Abstract:
A centrifugal compressor is provided. The centrifugal compressor includes a housing and a rotatable assembly mounted for rotation about an axis within the housing. The rotatable assembly includes an impeller forming part of a compressor stage and a motor rotor forming an armature of a motor for driving the rotating assembly about the axis. A first air intake is located at a first end of the apparatus, the first air intake providing an air source for the compressor stage and a second air intake is located at a second end of the apparatus, the second air intake providing an air source for at least two air cooling passageways.

Description:
[0001]    Priority is claimed to U.S. provisional patent application Ser. No. 61/462,801, filed Feb. 7, 2011, entitled “Centrifugal Compressor,” which is referred to and incorporated herein in its entirety by this reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to compressors. More particularly, the invention concerns a centrifugal compressor. 
       BACKGROUND OF THE INVENTION 
       [0003]    Centrifugal compressors have existed for many years, and there exist many different designs. Historically, compressed air (or a gas/air mixture) has been generated by various types of motor driven machines. To achieve high efficiency the motor must drive the centrifugal compressor at high rotational speeds. As rotational speeds become greater the overall machine size can be made smaller, while maintaining the same compressed air flows, pressures, and motor power. However, requirements for running at high speeds include properly designed rotating and non-rotating assemblies and bearings to support the high speed rotating shaft, typically ranging from 30,000 rpm to 200,000 rpm. 
         [0004]    Air or water cooling may be employed to dissipate heat that is generated. However, liquid cooling has several drawbacks including additional system complexity and increased manufacturing and unit cost, and the potential for fluid leaks into the compressor/motor internals is also a concern. For the very small machines, power density is exceptional and therefore the ability to reject heat from the machines relatively little surface area becomes challenging. 
         [0005]    Therefore, there remains a need to overcome one or more of the limitations in the above-described, existing art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  comprises a perspective view of one embodiment of the centrifugal compressor of the present invention; 
           [0007]      FIG. 2  comprises a cross-sectional view of the embodiment of  FIG. 1 ; 
           [0008]      FIG. 3  comprises a perspective view of a finned heat exchange element included in the embodiment of  FIG. 1 ; 
           [0009]      FIG. 4  comprises a cross-sectional view showing the internal air passageways located within the embodiment of  FIG. 1 ; 
           [0010]      FIG. 5  comprises a perspective view of a second embodiment of the centrifugal compressor of the present invention, the second embodiment comprising a fluid-cooled centrifugal compressor; and 
           [0011]      FIG. 6  comprises a cross-sectional view of the embodiment of  FIG. 5 . 
       
    
    
       [0012]    It will be recognized that some or all of the Figures are schematic representations for purposes of illustration and do not necessarily depict the actual relative sizes or locations of the elements shown. The Figures are provided for the purpose of illustrating one or more embodiments of the invention with the explicit understanding that they will not be used to limit the scope or the meaning of the claims. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the centrifugal compressor of the present invention. It will be apparent, however, to one skilled in the art that the centrifugal compressor may be practiced without some of these specific details. Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than as limitations on the centrifugal compressor. That is, the following description provides examples, and the accompanying drawings show various examples for the purposes of illustration. However, these examples should not be construed in a limiting sense as they are merely intended to provide examples of the centrifugal compressor rather than to provide an exhaustive list of all possible implementations of the centrifugal compressor. 
         [0014]    Referring now to  FIGS. 1-6 , the centrifugal compressor  10  includes many novel features including, among others, an air-cooled design that provides reduced system complexity and cost and that also eliminates the possibility of fluid internal leaks into the motor/compressor internals. The air cooled design comprises multiple air-cooling circuits that ensure sufficient cooling air supply for the unit. 
         [0015]    Another feature is a foil air bearing system that supports the impeller shaft. The foils eliminate the need for costly high-temperature coatings on the foil bearing surfaces, which are usually required on units that operate at higher operating temperatures. 
         [0016]    Yet another feature comprises a heat exchanging element that efficiently transfers heat generated by the electric motor stator and also allows cool air flow passage, thereby dissipating the generated heat. 
         [0017]    Other features include a compact, lightweight design that eliminates many seals, gaskets and other elements found in conventional compressors. Yet, the pneumatic power i.e., flow and pressure rise (aka “process air”) equals the output of much larger and heavier units, thereby enabling the installation of the centrifugal compressor  10  in aircraft to provide on-board inert gas and on-board oxygen generation (aka OBIGGS &amp; OBOGS). In at least one exemplary case, a 20 horsepower compressor is attained in a package weight totaling 12 pounds. 
         [0018]    As a background, the specific speed of the centrifugal machine is of primary importance to the designer as it relates and balances the general size, i.e., impeller diameter against the rotational speed for a given head rise. For example, impeller diameter may be traded for rotational speed to yield the same head rise. However, there is a limit as to how big a diameter may be traded for reduced rotational speed, without incurring significant losses. For designs that require relatively high pressure rise at relatively low flows, a smaller, faster rotating machine is desired in order to yield an acceptable specific speed. 
         [0019]    Referring now to  FIGS. 1-2 , the centrifugal compressor  10  is illustrated. A first end of the centrifugal compressor  10  comprises the centrifugal compressor axial air inlet  15 , with the other, second end comprising the main cooling air inlet  20 . 
         [0020]    The volute  25  couples to the base housing  30  through the back plate assembly  35  and V-band clamp  40 . The base housing  30  includes a heat exchange element  45  (shown in  FIG. 3 ) comprising a plurality of fin elements  50  densely arranged to allow air to pass between the fins along the main axis  55  of the base housing  30 . 
         [0021]    As shown in  FIG. 2 , the main cooling air inlet  20  attaches to a fan cover  60  that fits over the rear cover  65 , which provides a thrust bearing surface  70  for the impeller shaft assembly, or rotatable assembly  75  that rides on two air bearing journals  80 . 
         [0022]    Referring again to  FIGS. 1-2 , the centrifugal compressor  10  is generally symmetric about the compressor axis  55 . The compressor inlet  15  receives a fluid medium, generally air, to be compressed, which is discharged as compressed fluid at volute exit  85 . The inlet leads to a single centrifugal compressor stage comprised of an impeller  90  with the volute  25  surrounding the impeller  90  and the inlet  15 . 
         [0023]    As shown in  FIG. 2  the rotating assembly, or impeller assembly  75  includes the impeller  90 , a shaft  92 , a first air bearing journal  80 , a permanent magnet motor rotor  95 , and second air bearing journal  80 , a thrust load bearing  100  that balances the pressure load of the impeller  90 . The impeller assembly  75  also includes a fan  105  located at the distal end of the impeller assembly  75 , the fan  105  located within the cooling air inlet  15 . 
         [0024]    The motor rotor  95  in the rotating assembly  75  forms the armature of a electrically driven permanent magnet, high speed motor in which the stator  110  is fixedly retained within the finned heat exchanger  45 , as shown in  FIG. 2 . The motor rotor  95  includes a permanent magnet for enabling operation of the electric motor. 
         [0025]    The rotating assembly  75  consisting of the impeller  90 , the thrust load balancing disk  100  and the rotor motor  95  are supported for high speed rotation within the housing by means of oil-less air bearings (not shown) that are located between the rotating assembly shaft  92  and the air bearing journals  80 . The foil air bearings have numerous performance, maintenance and contamination-free advantages over conventional roller or ball bearings. 
         [0026]    Specifically, once the rotating assembly  75  is spinning quickly enough, the working fluid (usually air) pushes the foil away from the shaft  92  so that there is no more contact. The shaft  92  and foil are separated by the air&#39;s high pressure which is generated by the rotation which pulls gas into the bearing via viscosity effects. A high speed of the shaft  92  with respect to the foil is required to initiate the air gap, and once this has been achieved, no wear occurs. Unlike aero or hydrostatic bearings, foil bearings require no external pressurization system for the working fluid, so the hydrodynamic bearing is self-starting. 
         [0027]    Unlike contact-roller bearings, an air bearing (or air caster) utilizes a thin film of pressurized air to provide an exceedingly low friction load-bearing interface between surfaces. The two surfaces don&#39;t touch. Being non-contact, air bearings avoid the traditional bearing-related problems of friction, wear, particulates, and lubricant handling, and offer distinct advantages in precision positioning, such as lacking backlash and stiction, as well as in high-speed applications. 
         [0028]    The air cooling feature of the centrifugal compressor  10  will now be discussed with reference to  FIG. 4 . The centrifugal compressor  10  incorporates four (4) separate air passageways, or circuits:
       1) Compressor Air passageway  120 —also known as “process air” that enters through the compressor air inlet  15  and exits at the volute exit  85 . This air is used for any number of applications, ranging from aircraft to automotive to industrial applications.   2) Secondary Air Bleed passageway  125 —This air ‘bleeds’ past the impeller  90  periphery and flows though a first air bearing journal  80 , then the electric motor  95 , past the second air bearing journal  80 , then against the forward thrust bearing pad element  115 , and then exits into the heat sink area (shown in  FIG. 3 ). Exit ports  130  for the air are provided in the rear cover  65  (shown in  FIG. 6 ).   3) Cooling Air passageway  135 —This air is supplied by the rear-mounted cooling fan  105  which draws air from the cool ambient environment. Air is forced through and around the heat sink fins  50  in the finned heat exchanger  45  (shown in  FIG. 3 ) that have a large surface area and high convective heat transfer. A low pressure region is created by the cooling air flow passing over the exit ports  130  of the rear cover  65 , at the point where secondary air bleed flow enters the heat sink. This further enhances secondary air flow through the bearing and motor system, hence improving cooling efficiency. The aft-mounted cooling fan  105  is directly coupled to the high-speed impeller shaft  92 . This fan is sized to provide on the order of 35 cubic feet per minute (“CFM”) of airflow at only a moderate pressure rise.   4) Tertiary Air Bleed passageway  140 —The fourth air circuit is an additional air-bleed which is obtained from the periphery of the cooling fan  105  (i.e., bled off), and is directed against the aft section of the thrust bearing surface  70 . This bleed air exits into the heat sink cooling fins  50 . Tertiary air bleed flow is also enhanced by the same low pressure region at the exit ports  130  of rear cover  65 .       
 
         [0033]    By including four (4) distinct air passageways the centrifugal compressor  10  can be compact yet extremely efficient. For example, the highly effective heat sink fin  50  arrangement is designed to reject 1 kilowatt (“kW”) of heat or more, resulting in only a moderate temperature rise of the supplied cooling air. In one example, the electric motor total thermal losses of 900 watts will result in cooling air discharged at approximately 100° C., with the cooling air inlet at approximately 45° C. This data relates to a sustained, full load, high-speed, thermally stabilized operating condition. Parasitic power loss operates the cooling fan, but this amount on the order of 75 Watts of shaft power, or 0.5% of the maximum 15 kW power rating of the centrifugal compressor  10 . 
         [0034]    Referring now to  FIGS. 5-6 , a second embodiment centrifugal compressor  10  that includes water cooling in addition to air cooling is illustrated. The housing  30  includes a cooling inlet  145  and cooling outlet  150  for circulating a liquid cooling medium through a liquid heat exchanger  155 . In this embodiment, the finned heat exchanger  45 , shown in  FIG. 3 , is replaced with a liquid heat exchanger  155  that includes the liquid inlet  150  and outlet  145 . The four air passageways described above are still included, but instead of passing through the fin elements  50 , the air, after flowing through the passageways described above, exits the air exit ports  130  in the rear cover  65 . 
         [0035]    Not illustrated is an electronic module. The electronic module controls the centrifugal compressor  10  through use of a Hall effect sensor, software and other elements as required. For example, the electronic module may include computer hardware and software and may include a computer program product which is embodied on one or more computer-usable storage media having computer-usable program code embodied therein. Computer program instructions may also be stored in a computer-readable memory that can direct the centrifugal compressor  10  to function in a particular manner, such that the instructions stored in the computer-readable memory produce an operating cycle. 
         [0036]    Thus, it is seen that a centrifugal compressor is provided. One skilled in the art will appreciate that the present invention can be practiced by other than the above-described embodiments, which are presented in this description for purposes of illustration and not of limitation. The specification and drawings are not intended to limit the exclusionary scope of this patent document. It is noted that various equivalents for the particular embodiments discussed in this description may practice the invention as well. That is, while the present invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those of ordinary skill in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims. The fact that a product, process or method exhibits differences from one or more of the above-described exemplary embodiments does not mean that the product or process is outside the scope (literal scope and/or other legally-recognized scope) of the following claims.