross-section of the passageway; each coalescing element includes a box like frame 49 (Figure 4) supported upon a framework 50, the box like frame including porous medium through which the gas flow can pass. The framework 50 includes cross- members 51 and uprights 52, the cross-members 51 defining support channels into which the box-like frames 49 can be slidingly fitted. At least one side of the housing 31 is provided with a removable access door (not shown) to provide access to the support channels defined by the cross-members to enable the coalescing elements to be removed/inserted.The porous medium preferably defines a tortuous path of travel for the gas flow so as to encourage collision between the fine droplets and thereby encourage them to coalesce with one another. Preferably the porous medium comprises a fibre wad, for instance a hollow polyester fibre wad. The depth of the porous medium through which the gas needs to pass is chosen to be sufficient to achieve the desired size of coalesced droplets; .conveniently in the illustrated embodiment the wad is about 12 inches deep (about 30.5 cms) . An example of a suitable material is FILTAIR CC 1500 as supplied by Auchard Development Company Ltd. The fibre wad may be supplemented or replaced by a suitable wire mesh filter screen. In addition the depth of the coalescing assembly 40 and the amount of cooling created by the cooling means is chosen such that the temperature on the downstream side of the coalescing means is sufficiently cool for mineral oils to liquify and drain from the filter medium but sufficiently hot for waxes to remain in solution within the mineral oils. Thus any deposited wax is washed from the filter by the mineral oil. Accordingly on the upstream side of the assembly 40 liquids of a relatively high boiling point, predominantly water, tend to condense first and drain away. On the downstream side of the assembly 40 mineral oils and waxes in solution tend to collect and drain away. In the typical example described herein with reference to the drawings a gas flow rate of about 12,000 cubic feet per minute flows through the passageway 32 and the cooling means is arranged to achieve a temperature of about 40-50°c on the upstream side of the coalescing assembly 40. Gas flow through the apparatus is achieved by a suction fan 5 (Figure 4) Located downstream of the coalescing assembly 40 is a filter 60 for removing coalesced fine droplets from the gas flow which are still gas borne as well as fine particulate material. Preferably the filter 60 includes a first array of filter elements 62 and a second array of filter elements 64 located downstream of the first array. The filter elements 62 and 64 each include a box-like frame 89 the upper and lower sides of which are provided with a pair of projecting flanges which slidingly engage in pairs of upper channels 91 and lower channels 92 formed on cross- members extending across the pasageway 32. The first array of filter elements 62 include filter medium capable of removing the coalesced fine droplets. These tend to further coalesce when trapped in the filter medium and then gradually drain therefrom under gravity. Preferably the filter medium used in the first filter elements 62 is rated on ASHRAE 52-76 Test Standard at 90-95% efficiency and thereby removes droplets and particulate material in excess of about 0.5 microns in size. The filter medium in the first filter elements 62 is preferably a relatively thin sheet of wad material formed from a microfine glass fibre and reinforced with wire. As seen in Figures 5 to 7, the sheet 67 of wad material is folded to define a wave-like form extending across the width of the passageway 32 and in which the fold lines 66 defining the undulation of the wave-like form extend vertically. Located between each folded portion of the sheet is a support member 80 which is preferably on undulating plate formed from a relatively thin metal sheet such as aluminium. The undulations on the plate extend in the same direction as the air flow so that the surface of each folded portion is exposed to the air flow. Preferably the peaks of the undulations of the plate contact the opposed surfaces of the folded portions to provide suppport thereto and maintain them apart to provide a desired spacing for passage of the air flow at a desired flow rate. The folded sheet is located in a surrounding box¬ like casing 89 and the upper and lower edges of the folded sheet are each embedded in a sealing compound, such as a polyurethane adhesive, to sealingly connect these edges to the casing. Preferably the filter elements 64 are of a similar construction to filter elements 62 with the exception that the filter medium used is different. The filter medium used for the filter element 64 is preferably a glass fibre filter paper which is preferably 99.95% efficient at 0.3 microns. Thus the filter elements 64 are capable of removing from the air stream any particles of greater than 0.3 microns emerging from the filter elements 62. The filter paper may be of single thickness or provided with a support backing, for example a layer of the filter sheet material used in filter elements 62. Preferably the folds 64a on the downstream side of the filter elements 64 are adapted to be impervious to the air flow, for example, the folds may be coated with an impervious material such as a suitable paint or varnish. This has the effect of the fold creating a vertical channel down which condensed oils, waxes etc can drain away. Treatment of the folds in this way also strengthens the filter paper in the area of these folds and helps to resist perforation during use. It is envisaged that in -certain applications, for example where large quantities of fine particulate material is contained in the air flow, the second array of filter elements 64 may be replaced by an electrostatic filter, eg a dry plate type. A final filter 100 is conveniently provided to catch air borne droplets which may have become entrained in the air flow after draining from the filter 60. The filter 100 preferably comprising a woven wire mesh. Preferably, flow control means are provided for shutting off air flow through a region of the filter 60 so that liquids held in the region closed off are able to drain away. The flow control means operate to cyclically change the region shut off so that during a cycle all regions of the filter are successively closed off for a predetermined length of time. In the embodiment illustrated, the flow control means includes a pair of valve elements 100 defined by elongate plate members 101 each of which is supported at opposite ends on a pair of continuous chains 110. The chains 110 are entrained about sprocket wheels 112 mounted on shafts 114. One of the shafts 114 is driven by drive means (not shown) so that one valve element passes either intermittently or continuously at a slow rate of travel, across the upstream face of the array of filter elements 62. The valve elements are spaced apart such that when one valve element exits from one side of the array of filter elements 62 the other valve element eters the opposite side. If desired more than one pair of valve elements may be provided such that at any one time more than one valve element is acting to shut off air flow to the array of valve elements 62. In order to constrain the air flow through the filter 60 and thereby ensure that the region immediately downstream of a valve element is starved of air flow, the support plates are chosen to be imperforate and thereby act as baffles to constrain air flow in a longitudinal direction of the passageway. In the region between the arrays of filter elements 62 and 64 a series of vertically extending baffle plates 120 are provided to restrain air flow between the arrays of filter elements 62 and 64 in a longitudinal direction. The spacing between the baffle plates 120 is preferably chosen to be the same as or less than the width of the valve element 100. The flow control means not only allows fluids to easily drain from the filter 60 during use but also provides a benefit when the apparatus is started from a cold condition after a shut down. In this respect, since the region downstream of the valve element 100 has been permitted to drain whilst in a hot condition, solidification of waxes is less likely to occur in this region when the apparatus is shut down. Thus on start-up from cold, air-flow through at least this region is permitted thereby enabling the apparatus to operate at possibly a reduced level until normal working temperatures are reached. To assist drainage of liquid from the array of filter elements 64 in the filter 60 it is envisaged that the filter medium can be coated with a substance having a low co-efficient of friction which thereby resists the liquid contaminants adhering thereto.

Claims: CLAIMS 1. A filtration apparatus for removal of vaporised liquid contaminants from a hot gas including a housing having a gas inlet and a gas outlet, the housing defining a gas flow path between said gas inlet and outlet, cooling means located downstream of the inlet for cooling said gas flow to a temperature sufficiently low to achieve condensation of said vaporised contaminants, coalescing means located downstream of the cooling means and arranged such that the gas flow passes therethrough, and filter means located downstream of the coalescing means and _ arranged such that the gas flow passes therethrough, the coalescing means serving to cause the condensed vapour to coalesce and form droplets of a size which are sufficiently large to be removed from the gas flow by said filter means. 2. A filtration apparatus according to claim 1 wherein additional filter means are located between the inlet and cooling means and are arranged to permit the hot gas and vaporised contaminants to pass therethrough whilst filtering out of the gas flow gas borne solid particulate contaminants. 3. A filter apparatus according to claim 2 wherein the additional filter means includes a glass fibre mat which has a density which progressively increases in the direction of flow of gas. 4. A filter apparatus according to claim 1, 2 or 3 wherein the coalescing means comprises a medium defining a tortuous path for the gas to encourage collision between fine droplets of gas borne liquid contaminants, the medium also permitting condensed liquids to drain therefrom. 5. A filter apparatus according to claim 4 wherein the temperature of the gas emerging from the coalescing means is arranged to be sufficiently low to enable mineral oils to condense out but sufficiently high to enable waxes to be maintained in solution in the mineral oils. 6. A filter apparatus according to any preceding claim wherein the filter means located downstream of the cooling means includes a first filter assembly having a filter medium capable of removing particulate material in excess of 0.5 microns. 7. A filter apparatus according to claim 6 wherein the filter means includes a second filter assembly located downstream of the first filter assembly, the second filter assembly comprising a filter medium capable of removing particulate material of a size in excess of 3 microns. 8. A filter apparatus according to claim 6 wherein the filter means includes a second filter assembly located downstream of the first filter assembly, the second filter assembly comprising an electrostatic filter. 9. A filter apparatus according to any preceding cliam including flow control means for cyclically isolating regions of the filter means from the flow of gas through the housing and thereby enabling liquid to drain from an isolated region.

Inventors: GENT DAVID VICTOR, GENT, DAVID, VICTOR
Applicants: ENVIRONMENTAL POLLUTION CONTRO, ENVIRONMENTAL POLLUTION CONTROL LIMITED
Application year: 1990
Main classification: B01D50
IPC codes: B01D46, B01D50, B01D51, F26B25