METHOD FOR CONTROLLING THE TEMPERATURE OF A VACUUM PUMP, AND ASSOCIATED VACUUM PUMP AND INSTALLATION

Abstract

A method is provided for controlling a temperature of a vacuum pump of a dry vacuum pump type subjected to variable pumping loads, the pump including a stator, a pumping stage, two shafts extending into the stage and respectively bearing a rotor to rotate synchronously in opposite directions in the stator and to drive a gas from a suction inlet to a delivery outlet, a cooling element coupled to the stator, a temperature sensor to measure a stator temperature, and a controller to control the stator temperature by the cooling element and the temperature sensor; and the method including controlling the pump temperature by the cooling element based on a temperature setpoint and a measurement of the stator temperature, and monitoring whether a value of a pumping load parameter is below a load threshold and, if so, increasing the setpoint. A vacuum pump and a system are also provided.

Description

The present invention relates to a method for controlling the temperature of a vacuum pump of the dry vacuum pump type. The invention also relates to a vacuum pump of the dry vacuum pump type comprising means for implementing the said control method. The invention also relates to an installation comprising the said vacuum pump.

Rough vacuum pumps of the dry vacuum pump type comprise several pumping stages in series through which stages a gas that is to be pumped circulates between a suction inlet and a delivery outlet. Amongst known rough vacuum pumps, a distinction is made between those which have rotary lobes, also known by the name of “roots” pumps, which have two or more lobes, pumps known as “claw” pumps, and screw pumps. Vacuum pumps of the roots compressor (or more commonly “roots blower”) type having one or two stages and which are used upstream of the rough vacuum pumps in order to increase the pumping capability under very high flow conditions are also known.

These vacuum pumps are referred to as “dry” because, in operation, the rotors rotate inside the stator without any mechanical contact with each other or with the stator, which makes it possible not to use oil in the pumping stage. More and more applications are requiring the ability to vary the flows of gas that is to be pumped significantly and quickly between, on the one hand, process steps for which the vacuum pump needs to circulate high flows of gas, of the order of several slm (standard litres per minute) or several tens of slm and, on the other hand, idle (or standby) steps in which the vacuum pump is operating at what is known as the “ultimate vacuum pressure”, the flow of gas to be pumped being zero or very low.

Pumping high flows of gas leads to significant heating of the vacuum pump as a result of the compression. This rise in temperature makes it possible to avoid the condensing or solidification into a powder of pollutant gaseous species inside the vacuum pump. However, it is necessary to cool the bearings of the vacuum pumps in order to avoid any malfunctioning. Furthermore, in certain applications, the stator temperature needs to be controlled so that it does not exceed a predefined maximum beyond which the gaseous species being pumped could agglomerate in the pump and cause the pump to seize.

The cooling of the stator is generally achieved by circulating water at ambient temperature through cooling circuits in thermal contact with the stator.

However, in the situations described hereinabove for which the flow of gas to be pumped drops sharply, the vacuum pump is then without own heating and may cool down just as sharply. The stator in contact with the cooling circuits then experiences a drop in temperature whereas the rotors, which are not directly cooled, remain hot.

This difference in temperature between the rotors and the stator may be accentuated by the fact that the point at which the temperature is measured for controlling the cooling circuits is not necessarily situated at a suitable location that allows a rapid change in temperature due to a change in pumping load to be detected.

The measured temperature may thus be overestimated and lead to a continuing command to cool the stator even though at the bearings for example the temperature has already dropped significantly. The response time needed to actually recognize a drop in stator temperature may be relatively long, and this may lead to a widening of the discrepancy between the temperatures.

This discrepancy in temperature may give rise to a loss of clearance between the stator and the rotors because of the various thermomechanical behaviours, and notably to a loss in the axial clearance because the cooling circuits are generally arranged at each axial end of the vacuum pump in the region of the bearings, and to a reduction in the inter-axis distance because of the contraction of the shaft supports. These losses in clearance may lead to pump seizure or to rotors touching one another.

One of the objects of the present invention is to propose a vacuum pump of the dry vacuum pump type and a method for controlling the temperature of the vacuum pump that make it possible to redress at least one of the aforementioned disadvantages, notably by limiting the losses of clearance and seizure.

To this end, one subject of the invention is a method for controlling the temperature of a vacuum pump of the dry vacuum pump type subjected to variable pumping loads, the vacuum pump comprising:

• • a stator,
  • at least one pumping stage,
  • two shafts extending into the at least one pumping stage and respectively bearing at least one rotor, the rotors being configured to rotate synchronously in opposite directions in the stator in order to drive a gas that is to be pumped from a suction inlet of the vacuum pump to a delivery outlet,
  • at least one cooling element coupled to the stator,
  • at least one temperature sensor configured to take a measurement of the temperature of the stator, and
  • a control unit configured to control the temperature of the stator by means of the at least one cooling element and of the at least one temperature sensor,

in which the temperature of the vacuum pump is controlled by means of the at least one cooling element coupled to the stator on the basis of a temperature setpoint and of a measurement of the temperature of the stator,

characterized in that monitoring monitors whether the value of a parameter indicative of the pumping load, chosen from either a current drawn or a power consumed by the vacuum pump, is below a load threshold and, if the value of the parameter indicative of the pumping load is below the load threshold, then the temperature setpoint is increased.

The change in temperature setpoint thus makes it possible to shut off the cooling of the stator by the cooling element sooner, leaving the stator to warm up in the vicinity of the cooling element. Increasing the temperature setpoint during low pumping-load steps allows the stator to be kept as hot as during high-load steps, and this makes it possible to limit the risks of seizure or of rotors touching one another.

This temperature, which is kept high during low-load steps, also makes it possible to avoid the creation of cold zones where condensable pollutant species could solidify or condense.

The change in temperature setpoint that is triggered by monitoring the pumping load also allows the method to be highly responsive.

This monitoring may furthermore be performed on the basis of the information already available from sensors of the vacuum pump, by incorporating the thermal behaviour of the vacuum pump into the determination of the temperature control, without the need to add additional temperature sensors, without information regarding the process taking place in the chamber and without changing the position of the at least one temperature sensor or the structure of the cooling elements.

The temperature control method may also comprise one or more of the features described hereinafter, considered alone or in combination.

According to one exemplary embodiment, the temperature setpoint is increased at least for control of the temperature by means of a cooling element coupled to a pumping stage, referred to as a low-pressure pumping stage, of the vacuum pump.

According to one exemplary embodiment, after the temperature setpoint has been increased, monitoring monitors whether the value of the parameter indicative of the pumping load is above the load threshold and, if the value of the parameter indicative of the pumping load is above the load threshold, then an increased temperature setpoint is maintained for a predefined additional length of time.

The predefined additional length of time is, for example, greater than ten minutes.

The increase in the temperature setpoint is, for example, greater than 3° C.

The increase in the temperature setpoint is, for example, less than 20° C.

Another subject of the invention is a vacuum pump of the dry vacuum pump type, comprising:

• • a stator,
  • at least one pumping stage,
  • two shafts extending into the at least one pumping stage and respectively bearing at least one rotor, the rotors being configured to rotate synchronously in opposite directions in the stator in order to drive a gas that is to be pumped from a suction inlet of the vacuum pump to a delivery outlet, at least one cooling element coupled to the stator,
  • at least one temperature sensor configured to take a measurement of the temperature of the stator, and
  • a control unit configured to control the temperature of the stator by means of the at least one cooling element and of the at least one temperature sensor,

characterized in that the control unit is configured to implement a temperature control method as described hereinabove.

The vacuum pump of the dry vacuum pump type may be a multistage rough vacuum pump, which means to say one comprising at least two pumping stages mounted in series. The vacuum pump may equally be a vacuum pump of the roots blower type comprising one or two pumping stages mounted in series.

According to one exemplary embodiment, the vacuum pump of the dry vacuum pump type comprises two cooling elements coupled to the stator, one cooling element being arranged at each axial end of the vacuum pump.

Another subject of the present invention is an installation comprising a chamber, characterized in that it comprises a vacuum pump of the dry vacuum pump type as described hereinabove, connected to the chamber for pumping therein.

Further features and advantages of the invention will become apparent from the following description, given by way of nonlimiting example, with reference to the attached drawings in which:

FIG. 1 shows a schematic view of an installation according to the invention.

FIG. 2 shows a partial and partially sectioned schematic view of a vacuum pump of the installation of FIG. 1, in the dismantled state, depicting only the elements necessary for operation.

FIG. 3 is a schematic view showing various steps of a temperature control method for the vacuum pump of FIG. 2.

FIG. 4 is a graph showing an example of curves obtained as a function of time (in minutes), for:

• • the power (in watts, ordinate axis on the right) consumed by the vacuum pump of FIG. 2 (curve A),
  • the stator temperature (in ° C., ordinate axis on the left), measured by a vacuum pump temperature sensor (curve B), and
  • the stator temperature measured by way of indication by two test temperature sensors fixed to the centre of a vacuum pump cooling element (curves C and D).

In these figures, elements that are identical bear the same reference numerals. The following embodiments are merely examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference relates to the same embodiment or that the features apply solely to one single embodiment.

Simple features of various embodiments can also be combined or interchanged in order to provide other embodiments.

FIG. 1 depicts a first example of an installation 1 comprising a vacuum pump 2 of the dry vacuum pump type and a chamber 3 to which the vacuum pump 2 is connected for example via a valve 4, for pumping in the chamber 3.

Significant gas flows, of the order of several slm or several tens of slm, may be introduced into the chamber 3, for example cyclically, during steps referred to as “process” steps P1, P2 (FIG. 3). These process steps P1, P2 may precede and succeed steps referred to as “idle” steps I, during which the flows of gas introduced are low or zero. During the idle steps I, the vacuum pump 2 is operating at what is known as the “ultimate vacuum pressure” for periods of time in excess of several minutes, for example so as to allow the chamber 3 to be cleaned out. This succession of steps occurs for example during processes for manufacturing semiconductors, such as processes known as “HarpXT” processes.

As can best be seen in FIGS. 1 and 2, the vacuum pump 2 comprises a stator 5, at least one pumping stage T1-T5, two shafts 6, 7 extending into the at least one pumping stage T1-T5 and respectively bearing at least one rotor 8, at least one cooling element 11a, 11b coupled to the stator 5, at least one temperature sensor 12a, 12b configured to take a measurement of the temperature of the stator 5, and a control unit 13 configured to control the temperature of the stator 5 by means of the at least one cooling element 11a, 11b and of the at least one temperature sensor 12a, 12b.

The rotors 8 are configured to rotate synchronously in opposite directions in the stator 5 in order to drive a gas G that is to be pumped from a suction inlet 9 of the vacuum pump 2 to a delivery outlet 10 of the pump 2.

The rotors 8 have, for example, lobes with identical profiles, such as of the “roots” type (with a cross section in the shape of a “Figure-8” or of a “kidney-bean”) or of the “claw” type. According to another example, the pumping rotors 8 are of the “screw” type.

The vacuum pump 2 comprises for example at least two pumping stages, such as five pumping stages. Each pumping stage T1-T5 comprises a respective inlet and outlet. The successive pumping stages T1-T5 are connected in series one after the other by respective inter-stage ducts 14 that connect the outlet (or delivery outlet) of the preceding pumping stage to the inlet (or suction inlet) of the next stage.

During rotation, the gas drawn from the inlet is trapped in the volume generated by the rotors 8 and then driven by the rotors 8 towards the delivery outlet 10 (the direction in which the gases circulate is illustrated by the arrows G in FIGS. 1 and 2). The vacuum pump 2 is notably said to be “dry” because, in operation, the rotors 8 rotate inside the stator 5 without any mechanical contact with one another or with the stator 5, making it possible not to use oil in the pumping stages T1-T5.

In this exemplary embodiment, the vacuum pump 2 of the dry vacuum pump type is a multistage rough vacuum pump. A rough vacuum pump is a positive-displacement vacuum pump which, using two rotors, draws in, transfers then delivers the gas that is to be pumped at atmospheric pressure. According to another example, the vacuum pump 2 is of the roots blower type and comprises one or two pumping stages. Vacuum pumps of the roots blower type are mounted in series and upstream of a rough vacuum pump.

According to one exemplary embodiment, the cooling element 11a, 11b comprises a hydraulic circuit 16 to allow the circulation of water, for example at ambient temperature (FIG. 2).

The hydraulic circuit 16 is, for example, incorporated into the stator 5. It is, for example, in the shape of a “U” surrounding the bearings of the shafts 6, 7, in order to cool them.

The cooling element 11a, 11b further comprises for example a valve 17 which can be operated in order to allow or cut off the circulation of water (“all or nothing” control).

The vacuum pump 2 comprises for example two cooling elements 11a, 11b coupled to the stator 5, one cooling element 11a, 11b being arranged at each axial end of the vacuum pump 2 (FIG. 2). A cooling element 11 a is coupled to a pumping stage T1 referred to as a low-pressure pumping stage, the inlet of which communicates with the suction inlet 9 of the pump 2. A cooling element 11b is coupled to a pumping stage T5 referred to as a high-pressure pumping stage, the outlet of which communicates with the delivery outlet 10 of the pump 2.

The vacuum pump 2 comprises for example two temperature sensors 12a, 12b arranged on the stator 5 and spaced apart from one another. One temperature sensor 12a is, for example, associated with the cooling element 11a situated on the side of the suction inlet 9. The temperature sensor 12a is, for example, mounted on the stator 5 in the region of the low-pressure pumping stage T1 (on the side of the suction inlet 9). A temperature sensor 12b is, for example, associated with the cooling element 11b situated on the side of the delivery outlet 10. The temperature sensor 12b is, for example, mounted on the stator 5 in the region of the high-pressure pumping stage T5 (on the side of the delivery outlet 10).

The temperature sensors 12a, 12b are, for example, situated on the stator 5 at a mid-point between the two shafts 6, 7, and aligned on a straight line parallel to the axes of the shafts 6, 7 (FIG. 1).

The control unit 13 comprises one or more controllers or microcontrollers or processors and a memory to execute series of program instructions implementing a method 100 for controlling the temperature of the vacuum pump 2, in which method the temperature of the vacuum pump 2 subjected to variable pumping loads is controlled by means of the said at least one cooling element 11a, 11b coupled to the stator 5, on the basis of a temperature setpoint and of a measurement of the temperature of the stator 5.

In order to do that, the control unit 13 is connected to at least one temperature sensor 12a, 12b to receive a measurement of the temperature of the stator 5, and is connected to at least one cooling element 11a, 11b, for example to operate the opening/closing of the associated valve 17 of the hydraulic circuit 16. The temperature control may be performed independently on each cooling element 11a, 11b according to its own temperature setpoint and according to an associated individual temperature measurement.

In operation, the vacuum pump 2 is subjected to variable pumping loads which may vary between high or low gas flows.

The control unit 13 monitors whether the value of a parameter indicative of the pumping load is below a load threshold S (diagnostics step 101, FIG. 3).

The parameter indicative of the pumping load is, for example, the current drawn by the vacuum pump 2 or the power consumed by the vacuum pump 2. The control unit 13 calculates, for example, a mean of the current or of the power drawn or consumed over a period of time equal to or greater than the duration of a cycle of a process step P1, P2. To do that, the control unit 13 is, for example, connected to an output of a variator that varies the speed of the motor of the vacuum pump 2.

If, and as long as, the value of the parameter indicative of the pumping load is above the load threshold S, then it is considered that a process step P1, P2 is taking place in the chamber 3.

In that case, the control unit 13 controls the temperature of the vacuum pump 2 in order to achieve the temperature setpoint, using the cooling elements 11a, 11b, for example by closing the valves 17 in order to cut off the circulation of water when the temperature measurement is below the temperature setpoint and by opening the valves 17 to allow water to circulate when the temperature measurement is equal to or higher than the temperature setpoint (process-phase regulation step 102).

The temperature setpoint is, for example, higher than 70° C.

If, and as long as, the value of the parameter indicative of the pumping load is below the load threshold S, then it is considered that an idle step I is taking place in the chamber 3. In that case, the control unit 13 increases the temperature setpoint in order to control the temperature of the vacuum pump 2 by means of at least one cooling element 11a (idle-phase regulation step 103).

The temperature setpoint may be increased to control the temperature by means of both of the cooling elements 11a, 11b or just one of them, but for preference at least by means of the cooling element 11a coupled to the low-pressure pumping stage T1, which is more difficult to regulate in terms of temperature because the capacity for exchange of heat between the rotors 8 and the stator 5 is not so good at low pressure.

The increase in the temperature setpoint corresponds for example to at least 3% of the temperature setpoint, such as, for example, to more than 3° C. The increase in the temperature setpoint corresponds for example to at most 20% of the temperature setpoint, such as, for example, to under 20° C. The increase in the temperature setpoint is, for example, of the order of 6% of the temperature setpoint, such as 5° C.

The control unit 13 controls the temperature of the vacuum pump 2 in order to achieve the increased temperature setpoint as achieved during the process step P1, P2, by means of the cooling elements 11a, 11b, for example by actuating the water circulation valves 17.

VVhen the parameter indicative of the pumping load has increased beyond the load threshold S, it is considered that a further process step P1, P2 is taking place in the chamber 3.

Provision may then be made to maintain an increased temperature setpoint for a predefined additional length of time (reconditioning step 104) before switching the increased temperature setpoint back to the initial temperature setpoint.

The additional length of time is predefined, making it possible to dispense with the need for a sensor. It is, for example, longer than 10 minutes, such as 15 minutes. This reconditioning step 104 allows the stator 5 to be given time to warm up as a result of the higher pumping load of the process step P1, P2. That makes it possible to avoid generating a further discrepancy between the temperatures of the rotors 8 and the stator 5 when returning to the initial temperature setpoint.

A better understanding of this may be gained by viewing the graph in FIG. 4 which shows one example of a curve of the power consumed by the vacuum pump 2 subjected to a pumping load (curve A), a curve of the temperature of the stator 5 as measured by the temperature sensor 12a in the vicinity of the low-pressure pumping stage T1 (curve B) and two curves of the temperature of the stator 5 as measured by way of indication in order to provide an understanding of the invention, at the centre of the cooling element 11 a coupled to the stator 5 of the low-pressure pumping stage T1 (curves C and D).

During the first two hours, a flow of gas of 80 slm (135.12 Pa·m³/s) is introduced cyclically into the chamber 3. The flow of gas thus alternates between 80 slm for 5 minutes and 0 slm for 3 minutes. The consumed power, indicative of the pumping load, therefore varies in a square-wave pattern between 500 and 2000 W (curve A), above a load threshold for example of 600 W for a duration in excess of 3 minutes (duration equal to a flow-free phase of a process step).

The control unit 13 controls the temperature of the vacuum pump 2 in order to achieve a temperature setpoint of 83° C. by means of the cooling elements 11a, 11b (process-phase regulation step 102). It may be seen that the temperature of the stator 5, as measured by the temperature sensor 12a, thus fluctuates between 81° C. and 86° C. about the setpoint temperature because of the all-or-nothing regulation mode (curve B). It may also be seen that the temperature measured at the centre of the cooling element 11a (by way of indication) fluctuates between 84 and 87° C. (curves C and D).

The consumed power then drops below the load threshold S. From this, the control unit 13 concludes that an idle step I is taking place in the chamber 3. The control unit 13 therefore increases the temperature setpoint by 5° C. (idle-phase regulation step 103) and controls the temperature of the vacuum pump 2 to 88° C. by means of the cooling element 11a of the low-pressure pumping stage T1 and to 83° C. or 88° C. by means of the cooling element 11b of the high-pressure pumping stage T5.

It may be noted that the temperature of the stator 5, as measured by the temperature sensor 12a associated with the cooling element 11a, has jumped by around 5° C. to fluctuate between 86° C. and 90° C. (curve B).

It may also be seen that the temperature measured at the centre of the cooling element 11a has increased rapidly because of the increase in the temperature setpoint and then decreased because of the reduction in pumping load until it tends to stabilize at a temperature close to that of the process step P1 (curves C and D).

The change in temperature setpoint thus makes it possible to shut off the cooling of the stator 5 by the cooling element 11a sooner, leaving the stator 5 to warm up in the vicinity of the cooling element 11a. Despite the drop in temperature, the temperature of the stator 5, as measured at the cooling element 11a, has dropped below the temperature of the process step P1 little, if at all. The discrepancy in temperature between the stator 5 and the rotors 8 is therefore substantially the same during the process step P1 as during the idle step I, given that the rotors 8 remain hot.

The power consumed then increases beyond the load threshold S (curve A), indicating that a further process step P2 is taking place in the chamber 3. The temperature setpoint remains increased to 88° C. for 15 minutes (reconditioning step 104): it may be seen that the temperatures of the stator 5 in the region of the cooling element 11a begin to rise again as the vacuum pump 2 heats up (curves C and D).

After the predefined additional length of time has elapsed, because the temperatures at the centre of the cooling element 11a have more or less returned to the previous values of the process step P1, the control unit 13 decrements the temperature setpoint which returns to 83° C. (process-phase regulation step 102). The temperatures at the centre of the cooling element 11a decrease by the difference in temperature setpoint, then rise again slowly with the value of the setpoint to 83° C. During the idle step 1 and the process step P2 that follows, the temperature has remained above 83° C. in the region of the stator 5 near to the cooling element 11a.

Increasing the temperature setpoint during the idle step I at low pumping load allows the stator 5 to be kept as hot in the centre of the cooling element 11a as during the process steps P1, P2, and this makes it possible to limit the risks of seizure or of rotors 8 touching one another during the idle step 1, which risks are associated with the differences in thermal expansion between the rotors 8 and the stator 5.

This temperature which is kept high during the idle step 1 also makes it possible to avoid the creation of cold zones where pollutant condensable species could solidify or condense.

The change in temperature setpoint triggered by monitoring the pumping load also allows the method to be highly responsive.

This monitoring may furthermore be performed on the basis of the information already available from the sensors of the vacuum pump 2, by incorporating the thermal behaviour of the vacuum pump 2 into the determination of the temperature control, without the need to add additional temperature sensors, without information on the process taking place in the chamber 3 and without changing the positioning of the at least one temperature sensor 12a, 12b or the structure of the cooling elements 11a, 11b.

Claims

1.-10. (canceled)

11. A method for controlling a temperature of a vacuum pump of a dry vacuum pump type subjected to variable pumping loads, the vacuum pump comprising: a stator,at least one pumping stage,two shafts extending into the at least one pumping stage and respectively bearing at least one rotor, the at least one rotor being configured to rotate synchronously in opposite directions in the stator and to drive a gas that is to be pumped from a suction inlet of the vacuum pump to a delivery outlet,at least one cooling element coupled to the stator,at least one temperature sensor configured to measure a temperature of the stator, anda controller configured to control the temperature of the stator by means of the at least one cooling element and of the at least one temperature sensor; andthe method comprising: controlling the temperature of the vacuum pump by means of the at least one cooling element coupled to the stator on the basis of a temperature setpoint and of a measurement of the temperature of the stator, andmonitoring whether a value of a parameter indicative of a pumping load, chosen from either a current drawn or a power consumed by the vacuum pump, is below a load threshold and, if the value is below the load threshold, then the temperature setpoint is increased.

12. The method according to claim 11, the method further comprising increasing the temperature setpoint at least for control of the temperature of the vacuum pump by means of a cooling element coupled to a low-pressure pumping stage of the vacuum pump.

13. The method according to claim 11, wherein, after the temperature setpoint is increased, the monitoring monitors whether the value is above the load threshold and, if the value is above the load threshold, then an increased temperature setpoint is maintained for a predefined additional length of time.

14. The method according to claim 13, wherein the predefined additional length of time is greater than ten minutes.

15. The method according to claim 11, wherein an increase in the temperature setpoint is greater than 3° C.

16. The method according to claim 11, wherein an increase in the temperature setpoint is less than 20° C.

17. A vacuum pump of a dry vacuum pump type, comprising: a stator;at least one pumping stage;two shafts extending into the at least one pumping stage and respectively bearing at least one rotor, the at least one rotor being configured to rotate synchronously in opposite directions in the stator and to drive a gas that is to be pumped from a suction inlet of the vacuum pump to a delivery outlet;at least one cooling element coupled to the stator;at least one temperature sensor configured to measure a temperature of the stator; anda controller configured to control the temperature of the stator by means of the at least one cooling element and of the at least one temperature sensor, andimplement a temperature control method according to claim 11.

18. The vacuum pump according to claim 17, wherein the vacuum pump is a rough vacuum pump.

19. The vacuum pump according to claim 17, further comprising two cooling elements coupled to the stator, one cooling element being arranged at each axial end of the vacuum pump.

20. A vacuum pump system, comprising: a chamber; anda vacuum pump of the dry vacuum pump type according to claim 17,wherein the vacuum pump is connected to the chamber and configured for pumping in the chamber.