Note: Descriptions are shown in the official language in which they were submitted.
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LUBRICATION-FREE CENTRIFUGAL COMPRESSOR
Cross-Reference to Related Application
This application claims the benefit of United States provisional application
no.
62/769,323, filed 19 November 2018, which is hereby incorporated by reference
as
though fully set forth herein.
Field of the Invention
The present invention pertains to the field of gas compression equipment. In
particular, the present invention refers to compact electric devices or
equipment which
use centrifugal compressors to increase a gas pressure.
Background
Gas compression by mechanical means is currently carried out through different
technologies which are widely spread and tested from some years ago. Among the
most
used methods are centrifugal, axial, alternative and screw compressors.
Alternative, or piston, compressors are similar in construction to internal
combustion engines. A piston or plunger displaces longitudinally in an
alternate manner
inside a cylinder, against which a series of seals are located. A set of
valves allows the
entrance of gas during the expansion phase of the chamber formed by the
cylinder and
the plunger, while the latter gets back. When it advances again, the plunger
reduces the
volume of the chamber where the gas is located, thus producing the subsequent
desired
pressure increase. Finally, another set of valves allows the escape of gas at
high
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pressure. These compressors are widely used in small and medium scale
applications.
They are of simple construction and their efficiency is relatively high. They
have numerous
mobile components but they move at low speed relative to each other,
therefore, they can
use traditional lubricants for their seals and bearings. By functioning at low
speed (around
1,000 to 5,000 rpm) they usually couple to an electric motor or combustion
engine in a
direct manner, with no gear assemblies.
The main drawback of this technology lays in the intensive maintenance
required
by the lubrication system, which is needed to assure the sealing between
plungers and
cylinders, the integrity and water tightness of valves and to avoid damage in
bearings.
With a proper maintenance, alternative compressors may function with no
inconveniences for many years. However, all the above mentioned components
suffer
from mechanical wear and should be periodically replaced, even when they are
maintained in optimal lubricant conditions. Due to the number of mechanisms
involved,
alternative compressors are the most bulky of the four types mentioned (with
low specific
power).
Screw compressors, as well as the alternative ones, belong to a group of
machines
called "positive displacement machines", i.e., the change in gas pressure is
achieved from
a change in the volume of the chamber containing it. In a screw compressor,
two screw-
shaped helical axes, located parallel between each other, turn in a unified
manner, one
against the other. Both helicoids are within a chamber which has walls that
are very close
to the edge of their threads, thus forming a mechanical seal with or without
help of
lubricants. The turning of screws against the chamber generates hermetically
isolated
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volumes containing gas and which progressively reduce their size while they
displace
from the suction port to the discharge one.
This type of compressor is typically used in medium scale applications and
shows
some advantages such as a reduced size and high compression rates. The
mechanical
components turn at moderate speeds (for example, between 3,000 and 10,000
rpm),
therefore, they may require gear assemblies to couple to electric motor or
combustion
engines, which are typically slower. In opposition, they have fewer mobile
components
than the alternative compressors; therefore, the mechanical wear is less
relevant as
regards maintenance. The major drawback lays in the lubrication system
requirements,
which should be maintained free of solid particles and should be constantly
cooled. The
lubricant plays a key role by acting as a seal between the helicoids, and as
the means to
transmit torque between them with no mechanical friction. Additionally, the
lubricant
present in the compression chamber is mixed with the compressed gas
(especially if they
are both similar fluids, such as hydrocarbons) and should be separated and
recycled in a
set of specific devices. For these reasons, the reduced size of a screw
compressor is
offset by the high number of auxiliary systems accompanying it.
Centrifugal compressors belong to the group of turbo machines along with axial
compressors, turbines and centrifugal pumps. In a centrifugal compressor, the
increase
in gas pressure is achieved indirectly, by first increasing the speed of the
gas and then
converting this kinetic energy into potential energy. A disk with blades
called an impeller
receives gas at low pressure and rotationally accelerates it, at the same time
it displaces
it to the periphery. When the high speed gas exits the rotor, another fixed
component
called a diffuser is in charge of gradually deaccelerating it, thus increasing
its pressure.
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Centrifugal compressors show some important advantages over the positive
displacement technologies: First, for the same application (flow rate and
pressure) their
size is much smaller than an alternative or a screw compressor (they have high
specific
power). Second, since they do not use seals to generate waterproof chambers,
the
internal components, such as the impeller, do not suffer from mechanical wear.
The
function of the lubricant used in these compressors is to reduce the friction
on the auxiliary
components such as external seals and/or bearings.
The problems faced by these compressors are related to the high speed at which
their impellers should spin (typically between 20,000 and 100,000 rpm).
Therefore, there
is a need to use gear assemblies to multiply the speed of driving sources such
as electric
motor or combustion engines. Gear assemblies are a key component, both due to
the
mechanical wear and the loss of energy they imply. Another problem appears on
the seals
that keep the process gas isolated from the atmosphere. Said seals allow the
communication between the rotor axis and the gear assembly, thus keeping the
high gas
pressure inside the compressor body and low pressure in the outside. The seals
are a
key component as regards wear, in which lubrication plays a key role.
The above mentioned complexities have limited the use of centrifugal
compressors
to large scale applications. However, centrifugal compressors that are
suitable for
medium and small scale have recently been developed thanks to the
implementation of
such technologies as high speed electric engines. The firm Aerzen , for
example, offers
air compressors in which a radial flow electric motor is mounted on the same
axis as the
rotor of the compressor. This way, the need of a gear assembly and its
associated
operating difficulties are eliminated.
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The main disadvantage appearing on the use of high speed radial flow motors
lays
on the great amount of ferromagnetic material present on stators. The high
electric
frequency used at high speed implies huge losses of energy in said cores as
parasitic
currents and magnetic hysteresis. An alternative to this technology is that of
axial flow
electric motors, which has gained great interest over recent years due to a
series of
advantageous constructive features.
First, the geometry of axial flow motors allows obtaining machines with higher
specific power than radial flow motors. That is; in order to develop the same
power, an
axial flow motor requires a smaller size than its radial flow equivalent, with
the subsequent
reduction in weight and cost. As well as the radial flow motors, those of
axial flow may be
either inductive and use winded conductors equivalent to a "squirrel cage" or
synchronous
and use permanent magnets. The latter show the highest power density of all
possible
configurations.
Second, and with more relevance for the application of this invention, the
arrangement of coils in an axial flow motor requires the use of much less
ferromagnetic
material in their core than in a radial flow motor. In high electric frequency
applications
(high spinning speed) a reduction of the ferromagnetic material implies a
decrease in
energy losses due to magnetic hysteresis and parasitic currents, therefore,
significantly
increasing the performance of the machine and reducing the cooling
requirements. The
industrial use of axial flow synchronous motors in high speed applications,
which is not
very well known in current art, promises important improvements as regards
size
reduction and energetic efficiency increase.
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The firm ICR Turbine Corporation (Patent Publication No. US20140306460)
has developed a compact Brayton cycle implementing several compression and
expansion stages to increase the global performance of the system and to boost
a radial
flow electric generator coupled to an independent turbine. Each one of the
compression
and expansion stages comprises a radial compressor and a radial turbine, both
mounted
on a single axis. At the back of each radial compressor a small axial flow
motor has been
mounted that allows for the starting of the system, thus driving the rotor to
its minimum
holding speed. These engines are the inductive type and comprise a winded
stator and a
planar squirrel cage rotor.
Additionally, several technologies have been developed to support the motor-
compressor rotating assembly with no need to use lubricating oils, thus
eliminating
another complexity associated with the high speed of these machines. One of
them is
known as "air foil bearing" or simply "foil bearing" in which the same process
gas is used
as lubricating fluid. The foil bearings are typically used in air compressors,
such as the
ones from Aerzen .
Another more promising technology is the one of magnetic bearings. In this
case,
the rotating set is supported by the magnetic fields and not hydrodynamically
as in the
prior case, thus making the system independent of the process fluid. The firm
Danfoss@,
for example, offers the "Turbocor " compressors for cooling gases in which the
rotor is
suspended by active magnetic bearings (AMB). The engine-compressor assembly is
located in a water tight chamber containing the process gas, this way also
eliminating the
need for mechanic seals. Another example of this technology is found in the
firm
SIEMENS (Patent ES2309173) which has developed a large scale centrifugal
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compressor, in which the engine and the compressor are mounted on the same
axis and
housed in the same pressure containment. In the case of SIEMENS the bearings
are
also of the AMB type and also the same process gas is used to cool the motor.
The complexity shown by the AMB lays in the need for an axis position sensor
set
and an active control electronic system which externally energizes and
commands the
bearings. In the event of an energy interruption of the control system, the
rotating axis
may lose support and contact the stator spinning at high speed and producing
significant
and permanent damage.
SUMMARY OF THE INVENTION
The present invention comprises a compact sized centrifugal compressor, for
medium and small scale applications, which does not use lubrication systems,
speed
multiplication systems or mechanical seals and which works usually at very
high spinning
speeds.
This compressor uses, as the driving source, an axial flow and permanent
magnet
electric motor operating synchronically at high speed and high electric
frequency. Said
motor operates efficiently at high speed, since it has less ferromagnetic core
in its stator
than the equivalent radial flow motors. In fact, this motor may work
efficiently even with
no ferromagnetic core in its stator. The compressor impeller is directly
coupled to the
motor on a single axis, forming a single mobile part of the device. The high
efficiency of
the axial flow motor and the absence of speed multiplying gears give this
invention higher
reliability and global energy efficiency over the current art.
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The motor-impeller rotating assembly is supported by a combination of magnetic
radial bearings and electrodynamic thrust (or axial) bearings, such that there
is no
mechanical contact with the stator. Said bearings use permanent passive
magnets,
therefore they do not require control systems, sensors or external energy
supply. This
gives the invention a higher operating simplicity and reliability than the one
of the current
art.
The complete rotor, formed by the motor-impeller assembly and the magnetic and
electrodynamic bearings, is located inside a waterproof chamber
pressostatically linked
to the stator of the compressor and flooded by the same process gas. In this
way the use
of mechanic seals is avoided as well as the subsequent wear by friction, which
gives this
invention the feature of requiring less maintenance than other similar current
art
equipment.
By not requiring lubrication for bearings, gears or seals, the device of this
invention
does not use auxiliary systems for cooling, filtering, separation or feeding
of lubricants.
This allows the invention for higher simplicity and smaller size than other
similar devices
of the current art.
Brief description of figures
Figure 1 is a side view of an embodiment of the invention which shows only the
motor-impeller rotating assembly and the stator coils of the motor.
Figure 2a is a cross-sectional side view of the main components of the axial
flow
motor.
Figure 2b is an exploded perspective view of the main components of the axial
flow motor.
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Figure 3 is a cross-sectional view of an embodiment of the invention including
all
the components of the device.
Figure 4 is a full perspective view of an embodiment of the device and two
orthogonal views of same, being the latter compared to the silhouette of an
average adult
for dimensional reference.
Detailed Description of the Invention
The present invention is a compact device for gas compression driven by an
axial
flow synchronous electric motor with no use of any kind of lubricants.
Figure 1 shows the single mobile piece of the motor-impeller rotating assembly
(hereinafter, the rotor). Said rotor has a centrifugal impeller 1 in charge of
delivering
kinetic energy to the process gas. In the embodiment of Figure 1 only one
impeller is
shown but more than one may be used. Said impeller is mounted on an axis 5 and
fixed
thereto. If more than one impeller is used, all of them may be mounted on and
fixed to
the same axis.
An axial flow synchronous and permanent magnet electric motor 2 is formed by a
stationary stator section and a rotating section. The stator section of said
motor contains
the coils, different auxiliary pieces for support and may optionally contain
part of the
ferromagnetic core. This section may be formed by one or more assemblies
located
between assemblies of a rotating section. In the embodiment of Figure 1, two
stator
assemblies 4 are shown located between rotating assemblies 03. In other
embodiments
this could be applied to more than two stator assemblies or only one may be
used.
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Figures 2a and 2b show an embodiment of the axial electric motor in which it
is
comprised of a single stator assembly 4. Said assembly contains coils 10 and
different
portions of the ferromagnetic core 11. The assembly 4 is placed between two
rotating
disks 3 that contain the permanent magnets 12 and may optionally contain
another part
of the ferromagnetic core 13. These discs form the rotating section 3 of the
axial flow
motor. Said rotating section is mounted onto the axis 5 (see Fig. 1) and fixed
thereto.
The magnetic flow is established between each opposing pair of permanent
magnets 12, which are placed in an attraction configuration. Said magnetic
flow passes
through the coils through air or any other means in which the motor is
immersed. If the
stator assembly contains portions of ferromagnetic core 11, the magnetic flow
is
concentrated through these. If a rotor assembly 3 has a ferromagnetic core 13,
the flow
between opposite faces of its adjacent magnets is closed therethrough. An
external
electronic device monitors the relative position of magnets 12 as regards the
coils 10 and
activates a series of semiconductors (for example: mosfet, IGBT, SSR, etc.)
that inject
current to the latter. The moment and the duration of current pulses is such
that their
interaction with the magnetic field induces a force over the permanent magnets
resulting
in a torque applied onto the axis 5 (see Fig 1). The stator section of the
motor 04 may
contain one or more coils 10 either electrically independent or linked to each
other.
In Figure 1 it may be seen that the rotating section of a first magnetic
radial bearing
6 is formed by one or more permanent magnets with ring geometry and is mounted
on
one end or near to an end of the axis 5. The rotating section of a second
magnetic radial
bearing 7 is formed by one or more magnets with ring geometry and is mounted
on one
the opposite end or near to the end opposite end of the axis 5. Said rotating
sections of
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radial bearings are part of the rotor. For each bearing, a second stator
section is formed
by permanent magnets with ring or cylinder geometry that circumferentially
surround the
rotating sections and are fixed to the housing (not shown). Being the magnets
of the same
polarity and great field intensity, by means of the magnetic interaction
between each
rotating section and its stator counterpart, magnetic repulsion forces are
developed that
allow radially supporting the rotor avoiding its mechanical contact with the
rest of the
device. In the embodiment of Figure 1 two magnetic radial bearings are shown,
one at
each end, but three or more bearings could be mounted on different zones of
the axis 5
in order to make the rotor support more rigid. Radial bearings 6 and 7 are
passive and
operate with no intervention of control systems, sensors or external energy
sources. The
materials for the manufacturing of magnetic bearings based on high intensity
passive
magnets such as AINiCo, SmCo, or NdFeB are well known in the art and are
available in
the market.
The concept in mechanics of rigidity refers to the capacity of an object to
resist a
deformation or displacement due to external forces. The more rigid the object,
the higher
force it generates against the same degree of deformation. This concept,
typically applied
to elastic systems such as springs and bearings, is also frequently used to
describe the
mechanical properties of active and passive magnetic bearings. When the forces
due to
the rigidity of the above mentioned object are such that they tend to
compensate the
deformation or the displacement that origins them, it is said to have negative
rigidity. In
the case of magnetic bearings, positive rigidity refers to a particular
behavior of these in
which the forces originated by a displacement tend to increase it, instead of
being
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opposite. It is useful to note the concept of positive rigidity since this
will be used below
to explain the functioning of some elements of this invention.
The rotor assembly shown in Figure 1 is supported and stabilized by the
passive
radial bearings since they give negative rigidity in the radial direction.
That is, if axis 5 is
displaced laterally (radially), said bearings react by generating an opposite
elastic force
returning the axis to its central position. However, as stated by the Earnshaw
Theorem,
this type of bearing gives positive rigidity in the axial direction. That is,
if axis 5 is displaced
along its axial direction, said bearings react by generating a force in the
same direction
and attempting to increase the displacement, therefore the problem arises that
these
bearings tend to push axis 5 out of its position in the axial direction. Thus,
the great
advantage of these bearings, originated from their completely passive nature,
involves
the disadvantage of showing a positive axial rigidity, therefore they cannot
be used as
single rotating links of the assembly. To counteract this effect, an
additional mechanism
should be implemented to operate by physical principles that are different to
the
interaction between permanent magnets and that fix the position of axis 5 in
the axial
direction. After various tests with different types of restriction links in
the axial direction,
the best results have been obtained by using the electrodynamic thrust
bearing.
In Figure 1 it may be seen that the rotating section of an electrodynamic
thrust
bearing 8 is formed by two disks having permanent magnets and ferromagnetic
cores.
Said rotating section is mounted on axis Sand fixed thereto, thus forming part
of the rotor.
A solid or perforated conductor disc 9 is located between said rotating discs
and forms
the stator section of the thrust bearing, linked to the housing (not shown).
The relative
movement between the magnets of the rotating section and the conductor
material of the
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stator section induces electric currents on the latter that generate repulsion
forces against
said magnets. In the arrangement shown in Figure 1, said repulsion forces give
negative
rigidity in the axial direction. This way, if axis 5 is displaced in the axial
direction, the
electrodynamic thrust bearing generates a force in the opposite direction
attempting to
reestablish the original position.
The electrodynamic thrust bearing works in a completely passive manner and
does
not require auxiliary control systems. However, said functioning only happens
if there is
relative movement between parts, that is, only if the rotor is spinning. Above
a minimum
rotation speed, the electrodynamic thrust bearing provides the rotor with
enough negative
axial rigidity to counteract the positive axial rigidity of magnetic radial
bearings. It is
possible to arrange the magnet supporting discs 8 as the rotating section and
the
conductor disc 9 as the static section, or vice versa. In the embodiment of
Figure 1, only
one thrust bearing is shown, but two or more could be stacked to provide
higher axial
rigidity to the rotor. In applications where the rotor is not horizontally
oriented, part of or
the whole axial component of its weight may be offset by the same positive
rigidity of
magnetic bearings. This way, the electrodynamic thrust bearing must only
provide
negative rigidity to the assembly but it is not used to support its weight.
The combination of magnetic radial bearings with electrodynamic thrust
bearings
allows the rotor to be completely supported in its axial and radial position,
above a specific
minimum spinning speed, thus avoiding its mechanical contact with the rest of
the device.
In opposition to active magnetic bearings (AMB), the combination of passive
components
in this invention assures its functioning with no external energy or control
requirements,
even with total interruptions of electric supply. This novel combination
allows the device
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to spin at the required speed by the impeller of the centrifugal compressor
without
suffering any wear, due to the absence of friction force that would generate a
great
amount of caloric and stopping energy.
Figure 3 shows an embodiment of the invention in which the above mentioned
rotor
is positioned horizontally and allocated inside the body forming the fixed
structure of the
device (hereinafter, the stator). The orientation of the rotor may be
horizontal, vertical or
any other orientation different from what is shown in this embodiment. A
flanged
connection or any other type of connection 15 allows the entering of low
pressure process
gas to a water tight chamber formed by the main stator walls 16 and other
components
sealed against them, such as a high pressure collector 17 or an axis end cap
21. The
number and arrangement of the components that form said water tight chamber
may be
different from the ones represented in Figure 3 but they always contain inside
the whole
rotor, thus assuring the water tightness character of the gas by means of
static seals such
as gaskets, 0-rings, etc.
The zone where the axial flow motor 2 is located, shown in Figure 3 as the
stacking
of 4 stator assemblies and is corresponding rotating sections, is crossed by
the low
pressure-low temperature gas. This gas flow allows removing the heat generated
in the
motor, acting as a coolant. Then, the gas enters a first compression stage
formed by an
impeller 1 and its corresponding diffuser. In the embodiment of Figure 3, the
rotor contains
a second impeller 14 immediately crossed by the gas after the first one. At
each
compression stage the gas increases its pressure and temperature until it
enters a high
pressure collector 17 and is conducted to a discharge connection of the
stator. Other
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embodiments of the invention may contain more or fewer impellers, as well as
more or
fewer assemblies forming the axial flow motor, according to each specific
application.
In the embodiment shown in Figure 3, the gas temperature at the entrance is
low
enough to act as a coolant of the power electronics in charge of commanding
the electric
motor. A series of power electronic components 19 is placed out of the gas
pressure
containment 16 and is thermally linked thereto. Said electronic components
take
advantage of the thermal conductivity of metal forming the pressure
containment to be
cooled with the same process gas. Finally, a cap 18 externally covers the
power
electronics to protect it from dust and ambient moisture. Other embodiments of
the
invention may locate said electronic components directly inside the pressure
contention,
flooded by the process gas. Other different embodiments may use other
conventional and
independent methods to cool the power electronics in case the gas temperature
at the
entrance of the compressor is extremely high.
Figure 4 shows an isometric view of an embodiment of the invention in which
the
rotor is oriented horizontally. Said view shows a flanged entrance connection
for gas co-
axially disposed with the rotor, being the latter out of sight inside the
pressure
containment. In this embodiment, the high pressure gas discharge is placed
laterally and
perpendicularly to the above mentioned rotor axis. Figure 4 also shows a side
and another
front view of the compressor along with the average human figure in order to
visualize a
representative size of the device. Other embodiments of the invention may vary
in size
and proportions and may place the entrance and exit connections for gas in
other
orientations such as, for example, both coaxial with the rotor axis or both
lateral, etc.
Innovative technical characteristics of the present device include:
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1. It uses an electric, synchronous, axial flow motor with permanent
magnets
as driving force mounted on the same axis as the impellers of the centrifugal
compressor.
This type of motor is more efficient and has higher power density than high
speed radial
flow motors, which gives this device a superior global performance and a
smaller physical
size compared to the current art.
2. It uses passive magnetic bearings and passive electrodynamic bearings
that do not require any energy supply, auxiliary system or monitoring or
control system.
This characteristic gives the device a high operating reliability, even in
case of sudden
electric supply fault. Additionally, the absence of control auxiliary systems
contributes to
its compact size.
3. It does not use mechanical seals since the rotor assembly is placed
totally
inside the same pressure containment as the process gas. The mechanical seals
suffer
from wear by friction and require frequent maintenance, especially in high
speed
applications. Its absence gives this device the feature that it requires less
maintenance
than other prior art equipment. Additionally, the absence of mechanical seals
contributes
to the global energy efficiency of the equipment.
4. It does not use any kind of lubricants for seals, gears or bearings.
This
characteristic contributes to the low maintenance requirement of the equipment
and also
to its reduced size, since there is no need of auxiliary systems for treatment
of lubricant,
such as coolers, filters, separators, or pumps.
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5. Under normal conditions, due to the novel contact-free rotating
support
system, the assembly rotates at the same speed as the impeller of the
compressor without
suffering any mechanic wear.
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