KR20060058057A - Combination of compressor and permanent magnet motor for sewage aeration - Google Patents

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.

Description

Sewage aeration turbo compressor {COMBINATION OF COMPRESSOR AND PERMANENT MAGNET MOTOR FOR SEWAGE AERATION}

The present invention relates to sewage aeration, and more particularly to a sewage aeration system comprising a centrifugal air compressor.

Water treating plants generate large amounts of sewage sludge. In a properly designed aeration tank it is necessary to continuously aeration the tank of sewage sludge by delivering compressed air to the sludge. Currently, there are three different types of air compressors: positive displacement blowers, single or multi-stage centrifugal radial flow fans, and mixed flow turbo compressors. compressors) are used.

Constant displacement blowers have a rating of 60% and single or multi-stage centrifugal radial flow fans have an efficiency in the range of 60 to 70%, which is lowered at higher pressures, whereas turbocompressors are generally It has an efficiency of at least 80% when operating in the state of maximum efficiency called the "duty point". Clearly, in situations with operating conditions that keep the turbo compressor substantially constant, it is considerably more efficient than other alternatives.

Turbo compressors dominate the sewage aeration market because of two main reasons: first, higher capital costs than other alternatives, and secondly, they cannot maintain high efficiency in applications that require widely varying flow rates. It does not occupy. The operator of the sewage aeration plant is sensitive to both capital and long term operating costs, thus monitoring the oxygen demand in the treatment plant and reducing the air volume supplied when indicated that the oxygen demand has been reduced. This means that for many applications the compressor must be delivered at a value that can be as low as 50%, ie between 50% and 100% of the maximum output.

Turbo compressors can be considered to have one of two general design types: variable geometric design and fixed geometric design. In a variable geometry design, the geometry of the passages in the compressor can be varied by rotating the compressor to adjust compressor characteristics to meet varying conditions such as speed or load. Conversely, in the case of a stationary geometric design, geometrical adjustments in operation are not possible. Assuming that the efficiency of a conventional turbocompressor used for sewage aeration rapidly decreases as the speed of the turbo impeller is away from the typical duty point speed, an approach that allows to control the turndown of the turbocompressor is generally It depends on using variable inlet guide vanes located upstream of the impeller. The constant speed induction motor drive is coupled to the turbo compressor by a fixed ratio gearbox so that the turbo compressor rotates at a constant speed faster than the motor speed.

In a typical geared turbocompressor driven by an induction motor, the energy loss is approximately 7% in the motor, 5% in the gearbox, system bearings, even when the turbocompressor is a complex design including both variable inlet and diffuser vanes. 2% at and 19% at the turbo compressor itself. In particular, the combination of high capital costs for variable vane turbocompressors and inefficiencies in turbocompressor drive trains is encouraged in the sewage aeration industry, which continues to use relatively inefficient positive displacement and multi-stage radial flow centrifugal fans. have.

Turbo compressors are known which are driven by conventional induction motors operating at six synchronous speeds, which are coupled to a turbo compressor which does not require a gearbox. This motor is controlled by an inverter and turndown is achieved by controlling the frequency of AC power supplied to the motor by the inverter. Such a device avoids gearbox power losses but is advantageous because of the increased cost of power losses occurring in the inverter / motor combination. However, this loss is significant and it is not easy to achieve significant power savings.

In an induction motor, an alternating current is used to direct current through a first winding on one member (generally a stator). The second winding on the other member (generally the rotor) carries only the current induced by the magnetic field of the first winding. In contrast, in permanent magnet motors, the stator windings are supplied from a DC source through the power switch of the inverter. The rotor supports the permanent magnets. The stator winding switch is normally opened and closed to be performed at a time determined by the controller in response to the speed command and the input representing the measurement or estimation of the rotor position. The interaction between the magnetic field generated by the permanent magnet and the magnetic field generated by the stator winding rotates the rotor. Although relatively high efficiencies can be achieved with permanent magnet motors, these motors are generally used only for applications of relatively low power. Permanent magnet motors are not typically used in sewage aeration applications requiring a power rating of 300 kW.

Summary of the Invention

It is an object of the present invention to provide a sewage aeration compressor that prevents or mitigates the above-mentioned problems.

According to the present invention, a sewage aeration turbocompressor for continuously delivering air to a sewage sludge treatment plant, comprising: a housing, an impeller mounted on an impeller shaft in the housing, and an output shaft coupled to and synchronously rotating the impeller shaft. An electric motor having an axial air inlet extending to said impeller, a diffuser passage extending radially outward from said impeller, a volute extending from said diffuser to an air outlet, said electric The motor is a variable speed permanent magnet motor controlled by an inverter, the motor being designed to drive the compressor at a speed within a range limited by the maximum and minimum design speeds, the compressor being driven by the motor at the maximum design speed. If the pressure between the inlet and outlet does not exceed 1500 millibars A fixed geometric compressor with a vaneless diffuser designed to deliver lift, the compressor being provided with a sewage aeration turbocompressor designed to deliver maximum efficiency when the motor is driven at a slower speed than the maximum design speed.

By limiting the duty pressure rise below 1500 millibars, a highly efficient impeller can be designed in combination with a vaneless diffuser to create an unchanged efficiency versus flow curve. This configuration is very efficient compared to a wide range of motor speeds.

Preferably, the pressure rise is in the range of 850 to 1200 millibars. Maximum efficiency may be in the range of 1000 to 1050 millibars. The impeller design can be optimized to suit the particular application. Likewise, the volute can be designed to optimize the efficiency given the vaneless nature of the diffuser. Preferably, vanes are not provided in the air inlet to avoid energy losses in at least some extent of the possible impeller rotational speed. The diffuser passage may be a simple annular passage of uniform width in the axial direction.

The inverter can be controlled by an oxygen demand sensor coupled to monitor the oxygen content of the sludge in the sludge treatment plant.

Exemplary embodiments of the invention are described in detail by way of example only with reference to the accompanying drawings.

1 is a schematic block diagram showing components included in an embodiment of the present invention;

FIG. 2 is a schematic perspective view of a turbo compressor included in the system shown in FIG. 1;

3 is a schematic perspective view of an impeller and a volute of the turbo compressor shown in FIG. 2;

4 is a graph showing the relative efficiency with respect to the variable flow rate of the conventional sewage aeration turbo compressor including the turbo compressor and the diffuser vanes shown in FIGS.

5 is a graph showing isentropic efficiency versus mass flow rate of an impeller, diffuser and impeller / diffuser combination in a turbocompressor according to the present invention.

Referring to FIG. 1, the illustrated system comprises a turbo compressor 1 which delivers the air flow represented by the line 2 to the aeration vessel 3, the delivered air being for example sewage retained in the vessel 3. Bubbles are generated through the sludge. Typically, the output pressure of a turbocompressor is relatively low, for example 1.2 bar, and has a maximum flow rate of 11000 m ³ / h.

The turbo compressor 1 is driven by a permanent magnet motor 4 having an output shaft 5 directly coupled to the input shaft of the turbo compressor. Accordingly, the motor 4 and the turbo compressor 1 rotate synchronously. Inverter 6 controls to supply power to motor 4, which delivers a current in the range of 200 to 480 A to produce a useful power output rated above 300 kW. The power supplied to the motor 4 by the inverter 6 is controlled by the input 7 to an inverter provided by an oxygen demand sensor 8 that 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 the maximum speed, which is equal to the speed of the turbo compressor that delivers the maximum capacity of air to the vessel 3. same. If the sensed oxygen demand falls below the threshold, the motor speed is reduced to match the supplied air capacity to the oxygen demand.

2 and 3, the structure of the turbo compressor 1 will be described. The turbo compressor comprises a drive shaft 9 directly coupled to the output shaft 5 of the motor 4 and synchronously rotating thereto (see FIG. 1). The turbo compressor shaft 9 is mounted on an appropriate bearing and supports an impeller 10 having a central hub, from which an array of impeller vanes extends. The hub is shown in FIG. 2, but is not shown in FIG. 3 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 rotates, the impeller 10 introduces air through the inlet and has a uniform width in the aeration direction and has a radius from the impeller 10. Pressurized air is delivered to a diffuser 12 in the form of an annular vaneless slot extending outwardly. The diffuser 12 communicates with a volute 13 which is alternately 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 this edge is indicated by reference numeral 14 in FIG. 2.

Turbo compressors having a vaneless inlet and diffuser of the general type shown in FIGS. 2 and 3 are known as criteria to apply, for example, to the design of impeller vanes to deliver a desired flow rate and output pressure for a given impeller speed. However, it is not known to use a turbo compressor with a permanent magnet motor that delivers air to the aeration vessel in the sewage plant. However, using a turbo compressor in such a situation provides substantial benefits as discussed with reference to FIG. 4.

Referring to FIG. 4, line 15 shows the relationship between the isentropic efficiency and the percent of maximum flow rate for the turbo compressor of FIG. 2. The efficiency peaks at about 70% of the maximum flow just above 85% and drops a few percentage points at 100% of the maximum flow. The efficiency is always above 80%. Conversely, line 16 is designed to maximize efficiency in a conventional manner, i.e. achieving the highest possible efficiency for a relatively narrow range of impeller speeds, such that between isentropic efficiency and maximum percent flow rate in turbocompressors with vane diffusers. Indicates a relationship. Line 16 exhibits a maximum efficiency of 87%, which drops to an increased flow rate up to 82%, but increases very rapidly at a reduced flow rate.

The results shown in FIG. 4 are significantly better than what can be achieved with a modified sewage aeration system. This is summarized in the table below, where column 1 represents the direct drive, permanent magnet motor and high efficiency vaneless diffuser compressor combination according to the invention, and row 2 represents the gearbox, induction motor and variable vane diffuser combination. Column 3 represents the direct drive, induction motor and vaneless diffuser combination, column 4 represents the positive displacement belt driven blower, and this table shows the gas compressor (compressor or blower), drive (motor and drive train). And four variants of the efficiency of a combination of gas compression and drive systems for both duty (100% of maximum speed) and 40% turndown (60% of maximum speed).

TABLE 1

efficiency Duty 40% turndown gas Driving body sum gas Driving body sum One 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 shown in the table above, induction motors / gearboxes and induction motors / inverter drives have energy losses of approximately 11% and 8% at duty flow, respectively, while the drive comprises a 300 kW permanent magnet motor according to the invention The system exhibits approximately 3% drive loss. Overall efficiency is approximately 82%. This significant efficiency is maintained over the full duty range, i.e. for all expected flow rates and absorbed power.

Assuming that a relatively low percentage of the maximum flow rate, such as 50%, in the sewage treatment plant may be an extended period of time required, the efficiency rapidly dropped to the reduced maximum flow rate percentage represented by line 16 may degrade the overall efficiency. Thus, in combination with a high efficiency variable speed motor, a permanent magnet motor coupled directly to the drive shaft of a turbogenerator with a vaneless inlet and a vaneless diffuser, in particular assuming that the vaneless turbo compressor is relatively easy to manufacture and maintain, the system Increase the overall efficiency, which significantly reduces the overall cost of At least 80% overall efficiency can be achieved. This compares with a modified turbo compressor system that delivers up to approximately 69% efficiency at full turndown. Assuming an efficiency difference in current electricity costs, the cost of ownership is reduced by £ 20,000 per year, the system delivers on average 234 kW of gas compression power. Approximately £ 75,000 is saved annually compared to inverter-driven positive displacement blowers with up to 51% total efficiency. Although the initial cost of the positive displacement blower is lower than that of the turbocompressor system according to the invention, the savings in operating costs should be sufficient to cover the cost increase in a relatively short time, for example less than two years.

Thus, conventional turbo compressors are applied to sewage aeration depending on fixed speed motors and gearboxes, whereas turbocompressors and gearboxes cannot achieve high efficiency. In contrast, the described embodiment of the present invention relies on a high efficiency motor, and a highly efficient impeller / vaneless diffuser compressor delivers high efficiency over a wide range of compressor speeds. Variable speed drive motors do not require an inverter for controlling the motor, but the energy loss in the inverter is relatively small, especially if the turbo compressor is designed to deliver a relatively low pressure flow rate of air required for most sewage aeration applications. More significantly better overall efficiency.

Claims (5)

In a sewage aeration turbo compressor that continuously delivers air to a sewage sludge treatment plant, An electric motor having a housing, an impeller mounted on an impeller shaft within the housing, and an output shaft coupled to and rotating synchronously with the impeller shaft, the housing defining an axial air inlet extending to the impeller, The diffuser passage extends radially outward from the impeller, the volute extends from the diffuser to the air outlet, The electric motor is a variable speed permanent magnet motor controlled by an inverter, wherein the motor is designed to drive the compressor at a speed within a range limited by the maximum and minimum design speed, the compressor being configured to drive the motor at the maximum design speed. A fixed geometric compressor having a vaneless diffuser designed to deliver a pressure rise between the inlet and outlet not exceeding 1500 millibars when driven, the compressor having maximum efficiency when the motor is driven at a speed slower than the maximum design speed Designed to convey Sewage aeration turbo compressor. The method of claim 1, The compressor is designed to deliver a pressure rise between 850 millibars when the motor is driven at the minimum design speed and 1200 millibars when the motor is driven at the maximum design speed. Sewage aeration turbo compressor. The method according to claim 1 or 2, The diffuser is an annular passage of uniform width in the axial direction. Sewage aeration turbo compressor. The method according to any one of claims 1 to 3, The inverter is controlled by an oxygen demand sensor arranged to monitor the oxygen content of the sludge in the sludge treatment plant. Sewage aeration turbo compressor. As substantially described above with reference to the accompanying drawings Sewage aeration turbo compressor.

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