EP2980500A1

EP2980500A1 - Air cycling fan unit silencer and method of silencing an air cycling fan unit

Info

Publication number: EP2980500A1
Application number: EP15177510.3A
Authority: EP
Inventors: Phil GILLATT
Original assignee: SilentAire Group Ltd
Current assignee: SilentAire Group Ltd
Priority date: 2014-07-28
Filing date: 2015-07-20
Publication date: 2016-02-03
Also published as: GB201413324D0; GB201500593D0; GB2528734A; GB2528686A

Abstract

An air cycling fan unit silencer suitable to be placed at an outlet and/or inlet of a air cycling fan unit, wherein the silencer includes a first air flow changer. Furthermore, a second air flow changer may be spaced from the first air flow changer and arranged to create a plurality of substantially non-linear air flows. Although the non-linear air flows create turbulence, which is typically a source of noise generation at a frequency of around 500Hz - 2000Hz, advantageously it has been found that by forming one of the air flow changers and preferably both air flow changers from a sound absorbing material such as acoustic foam, the sound generated by air movement on the supply side of the unit can be substantially reduced.

Description

The present invention relates to a silencer for an air cycling fan unit and in particular, though not exclusively, to a silencer that can be retro-fitted to an existing air cycling fan unit.
Air cycling fan units are known. Types of air cycling fan units include air handling units, air conditioning units, heat reclaim units and ducted fan coil units, for example. In particular, air conditioning units are widely known. Typically, an air conditioning unit comprises a housing that houses a fan to pull air through an inlet, over a heat exchanger and out through an exit. The cooled air is expelled through the exit providing air conditioning to a room. Operation of the air conditioning unit generates noise through movement of the various parts and air turbulence. It is known to silence the air conditioning unit by insulating the housing and reducing turbulence.
Air conditioning units are widely used in hotel rooms with a single unit in each room that circulates the air within the room. However, some air conditioning units can be too noisy for guests, particularly since the small size of the units means smaller fans must be used that operate at higher speeds creating greater air movement and therefore greater noise. It is often not practical for hotels to replace noisy air conditioning units and so a need exists for a method of silencing existing air conditioning units within a confined space and without easy access to the air conditioning unit itself. Typically, hotel rooms have around 0.15m between the air-conditioning unit and the bulkhead. Standard techniques of silencing the unit by fitting a long plenum are not possible. Here, the plenum is typically a box like structure with insulated sides.
It is an object of the present invention to attempt to overcome at least one of the above or other disadvantages. It is a further aim to provide a silencer for an air cycling fan unit that does not involve using long sound proofed ducting.
According to the present invention there is provided an air cycling fan unit silencer as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.
An air cycling fan unit silencer suitable to be placed at an outlet and/or inlet of an air cycling fan unit, such as an air conditioning unit or the like, wherein the silencer includes a first air flow changer having apertures to allow air to pass through wherein the density of the apertures to the area of the air flow changer is between about 25% and about 45%. The aperture density is defined as the proportion of total aperture area to total air flow changer surface area on which the apertures are disposed. Therefore, in this instance, the total aperture area of the air flow changer is between 25% and 45% of the total air flow changer surface area on which the apertures are disposed. The term air flow changer may be described as an air flow restrictor, manipulator, disturber or baffle that is suitable for redirecting, manipulating, disturbing or impeding the flow of air, particularly when the inlet and outlet are coaxial. Therefore, these terms may be used interchangeably because the function of the air flow changer is the same. It is believed that the or each air flow changer does not reduce the volumetric air flow rate through the air cycling fan unit relative to an air cycling fan unit without any air flow changers. This is because the air flow changer re-directs air in order to elongate the air flow path. This prevents the build-up of back pressure on the fan because air can smoothly transfer through the air cycling fan unit. Although the air flow changer will inevitably cause some turbulence, it is believed that the air flow changer also helps to reduce eddy currents and swirling motion within the air cycling fan unit, in order to reduce noise. Therefore, it is the motion of air that is affected by the air flow changer, rather than the volume flow of air. The silencer may include a second air flow changer spaced from the first (e.g. by a gap). The first and second air flow changers may be arranged to create a plurality of substantially linear air flows or may be arranged to create a plurality of substantially non-linear air flows. Although the linear or non-linear air flows create turbulence, which is typically a source of noise generation at a frequency of around 50Hz - 2000Hz, advantageously it has been found that by forming one of the air flow changers and preferably both air flow changers from a sound absorbing material such as acoustic foam, the sound generated by air movement on the supply side of the unit can be substantially reduced.
Acoustic foam is a term of art used to describe materials that absorb sound from turbulent air and airborne sound waves. Air movement noise is typically around 500Hz to 2000Hz. Materials can be classified with a sound absorbing coefficient. The more sound the material absorbs the better the sound silencing properties. The acoustic foam can be coated or covered with fabric to improve the sound absorption properties. The minimum sound absorption coefficient for the absorbing material is between 0.1 at 100Hz and 0.94 at 4kHz. The sound absorption coefficient is measured in accordance with EN 150 11654:1997.
In the exemplary embodiments, ducting extends from the inlet / outlet to create an enclosed duct having substantially the same and consistent cross-section to the inlet / outlet. The inlet and outlet may not be orthogonal to each other in order to improve flow. For example, the inlet and outlet may be substantially coaxial. In addition, an air flow changer may be arranged at the inlet and another air flow changer at the outlet of the duct. As will be appreciated, advantageously, due to the arrangement of the air flow changers, the length of the ducting can be substantially reduced from known plenums and can typically be less than 0.3m or less than 0.25m or less than 0.2m in length. The ducting can be lined with sound insulating material such as acoustic foam as with known plenums. At least one air flow changer may be a plate that extends across the air ducting. That is, the at least one air flow changer may extend typically orthogonal to the sides of the ducting. The at least one air flow changer may be housed within the ducting, that is the at least one air flow changer may be contained within the ducting and extend radially from the axis. The air flow changers therefore create turbulence by allowing air to freely pass through apertures formed through the air flow changers, whilst substantially restricting air movement through parts of the air flow changer without apertures. Alternatively, or in addition, the air flow changers may be substantially parallel such that the degree of inclination between a surface of the air flow changer with respect to an adjacent surface of an adjacent air flow changer is less than 1 %. That is, the air flow changers may be provided in series and arranged side-by-side. The movement of the air flow changers may be restricted with respect to the ducting (i.e. the housing) so that the air flow changers do not move along the longitudinal axis of the ducting and are fixed in place and are anchored to the ducting. This provides increased stability, reduces the complexity of the unit and also avoids unnecessary restrictions within the air flow. The density of the holes to the cross sectional area of the duct is preferably between around 40% and 10%. Preferably, the density of the holes to the cross sectional area is around 20-40%. In the exemplary embodiments the air flow changers are plates sized to fit in the ducting and formed with apertures having a combined area of around 10% to 40% of the plate and preferably between 20% to 25%.
The aperture through the air flow changers can be formed from in a number of various ways. Preferably, the apertures within each air flow changer and between each air flow changer are arranged axially within the ducting. That is, each aperture within the unit is orthogonal to the longitudinal direction of the ducting and parallel to each other aperture. Preferably, each quarter of the air flow changers when viewed as a cross section orthogonally through the duct has an aperture density of not less than around 50% of the quarter having the greatest aperture density. More preferably, the aperture density in each quarter may be at least 5% or 10% less than the quarter having the greatest density. The area may be divided into quarters for the aperture density by imaginary dividing lines that separate the air flow changers at equal distances along one axis, along the other axis or along both axes. Preferably the estimation of aperture density is approximately correct for two of the methods of separating the cross-sectional area into quarters or yet more preferably all three. Notwithstanding the approximation, it is preferable if the arrangement of the apertures across at least one air flow changer or the air flow changers is random. That is, there is no discernable geometric pattern to the layout of the apertures and each aperture has a different pattern. Preferably, the aperture arrangement does not have reflectional, rotational or translational symmetry. For example, the aperture arrangement may not be symmetrical about a bisectional plane, despite the air flow changer as a whole being substantial symmetrical either side of said plane. Therefore, although at least one air flow changer may, as a whole have reflectional symmetry, the aperture arrangement may not. Furthermore, between air flow changers the pattern may be different, that is, no aperture pattern may be the same or within ±2% or ±5% misalignment with each another.
Although it can be envisaged the aperture having a complex shape, in the exemplary embodiments, the aperture is formed from a series of discrete apertures. Here, each aperture has an area of ± 50% of the mean aperture area, but preferably the aperture areas are substantially equally sized. At least one aperture may be different in size. For example, the apertures adjacent the perimeter or at opposite sides of the perimeter may be at least 5% larger than the central apertures. Advantageously, a better distributed air flow may be provided. In the exemplary embodiments, 5 to 10 apertures are provided. Again, although a complex aperture outline is possible, in the exemplary embodiments a geometric shape such as a circle or a square is used. Preferably a circular hole is formed or drilled or punched, or cut to form each aperture.
In the exemplary embodiments, the at least one of the air flow changers may have a thickness in the direction of the air flow of at least 0.02m or at least 0.04m and preferably around 0.05m. The air flow changers may be sheet-like. That is, the thickness of the air flow changers may be less than 10% of the width or height. As mentioned, the air flow changers are suitably formed from acoustic foam. The first and second air flow changers are separated by a gap. The gap has a cross sectional area substantially the same as the ducting. The gap allows the air to mix. Preferably, the gap is an airspace and is free from restrictions. That is, the gap may not contain filters or a layer of material that hinders airflow. Preferably, the gap spaces the first and second air flow changers between about 90% and about 110% of the thickness of at least one air flow changer, or both air flow changers when the air flow changers have substantially the same thickness. The gap may longitudinally (i.e. axially) space the air flow changers. Alternatively, the gap may have a thickness of at least 190% of the thickness of the or each insulated or non-insulated air flow changer and preferably around 200%. Extending the gap distance increases the sound insulation factor, but the affect is limited above around 300% of the thickness of the air flow changers. The area of the apertures in one air flow changer may be substantially aligned with the aperture area of the other air flow changer to thereby create linear flow. Alternatively, the apertures through the first air flow changers and second air flow changers may not be substantially aligned to thereby create the non-linear flow. That is (for non-linear flow), when viewed in the direction of the air flow, the apertures in the first air flow changer substantially cover closed areas of the second air flow changer. In the exemplary embodiments, some overlap may be acceptable. It has been found that the overlap should be kept below 30% of the aperture area. That is, when the first air flow changer is overlaid on the second air flow changer, 70% or more of the aperture area of the first plate should overlay closed areas of the second air flow changer. Preferably, in the embodiments having a plurality of discrete apertures, less than 30% of each aperture should overlay an aperture of the second air flow changer. Preferably, at least one aperture is aligned with another aperture by at least 10% of the aperture area. Additionally, or alternatively, the amount of overlap of each aperture is different. The range of overlap between the minimum overlap to the maximum overlap may be at least 5%. Furthermore, at least one aperture on an air flow changer (or on each air flow changer) may overlap with a plurality of apertures on another air flow changer. Therefore, an aperture may have two regions of overlap.
In one exemplary embodiment a third air flow changer is provided spaced from the second. The relationship between the second and third air flow changers is preferably substantially the same as the relationship between the first and second air flow changers.
A method of silencing an air cycling fan unit is provided. The method comprises fitting an air cycling fan silencer as described to an inlet and/or outlet of an air cycling fan unit. The method may further comprise installing the silencer in a plenum of an existing air cycling fan installation.
In the exemplary embodiments, the supply of air into the room is expelled through a vent grill. The vent grill may be integral to the second air flow changer, or may be a decorative face plate. Consequently, according to a further exemplary embodiment, there is provided a vent grill for an air cycling fan or air-conditioning duct. The vent grill is arranged to be connected to air cycling fan ducting and have a generally planar construction in a main plane. The vent grill comprises a plurality of apertures arranged through a front face to provide a plurality of fluid passageways between a rear surface and the front face. Advantageously, the front face is arcuate such that an approximate plane of the front of one aperture is angled with respect to an approximate plane of the front of an adjacent aperture. This has been found to provide an improved air flow distribution into the room. In particular, an arrangement of adjacent apertures at an angle to each other such that an aperture towards an edge of the front face is arranged at a greater angle to the main plane than an aperture spaced further from the edge has been found to provide a divergent air flow distribution. Consequently, when installed in a room without direct, linear directionality from the air cycling fan ducting to the area to be cooled, use of the improved vent grill enables a divergent flow to be achieved and thereby reaching the area more effectively.
It is envisaged that the vent grill would require improved distribution in one direction only. Typically when installed this would be in a horizontal direction. However, should the need arise, the vent grill could be arcuate in two orthogonal planes, wherein the approximate planes of adjacent apertures in both directions are angled.
In the exemplary embodiments, the apertures through the front face of the vent grill may be a grill or mesh but are preferably discrete apertures. Preferably, the apertures have a shape and area as described above. When the vent grill is separate to the second air flow changers, the apertures in the vent grill may have a similar overlapping relationship with the second air flow changers as described above. Preferably, for aesthetic reasons, the apertures may be uniformly spaced on the vent grill.
In the exemplary embodiments, the rear face of the vent grill is also arcuate. Suitably, the thickness of the front face is constant. For instance, the vent grill may be pressed or moulded from a flat sheet, though other forming processes are envisaged. Again, the apertures may be stamped or machined prior to forming the arcuate face or may be formed as part of the forming process.
In a further exemplary embodiment, a vent grill for use with an air cycling fan unit is provided. The vent grill may have a generally planar construction in a main plane and include a plurality of apertures arranged through a front face to provide a plurality of fluid passageways between a rear surface and the front face, wherein the front face may be arcuate such that an approximate plane of the front of one aperture is angled with respect to an approximate plane of the front of an adjacent aperture.
Preferably, the density of the apertures to the area of the vent grill is between 10% and 30%. Preferably, a quarter of the air flow changer has an aperture density of not less than 50% of a quarter having the greatest aperture density. Preferably, each aperture is substantially equally sized.
According to a further exemplary embodiment an air cycling fan unit is provided including a silencer as described and / or a vent grill as described.
For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which:

Figure 1 is a plan schematic view of an air conditioning arrangement;
Figures 2 and 3 are plan views of first and second air flow changers, respectively;
Figure 4 is a plan view showing the first and second reflectors overlaid;
Figure 5 is a plan view of a vent grill;
Figure 6 is a plan view of the vent grill of Figure 4;
Figure 7 is a table showing experimental data obtained from testing an air conditioning arrangement having a linear air flow path between air flow changers and, wherein each air flow changer (supply and/or return) comprises a single air flow changer;
Figure 8 is a table showing experimental data obtained from testing an air conditioning arrangement having a non-linear air flow path between air flow changers and, wherein each silencer (supply and/or return) comprises two air flow changers;
Figure 9 is a chart highlighting the noise reduction effect with airflow for different air conditioning arrangements comprising both supply and return silencers;
Figure 10 is a chart highlighting the noise reduction effect with airflow for different air conditioning arrangements comprising supply silencers only; and
Figure 11 is a chart highlighting the noise reduction effect with airflow for different air conditioning arrangements comprising return silencers only.

Referring to Figure 1, an exemplary air cycling fan unit is explained in relation to air conditioning arrangement and comprises an air conditioning unit 100, a supply silencer 200, a return silencer 300 and a vent grill 400. The air conditioning unit 100 is a standard unit that typically includes an air flow means 101, an inlet 102 and an outlet 104, and a heat exchanger 105. The air flow means 101 draws air in to the unit through the inlet 102, over the heat exchanger 105 and expels the air through the outlet 104. Silencers 200, 300 are attached to the outlet 104 and inlet 102 respectively. The silencers control the air flow to provide sound damping in a room being conditioned. The supply silencer 200 reduces air turbulence directly emitted in to the room. Applying a silencer to the return air reduces the noise level of the air travelling through the air conditioning unit 100 and therefore has an effect on the noise experienced within the room being conditioned.
It is envisaged that the silencers may be fitted as integral parts of new air conditioning units. However, the silencers are particularly suitable to be retro fitted to existing air conditioning units where access to the air conditioning unit 100 is not practical and where space is limited or at a premium for plenum ducting. Consequently, the silencers are particularly suitable for retrofitting within the hotel industry. Hotel room layouts are reasonably standard and typically use standard fan coil air conditioning unit packaged in a 0.2x0.7x0.62m envelope and housed in the ceiling space above the entrance hallway. Air is drawn in from the corridor and expelled out to one side of the room relative to the bed. Space for outlet or inlet ducting is limited by the bulkhead and is typically less than 0.3m at either side. Traditional box ducting with acoustic foam applied to each internal face of the ducting is not effective over such short distances.
Silencers 200 and 300 are substantially the same and so a description of only silencer 200 is given below. Silencer 300 is shown as having an additional gap between the silencer and inlet 102 as extra space is provided. However, a gap between air flow changers may also be increased. However, extending the gap to be greater than seven or eight times the thickness of air flow changers does not seem to have significant affect on the sound decrease.
Silencer 200 includes a first air flow changer 210 and second air flow changer 220. The air flow changers are spaced from each other to be separated by a gap 230. Air from the air-conditioning unit is passed through the first air flow changer 210 into the gap 230. The air continues to pass from the gap 230 through second air flow changer 220. The air flow changers cause turbulence air flow within the gap. The gap 230 provides an expansion for the air after exiting the first air flow changer. The air flow changers are arranged to provide a nonlinear flow path, relative to the flow direction of the air approaching the first air flow changer. As the airflow approaching the first air flow changer will typically be orthogonal to the air flow changers, the first and second air flow changers are arranged to not substantially provide a straight fluid path across the gap.
Referring to Figures 2 and 3, the non-linear air flow is achieved using off-set apertures through each airflow air flow changer. Whilst the apertures may be a single aperture formed in a complex shape, in the exemplary embodiments, six or seven equally sized apertures have been found to be particularly suitable. It is believed, the apertures need to be a reasonable size to avoid creating too much turbulence and therefore noise that the system can't absorb. However, the apertures cannot be too large as otherwise they would not create the turbulent flow into the gap. In the exemplary embodiments, the apertures are between 1.5% and 4.5% of the area of the air flow changers and preferably around 3.6%. Although any number of shapes is envisaged, the apertures are shown in the figures as being circular having a diameter between 0.09m and 0.05m and preferably around 0.08m based on a rectangular air flow changers having dimensions of 0.7m by 0.2m.
The apertures are randomly spaced but preferably have a generally even spread so that each quarter of the air flow changer has an aperture to area ratio of not less than around 50% of the aperture to area ratio of each of the other quarters. Here, the air flow changer can be divided into quarters by imaginary lines draw orthogonal to an axis and having equal areas. The imaginary lines may be drawn along a first axis, or a second axis or may split the air flow changers in to four by imaginary lines along the centre of the airflow or the air flow changer in both axis.
The apertures of the exemplary embodiments are arranged to not substantially overlap in order to achieve the required non-linear flow, see Figure 4. By overlap, it is meant that when the two air flow changers are viewed from the direction of airflow approaching the first air flow changer, that is, perpendicular to the first air flow changer, the apertures in the first air flow changer are arranged to cover portions of the second air flow changer. Some overlap of the apertures 219 is permissible whilst still achieving the non-linear air flow. It has been found that up to 30% overlap is allowable. That is up to 30% of the aperture area or of each aperture in one air flow changer is able to overlie a respective aperture in the other air flow changer.
The air flow changers are formed from a sound absorbing material. In the exemplary embodiments, acoustic foam having a thickness of 0.05m was used and a sound absorption coefficient for the absorbing material is 0.18 at 100Hz, 0.44 ay 250Hz, 0.84 at 500Hz, 0.94 at 1000Hz, 0.87 at 2000Hz and 0.94 at 4000Hz. The sound absorption coefficient is measured in accordance with EN 150 11654:1997.
The gap 230 allows turbulent air to flow between the apertures of the air flow changers. The gap extends the full cross section of the ducting. The width of the gap has been found to have an impact on the silencing function. If the gap is too small it is thought that the air does not have sufficient space to be turbulent and for sound absorption. If the gap is too large, the air is not sufficiently turbulent at the second air flow changer to be absorbed. It has been found that the gap having a thickness of between 190% and 300% of the thickness of the first air flow changer provides good sound absorption and a thickness of around 200% is particularly suitable.
In one exemplary embodiment, referring to silencer 300 of Figure 1, an air flow changer 330 is provided after the first and second air flow changers 310, 320. The third air flow changer is substantially the same as the first and second air flow changers in terms of form and apertures and suitably has the same relationship with the second air flow changer as the second air flow changer has with the first air flow changer. The first and third air flow changers may have the same layout of apertures or may be different again. That is the first and third air flow changers may be substantially the same part separated by the second air flow changers with non-overlapping apertures to create non-liner flow between the first and second and second and third air flow changers.
It has been found that by utilising the silencers 200, 300 as described above, the operating noise of a standard air-conditioning unit can be reduced by around 10dB.
Referring to Figure 1, a vent grill is shown on the exit of the supply side to the room. The vent grill is used to provide a decorative face plate to the exit of the air conditioning plenum and typically covers the join between the wall finish and opening for the air-conditioning plenum. It will be appreciated therefore that the vent grill 400 includes appropriate fixings and mountings as required. For instance, the vent grill may include a flange 410 arranged to fix flush to the wall. The flange therefore extends orthogonally to the flow path of the plenum. Referring to Figures 5 and 6, the exemplary vent grill is described. Importantly, the vent grill includes a non-planar front face that has an arcuate profile. The arcuate profile is shown as being relative to an installed, horizontal direction. However, the arcuate profile may equally be arranged to be relative to the vertical direction or both. The result of arranging the front face to be arcuate is to achieve a more suitable directionality on the expelled air. With a planar front face, the air is directed in a substantially parallel fashion. In contrast, by arranging the front face to be arcuate, the air is caused to be less collimated and to have a spread and directionality that better suits the position of the vent within the room. That is, typically the vent is situated above a corridor of a hotel room, directly opposite dead space, whereas using the vent grill 400, the conditioned air can be directed more efficiently towards the parts of the room that need to be cooled.
Referring to Figure 6, exit apertures 420 are formed on the front face. Because the front face is arcuate, the apertures each have an approximate plane angled with respect to adjacent apertures. Here the approximate plane is the plane comprising a tangent to the centre of the aperture. The figures show seven discrete apertures. However, the apertures are preferably as described herein in relation to the apertures of the first and second air flow changers. Here, the relationship would be in terms of the apertures 420and area of the vent grill and the apertures of the air flow changer adjacent the vent grill. However, for aesthetic reasons it is preferable for the apertures 420 of the vent grill to be geometrically arranged on a regimented grid pattern.
Figures 7 and 8 show experimental test data of the various air conditioning arrangements as previously described, wherein the data is further represented graphically in Figures 9 to 11. The size, shape and number of apertures were kept constant for each silencer. However, the number of silencers, their position and the arrangement of a linear or non-linear air path was varied in order to examine the effects of noise reduction compared to the use of no silencer.
As clearly shown in Figures 9-11, the noise reduction benefits of an air cycling fan unit silencer are self-evident. The greatest advantage is shown in Figure 9, particularly for the non-linear air path arrangement. Here, a supply and return silencer is used, wherein each silencer comprises two air flow changers separated by an airspace gap. The amount of noise reduction when the airflow increases between about 8 m³/min to about 9.72 m³/min, is about 10 dBA to about 6 dBA, respectively. Therefore, the greatest noise reduction was seen at around 10 dBA for the non-linear air path arrangement. In relative terms, this equates to a noise reduction of about 15% to about 25% when using a linear or non-linear air path arrangement with a single or two air flow changers, respectively for each supply and return silencers.
Notwithstanding the marginally more favourable results achieved with the non-linear air path arrangement (having two air flow air flow changers s for a given silencer), the linear air path arrangement (having a single air flow changer for a given silencer) performed comparatively well. Advantageously, the linear air path arrangement provides similar noise reduction benefits to the non-linear air path arrangement but with the further advantage that fewer parts are needed and the complexity is reduced.
Although sound absorbing materials such as acoustic foam will aid sound reduction, the noise reduction benefits given above are independent of the material used for the air flow changer. However, the material should have around 25% to around 45% of free air space, i.e. the total area of the apertures is around 25% to around 45% of the total surface area of each air flow changer. In addition, a silencer having two air flow changers placed side-by-side with a gap between them equal to around 100% of the thickness of each air flow changer will produce favourable results, particularly when the air flow changer has a thickness of around 0.025m.
Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

Claims

An air cycling fan unit silencer suitable to be placed at an outlet and/or inlet of an air cycling fan unit, wherein the silencer includes a first air flow changer having apertures to allow air to pass through wherein the density of the apertures to the area of the air flow changer is between 25% and 45%.
The air cycling fan unit silencer of claim 1, wherein the silencer includes a second air flow changer spaced by a gap from the first.
The air cycling fan unit silencer of claim 2, wherein the first and second air flow changers are arranged to create a plurality of substantially non-linear air flows.
The air cycling fan unit silencer of any preceding claim, wherein at least one air flow changer is formed from a sound absorbing material.
The air cycling fan unit silencer of any preceding claim, wherein at least one air flow changer is a plate that extends across an air ducting, orthogonal to sides of the ducting.
The air cycling fan unit silencer of any preceding claim, wherein a quarter of the air flow changer has an aperture density of not less than 50% of a quarter having the greatest aperture density.
The air cycling fan unit silencer of any preceding claim, wherein each aperture is substantially equally sized.
The air cycling fan unit silencer of claim 2, wherein the gap spaces the first and second air flow changers between 90% and 110% of the thickness of the air flow changers.
The air cycling fan unit silencer of claim 3, wherein the gap spaces the first and second air flow changers between 190% and 300% of the thickness of the air flow changers.
The air cycling fan unit silencer of claim 2, wherein the area of the apertures in one air flow changer is substantially aligned with the aperture area of the other air flow changer.
The air cycling fan unit silencer of claim 3, wherein less than 30% of the area of the apertures in one air flow changer is aligned with the aperture area of the other air flow changer.
The air cycling fan unit silencer of claim 2 including a third air flow changers.
The air cycling fan unit silencer of any preceding claim, wherein the arrangement of apertures across the at least one air flow changer is random.
A method of silencing an air cycling fan unit comprising fitting an air cycling fan silencer of any preceding claim to an inlet or outlet of an air cycling fan unit.
The method of silencing an air cycling fan unit of claim 14, wherein the method comprises installing the silencer in a plenum of an existing air cycling fan installation.