Note: Descriptions are shown in the official language in which they were submitted.
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COMBINATION OF COMPRESSOR AND PERMANENT MAGNET MOTOR FOR SEWAGE AERATION
The present invention relates to sewage aeration, and in particular to a
sewage
aeration system including a centrifugal air compressor.
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.
Positive displacement blowers have efficiencies of the order of 60%, rnulti-
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.
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
vaxying
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.
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 fox sewage aeration reduces rapidly as
the
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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.
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.
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 axe 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.
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) carnes 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
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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 300kW are required.
It is an object of the present invention to provide a sewage aeration
compressor which obviates or mitigates the problems outlined above.
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.
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.
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.
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.
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An embodiment of the present invention will now be described, by way of
example, with reference to the accompanying drawings, in which;
Figure 1 is a schematic block diagram illustrating components incorporated in
an embodiment of the present invention;
Figure 2 is an axial section through a turbocompressor incorporated in the
system illustrated in Figure 1;
Figure 3 is a schematic perspective view of an impeller and volute of the
turbocompressor shown in Figure 2;
Figure 4 represents the relative efficiencies at variable flow rates of the
turbocompressor shown in Figures 2 and 3 and a conventional sewage aeration
turbocompressor incorporating diffuser vanes; and
Figure 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.
Referring to Figure 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 11000m3 per hour.
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
300kW. 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.
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Referring to Figures 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
Figure 1).
The turbocompressor shaft 9 is mounted on suitable bearings and supports an
impeller
having a central hub from which an array of impeller vanes extend. The hub is
shown in Figure 2 but is not shown in Figure 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 Figure 1. In
Figure 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 Figure 2.
Turbocompressors having vaneless inlets and diffusers of the general type
illustrated in Figures 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 Figure 4.
Referring to Figure 4, the line 15 shows the relationship between isentropic
efficiency and the percentage of maximum flow for the turbocompressor of
Figures 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.
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The results represented in figure 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);
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
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 300kW 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.
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
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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 iurbocompressor 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 ~20,000 per year assuming the
system
delivers on average a gas compression power of 234kW. 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 ~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.
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.
