Gas, which follows along with the pulp, causes problems at pumping by centrifugal pumps. The gas in the pulp gathers to bubbles, which accumulate in front of the impeller and thereby deteriorate the capacity of the pump. This problem can be remedied by evacuating the gas from the space in front of the impeller, for example by means of vacuum pumps of liquid ring type. The pumps are often a separate vacuum pump, the suction pipe of which is connected to the sealing space of the pulp pump. They can also be designed as a separate vacuum impeller in the form of a liquid ring impeller located on the shaft of the pulp pump so that a common drive for both impellers is obtained.

In those cases when a separate vacuum pump is used, the pulp pump becomes relatively complicated and expensive, because more components are comprised in the system. When a separate liquid ring impeller of conventional type is placed on the same shaft as the pulp pump, the capacity of the liquid ring impeller is restricted for space reasons.

The present invention implies that the afore-said problems can be eliminated in that the vaccum impeller is designed integrated in the impeller of the pulp pump. The construction is compact and simple, and the capacity of the vacuum pump can be easily controlled. Moreover, the arrangement is formed so that it can easily be combined with a separate vacuum pump so that both pumps can be used in parallel. Alternatively, the separate vacuum pump can be connected at an operation drop-out of the integrated vacuum pump according to the invention in order thereby to ensure the accessability for the centrifugal pump.

The characterizing features of the invention are apparent from the attached claims.

The invention is described in greater detail in the following with reference to the accompanying Figures illustrating an embodiment of the invention.

Fig. 1 shows by way of section a centrifugal pump according to the invention.

Fig. 2 is a section according to A-A in Fig. 1.

The centrifugal pump shown comprises a pump housing 10 with an inlet 11 and an outlet 12 for the pulp suspension. Such pumps are used at relatively high pulp concentration, for example 8-12 %. On a rotary shaft 13 in the pump housing 10 an impeller 14 with pump blades 15 is located which extend from a hub 16 outward along a wheel disk 17. On the rear side of the wheel disk 17 rear blades 18 are located. The wheel disk 17 is formed with openings 19 for the discharge of gas collected in front of the impeller. The openings preferably are designed as slits extending from the hub 16 outward.

Behind the wheel disk 17 the hub 16 is provided with vacuum pump blades 20, which are surrounded by a vacuum pump housing 21. The vacuum pump housing 21 comprises a cylindric outer wall 22 and a rear wall 23. The cylindric wall 22 is eccentric in relation to the vacuum pump blades 20 so that a liquid ring pump is formed. In the rear wall 23 a suction port 24 and, respectively, an exhaust port 25 for gas are located. The ports 24,25 are spaced from each other in the circumferential direction, counted for gas suction into the vacuum pump housing and, respectively, gas exhaust out of the vacuum pump housing. This effect is brought about by the rotation of the vacuum pump blades and the variations in volume in the circum- ferential direction of the vacuum pump housing as a consequence of the eccentric location of the wall 22. The exhaust port 25 communicates with a chamber 26 behind the rear wall 23 for gas evacuation through a gas outlet 27. In a sealing housing 28 in the pump housing 10 outside the outer wall 22 a gas passage 29 is located for communication between the openings 19 in the wheel disk 17 and suction port 24.

The vacuum pump housing 21 is axially adjustable in the sealing housing 28 by an adjustment means 30 for setting a gap 31 between the hub 16 of the wheel disk and the outer wall 22.

An inlet 32 for liquid is connected to the sealing housing 28 for the supply of liquid in the gas passage 29. The liquid supplied is utilized both as drive liquid for the liquid ring in the vacuum pump as well as flush liquid in the passage 29 and in the sealing housing 28 for external flushing of the shaft sealing 34.

An additional connection opening 33 in the sealing housing 28 to the gas passage 29 can preferably be provided for rendering it possible to connect a separate vacuum pump (not shown).

At the pumping of a pulp suspension from the inlet 11 to the outlet 12 gas enclosed in the pulp will be collected in front of the impeller 14. The vacuum pump integrated in the hub 16 will then suck in the gas through the openings 19 in the wheel disk 17 and from there via the gas passage 29 into the vacuum pump through the suction port 24.

By the pumping effect brought about by the vacuum pump blades 20 together with the rotating liquid ring in the vacuum pump housing 21 eccentrical with the hub 16, the gas is pumped through the exhaust port 25 and evacuated through the gas outlet 27.

By adjusting the gap 31 the capacity of the vacuum pump can be controlled. By increasing the gap width the liquid leakage through the gap will increase, whereby the capacity of the vacuum pump is reduced, and vice versa.

In the case when a separate vacuum pump is connected to the connection 33, this vacuum pump can serve in parallel with the integrated vacuum pump according to the invention or be connected at an operational drop-out of the integrated vacuum pump.

The operation of the centrifugal pump is thereby ensured.

The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the claims.