Abstract:
A variable speed blower for Continuous Positive Airway Pressure (CPAP) ventilation of patients includes two impellers in the gas flow path that cooperatively pressurize gas to desired pressure and flow characteristics. Thus, the blower can provide faster pressure response and desired flow characteristics over a narrower range of motor speeds, resulting in greater reliability and less acoustic noise.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an apparatus for supplying breathable gas to a human, used in, for example, Continuous Positive Airway Pressure (CPAP) treatment of Obstructive Sleep Apnea (OSA), other respiratory diseases and disorders such as emphysema, or the application of assisted ventilation. 
       DESCRIPTION OF RELATED ART 
       [0002]    CPAP treatment of OSA, a form of Noninvasive Positive Pressure Ventilation (NIPPV), involves the delivery of a pressurized breathable gas, usually air, to a patient&#39;s airways using a conduit and mask. Gas pressures employed for CPAP can range from 4 cm H 2 O to 28 cm H 2 O, at flow rates of up to 180 L/min (measured at the mask), depending on patient requirements. The pressurized gas acts as a pneumatic splint for the patient&#39;s airway, preventing airway collapse, especially during the inspiratory phase of respiration. 
         [0003]    Typically, the pressure at which a patient is ventilated during CPAP is varied according to the phase of the patient&#39;s breathing cycle. For example, the ventilation apparatus may be pre-set to deliver two pressures, an inspiratory positive airway pressure (IPAP) during the inspiration phase of the respiratory cycle, and an expiratory positive airway pressure (EPAP) during the expiration phase of the respiratory cycle. An ideal system for CPAP is able to switch between IPAP and EPAP pressures quickly, efficiently, and quietly, while providing maximum pressure support to the patient during the early part of the inspiratory phase. 
         [0004]    In a traditional CPAP system, the air supply to the patient is pressurized by a blower having a single impeller. The impeller is enclosed in a volute, or housing, in which the entering gas is trapped while pressurized by the spinning impeller. The pressurized gas gradually leaves the volute and travels to the patient&#39;s mask. 
         [0005]    There are currently two common ways in which the blower and impeller can be configured to produce the two different pressures, IPAP and EPAP, that are required in an ideal CPAP system. A first method is to set the motor/impeller to produce a constant high pressure and then employ a diverter valve arrangement that modulates the high pressure to achieve the required IPAP and EPAP pressures. CPAP systems according to the first method are called single-speed bi-level systems with diverters. A second method is to accelerate the motor that drives the impeller to directly produce IPAP and EPAP pressures. CPAP systems according to the second method are called variable-speed bi-level systems. 
         [0006]    Variable-speed bi-level CPAP systems have a number of particular disadvantages. A first disadvantage is that in order to switch rapidly between IPAP and EPAP, the impeller must be accelerated and decelerated rapidly. This causes excessive stress on the impeller, motor, and bearings. However, if the impeller is accelerated slowly, the pressure rise may be unsatisfactorily slow, and thus, the patient may not receive adequate treatment. 
         [0007]    Rapid acceleration and deceleration of the motor and impeller also result in excessive heat generation and undesirable acoustic noise. (“Undesirable” acoustic noise, as the term is used here, refers to acoustic noise that is overly loud, as well as acoustic noise which occurs at a frequency that is irritating to the user, regardless of its volume.) In addition, design engineers are often forced to make a compromise, sacrificing optimal pressure and flow characteristics in favor of achieving a desired peak pressure. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention, in one aspect, relates to variable speed blowers providing faster pressure rise time with increased reliability and less acoustic noise. Blowers according to the present invention comprise a gas flow path between a gas inlet and a gas outlet, a motor, and an impeller assembly. 
         [0009]    Preferably, the impeller assembly may include a shaft in communication with the motor for rotational motion about a first axis and first and second impellers coupled, e.g., fixedly secured, to the shaft. The impellers are placed in fluid communication with one another by the gas flow path such that both impellers are disposed between the gas inlet and the gas outlet to cooperatively pressurize gas flowing from the gas inlet to the gas outlet. 
         [0010]    In one embodiment, the impellers are disposed in series between the gas inlet and the gas outlet. The blower may also comprise a housing, portions of the housing being disposed around each of the first and second impellers. In particular, the housing may include first and second volutes, the first volute containing gas flow around the first impeller and the second volute containing gas flow around the second impeller. The gas inlet may be located in the first volute and the gas outlet may be located in the second volute. 
         [0011]    The impellers may be arranged such that they are vertically spaced from one another along the first axis. In particular, they may be disposed at opposite ends, respectively, of the blower housing. 
         [0012]    A blower according to the present invention may have varying configurations. In one embodiment, the two impellers are designed to rotate in the same direction. In another embodiment, the two impellers are designed to rotate in opposite directions. 
         [0013]    Another aspect of the invention relates to an in-plane transitional scroll volute for use in either a double- or single-ended blower. The in-plane transitional scroll volute gradually directs pressurized air away from a spinning impeller. 
         [0014]    These and other aspects of the present invention will be described in or apparent from the following detailed description of preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Various exemplary embodiments will be described with reference to the following drawings, in which like reference characters represent like features, wherein: 
           [0016]      FIG. 1  is a perspective view of a double-ended blower according to a first embodiment of the present invention; 
           [0017]      FIG. 2  is a partially sectional perspective view of the double-ended blower of  FIG. 1 ; 
           [0018]      FIG. 3  is a perspective view of a double-ended blower according to a second embodiment of the present invention; 
           [0019]      FIG. 4  is a sectional perspective view of the double-ended blower of  FIG. 3 ; 
           [0020]      FIG. 5  is a rear perspective view of the double-ended blower of  FIG. 3 , illustrating the flow therethrough; 
           [0021]      FIG. 6  is a perspective view of an in-plane transitional scroll volute suitable for use in blowers according to the present invention; 
           [0022]      FIG. 7  is an exploded perspective view of a double-ended blower according to another embodiment of the present invention; 
           [0023]      FIG. 8  is an assembled perspective view of the double-ended blower of  FIG. 7  from one side; and 
           [0024]      FIG. 9  is an assembled perspective view of the double-ended blower of  FIG. 7  from another side. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Referring now to the Figures,  FIG. 1  is a perspective view of a double-ended blower  100  according to a first embodiment of the present invention. Blower  100  has a generally cylindrical shape with impeller housings, or volutes  112 ,  113 , disposed at each end. Thus, blower  100  accommodates two impellers  114 ,  115 , which are best seen in the cut-away perspective view of  FIG. 2 . 
         [0026]    Referring to  FIGS. 1 and 2 , the two impellers  114 ,  115  are placed in fluid communication with one another by an airpath  116 . The airpath  116  of blower  100  is comprised of piping that extends from the first volute  112  to the second volute  113 , the terminal ends of the airpath  116  being contoured around, and gradually fusing with, the body of blower  100  proximate to the volutes  112 ,  113  to form a single, integral structure. The airpath  116  may be comprised of rigid piping that is integrally molded with the other components of the blower  100 , or it may be comprised of flexible piping (e.g., metallic or plastic flexible piping). 
         [0027]    Blower  100  has a single air intake  118  positioned such that air, or another suitable gas, flows directly into the first volute  112  and can be drawn in by the turning impeller  114  inside the first volute  112 . Once drawn into the air intake  118 , the air is circulated and pressurized by the motion of the impeller  114  before gradually exiting the volute  112  and entering the airpath  116 . Once in the airpath  116 , the air travels to the second volute  113 , where it is further circulated and pressurized by the impeller  115  of the second volute  113  before exiting the blower  100  through the outflow conduit  120 . The path of the air in blower  100  is indicated by the arrows in  FIG. 1 . As shown, in blower  100 , air from the first volute  112  travels along a relatively straight section of the airpath  116  and enters the second volute  113  through an intake cavity just above the second volute  113  (not shown in  FIG. 1 ). 
         [0028]    Blower  100  could have two air intakes  118 , one for each volute  112 ,  113 , if the impellers  114 ,  115  are designed to work in parallel, rather than in series. This type of parallel impeller arrangement may be beneficial if installed in a low-pressure CPAP device requiring high flow rates. However, other means for generating high flow rates in a low-pressure CPAP device are known in the art. 
         [0029]    The design of the airpath  116  can effect the overall performance of the blower  100 . In general, several design considerations influence the design of an airpath for use in blowers according to the present invention. First, airpaths to be used in blowers according to the present invention are most advantageously configured to provide low flow resistance, because low flow resistance in the airpath minimizes the pressure drop between the two volutes  112 ,  113  in the blower. Second, airpath are best configured such that the air entering the second volute  113  enters from a direction for which the blades of the impeller  115  were designed. (As will be described in more detail below, the two impellers of a blower according to the present invention may be designed to spin in the same or different directions.) Finally, airpaths for blowers according to the present invention are most advantageously of a compact design. 
         [0030]    The design considerations set forth above are best embodied in an airpath having long, sweeping bends to minimize the pressure drop around the bends. It is also beneficial to have a relatively straight section after a bend in the airpath, because a relatively straight section after a bend allows the gas flow to become more fully developed before entering a volute. An appropriate length for a straight airpath section following a bend is about three times the diameter of the airpath. The relatively straight section also ensures that the flow entering the second volute  113  is axial, the flow orientation for which many impellers are designed. If additional flow shaping is desired, stator vanes or other similar flow directing structures may be added to the blower, however, stator vanes may be costly in terms of flow impedance and pressure drops. 
         [0031]    In view of the three major airpath design considerations set forth above, the airpath  116  of the embodiment depicted in  FIG. 1  has a long, relatively straight section because the relatively straight section is one of the shortest possible paths between the two volutes  112 ,  113 . Those skilled in the art will realize that the airpath  116  need not be straight at all. 
         [0032]    Blowers according to the invention may be designed manually, using prototypes and experimental measurements of air flows and pressures in those prototypes to optimize the design of the airpath  116  and other components. Alternatively, they may be designed, either as a whole or in part, by using computational fluid dynamics computer simulation programs. A variety of computational fluid dynamics programs are known in the art. Computational fluid dynamics programs particularly suited for the design of blowers according to the invention include FLOWORKS (NIKA GmbH, Sottrum, Germany), ANSYS/FLOTRAN (Ansys, Inc., Canonsburg, Pa., USA), and CFX (AEA Technology Engineering Software, Inc., El Dorado Hills, Calif., USA). Such simulation programs give the user the ability to see the effects of airpath design changes on a simulated gas flow. 
         [0033]    Many different types of configurations for the two volutes  112 ,  113  and airpath  116  are possible in a double-ended blower according to the present invention. In general, each volute is designed to retain the gas around the impeller for a short period of time, and to permit a gradual exit of gas into the airpath. The exact configuration of the airpath may depend on many factors, including the configuration of the volutes and the “handedness,” or direction of airflow, around each impeller. 
         [0034]    The design of the volutes is an art unto itself, as improperly designed volutes may cause a noise, or may interfere with the generation of the desired pressure and flow characteristics. The computational fluid dynamics computer programs described above may also be useful in designing the volutes, although the number of variables involved in volute design usually precludes the volute from being entirely computer-designed. 
         [0035]    One common problem with volutes  112 ,  113  is that they may provide too abrupt of a transition into the airpath  116 . An abrupt transition between the volute  112 ,  113  and the airpath  116  usually leaves a forked path or “lip” around the opening. When the impeller blades pass by this lip, a noise called “blade passing frequency” is created. Double-ended blowers according to the present invention are particularly suited for use with volutes that are constructed to reduce the occurrence of “blade passing frequency” and other noise. 
         [0036]      FIG. 6  is a perspective view of an in-plane transitional scroll volute  300  suitable for use in a blower according to the present invention. Additionally, the volute  300  may be employed in any conventional blower apparatus. In the view of  FIG. 6 , the volute  300  is provided with its own motor  302 , although it may be adapted for use in a double-ended blower having a single motor driving the impellers in two volutes. As shown, the volute  300  is comprised of two halves  304 ,  306 , the two halves defining upper and lower portions of the volute  300 , respectively. The air intake of the volute  308  is located at the center of the top half  304 . The two halves  304 ,  306  define a path which slowly “peels” away from the air rotating with the impeller. In the path defined by the two halves, there is no sudden “lip” or “split” as in conventional volutes, therefore, “blade passing frequency” is reduced or eliminated entirely. The volute  300  depicted in  FIG. 6  is particularly suitable for relatively short, wide motors. 
         [0037]    Alternatively, any common type of volute may be used, depending on the dimensions of the motor installed in the blower. Another suitable type of volute is the axial volute disclosed in U.S. patent application Ser. No. 09/600,738, filed on Jul. 21, 2000, the contents of which are hereby incorporated by reference herein in their entirety. 
         [0038]    One important design consideration for a double-ended blower according to the present invention is the “handedness,” or direction of airflow, around each impeller. This “handedness” may be determined by the direction in which the impeller spins, or it may be determined by the orientation and configuration of the individual blades or vanes of the impeller. For example, one impeller may be spun or the blades oriented to drive the air in a clockwise direction, and the other impeller may be spun or the blades oriented to drive the air in a counterclockwise direction, resulting in a “opposing-handed” double-ended blower. Alternatively, both impellers could be driven in the same direction, resulting in a “same-handed” double-ended blower. Blower  100  of  FIG. 1  is an example of an “opposite-handed” type of double-ended blower. 
         [0039]    A “same-handed” blower is advantageous because the two impellers can be identical, reducing the part count and cost of the blower. However, it should be noted that a designer may choose to design a “same-handed” blower in which the two impellers are each designed and optimized for the air flow in their respective volutes. 
         [0040]    An “opposing-handed” blower permits the designer to reduce the length of the shaft on which the impellers are mounted. This may increase the stability of the shaft itself, because it reduces the problems associated with having an imbalance on a long, cantilevered shaft rotating at high speed. 
         [0041]      FIG. 3  illustrates a “same-handed” blower  200  according to the present invention. Blower  200  also has two volutes  212 ,  213 , an airpath  216 , an air intake  218  and an air outlet  220 . However, as is shown in  FIG. 3 , the airpath  216  has the shape of a spiral. That is, airpath  216  transitions away from the first volute  212  and then slopes downward as it follows the circumference of the blower  200 , before bending and gradually fusing with an intake cavity located between the motor  150  and the arcuate flange  160 , which acts as an air intake in blower  200 . The airflow through the blower  200  is illustrated by the arrows in the perspective view of  FIG. 5 . 
         [0042]    The internal configuration of blower  200  is shown in the partially sectional perspective view of  FIG. 4 . The internal arrangements of blowers  100  and  200  are substantially similar, and will be described below with respect to components of both blowers, Where applicable. As shown in  FIG. 4 , an electric motor  150  is installed in the center of the blowers  200 . Various types of known brackets and mountings may be used to support the motor and to secure it to the interior of the blower  200 , although for simplicity, these are not shown in  FIG. 4 . 
         [0043]    The motor  150  drives a single shaft  152 . The shaft  152  traverses substantially the entire length of the blower  100 ,  200  along its center, and is secured to an impeller  114 ,  115 ,  214  at each end. The shaft may be round, square, keyed, or otherwise shaped to transmit power to the two impellers  114 ,  115 ,  214 . The connection between the impellers  114 ,  115 ,  214  and the shaft  152  may be created by an interference fit between the two parts, a weld, an adhesive, or fasteners, such as set screws. In blowers  100  and  200 , the connection between the shaft  152  and the impellers  114 ,  115 ,  214  is by means of a vertically oriented (i.e., oriented along the axis of the shaft  152 ) annular flange  154  formed in the center of the impellers  114 ,  115 ,  214 . In  FIGS. 3 and 4 , the connection between the impellers  114 ,  115 ,  214  and the shaft is shown as an interference fit. 
         [0044]    The impeller  114 ,  115 ,  214  is substantially annular in shape. The center section  156  of the impeller  114 ,  115 ,  214 , is a thin plate which extends radially outward from the shaft  152  to the blades  158 , and is upswept, gradually curving downward as it extends outward from the shaft  152  towards the blades  158 . The actual diameter of each impeller  114 ,  115 ,  214  may be smaller than that of a conventional blower with a single impeller. Fast pressure rise time in a blower requires a low rotational inertia, which varies as the diameter to the fourth power. Because impellers  114  and  214  of blowers  100  and  200  are smaller in diameter, they have less rotational inertia, and thus, are able to provide a faster pressure rise time. In addition to diameter, other design parameters of the impellers  114 ,  214  may be modified to achieve a lower rotational inertia. Other techniques to reduce rotational inertia include “scalloping” the shrouds to produce a “starfish-shaped” impeller, using an internal rotor motor, and using materials, such as liquid crystal polymer, that can be molded into thinner wall sections, so that impeller blades can be hollowed out and strengthened by ribs. 
         [0045]    Referring to  FIGS. 4 and 5 , which show the same-handed, double-ended blower, the top of the first volute  212  is open, forming the air intake  118 . At the air intake  118 , the top surface  120  of the blower  100  curves arcuately inward, forming a lip  122  over the top of the impeller  214 . The upswept shape of the impeller center section  156  and the lip  122  of the top surface  120  confine the incoming air to the blower volume inside the first volute  212  and help to prevent air leakage during operation. An arcuate flange  160  similar to the arcuate top surface  120  extends from the lower interior surface of the blower  200 , forming the top of the second volute  213 . A contoured bottom plate  162 ,  262  forms the bottom of the second volute  113 ,  213  of each blower  100 ,  200 . The bottom plate  162  of blower  100  has a hole in its center, allowing the airpath  116  to enter, while the bottom plate  262  of blower  200  has no such hole. As described above, the arcuate flange  160  acts as the air intake for the second volute  213  of blower  200 . In blower  200 , stator vanes and additional flow shaping components may be added to the cavity between the motor  150  and the arcuate flange  160  to assist in distributing the incoming air so that it enters the second volute  213  from all sides, rather than preferentially from one side. 
         [0046]    As is evident from  FIGS. 2 and 4 , blowers according to the present invention may have many intricate and contoured surfaces. Such contours are used, as in the case of the arcuate top surface  120  and arcuate flange  160 , to direct gas flow and prevent gas leakage. The no-leak requirement is particularly important when the gas flowing through the blower  100 ,  200  has a high concentration of oxygen gas. If high-concentration oxygen is used, gas leakage may pose a safety hazard. Also, apart from any safety considerations, leaking gas may produce unwanted noise, and may reduce blower performance. 
         [0047]    The number of intricate, contoured surfaces present in blowers according to the present invention makes a production method such as investment casting particularly suitable. Although relatively expensive, investment casting can produce a single part with many hidden and re-entrant features, whereas other methods of production may require that a design be split into many parts to achieve equivalent function. However, a large number of parts is generally undesirable—in order to minimize the potential for gas leaks, the number of parts is best kept to a minimum and the number of joints between parts is also best kept to a minimum. 
         [0048]    There are also a number of materials considerations for blowers according to the present invention. Metals are typically used in investment casting, but some metals are particularly sensitive to oxidation, which is a concern because medical grade oxygen gas may be used in blowers according to the present invention. One particularly suitable material for the blowers  100 ,  200  is aluminum. Whereas steel may rust on exposure to high concentrations of oxygen, aluminum oxidizes quickly, the oxide forming an impervious seal over the metal. Whichever metal or other material is used, it is also important that the material has a high thermal conductivity and is able to draw heat away from the airpath, to prevent any heat-related ignition of oxygen. 
         [0049]    While the use of aluminum has many advantages, it does have a tendency to “ring,” or resonate, during blower operation. Therefore, damping materials may be installed in an aluminum blower to reduce the intensity of the vibration of the aluminum components. 
         [0050]    In blowers  100  and  200 , the electric motor  150  is driven at variable speeds to achieve the desired IPAP and EPAP pressures. The double-ended (i.e., two-stage) design of the blowers means that the range of motor speeds traversed to achieve the two pressures is reduced. The narrower range of motor speeds results in a faster pressure response time than that provided by a single-stage blower having similar motor power and drive characteristics. In addition, the narrower variation in speed applies less stress to the rotating system components, resulting in increased reliability with less acoustic noise. 
         [0051]    The performance of blowers  100  and  200  is approximately equal to the combined performance of the two impeller/volute combinations, minus the pressure/flow curve of the airpath  116 ,  216  between the two volutes  112 ,  113 ,  212 ,  213 . For a variety of reasons that are well known in the art, the actual performance of the blowers  100 ,  200  will depend upon the instantaneous flow rate of the particular blower  100 ,  200 , as well as a number of factors. At higher flow rates, the pressure drop in the airpath  116 ,  216  is generally more significant. 
         [0052]    Double-ended blowers according to the present invention may be placed in a CPAP apparatus in the same manner as a conventional blower. The blower is typically mounted on springs, or another shock-absorbing structure, to reduce vibrations. 
       A Further Embodiment 
       [0053]    One further embodiment of the present invention is illustrated in  FIG. 7 , an exploded perspective view of a double-ended blower  400  according to the present invention. The motor and stator blade portion  402 , located in the center of the exploded view, is investment cast from aluminum in this embodiment, although other manufacturing methods are possible and will be described below. The aluminum, as a good conductor of heat, facilitates the dissipation of heat generated by the accelerating and decelerating motor. Each end of the shaft  404  is shown in  FIG. 7 , but the motor windings, bearing and cover are not shown. The motor power cord  406  protrudes from the motor and stator blade portion  402  and exits the blower  400  through a sealed orifice  450 . The motor and stator blade portion  402  includes, at its top, a bottom portion of the upper volute  408 . 
         [0054]    As a variation of the design illustrated in  FIG. 7 , the motor and stator blade portion  402  may be made separately from the bottom portion of the upper volute  408 . If the two components are made separately, investment casting would not be required. For example, the motor body may be die cast, while the bottom portion of the upper volute  408  may be injection molded. 
         [0055]    Secured to the motor and stator blade portion  402  by bolts or other fasteners is a circular plate  410 , in which a hole  412  is provided for the passage of the shaft  404 . An impeller  414  rests atop the circular plate. The impeller  414  is scalloped along its circumference to reduce its rotational inertia, giving it a “starfish” look. 
         [0056]    An upper endcap  416  is secured above the impeller  414 , and provides the top portion of the upper volute. The upper and lower volutes in this embodiment are versions of the in-plane transitional scroll volute  300  illustrated in  FIG. 6 . An aperture  418  in the center of the upper endcap  416  serves as the air intake of the blower  400 . 
         [0057]    On the lower end of the blower  400 , a contoured plate  420  forms the top portion of the lower volute. The top of the contoured plate  420  is raised and curves arcuately downward toward a hole  422 . As was explained above, the contoured plate  420  helps to shape the airflow and to ensure that it enters the impeller cavity from all sides, rather than preferentially from a single direction. Beneath the contoured plate  420 , a lower impeller  414  rotates proximate to a lower endcap  428 . The two endcaps,  416 ,  428  may be die cast (e.g., from aluminum or magnesium alloy) or they may be injection molded from an appropriate metal. 
         [0058]    The airpath  454  between the upper and lower volutes is an integral part of the left  424  and right  426  side casings, onto which the other components are secured. The left side casing  424  also provides the air outlet  442  for the blower  400 . The left  424  and right  426  side casings are secured together with bolts or other removable fasteners. On the top surface of the side casings  424 ,  426  are square flanges  430 ,  432  having protrusions  434 ,  436  that allow the blower  400  to be mounted on springs inside a CPAP apparatus. In  FIG. 7 , the protrusions  434 ,  436  are shown as having different sizes and shapes, however, in  FIGS. 8 and 9 , the protrusions  434  are shown as having the same shape. It will be realized that the protrusions  434 ,  436  may take either of the depicted shapes, or any other shape, depending on the properties and arrangement of the springs onto which the blower  400  is mounted. 
         [0059]    The double-ended blower  400  also includes two damping sleeves  438 ,  440 . The damping sleeves  438 ,  440  are rubber or foam rubber components that are injection molded to match the internal contours of the left  424  and right  426  side casings, respectively. In one implementation, the damping sleeves  438 ,  440  are  40  Shore A hardness polyurethane formed from a rapid prototype silicone mold. Alternatively, the damping sleeves  438 ,  440  could be silicone, or another elastomer that is stable&#39;at the high temperatures generated by the motor. 
         [0060]    The damping sleeves  438 ,  440  serve three major purposes in blower  400 : they form the actual airpath  454 , they provide a seal between the other components, and they dampen the vibrations of the other parts. The rubber or foam rubber material of the damping sleeves  438 ,  440  is particularly suitable for the airpath  454 , as it allows for re-entrant molds (i.e., undercuts). The damping properties of the damping sleeves  438 ,  440  reduce the “ringing” of the aluminum components that would otherwise be experienced. 
         [0061]      FIG. 8  is an assembled perspective view of blower  400  from one side. The assembled air outlet  442  is shown in  FIG. 8 , as is the seam  444  between the left  424  and right  426  side casings. As shown in  FIG. 8 , and in the rotated perspective view of  FIG. 9 , flanges  446 ,  448  protrude laterally from the edge of each side casing  424 ,  426  and abut to form the seam  444 . The two side casings  424 ,  426  are secured together by bolts  452  that pass through the flange  446  provided in the right side casing  426  and into threaded holes provided in the flange  448  of the left side casing  424 . 
         [0062]    Blower  400  has several advantages. First, investment casting is not required to produce blower  400 , which reduces the cost of the blower. Additionally, because the components of blower  400  have fewer hidden and intricate parts, the castings can be inspected and cleaned easily. Finally, blower  400  is easier to assemble than the other embodiments because the components are clamped together using the two side casings  424 ,  426 , which can be done with simple fasteners. 
         [0063]    While the invention has been described by way of example embodiments, it is understood that the words which have been used herein are words of description, rather than words of limitation. Changes may be made without departing from the scope and spirit of the invention in its broader aspects. Although the invention has been described herein with reference to particular embodiments, it is understood that the invention is not limited to the particulars disclosed. The invention extends to all appropriate equivalent structures, uses and mechanisms.