Cooling system and method for magnetic bearing compressors FIELD OF THE INVENTION

[0001] The present invention relates to magnetic bearing compressors. More specifically, the present invention is concerned with a cooling system and method for magnetic bearing compressors.

BACKGROUND OF THE INVENTION

[0002] Many commercially available magnetic bearing centrifugal compressors rely on liquid refrigerant created in the condenser to be passed through passages around the motor stator and through the "air" gap in between the rotor and stator in order to maintain temperatures within safe limits for the materials, such as for example magnets, motor winding and other composite materials that may be used in the rotor constructions.

[0003] Additionally if the compressor is a variable speed driven compressor then there may be additional cooling requirements associated with the power electronic devices that make up the variable speed drive. Typically these power electronic devices are mounted on heat sink plates that circulate a coolant fluid through them.

[0004] In all variants of refrigerant piping and cooling circuit systems relating to the above there is a requirement for a refrigerant metering device such as a thermostatic expansion valve, electronic expansion valve such as a stepper motor driven device, float style valve or an evaporator pressure regulating device that is used to meter the flow rate of liquid refrigerant that enters the cooling circuit. [0005] If the flow rate of motor and variable frequency drives (VFD) cooling is not controlled, the motor, rotor and VFDs may be over cooled or, in the case of over feeding a high speed rotor, the windage loss may increase significantly, which will decrease the system's overall efficiency as the refrigerant is directed away from useful work in the evaporator.

[0006] When the cooling system in the compressor utilizes a direct or dry expansion style heat exchanger that operates at the evaporator pressure or inter-stage pressure of the compressor, so that the refrigerant that exits the cooling circuit either returns to the inlet of the compressor at low pressure or the inlet of an intermediate impellor on a multiple impellor machine, another use for the refrigerant metering device is to create a pressure drop that allows expansion.

[0007] In all systems described above the heat exchangers used in VFDs and cooling paths through the compressors, motors and rotors have a pressure drop associated with them due to circuit frictional losses. In addition, metering valves also have a frictional loss and often a minimum pressure drop requirement to operate. These valve and circuit pressure drops typically dictate the minimum pressure ratio the compressor can operate at and thus define the minimum lift requirement for a chiller or an air conditioning unit. A typical value for the minimum lift in a system that has a correctly sized liquid line piping from the condenser is a pressure ratio of 1.5:1 (see Figure 1).

[0008] A desirable aspect of the centrifugal compressor, especially when fitted with a variable speed drive, is its ability to reduce compression power when the lift or pressure ratio is reduced. In most all applications however, the level of power reduction possible is limited by the minimum lift requirement to facilitate cooling to the compressor and drive.

[0009] In many all-years round cooling applications, such as data centers, hospitals and manufacturing processes, the limitation of lift reduction causes systems to operate less efficiently than they could, especially in case of installations in colder climates (see Figure 2). [0010] There is still a need in the art for a cooling system and method for magnetic bearing compressors.

SUMMARY OF THE INVENTION

[0011] More specifically, in accordance with the present invention, there is provided a cooling system, comprising: a condenser; a magnetic bearing centrifugal refrigeration compressor that uses liquid refrigerant from the condenser to supply cooling to its motor assembly; and a refrigerant pump installed in a motor cooling refrigerant supply line of the condenser; wherein the refrigerant pump extends a low lift operating range of the compressor by allowing a controlled amount of liquid refrigerant to be pumped from the condenser.

[0012] There is further provide a method for compression refrigeration, comprising providing a magnetic bearing centrifugal refrigeration compressor that uses liquid refrigerant from a condenser to supply cooling to its motor assembly; providing a cooling line including an evaporator connected to the compressor by a refrigerant pipe; providing a refrigerant pump in a motor cooling refrigerant supply line of the condenser; and varying the pump speed directly in relation to temperatures measured inside the compressor.

[0013] There is further provide a method for controlling a cooling system comprising a magnetic bearing centrifugal refrigeration compressor that uses a pressure sub cooled liquid refrigerant from a condenser to supply cooling to its motor assembly, comprising: providing a centrifugal refrigerant pump in the motor cooling refrigerant supply line of the condenser, increasing the condenser capacity and turning on the centrifugal refrigerant pump.

[0014] There is further provide a method for starting a cooling system comprising a magnetic bearing centrifugal refrigeration compressor that uses a pressure sub cooled liquid refrigerant from an outdoor air-cooled or evaporative condenser to supply cooling to its motor assembly, comprising: providing a centrifugal refrigerant pump in the motor cooling refrigerant supply line of the condenser and that allowing an amount of liquid refrigerant to be pumped from the outdoor air cooled condenser or evaporative condenser.

[0015] Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings

BRIEF DESCRIPTION OF THE DRAWINGS [0016] In the appended drawings:

[0017] Figure 1 shows a sample compressor map at varied pressure ratio;

[0018] Figure 2 shows power reduction at constant load; and

[0019] Figure 3 is a diagrammatic view of a system according to an embodiment of an aspect of the present invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0020] This invention relates to centrifugal magnetic bearing compressors that use condenser pressure sub cooled liquid refrigerant as a means to supply cooling to a motor assembly, i.e. to the stator and the rotor and optionally a variable speed drive package that may include insulated gate bipolar transistor (IGBT), SCRs and liquid cooled inductors.

[0021] Figure 3 shows a cooling system 10 comprising a magnetic bearing centrifugal refrigeration compressor 60 that uses a pressure sub cooled liquid refrigerant from a condenser 50 to supply cooling to its motor assembly, i.e. stator and rotor, and optionally a variable speed drive (VFD) unit that may include IGBT, SCRS and liquid cooled inductors for example. The cooling line includes an evaporator 52 connected to the compressor 60 by a refrigerant pipe 54, and the condenser 50 connected to the compressor 60 by a discharge pipe 58. The refrigerant pipe 54 comprises an electronic expansion valve 56.

[0022] The condenser 50 may be air cooled, or water cooled or evaporatively cooled. The condenser 50 may be an an outdoor device.

[0023] A magnetically coupled centrifugal or magnetic bearing centrifugal refrigerant pump 30 coupled with a refrigerant receiver tank 20 is installed in the motor cooling refrigerant supply line 40 of the condenser 50. The pump 30 may or may not be fitted with a variable speed drive.

[0024] The pump 30 acts as a motor cooling liquid refrigerant pressure amplifier in situations where the lift or pressure ratio is below a value that provides adequate cooling to the compressor motor, stator and power electronics in the motor and variable frequency drives (VFD).

[0025] The pump 30 allows extending the operational envelope of the compressor 60 by overcoming a minimum lift limitation related to the motor or drive cooling requirement.

[0026] In colder weather or climates cooling installations that operate all year round, such as data centers for example, the annual energy use may thus be reduced by as much 35% when compared to a conventional magnetic bearing refrigeration compressor system.

[0027] In cold weather climates the pump 30 may also be used to start the compressor and refrigeration system safely when refrigerant migration from the evaporator 52 to the condenser 50, which may be an outdoor device in some cases, has occurred, by providing an additional valve and start sequence, thereby improving the starting reliability of the system.

[0028] The pump 30 has a fully sealed design requiring no lubrication other than the refrigerant passing through it, which may be used with an oil free magnetic bearing compressor 50.

[0029] The pump 30 to the motor/ drive cooling piping of the refrigeration system allows extending the low lift operating range of the magnetic bearing centrifugal refrigeration compressor 50.

[0030] The pump 30 allows a small amount of liquid refrigerant to be pumped from an outdoor air cooled condenser or evaporative condenser 50 for example, to enable reliable starting of the system with cold ambient temperatures (low ambient starting sequence).

[0031] The present method allows reducing pumping energy in a cooling system comprising a magnetic bearing centrifugal refrigeration compressor that uses a pressure sub cooled liquid refrigerant from a condenser to supply cooling to its motor assembly, i.e. stator and rotor, and optionally a variable speed drive (VFD) unite that may include IGBT, SCRS and liquid cooled inductors for example, by providing a centrifugal refrigerant pump in the motor cooling refrigerant supply line of the condenser and varying the pump speed directly in relation to temperatures measured inside the compressor such as rotor gap temperature, winding temperature and VFD heat sink plate or IGBT temperatures.

[0032] The present method allows compressor/ cooling control in a cooling system comprising a magnetic bearing centrifugal refrigeration compressor that uses a pressure sub cooled liquid refrigerant from a condenser to supply cooling to its motor assembly, i.e. stator and rotor, and optionally a variable speed drive (VFD) unit that may include IGBT, SCRS and liquid cooled inductors for example, by providing a centrifugal refrigerant pump in the motor cooling refrigerant supply line of the condenser, and decreasing the system lift by increasing the condenser fan or water pump speed/ capacity and then turning on the centrifugal refrigerant pump.

[0033] The present method provides pumping energy reduction by allowing varying the cooling pump speed directly in relation to temperatures measured inside the compressor such as rotor gap temperature, winding temperature and VFD heat sink plate or IGBT temperatures.

[0034] The present invention provides that the compressor operational envelope can be dramatically extended as there no longer exists a minimum lift limitation as a result motor/ drive cooling requirement.

[0035] The scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.