Abstract:
A compressor, as well as a lightweight and strong casting for a compressor, are disclosed. The compressor, which may be a reciprocating compressor for use in compressing high-pressure refrigerants such as CO 2 , includes substantially reduced wall thicknesses (t) compared to prior art castings. The side walls of the compressor can be manufactured to such reduced thicknesses (t) through the use of a bridge spanning across the crankcase. This not only allows the opposing side walls to be manufactured of a thinner material, but the bottom cover removably mounted to the crankcase can be manufactured from a thinner and lighter material as well. Through the use of such a bridge, the resulting compressor is not only able to satisfy current strength requirements, but at significant weight, size and cost savings as well.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a US national phase patent application of International Patent Application No. PCT/US10/03569 filed on May 19, 2010 filed pursuant to the Patent Cooperation Treaty and claim priority under 35 USC §119(e) to U.S. Provisional Patent Application Ser. No. 61/179,514 filed on May 19, 2010. 
    
    
     BACKGROUND 
     1. Technical Field 
     This disclosure is directed to air conditioning and refrigeration compressor control and more particularly, to variable speed compressors that incorporate continuous variable transmissions (CVTs). 
     2. Description of the Related Art 
     Refrigerant systems are utilized in many air conditioning and heat pump applications for cooling and/or heating the air entering an environment. The cooling or heating load on the environment may vary with ambient conditions, and as the temperature and/or humidity levels demanded by the occupant of the environment change. A compressor is used to compress a working fluid (i.e., the refrigerant) from initial (suction) conditions to compressed (discharge) conditions. 
     In some refrigerant systems, a single compressor is utilized to compress the refrigerant and move the refrigerant through the cycle connecting indoor and outdoor heat exchangers in a closed loop. However, under many circumstances, it is desirable to have the ability to vary the capacity, or amount of cooling or heating provided by the refrigerant system. 
     To vary the capacity of a compressor, variable speed drives (VSDs) are known for driving compressors at variable speeds in a refrigerant system. By driving the compressor at a higher or lower speed, the amount of refrigerant that is compressed per unit of time changes, and thus the system capacity can be adjusted. A VSD also allows the removal of all unloading hardware from the compressor system. In typical applications involving more than one compressor, such as multiple circuit chillers, multiplexed compressor chillers, refrigeration and compressor racks, a VSD may be used with each compressor to selectively unload compressors as necessary based on system demand. In general, early VSD designs required a constant voltage:frequency ratio. On the other hand, modern inverters within VSDs provide the ability to adjust both frequency and voltage independently of each other, but the voltage:frequency ratio is preset when the VSD is matched with a motor for a given application. 
     VSDs are expensive and therefore, multiple compressor systems requiring multiple variable speed drives are also expensive. In addition, the need for multiple VSDs adds to the complexity and size of the air conditioning or refrigeration system. 
     As illustrated in  FIG. 1 , an exemplary compressor  10  is powered by a hermetic motor  11  which is, in turn, powered by a variable speed drive VSD  12 . The VSD  12  supplies a modulated alternating current output having a characteristic output voltage and output frequency. The VSD  12  receives power from a power supply (e.g., 460 VAC, 60 Hz). A gearbox or variable ratio transmission  13  enables the ratio of motor  11  speed to compressor  10  speed to vary. The hermetically sealed casing is shown schematically at  14 . 
     The torque required by the compressor  10  (and thus supplied by the motor  11 ) will essentially be a function of the load (e.g., the air conditioning load), the saturated suction temperature (SST) and saturated discharge temperature (SDT). A given motor speed is associated with a proportional frequency position on the fixed voltage/frequency curve of the VSD. At a given point on the voltage/frequency curve, however, the current draw of the drive will accordingly be determined by the SST and SDT values. For example, at a given voltage and frequency, if the SDT were to increase suddenly, the torque would increase at a given speed thus necessitate a power increase from the VSD and, accordingly, a current increase. As a result, the operating efficiency of a variable speed compressor equipped with currently available VSDs remains a concern. 
     A water-cooled chiller is a machine that removes heat from water via a vapor-compression or absorption refrigerant cycle. A vapor-compression water chiller comprises the four major components of the vapor-compression refrigerant cycle: compressor; evaporator; condenser; and some form of metering device. Water-cooled chillers can employ a variety of refrigerants. Chilled water is often used to cool and dehumidify air in mid- to large-size commercial, industrial, and institutional facilities. Chillers can be water-cooled, air-cooled or evaporatively cooled. Water-cooled chillers may incorporate the use of cooling towers which improve the thermodynamic effectiveness of the chiller as compared to air-cooled chillers. 
     Large tonnage water-cooled chillers (above 2000 tons) typically use open drive centrifugal compressors powered by electric motors. This is due to unavailability of hermetic motors like the one shown at  11  in  FIG. 1  that can deliver power levels above 2000 hp (˜1500 kW). Also, the input power for these applications tends to be between 4.16 kV to 11 kV. VSDs for medium (4.16 kV) and high voltage (6.9 kV) compressors are very expensive and consume a lot of space, thereby presenting installation problems. Still further, in the Middle East, the input power for large tonnage water-cooled chillers is 11 kV. VSDs for this voltage are not commercially available and must be custom built. 
     Accordingly, there is a need for an improved variable speed compressor design that does not rely upon a variable speed drive or a VSD. 
     SUMMARY OF THE DISCLOSURE 
     In satisfaction of the above-described needs, a cooling system is disclosed that comprises a drive motor that is connected to a proximal drive shaft. The proximal drive shaft is connected to a continuous variable transmission (CVT). The CVT is connected to an output drive shaft that is connected to a rotor disposed within a compressor. The compressor is connected to an evaporator, which is connected to a condenser, which is connected to the compressor thereby forming a vapor-compression refrigerant cycle. A controller is linked to the CVT and a sensor for detecting a leaving chilled water temperature in the evaporator. The controller increases the output of the CVT to the output drive shaft when the leaving chilled water temperature is above a set point. In contrast, the controller decreases the output of the CVT to the output drive shaft when the leaving chilled water temperature is below the set point. The CVT may be a hydrostatic or hydraulic, hydro-mechanical or mechanical (e.g., adjustable pulley) type CVT. 
     A method of cooling air is disclosed that comprises providing a cooling system comprising a drive motor that is connected to a proximal drive shaft. The proximal drive shaft is connected to a continuous variable transmission (CVT). The CVT is connected to an output drive shaft that is connected to a rotor disposed within a compressor. The compressor is connected to an evaporator which is connected to a condenser, which is connected to the compressor thereby forming a vapor-compression refrigerant cycle. The method includes measuring a leaving chilled water temperature in the evaporator, comparing the measured leaving chilled water temperature with a predetermined set point, increasing the output of the output drive shaft when the leaving chilled water temperature is above the set point, and decreasing the output of the CVT to the output drive shaft when the leaving water temperature is below the set point. 
     Other advantages and features will be apparent from the following detailed description when read in conjunction with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the disclosed methods and apparatuses, reference should be made to the embodiments illustrated in greater detail in the accompanying drawings, wherein: 
         FIG. 1  schematically illustrates a variable speed compressor with a variable frequency drive or variable speed drive (VSD); 
         FIG. 2  illustrates a disclosed variable speed compressor equipped with a continuously variable transmission (CVT); and, 
         FIG. 3  schematically illustrates a disclosed hydrostatic or hydraulic CVT for use with a disclosed variable speed compressor; 
         FIG. 4  schematically illustrates a disclosed hydro-mechanical CVT for use with a disclosed variable speed compressor; and 
         FIG. 5  schematically illustrates a disclosed pulley-based CVT for use with a disclosed variable speed compressor. 
     
    
    
     It should be understood that the drawings are not necessarily to scale and that the disclosed embodiments are sometimes illustrated diagrammatically and in partial views. In certain instances, details which are not necessary for an understanding of the disclosed methods and apparatuses or which render other details difficult to perceive may have been omitted. It should be understood, of course, that this disclosure is not limited to the particular embodiments illustrated herein. 
     DETAILED DESCRIPTION 
     Turning to  FIG. 2 , a compressor  10   a  and motor  11   a  are coupled to a continuous variable transmission (CVT)  15 . The motor  11   a  may or may not be hermetically sealed within a casing  14   a . Thus, the design of  FIG. 2  is applicable to both hermetic and open systems. The CVT  15  eliminates the need for the costly combination of a gearbox  13  and VSD  12  of the prior art system shown in  FIG. 1 . Further, the prior art system shown in  FIG. 1  can only be constructed economically as a hermetic system with a hermetic casing  14  and lower capacity motor  11  as VSDs  12  are not readily available for larger motors  11   a , which can provide power exceeding 2000 hp or 1500 kW. Systems requiring higher capacities require multiple compressors  10 , gear boxes  13 , motors  11 , VSDs  12  and the costs associated therewith. Of course, in addition to be suitable for higher capacities, the system illustrated in  FIG. 2  is also applicable to lower capacity systems. 
     Suitable designs for CVTs  15 ,  15   a ,  15   b  are illustrated in  FIGS. 3-5  and include hydraulic or hydrostatic, hydro-mechanical and purely mechanical CVTs. Referring first to  FIG. 3 , a hydrostatic or hydraulic CVT  15  is illustrated whereby input power from the motor  11   a  is delivered to a hydraulic pump  21  through an input shaft  18 . The motor  11   a  may be operated at a constant speed. A continuous loop  45  connects the hydraulic pump  21  to a hydraulic motor  22 . The variable-displacement hydraulic pump  21  is used to vary the fluid flow into hydrostatic motor  22 . The rotational motion of the motor  11   a  operates the hydrostatic pump  21  and the pump  21  converts the rotational motion into fluid flow through the loop  45 . Then, with the hydrostatic motor  22  located on the driven side of loop  45 , the fluid flow is converted back into rotational motion of an output shaft  19 . 
     The output shaft  19  is connected to an impeller  33  of the compressor  10   a . The compressor  10   a  is part of a refrigerant cycle  60  which includes a condenser  52  that receives fluid from the compressor  10   a  and delivers evaporated fluid to the evaporator  53  through a restriction orifice  55  and back into the compressor  10   a.    
     A controller  50  is utilized to control the speeds of the hydraulic pump  21  and hydraulic motor  22  based on the “leaving chilled water temperature” at the evaporator  53  indicated at  54  in  FIG. 3  (and  FIGS. 4-5 ). While the disclosed systems  15 ,  15   a ,  15   b ,  60  are particularly adaptable to large water chillers, other applications will be apparent to those skilled in the art the controller  50  will control the speed of the hydraulic pump  21  and hydraulic motor  22  in response to changes in the load to the refrigerant cycle  60 , as measured by the leaving chilled water temperature at  54 . Thus, the leaving chilled water temperature at  54  are shown as input signals to the controller  50  in  FIGS. 3-5  and the links between the controller  50  and the hydraulic pumps  21 ,  21   a , hydraulic motors  22 ,  22   a  and pulley  41  are shown as output signals in  FIGS. 3-5 . 
     In  FIG. 4 , a hydrostatic CVT like the one shown at  15  in  FIG. 3  is combined with a planetary gear set  17  and appropriate clutches (not shown) to create a hybrid system referred to as a hydro-mechanical CVT  15   a . The hydro-mechanical CVT  15   a  transfers power from the drive motor  11   a  to the compressor  10   a  in three different modes. At a low speed, power is transmitted hydraulically using the hydraulic pump  21   a  and hydraulic motor  22   a ; at a high speed, power is transmitted mechanically by the drive motor  11   a  through the gear set  17 ; between these extremes, the CVT  15   a  uses both hydraulic and mechanical means to transfer power to the output shaft  19   a.    
     The drive motor  11   a  is connected to the planetary gear set  17  by the proximal drive shaft  18   a . The planetary gear set  17  divides the power delivered by the proximal drive shaft  18   a  from the motor  11   a  into two output power paths: one output power path passing to the distal output shaft  19   a  through the ring gear  23 , planetary gear  30 , carrier  26  and sun gear  24 ; and the second output power path that drives a hydraulic pump  21   a  through the action of the spur gear  27  and input pump gear  32 . The pump  21   a , in turn, drives a hydrostatic or hydraulic motor  22   a  via the loop  45   a . The hydraulic motor  22   a  is linked or coupled to the output shaft  19   a  via the gears shown at  31 ,  28 . 
     Regarding the power path from the motor  11   a  to the output shaft  19 , the proximal drive shaft  18   a  is connected to a ring gear  23  of the planetary gearset  17 . The ring gear  23  is coupled to a sun gear  24  by the planetary gear  30  and carrier  26 . The sun gear  24  is coupled to or connected to the output drive shaft  19   a.    
     Regarding the second mechanical power path to the pump  21   a , rotational power is delivered from the motor  1   a  to the pump  21   a  via the gears  27 ,  32 . The hydraulic pump  21   a  drives the hydraulic motor  22   a  that may be geared to the output shaft  19   a  through the hydraulic motor output shaft  29 , output gear  31  and the shaft gear  28  that is connected to the output shaft  19   a.    
     The power that is delivered to the planetary gear set  17  is therefore split to drive the hydraulic pump  21   a  and to drive the output shaft  19   a  directly. If the hydraulic pump  21   a  is at zero or its minimum displacement and the hydraulic motor  22   a  is at its maximum displacement, the hydraulic pump  21   a  will ‘freewheel’ and rotate without producing any flow or pressure to the hydraulic motor  22   a . As the hydraulic pump  21   a  cannot put any reaction torque on the planetary gearset  17  when the hydraulic pump  21   a  is at zero or its minimum displacement, there can be no torque (and hence no power) going to the output shaft  19   a  from the hydraulic motor  22   a . All power to the output shaft  19   a  is provided by the drive motor  11   a  through the carrier  26  and sun gear  24 . In this scenario, the hydraulic pump  21   a  is set at its minimum displacement in the hydraulic motor  22   a  at its maximum displacement. The gear  31  spins at a high rotational velocity but the rotational velocity of the output shaft  19   a  and compressor  10   a  is dictated by the drive motor  11   a.    
     On the other hand, if a small amount of displacement is given to the hydraulic pump  21   a , the gear  27  will engage the gear  32  causing the pump  21   a  to generate high pressure and a small flow rate to the hydraulic motor  22   a . This high pressure and small flow rate acts upon the hydraulic motor  22   a  to produce high torque and low speed that is transmitted to the output drive shaft  19   a  via the gears  31 ,  28 . The hydraulic pump  21   a  under these conditions is also creating a reaction torque on the planetary gearset  17  which results in mechanical torque (and hence power) going directly to the output shaft  19   a  through the ring gear  23 , carrier  26 , and sun gear  24 , which combines with the power delivered from the hydraulic motor  22   a  via the gears  31 ,  28 . 
     When the hydraulic pump  21   a  is at its maximum displacement and the hydraulic motor  22   a  is at its minimum displacement, the hydraulic motor  22   a  cannot accept the flow rate from the hydraulic pump  21   a . This has the effect of locking the ring gear  23  to the planetary gear  30 , carrier  26  and sun gear  24  so the sun gear  24  (and hence the output shaft  19   a ) rotates at its highest rotational velocity, and about 100% of the power of the hydraulic pump  21   a  is translated mechanically to the output shaft  19   a . The hydraulic motor  22   a  is now freewheeling at zero displacement or torque to the output shaft  19   a . Because all power from the hydraulic pump  21   a  is now going to the output shaft  19   a  mechanically via gears  23 ,  30 ,  26 ,  24 , the CVT  15   a  efficiency is very high when the hydraulic pump  21   a  is operating at its maximum displacement. 
     The CVT  15   a  as illustrated in  FIG. 4  is ideal for large-scale centrifugal compressors  10   a  because the power requirement increases with the cube of the impeller  33  speed, not linearly. As the CVT  15   a  increases the impeller  33  speed within the compressor  10   a , additional power is fed mechanically to the output shaft  19   a  and not hydraulically through the hydraulic motor  22   a  of the CVT  15   a . The supplemental use of mechanical power from the hydraulic pump  21   a  through the gears  32 ,  27 ,  23 ,  30 ,  26 ,  24  to the output shaft  19   a  reduces power loss when the CVT  15   a  is not operating at its fastest output speed, but the use of supplemental mechanical power also significantly reduces the size and power capacity requirements of the hydraulic circuit  21   a ,  22   a ,  45   a . For a 2000 hp (˜1500 kW) compressor  10   a , the CVT  15   a  needs a maximum of only about 200 hp (˜150 kW) transmitted thru the hydraulic circuit  21   a ,  22   a ,  45   a.    
     The CVT  15   a  eliminates the need for varying the speed of the drive motor  11   a  or utilizing a controller  50  to control the speed of the drive motor  11   a . The controller  50  may be used to adjust the displacements of the hydraulic pump  21   a  and hydraulic motor  22   a  in response to the leaving chilled water temperature measured at  54  on the leaving chilled water side in the evaporator  53 . Instead, the drive motor  11   a  may be operated at a constant speed. 
     Turning to  FIG. 5 , the drive motor  11   a  is connected to a pulley-based CVT  15   b  by the drive shaft  18   b  which is connected to a variable-diameter pulley  41 . The pulley  41  includes a pair of opposing cones  44 ,  45 . A belt  43  rides in the groove between the two cones  44 ,  45 . V-belts are preferred if the belt is made of rubber although steel belts are becoming commonplace. 
     When the two cones  44 ,  45  of the pulley are spread apart, the effective pulley diameter increases, the belt  43  rides lower in the groove, and the radius of the belt loop going around the pulley gets smaller. When the cones  44 ,  45  are pulled closer together, the effective pulley diameter decreases, the belt  42  rides higher in the groove, and the radius of the belt loop going around the pulley gets larger. The CVT  15   b  of  FIG. 5  may use hydraulic pressure, centrifugal force or spring tension to create the force necessary to adjust the pulley halves  44 ,  45 . A controller  50  may be utilized to control the mechanism pulling the halves  44 ,  45  apart and moving the halves  44 ,  45  closer together. Input to the controller  50  will again include the leaving chilled water temperature at  54 . 
     INDUSTRIAL APPLICABILITY 
     In satisfaction of the above-described need, a large tonnage water chiller  60  is disclosed that comprises a drive motor  11   a  connected to a shaft  18 . The drive shaft  18  is connected to a continuous variable transmission (CVT)  15 ,  15   a ,  15   b . The CVT  15 ,  15   a ,  15   b  that is linked to a controller  50 . The controller  50  may controls the operation of the CVT  15 ,  15   a .  15   b  based on the leaving chilled water temperature at  54 . The output shaft  19 ,  19   a ,  19   b  of the CVT  15 ,  15   a ,  15   b  is linked to a refrigerant cycle  60 . 
     The refrigerant cycle  60  includes the compressor  10   a , evaporator  53 , condenser  52  and metering orifice  55 . The controller  50  may control the flow of fluid through the refrigerant cycle  60  by controlling the speed of the hydraulic pump  21 ,  21   a , hydraulic motor  22 ,  22   a  or spacing of the pulley halves  44 ,  45 . In operation, the controller  50  may take an input signal from the leaving chilled water temperature at  54  (i.e. and compares to a set point). If the chilled water temperature leaving the chiller evaporator  53  is below the set point, then the output speed of the hydraulic pump  21 ,  21   a  is reduced by increasing the displacement of the hydraulic motor  22 ,  22   a  and reducing the hydraulic pump  21 ,  21   a  displacement. Alternatively, the output of the variable diameter pulley  41  is reduced. If the leaving chilled water temperature at  54  is above the set point then the output speed of the hydraulic pump  21 ,  21   a  is increased by increasing the pump  21 ,  21   a  displacement and reducing the hydraulic motor  22 ,  22   a  displacement. Alternatively, the output of the variable diameter pulley  41  is increased. 
     While only certain embodiments have been set forth, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure and the appended claims.