Abstract:
A screw-type vacuum pump ( 1 ) is tempered such that characteristics of the pump are not substantially altered when the pump is subjected to thermal stress. In order to achieve said aim, cooling is adjusted according to an operating state of the screw-type vacuum pump ( 1 ), preferably to maintain a substantially constant pump gap ( 4 ).

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a method for tempering a screw-type vacuum pump. Moreover, the invention relates to a screw-type vacuum pump suited for implementing said method. 
   From DE-A-198 20 523 a screw-type vacuum pump of the here affected kind is known. The multitude of heat problems has been disclosed. Cooling of the rotors revolving in a pump chamber involves special difficulties when the threads of the rotors exhibit a pitch which decreases from the intake side to the delivery side, frequently even also in combination with an increase in the width of the thread ridges. Rotors of this kind are subjected during operation to severe thermal stresses, in particular in the area of their delivery side, since the compression of the pumped gases produces a not insignificant amount of heat. Since the quality of a screw-type vacuum pump depends significantly on the gap between the rotors and the pump chamber housing, the manufacturers strive to keep this gap very small. However, opposed to this aim is the thermal expansion of the thermally highly stressed areas, rotors and housing. The pump chamber housing does not, or only slightly, take part in the thermal expansion of the rotors. A sufficiently large gap must be present. It was previously only in this manner possible to prevent the rotors from making contact with the housing with the attendant risk of standstill seizing. The problem detailed grows to be particularly grave when the rotors and the housing consist of different materials. In the instance of the coefficient of expansion of the housing being smaller than the expansion of coefficient of the rotor material (for example, housing made of cast iron, rotors of aluminium) there exists the risk of the rotors running against the housing. If the reverse expansion conditions exist, the pump&#39;s gap can increase such that the performance of the pump decreases. 
   It is the task of the present invention to design and be able to operate a screw-type vacuum pump of the here affected kind such that during thermal stresses its properties will not change substantially. 
   SUMMARY OF THE INVENTION 
   Through the present invention it is possible to have an influence on the effect of the cooling, respectively tempering, with the aim of permitting a temperature increase in the pump chamber housing which does not exceed inadmissible limits. During an increased thermal stress on the pump, the only slightly cooled pump chamber housing expands jointly with its rotors. The risk of making contact does no longer exist. The cooling system is controlled expediently such that the size of the gaps in the pump chamber housing remains substantially unchanged during the different operating conditions. 
   For example, the outside temperature of the pump chamber housing may be employed as the controlled variable. 
   If the screw-type vacuum pump is air cooled, then the cooling air flow may be controlled depending on the operating status of the pump, for example by controlling the rotational speed of a fan producing the cooling air flow. This requires that the fan be equipped with a drive being independent of the drive motor of the pump. If the fan is linked to the drive of the pump, control of the cooling air flow can be implemented with the aid of adjustable screens, throttles or alike. If the pump is cooled by liquids, control can be effected by adjusting the quantity (flow rate) or the temperature of the cooling liquid. 
   If the pump is air cooled from the outside and if its rotors are equipped with a liquid cooling system, it is expedient to arrange a heat exchanger in the cooling air flow so as to dissipate the heat dissipated by the liquid (oil, for example). When said heat exchanger is arranged, with respect to the direction of the flowing cooling air, upstream of the pump chamber housing, well-aimed tempering of the pump chamber housing is possible. Again, the outside temperature of the pump chamber housing may serve as the controlled variable; also the temperature of the cooling liquid may be employed as the controlled variable. Arrangements of this kind allow, above all, cooling of the pump to be controlled such that the gap between the rotors and the housing is maintained during operation of said pump at a substantially constant width. 
   Moreover, it is expedient when the pump is equipped with an inner rotor cooling system (liquid) and a housing cooling system (from the outside with liquid), and where both cooling systems are controlled matched to each other such that during all operating modes of the pump a substantially constant gap is maintained. The desired control with the aim of a constant gap is effected such that the quantities of liquid supplied to the cooling systems, for example with the aid of a heat exchanger, are controlled depending on cooling demand. 
   In order to be able to implement the desired control, the utilization of sensors is required. These may be temperature sensors, the signals of which are supplied to a control center. The control center in turn regulates the intensity of the cooling, preferably in such a manner that the pump gap is maintained at a substantially constant width. Instead of one or several temperature sensors, also a distance sensor may be employed which supplies direct information on the size of the gap. 
   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. 
       FIG. 1  is a longitudinal section of an air cooled screw-type vacuum pump and cooling system: 
       FIGS. 2 and 3  each illustrate an air and liquid cooled screw-type vacuum pump; and 
       FIG. 4  illustrates a screw-type vacuum pump equipped with two liquid cooling systems. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the Figure, a screw-type vacuum pump to be cooled is designated as  1 , its pump chamber housing with  2 , its rotors with  3 , the gap on the delivery side between the rotors  3  and pump chamber housing  2  with  4 , and its inlet with  5 . The gear/motor chamber housing adjacent the pump chamber housing  2  containing the rotors  3  is designated as  6 . It is only schematically outlined that the rotors  3  are equipped with threads, with their pitch and ridge width decreasing from the intake side to the delivery side. An outlet located on the delivery side is not depicted. Located in housing  6  is the gear chamber  7 , the motor chamber  8  with the drive motor  9  and a further chamber  10 , being the bearing chamber ( FIG. 1 ) or part of a cooling liquid circuit for the rotors  3  ( FIGS. 2 and 3 ). 
   The rotors  3  are equipped with shafts  11 ,  12  which penetrate the gear chamber  7  and the motor chamber  8 . By means of bearings in the separating walls between the pump chamber and the gear chamber  7  (separating wall  14 ) as well as motor chamber  8  and bearing respectively a cooling liquid chamber  10  (separating wall  14 ), the rotors  3  are suspended in a cantilevered manner. The separating wall between gear chamber  7  and motor chamber  8  is designated as  15 . Located in the gear chamber  7  is the pair of toothed wheels  16 ,  17  effecting the synchronous rotation of the rotors  3 . The rotor shaft  11  forms simultaneously the drive shaft of the motor  9 . The motor  9  may exhibit a drive shaft different from the shafts  11 ,  12 . In the instance of such a solution, the drive shaft of said motor terminates in gear chamber  7  and is there equipped with a toothed wheel, which engages with one of the synchronising toothed wheels  16 ,  17  (or a further toothed wheel, not depicted, of the shaft  12 ). 
   In the embodiments according to the  FIGS. 1 to 3 , cooling of the housings  2  and  6  of the pump  1  is effected with aid of an air flow produced by the wheel or impeller  20  of a fan  21 . A housing  22  encompassing the pump  1  serves the purpose of guiding the air movement produced by the blade wheel  20 , said housing being open (apertures  23 ,  24 ) in the area of both its sides. Fan  21  is arranged such that the aperture  24  on the fan/motor side of the housing  22  forms the air inlet aperture. 
   In the embodiments according to the  FIGS. 1 and 2 , the fan  21  has a drive motor  25  independent of the drive motor  9  of the pump  1 . This solution is advantageous for screw-type vacuum pumps. The motor  9  of which is depicted as a canned motor, is thereby encapsulated. 
   In the embodiments according to the  FIGS. 3 and 4 , the shaft  11  penetrates the chamber  10 , is run out of the housing  6  of the pump  1 , and carries at its unoccupied end the wheel  20  of the ventilator or fan  21 . 
   In all Figures, a control facility or module is in each instance schematically represented by way of block  26 . It is linked through lines depicted by way of dashed lines to sensors supplying the signals of desired manipulated variables. As examples, two alternatively or simultaneously employable temperature sensors  27  and  28  are depicted. Sensor  27  supplies signals corresponding to the temperature of the housing  2 . Said sensor is preferably affixed at the housing  2  in the area of the delivery side of the rotors  3 . Sensor  28  is located in the motor chamber and supplies signals which correspond to the temperature of the cooling liquid, preferably oil temperature. Through further lines the control facility is linked in each instance to facilities aiding controlled cooling of the pump  1  in the desired manner. 
   In the embodiment according to  FIG. 1 , the air flow produced by the fan  21  is controlled. For this purpose the control facility  26  is connected through the line  29  to the drive motor  25 . Corresponding to the signals supplied by one or both sensors  27  or  28 , control of the rotational speed of the blade wheel  20  is effected. Since the signals supplied by the sensor  27  provide information on the housing temperature and the signals supplied by the sensor  28  provide information on the rotor temperature, the utilization of both sensors can be employed to perform a differential control with respect to the gap  4 . 
   In the instance of an alternative solution, only one sensor  27 ′ may be provided instead of the two temperature sensors  27 ,  28 , said sensor  27 ′ being located, for example, at the location of the temperature sensor  27 , i.e. in the area of the delivery side of the pump chamber  2 . The sensor  27 ′ is a distance sensor which supplies direct information as to the magnitude of the pump gap  4 . Sensors of this kind are basically known. Changes in capacitance or—preferably—changes in an eddy current which occur depending on the size of the gap are employed for producing the sensor signals. 
   Alone depending on one sensor  27 ′ of this kind, tempering of the pump  1  can be controlled. If, for example, during operation of the pump the size of the gap decreases in that the rotors  3  expand, cooling of the housing  2  is reduced by reducing the quantity of cooling air by a reduction in speed of the ventilator  20 . Thus the housing expands so that the decrease in gap size can be compensated. If during operation of the pump  1  the gap size increases, this increase may be compensated by increasing the cooling effect (shrinking of housing  2 ). 
   The embodiment according to  FIG. 2  differs from the embodiment according to  FIG. 1  in that the pump  1  is equipped with a liquid cooling system for the rotors. The cooling liquid circuit for cooling the rotors  3  is only outlined schematically. In patent/applications U.S. Pat. No. 6,544.020, DE 199 63 171.9, US 2003/147764, cooling systems of this kind are described in detail. The shafts  11  and  12  serve the purpose of transporting the coolant (oil, for example) to and from the rotors  3 . In the example of an embodiment presented, the coolant exiting the rotors  3  collects in the motor chamber  8 . From there it is supplied through the line  31  to a heat exchanger  32 . The heat exchanger  32  may be air or water cooled. Especially expedient—as depicted—is an arrangement where the air flow produced by the fan  21  dissipates the heat dissipated by the cooling liquid in the rotors  3 . The liquid exiting the heat exchanger  32  is supplied through the line  33  into the chamber  10 . In a manner not depicted in detail said cooling liquid passes from there through bores located in the shafts  11 ,  12  to the rotors  3 , flows there through cooling ducts and passes through the shafts  11 ,  12  back into the motor chamber  8 . 
   In order to control the liquid cooling system, two alternatives for the actuating variable (already described sensors  27 ,  28 ) and two alternatives for controlled cooling of the cooling liquid in the heat exchanger  32  are depicted in  FIG. 2 . Either, as depicted in  FIG. 1 , the rotational speed of a blade wheel  20  is controlled depending on one of the manipulated variables. In the instance of the other alternative, a control valve  35  in line  31  defines the quantity of cooling liquid flowing through the heat exchanger per unit of time. 
   In the instance of the solution according to  FIG. 2 , the pump  1  may be tempered in addition by the air flow of the fan  21 . In this instance, it is expedient to arrange the heat exchanger  32  and fan  21  in the area of the aperture  24 . The advantage of this arrangement is such 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  which during operation of the pump  1  attain relatively high temperatures, will not make contact with the housing  2 . Preferably, the housing  2  and the rotors  3  consist of aluminium for the purpose of improving heat conductance. Moreover, the housing  2  may exhibit fins for improving thermal contact and heat transfer. 
   Irrespectively whether the air flow produced by fan  21  cools only the heat exchanger  32  or the heat exchanger  32  and the housing  2 ,  6  of the pump, it is expedient to locate the heat exchanger  32  upstream of the blade wheel thereby providing a means of touch protection, i.e., a guard which prevents operator contact with the fan blade. 
   In the instance of the solution according to  FIG. 3 , the blade wheel  20  is coupled to the motor shaft  11 . Since screw-type vacuum pumps are commonly operated at constant rotational speeds, there no longer exists the possibility of controlling the air flow with the aid of the fan  21 . For the purpose of controlling the air flow, a controllable aperture  36  (iris aperture, for example), throttle or alike is provided in the instance of the embodiment according to  FIG. 3 . Said aperture is located between the blade wheel  20  and the heat exchanger  32 , is only depicted schematically with reference number  36 . Through the line  37  the aperture  36  is connected to the control facility  26 . Control of the magnitude of the cooling air flow and/or cooling of the liquid is effected corresponding to the control arrangement detailed for  FIG. 2  by controlling the flow cross-section of the air flow, preferably with respect to a constant gap size. 
   Additionally, the cooling liquid circuit in the instance of the solution according to  FIG. 3  is equipped with a thermostatic valve  38 . It is located in the line  31  and is preferably also controlled by the control module or facility  26 . During the phase of operational start-up of pump  1  in which the cooling liquid has not yet attained its operating temperature, said thermostatic valve has the task of blocking the line  31  and supplying the cooling liquid through the bypass line  39  directly into line  33  bypassing the heat exchanger. 
   When the temperature of the cooling liquid has attained its operating temperature, line  39  is blocked and line  31  is opened (drawn position of the valve  38 ). The bypass solution reduces the time needed for the start-up phase. 
   In the example of the embodiment according to  FIG. 4 , the screw-type vacuum pump is equipped with the already described inside cooling system for the rotors as well as with a housing cooling system  41  operated with a liquid. Said housing cooling system comprises a cooling jacket  42  (filled with liquid, for example) located at the outlet area of the rotor housing  2 . A cooling coil  43  through which the actual coolant flows is located in the cooling jacket  42 . Alternatively the cooling liquid may flow also through the cooling jacket  42  itself. 
   In the presented example of an embodiment, the outlet of the housing cooling system is linked to the motor chamber  8  into which also the cooling liquid exiting the internal rotor cooling system flows. Through the line  31  the cooling liquid passes into the heat exchanger  32 . Connected downstream thereto is the line  44  with a 3/2 way valve  47  which selectively splits the quantities of the cooling liquid supplied between the lines  45  and  46 . 
   Line  45  is linked to the inlet of the internal rotor cooling system, line  46  is linked to the inlet of the outer housing cooling system  41 . The valve  47  is a control valve controlled by the controller  26 . 
   In the example of the embodiment according to  FIG. 4  the ventilator  20  and the heat exchanger  32  are located, as in the instance of the embodiments according to  FIGS. 2 and 3 , in the area of the aperture  24  of the housing  22 . Since cooling by an air flow is no longer an absolute necessity (it only cools the motor and gear housing  6 ), the heat exchanger  32  and its cooling system (air or liquid) may also be arranged at a different location and independently of the drive motor  9 . For both cooling circuits also separate heat exchangers may be provided. Finally, the housing  22  need not be present. 
   In the embodiment according to  FIG. 4  tempering of the pump  1  may—as also in the instance of all other examples of embodiments—be effected such that its pumping gap  4  is maintained substantially constant. The sensors  27  and  28  supply signals which are related to the temperatures of the housing  2  on the one hand and the rotors  3  on the other hand. Depending on these signals, the valve  45  splits of the cooling liquid shares to both cooling systems in ratios set by the control module  26 . 
   In all, the features according to the present invention permit a further increase in performance density of a screw-type pump. The pump may be designed to be smaller and may be operated at higher surface temperatures. The outer housing  22  serving the purpose of guiding the air also serves the purpose of providing a means of touch protection. It has been found expedient to adjust the cooling such that in the instance of two cooling systems (inner rotor cooling system and outer housing cooling system) approximately half of the heat produced by the pump is dissipated by each of the two cooling systems. 
   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.