EP2650543A2 - Radial compressor with a diffuser including passive flow control - Google Patents

Abstract

The diaphragm (4) has a set of passages (9) extending between two mutually opposite faces of the diaphragm, so as to create a flow of gas between the opposite faces depending on a pressure difference prevailing on either side of the diaphragm. The set of passages is arranged regularly in the diaphragm, where each passage extends, on one of the faces of the diaphragm, between two fixed blades provided on the diaphragm, and the number of passages corresponds to the number of fixed blades of the diaphragm. An independent claim is also included for a compression stage of a compressor.

Description

The present invention relates to compressors and, in particular, centrifugal compressors. It relates more particularly to the control of the aerodynamic stability of the compressors, at low flow rate.

The compressors generally comprise an electric motor and a compressor unit, for example multi-stage, which comprises a set of paddle wheels carried by a shaft. The motor rotates a rotor which is coupled to the compressor shaft. In operation, the manipulated gas is admitted to the inlet of the compressor and is transmitted to the successive compression stages of the compressor unit to be discharged at the outlet of the compressor.

The characteristic curve of the compression provided by a compressor, which represents the discharge pressure as a function of the flow rate, is a function of a series of parameters, such as the suction pressure, the speed of rotation of the rotor, the nature of the gas compressed, ...

It has been found that, at low flow rates, that is to say at flow rates lower than the stability range of the compressor, aerodynamic instabilities corresponding to an occurrence of detachment of the gas flow manipulated by the compressor and the phenomenon known by the term "pumping".

Such instabilities, downstream of the impeller, are likely to cause vibrations, or even alterations of the constituent elements of the compressor and in any case limit the operating range of the machine.

This is the reason why it has been proposed, for low pressure applications, to provide the compressors and, in particular the diffusers provided downstream of the wheels, variable pitch vanes, with regard to the diffusers aubés or moving walls, with regard to smooth diffusers. However, such bladders are generally moved by mechanical devices that require holes in the body of the rotating machine and, consequently, to provide additional devices to ensure sealing.

Such solutions therefore limit their use at pressures generally below 20 bar.

It has also been proposed to equip the compressors antipumping devices, ensuring a communication between the discharge and suction, to artificially increase the compressor supply flow.

Such devices require the use of instrumentation to measure the flow rate through the compressor, as well as the suction and discharge pressures, as well as control devices driving a gas recirculation control valve.

Such solutions are therefore relatively expensive, difficult to implement and likely to be the seat of malfunctions.

The object of the invention is therefore to overcome the drawbacks associated with the solutions according to the state of the art and to reduce the risks of occurrence of pumping phenomena and, in particular, the phenomena of instability of the gases flowing in the diffuser. of the compressor.

The invention therefore has, according to a first aspect, a diaphragm for a compression stage of a compressor, comprising an assembly of at least one through passage extending between two mutually opposite faces of the diaphragm, so as to create a diaphragm flow of gas between said faces according to the pressure difference prevailing on both sides of the diaphragm.

According to another characteristic, this diaphragm comprises a set of passages regularly arranged in the diaphragm and each extending on one of the faces of the diaphragm, between two fixed vanes provided on the diaphragm.

The subject of the invention is also, according to a second aspect, a compressor compression stage, comprising a diaphragm receiving a gas supplied at the inlet of said stage, a winding wheel providing a compression of the gas from the diaphragm and an outlet diffuser.

The diaphragm comprises an assembly of at least one through passage extending between a first face of the diaphragm facing gas suction in the compression stage and a second face facing the diffuser so as to create a flow of gas between the diffuser and the gas inlet zone in the diaphragm.

According to another characteristic, this compression stage comprises a set of passages regularly arranged in the diaphragm and each extending, on one of the faces of the diaphragm, between two fixed vanes provided on the diaphragm.

The invention also relates to a compressor, in particular a centrifugal compressor, comprising a diaphragm per compression stage as defined above.

• la figure 1 est une vue schématique en coupe d'un étage de compression d'un compresseur centrifuge selon l'invention ;
• la figure 2 est une vue de profil de l'étage de compression de la figure 1, montrant les passages de gaz en entrée de l'étage de compression ;
• la figure 3 est une vue de profil de l'étage de compression de la figure 1, montrant les passages de gaz au niveau du diffuseur ; et
• la figure 4 est une courbe caractéristique de compression d'un compresseur illustrant les avantages liés à l'invention.

Other objects, features and advantages of the invention will appear on reading the following description, given solely by way of nonlimiting example, and with reference to the appended drawings in which:

• the figure 1 is a schematic sectional view of a compression stage of a centrifugal compressor according to the invention;
• the figure 2 is a profile view of the compression stage of the figure 1 , showing the gas passages at the inlet of the compression stage;
• the figure 3 is a profile view of the compression stage of the figure 1 , showing the gas passages at the level of the diffuser; and
• the figure 4 is a compression characteristic curve of a compressor illustrating the advantages of the invention.

On the figure 1 , there is shown a sectional view of a compression stage of a multi-stage compressor according to the invention.

In the exemplary embodiment shown, the compressor is a centrifugal compressor and is intended to ensure the compression of a gas taken upstream of the stage and returned downstream at a higher pressure.

Moreover, Figures 1 to 3 , the compression stage is an intermediate stage disposed between two compression stages. But of course, the invention also applies to the input and output stages.

As can be seen, the compression stage, designated by the general numerical reference 1, comprises an impeller 2 driven in rotation by a rotor 3 itself coupled to a shaft (not shown) rotated by an electric motor.

Upstream of the wheel 2, that is to say on the gas suction side, the compression stage comprises a diaphragm 4 intended to communicate the inlet of the compression stage 1 with the exit of a compression stage immediately placed upstream, and a diffuser 5 for slowing down the speed of the gases coming from the impeller 2 and thus increasing the pressure at the outlet of the stage 1.

Referring also to the figure 2 , on which elements identical to those of the figure 1 have the same numerical references, the diaphragm 4 has a generally annular shape and is, with the diffuser 5, one of the stator parts of the compression stage.

It comprises two mutually opposite large faces 6 and 7, one of which, namely the face designated by the reference 6, is here provided with fixed vanes, such as 8, and the other of which delimits the diffuser 5.

It can be seen further that the diaphragm 4 is provided with a set of through-passages, such as 9, extending between the two faces 6 and 7 of the diaphragm so as to ensure communication between the diffuser 5 and the suction zone. of the compression stage located immediately upstream of the wheel 2.

In the exemplary embodiment shown, the number of passages 9 corresponds to that of the fixed vanes of the diaphragm 4, so that the passages each open, on the suction side, through an orifice 10 disposed between two fixed blades 8. Well In general, diaphragm 4 may comprise any number of such passages greater than one.

The passages are, however, preferably arranged in the thickness of the diaphragm 4 so as to extend in a radially external inclined manner between a zone converging towards the rotor 3, on the suction side, and a radial zone situated in the diffuser, downstream of wheel 2 ( figure 3 ).

It has been found that the presence of such passages made it possible to compensate for the phenomena of instability causing the pumping of the machine, in particular for diffusers of very small dimensions, or for relatively low flow rates.

In this respect, it is customary to represent the flow rate from the suction volume flow rate (Qv) of the front surface of the wheel S for a centrifugal stage and a peripheral speed U, from the following relation: $$\mathrm{\phi }=\frac{\mathit{qv}}{\mathit{S}\mathit{\cdot }\mathit{U}}$$

It has been found that the presence of the passages 9 within the diaphragm 4 made it possible to eliminate the aerodynamic instabilities and in particular the pumping phenomena for relatively low flow rates corresponding to flow rates of less than 0.01, by creating a flow of gas from the diffuser to the upstream part of the impeller through the passages, according to the pressure difference between the discharge zone and the suction zone, to increase the flow of the compressor and thereby maintain the compressor in its stability range ( figure 4 ).

It should be noted in this regard that the amount of recirculated gas, the discharge to the suction, through the passages 9, which depends on the differential pressure on either side of the diaphragm, is therefore greater for low flow rates. It is, on the contrary, less important for relatively higher flow rates for which the pumping phenomena are less likely to occur.

In other words, as illustrated on the figure 4 according to the invention, by creating this artificial circulation at the inlet of the compression stage, the suction flow is artificially increased, so that the operating point of the compressor is pushed back to its stability range.

It should also be noted that the number and the size of the gas passages are adapted to the flow coefficient of the centrifugal stage, to the number of rectifier blades of the diaphragm and to the quantity of gas necessary to maintain the operating stability.

Note further that the arrangement just described is an entirely passive device that does not require any other control device or measuring means and is therefore a very reliable system. It is also applicable to all types of gas and pressure levels, while remaining particularly interesting for stages with a low coefficient of flow inducing very small diffuser widths which are therefore difficult to implement and control.

The broadcaster just described has a number of additional advantages.

It has been found that no gas leakage to the outside was caused by the presence of passages in the diaphragm, which contributes to the protection of the environment.

The operating principle of the diaphragm remains available for all conditions of higher pressure.

It is also possible to use the technology of the centrifugal stages at very low rates while maintaining the operability in industrial installation.

It was finally found that the invention allowed stable operation over an extended operating range compared with centrifugal machines according to the state of the art.

Claims (3)

Diaphragm for a compressor compression stage, characterized in that it comprises an assembly of at least one through passage (9) extending between two mutually opposite faces (6, 7) of the diaphragm, so as to create a flow of gas between said faces according to the pressure difference prevailing on both sides of the diaphragm and in that it comprises a set of passages regularly arranged in the diaphragm and each extending, on one of the faces of the diaphragm, between two fixed vanes (8) provided on the diaphragm, the number of passages corresponding to the number of vanes of the diaphragm. Compressor compression stage comprising a diaphragm (4) receiving a gas supplied at the inlet of said stage, an impeller (2) providing a compression of the gas coming from the diaphragm and an outlet diffuser (5), characterized in that the diaphragm ( 4) comprises an assembly of at least one through passage (9) extending between a first face (6) of the diaphragm turned towards the suction of gas in the compression stage and a second face (7) facing towards the diffuser so as to create a flow of gas between the diffuser and the zone of gas admission into the diaphragm, and in that it comprises a set of passages (9) regularly arranged in the diaphragm and each extending on one of the faces of the diaphragm, between two fixed vanes provided on the diaphragm, the number of passages corresponding to the number of vanes of the diaphragm. Compressor, characterized in that it comprises one or more diaphragms according to claim 1.

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