GB2362585A

GB2362585A - Filter units for air supply systems

Info

Publication number: GB2362585A
Application number: GB0013017A
Authority: GB
Inventors: Mark Simon Ellis; Jeffrey Stamp
Original assignee: Pall Corp
Current assignee: Pall Corp
Priority date: 2000-05-26
Filing date: 2000-05-26
Publication date: 2001-11-28
Anticipated expiration: 2020-05-26
Also published as: GB0013017D0; GB2362585B; DE10125201A1

Abstract

A filter unit preferably for use in an aircraft cabin air recirculation system comprises an HEPA filter 12 having a rating of at least 99.99%, the filter 12 being without an enclosing housing, an outlet section 13 being provided downstream of the filter 12 for connection to a casing enclosing a fan 33 and the filter unit having a noise attenuation of at least 1.8 dB re<SP>-12</SP> watts across an octave band centre frequency of 63 Herz to 8000 Herz. The unit has a total pressure drop across the filter 12 of less than 11.5 mbar and a length of less than 510 mm. The filter 12 and sound deadening material in the outlet section 13 attenuate the noise from the fan 33. The filter 12 may be clamped to the outlet section 13 by a strap.

Description

2362585 1 AIR SUPPLY SYSTEMS The invention relates to filters and filter

units for air supply systems and, in particular, although not exclusively, for air supply systems for aircraft cabin recirculation systems.

In this specification, a reference to a filter having a particular rating expressed as a percentage is a reference to a sodium chloride retention rating in accordance with B S 3928.

A known filter unit comprises a filter through which air is passed by a fan. Air from a source passes through the filter of the filter unit which removes particulates in the air before the air is passed to the point at which it is used. In the case of an aircraft cabin recirculation system, the source of air is the cabin and the filter removes particulates before the air is passed back into the cabin after being conditioned.

Filter units, particularly those for aircraft cabin recirculation systems, have to meet a number of criteria. First, they must have a dust life that is acceptably long to prevent too frequent changes of the filter. "Dust life" refers to the accumulation of particulates within the filter and the need to change the filter once sufficient particulates have accumulated within the filter to cause an unacceptable pressure drop across the filter. Secondly, they must fit into a predefined space so that the maximum dimensions of 2 the filter unit are fixed. Thirdly, although some previous filter units have used filters with, for example, a rating of 65%, it is now required that HEPA filters are used having a rating of at least 99.99% so that the filter removes small particulates, bacteria and viruses as well as larger particulates.

In previously proposed air supply systems, the filter unit has been mounted in an enclosed housing, which provides significant noise attenuation, with an upstream separator. When, however, a filter unit is used incorporating an HEPA filter having a rating of at least 99.99%, it is necessary to provide a larger filter to provide an acceptable dust life, since an HEPA filter having a rating of at least 99. 99% will retain more particulates than a lower rated filter. Accordingly, there is no room for the housing or the separator and this tends to produce greater fan noise levels.

According to a first aspect of the invention, there is provided a filter unit comprising an HEPA filter having a rating of at least 99.99%, the filter being without an enclosing housing, an outlet being provided downstream of the filter for connection to a casing for enclosing a fan for drawing air through the filter, the filter unit having a noise attenuation of at least 1.8 dB re' watts across an octave band centre frequency of from 63Hz to 8000Hz.

As mentioned above, an HEPA filter having a rating of at least 99.99% will retain significantly more particulates than a lower rated filter. It is thus a problem, given the 3 inability to increase the length and diameter/width of the filter beyond certain predefined limits, to provide such an HEPA filter with an acceptable dust life.

One measurement that correlates to dust life is obtained by passing SAE fine test dust through the filter at a constant grams/sec (e.g. nominally 3 grams/sec) and then measuring the pressure drop across the filter, determining the dust life of the filter as the weight of dust passed to the filter when a predetermined pressure drop is achieved across the filter. The pressure drop can be measured at any point in the unit but an absolute measurement is the total pressure drop at the outlet side of the filter. An acceptable level of this pressure drop may be no more than 11. 5mbar.

According to a third aspect of the invention, there is provided a filter unit including an HEPA filter having a rating of at least 99.99% and having a total pressure drop across the filter of less than 11.5mbar with a dust feed of at least 600 grams, the filter having a length of less than 5 1 Omm.

The following is a more detailed description of an embodiment of the invention, by way of example, reference being made to the accompanying drawings in which:-

Figure I is a cross-section of an air supply system for incorporation into an aircraft cabin air recirculation system and including a filter unit and a fan assembly, 4 Figure 2 is a cross-sectional view of the filter unit of Figure 1, Figure 3 is a side elevation partly in section of a filter element of the filter unit of Figures 1 and 2, the section being on the line 3-3 of Figure 4, Figure 4 is an end elevation of the filter of Figure 3, Figure 5 is a sound power level graph for a fan of the fan assembly of Figure 1 without the filter unit (dotted line) for a fan of the fan assembly of Figure 1 with a known filter (chain dotted line) and for a fan of the fan assembly of Figure 1 with the filter of Figure 1 (continuous line), and Figure 6 is a graph plotting pressure loss in mbar across the filter of the filter unit of Figures I to 4 against total weight (in grams) of SAE fine test dust fed to the filter in an air flow of 0.35m'/sec.

Referring first to Figures 1-4, the system comprises a filter unit 10 and a fan assembly 11.

The filter unit 10, which is best seen in Figures 2, 3 and 4, comprises a filter element 12 and an outlet section 13. The filter element 12 comprises a filter 14 which is an HEPA filter having a rating of at least 99.99% formed from a filter material pleated and formed into a hollow cylinder. There may be from 100-140 pleats but preferably there are 122 pleats 5 pleats. The filter medium 14 may have a length of from 450 50Omm, preferably 50Omm 5 min. The outside diameter of the filter element 14 may be from 240-28Omm and is preferably 265mm 1Omm. The inside diameter of the filter medium 14 is preferably from 120-15Omm and more preferably 135mm 1Omm. The HEPA rated filter 14 is formed from randomly intertwined glass fibres having a base weight of 73g/m and is a filter material sold by Lydall Inc. as Grade 3428.

The filter 14 may be held between an inner core 15 and an outer cage 16. In addition, the filter medium is provided with end caps 17 The outlet section 13 comprises a cylindrical casing 18 whose axis 19 is coaxial with the axis 20 of the filter element 12. The casing 18 is sealed to the filter element 12 by an annular seal 21 that is attached to one end of the filter element 12 and to an end of the casing 18. The interior of the casing 18 is lined with an acoustic absorbent foam 22. The end of the casing 18 opposite the filter element 12 has an annular face 23 lying in a plane normal to the axis 20 and carrying a V band flange 25.

The filter element 12 is connected to the outlet section 13 by a clamping assembly including an end plate 26 which overlies the end of the filter element 12 opposite the casing 18 and lies in a plane generally normal to the filter element axis 19 and a 6 clamping strap 27 extending between the plate 26 and the outlet section 13 over the an exterior surface of the filter element 12. The mounting members may carry brackets, one of which is shown at 28 in Figure 2, by which the system can be supported from aircraft fuselage support points (not shown). As will be seen, the filter element 12 is not enclosed in a housing.

The fan assembly 11 includes a duct 29 having a male V band flange 30 which engages in the V band flange 25 of the outlet section 13 so that the axis 31 of the duct is coaxial with the axis 20 of the filter element 12 and the axis 19 of the outlet section 13. The duct 29 is lined with the sound deadening material 32. The duct 29 contains a fan 30 rotatable about the duct axis 3 1.

The system described above with reference to the drawings is incorporated in an aircraft cabin air recirculating system in which air from the cabin is led to the exterior of the filter element 12 and where the outlet to the duct 29 is connected to an air distribution system for feeding air to the cabin (both not shown). In use, the fan 33 runs continuously to draw air through the filter element 12 from the exterior surface to the hollow interior 34 and then passes the air through the outlet section 13 and the duct 29 to return it to the cabin. The filter unit described above with reference to the drawings is intended as a replacement for an existing filter unit and so the space for occupation by the filter unit is limited to a maximum length of 750mm and a maximum width of 270mm.

7 As mentioned above, the filter 14 does not have a surrounding casing and this allows the filter to be of increased size, in comparison with a filter enclosed in a casing and with an upstream separator, within the limited space available. The presence of a casing and a separator does, however, provide attenuation of noise from the fan. The presence of the HEPA filter 14 rated at at least 99.99% does not allow for the presence of a casing or an upstream separator and so this increases fan noise.

The filter unit 10 described above with reference to the drawings overcomes this problem in two ways. First, the filter 14 itself has good noise attenuation properties.

This noise attenuation is believed to arise from the nature of the filter material and/or the pleat arrangement and/or the dimensions of the filter. In addition, the presence of the sound deadening material 22 in the outlet section 13 also provides noise attenuation in conjunction with the filter 14.

Referring to Figure 5, the dotted line shows the sound power level of the fan 33 without any sound attenuation. The chain dotted line shows the sound power level of the fan 33 with a known filter encased in a casing and with an upstream separator in place of the filter unit 10 and the continuous line shows the sound power level of a filter unit 10 of the type described with reference to the drawings. As will be seen, 20 the sound power level of the air supply system of the type described above with reference to the drawings is significantly lower than that of both the fan 33 alone and the fan 3 3 with a known filter. As will be seen, the filter unit 10 described above with 8 reference to the drawings has a noise attenuation of at least 1.8 dB re' watts over an octave band centre frequency range of 6311z to 8000Hz. In particular, the filter unit may have a noise attenuation of at least 1.8 dB re- 12 watts over an octave band centre frequency range of 63Hz to 5 0011z and a noise attenuation of at least 10 c1B re- 12 watts over an octave band centre frequency range of 1000Hz to 8000Hz.

In addition, the filter 14 described above with reference to the drawings has satisfactory dust capacity for its size. The dust capacity of a filter is a measure of the mass of particulates that can be retained by the filter before the pressure drop across the filter reaches unacceptable levels. The filter 14, which is an HEPA filter with a rating of 99.99%, will retain more particulates per unit area than a lower rated filter.

The dust capacity of the filter 14 described above with reference to the drawings was tested in the following way. Air was passed through the filter 14 at 0.35m'/sec. An SAE fine test dust was passed into the airstream at a nominal 3 grams/second. The pressure drop across the filter was measured after periods of time corresponding to known weights of test dust, in this case at intervals corresponding to 100 grams of test dust. Two pressures were measured, a total pressure drop at the outlet of the filter 14 and a pressure drop at a tapping on the outlet section 13. The tapping is the point at which pressure drop is measured during operation for ease of access and is correlated to the total pressure drop.

9 Figure 6 shows the results of this test. The filter 14 is deemed to require change when the total pressure drop reaches 11.5mbar. This line is plotted on Figure 6 and it will be seen that the filter 14 only provides such a pressure drop after intercepting 659 grams of SAE fine test dust.

Claims

1. A filter unit comprising an HEPA filter having a rating of at least 99. 99%, the filter being without an enclosing housing, an outlet being provided downstream of the filter for connection to a casing enclosing a fan, the filter unit having a noise attenuation of at least 1.8 dB re' watts across an octave band centre frequency of from 63Hz to 8000Hz.

2. A filter unit according to claim 1 wherein the filter unit has a noise attenuation of at least 1.8 dB re-12 watts across an octave band centre frequency range of from 6.311z to 500Hz.

3. A filter unit according to claim 1 or claim 2 wherein the filter unit has a noise attenuation of at least 10 dB re' watts across an octave band centre frequency range of 1000Hz to 8000Hz.

4. A filter unit according to any one of claims 1 to 3 wherein the filter is formed from at least one pleated layer of filter medium formed into a hollow cylinder with an exterior surface and a hollow interior, the air passing from the exterior surface to the interior of the filter.

11 5. A filter unit according to claim 4 wherein the filter has from 100 to 140 pleats.

6. A filter unit according to claim 4 or claim 5 wherein the length of the filter is from 450 to 550mm.

7. A filter unit according to any one of claims 4 to 6 wherein the filter has an outside diameter of from 240mm to 280mm. and an inside diameter of from 120min to 150mm.

8. A filter unit according to any one of claims 1 to 7 wherein the outlet is lined with a sound absorbing material.

9. A filter unit including an HEPA filter having a rating of at least 99. 99% and having a total pressure drop across the filter of less than 11. 5mbar with an SAE fine dust feed of at least 600 grams, the filter having a length of less than 5 1 Omm.

10. A filter/filter unit substantially as hereinbefore described with reference to the accompanying drawings.

11. An aircraft cabin air recirculation system including an air supply system according to any one of claims 1 to 10.

12 12. An aircraft cabin air recirculation system substantially as hereinbefore described with reference to the accompanying drawings.