GB2131717A - Air filter - Google Patents

Abstract

An air filter comprises sheets 12 of filter medium separated by corrugated spacers 16 to define alternate inlet and outlet channels 20 between them. The corrugations decrease in height along the length of the corrugations with adjacent spacers 16 tapering in opposite directions, so that the flow channels 20 are similarly tapered. The corrugations may vary in cross- sectional shape along the length of the corrugations, and may have flat sides or be defined by curved surfaces. Sheets 12 may be separate sheets glued together at 14 or a single folded strip. <IMAGE>

Description

SPECIFICATION Air filter This invention relates to air filters in which sheets of filter medium are separated by a plurality of spacers.

In a known form of air filter constructed from sheets of filter paper arranged parallel to the air flow direction, adjacent sheets are separated by a uniformly corrugated spacer to provide flow channels between the sheets in the direction of the air flow. Adjacent sheets are joined together at alternate ends of the air filter, so that the flow channels on opposite sides of a sheet are open at opposite ends of the air filter. In such an air filter, 50% of the area of each end of the filter is therefore obstructed. Furthermore each flow channel is of constant cross-section throughout its length although the volume of air flowing along the flow channel varies as air passes through the sheet of filter paper defining a side of the channel, the air flow being greatest at the open end of each channel and least at the closed end of each channel.

According to the present invention there is provided an air filter comprising sheets of filter medium, and corrugated spacers located between adjacent sheets to separate the sheets and to define, in combination with the sheets, flow channels between the sheets extending in the direction of air flow, the height of the corrugations decreasing along the length thereof so that the spacers and thereby the flow channels taper in longitudinal section, adjacent spacers tapering in opposite directions, and adjacent sheets being joined together at the narrower end of each flow channel.

The corrugations may be of constant crosssectional shape, varying only in amplitude, from one end of each spacer to the other. Alternatively the corrugations may vary in cross-sectional shape from one end of each spacer to the other. The sides of the corrugations may be flat, or may be defined by curved surfaces, and the sheets may be flat, or may be of arcuate form to form an annular air filter.

The invention will now be further described by way of example only and with reference to the accompanying drawings, in which Figure 1 is a longitudinal sectional view of part of an air filter, incorporating corrugated spacers; Figure 2 is a sectional view on the line Il-Il of Figure 1; Figure 3 is a perspective view of one of the corrugated spacers of Figure 1; Figure 4 is a perspective view of an alternative spacer to that of Figure 3; Figure 5 is a perspective view of another alternative spacer to that of Figure 3; and Figure 6 is a perspective view of yet another alternative spacer to that of Figure 3.

Referring to Figures 1 and 2, an air filter 10 comprises a plurality of rectangular plane sheets 12 of glass-fibre filter paper, adjacent sheets 12 being joined together by a layer 14 of glue at alternate ends of the air filter 10. Adjacent sheets 12 are separated by corrugated spacers 16, see Figure 3, of length 280 mm, the corrugations on each spacer 16 being parallel to each other and tapering uniformly in height from 7 mm at one end to 2 mm at the other end, adjacent spacers 1 6 on opposite sides of a sheet 12 tapering in opposite directions. The sheets 12 and spacers 16 define channels 20 for air flow through the air filter 10 which taper along their length and are closed at their narrower ends by the glue layers 14.Thus, the channels 20 through which air enters the filter 10 become narrower in the flow direction, and the channels 20 through which air leaves the filter 10 become wider in the flow direction.

Although the air filter 10 has been described as comprising a plurality of sheets 12 of filter paper joined at alternate ends by layers 14 of glue, it will be understood that a long strip of filter paper may be folded into a zig-zag shape so as to form the rectangular plane sheets 12, in which case the layers 14 of glue are not required.

The spacer 16 is of thermoplastic plastics material and is formed by deforming a rectangular sheet of the material between heated shaped moulds. It will be understood that the spacer 16 cannot be made by creasing a rectangular sheet of non-stretch material such as paper, as the distance along the surface of the spacer 1 6 between any two parallel creases is greater at one end than at the other end of the spacer 1 6. The spacer 16 may however be made of such nonstretch material by creasing a sector of an annular sheet whose inner and outer radii are in the same ratio as the heights of the corrugations at the ends of the filter 10, or by creasing a helical strip the lengths of whose inner and outer edges are in the above ratio.

It will be appreciated that, for some purposes, a corrugated spacer made by creasing a rectangular sheet of non-stretch material may be preferred, and in Figure 4 is shown a corrugate spacer 30 which may be used in place of the spacer 1 6 in the air filter 10 of Figures 1 and 2. At one end 32 of the spacer 30 the corrugations define a triangular wave of height 7 mm, and at the other end 34 the corrugations define an approximately rectangular wave of height 2 mm. The spacer 30 is shown to scale, and it will be observed that the horizontal distance between a peak 36 and an adjacent trough 38 (i.e. half the wavelength of the triangular wave) is twice the vertical distance between the peak 36 and the trough 38, which is 7 mm.

For a spacer similar to that of Figure 4 with corrugations which at one end define a triangular wave of height H and at the other end define a rectangular wave of height H/a, the horizontal distance between a peak and an adjacent trough of the triangular wave must be given by bH, where:

if the spacer is to be made by creasing a rectangular sheet of material. If for example the heights at the ends of the spacer are to be 7 mm and 2 mm, then a = 3.5 and hence b = 1.65. As stated above, the spacer 30 of Figure 4 has the value b = 2, and consequently the wave at the end 34 of the spacer 30 is not exactly rectangular, being defined by horizontal portions 40 each of width 13.6 mm, and portions 41 which are inclined at 11 10 to the vertical in Figure 4.Because the portions 41 are inclined from the vertical, the spacer 20 can be made by passing a rectangular strip between appropriately shaped rollers, which would be more difficult if the portions 41 were exactly vertical.

When the spacers 30 are used in the air filter 10 of Figures 1 and 2, in place of spacers 16, it will be appreciated that parts of the sheets 1 2 of filter paper are obstructed by the spacer 30. Each triangular plane sector 43 or 45 of the spacer 30, with its base defined by one of the horizontal portions 40 at the end 34, and its vertex at one of the peaks 36 or troughs 38 at the end 32 of the spacer 30, is touching and therefore obstructing an adjacent part of one of the sheets 12. This consequently reduces the surface area of filter paper through which air can pass through the filter 10.

Referring now to Figure 5, an alternative corrugated spacer 50 is shown which may be used in place of the spacer 1 6 in the air filter 10 of Figures 1 and 2 and which may be made by creasing a rectangular sheet of material. At one end 52 of the spacer 50 the corrugations define a triangular wave of height 7 mm, and at the other end 54 of the spacer 50 the corrugations define a double triangular wave of height 2 mm. At the one end 52 the horizontal distance between a peak 56 and an adjacent trough 58 is 5 mm so that each edge 57 is of length 8.6 mm; while at the other end 54 the lengths of consecutive edges 60, 61, 62, 63, 64, 65 which form a complete wave-form are respectively 2.06 mm, 4.48 mm, 2.06 mm, 2.06 mm, 4.48 mm and 2.06 mm.The spacer 50 is preferred to the spacer 30 of Figure 4 because a much smaller part of each of the sheets 12 of filter paper is obstructed by the spacer 50, the obstructed parts being those defined by triangles either with vertices at a peak 56 at the one end 52 and at adjacent peaks 66 at the other end 54, or with vertices at a trough 58 at the one end 52 and at adjacent troughs 68 at the other end 54.

A spacer similar to that shown in Figure 5, with corrugations which at one end define a triangular wave of height H and at the other end define a double triangular wave of height H /c, must have c equal to at least 3 if the spacer is to be made by creasing a rectangular sheet of material. In the case when c = 3, the edges which form the double triangular wave must all be of the same length.

It will be appreciated that spacers embodying the invention may take a wide variety of shapes in addition to those described above. All the spacers described have had corrugations formed by sharp creases, and at each end have defined waves consisting of straight edges. Spacers may also have corrugations formed by curved surfaces, with waves at each end defined by curved edges, and in Figure 6, to which reference is now made, is shown a spacer 70 which may be used in place of the spacer 1 6 in the air filter 10 of Figures 1 and 2. The spacer 70 has corrugations the overall height of which decreases from 7 mm at one end 72 of the spacer 70 to 2 mm at the other end 74 of the spacer 70. At each end 72 and 74 the corrugations define a generally sinusoidal wave shape, three waves at the end 74 extending across the same distance as one wave at the end 72.

Although the spacers have been described as being incorporated in the air filter 10 comprising a plurality of rectangular plane sheets 12 it will be understood that alternatively each sheet may be curved in transverse section into a spiral for example, so as to form an annular air filter.

Claims (2)

1. An air filter comprising sheets of filter medium, and corrugated spacers located between adjacent sheets to separate the sheets and to define, in combination with the sheets, flow channels between the sheets extending in the direction of air flow, the height of the corrugations decreasing along the length thereof so that the spacers and thereby the flow channles taper in longitudinal section, adjacent spacers tapering in opposite directions, and adjacent sheets being joined together at the narrower end of each flow channel.

2. An air filter substantially as hereinbefore described and with reference to Figures 1,2 and 3, Figure 4, Figure 5 or Figure 6 of the accompanying drawings.