Patent Application
20080178879
The present invention relates generally to an impeller for positive air pressure devices. More particularly, the present invention relates to an impeller for wearable positive air pressure devices.
Obstructive sleep apnea (OSA) is a condition that affects an estimated 14 million Americans. The condition is caused by relaxation of the soft tissue in the palate during sleep, resulting in obstruction of the upper airway. OSA is characterized by a complete cessation of breathing during sleep for 10 or more seconds (apnea), or a reduction in breathing for 10 or more seconds causing a 4% or greater decrease in blood oxygen level (hypopnea). Individuals having five or more apneic or hypopneic events per hour are diagnosed as suffering from OSA. The obvious side effects of sleep apnea are daytime sleepiness and chronic fatigue. However, OSA is known to be a contributing factor in hypertension, heart disease, as well as other serious health conditions.
The most common treatment for OSA is positive pressure (above-ambient) applied at the patient's nose or mouth, or at both the nose and mouth. This creates what is known as a pneumatic splint, which prevents the closing of the airway that causes apnea and hypopnea. Treatment pressures typically range between 4 and 20 cm H2O, depending primarily on the severity of the condition.
The PAP device 10 may also include other components or features that are adapted to increase patient comfort, or that are used for diagnostic purposes. Examples of comfort enhancing features include: air humidifiers 32 and heaters 30 that are designed to prevent soreness of the airway and larynx, by providing cold humidification or heated humidification.
The usual operational configuration of the PAP device consists of the PAP device sitting on a night table beside the bed. However, recent versions of the PAP device now have the device configured to be worn on a patient's body. For example, the device disclosed in United States Patent Application Publication No. US 2006/0096596 A1—Occhialini et al., has the PAP device and power supply located on the patient's body.
There are three common means by which positive airway pressure treatment is administered. The physical arrangement for the three methods is the same, the only substantial difference being in the programming of the controller. The first method is known as a continuous positive airway pressure (CPAP). A CPAP device is designed to maintain a prescribed positive pressure in the patient's airway at all times. The second method is known as bi-level positive airway pressure. Bi-level is similar to CPAP, except that the pressure alternates between two prescribed levels: a higher pressure during inhalation, and a lower pressure during exhalation. The third method is automatic positive airway pressure (APAP) treatment. An APAP device monitors the patient's breathing, and adjusts the positive pressure in response to apneic and hypopneic events, or other abnormal breathing. Each of the above methods has been demonstrated to provide effective treatment of OSA. Subsequent references in this document to positive airway pressure (PAP) treatment or devices are intended to include any or all of the above methods.
Although positive airway pressure is known to be an effective treatment for OSA, only 50% of patients prescribed PAP treatment use their device regularly. According to patient studies, the primary reason for this lack of compliance is that patients find the devices cumbersome and uncomfortable to wear. The size, weight, and alternating current (AC) power requirement restrict the patient's freedom for travel with the device, and freedom for movement while in bed. During use, patient movement frequently causes the tubing to tug on the interface, which may wake the patient. Tugging on the interface may also cause improper fit of the device, resulting in loss of effectiveness of the treatment and increased noise of the device due to air leakage. The 6-foot hose that normally connects the PAP device to the patient interface may also restrict body movement. The above factors are all likely to cause frequent patient arousal during the night, and contribute to the low level of compliance of PAP treatment.
Commercially available positive airway pressure devices require either direct AC connection for power, or, for portable devices, a substantially sized battery pack. For example, the lightest and most compact bedside PAP device currently on the market has a total weight of 2.1 lb, which does not include a power source. The wearable PAP system disclosed by Occhialini et al. is a lightweight and portable blower system made up of small air pumps, and requires a power supply weighing 1.68 kg (3.7 lb). Another commercially available device, such as taught in U.S. Pat. No. 7,012,346 to Hoffman et al., provides airway pressure between 5 cm H2O and 12 cm H2O, and has a total weight of 4.7 lb, including the battery. In spite of the reduced size, increased portability, and quieter operation of modern devices compared to their predecessors, inconvenience and awkwardness remain significant contributors to low patient compliance. In order to increase the patient compliance and usage, PAP devices need to be further miniaturized, must be more lightweight, and consume even less power than current alternatives.
The most significant obstacle to making devices used in treatment of sleep-disordered breathing portable is the high power consumption of the blower unit, therefore requiring a large and heavy stored energy device (i.e. battery). Typically, the largest and heaviest component in a portable PAP treatment apparatus is the battery or batteries. Minimizing battery size and weight can only be achieved with significant improvements in the efficiency of the blower unit, thereby minimizing the power consumption.
a is a schematic representation of the rotating blower 38 of the conventional PAP device 10 shown in
Rotating blowers are most often classified as either axial or radial, with the classification describing the meridional direction of the gas flow path, particularly as it exits the impeller. With reference to
Examples of axial impellers include ducted fans and propellers, in which the flow through the machine is primarily axial, with negligible radial motion. Axial impellers are generally characterized by high flow rates and low discharge pressures for a given rotational speed. Radial impellers, such as squirrel-cage blowers and centrifugal pumps, are characterized by a through flow which is primarily radial, with no significant axial component. These machines are generally used when high discharge pressures and lower flow rates are required for a given rotational speed. A third, and somewhat less common, type is the mixed-flow blower, where the direction of the flow exiting the impeller has significant components in both the axial (z) and radial (r) directions. These machines are most appropriate in applications where the flow rate and discharge pressure are both moderate. At operating speeds typical of currently available brushless DC motors, the radial impeller provides the best efficiency for pressures and flow rates typical of PAP treatment devices.
For a given geometric design, the pressure developed by a rotating impeller is approximately proportional to the square of the velocity of the impeller at its outer periphery. Thus, the output pressure may be increased either by increasing the rotational speed of the impeller, or by increasing the impeller diameter. Heretofore, several attempts have been made to design impellers with improved efficiency based on the construction and geometry of the impeller blades or vanes.
U.S. Pat. No. 6,681,033 to Makinson et al. teaches an impeller having a plurality of impeller vanes molded over the permanent magnets of an electric motor rotor. The impeller vanes are arranged in an annular array on the face of a disc shaped rotor and in a plane substantially perpendicular to the plane of the disc. The impeller vanes have a curved profile. Makinson et al. teach an improved construction of the impeller to reduce the imbalances of the completed rotor product thereby improving the efficiency of the impeller.
U.S. Pat. No. 6,622,724 to Truitt et al. discloses an impeller having a plurality of impeller blades disposed on a face of the impeller body with an inlet area between each pair of adjacent blades being substantially equal to a corresponding outlet area for each pair of adjacent blades. Maintaining of the inlet area substantially equal to the outlet area is believed to provide a substantially constant pressure gas at the outlet, despite fluctuations in the flow rate typically encountered in a respiratory pressure support system. This is achieved by having the blades decrease in height as they extend radially outward from the hub.
Other methods for improving the efficiency of an impeller used in PAP devices include the use of two impellers as taught by Daly et al. in U.S. Pat. No. 6,910,483.
All of the impellers described above are radial impellers characterized by a through flow which is primarily radial, with no significant axial component. Although radial impellers provide the best performance for pressures and flow rates typical of PAP treatment devices, there is significant scope for improving the efficiency of impellers in order to achieve miniaturization of impellers needed for wearable PAP devices.
It is, therefore, desirable to provide a method and a PAP device with an improved impeller of smaller size that produces at least the same air pressure as prior art devices or an increased air pressure for a given size.
It is an object of the present invention to obviate or mitigate at least one disadvantage of previous impellers for wearable PAP devices. This is achieved with an impeller, which successively accelerates the pumped air in substantially radial and axial directions. The term acceleration in a substantially radial direction as used in this specification means that a major portion of the angular acceleration occurs in a direction perpendicular to the axis of rotation of the impeller. The term acceleration in a substantially axial direction means that a major portion of the angular acceleration occurs in a direction parallel to the axis of rotation of the impeller.
In a first aspect, the invention provides a method for increasing output pressure of a blower unit in a PAP device, the blower unit having a blower rotatable about an axis of rotation. The method includes the steps of ingesting air into the blower unit, successively accelerating the ingested air in a direction substantially radial to the axis of rotation and a direction substantially parallel to the axis of rotation for generating a flow of compressed air; and exhausting the accelerated air from the blower unit.
In a second aspect, the present invention provides a rotary impeller for a blower unit in a PAP device. The impeller has an axis of rotation and includes a rotatable impeller body, and radial vanes connected to the impeller body for accelerating air in a substantially radial direction upon rotation of the impeller body, to generate a generally radial air flow. Each radial vane has a pair of end portions which are, relative to the direction of the air flow, a leading portion with a leading edge and a trailing portion with a trailing edge respectively; and at least one of the radial vanes has an end portion which is curved for accelerating air in a substantially axial direction upon rotation of the impeller body.
In an embodiment of the present invention, there is provided a blower for use in a PAP device, including a housing, an impeller rotatably mounted in the housing, and a motor for rotating the impeller. The housing includes an inner casing and an outer casing, the impeller has a hub meridional line and a tip meridional line and at least one of the following applies: the impeller hub and/or tip meridional line(s) are within 20 degrees of perpendicular to the axis of rotation of the impeller at least at one point between an impeller inlet and an impeller outlet; at the impeller outlet the impeller hub meridian line and the impeller tip meridian line are within 20 degrees of the axis of rotation of the impeller; the impeller vanes are extended forward along the axis of rotation and also curved in the direction of rotation; the geometry of the vanes at their leading edge is chosen such that a direction of inlet flow relative to the rotating impeller blade is within 10 degrees of an angle of the vane; the leading edges of the vanes follows a curved path from a base edge of the vane to a free edge of the vane; the outer casing and impeller define an intermediate air flow path and pressurized air is bled from the flow path between the impeller and the inner casing of the blower and into contact with the motor for cooling of the motor; the housing includes bleed air channels for diverting the bled air to pass over and come into direct contact with the motor; the housing includes cooling members in thermal contact with the motor and extending into the bleed air channels for providing convective cooling of the motor.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:
a and 3b are front view and side view schematic representations, respectively, of a typical radial blower used in the prior art device of
a is a front view of an impeller according to a first embodiment of the present invention;
b is a side view of the impeller of
c is a schematic sectional view of the impeller of
a is a front view of an impeller according to a second embodiment of the present invention;
b is a side view of the impeller of
Generally, the present invention provides a method for improving the air pumping efficiency of a PAP device, and a PAP device with an improved impeller that generates increased air pressure compared to prior art devices or produces at least the same air pressure at a reduced size.
As discussed earlier, the pressure developed by a rotating impeller of given geometric design is approximately proportional to the square of the velocity of the impeller at its outer periphery. Thus, the output pressure may be increased either by increasing the rotational speed of the impeller, or by increasing the impeller diameter.
In view of the move to miniaturization of wearable PAP devices and the concurrent need for reduced impeller size and a compact design, increasing the rotational speed is preferred over increasing the size of the impeller. However, the maximum rotational speed of the impeller is limited by the capacity of the motor and/or the acceptable level of noise emitted by motors and impellers operating at high rotational speeds. Thus, a blower/impeller design is needed which maximizes the use of limited available space, and motor output.
This is achieved with an air compression method and impeller design in accordance with the present invention. The output pressure of a rotary air pump in a PAP device is increased by successively accelerating air ingested by the blower in substantially radial and axial directions. Preferably, the air is first accelerated in a substantially radial direction and subsequently in substantially axial direction. Most preferably, the air is successively accelerated in a substantially axial direction, a substantially radial direction and finally again in a substantially axial direction.
a shows a front view of an impeller according to a first embodiment of the present invention. The impeller 160 includes a hub 164 and a plurality of radial blades or vanes 122 and 122′ arranged in an annular array on the face of the impeller disc 160. In the illustrative example, vanes 122 extend radially outwardly from the hub 164 to the periphery of the impeller disc 160. Alternating vanes 122′ are optionally shorter in length and extend radially outwardly from the vicinity of the middle portion of the impeller disc 160 to the periphery thereof. The vanes 122 have end portions with leading edges 122a and trailing edges 122b respectively. The leading and trailing edges 122a, 122b of the vanes 122 are defined as the edges of the vanes 122 in the proximity of the hub 164 and the periphery of the impeller disc 160, respectively. The vanes 122′ have similar leading and trailing edges.
As shown in
Due to the geometry of the construction of the impeller 160 and curvature of the trailing edges 122b of the vanes 122 and 122′, the air drawn by the impeller 160 is successively accelerated in the radial direction, due to the rotation of the impeller, and in the axial direction by the curvature of the trailing edges 122b in the axial direction. Thus, the pressurized air exits the impeller 160 in a direction substantially parallel to the axis of rotation of the impeller 160 as shown in
In a preferred embodiment, the overall diameter of the device is larger than the impeller 160 only by the thickness of the housing and the necessary clearance between the impeller and housing to allow rotation of the impeller without contacting the housing. With this modification, the volute 180 can be offset from the impeller 160 axially, rather than radially as in the prior art, providing a more compact design in applications where the diameter of the blower is to be minimized. The volute 180 is preferably offset in the direction toward the motor 168, as shown in
Under typical load conditions, the winding temperature of brushless DC motors may exceed 300 degrees Fahrenheit (150 degrees Celsius), and the surface temperature of the motor casing may reach temperatures between 175 and 200 degrees Fahrenheit (80 and 100 degrees Celsius). Having the motor contained in an enclosed space, and in close proximity to the patient, presents a significant safety hazard if the device overheats or catches fire. In the device according to the first embodiment of the present invention, under extreme loading conditions, the motor 168 may require as much as 3 W of cooling to remain at a safe temperature. To minimize the risk of injury, the preferred embodiment of the device includes a means of cooling the motor 168 and safely removing excess heat from the device. As shown in
In the second embodiment of the present invention, the output pressure of a rotary air pump in a PAP device is increased by successively accelerating air ingested by the blower, first in the axial direction followed by a second acceleration in the radial direction as shown in
The impeller 260 shown in
Unlike the trailing edges 122b of the vanes 122 and the trailing edges of the vanes 122′, which are curved in the axial direction, trailing edges 222b of the vanes 222 and the trailing edges of the vanes 222′ are not curved in this embodiment. However, the leading edges 222a of the vanes 222 are extended forward along the axis of rotation of the impeller 260 and are also curved in the direction of rotation as illustrated in
In the third embodiment of the present invention, the output pressure of a rotary air pump in a PAP device is increased by successively accelerating air ingested in a substantially axial direction, a substantially radial direction and finally again in a substantially axial direction. This is achieved by combining the geometries of the vanes 122, 122′ and vanes 222 and 222′.
The impeller 360 shown in
The curvature of the leading edges 322a of the vanes 322 and the leading edges of the vanes 322′ is similar to the curvature of the leading edges 222a of the vanes 222 of the second embodiment. In addition, the curvature of the trailing edges 322b of the vanes 322 and the trailing edges of the vanes 322′ is similar to the curvature of the trailing edges 122b of the vanes 122 and that of the trailing edges of the vanes 122′ of the first embodiment. Thus, air drawn in a direction parallel to the axis of rotation of the impeller 360 is first accelerated in the axial direction followed by a second acceleration in the radial direction, and a third acceleration in the axial direction, thereby increasing the output pressure of the rotary air pump in the PAP device.
In a preferred embodiment, the leading edges 322b of the vanes 322 and the leading edges of the vanes 322′ are also shaped such that they follow a curved path from the hub 364 of the impeller 360 to the leading tip 321 of the vanes 322 and 322′ as shown in
In this example, the inner portion 382 and the outer portion 384 of the housing base 372 are shaped such that they form an annular space 380 downstream of the impeller 360. This annular space, which replaces the volute or scroll 180 described in previous embodiments, allows the pressurized air to exit the blower unit in a direction parallel to the axis of rotation of the motor 368. Since this is also parallel to the direction of the inlet flow, the change in direction between the inlet and the outlet, characteristic of conventional volute designs, is eliminated, and the blower unit is more easily fitted into a compact device. As in previous embodiments, the impeller 360 contains a section where the vane meridians at the base 323 and at the free edge 324 are directed primarily in the radial direction, thus taking advantage of the centrifugal compression characteristic of radial blowers. In a preferred embodiment of the invention, the inner portion 382 of the housing base 372 of the blower is in part or in whole, comprised of the outer housing of the motor 368, whereby the patient treatment air comes into direct contact with the motor housing. In order to better illustrate this principle,
Furthermore, the annular space 380 also contains two or more stationary protrusions 390, which are used to attach the outer portion 384 of the housing base to the inner portion of the housing base 372. Preferably, the number of stationary protrusions 390 is chosen such that it is not an integer multiple of the number of impeller vanes 322 and 322′, nor is the number of vanes 322 and 322′ an integer multiple of the number of stationary protrusions 390. This prevents more than one vane 322 or 322′ from simultaneously passing within close proximity of a stationary protrusion 390, thereby reducing the acoustic noise output by the device.
It is known in the art that higher aerodynamic efficiency is obtained in axial through-flow machines when the angular momentum of the flow, imparted by the rotating impeller, is reduced by means of a row of stator blades, placed downstream of the impeller. If properly designed, these stator blades convert a significant proportion of the angular kinetic energy of the flow into a static pressure, which would otherwise be lost if the flow were allowed to diffuse naturally. As shown in
To minimize the risk of patient injury due to burns or fire, the stationary protrusions 390 are also preferably constructed to function as cooling vanes for the motor 368. For that purpose, the protrusions 390 are in contact with both the motor 368 and the flow of air, and are constructed from a material having a low thermal resistance. Examples of suitable materials include stainless steel, aluminum alloys, and high-conductance polymer resins. This aspect of the invention serves to conduct heat from the motor 368 along the stationary protrusions 390, which are in turn cooled by the flow of air passing thereover. This allows cooling of the motor 368 to occur without the need to bleed air from the patient treatment circuit, thereby increasing the efficiency of the device, as compared to the embodiment shown in
In order to diagnose certain system faults in PAP devices, accurate measurement of the flow rate through the device is required. Flow rate is often also logged for clinical purposes. Commercially available flow metering devices, for the normal range of flow rates in use, typically measure 2-3 inches in length, and up to 1 inch in diameter. Alternatives, such as flow nozzles and orifice meters, are more compact, but require an additional pressure sensor to be present. Either of these options increases the weight, size, and manufacturing cost of the PAP device. Furthermore, the pressure sensing port, or ports, in these devices can become clogged with dust or other particles, causing failure of the device, or undesired behavior due to incorrect control input. Fortunately, for a given blower, flow rate (Q) can be correlated to the motor speed (N) and output pressure (ΔP). These quantities are normally already measured in the device, since they are used to control the motor 368 and also the output pressure. The form of the correlation is typically:
where the function f represents a curve fit to measured data. In most cases, a low-order polynomial (e.g. quadratic), or even a straight-line fit, provides an acceptable fit to the data, as shown in the example of
In PAP treatment, the mask or patient interface is typically connected to the blower unit by a flexible hose, which typically measures 6 feet or more in length. In an embodiment of the present invention, shown in
In the preferred embodiment of the invention shown in
In addition, for convenience of use, the device may also include a remote control unit, from which the patient may control the various settings of the unit. The remote-control unit is in wireless communication with the blower unit 138, and, at a minimum, allows the user to power the device on and off, and adjust the treatment pressure. The remote control unit also preferably allows for control of any additional accessories that may be present in the unit, including, but not limited to, humidification and heating of the treatment air. The remote control unit also comprises data acquisition, data processing, and memory storage devices that may be used to record unit performance and patient compliance, and to diagnose sleep disordered breathing events. The remote control unit preferably comprises a display screen that is used to communicate information to the patient, such as the device status, current treatment pressure, and remaining battery life, etc. Other optional display options may include the date and time, device usage, and other information as may be required for clinical purposes.
Thus, the PAP device according to the present invention is lightweight, wearable, and travel-friendly. In the present arrangement, the weight of the entire treatment apparatus, including the blower unit, power source, electronics, and patient interface, is approximately 1 lb (450 g) and is capable of producing treatment pressures typically used in PAP therapy, for example, up to 12 cm H2O, for periods of up to 8 hours on a single battery charge. Reducing the length of coupling unit 96, fixing the geometric relationship between the blower unit 138 and the patient interface 95, and maintaining all elements of the apparatus within close proximity to each other and to the patient, significantly reduces the frequency of interface leaks due to patient movement. This is expected to significantly contribute to increased patient compliance of PAP therapy.
