FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENT RULES, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)
Title of Invention: AIR FILTER
APPLICANT: 4Energy Limited
A Great Britain national company
Having address:
Block B, Phase 2, Debdale Lane Industrial Estate
Keyworth, Nottingham NG12 5HN
United Kingdom
The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The invention relates to air filters for use in ventilation and temperature control of for example, electrical equipment enclosures.
Enclosures for electrical equipment typically require ventilation to allow the equipment to expel waste heat to an external environment to avoid overheating. In applications where electrical equipment is to be used in remote locations, for example in mobile radio telecommunications base stations, equipment is typically left unattended for extended periods. To ensure that the temperature of the equipment is kept within a preferred temperature range, additional cooling and ventilation apparatus is normally required. Depending on complexity, this cooling equipment consumes energy, which is both expensive and sometimes in limited supply, and also requires maintenance to avoid the costly implications of failure.
An alternative to the use of air conditioning or large fans in providing cooling for electrical equipment is disclosed in published PCT application WO 2009/090405. An exemplary embodiment from this application is illustrated in figure 1. comprising a room or enclosure 110 having an air filter unit 70 installed on an external wall 120 for filtering air being drawn into the enclosure (indicated by arrow 75) for cooling electrical equipment within the enclosure, for example within an equipment cabinet 121. The air filter unit 70 comprises a number of plates 71 having outwardly extending bristles, the plates 71 being mounted within the unit and aligned vertically, i.e. substantially in the direction of air flow through the filter 70. An air outlet 73 extends into the internal volume 115 of the room 110, allowing air to enter the room vertically downwards, in the direction indicated by arrow 76. Air passes through the filter unit 70 vertically upwards and then enters the internal volume 115 of the room 110 vertically downwards. In certain alternative embodiments an evaporator unit may be connected to the air outlet 73 between the outlet and the floor of the room .110, to provide additional cooling to air entering into the room.
Under normal operating conditions, a proportion of the dirt trapped by the filter bristles will fall out of the filter by the action of gravity alone, since the use of a large air inlet reduces the speed of air entering the filter to an extent that the particles are not continually drawn upwards through the filter. Although the above described air filter arrangement functions effectively to remove entrained particles from air entering the enclosure, occasional cleaning of the filter may still be required to ensure that the filter does not become clogged, which would reduce the filter's effectiveness. Cleaning may. for

example, involve a 'wash-down' of the filter material, which would typically involve at least partially disassembling of the filter unit. This would necessitate a site visit, which adds to the cost of maintaining the equipment enclosure.
A further problem with the above existing air filter unit is that of condensation within the equipment enclosure. Installing an air conditioner unit within the enclosure, for cooling air entering the enclosure when required, leads to condensation forming within the enclosure, which could cause problems with nearby electrical equipment and necessitates consideration of drainage in the enclosure.
A further problem relates to the complexity of installation, in particular the number of individual items to be installed to form a working cooling system and the number of holes required in the wall to which the air filter is mounted. To install air conditioning as an additional feature would require further holes through which pipes leading to and from a condenser unit would be required to pass, potentially forming points where condensation could gather.
A further problem relates to the volume available within the equipment enclosure, which would typically be quite limited and may not be sufficient to accommodate a bulky-unit incorporating an air inlet with inline air conditioning.
It is an object of the invention to address one or more of the above mentioned problems.
In accordance with a first aspect of the invention there is provided an air filter unit configured for attachment to an external wall of an enclosure, the air filter unit comprising: an inlet at a base of the unit;
a filter formed by a passageway extending from the inlet and lined with bristles extending across the passageway along at least a portion of the passageway length; an outlet; and
a heat exchanger disposed between the filter and the outlet such that air passing through the inlet and filter passes through the heat exchanger prior to exiting the unit through the outlet,
wherein the air filter unit is configured for attachment such that the heat exchanger is disposed above the filter in use.
An advantage of the above arrangement is that the heat exchanger, which would typically comprise an evaporator unit of an air conditioning system, cools the air that has passed through the filter prior to the air exiting through the outlet. Any condensation that

forms as a result will tend to fall back down on to and through the filter, eventually falling out of the outlet of the unit at the base. An additional advantage of this arrangement is that this condensation water can provide a cleaning effect on the air filter bristles, thereby reducing the requirement for servicing visits. The heat exchanger may also be configured in some embodiments to heat air passing through it. if required.
The evaporator is preferably configured to allow passage of air through the evaporator from a first surface to a second opposing surface. This arrangement ensures a maximum available area for passage of air through the filter unit, reducing any obstructive effect the evaporator has on air flow.
The evaporator preferably comprises a plurality of fins extending between the first and second opposing surfaces, to maximise the surface area available for exchanging heal with air passing through the unit.
The evaporator is preferably disposed directly above the filter to allow condensation forming on the evaporator to fall on to and through the filter.
The air filter unit optionally comprises a fan disposed between the outlet and the heat exchanger for drawing air through the filter and heat exchanger and expelling filtered air out of the unit through the outlet. Incorporating a fan into the unit reduces the number of components required to be installed when fitting the air filler unit, thereby reducing installation costs. In order to reduce the size of the unit, the fan is preferably a centrifugal fan. which is configured to draw air upwards through the filter and heat exchanger and expel air laterally towards the outlet and into the enclosure.
In accordance with a second aspect of the invention there is provided an electronic equipment enclosure comprising an external wall on which is mounted an air filter unit according to the first aspect of the invention, the air filter unit being oriented such that the heat exchanger is disposed above the filter. The heat exchanger is disposed above the filter so as to allow condensation to fall on to and through the air filter, to provide a cleaning effect as well as addressing the aforementioned problem of condensation within the enclosure.
In accordance with a third aspect of the invention there is provided a method of operating an air filter unit according to the first aspect, the air filter unit being affixed to an external wall of an enclosure, the method comprising: drawing air into the unit through the inlet: drawing the air upwards from the inlet through the filter and heat exchanger: and expelling the air from the unit through the outlet into the enclosure. The method preferably

comprises condensing water vapour from the air drawn into the unit on the heat exchanger to form condensation, the condensation falling from the heat exchanger downwards through the air filter and out of the unit through the inlet. The condensation thereby acts to clean the air filter as it passes downwards through the unit.
The invention will now be described by way of example, and with reference to the enclosed drawings in which:
figure 1 illustrates an embodiment of a known air filter unit mounted for use on an equipment enclosure;
figure 2a is a first perspective view of an exemplary air filter unit according to the invention;
figure 2b is a second perspective view of the exemplary air filter unit of figure 2a;
figure 3 is an elevation view of the exemplary air filter unit according to the invention: and
figure 4 is a cut-away perspective view of the exemplary air filter unit in an installed configuration.
The air filter unit and associated equipment enclosure shown in figure 1 has already been described above in relation to the background to the invention.
Figures 2a and 2b illustrate an air filter unit 20 according to the invention, figure 2a showing an external view of the unit 20 while figure 2b shows the internal components of the unit 20 that are not visible when in use. The unit comprises an air inlet 21 in the form of a grille, which allows air to pass freely (indicated by arrow 25a) into an inlet plenum 22 before entering an air filter 23 (indicated by arrow 25b). The large air inlet 21 allows air to enter the unit unimpeded at a low velocity, reducing the possibility of drawing in large dirt particles and allowing particles that become trapped by the filter 23 to eventually fall out of the unit while the unit is operational. The plenum 22 separates the filter 23 from the air inlet 21 to allow space for materia! falling from the filter 23 to gather without affecting the air flow through the unit 20.
The air filter 23 is composed of a vertically oriented stack of panels, each panel having bristles extending from one or both planar faces. The panels may for example be mounted within slots defined on the internal walls of the filter unit body or alternatively may be provided as a self-contained pre-assembled block for ease of assembly and

replacement. A particular preferred type of air filter is assembled from panels having artificial turf covering one or both faces.
Disposed above and adjacent to the air filter 23 is a heat exchanger 24, through which air passes after having been filtered and before exiting the unit 20. The heat exchanger 24 is preferably in the form of an evaporator unit, which is specifically configured to fit within the air filter unit such that air flows through the heat exchanger 24 vertically upwards (indicated by arrows 25c). Air exiting the heat exchanger 24 flows through a centrifugal fan 26, which forces the air laterally out of the unit (indicated by arrow 25d) through an outlet 28 and info the enclosure to which the unit 20 is attached (see figure 4, described below). The outlet 28 will in typical embodiments be smaller than the inlet 21.
An alternative internal elevation view of the air filter unit 20 is shown in figure 3. illustrating the relative positions of each of the components of the unit 20. The unit 20 comprises a flange 27 extending around an outer perimeter, which allows the unit to be mounted on to a flat external wall of an enclosure.
A cut-away perspective view of an air filter unit 20 according to the invention is shown in figure 4, with the unit 20 attached to an external supporting wall 41, for example of an equipment enclosure. In this embodiment, the air inlet 21 comprises a grille extending across a bottom face of the unit 20. which allows dirt and condensation to exit the unit as well as air to enter the unit. The centrifugal fan 26 is mounted on a support bracket 44 within an air exit plenum 45, and air is forced by the fan 26 into the enclosure through a hole 42 provided in the supporting wall 41. The hole 42 can be made substantially smaller than the inlet 2 i of the air filter unit 20, since there is no requirement for the air speed to be low once it has passed the filter 23 and heat exchanger 24. The filter 23 within the unit 20 is formed from a stack o( filter cards oriented parallel to the supporting wall 41.
Other embodiments are intentionally within the scope of the invention as defined by the appended claims.

CLAIMS
1. An air filter unit configured for attachment to an external wall of an
enclosure, the air filter unit comprising:
an inlet at a base of the unit:
a filter formed by a passageway extending from the inlet and lined with bristles extending across the passageway along at least a portion of the passageway length:
an outlet; and
a heat exchanger disposed between the filter and the outlet such that air passing through the inlet and filter passes through the heat exchanger prior to exiting the unit through the outlet,
wherein the air filter unit is configured for attachment such that the heat exchanger is disposed above the filter in use.
2. The air filter unit of claim 1 wherein the heat exchanger unit comprises an evaporator of an air conditioning system.
3. The air filter unit of claim 2 wherein the evaporator is configured to allow passage of air through the evaporator from a first surface to a second opposing surface.
4. The air filter of claim 3 wherein the evaporator comprises a plurality of fins extending between the first and second opposing surfaces.
5. The air filter of any of claims 2 to 4 wherein the evaporator is disposed directly above the filter to allow condensation forming on the evaporator to fall on to and through the filter.
6. The air filter unit of any preceding claim comprising a fan disposed between the outlet and the heat exchanger for drawing air through the filter and heat exchanger and expelling filtered air through the outlet.
7. The air filter unit of claim 6 wherein the fan is a centrifugal fan.
8. An electronic equipment enclosure comprising an external wall on which is mounted an air filter unit according to any preceding claim, the air filter unit being oriented such that the heat exchanger is disposed above the filter.
9. A method of operating an air filter unit according to any one of claims I to 7, the air filter unit being affixed to an external wall of an enclosure, the method comprising:
drawing air into the unit through the inlet;

drawing the air upwards from the inlet through the filter and heat exchanger; and
expelling the air from the unit through the outlet into the enclosure. 10. The method of claim 9 comprising condensing water vapour from the air drawn into the unit on the heat exchanger to form condensation, the condensation falling from the heat exchanger downwards through the air filter and out of the unit through the inlet.