Abstract:
A screw-type vacuum pump ( 1 ) includes a pump housing ( 2, 6 ) with rotors ( 3, 4 ). A liquid cooling system cools the rotors and a drive motor ( 9 ). In order to improve the cooling of said pump, an external air-flow impelled cooling system is also provided for the pump housing ( 2, 6 ).

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a screw-type vacuum pump comprising a pump housing with rotors arranged therein, a liquid cooling system for the rotors and a drive motor. 
   DE-A-198 20 523 document discloses multiple problems involved in cooling screw-type vacuum pumps when these shall be built for, and operated at high performance densities—compact and operating at high rotational speeds. 
   It is the task of the present invention to improve the cooling system for a screw-type vacuum pump having the aforementioned characteristics. This task is solved through the present invention through the characterising features of the patent claims. 
   SUMMARY OF THE INVENTION 
   The additional cooling system in accordance with the present invention for cooling the pump housing from the outside, specifically by way of an impelled air flow produced by a fan linked to the motor, for example, considerably relieves the liquid cooling system for the rotors accommodated within the pump of stress. In addition, it is possible with the aid of the impelled air flow to also cool a heat exchanger through which the cooling liquid of the rotor cooling system flows. 
   The present invention allows the implementation of a cooling concept for a screw-type vacuum pump in which the entire machine is air cooled although in addition a liquid cooling system for the rotors is present. The produced heat is in fact dissipated by two different heat carriers (liquid for the inner rotor cooling system, outer cooling air flow). Finally the heat, however, is in total disspated by the cooling air flow. This also applies to the dissipation of secondary heat flows produced by motor losses, gear and bearing losses, etc. 
   Still further advantages of the present invention will be appreciated to those of ordinary skill in the art upon reading and understand the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. 
     The FIGURE is a longitudinal transverse view of a vacuum pump in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the drawing FIGURE, the screw-type vacuum pump which is to be cooled is designated as  1 , its pump or rotor chamber housing with  2  which defines a rotor or first chamber  2   a  in which rotors with  3 ,  4  are disposed and which has an inlet  5 , and the gear/motor chamber housing with  6 , the latter being adjacent with respect to the pump chamber housing  2  with the rotors  3 ,  4 . An outlet on the delivery side is not depicted. Accommodated in the housing  6  is a second chamber  7 ,  8 ,  10  including the gear chamber  7 , the motor chamber  8  with the drive motor  9  and a further chamber  10 , being a component of the liquid cooling circuit for the rotors  3 ,  4 . 
   The rotors  3 ,  4 , are equipped with shafts  11 ,  12  which penetrate the gear chamber  7  and the motor chamber  8 . Through bearings  13   a ,  13   b ;  14   a ,  14   b  in the separating walls between pump chamber and gear chamber  7  (separating wall  13 ) as well as motor chamber  8  and cooling liquid chamber  10  (separating wall  14 ), the rotors  3 ,  4  are suspended in a cantilevered manner. The separating wall between gear chamber  7  and motor chamber  8  is designated as  15 . Accommodated in the gear chamber  7  is a pair of toothed wheels  16 ,  17  effecting the synchronous rotation of the rotors  3 ,  4 . The rotor shaft  11  is simultaneously the drive shaft of the motor  9 . The motor  9  may even be equipped with a drive shaft differing from the shafts  11 ,  12 . In the instance of such a solution its drive shaft terminates in gear chamber  7  and is equipped there with a toothed wheel which intermeshes with one of the synchronizing toothed wheels  16 ,  17  (or a further toothed wheel, not depicted, of the shaft  12 ). 
   Shaft  11  penetrates the chamber  10 , is run out, of the housing  6  of the pump  1  and carriers at its occupied end the wheel  20  of a ventilator or fan  21 . A housing  22  encompassing the pump  1  serves the purpose of guiding the air flow produced by blade wheel  20  in an air flow direction  24   a , said housing being open (apertures  23 ,  24 ) in the area of both face sides. 
   In the sense of the present invention, the fan  21  is operated such that the aperture  24  on the fan/motor side forms the air inlet aperture. Assigned to this aperture is a heat exchanger  25  through which the cooling liquid of the internal rotor cooling system flows. Expediently, the heat exchanger  25  is located upstream of fan  21  so that it simultaneously forms a means of touch protection for the blade wheel  20 . The advantage of this arrangement is, that the air flow cooling the pump chamber housing  2  of the pump  1  is pre-warmed. In this manner it is achieved that thermal expansions of the pump chamber housing  2  are allowed to such an extent that the rotors  3 ,  4  attaining during operation of pump  1  relatively high temperatures, do not come into contact with the housing  2 . Preferably, the housing  2  and the rotors  3 ,  4  are made of aluminium for the purpose of improving heat conductance. Moreover, the housing  2  may exhibit fins  30  for improving the thermal contact. Through the size of the heat exchanger  25  and also through the degree by which the pump chamber housing  2  is equipped with fins, the gap between the rotors  3 ,  4  and the housing  2  is adjusted. 
   The cooling liquid circuit for cooling to rotors  3 ,  4  is depicted only schematically. In U.S. Pat. No. 6,544,020, DE 199 63 171 and U.S. 2003/147764, cooling systems of this kind are described in detail. The shafts  11  and  12  serve the purpose of conveying the coolant (oil, for example) to and from the rotors  3 ,  4 . In the example of the depicted embodiment, the coolant exiting the rotors  3 ,  4 , collects in the motor chamber  8 . From there the coolant is supplied through the line  26  to the heat exchanger  25 . The air flow produced by fan  21  dissipates the heat which was dissipated by the cooling liquid in the rotors,  3 ,  4 . The liquid exiting the heat exchanger  25  is supplied through the line  26  to the chamber  10 . In a manner not depicted in detail it passes from there through bores in the shafts  11 ,  12  to the rotors  3 ,  4 , flows there through cooling ducts and passes through the shafts  11 ,  12  back into the motor chamber  8 . 
   It has been found to be expedient to adjust the cooling system such that approximately half of the heat generated by the pump is first dissipated by the cooling liquid and thereafter removed through the heat exchanger  25 , and such that the other half is dissipated directly by the cooling air flow. 
   In all, the characteristics in accordance with the present invention allow a further increase in the performance density of a screw-type pump. The pump may be designed to be smaller and may be operated at higher surface temperatures. 
   The housing  22  serving the purpose of guiding the outer air flow has, in addition, the function of providing a means of touch protection. 
   The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.