U.S. patent application number 10/559394 was filed with the patent office on 2006-12-07 for combination of compressor and permanent magnet motor for sewage aeration.
Invention is credited to Kevan Gradwell, Jean Mangnall, Keith Mangnall.
Application Number | 20060275114 10/559394 |
Document ID | / |
Family ID | 9959533 |
Filed Date | 2006-12-07 |
United States Patent
Application |
20060275114 |
Kind Code |
A1 |
Mangnall; Keith ; et
al. |
December 7, 2006 |
Combination of compressor and permanent magnet motor for sewage
aeration
Abstract
A sewage aeration turbocompressor for continuously delivering
air at a relatively low pressure to a sewage sludge treatment
plant. The compressor has a housing, an impeller (10) mounted on an
impeller shaft within the housing, and an electric drive motor
having an output shaft coupled to and rotating in synchronism with
the impeller shaft (9). The housing defines an axial air inlet (4)
extending to the impeller, a diffuser passageway (12) extending
radially outwards from the impeller, and a volute (13) extending
from the diffuser to an air outlet. The electric motor is a
variable speed permanent magnetic motor controlled by an inverter
and the diffuser is vaneless. High levels of efficiency are
achieved over a wide range of impeller speeds, enabling the
compressor to deliver large volumes of air across a wide range of
delivery rates, by designing the system to deliver optimum
efficiency at a relatively low pressure rise less than 1500
millibar.
Inventors: |
Mangnall; Keith; (Bolton,
GB) ; Gradwell; Kevan; (Lostock, Bolton, GB) ;
Mangnall; Jean; (Bolton, GB) |
Correspondence
Address: |
THE BOC GROUP, INC.
575 MOUNTAIN AVENUE
MURRAY HILL
NJ
07974-2064
US
|
Family ID: |
9959533 |
Appl. No.: |
10/559394 |
Filed: |
April 7, 2004 |
PCT Filed: |
April 7, 2004 |
PCT NO: |
PCT/GB04/01532 |
371 Date: |
April 17, 2006 |
Current U.S.
Class: |
415/206 |
Current CPC
Class: |
C02F 2209/22 20130101;
C02F 3/00 20130101; F04D 27/0261 20130101; Y02B 30/70 20130101;
Y02W 10/10 20150501; F04D 25/08 20130101; C02F 3/006 20130101 |
Class at
Publication: |
415/206 |
International
Class: |
F04D 29/44 20060101
F04D029/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 7, 2003 |
GB |
0313143.0 |
Claims
1. A sewage aeration turbocompressor for continuously delivering
air to a sewage sludge treatment plant comprising: a compressor
having a housing, an impeller mounted on an impeller shaft within
the housing, and an electric motor having an output shaft coupled
to and rotating in synchronism with the impeller shaft, the housing
defining an axial air inlet extending to the impeller, a diffuser
passageway extending radially outwards from the impeller, and a
volute extending from the diffuser to an air outlet, wherein the
electric motor is a variable speed permanent magnet motor
controlled by an inverter, the motor is deigned to drive the
compressor at speeds within a range limited by maximum and minimum
design speeds, the compressor is a fixed geometry compressor with a
vaneless diffuser designed to deliver a pressure rise between the
inlet and outlet of not more than 1500 millibar when the motor is
driven at the maximum design speed, and the compressor is designed
to deliver maximum efficiency when the motor is driven at a speed
less than the maximum design speed.
2. A sewage aeration turbocompressor according to claim 1, wherein
the compressor is designed to deliver a pressure rise of between
850 millibar when the motor is driven at the minimum design speed
and 1200 millibar when the motor is driven at the maximum design
speed.
3. A sewage aeration turbocompressor according to claim 1 wherein
the diffuser is an annular passageway of uniform width in the axial
direction.
4. A sewage aeration turbocompressor according to claim 3 wherein
the inverter is controlled by an oxygen demand sensor arranged to
monitor the oxygen content of sludge in the sludge treatment
plant.
5. (canceled)
6. A turbocompressor for delivering a gas to a sewage treatment
plant comprising: a compressor having a housing; an impeller
mounted on an impeller shaft within the housing; and an electric
motor having a maximum design speed and an output shaft coupled to
the impeller shaft; and wherein the housing is adapted to define an
axial gas inlet extending to the impeller and a diffuser passageway
extending radially outwards from the impeller and a volute
extending from the diffuser to a gas outlet.
7. The turbocompressor of claim 6 wherein the electric motor is a
variable speed permanent magnet motor.
8. The turbocompressor of claim 7 further including an
inverter.
9. The turbocompressor of claim 8 wherein the compressor is a fixed
geometry compressor and further includes a vaneless diffuser.
10. The turbocompressor of claim 9 wherein the compressor has a
capacity to deliver a pressure rise between the inlet and the
outlet at less than 1500 millibar when the motor is operating at
the maximum design speed.
11. The turbocompressor of claim 10 wherein the compressor has a
maximum efficiency when the motor operates at a speed less than the
maximum design speed.
12. The turbocompressor of claim 11 wherein the compressor delivers
a pressure rise between the inlet and the outlet of 850 millibar
when the motor is driven at a minimum design speed and 1200
millibar when the motor is driven at the maximum design speed.
13. The turbocompressor of claim 12 wherein the diffuser is an
annular passageway of uniform width extending in an axial
direction.
14. The turbocompressor of claim 13 further comprising an oxygen
demand sensor for monitoring the oxygen content of sewage in the
sewage treatment plant, and wherein the inverter is responsive to
an output of the oxygen demand sensor.
Description
[0001] The present invention relates to sewage aeration, and in
particular to a sewage aeration system including a centrifugal air
compressor.
[0002] Water treatment plants generate large volumes of sewage
sludge. It is necessary to continuously aerate tanks of sewage
sludge by delivering compressed air to the sludge in appropriately
designed aeration tanks. Currently three different types of air
compressors are used, that is positive displacement blowers, single
or multi-stage centrifugal radial flow fans, and mixed flow turbo
compressors.
[0003] Positive displacement blowers have efficiencies of the order
of 60%, multi-stage centrifugal fans have efficiencies in the range
of 60 to 70%, the efficiency being lower at higher pressures,
whereas turbocompressors have efficiencies above 80% when operating
in conditions of maximum efficiency, those conditions generally
being referred to as the "duty point". Clearly in circumstances
where operating conditions can be maintained substantially constant
turbocompressors are significantly more efficient that the
alternatives.
[0004] Turbocompressors have not dominated the sewage aeration
market for two main reasons, that is firstly high capital cost as
compared to the alternatives, and secondly an inability to maintain
high efficiency in applications where widely varying flow rates are
demanded. The operators of sewage aeration plant are sensitive to
both capital cost and long term operating costs and therefore
monitor oxygen demand in treatment plants and reduce the volume of
air supplied if a reduced oxygen demand is indicated. This means
that in many applications a compressor must be able to be turned
down by as much as 50%, that is to deliver anything between 50% and
100% of maximum output.
[0005] Turbocompressors can be considered as belonging to one of
two general design types, that is variable geometry and fixed
geometry designs. In variable geometry designs, the geometry of
passageways within the compressor can be varied as the compressor
is rotating so as to adjust compressor characteristics to match
varying conditions such as speed or load. In contrast, with a fixed
geometry design, no geometry adjustments are possible during
operation. Given that the efficiency of a conventional
turbocompressor as used for sewage aeration reduces rapidly as the
speed of the turbo impeller moves away from the normal duty point
speed the approach adopted to enable turndown of a turbocompressor
has generally depended upon the use of variable inlet guide vanes
upstream of the impeller. A constant speed induction motor drive is
coupled to the turbocompressor by a fixed ratio gearbox such that
the turbocompressor rotates at a constant speed higher than the
motor speed.
[0006] In a typical geared turbocompressor assembly driven by an
induction motor, energy losses of approximately 7% occur at the
motor, 5% at the gearbox, 2% in the system bearings, and 19% in the
turbocompressor itself even if the turbocompressor is a complex
design including for example both variable inlet and diffuser
vanes. The combination of high capital cost, particularly for
variable vane turbocompressors, and inefficiencies in the
turbocompressor drive train have encouraged the sewage aeration
industry to continue to use the relatively inefficient positive
displacement and multistage radial flow centrifugal fans.
[0007] A turbocompressor is known which is driven by a conventional
induction motor operating at six times synchronous speed, the motor
being directly coupled to the turbocompressor to avoid the need for
a gear box. The motor is controlled by an inverter, turndown being
achieved by controlling the frequency of the AC power supplied to
the motor by the inverter. This arrangement is advantageous as gear
box power losses are avoided, but at the cost of increased power
losses arising in the inverter/motor combination. These losses are
substantial however and thus significant power savings cannot be
readily achieved.
[0008] In induction motors, an alternating current is used to
energise a primary winding on one member (usually the stator). A
secondary winding on the other member (usually the rotor) carries
only current induced by the magnetic field of the primary. In
contrast, in a permanent magnet motor, stator windings are supplied
from a DC source through power electronic switches of an inverter.
The rotor supports permanent magnets. The stator winding switches
are switched so as to be conducting at times determined by a
controller which in general is responsive to inputs representing a
speed command and a measurement of or estimate of rotor position.
Interaction between the magnetic fields produced by the permanent
magnets and the magnetic fields generated by the stator windings
causes the rotor to rotate. It is known that relatively high
efficiencies can be achieved with permanent magnet motors but
generally such motors are only used in relatively low power
applications. The use of permanent magnet motors has not been
considered in sewage aeration applications where typically powers
of the order of 300 kW are required.
[0009] It is an object of the present invention to provide a sewage
aeration compressor which obviates or mitigates the problems
outlined above.
[0010] According to the present invention, there is provided a
sewage aeration turbocompressor for continuously delivering air to
a sewage sludge treatment plant, comprising a compressor having a
housing, an impeller mounted on an impeller shaft within the
housing, and an electric motor having an output shaft coupled to
and rotating in synchronism with the impeller shaft, the housing
defining an axial air inlet extending to the impeller, a diffuser
passageway extending radially outwards from the impeller, and a
volute extending from the diffuser to an air outlet, wherein the
electric motor is a variable speed permanent magnet motor
controlled by an inverter, the motor is deigned to drive the
compressor at speeds within a range limited by maximum and minimum
design speeds, the compressor is a fixed geometry compressor with a
vaneless diffuser designed to deliver a pressure rise between the
inlet and outlet of not more than 1500 millibar when the motor is
driven at the maximum design speed, and the compressor is designed
to deliver maximum efficiency when the motor is driven at a speed
less than the maximum design speed.
[0011] By limiting the duty pressure rise to less than 1500
millibar a very efficient impeller can be designed which in
combination with a vaneless diffuser produces a flat efficiency
verses flow curves. Such an arrangement is highly efficient over a
wide range of motor speeds.
[0012] Preferably the pressure rise ranges from 850 to 1200
millibars. Maximum efficiency may be in the range 1000 to 1050
millibars. The impeller design can be optimised to suit the
particular application. Similarly the volute can be designed to
optimise efficiency given the vaneless nature of the diffuser.
Preferably no vanes are provided in the air inlet, again avoiding
energy losses across at least some of the range of possible
impeller rotational speeds. The diffuser passageway may be a simple
annular passageway of uniform width in the axial direction.
[0013] The inverter may be controlled by an oxygen demand sensor
coupled so as to monitor the oxygen content of sludge in the sludge
treatment plant.
[0014] An embodiment of the present invention will now be
described, by way of example, with reference to the accompanying
drawings, in which:
[0015] FIG. 1 is a schematic block diagram illustrating components
incorporated in an embodiment of the present invention;
[0016] FIG. 2 is an axial section through a turbocompressor
incorporated in the system illustrated in FIG. 1;
[0017] FIG. 3 is a schematic perspective view of an impeller and
volute of the turbocompressor shown in FIG. 2;
[0018] FIG. 4 represents the relative efficiencies at variable flow
rates of the turbocompressor shown in FIGS. 2 and 3 and a
conventional sewage aeration turbocompressor incorporating diffuser
vanes; and
[0019] FIG. 5 represents the variation of isentropic efficiency
with mass flow for the impellor, diffuser and impeller/diffuser
combination in a turbocompressor according to the invention.
[0020] Referring to FIG. 1, the illustrated system comprises a
turbocompressor 1 delivering a flow of air represented by line 2 to
an aeration vessel 3, the delivered air being for example bubbled
through sewage sludge retained in the vessel 3. Typically the
output pressure of the turbocompressor will be relatively low, for
example 1.2 bar, with a maximum flow rate of for example 11000
m.sup.3 per hour.
[0021] The turbocompressor 1 is driven by a permanent magnet motor
4 having an output shaft 5 which is directly coupled to an input
shaft of the turbocompressor. Thus the motor 4 and turbocompressor
1 rotate in synchronism. An inverter 6 controls the supply of power
to the motor 4, the inverter delivering a current in the range of
200 to 480 Amps to produce a useful power output of the order of up
to 300 kW. The power supplied to the motor 4 by the inverter 6 is
controlled by an input 7 to the inverter provided by an oxygen
demand sensor 8 which senses the oxygen demand in the vessel 3.
Thus if the oxygen demand is above a predetermined maximum
threshold, the inverter 6 drives the motor 4 at full speed, that
speed equating to the turbocompressor speed which will deliver the
maximum volume of air to the vessel 3. When the sensed oxygen
demand falls below the threshold, the motor speed is reduced to
match the volume of air supplied to the oxygen demand.
[0022] Referring to FIGS. 2 and 3, the structure of the
turbocompressor 1 will be described. The turbocompressor comprises
a drive shaft 9 which is directly coupled to and rotates in
synchronism with the output shaft 5 of the motor 4 (see FIG. 1).
The turbocompressor shaft 9 is mounted on suitable bearings and
supports an impeller 10 having a central hub from which an array of
impeller vanes extend. The hub is shown in FIG. 2 but is not shown
in FIG. 3 so as to make it easier to see the shape of the impeller
vanes. The impeller extends into a vaneless axial inlet 11 such
that when the shaft is rotated the impeller 10 draws air in through
that inlet and delivers pressurised air to a diffuser 12 which is
in the form of an annular vaneless slot which is of uniform width
in the aerial direction and which extends radially outwards from
the impeller 10. The diffuser 12 communicates with a volute 13
which in turn is coupled to an air delivery line corresponding to
the line 2 of FIG. 1. In FIG. 3, the radially inner edge of the
diffuser 12 is indicated by line 14 and the position of that edge
is indicated by numeral 14 in FIG. 2.
[0023] Turbocompressors having vaneless inlets and diffusers of the
general type illustrated in FIGS. 2 and 3 are known, as are the
criteria which apply to the design of for example the impeller
vanes so as to deliver a given rate of flow and output pressure for
a given impeller speed. The use of such a turbocompressor with a
permanent magnet motor to deliver air to an aeration vessel in a
sewage plant is not however known. The use of such a
turbocompressor in those circumstances does however provide
substantial benefit as discussed with reference to FIG. 4.
[0024] Referring to FIG. 4, the line 15 shows the relationship
between isentropic efficiency and the percentage of maximum flow
for the turbocompressor of FIGS. 2 and 3. It will be noted that
efficiency peaks at around 70% of maximum flow at just above 85%
and falls by a few percentage points at 100% of maximum flow. At
all times the efficiency is well above 80%. In contrast, the line
16 represents the relationship between isentropic efficiency and
percentage maximum flow in a turbocompressor with a vaned diffuser
designed to maximise efficiency in a conventional manner, that is
by achieving the highest possible efficiency over a relatively
narrow range of impeller speeds. The line 16 indicates a maximum
efficiency of 87%, the efficiency falling off with increasing flow
to 82% but decreasing very rapidly with decreasing flow.
[0025] The results represented in FIG. 4 are significantly better
than what can be achieved with alternative sewage aeration systems.
This is summarised in the table below, where row 1 represents a
direct drive, permanent magnet motor and high efficiency vaneless
diffuser compressor combination in accordance with the invention,
row 2 represents a gear ox, induction motor and variable vane
diffuser combination, row 3 represents a direct drive induction
motor vaneless diffuser combination, and row 4 represents a
positive displacement belt driven blower, the table showing for
each of the four alternatives the efficiency of the gas compression
device (compressor or blower), the drive (motor and drive train),
and the combination of the gas compression and drive systems
(total) for both duty (100% of maximum speed) and 40% turndown (60%
of maximum speed); TABLE-US-00001 EFFICIENCY Duty 40% Turndown Gas
Drive Total Gas Drive Total 1 85 97 82 82 95 78 2 87 89 77 77 86 66
3 80 92 74 78 88 69 4 63 88 55 59 86 51
[0026] As represented in the above table, whereas induction
motor/gearbox and induction motor/inverter drives have efficiency
losses of approximately 11% and 8%, at duty flow, respectively the
drive system incorporating a 300 kW permanent magnet motor in
accordance with the invention shows drive losses of approximately
3%. Overall efficiency is approximately 82%. This remarkable
efficiency is maintained over the full duty range, that is for all
flows and absorbed powers that are contemplated.
[0027] Given that in a sewage treatment plant there can be
prolonged periods during which a relatively low percentage maximum
flow such as 50% is required, the rapid fall off in efficiency with
reducing maximum flow percentage indicated by line 16 can result in
poor overall efficiency. Thus, combining a high efficiency variable
speed motor such a permanent magnet motor coupled directly to the
driveshaft of a turbo generator with vaneless inlet and vaneless
diffuser results in an overall increase in efficiency which
significantly reduces the overall cost of the system, particularly
given that a vaneless turbocompressor is relatively easy to
manufacture and maintain. Overall efficiencies of greater than 80%
can be achieved. This compares with alternative turbocompressor
systems delivering at most approximately 69% efficiency at full
turndown. Given current costs of electricity this efficiency
difference translates into a cost of ownership saving of the order
of .English Pound.20,000 per year assuming the system delivers on
average a gas compression power of 234 kW. Compared with an
inverter driven positive displacement blower a solution where the
total efficiency will be at most of the order of 51%, the annual
saving is approximately .English Pound.75,000. Although the initial
cost of a positive displacement blower is lower than a
turbocompressor system in accordance with the invention, the
running cost savings should be sufficient to cover the increase in
cost in a relatively short time, for example less than two
years.
[0028] Thus, whereas in the prior art turbocompressor systems
applied to sewage aeration relied upon fixed speed motors and a
gearbox, supplemented by variable vane structures, the motor,
turbocompressor and gearbox losses are such that high overall
efficiencies cannot be achieved. In contrast, the described
embodiment of the present invention relies upon a high efficiency
motor, and a very efficient impeller/vaneless diffuser compressor
delivering a high efficiency across a wide range of compressor
speeds. The variable speed drive motor does require an inverter for
motor control, but energy losses in the inverter are relatively
small, enabling an overall efficiency significantly better than any
of the other alternatives, particularly if the turbocompressor is
designed to deliver a relatively low pressure flow of air which is
what is required in most sewage aeration applications.
* * * * *