Centrifugal compressor

Details Centrifugal compressor Centrifugal compressor

1999-05-21

2001-02-20

Look, Edward K. (Department: 3745)

Rotary kinetic fluid motors or pumps

Including means for handling working fluid leakage

C415S001000, C415S116000

Reexamination Certificate

active

06190123

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method of operating a centrifugal compressor as described in the preamble amble to claim
1
and to a corresponding centrifugal compressor as described in the preamble to claim
6
.
BACKGROUND OF THE INVENTION
Contactless seals, in particular labyrinth seals, are widely used for sealing rotating systems in turbomachine construction. Because of the aerodynamic boundary layers which form, a high frictional power appears in the separating gap through which fluid flows between the rotating and stationary parts. This causes heating of the fluid in the separating gap and therefore also causes heating of the components surrounding the separating gap. The high material temperatures cause a reduction in the life of the corresponding components.
Depending on their design, exhaust gas turbochargers have an axial thrust from the exhaust gas turbine which acts against or in the same direction as that from the centrifugal compressor. In the latter case, the resulting pressure in the separating gap between the rotating rear wall of the compressor impeller and the adjacent stationary compressor casing has to be reduced. For this reason, such separating gaps have very tight tolerances. In addition, they usually have a contactless seal. Such narrow separating gaps involve a particularly high frictional power. In addition, the deflection and the eddying of the working fluid flowing through the separating gap lead to repeated mixing of the working fluid at the throttle locations of the seal and this is associated with a high level of momentum and heat exchange. Downstream of the throttle location, the working fluid has to be accelerated afresh each time in the peripheral direction on the rotating component so that the frictional power, and therefore the generation of heat, increases further in this region.
A cooling appliance for centrifugal compressors with sealing elements arranged on the rear wall of the compressor impeller, in the separating gap between the latter and the compressor casing, is known from EP 0 518 027 B1. In this arrangement, a cold gas which is provided with a pressure which is higher than that present at the outlet from the compressor impeller is fed through the seal. This gas impinges on the rear wall of the compressor impeller and simultaneously acts there as sealing air to prevent a flow of hot compressor air from the outlet of the compressor impeller through the labyrinth gap. The service life of such a compressor wheel provided with sealing geometry can be markedly increased by this means. In this solution, it is found to be a disadvantage that the specially shaped seal complicates the overall design and the assembly of the compressor and makes it more expensive. Because the clearance of the separating gap is in the range of tenths of a millimeter, furthermore, there is always a latent danger of the rotating compressor impeller rubbing on the compressor casing.
In contrast to this, no reduction in pressure in the separating gap is necessary in the case of an axial thrust of the exhaust gas turbine acting against the centrifugal compressor so that its clearance is in the range of millimeters and it becomes unnecessary to seal the separating gap in the region of the rear wall of the compressor impeller. A centrifugal compressor without such sealing elements is known from DE 195 48 852. It is simple in construction and therefore can be manufactured at favorable cost. There is no danger of the rotating compressor impeller rubbing against the compressor casing. Nevertheless, even in this case the frictional heat resulting from aerodynamic shear layers on the rear wall of the compressor impeller ensures heating of the compressor impeller and, therefore, a reduction in its life. No solution for reducing the generation of heat in the case of centrifugal compressors without sealing elements in the region of the rear wall of the compressor impeller is known.
SUMMARY OF THE INVENTION
The invention attempts to avoid all these disadvantages and, accordingly, one object of the invention is to provide a novel method of operating a simply constructed centrifugal compressor equipped, in the region of the rear wall of the compressor impeller, with no sealing elements in the separating gap between the compressor impeller and the compressor casing, which method increases the service/life of the centrifugal compressor. In addition, an appliance is made available for carrying out the method.
In a method according to the invention, this is achieved by a cooling medium being introduced into the separating gap downstream of the leakage flow of the working medium and the cooling medium being finally removed again after heat exchange has taken place. For this purpose, in an appliance according to the invention, at least one supply duct for a gaseous cooling medium, said duct penetrating the compressor casing, opening into the separating gap in the region of the rear wall, of the compressor impeller and directed onto the rear wall, and at least one removal duct for the cooling medium are arranged in the compressor casing.
On the basis of this method and the corresponding configuration of the centrifugal compressor, the rear wall of the compressor impeller can be effectively cooled by means of the gaseous cooling medium and the service life of the centrifugal compressor can therefore be increased. Because cooling of the hot leakage flow of the working medium by the cooling medium is already sufficient for this purpose, it is not necessary to prevent the penetration of the leakage flow into the separating gap. In consequence, even the supply of relatively small quantities of the cooling medium are sufficient so that a simple supply arrangement can be employed.
Because the pressure of the leakage flow of the working medium is reduced when supplied into the separating gap, as compared with the pressure of the main flow of the working medium, the cooling medium can be advantageously introduced into the separating gap at a pressure which is either higher or lower than the pressure of the main flow of the working medium. For this purpose, a sealing element is arranged in the separating gap upstream of the rear wall of the compressor impeller. The removal of the used cooling medium takes place through the compressor casing, either to the atmosphere or to the main flow of the working medium of the centrifugal compressor, for which purpose the removal duct for the cooling medium either opens into the ambient air or into the flow duct of the centrifugal compressor. In this way, numerous variation possibilities follow for the cooling the compressor impeller and these permit optimum adaptation of the centrifugal compressor to the conditions present in its application.
The supply duct for the cooling medium is arranged to open into the separating gap approximately parallel or approximately diagonally to the shaft of the compressor impeller, or else approximately tangentially to the rear wall of the compressor impeller. Impingement cooling is achieved in the case of a supply of the cooling medium taking place parallel to the direction of the shaft. In this way, particularly endangered positions on the rear wall of the compressor impeller can be directly and effectively cooled. On the other hand, film cooling is achieved by a radial feed of the cooling medium, with the aid of which even larger regions of the rear wall of the compressor impeller can be cooled. The diagonal feed of the cooling medium combines the advantages of the solutions previously described, although with lower cooling effectiveness. In order to provide compensation for this disadvantage, at least one of the supply ducts accommodates a tube projecting into the separating gap and directed onto the rear wall of the compressor impeller. It is particularly advantageous for each of the tubes to open into the separating gap in the region of the radially outer wall part of the rear wall of the compressor impeller. An effective employment of the cooling medium can be achieved by this means because the

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