Abstract:
A method and system for reducing windage losses in compressor motors is provided. The compressor motor is cooled by circulating refrigerant from a closed refrigerant loop incorporating the compressor. A pumping device coupled to a liquid expander in the closed refrigerant loop circulates refrigerant through the motor cavity and produces a motor cavity pressure lower than evaporating pressure. The lower pressure in the motor cavity reduces the density of the gasses in the motor cavity, resulting in reduced windage losses of the motor. Additionally, the pumping device is powered by the recovered liquid expansion energy between the condenser and the evaporator.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a system and method of cooling a compressor motor by circulating refrigerant gas over the motor components. More specifically, the present invention is directed to reducing windage losses in a compressor motor by lowering the pressure and density of the refrigerant gas within the motor cavity. 
         [0002]    High-speed motors typically have large windage losses, in part because of the large amount of cooling gas induced windage friction caused during high-speed rotor rotation, which impacts the motor&#39;s performance and efficiency. To reduce the windage losses, factors directly related to the motor such as the peripheral speed of the rotor, the flow of motor cooling gas around the motor, the rotor surface area and the roughness of the rotor surface are manipulated and controlled to optimize the performance of the motor. 
         [0003]    One method for reducing energy losses in motors while cooling the motor is by suctioning refrigerant toward the motor windings. The reduction in temperature of the motor windings prevents the motor components from overheating and creates more operating efficiency. Another method for reducing energy losses in motors is to maintain constant pressure throughout the motor cavity. A pressure valve can be placed within the motor cavity to release higher-pressure gas build up that occurs in the motor cavity during operation. As the pressure in the cavity increases, the valve opens, thereby releasing high-pressure gases. The maintenance of constant pressure in the cavity increases motor efficiency. However, this method uses mechanical equipment and is not optimal for maintaining a true constant pressure in the motor cavity. Additionally, this method does not address the issue of the motor cavity temperature. 
         [0004]    An additional method controls energy losses in motors by maintaining a constant pressure in the motor cavity, while also preventing the oil losses between motor components. The preservation of oil in the motor bearing components allows for greater lubrication for the movement of parts thereby reducing friction while not allowing oil to escape into the motor cooling cavity, preventing excessive oil churning and reducing energy losses. A hermetically sealed housing containing the refrigeration compressor transmission and oil supply reservoir is connected to the suction side of the compressor to equalize the pressure in the housing. The focus of the method is to prevent the boiling of refrigerant from the oil reserve. However, this system only holds the pressure in the motor cavity at a constant level, and only assists in reducing energy losses, rather than optimizing the motor efficiency. 
         [0005]    For very high speed motors however, windage losses can still be substantial even after factors such as the peripheral speed of the rotor, the density and flow of motor cooling gas around the motor, the rotor surface area and/or the roughness of the rotor surface are optimized. The only remaining factor that can be manipulated to reduce windage losses is the density of the gas in the motor cavity. Windage losses decrease as the density of the gas in the motor cavity decreases resulting in better motor efficiency. 
         [0006]    To reduce the gas density in these higher-speed motor cavities, vacuum pumps are used to lower the pressure surrounding the motors to reduce windage losses as much as possible. However, these systems lack the ability to both adequately cool the motor while providing a vacuum surrounding the motor cavity. One attempt to lower the gas density in the motor cavity while simultaneously cooling the motor involves the use of auxiliary positive displacement gas compressors powered by an independent power source to “pump down” the motor cavity while a complete chiller system is in operation. However in these systems, the auxiliary compressors consume more energy than they are saving in motor windage losses, therefore these systems are not a good solution to increasing motor efficiency. 
         [0007]    Therefore, there is a need for a system that can reduce windage and other energy losses in a compressor motor while not expending more energy than is being saved. 
       SUMMARY OF THE INVENTION 
       [0008]    One embodiment of the present invention is directed to a refrigeration system including a compressor, an evaporator and a condenser connected in a closed refrigerant loop. A motor is connected to the compressor to provide power to the compressor. A liquid expander is connected in the refrigerant loop between the condenser and the evaporator. In conjunction with the refrigeration system, a motor coolant system is used to cool the compressor motor. The motor coolant system has a first connection with the refrigerant loop to receive refrigerant from the evaporator to the motor cavity for cooling, and a second connection with the refrigerant loop to return refrigerant to the evaporator from the motor cavity. The motor coolant system also has a pumping device to circulate refrigerant from the first connection through the motor cavity and to the second connection. The pumping device is powered by operation of the liquid expander and the pumping device lowers the pressure and density of the gaseous refrigerant in the motor cavity to reduce windage losses in the motor. 
         [0009]    A second embodiment of the present invention is directed to a motor coolant system for a chiller system including a compressor, an evaporator and a condenser connected in a closed refrigerant loop. The motor coolant system includes a motor housing for a motor that powers the compressor of the chiller system. The motor coolant system also includes a liquid expander that is connectable in the closed refrigerant loop between the condenser and the evaporator of the chiller system. Additionally, the motor coolant system has a first connection connectable to the closed refrigerant loop to receive refrigerant from the evaporator and provide refrigerant to the motor housing and a second connection connectable to the closed refrigerant loop to return refrigerant to the evaporator. A pumping device is disposed in the second connection and is used to circulate refrigerant from the first connection through the motor housing to the second connection to cool the motor and maintain a predetermined pressure in the motor cavity. The pumping device is coupled to a liquid expander and is powered by operation of the liquid expander. Further, the predetermined pressure in the motor cavity is maintained at a constant level throughout the operation of the motor coolant system. 
         [0010]    Another embodiment of the invention is a method for cooling a motor of a chiller system including the steps of providing a first connect with a refrigerant loop, where the first connection is configured to receive refrigerant from an evaporator. The next step involves providing a second connection with the refrigerant loop, where the second connection is configured to return refrigerant to the evaporator, and then providing a motor in a motor cavity, where the motor cavity is connected to the first connection and the second connection. The next step involves circulating refrigerant from the first connection through the motor cavity to the second connection with a pumping device, and then powering the pumping device with energy of expansion from a liquid expander, where the liquid expander is configured to expand refrigerant in the refrigerant loop between a condenser and the evaporator, wherein the circulation of refrigerant in the motor cavity by the pumping device cools the motor and lowers the pressure and gas density of a refrigerant in the motor cavity thereby reducing windage losses of the motor. 
         [0011]    One advantage of the present invention is the reduction in windage and energy losses in the motor. 
         [0012]    Another advantage of the present invention is the recycling of discharged energy by the liquid expander. 
         [0013]    Still another advantage of the present invention is that the system effectively lowers the pressure of refrigerant gas in the motor cavity, cools the motor, and keeps energy expenses at a minimum. All this optimizes the reduction of windage losses and increases the efficiency of the motor. 
         [0014]    Additionally, another advantage of the present invention is that the compressor for the motor cooling loop is load dependant. Therefore, the system only operates at the necessary level for the current load of the system and does not consume unnecessary energy. 
         [0015]    Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a block diagram of an embodiment of the present invention. 
           [0017]      FIG. 2  is a block diagram of another embodiment of the present invention. 
           [0018]      FIG. 3  illustrates a cross section of a motor and compressor housing. 
           [0019]      FIG. 4  illustrates a detailed view of the connection between the pumping device and the expander. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. Referring to  FIG. 1 , the HVAC, refrigeration or liquid chiller system includes a compressor  302 , a condenser arrangement  112 , and a liquid chilling evaporator arrangement  114  connected in a refrigerant loop. In a preferred embodiment, the chiller system has a capacity of 250 tons or greater and even more preferably, has a capacity of 1000 tons or greater. A motor  106  is connected to the compressor  302  to power the compressor  302 . The motor  106  and compressor  302  are preferably housed in a common hermetic enclosure, but can be housed in separate hermetic enclosures. The compressor  302  compresses a refrigerant vapor and delivers high pressure vapor to the condenser  112  through a discharge line. The compressor  302  is preferably a centrifugal compressor; however, the compressor  302  can be any suitable type of compressor including a screw compressor, a reciprocating compressor, a scroll compressor, a rotary compressor or any other type of compressor. 
         [0021]    The high pressure refrigerant vapor delivered by the compressor  302  to the condenser  112  enters into a heat exchange relationship with a fluid, such as air or water, and undergoes a phase change to a high pressure refrigerant liquid as a result of the heat exchange relationship with the fluid. The high pressure liquid refrigerant from the condenser  112  flows through an expander  128  to enter the evaporator  114  at a lower pressure. The liquid refrigerant delivered to the evaporator  114  enters into a heat exchange relationship with a fluid, e.g., air or water, and undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the fluid. The vapor refrigerant in the evaporator  114  exits the evaporator  114  and returns to the compressor  302  by a suction line to complete the cycle. It is to be understood that any suitable configuration of condenser  112  and evaporator  114  can be used in the system, provided that the appropriate phase change of the refrigerant in the condenser  112  and evaporator  114  is obtained. 
         [0022]    A motor cooling loop is connected to the refrigerant loop discussed above to provide cooling to the motor  106 . The motor cooling loop has a connection near the suction inlet of the compressor  302  that leads to the motor cavity of the motor  106 . The circulated refrigerant gas for cooling the motor  106  exits the motor cavity and is sent to the evaporator  114 . As discussed in greater detail with regard to  FIGS. 3 and 4 , a pumping device  130  is used to circulate the refrigerant through the motor cavity from the refrigerant loop near the suction inlet of the compressor  302  and return the refrigerant to the refrigerant loop near the evaporator  114 . The circulation of the refrigerant from the refrigerant loop into the motor cavity and the removal of the heated refrigerant gas from the motor cavity by the pumping device  130  helps to cool and lower windage losses in the motor  106  and raise the overall motor efficiency. In particular, the operation of the pumping device  130  is used to maintain a substantial constant predetermined pressure and density of refrigerant gas in the motor cavity to lower windage losses. The predetermined pressure and density of refrigerant gas in the motor cavity is less than the suction pressure of the compressor and can approach a vacuum type condition. The HVAC or refrigeration system can include many other features that are not shown in  FIG. 1 . These features have been purposely omitted to simplify the drawing for ease of illustration. 
         [0023]    Similar to  FIG. 1 ,  FIG. 2  also has a compressor  302 , a condenser  112 , and an evaporator  114  connected in a closed refrigerant loop. The compressor  302  compresses the refrigerant vapor and delivers high pressure vapor to the condenser  112  through a discharge line. The high pressure refrigerant vapor delivered to the condenser  112  enters into a heat exchange relationship with a fluid from a cooling tower, e.g., water, and undergoes a phase change to a high pressure refrigerant liquid as a result of the heat exchange relationship with the fluid. The high pressure liquid refrigerant from the condenser  112  flows through the expander  128  and enters the evaporator  114  at a lower pressure. The evaporator  114  includes connections for a supply line and a return line of a cooling load. A secondary liquid, e.g., water, ethylene glycol, calcium chloride brine or sodium chloride brine, travels into the evaporator  114  via a return line and exits the evaporator  114  via a supply line for a cooling load. The liquid refrigerant in the evaporator  114  enters into a heat exchange relationship with the secondary liquid to lower the temperature of the secondary liquid. The refrigerant liquid in the evaporator  114  undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the secondary liquid. The vapor refrigerant in the evaporator  114  exits the evaporator  114  and returns to the compressor  302  by a suction line to complete the cycle. 
         [0024]    As in  FIG. 1 , the motor cooling loop is connected to the refrigerant loop to provide cooling to the motor  106 . The motor cooling loop has a connection near the suction inlet of the compressor  302  that leads to the motor cavity for the motor  106 . However, different from the embodiment in  FIG. 1 , the circulated motor coolant refrigerant gas, after cooling the motor  106  and passing through the pumping device  130 , is passed through a heat exchanger  134  to lower the temperature of the superheated refrigerant gas before the refrigerant gas is sent to the evaporator  114 . The heat exchanger  134  has a connection with the supply line between the cooling tower  132  and the condenser  112  to receive cooling water from the cooling tower  132 . Water from the cooling tower  132  is used to cool the refrigerant gas exiting the pumping device  130 , by de-superheating the refrigerant as it flows through heat exchanger  134 . After the cooling water exchanges heat with the refrigerant, the cooling water is returned to the cooling tower  132  with a connection to the return line between the condenser  112  and the cooling tower  132 . The HVAC or refrigeration system can include many other features that are not shown in  FIG. 2 . These features have been purposely omitted to simplify the drawing for ease of illustration. 
         [0025]    As shown in both  FIGS. 1 and 2 , the pumping device  130  is coupled to the expander  128  from the refrigerant loop. The pumping device is preferably a compressor, and can be any one of a screw compressor, a reciprocating compressor, a scroll compressor, a vane type compressor or other suitable compressor. For example, in a 1000 ton capacity chiller system, the pumping device or compressor  130  preferably has a swept volume of at least about 310 CFM and a volume ratio of at least about 3.3 to deliver the necessary pressures. The pumping device  130  and the expander  128  can be mechanically coupled via a common shaft, or by having two separate mechanical components that are tied together electrically where the expander  128  is coupled to a type of electric generator, and the pumping device  130  is powered by an electric motor that uses the required portion of the electric that is generated. The pumping device  130  and the expander  128  can also be integrated into a single system unit having either a mechanical or electrical connection with a common shaft. A single system unit utilizes a control valve to control or limit the amount of expander power extraction so that the depressed pressure in the motor cavity can be controlled. In utilizing a control valve, the excess expansion refrigerant is essentially expanded through a part of the slide control orifice to satisfy the cooling load liquid refrigerant flow requirements into the evaporator. With the single system unit having the pumping device  130  and the expander  128  with a control valve to regulate motor cavity pressure and control expansion of the liquid refrigerant, only four refrigerant connections are required on an efficient chiller component with no shaft seals. When positive-displacement compression technology is used for the pumping device  130  and the expander  128 , the required pressure ratios and volume ratios are attainable. If aerodynamic compression technology is utilized, the required pressure ratios and volume ratios are achieved through the incorporation of additional aerodynamic stages on the pumping device  130  and/or the expander  128  to achieve the required pressure ratios and volume ratios for proper operation. Preferably, the expander  128  is one of an eductor, a positive displacement expander, or turbine type centrifugal expander. For example, in a 1000 ton capacity chiller system, the expander  128  preferably is sized for at least 300 GPM liquid refrigerant inlet flow with a volume ratio of at least about 13.8 to fully expand the liquid as needed for the system. It is to be understood that the particular swept volume and volume ratio minimums for the expander  128  and pumping device  130  are dependant on a variety of factors such as the type of refrigerant used and the capacity of the refrigeration system. The expander  128  provides power to the pumping device  130  by recovering the discharged energy from the expansion of the liquid refrigerant. The use of recovered energy to power the pumping device  130  reduces energy losses of the motor coolant system and also reduces the amount of total power needed to operate the motor coolant system. 
         [0026]    In addition, the connection of the pumping device  130  to the expander  128  permits the operation of the motor coolant system to be load dependant. When the load on the motor is reduced, the motor operates at a lower speed and can have a corresponding reduced cooling demand. Additionally, at lower load capacity, the coupled pumping device  130  receives less power from the expander  128  due to reduced flow of refrigerant through the primary refrigerant loop and the pumping device correspondingly provides a lower amount of suction on the motor cavity to siphon off refrigerant gasses cooling the motor  106 . Since the system is load dependant, it never reduces the gas density of the refrigerant in the motor cavity lower than necessary or expends more energy than necessary. 
         [0027]    As shown in  FIG. 3 , an aerodynamic compressor  302  is powered by a hermetic motor  106 . The compressor  302  can be any one of a single stage compressor, or a multiple-stage compressor configured on a common shaft with the motor  106 , or with the motor  106  disposed between the multiple stages. The motor  106  includes a stator  502  having a plurality of projecting poles (i.e. motor windings), and a rotor  504  also having a plurality of poles. In the cross-sectional drawing in  FIG. 3 , there are shown only one pair of poles for each of the stator  502  and the rotor  504 , although the motor  106  normally had multiple pole-pairs on each of the stator  502  and the rotor  504 . The stator  502  typically has a greater number of poles than the rotor  504 . The rotor  504  is attached to a shaft  508  that is connected to and drives the impeller  510  of the compressor  302 . A plurality of electrical connectors  518  connects the poles of the stator  502  to impart rotation to the rotor  504  and the impeller  510 . The motor  106  is shown within the hermetic enclosure  516  that encloses the compressor  302  and its associated components. 
         [0028]    The motor  106  and motor cavity are maintained at a pressure much lower than the suction pressure of the compressor  302  at the suction line  524  to reduce windage losses. The motor  106  and motor cavity are in fluid communication with the suction line  524  and the compressor chamber  528  via conduit  526  (shown schematically in  FIG. 3 ). The conduit  526  is in fluid communication with motor passages  530  that exist between the rotor  504  and the stator  502 . The refrigerant gas inside the motor  106  is drawn from the compressor chamber  528  into the motor passages  530  thereby circulating refrigerant vapor inside the motor  106  and motor cavity to cool the motor  106 . The now heated refrigerant gas is drawn from the motor cavity by the pumping device  130  and then sent to the heat exchanger  134  and/or the evaporator  114  by the pumping device  130 . 
         [0029]    Referring to  FIG. 4 , a cross sectional illustration of one connection between the expander  128  and the pumping device  130  is shown. The expander  128  and the pumping device  130  are shown connected by a mechanical connection. The expander  128  and the pumping device  130  operate on a common shaft, where the expander  128  drives the compressor  130  based on the amount of refrigerant from the condenser  112  flowing through the expander  128 . The pumping device  130  receiving gasses directly from the motor cavity, and the expander  128  receives liquid refrigerant from the condenser  112 . The pumping device  130  transfers the discharged motor gas to the heat exchanger  134  and/or the evaporator  114 . The expander  128  uses the excess energy from the expansion of the refrigerant to power the pumping device  130 . As the expander  128  processes the excess energy, the energy is transferred to the connected pumping device  130 , thereby supplying power to the pumping device  130 . The refrigerant is then discharged from the expander  128  to the evaporator  114  before returning to the compressor  302 . 
         [0030]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.