COOLING SYSTEM AND METHOD FOR MAGNETIC BEARING COMPRESSORS

SYSTEME ET PROCEDE DE REFROIDISSEMENT DE COMPRESSEURS A PALIERS MAGNETIQUES

English Abstract

A cooling method and system, the system comprising 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; the refrigerant pump extending a low lift operating range of the compressor by allowing a controlled amount of liquid refrigerant to be pumped from the condenser.

French Abstract

La présente invention concerne un procédé et un système, le système comprenant un compresseur de réfrigération centrifuge à paliers magnétiques qui utilise un réfrigérant sous forme liquide provenant du condenseur pour permettre le refroidissement de son ensemble moteur ; et une pompe de réfrigérant installée dans une conduite d'alimentation en réfrigérant de refroidissement de moteur du condenseur ; la pompe de réfrigérant augmentant la plage de mise en uvre à basse pression du compresseur en permettant de pomper une quantité régulée de réfrigérant sous forme liquide du condenseur.

Claims

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1. A system for extending a low lift operating range of a magnetic
bearing centrifugal compressor in a cooling system comprising a condenser
and an evaporator, the system comprising a refrigerant pump installed in a
motor cooling refrigerant supply line of the condenser, wherein said
refrigerant
pump is configured to pump a pressure sub cooled liquid refrigerant from said
condenser to said compressor when a lift or pressure ratio is below a
threshold
value, said refrigerant pump pumping an amount of the pressure sub cooled
liquid refrigerant controlled directly in relation to temperatures measured
inside
the compressor.
2. The system of claim 1, wherein said condenser is one of: i) air
cooled, ii) water cooled and iii) evaporatively cooled.
3. The system of any one of claims 1 and 2, wherein said
condenser is an outdoor device.
4. The system of any one of claims 1 to 3, wherein said refrigerant
pump is one of: i) a magnetically coupled centrifugal pump and ii) a magnetic
bearing centrifugal pump.
5. The system of any one of claims 1 to 4, wherein said refrigerant
pump is fitted with a variable speed drive.
6. The system of any one of claims 1 to 5, wherein said refrigerant
pump is coupled with a refrigerant receiver tank.
7. The system of any one of claims 1 to 6, wherein said refrigerant
pump acts as a motor cooling liquid refrigerant pressure amplifier.
8. The system of any one of claims 1 to 7, wherein said refrigerant
pump is a fully sealed pump.
9. The system of any one of claims 1 to 8, wherein the amount of
the pressure sub cooled liquid refrigerant pumped from said condenser to said

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compressor enables starting of the system with cold ambient temperatures.
10. A method for extending a low lift operating range of a
magnetic bearing centrifugal compressor in a cooling system comprising a
condenser and an evaporator, comprising pumping a controlled amount of
pressure sub cooled liquid refrigerant from the condenser to the compressor
using a refrigerant pump installed in a motor cooling refrigerant supply line
of
the condenser when a lift or pressure ratio is below a threshold value and
varying the pump speed directly in relation to temperatures measured inside
the compressor.
11. The method of claim 10, comprising increasing the condenser
capacity and turning on the refrigerant pump.

Description

1
TITLE OF THE INVENTION
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.
Date Recue/Date Received 2020-05-12

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[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).
Date Recue/Date Received 2020-05-12

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[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 provided 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 provided 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 provided 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:
Date Recue/Date Received 2020-05-12

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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), silicon controller rectifiers (SCRs) and liquid
cooled inductors.
[0021] Figure 3 shows a cooling system 10 comprising a magnetic
bearing centrifugal
Date Recue/Date Received 2020-05-12

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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,
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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
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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.
Date Recue/Date Received 2020-05-12

Representative Drawing