"FAN UNIT WITH BYPASS VENT HOLES"

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to a gases supply and gases humidifϊcation apparatus, particularly but not solely for providing respiratory assistance to patients or users who require a supply of gas at positive pressure for the treatment of diseases such as Obstructive Sleep Apnea (OSA), snoring, or Chronic Obstructive Pulmonary Disease (COPD) and the like. In particular, this invention relates to a compressor or blower for use in a gases supply apparatus which in use is integral with the gases supply apparatus. Summary of the Prior Art

Devices or systems for providing a humidified gases flow to a patient for therapeutic purposes are well known in the art. Systems for providing therapy of this type, for example CPAP therapy, have a structure where gases at the required pressure are delivered from a blower (also known as a compressor, an assisted breathing unit, a fan unit, a flow generator or a pressure generator) to a humidifier chamber downstream from the blower. As the gases are passed through the heated, humidified air in the humidifier chamber, they become saturated with water vapour. The gases are then delivered to a user or patient downstream from the humidifier, via a gases conduit.

Humidified gases can be delivered to a user from a modular system that has been assembled from separate units (that is, a system where the humidifier chamber/heater and the breathing unit/blower are separate items) connected in series via conduits. A schematic view of a user 1 receiving air from a known (prior art) modular assisted breathing unit and humidifier system is shown in Figure 1. Pressurised air is provided from an assisted breathing unit or blower 2a via a connector conduit 10 to a humidifier chamber 4a. Humidified, heated and pressurised gases exit the humidifier chamber 4a via a user conduit 3, and are provided to the patient or user 1 via a user interface 5.

It is becoming more common for integrated blower/humidifier systems to be used. A typical integrated system consists of a main blower or assisted breathing unit which provides a pressurised gases flow, and a humidifier unit that mates with or is otherwise rigidly connected to the blower unit. This mating occurs for example by a slide-on or push connection, so that the humidifier is held firmly in place on the main blower unit. A schematic view of the user 1 receiving air from a known, prior art integrated blower/humidifier unit 6 is shown in Figure 2.

The system operates in the same manner as the modular system shown in Figure 1, except that humidifier chamber 4b has been integrated with the blower unit to form the integrated unit 6.

The user interface 5 shown in Figures 1 and 2 is a nasal mask, covering the nose of the user 1. However, it should be noted that in systems of these types, a mask that covers both the mouth and nose, a full face mask, a nasal cannula, or any other suitable user interface could be substituted for the nasal mask shown. A mouth-only interface or oral mask could also be used. Also, the patient or user end of the conduit can be connected to a tracheostomy fitting, or an endotracheal intubation.

US 7,111,624 includes a detailed description of an integrated system. A 'slide-on' water chamber is connected to a blower unit in use. A variation of this design is a slide-on or clip-on design where the chamber is enclosed inside a portion of the integrated unit in use. An example of this type of design is shown in WO 2004/112873, which describes a blower, or flow generator 50, and an associated humidifier 150.

For these systems, the most common mode of operation is as follows: air is drawn by the blower through an inlet into the casing which surrounds and encloses at least the blower portion of the system. The blower pressurises the air stream from the flow generator oudet and passes this into the humidifier chamber. The air stream is heated and humidified in the humidifier chamber, and exits the humidifier chamber via an oudet. A flexible hose or conduit is connected either direcdy or indirecdy to the humidifier oudet, and the heated, humidified gases are passed to a user via the conduit. This is shown schematically in Figure 2.

Impeller type fans or blowers are most commonly used in breathing systems of this type. An impeller blade unit is contained within an impeller housing. The impeller blade unit is connected to a drive of some form by a central spindle. A typical impeller housing is shown in Figures 3 and 4. A typical rotating impeller blade unit which in use is located inside the housing is shown in Figures 5 and 6. Air is drawn into the centre of the impeller unit through an aperture, and is then forced outwards from the centre of the housing towards an exit passage (usually located to one side of the housing) by the blades of the rotating impeller unit. An impeller blower suitable for use with a breathing system is described in US 6,881,033.

Generally, domestic users receive treatment for sleep apnea or similar. It is most common for a nasal mask, or a mask that covers both the mouth and nose, to be used. If a nasal mask is used, it is common to strap or tape the mouth closed, so that the use of the system is effective (mouth leak and the associated pressure drop are substantially reduced or eliminated). For the range of flows dictated by the user's breathing, the CPAP device pressure generator

provides a flow of gases at a substantially constant pressure. The pressure can usually be adjusted before use, or during use, either by a user, or a medical professional who sets up die system. Systems that provide variable pressure during use are also known — for example BiPAP machines that provide two levels of pressure: One for inhalation (IPAP) and a lower pressure during the exhalation phase (EPAP).

A person using a breathing assistance apparatus will inhale and exhale as part of dieir breathing cycle. As the user exhales, they are exhaling against the incoming gases stream provided by the blower. It is well-known in this field of technology to add a one-way or bias valve to the system, on or close to the mask or interface. A mask vent is described in US 6,662,803. This allows exhaled air to be vented to atmosphere.

A mask vent of different design is described in EP 1275412.

US 6,123,074 discloses a system where the mask includes an exhaust port, and where pressure in the breathing system is constandy monitored and a pressure controller downstream of the flow generator (between the mask and the flow generator) acts to maintain a constant pressure within the conduit.

US 6,526,974 discloses a CPAP device where the size of the inlet to the blower or flow generator can be varied, or where the size of the inlet is automatically varied, in response to the needs of the user. An exhalation path is also provided.

The term "comprising" as used in this specification means "consisting at least in part of. When interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a breathing assistance apparatus which goes some way to overcoming the abovementioned disadvantages or which at least provides the public or industry with a useful choice.

Accordingly, the invention may broadly be said to consist in a fan unit which in use forms part of a gases supply unit suitable for use as part of a system for providing heated humidified gases to a user, said fan unit comprising: a casing having an inlet aperture and an outlet passage, said oudet passage including an exit aperture,

a fan located inside said casing and adapted for connection to a motor to drive rotation of said fan in use, so that in use said fan draws gases into said casing via said inlet aperture, and forces said gases out of said casing via said oudet passage as a gases stream, and wherein said oudet passage includes at least one vent hole, independent of said exit aperture and arranged at an angle to the path of said gases stream dirough said oudet passage.

Preferably said oudet passage and said at least one vent hole are configured so that if the oudet of said oudet passage is substantially blocked, the flow in use dirough said vent hole or holes matches the flow through said oudet passage at which the onset of surge would occur in a substantially similar, substantially blocked oudet passage without a vent hole or holes. Preferably said exit aperture is larger than the cross-sectional area of any one of the individual vent hole or holes by approximately three orders of magnitude.

Preferably said exit aperture is larger than total cross-sectional area of said vent hole or holes by approximately one to two orders of magnitude.

Preferably said angle of said at least one vent hole or holes to said path of said gases stream is substantially perpendicular.

Preferably said oudet passage includes a plurality of said vent holes. Preferably the total number of said vent holes is between 65 and 75.

Preferably each of said vent holes is substantially the same size as the others of said vent holes. Preferably said exit aperture has a cross-sectional area of substantially between 210mm ² and 270mm ² .

Preferably the cross-sectional area of each of said vent holes on die inner surface of said oudet passage is substantially between 0.22mm ² and 0.26mm ² .

Preferably the total cross-sectional area of said vent hole or holes on the inner surface of said oudet passage is substantially between 15.00mm ² and 17.00mm ² .

Preferably said vent hole or holes are formed with a draft of substantially between 5° and 15°.

Even more preferably said vent holes are formed with a draft of 10°.

Preferably said casing has a generally circular form and said oudet passage is tangential to said casing.

Preferably said exit aperture has a generally rectangular cross-section. Preferably said vent holes are located on two opposed sides of said passage.

Preferably substantially the same number of vent holes are formed on each of said opposed sides. Preferably said inlet aperture is located on the lower face of said casing in use.

Preferably said inlet aperture is located generally centrally on said casing. Preferably said fan is an impeller unit.

Preferably said impeller unit includes a spindle, adapted for connection to a motor in use to drive said impeller unit, said spindle passing out of the bottom of said casing. This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. BRIEF DESCRIPTION OF THE DRAWINGS

A preferred form of the present invention will now be described with reference to the accompanying drawings.

Figure 1 shows a schematic view of a user receiving humidified air from a modular blower /humidifier system of a known, prior art, type. Figure 2 shows a schematic view of a user receiving humidified air from an integrated blower/ humidifier system of a known, prior art, type.

Figure 3 shows a top view of an impeller casing or fan housing of a known, prior art, type which can be used with the blower or integrated blower/humidifier of Figures 1 and 2.

Figure 4 shows a side view of the fan housing of Figure 3. Figure 5 shows a top perspective view of an impeller unit such as might be used as part of the fan of Figures 3 and 4.

Figure 6 shows a bottom perspective view of an impeller unit such as might be used as part of the fan of Figures 3 and 4.

Figure 7 shows an integrated blower/humidifier which forms part of the present invention, or which the present invention can be used with.

Figure 8 shows an exploded view of the integrated blower/humidifier of Figure 7. Figure 9 shows a bottom perspective view of the blower of Figures 7 and 8. Figure 10 shows a side bottom perspective view of the blower of Figures 7 and 8

Figure 11 shows a perspective top view of a fan casing for use with a system that provides heated, humidified air to a user, the casing having an inlet and an oudet passage.

Figure 12 shows a top view of the fan casing of Figure 11.

Figure 13 shows a perspective bottom view of the fan casing of Figures 11 and 12.

Figure 14 shows a bottom view of the fan casing of Figures 11, 12 and 13.

Figure 15 shows detail of vent holes formed on the top surface of the oudet passage of fan casing of Figures 11 — 14.

Figure 16 shows detail of vent holes formed on the lower surface of the oudet passage of fan casing of Figures 11 - 14.

Figure 17 shows a view from above of an alternative form of fan casing, with an oudet aperture on the circumference of the housing. Figure 18 shows a view from below of an alternative form of fan casing, with an oudet aperture on the circumference of the housing.

Figure 19 shows a comparison graph of the pressure plotted against flow for a fan casing with vent holes and a fan casing without vent holes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to a system where the humidifier chamber is integrated with the gases supply unit (also referred to as a respirator unit or blower unit). However, it should be noted that the system is equally applicable to a modular system.

An integrated gases supply unit 7 with which the present invention can be used is shown in Figure 7. The integrated unit 7 comprises two main parts: a gases supply unit or blower unit 8 and a humidifier unit 9. Humidification unit 9 is partially enclosed within the external shell 80 of the blower unit 8 in use, except for the top of the humidification unit 9. It is not necessary to describe the structure and operation of the humidification unit 9 in detail in order to fully describe the present invention.

The body of die gases supply unit 8 has the form of a generally rectangular block with substantially vertical side and rear walls, and a front face that is angled slighdy rearwards (all the

walls can be angled inwards slightly if required). In the preferred embodiment, the walls, base and top surface are all manufactured and connected as far as possible to minimise the occurrence of seams, and any necessary seams are sealed. As shown in Figure 3, the gases supply unit 8 includes a control knob 11 , located on the lower section of the front face of the gases supply unit 8, with a control display 12 located direcdy above the knob 11. A patient outlet 30 is shown passing out of the rear wall of the gases supply unit 8. In the preferred embodiment, the free end of the outlet 30 faces upwards for ease of connection. The patient oudet 30 is adapted to allow both pneumatic and electrical connection to one end of a conduit — e.g. conduit 3 — running between the integrated unit 7 and a patient interface — e.g. interface 5. An example of the type of connector that can be used and the type of dual connection that can be made is described in US 6,953,354. It should be noted mat for the purposes of reading this specification, the patient interface can be thought of as including both the interface 5 and the conduit 3 where it would be appropriate to read it in this manner.

The internal structure and components of the gases supply unit 8 will now be described widi reference to Figures 8, 9 and 10. The gases supply unit 8 includes an enclosing external shell 80 which forms part of, and encloses, the gases supply unit 8. The shell 80 includes internal air passages for ducting air passing through the gases supply unit 8, and also internal recesses, cavities or slots into which componentry of the gases supply unit 8 is located in use. The shell 80 of the gases supply unit 8 is furdier adapted to include an open-topped compartment 13. In use, humidifier chamber 9 is located within the compartment 13. Blower unit 8 includes a heater base or heater plate (not shown), located at the bottom of the compartment 13. A humidifier inlet aperture 15 and humidifier oudet aperture 16 are located on the wall of the compartment 13, towards the top of the compartment 13. In the preferred embodiment, the inlet and outlet apertures 15, 16 are aligned so as to mate with inlet and oudet humidifier ports 17, 18 located on the humidifier chamber 9, when the system is in use. It should be noted that other forms of humidifier inlet are possible. For example a conduit running between the gases supply unit 8 and e.g. die lid of the humidifier chamber 9. Also, if the humidifier chamber is a separate item (that is, not rigidly connected to the gases supply unit in use), the humidifier inlet aperture 15 will not be connected directly to the humidifier chamber, but will be connected instead to one end of a conduit or similar leading from the humidifier inlet aperture on the gases supply unit, to die humidifier chamber.

Air from atmosphere is drawn into the shell of the gases supply unit 8 dirough an atmospheric inlet vent 19. This vent 19 can be located wherever is convenient on die external surface of the shell of the gases supply unit 8. In the preferred embodiment, as shown in Figure

9, the inlet vent 19 is located on the rear face of the shell of the gases supply unit 8, on the right hand side of the rear face (right hand side when looking forwards). In the preferred embodiment, air is drawn in through the inlet vent 19 by means of a fan unit 20 which forms part of the gases supply unit 8, and which is located inside the enclosing external shell of the gases supply unit 8. The fan unit 20 provides a pressurised gases stream for the gases supply unit and therefore the assisted breathing system. The fan unit 20 will be described in more detail below. The air is drawn into the fan unit 20 indirecdy, via a curved inlet path 22 formed through the shell of die gases supply unit 8. Patii 22 runs from the inlet vent 19 to an aperture 23 formed in the gases supply unit shell 80, die aperture 23 passing into a recess 21 which is formed in the gases supply unit shell 80, in which the fan unit 20 is located.

The gases stream passes through the fan unit 20 to the humidifier inlet aperture 15 as follows: the shell of the gases supply unit 8 includes a chamber or oudet duct 26 which forms at least part of an oudet air padi to allow gaseous communication between the fan unit 20 and the humidifier inlet aperture 15. In the preferred embodiment, the oudet duct 26 runs up between the right hand side wall of the gases supply unit 8 (from behind looking forwards) and the front wall, up to die humidifier inlet aperture 15. As shown in Figures 9 and 10, air exiting the fan unit 20 enters the duct 26.

In use, air exits the shell of the gases supply unit or blower 8 via the humidifier inlet aperture 15 and enters the humidifier chamber 9. In the preferred form, the humidifier inlet aperture 15 forms an oudet at die end of the duct 26. The gases are humidified and heated in die chamber 9, before passing out of die chamber 9 through die humidifier outiet aperture 16, which is direcdy or indirecdy connected to the patient oudet 30 (it should be noted that the oudet of die humidifier chamber 9 could also be completely separate from the gases supply unit 8). The heated humidified gas is then passed to die user 1 via conduit 3. The patient oudet 30 is adapted to enable pneumatic attachment of the patient conduit 3, and in the preferred embodiment, oudet 30 is also adapted to enable electrical connection via an electrical connector. A combined electrical and pneumatic connection can be useful for example if the conduit 3 is to be heated. Electrical heating of a conduit such as conduit 3 can prevent or minimise the occurrence of condensation widiin the conduit 3. It should also be noted that the oudet connection does not have to be via the shell of die integrated unit 7. If required, the connection for the conduit 3 could be located direcdy on an oudet from humidifier chamber 9.

The blower unit 8 in use is set to a user-specified pressure level. The flow rate for the preferred embodiment will vary during use, depending on the users breathing. The power to fan

unit 20 can be altered, to change the speed at which the impeller 24 is rotating, and therefore the pressure.

Fan Unit and Air Path

The structure of the fan unit 20 shall now be described, with particular reference to Figures 11 to 16. The fan unit 20 is located in recess 21 of the shell of the gases supply unit 8 in use, as described above with reference to Figures 9 and 10. In the preferred form, the fan unit 20 comprises a rotating impeller unit 24 located inside a casing having the form of a snail or scroll casing 25. However, it should be noted that any suitable type of fan or compressor could be used. For example a scroll compressor could be used instead of the impeller unit 24. The compressor or fan located inside the casing 25 will be referred to in the general as a 'fan' for the purposes of this specification, and as 'impeller unit 24' for the specific preferred embodiment. It can be seen that the fan unit 20 appears generally circular in plan view, as shown in Figures 12 and 14. The fan casing 25 includes an inlet aperture 27. In the preferred form, inlet aperture 27 is a circular hole located in approximately the centre of the lower face of the casing 25 and passing from the outside of the casing to the inside. Air from the inlet path 22 enters the fan casing 25 via the inlet aperture 27. It should be noted that where it would be appropriate to include the aperture 23 and at least part of the recess 21 as part of the air inlet path, the specification should be read as including these elements. The preferred form of the casing 25 of the fan unit 20 also includes an outlet passage 28. In the preferred form, the outlet passage 28 is a short passage formed as an integral part of the casing 25 and aligned substantially circumferentially to the remainder of the generally circular casing 25. A fan casing oudet aperture or exit aperture 29 is located at the outer end of the passage 28. It should be noted that the fan casing exit aperture 29 could be located wherever is convenient on the passage 28 (Le. it does not have to be at the end of the passage, it could be through the passage wall partway along its length). Exit aperture 29 opens into die duct 26.

The oudet passage 28 forms part of the air path from the fan to the humidifier inlet aperture 15. The fan casing 25 encloses the fan in use, except for the inlet aperture 27 and the exit aperture 29 of the passage 28.

In the preferred embodiment, rotation of the fan unit 20 is driven by a motor (not shown) located outside the casing 25, the fan or impeller unit 24 being adapted for connection to the motor. In the preferred embodiment, the motor is located below the casing 25 in the recess - 21, and is an electromagnetic motor. Impeller unit 24 includes a spindle 60 which passes vertically downwards out of the casing 25 to connect with the motor. In use, the motor is

powered to rotate the spindle, which causes rotation of the impeller unit 24. In alternative embodiments, the fan could be run indirectly by the motor, for example by gears or similar connecting the fan to the motor, or by magnetic induction or similar. Air or gases are drawn through inlet aperture 27 in the centre of the casing 25, into the centre of the impeller unit 24, and are then forced outwards as a gases stream through the exit aperture 29 of the outlet passage 28 by the impeller blades 31 as the impeller unit 24 rotates. In the preferred form, the fan outlet passage or exit passage 28 has a generally rectangular cross-section, and the exit passage 28 is aligned substantially tangentially to the casing 25. However, the cross-section of the fan oudet passage 28 could be any suitable shape, such as oval, rectangular or circular. The fan outlet passage 28 could also be arranged at any suitable angle to the impeller unit, for example facing radially outwards, or at any suitable angle between tangential and radial. The fan oudet passage 28 causes the gases forced outwards by the impeller unit 24 to coalesce as a fluidic gases stream, and dictates the direction in which the gases stream flows. There will inevitably be some swirling of the gases within the passage. However, the overall path or overall direction of the gases flow will be along the passage from the fan towards the fan casing exit aperture 29.

In alternative embodiments, and as shown in Figures 17 and 18, the fan casing 25 is formed without an integral passage, and has a fan casing oudet aperture 50 on or close to the circumference of the main portion of the casing. In use, this fan casing oudet aperture 50 mates with an oudet passage similar to that described above, but which is either formed as part of die shell 80 of the gases supply unit 8, or as a separate element to both the fan casing 25 and the shell 80 of the gases supply unit 8. If the oudet passage is formed as a separate element, die oudet passage is connected into position during assembly by any suitable mechanism such as glue, friction fit, latches, screws, or similar. In this specification, 'exit aperture' will be used to refer to the exit 29 of the passage 28 (or similar), and Outlet aperture' will be used to refer to an oudet on or close to die circumference of the main portion of the fan casing 25, which either connects with an integral passage (e.g. passage 28), or which connects with a separate passage, formed as part of the shell of the gases supply unit 8, or formed as a completely separate passage which connects separately to both the fan unit 20 and the shell 80 of the gases supply unit 8. The connection formed in each of these cases is a connection to allow a fluidic connection between the components - so that the gases can pass from one portion of the gases supply unit 8 to another.

The air exiting the impeller unit 24 has both tangential and radial velocity components. The rotational speed of the impeller determines the tangential velocity. The flow drawn by die

user will determine the rate at which air exits the impeller, hence the flow rate determines the radial velocity.

A person using a breathing assistance apparatus will inhale and exhale as part of their breathing cycle. As the user exhales, they are exhaling against the incoming gases stream provided by the blower, throttling the gases stream flow. If the gases stream flow from the fan to the user is throtded, then the flow rate diminishes and the radial velocity exiting the impeller decreases. This can cause the flow exiting the impeller to fall back on itself, which causes the impeller unit 24 to stall or surge. Stall or surge can result in high frequency fluctuations in the pressure of the delivered gases stream. The fluctuations can be felt by the user through the gases stream and can cause audible noise, both of which are disturbing for a user. The fluctuations can also introduce vibration into mechanical structures of the system that can cause additional noise that is disturbing for a user.

Surprisingly, it has been found that die addition of bypass vent holes to the fan unit 20 helps to prevent the onset of flow instability, and most surprisingly, at the same time allows a flow rate through the fan housing 25 and the impeller 24 to be maintained (i.e. the overall flow rate is not negatively impacted by the addition of the bypass vent holes), . At least a portion of the flow exiting the fan or impeller 24 is diverted through the vent holes instead of stopping when it is choked.

The preferred form of bypass vent holes on the fan unit are described below. Figure 19 shows an experimental comparison of a fan unit which is very similar to fan unit 20, the experimental fan unit tested in two states: without vent holes (line 90), and with vent holes (line 91). For the un-vented fan unit, it has been found by way of experimentation that surge occurs in that portion of curve 90 where the pressure/ flow slope is increasing ('rising') or flat - from zero flow, up to approximately 50 Litres/minute. It can be seen that the line plotted for experimental data gathered from the vented fan unit (line 91) shows a curve that almost exactly parallels line 90 (the curve for the unvented blower), except that it is shifted to the left (along the x-axis). Because curve 91 is shifted to the left, the 'rising' component is removed during normal operating conditions, or lies outside normal operating parameters, and therefore surge is less likely to occur. Gases supply unit bypass vent holes

As can be seen in Figures 11 to 16, the preferred form of the fan outlet passage 28 includes vent holes 32 passing through the walls or sides of the fan outlet passage 28. Detail of

the vent holes is shown in Figures 11 and 12. These vent holes are separate to, or independent of, the fan casing outlet aperture 29 of the fan outlet passage 28. It can be seen that the vent holes 32 will be at an angle to the general overall path of the gases stream. That is, they are arranged at an angle to the general overall path of the gases stream. For a rectangular passage (as in the preferred embodiment), the holes are aligned substantially perpendicular to the general overall path of the gases stream. In the preferred from, the holes are each approximately 0.55mm in diameter, and have a 10° draft (wider at the external surface of the passage than the internal surface). The draft is beneficial for noise reduction as it allows the flow to escape smoothly — the draft acts as a diffuser, which slows the flow and therefore reduces turbulence and noise. The draft also offers a manufacturing advantage, as it allows the passage to be formed by e.g. injection moulding in one operation including the holes. The holes do not have to be added as a separate manufacturing operation — e.g. by drilling them post-moulding.

When a patient is inhaling, litde to no air from the fan unit 20 passes through these holes, as the gases stream will follow the general overall path, or the path of least resistance, from the impeller unit 24 down the passage 28 and through the exit 29. When a user exhales, this can cause throttling in the system, disrupting the smooth flow of the air from the fan unit 20, as described above. The vent holes 32 allow gases to vent from inside the fan oudet passage 28 - the outlet passage portion of the fan casing 25. This allows a rate of flow to be maintained through the fan casing 25 that helps to prevent the onset of flow instability. The result is decreased turbulence and therefore less noise. Once a patient finishes exhaling and begins their inhalation phase, one potential cause of throttling in the system is removed. Air ceases to vent through the vent holes 32 and resumes following a path of least resistance from the impeller to the aperture 29, so that substantially all the air from the fan unit 20 passes into die duct 26.

In the preferred form, the blower unit is set by a user to a constant pressure setting, which can be adjusted to different (constant pressure) levels according to the users needs. The flow rate delivered by the CPAP unit or blower unit 8 for any particular constant pressure setting is variable, and depends on an individual user's breathing pattern. Ideally, a CPAP device would deliver a constant pressure for all flow rates. However, in use, for any given pressure setting, die blower unit 8 will actually deliver a variable pressure and flow rate as a user breathes. The experimental plots of Figure 19 are for a constant fan speed. The experiment was carried out with a single fan speed setting. The experiment was carried out under laboratory conditions (without a user connected), so the flow rate was substantially constant. The experiment was also carried out with the fan unit alone — i.e. not mounted in a blower shell. It

can be seen from the plots that the vented fan unit (line 91) has a static pressure of just under 16 cmH ₂ 0, this static pressure substantially constant for a flow rate of between 0 — 50 Litres/minute. In contrast, it can be seen that the plot for the unvented fan unit includes a rising component for a flow rate between 0 — 50 Litres/minute. The preferred form of fan unit is speed adjustable, to provide a range of pressures preferably between approximately 4 CmH ₂ O and 20 cmH ₂ 0.

The vent holes should be sized so that the flow through die vent holes matches the outlet flow (flow through the exit aperture 29) at the onset of the stall or surge condition. The total area necessary will depend on the size of the individual holes. Smaller vent holes are preferred as these will have an increased resistance to flow at higher rates of overall flow through the oudet passage 28 (and therefore gases will only vent through the holes when the flow is throtded by back pressure as a user exhales). It has also been found by carrying out testing with vent holes having diameters of between 0.5mm and 1.5mm that the holes with smaller diameters tend to be more effective at reducing noise and vibration. For vent holes widi larger diameters, sputtering can occur (periodic, random, spitting or popping sounds, or instability), which is undesirable.

In the preferred form, fan casing oudet aperture 29 is larger than the cross-sectional area of any one of the individual vent holes 32 by between approximately two and four, and more specifically substantially three orders of magnitude. The fan casing oudet aperture 29 is larger than the total cross-sectional area of all of the vent holes 32 by between approximately one and two orders of magnitude. Because the fan casing oudet aperture 29 is aligned in substantially the same direction as the general overall path in which die air is already travelling, substantially all of the air from the impeller unit 24 passes along the fan casing oudet passage 28, through the fan casing oudet aperture 29 and into the duct 26. In the preferred form, the holes 32 of the preferred form have a total cross-sectional area of approximately 16mm ² to 17mm ² , and are evenly spaced in a generally rectangular pattern. It is preferred that the same number of holes, of the same size, distributed in substantially the same number and the same pattern, are used on the two opposed surfaces, and that the upper and lower opposed surfaces of the fan oudet passage 28 are used, as this offers a manufacturing advantage for the preferred form of fan casing. However, the holes could be placed wherever is convenient, and in whatever pattern is convenient. It should also be noted that different oudet shapes would also be effective — for example, slots or a grid pattern could be used instead. It should also be noted that aldiough a generally rectangular oudet passage has been described, diis

could be oval, circular, or any other suitable cross-sectional shape. If the passage is circular or oval, for example, the holes could be on two opposed 'sides' of the passage — that is, in two areas which are generally opposed or at generally equal distances from one another around the perimeter of the passage. In the most preferred form, the cross-sectional area of the exit aperture 29 is approximately 240mm ² . In the preferred form, the passage 28 has a rectangular cross-section, with the vent holes 32 located on two opposed sides of the rectangular passage. As shown in Figures 15 and 16, in the most preferred form, a total of 69 vent holes are formed in the upper and lower surfaces of the passage 28. Each has a diameter on the inside surface of approximately 0.55mm. The cross-sectional area of each of the holes passing through the inner surface is therefore approximately 0.24mm ² . The total cross-sectional area of the outlet holes is therefore 16.39mm ² for the most preferred form. It should be noted that this is the most preferred form of the invention. Testing has been carried out using different numbers of vent holes with different diameters. It has been found that the effective total or overall size for the vent holes remains substantially constant and is almost completely independent of the size of each individual hole. An overall 'bleed', 'diversion' or vent hole cross-sectional area of between 15.5mm ² and 17mm ² is preferred, but it has been found experimentally that the benefits of the invention can be realised with a total vent hole cross-sectional area from as low as approximately 12.0mm ² (seven vent holes, each with a diameter of 1.5mm). Tests with vent holes having diameters of 0.55mm, 0.75mm, 1.00mm, 1.25mm and 1.5mm have also been carried out. The number of holes of each diameter needed to prevent excessive mechanical noise or 'rumble' was varied (although it should be noted that this was a subjective test, and that no fixed parameter was measured to determine a suitable noise level). The number of holes necessary to prevent rumble for each of the different diameters is as follows: 56 (0.55mm), 28 (0.75mm), 17, (1.00mm), 11 (1.25mm), and 7 (1.5mm).

As noted above, the test equipment was slighdy different from the preferred form of fan casing 25. the number and size of holes in the most preferred form was decided as 69 holes, each of 0.55mm diameter.

This testing indicates that the benefits of the invention can be realised with a single vent hole having a cross-sectional area in the region of between 12.0mm ² to 17mm ² . Although it has been found that a pattern of small holes each having an area of approximately 0.24mm ² is more effective than using one large hole, the invention can still be realised using one larger hole, or a smaller number of larger holes. This will still produce the same reduction in throttling side

effects, as gases can vent in a throttle condition, and flow through the impeller can be maintained. However, there can be other undesirable side-effects from using large holes (rather than a larger number of smaller holes with the same cross-sectional area). For example, gases will vent more easily through the holes during the inspiratory phase as well as the expiratory phase. This leads to a lower overall system pressure, and reduces the effectiveness of the holes in minimising back pressure effects as a certain amount of gas will already be flowing through them when a user starts to exhale. Also, as noted above, it has been found that smaller holes tend to be more effective at reducing noise and vibration. For vent holes with larger diameters, sputtering can occur (periodic spitting or popping sounds), which is undesirable. Although round vent holes have been described, differently shaped holes with the same area could be used if this is preferred. A mix of holes of different shapes and sizes could also be used if this is preferred, or if this configuration would be beneficial in certain situations.

The invention as outlined above is beneficial in reducing noise and vibration in assisted breathing systems. Preferred Features (US claims)

1. A fan unit which in use forms part of a gases supply unit suitable for use as part of a system for providing heated humidified gases to a user, said fan unit comprising: a casing having an inlet aperture and an outlet passage, said oudet passage including an exit aperture, a fan located inside said casing and adapted for connection to a motor to drive rotation of said fan in use, so diat in use said fan draws gases into said casing via said inlet aperture, and forces said gases out of said casing via said outlet passage as a gases stream, and wherein said outlet passage includes at least one vent hole, independent of said exit aperture and arranged at an angle to the path of said gases stream through said outlet passage.

2. A fan unit as outlined above in paragraph 1 wherein said oudet passage and said at least one vent hole are configured so that if the outlet of said oudet passage is substantially blocked, the flow in use through said vent hole or holes matches the flow through said oudet passage at which the onset of surge would occur in a substantially similar, substantially blocked oudet passage without a vent hole or holes.

3. A fan unit as outlined above in paragraph 1 wherein said outlet passage includes a plurality of said vent holes.

4. A fan unit as outlined above in paragraph 3 wherein said exit aperture is larger than the cross-sectional area of any one of the individual vent holes by approximately three orders of magnitude.

5. A fan unit as outlined above in paragraph 3 or paragraph 4 wherein said exit aperture is larger than the total cross-sectional area of said vent holes by approximately one to two orders of magnitude.

6. A fan unit as outlined above in paragraph 5 wherein said vent holes are located on two opposed sides of said passage.

7. A fan unit as outlined above in paragraph 6 wherein substantially the same number of vent holes are formed on each of said opposed sides.

8. A fan unit as outlined above in paragraph 7 wherein the total number of said vent holes is between 65 and 75. 9. A fan unit as outlined above in paragraph 8 wherein said angle of said vent holes to said path of said gases stream is substantially perpendicular.

10. A fan unit as outlined in paragraph 9 wherein each of said vent holes is substantially the same size as the others of said vent holes.

11. A fan unit as oudined above in paragraph 10 wherein said exit aperture has a cross- sectional area of substantially between 210 and 270mm .

12. A fan unit as outlined above in paragraph 11 wherein the cross-sectional area of each of said vent holes on the inner surface of said outlet passage is substantially between 0.22mm ² and 0.26mm ² .

13. A fan unit as outlined above in paragraph 12 wherein the total cross-sectional area of said vent holes on the inner surface of said oudet passage is substantially between 15.00mm ² and

17.00mm ² .

14. A fan unit as outlined above in paragraph 13 wherein said vent holes are formed with a draft of substantially between 5° and 15°.

15. A fan unit as outlined above in paragraph 14 wherein said vent holes are formed with a draft of 10°.

16. A fan unit as outlined above in paragraph 15 wherein said casing has a generally circular form and said oudet passage is tangential to said casing.

17. A fan unit as outlined above in paragraph 16 wherein said exit aperture has a generally rectangular cross-section. 18. A fan unit as outlined above in paragraph 17 wherein said inlet aperture is located on the lower face of said casing in use.

19. A fan unit as outlined above in paragraph 18 wherein said inlet aperture is located generally centrally on said casing.

20. A fan unit as outlined above in paragraph 19 wherein said fan is an impeller unit. 21. A fan unit as outlined above in paragraph 20 wherein said impeller unit includes a spindle, adapted for connection to a motor in use to drive said impeller unit, said spindle passing out of die bottom of said casing.

22. A housing for a fan unit which in use forms part of a gases supply unit suitable for use as part of a system for providing heated humidified gases to a user, said housing comprising: an outer casing having an inlet aperture and an outlet passage, said oudet passage including an exit aperture, said casing adapted to enclose a fan in use, said outlet passage including at least one vent hole, independent of said exit aperture and arranged at an angle to the overall path of a gases stream passing along said oudet passage.

23. A housing for a fan unit as oudined above in paragraph 22 wherein said oudet passage and said at least one vent hole are configured so that if the oudet of said oudet passage is substantially blocked, die flow in use through said vent hole or holes matches die flow through said oudet passage at which die onset of surge would occur in a substantially similar, substantially blocked oudet passage without a vent hole or holes.

24. A housing for a fan unit as oudined above in paragraph 22 wherein said oudet passage includes a plurality of said vent holes.

25. A housing for a fan unit as outlined above in paragraph 24 wherein said exit aperture is larger than the cross-sectional area of any one of the individual vent holes by approximately three orders of magnitude.

26. A housing for a fan unit as outlined above in paragraph 24 or paragraph 25 wherein said exit aperture is larger than the total cross-sectional area of said vent holes by approximately one to two orders of magnitude.

27. A housing for a fan unit as outlined above in paragraph 24 wherein said vent holes are located on two opposed sides of said passage.

28. A housing for a fan unit as oudined above in paragraph 27 wherein substantially the same number of vent holes are formed on each of said opposed sides. 29. A housing for a fan unit as outlined above in paragraph 28 wherein the total number of said holes is between 65 and 75.

30. A housing for a fan unit as outlined above in paragraph 29 wherein said angle of said vent holes to said path of said gases stream is substantially perpendicular.

31. A housing for a fan unit as outlined above in paragraph 30 wherein each of said vent holes is substantially the same size as the others of said vent holes.

32. A housing for a fan unit as outlined above in paragraph 31 wherein said exit aperture has a cross-sectional area of substantially between 210 and 270mm ² .

33. A housing for a fan unit as outlined above in paragraph 32 wherein the cross-sectional area of each of said vent holes on the inner surface of said outlet passage is substantially between 0.22mm ² and 0.26mm ² .

34. A housing for a fan unit as outlined above in paragraph 33 wherein die total cross- sectional area of said vent holes on the inner surface of said oudet passage is substantially between 15.00mm ² and 17.00mm ² .

35. A housing for a fan unit as outlined above in paragraph 34 wherein said vent holes are formed with a draft of substantially between 5° and 15°.

36. A housing for a fan unit as outlined above in paragraph 35 wherein said vent holes are formed with a draft of 10°.

37. A housing for a fan unit as outlined above in paragraph 36 wherein said casing has a generally circular form and said oudet passage is tangential to said casing. 38. A housing for a fan unit as oudined above in paragraph 37 wherein said exit aperture has a generally rectangular cross-section.

39. A housing for a fan unit as outlined above in paragraph 38 wherein said inlet aperture is located on the lower face of said casing in use.

40. A housing for a gases supply unit of the type that in use forms part of a system for providing heated humidified gases to a user and which is adapted to in use provide a pressurised gases stream, said housing comprising: an enclosing external shell that encloses and forms part of said gases supply unit, said shell including an atmospheric inlet adapted to allow gases from atmosphere to enter said shell in use, and a humidifier inlet aperture adapted to allow a pressurised gases stream to exit said shell, an internal recess adapted to hold a fan casing, so that in use a motor included as part of said gases supply unit can be connected to a fan located inside said fan casing to drive said fan and draw air into said gases supply unit shell through said atmospheric inlet, an outlet duct, having an oudet end adjacent to and fluidically connected to said humidifier inlet aperture, and an inlet end, a passage which in use fluidically connects the outlet of said fan casing and said inlet end of said outlet duct, said passage including at least one vent hole, arranged at an angle to the overall path of said gases stream through said passage. 41. A housing for a gases supply unit as outlined above in paragraph 40 wherein said passage is formed as an integral part of said housing.

42. A housing for a gases supply unit as outlined above in paragraph 40 wherein said passage is formed as a separate item to said shell, and is connected to said shell in use.

43. A housing for a gases supply unit as outlined above in paragraph 40 wherein said passage is located and arranged in said shell so that gases from said oudet aperture enter said passage substantially at a tangent to said fan.

44. A housing for a gases supply unit as outlined above in any one of paragraphs 40 to 43 wherein said passage and said at least one vent hole are configured so that if the outlet of said outlet passage is substantially blocked, the flow in use through said vent hole or holes matches the flow through said oudet passage at which the onset of surge would occur in a substantially similar, substantially blocked oudet passage without a vent hole or holes.

45. A housing for a gases supply unit as outlined above in paragraph 44 wherein said passage includes a plurality of said vent holes.

46. A housing for a gases supply unit as outlined above in paragraph 45 wherein said exit aperture is larger than the cross-sectional area of any one of the individual vent holes by approximately three orders of magnitude.

47. A housing for a gases supply unit as outlined above in paragraph 46 wherein said exit aperture is larger than the total cross-sectional area of said vent holes by approximately one to two orders of magnitude.

48. A housing for a gases supply unit as outlined above in paragraph 47 wherein said vent holes are located on two opposed sides of said passage.

49. A housing for a gases supply unit as oudined above in paragraph 48 wherein substantially the same number of vent holes are formed on each of said opposed sides.

50. A housing for a gases supply unit as outlined above in paragraph 49 wherein the total number of said holes is between 65 and 75. 51. A housing for a gases supply unit as outlined above in paragraph 50 wherein said angle of said vent holes to said path of said gases stream is substantially perpendicular.

52. A housing for a gases supply unit as oudined above in paragraph 51 wherein each of said vent holes is substantially the same size as the others of said vent holes.

53. A housing for a gases supply unit as outlined above in paragraph 52 wherein said exit aperture has a cross-sectional area of substantially between 210 and 270mm ² .

54. A housing for a gases supply unit as outlined above in paragraph 53 wherein the cross- sectional area of each of said vent holes on the inner surface of said oudet passage is substantially between 0.22mm ² and 0.26mm ² .

55. A housing for a gases supply unit as oudined above in paragraph 54 wherein the total cross-sectional area of said vent holes on the inner surface of said outiet passage is substantially between 15.00mm ² and 17.00mm ² .

56. A housing for a gases supply unit as oudined above in paragraph 55 wherein said vent holes are formed with a draft of substantially between 5° and 15°.

57. A housing for a gases supply unit as oudined above in paragraph 56 wherein said vent holes are formed with a draft of 10°.

58. A housing for a gases supply unit as outlined above in paragraph 57 wherein said exit aperture has a generally rectangular cross-section.

59. A housing for a gases supply unit as outlined above in paragraph 58 wherein said casing has a generally circular form and said oudet passage is tangential to said casing.

60. A housing for a gases supply unit as outlined above in paragraph 59 wherein said inlet aperture is located on the lower face of said casing in use.

61. A gases supply unit which in use forms part of a system for providing heated humidified gases to a user, said gases supply unit comprising: a fan unit having a fan casing with an inlet aperture and an oudet aperture, and a fan located in use inside said casing, said fan unit adapted to provide a pressurised gases stream through said outlet aperture in use, a motor, adapted for connection to said fan unit to drive said fan in use, said gases supply unit including a housing as oudined above in any one of paragraphs 40 to 60 above.

62. A medical breathing system suitable for supplying heated humidified gases to a user, comprising: a gases supply unit adapted to provide a pressurised gases stream, a humidifier chamber, said chamber fluidically connected to said gases supply unit in use so that said gases stream can pass into and through said chamber, said chamber containing a volume of water which in use is heated to heat and humidify said gases stream as said gases stream passes through said chamber, a patient interface adapted to provide said heated humidified gases stream to a user, said patient interface including a conduit to transport said heated humidified gases from said humidifier chamber to said user, said gases supply unit comprising: a fan unit having a fan casing with an inlet aperture and an oudet aperture, and a fan located in use inside said casing, said fan unit adapted to provide a pressurised gases stream through said oudet aperture to said humidifier chamber in use, a motor, adapted for connection to said fan unit to drive said fan in use, said gases supply unit including a housing as outlined above in any one of paragraphs 40 to 60.

63. A medical breathing system as oudined above in paragraph 62 wherein said external shell is adapted to at least partially enclose said humidifier chamber in use.

64. A medical breathing system as outlined above in paragraph 63 wherein said external shell and said humidifier chamber are mutually adapted so that said outlet aperture can mate directly with an inlet port to said humidifier chamber in use.

65. A medical breathing system as outlined above in paragraph 64 wherein said gases supply unit includes a heater plate, said gases supply unit powering said heater plate in use to heat said volume of water.

66. A fan unit which in use forms part of a gases supply unit suitable for use as part of a system for providing heated humidified gases to a user, said fan unit comprising: a casing having an inlet aperture and an outlet passage, said outlet passage including an exit aperture, a fan located inside said casing and adapted for connection to a motor to drive rotation of said fan in use, so that said fan draws gases into said casing via said inlet aperture, and forces said gases out of said casing via said outlet passage as a gases stream, and wherein said outlet passage includes a plurality of vent holes, independent of said exit aperture and arranged at an angle to the path of said gases stream through said outlet passage.

67. A fan unit as outlined above in paragraph 66 wherein said oudet passage and said vent holes are configured so that if the oudet of said oudet passage is substantially blocked, the flow in use through said vent holes matches the flow through said oudet passage at which the onset of surge would occur in a substantially similar, substantially blocked oudet passage without a vent hole or holes.

68. A fan unit as outlined above in paragraph 66 wherein said exit aperture is larger than the cross-sectional area of any one of the individual vent holes by approximately three orders of magnitude. 69. A fan unit as outlined above in paragraph 68 wherein said exit aperture is larger than the total cross-sectional area of said vent holes by approximately one to two orders of magnitude.

70. A fan unit as outlined above in paragraph 69 wherein said vent holes are located on two opposed sides of said passage.

71. A fan unit as outlined above in paragraph 70 wherein substantially the same number of vent holes are formed on each of said opposed sides.

72. A fan unit as outlined above in paragraph 71 wherein the total number of said vent holes is between 65 and 75.

73. A fan unit as outlined above in paragraph 72 wherein said angle of said vent holes to said path of said gases stream is substantially perpendicular. 74. A fan unit as outlined above in paragraph 73 wherein each of said vent holes is substantially the same size as the others of said vent holes.

75. A fan unit as outlined above in paragraph 74 wherein said exit aperture has a cross- sectional area of substantially between 210 and 270mm ² .

76. A fan unit as oudined above in paragraph 75 wherein the cross-sectional area of each of said vent holes on the inner surface of said outlet passage is substantially between 0.22mm ² and

0.26mm ² .

77. A fan unit as outlined above in paragraph 76 wherein the total cross-sectional area of said vent holes on the inner surface of said outlet passage is substantially between 15.00mm ² and 17.00mm ² . 78. A fan unit as outlined above in paragraph 77 wherein said vent holes are formed .with a draft of substantially between 5° and 15°.

79. A fan unit as outlined above in paragraph 78 wherein said vent holes are formed with a draft of 10°.

80. A fan unit as outlined above in paragraph 79 wherein said casing has a generally circular form and said outlet passage is tangential to said casing.

81. A fan unit as outlined above in paragraph 80 wherein said exit aperture has a generally rectangular cross-section.

82. A fan unit as outlined above in paragraph 81 wherein said inlet aperture is located on the lower face of said casing in use. 83. A fan unit as outlined above in paragraph 82 wherein said inlet aperture is located generally centrally on said casing.

84. A fan unit as outlined above in paragraph 83 wherein said fan is an impeller unit.

85. A fan unit as outlined above in paragraph 84 wherein said impeller unit includes a spindle, adapted for connection to a motor in use to drive said impeller unit, said spindle passing out of the bottom of said casing.

Approved features (EP paragraphs)

86. A housing for a gases supply unit of the type that in use forms part of a system for providing heated humidified gases to a user and which is adapted to in use provide a pressurised gases stream, said housing comprising: an enclosing external shell that encloses and forms part of said gases supply unit, said shell including an atmospheric inlet adapted to allow gases from atmosphere to enter said shell in use, and a humidifier inlet aperture adapted to allow a pressurised gases stream to exit said shell, an internal recess adapted to hold a fan casing, so that in use a motor included as part of said gases supply unit can be connected to a fan located inside said fan casing to drive said fan and draw air into said gases supply unit shell through said atmospheric inlet, an oudet duct, having an oudet end adjacent to and fluidically connected to said humidifier inlet aperture, and an inlet end, a passage which in use fluidically connects the oudet of said fan casing and said inlet end of said oudet duct, said passage including at least one vent hole, arranged at an angle to the overall padi of said gases stream through said passage.

87. A housing as outlined above in paragraph 86 wherein said oudet passage and said at least one vent hole are configured so that if the oudet of said oudet passage is substantially blocked, the flow in use dirough said vent hole or holes matches die flow through said oudet passage at which the onset of surge would occur in a substantially similar, substantially blocked oudet passage without a vent hole or holes.

88. A housing as outlined above in paragraph 86 or 87 wherein said exit aperture is larger than the cross-sectional area of any one of the individual vent hole or holes by approximately three orders of magnitude.

89. A housing as outlined above in any one of paragraphs 86 to 88 wherein said exit aperture is larger than the cross-sectional area of the total cross-sectional area of said vent hole or holes by approximately one to two orders of magnitude.

90. A housing as outlined above in any one of paragraphs 86 to 89 wherein said angle of said at least one vent hole or holes to said path of said gases stream is substantially perpendicular.

91. A housing as outlined above in any one of paragraphs 86 to 90 wherein said oudet passage includes a plurality of said vent holes.

92. A housing as outlined above in paragraph 91 wherein said vent holes are located on two opposed sides of said passage.

93. A housing as outlined above in paragraph 92 wherein substantially the same number of vent holes are formed on each of said opposed sides. 94. A housing as outlined above in any one of paragraphs 91 to 93 wherein the total number of said vent holes is between 65 and 75.

95. A housing as outlined above in any one of paragraphs 91 to 94 wherein each of said vent holes is substantially the same size as the others of said vent holes.

96. A housing as outlined above in any one of paragraphs 86 to 95 wherein said exit aperture has a cross-sectional area of substantially between 210 and 270mm ² .

97. A housing as outlined above in any one of paragraphs 86 to 96 wherein the cross- sectional area of each of said vent holes on the inner surface of said outlet passage is substantially between 0.22mm ² and 0.26mm ² .

98. A housing as outlined above in any one of claims 91 to 97 wherein the total cross- sectional area of said vent holes on the inner surface of said outlet passage is substantially between 15.00mm ² and 17.00mm ² .

99. A housing as outlined above in any one of paragraphs 86 to 98 wherein said vent hole or holes are formed with a draft of substantially between 5° and 15°.

100. A housing as outlined above in claim 99 wherein said vent hole or holes are formed with a draft of 10°

101. A housing as outlined above in any one of paragraphs 86 to 100 wherein said casing has a generally circular form and said outlet passage is tangential to said casing.

102. A housing as outlined above in any one of paragraphs 86 to 101 wherein said exit aperture has a generally rectangular cross-section. 103. A housing as outlined above in any one of paragraphs 86 to 102 wherein said inlet aperture is located on the lower face of said casing in use.

104. A housing as outlined above in any one of paragraphs 86 to 103 wherein said inlet aperture is located generally centrally on said casing.

105. A housing as outlined above in any one of paragraphs 86 to 104 wherein said fan is an impeller unit.

106. A housing as outlined above in paragraph 105 wherein said impeller unit includes a spindle, adapted for connection to a motor in use to drive said impeller unit, said spindle passing out of the bottom of said casing.

107. A housing for a gases supply unit which in use forms part of a system for providing heated humidified gases to a user, said gases supply unit adapted to in use provide a pressurised gases stream, said housing forming an enclosing external shell which includes an atmospheric inlet adapted to allow gases from atmosphere to enter said shell in use, and a humidifier inlet aperture adapted to allow a pressurised gases stream to exit said shell, said shell including an internal recess adapted to hold a fan casing and a motor, so that in use said motor can drive a fan located inside said fan casing to draw air into said gases supply unit shell through said atmospheric inlet, said gases supply unit further including an oudet duct, having an oudet end adjacent to and fluidically connected to said humidifier inlet aperture, and an inlet end, and wherein said housing includes a passage fluidically connecting said fan unit oudet aperture and said inlet end of said oudet duct, said passage including at least one vent hole, arranged at an angle to the overall path of said gases stream through said passage.

108. A housing for a gases supply unit as oudined above in claim 107 wherein said passage is formed as an integral part of said shell.

109. A housing for a gases supply unit as oudined above in claim 108 wherein said passage is formed as a separate item to said shell, and is connected to said shell in use. 110. A housing for a gases supply unit as outlined above in any one of paragraphs 107 to 109 wherein said passage is located and arranged in said shell so that gases from said oudet aperture enter said passage substantially at a tangent to said fan.

111. A housing for a gases supply unit as outlined above in any one of paragraphs 107 to 110 wherein said passage and said at least one vent hole are configured so that if the oudet of said oudet passage is substantially blocked, die flow in use through said vent hole or holes matches the flow through said oudet passage at which the onset of surge would occur in a substantially similar, substantially blocked oudet passage without a vent hole or holes.

112. A housing for a gases supply unit as outlined above in any one of paragraphs 107 to 111 wherein said passage includes a plurality of said vent holes. 113. A housing for a gases supply unit as oudined above in any one of paragraphs 107 to 112 wherein said exit aperture is larger than the cross-sectional area of any individual one of said vent hole or holes by approximately three orders of magnitude.

114. A housing for a gases supply unit as outlined above in any one of paragraphs 107 to 113 wherein said exit aperture is larger than the cross-sectional area of any individual one of said vent hole or holes by approximately one to two orders of magnitude.

115. A housing for a gases supply unit as outlined above in any one of paragraphs 112 to 114 wherein said vent holes are located on two opposed sides of said passage.

116. A housing for a gases supply unit as outlined above in any one of paragraphs 112 to 115 wherein substantially the same number of vent holes are formed on each of said opposed sides.

117. A housing for a gases supply unit as outlined above in any one of paragraphs 112 to 116 wherein the total number of said holes is between 65 and 75. 118. A housing for a gases supply unit as outlined above in any one of paragraphs 107 to 117 wherein said angle of said vent hole or holes to said path of said gases stream is substantially perpendicular.

119. A housing for a gases supply unit as oudined above in any one of paragraphs 112 to 118 wherein each of said vent holes is substantially the same size as die others of said vent holes. 120. A housing for a gases supply unit as outlined above in any one of paragraphs 107 to 119 wherein said exit aperture has a cross-sectional area of substantially between 210 and 270mm ² .

121. A housing for a gases supply unit as oudined above in any one of paragraphs 112 to 120 wherein the cross-sectional area of each of said vent holes on the inner surface of said oudet passage is substantially between 0.22mm ² and 0.26mm ² . 122. A housing for a gases supply unit as outlined above in any one of paragraphs 112 to 121 wherein die total cross-sectional area of said vent holes on the inner surface of said oudet passage is substantially between 15.00mm ² and 17.00mm ² .

123. A housing for a gases supply unit as outlined above in any one of paragraphs 107 to 122 wherein said vent hole or holes are formed with a draft of substantially between 5° and 15°. 124. A housing for a gases supply unit as outlined above in claim 123 wherein said vent holes are formed with a draft of 10°.

125. A housing for a gases supply unit as outlined above in any one of paragraphs 107 to 124 wherein said exit aperture has a generally rectangular cross-section.

126. A housing for a gases supply unit as outlined above in any one of paragraphs 107 to 125 wherein said casing has a generally circular form and said oudet passage is tangential to said casing.

127. A housing for a gases supply unit as outlined above in any one of paragraphs 107 to 126 wherein said inlet aperture is located on the lower face of said casing in use.

128. A gases supply unit which in use forms part of a system for providing heated humidified gases to a user, said gases supply unit comprising: a fan unit having a fan casing with an inlet aperture and an outlet aperture, and a fan located in use inside said casing, said fan unit adapted to provide a pressurised gases stream through said outlet aperture in use, a motor, adapted for connection to said fan unit to drive said fan in use, said gases supply unit including a housing as outlined above in any one of paragraphs 107 to 127.

129. A medical breathing system suitable for supplying heated humidified gases to a user, comprising: a gases supply unit adapted to provide a pressurised gases stream, a humidifier chamber, said chamber fluidically connected to said gases supply unit in use so that said gases stream can pass into and through said chamber, said chamber containing a volume of water which in use is heated to heat and humidify said gases stream as said gases stream passes through said chamber, a patient interface adapted to provide said heated humidified gases stream to a user, said gases supply unit comprising: a fan unit having a fan casing with an inlet aperture and an oudet aperture, and a fan located in use inside said casing, said fan unit adapted to provide a pressurised gases stream through said oudet aperture to said humidifier chamber in use, a motor, adapted for connection to said fan unit to drive said fan in use, said gases supply unit including a housing as outlined above in any one of paragraphs 107 to 127.

130. A medical breathing system as outlined above in paragraph 129 wherein said external shell is adapted to at least partially enclose said humidifier chamber in use.

131. A medical breathing system as outlined above in paragraph 129 or paragraph 130 wherein said external shell and said humidifier chamber are mutually adapted so that said oudet aperture can mate directly with an inlet port to said humidifier chamber in use.

132. A medical breathing system as outlined above in any one of paragraphs 129 to 131 wherein said gases supply unit includes a heater plate, said gases supply unit powering said heater plate in use to heat said volume of water.

133. A fan unit which in use forms part of a gases supply unit suitable for use as part of a system for providing heated humidified gases to a user, said fan unit comprising: a casing having an inlet aperture and an outlet passage, said oudet passage including an exit aperture, a fan located inside said casing and adapted for connection to a motor to drive rotation of said fan in use, so that said fan draws gases into said casing via said inlet aperture, and forces said gases out of said casing via said outlet passage as a gases stream, and wherein said oudet passage includes a plurality of vent holes, independent of said exit aperture and arranged at an angle to the path of said gases stream through said oudet passage.

134. A fan unit as outlined above in paragraph 133 wherein said oudet passage and said vent holes are configured so that if the oudet of said oudet passage is substantially blocked, the flow in use through said vent holes matches die flow through said oudet passage at which the onset of surge would occur in a substantially similar, substantially blocked oudet passage without a vent hole or holes.

135. A fan unit as outlined above in claim 133 or claim 134 wherein said exit aperture is larger than the cross-sectional area of any one of the individual vent holes by approximately three orders of magnitude.

136. A fan unit as oudined above in any one of paragraphs 133 to 135 wherein said exit aperture is larger than the cross-sectional area of any one of the individual vent holes by approximately one to two orders of magnitude. 137. A fan unit as outlined above in any one of paragraphs 133 to 136 wherein said vent holes are located on two opposed sides of said passage.

138. A fan unit as oudined above in claim 137 wherein substantially the same number of vent holes are formed on each of said opposed sides.

139. A fan unit as outlined above in any one of paragraphs 133 to 138 wherein the total number of said vent holes is between 65 and 75.

140. A fan unit as outlined above in any one of paragraphs 133 to 139 wherein said angle of said vent holes to said path of said gases stream is substantially perpendicular.

141. A fan unit as outlined above in any one of paragraphs 133 to 140 wherein each of said vent holes is substantially the same size as the others of said vent holes. 142. A fan unit as oudined above in any one of paragraphs 133 to 141 wherein said exit aperture has a cross-sectional area of substantially between 210 and 270mm .

143. A fan unit as outlined above in any one of paragraphs 133 to 142 wherein the cross- sectional area of each of said vent holes on the inner surface of said oudet passage is substantially between 0.22mm ² and 0.26mm ² . 144. A fan unit as oudined above in any one of paragraphs 133 to 143 wherein the total cross- sectional area of said vent holes on the inner surface of said oudet passage is substantially between 15.00mm ² and 17.00mm ² .

155. A fan unit as outlined above in any one of paragraphs 133 to 144 wherein said vent holes are formed with a draft of substantially between 5° and 15°. 156. A fan unit as outlined above in claim 155 wherein said vent holes are formed with a draft of 10°.

157. A fan unit as oudined above in any one of paragraphs 133 to 156 wherein said casing has a generally circular form and said oudet passage is tangential to said casing.

158. A fan unit as oudined above in any one of paragraphs 133 to 157 wherein said exit aperture has a generally rectangular cross-section.

159. A fan unit as outlined above in any one of paragraphs 133 to 158 wherein said inlet aperture is located on the lower face of said casing in use.

160. A fan unit as outlined above in claim 159 wherein said inlet aperture is located generally centrally on said casing. 161. A fan unit as outlined above in any one of paragraphs 133 to 160 wherein said fan is an impeller unit.

162. A fan unit as outlined above in claim 161 wherein said impeller unit includes a spindle, adapted for connection to a motor in use to drive said impeller unit, said spindle passing out of the bottom of said casing.