SYSTEM AND METHOD FOR REDUCING WINDAGE LOSSES IN COMPRESSOR MOTORS

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

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.

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'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.

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.

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.

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.

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.

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

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.

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.

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.

One advantage of the present invention is the reduction in windage and energy losses in the motor.

Another advantage of the present invention is the recycling of discharged energy by the liquid expander.

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.

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.

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

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a block diagram of another embodiment of the present invention.

FIG. 3 illustrates a cross section of a motor and compressor housing.

FIG. 4 illustrates a detailed view of the connection between the pumping device and the expander.

DETAILED DESCRIPTION OF THE INVENTION

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Claims

1. A refrigeration system comprising:

a compressor, an evaporator and a condenser connected in a refrigerant loop;

a motor connected to the compressor to power the compressor, the motor being disposed in a motor cavity;

a liquid expander connected in the refrigerant loop between the condenser and the evaporator; and

a motor coolant system, the motor coolant system comprising: a first connection with the refrigerant loop to receive refrigerant from the evaporator; a second connection with the refrigerant loop to return refrigerant to the evaporator; a pumping device to circulate refrigerant from the first connection through the motor cavity to the second connection, the pumping device being powered by operation of the liquid expander; and wherein the pumping device lowers a pressure and gas density of the refrigerant in the motor cavity to reduce windage losses of the motor.

2. The refrigeration system of claim 1 wherein the liquid expander is configured to expand high-pressure refrigerant liquid from the condenser to low-pressure refrigerant liquid for the evaporator.

3. The refrigeration system of claim 2 wherein the liquid expander powers the pumping device by recovering energy from the expansion of refrigerant in the liquid expander.

4. The refrigeration system of claim 3 wherein the liquid expander comprises one of an eductor, a positive displacement expander or a turbine centrifugal expander.

5. The refrigeration system of claim 1 wherein the pumping device is a gas compressor.

6. The refrigeration system of claim 5 wherein the gas compressor comprises one of an aerodynamic compressor or a positive displacement compressor.

7. The refrigeration system of claim 6 wherein the gas compressor comprises one of a screw compressor, a reciprocating compressor, a scroll compressor, or a vane type compressor.

8. The refrigeration system of claim 7, wherein the refrigeration system has as a 1000 ton capacity, the gas compressor has a swept volume of about 310 CFM and a volume ratio of about 3.3, and the liquid expander is configured for a flow of at least 300 GPM and has a volume ratio of about 13.8.

9. The refrigeration system of claim 1 wherein the liquid expander is coupled to the pumping device by one of a mechanical connection or an electrical connection.

10. The refrigeration system of claim 1 wherein the liquid expander and the pumping device are combined as a single unit.

11. The refrigeration system of claim 1 further comprising a heat exchanger connected between the pumping device and the evaporator, the heat exchanger being configured to de-superheat refrigerant discharged from the pumping device.

12. The refrigeration system of claim 11 wherein the heat exchanger is configured to de-superheat refrigerant from the pumping device with condenser cooling tower water.

13. A motor coolant system for a chiller system having a compressor, an evaporator and a condenser connected in a closed refrigerant loop, the motor coolant system comprising:

a motor housing for the motor;

a liquid expander connectable to the closed refrigerant loop between the condenser and the evaporator of the chiller system;

a first refrigerant connection connectable to the closed refrigerant loop to receive refrigerant from the evaporator and provide refrigerant to the motor housing;

a second refrigerant connection connectable to the closed refrigerant loop to return refrigerant to the evaporator; and

a pumping device disposed in the second refrigerant connection to circulate refrigerant from the first refrigerant connection through the motor housing to the second refrigerant connection to cool the motor and maintain a predetermined pressure in the motor housing, the pumping device being coupled to the liquid expander and powered by operation of the liquid expander.

14. The motor coolant system of claim 13 wherein the predetermined pressure in the motor housing is maintained at a predetermined level throughout the operation of the motor coolant system.

15. The motor coolant system of claim 13 wherein the coupled pumping device and liquid expander are connected by one of a mechanical connection or an electrical connection.

16. The motor coolant system of claim 13 wherein the coupled pumping device and liquid expander are connected as one unit.

17. The motor coolant system of claim 13 comprising a heat exchanger disposed in the second refrigerant connection between the pumping device and the evaporator, the heat exchanger lowering the temperature of the refrigerant in the second refrigerant connection.

18. The motor coolant system of claim 13 wherein the pumping device lowers the density of the refrigerant within the motor housing to reduce windage losses of the motor.

19. The motor coolant system of claim 13 wherein the pumping device comprises one of an aerodynamic compressor or a positive displacement compressor.

20. The motor coolant system of claim 19 wherein the pumping device comprises one of a screw compressor, a reciprocating compressor, a scroll compressor, or a vane type compressor.

21. The motor coolant system of claim 13 wherein the liquid expander comprises one of an eductor, positive displacement expander or a turbine centrifugal expander.

22. A method for cooling a motor of a chiller system comprising the steps of:

providing a first connection with a refrigerant loop, the first connection being configured to receive refrigerant from an evaporator;

providing a second connection with the refrigerant loop, the second connection being configured to return refrigerant to the evaporator;

providing a motor in a motor cavity, the motor cavity being connected to the first connection and the second connection;

circulating refrigerant from the first connection through the motor cavity to the second connection with a pumping device;

powering the pumping device with energy of expansion from a liquid expander, the liquid expander being configured to expand refrigerant in the refrigerant loop between a condenser and the evaporator; and

wherein the circulation of refrigerant in the motor cavity by the pumping device cools the motor and lowers a pressure and gas density of a refrigerant in the motor cavity thereby reducing windage losses of the motor.

23. The method of claim 23 further comprising the step of connecting the pumping device and the liquid expander by one of an electrical connection or a mechanical connection.

24. The method of claim 24 wherein the pumping device and liquid expander are combined as a single unit.

25. The method of claim 23 further comprising the step of cooling the refrigerant in the second connection with a heat exchanger.

26. The method of claim 26 wherein the heat exchanger uses a cooling liquid for the condenser to cool the refrigerant in the second connection.