Vacuum pump and method for temperature control of a vacuum pump

Abstract

A vacuum pump 10 having a high temperature zone 12, 12' and a low temperature zone 14, 14', the vacuum pump 10 comprising a stator 11 defining at least one pumping chamber 13; at least one rotor 131 a

Description

The present invention relates to a vacuum pump and a method for temperature control of a vacuum pump. Temperature control or temperature management is an important factor in vacuum pump operation, particularly in dry vacuum pump operation. In conventional dry vacuum pumps, heaters are attached to the stator to heat the stator to a required set point temperature. A heater module may be used to control the heaters to switch on and off as required to maintain the set point temperature. Furthermore, coolers, for example cooling blocks, are attached to the stator to assist with maintaining the stator temperature when the temperature exceeds the set point temperature. The cooling blocks may interface with one or more cooling pipes transporting a coolant fluid, such as water. More specifically, during the startup of a vacuum pump, the heaters are used to heat the stator up to the set point temperature. This is because the rotor, which is compressing gas within the vacuum pump, tends to heat up quickly and expand. The stator however does not heat up at the same rate. Without the application of the additional heating to the stator using the heaters, there tends to be a risk of rotor-stator contact. The heaters are turned off when the temperature of the stator is close to the set point temperature i.e., a set degree below the set point temperature. By way of example, if the set point temperature is 150°C, the heaters may be controlled to turn off at 145°C. Despite the heaters being turned off, if the heat generated during operation of the vacuum pump (i.e., through compression of gases) entering the stator exceeds the natural heat loss from the stator to the surrounding environment, the temperature of the stator may still increase. Hence, once the temperature exceeds the set point, the coolers may be operated in an attempt to cool the stator and mitigate the temperature from rising further. In addition, with the heaters turned off, the heat generated during operation of the vacuum pump that enters the stator may be less than the natural heat loss from the stator to the surrounding environment, resulting in the temperature of the stator decreasing for instance below the set point temperature. Consequently, the heaters may need to be operated again to bring the temperature of the stator back to the set point. Generally, this is how temperature control about a set point temperature is achieved for a conventional vacuum pump. When a conventional vacuum pump is operated, most of the gas compression occurs proximate the exhaust region making it significantly hotter than the inlet region of the vacuum pump. This tends to result in a temperature differential between the exhaust region and inlet region i.e., a temperature gradient between the two regions or'zones'. These temperature gradients tend to result in uneven thermal expansion of the stator as the differential heating conducts through the stator. This tends to lead to distortion of the stator itself. Distortion of the stator is highly undesirable as it can result in changes in clearances between the rotating components of the vacuum pump and the stator, and hence affect performance of the vacuum pump. Depending on the severity of distortion, this may even result in rotor-stator contact and eventually seizure of the vacuum pump. In order to reduce temperature gradients in a vacuum pump, multiple zone heaters may be attached around the stator. This approach to temperature control requires a plurality of active heat control devices, particularly heaters and / or coolers. Resultantly, extensive cabling is necessary. In a more specific example, in prior art temperature control systems, heaters may be positioned at the inlet region of a stator (which may be the region from where the gas enters the pumping chamber) and at the exhaust region of the stator (which may be the region from where the compressed gas exits the pumping chamber) of a vacuum pump. However, the exhaust region can also get exposed to a greater amount of additional heat generated through compression of gases, than the inlet region, during operation of the vacuum pump. This tends to result in the exhaust region becoming hotter than the inlet region. Some of the heat at the exhaust region is conducted via the stator to the inlet region, but for some vacuum pumps, this conduction alone is not sufficient to equalize the temperatures and prevent a temperature differential from becoming established. Temperature measurement means such as a thermistor tend to be located proximate or in the exhaust region and detect when the temperature in this region exceeds a set point temperature. Although it would be ideal to attach more than a single thermistor in a vacuum pump, this tends not to be feasible owing to factors including cost and servicing. Hence the control of all heaters and coolers tends to be based on data collected by one thermistor in the exhaust region. Once the stator has reached the desired set point temperature, the thermistor sends data to a controller which then signals a solenoid valve to open allowing the flow of cooling water into the coolers (i.e., cooling blocks). It should be noted that when it's referred that the temperature of the stator has reached a set point, it is the location where the thermistor is located in / near the exhaust region that has reached that set point temperature (not the whole stator). A set degree before the set point is reached, the heaters are turned off. If the heat generated during gas compression entering the stator is more than the heat loss from the stator to the surrounding environment, the temperature of the stator will continue to rise. Consequently, the heaters will remain switched off. Once the temperature exceeds the set point temperature, coolers will be operated and the bottom of the stator will be cooled to bring it back to the set point temperature or at least mitigate further temperature rise. At this instance, a temperature differential already exists between the exhaust region and the inlet region of the stator. The temperature delta exists during start up even when both of the inlet region and the exhaust region are heated by heaters. This is because the exhaust region also experiences additional heat generated during compression, some of which can't be conducted effectively via the body of the stator to low temperature zones. The temperature delta remains even after the stator has reached it's set point, and the heaters have been turned off, with the temperature continuing to rise owing to compression heat, which is then limited by the coolers / cooling water. The temperature delta exists here because the exhaust region is still being heated from compression heat, but the inlet region is not receiving additional heat from the heaters. Expressed differently, at this instance the exhaust region is being cooled by dumping heat via the coolers (i.e., into a coolant such as cooling water), while the inlet region is requiring additional heat. It is apparent that useful heat is being wasted that could otherwise be exploited to reduce the temperature differential. Thus, it is an object of the present invention to provide a vacuum pump and a method for temperature control of a vacuum pump. In a first aspect, there is provided a vacuum pump having a high temperature zone and a low temperature zone, the vacuum pump comprising: a stator defining at least one pumping chamber; at least one rotor arranged within the at least one pumping chamber; and a heat transfer device comprising at least one heat pipe for transferring heat from the high temperature zone to the low temperature zone, wherein the at least one heat pipe comprises: a first side arranged to receive heat from the high temperature zone via the stator and / or to receive heat via the at least one rotor; a second side arranged to provide said heat to the low temperature zone via the stator and / or via the at least one rotor; and a working fluid sealed within the at least one heat pipe; such that in-use the heat can be transferred from the high temperature zone to the low temperature zone by evaporation of the working fluid at the first side and condensation of the working fluid at the second side of the at least one heat pipe. In a second aspect, there is further provided a method of temperature control of a vacuum pump according to the first aspect, by transferring heat from the high temperature zone to the low temperature zone using thermal conductivity, the method comprising: introducing heat into the high temperature zone; evaporating the working fluid at the first side of the at least one heat pipe using heat from the high temperature zone; and condensing the working fluid at the second side of the at least one heat pipe thereby transferring heat to the low temperature zone. The first side may be referred to herein as comprising the evaporator side or the evaporator. The second side may be referred to herein as comprising the condenser side or the condenser. The vacuum pump of the invention preferably is a dry vacuum pump, more preferably a dry vacuum pump for semiconductor and electronics manufacturing. Particularly, the vacuum pump is a one or multi stage pump. The vacuum pump comprises a stator-rotor arrangement. The vacuum pump may be referred to as a booster pump. The vacuum pump has a high temperature zone and a low temperature zone. The vacuum pump comprises a heat transfer device adapted to transfer heat between the low temperature zone and the high temperature zone. The heat transfer device comprises at least one heat pipe. Preferably, the heat transfer device is adapted to balance the temperature between the low temperature zone and the high temperature zone, for example to reduce the gradient between the low temperature zone and the high temperature zone. "Heat" in accordance with the present disclosure may refer to heat energy. Particularly, the heat transfer device is adapted such that the low temperature zone is implemented as a heat sink, preferably for the high temperature zone. The at least one heat pipe may comprise at least one of: a diode heat pipe, a variable conductance heat pipe, a vapor chamber heat pipe, a flat heat pipe, a thermosyphons heat pipe, a loop heat pipe, an oscillating heat pipe, or a pulsating heat pipe. The heat pipe comprises a first side in thermal contact with the high temperature zone via the stator and / or the at least one rotor. The heat pipe comprises a second side in thermal contact with the low temperature zone via the stator and / or the at least one rotor. The heat pipe may be linear or bent. The heat pipe may be formed from copper. The heat pipe comprises a working fluid sealed within the heat pipe under vacuum. This tends to allow the working fluid to turn to vapor even at relatively low temperatures. The working fluid may be water, for instance, although another refrigerant for use at the operating temperatures of the vacuum pump may be used. For example, alcohol based working fluid such as Ethanol. Generally, a heat pipe tends to be a very good conductor of heat. At the first side (the evaporator side), the working fluid sealed within the heat pipe is evaporated using heat conducted via the stator and / or the at least one rotor from the high temperature zone. The vapor travels along the heat pipe to the second side (the condenser side), which is cooler than the evaporator side, owing to the thermal contact with the low temperature zone via the stator and / or at least one rotor. The vapor condenses to a liquid at the second side of the heat pipe, releasing latent heat to the stator and / or the at least one rotor and thus through conduction to the low temperature zone. The working fluid then returns to the first side of the heat pipe where the cycle is repeated. The working fluid returning to the first side of the heat pipe may be as a result of capillary action, centrifugal force or gravity, for instance, depending on the configuration of the vacuum pump. The efficiency of heat pipes tends to allow for temperature gradients between the high and low temperature regions to be mitigated i.e., tends to allow for temperatures in the high and low temperature regions to become balanced. This tends to provide improved heat transfer compared to relying solely on the conduction via the stator of the vacuum pump, particularly where the stator size is large, or where the stator comprises a material having a relatively low thermal conductivity. In this regard, a stator comprising a relatively low thermal conductivity material may result in a large temperature delta developing in the stator. A high thermal conductivity material is not always the most suitable material for a stator owing to other material properties such as corrosion resistance, manufacturability, cost of materials, for instance. By providing the heat pipes the temperature delta in the stator can be addressed, mitigating differential thermal expansion of the stator elements. The high temperature zone may correspond to a high-pressure zone and / or an outlet area of the vacuum pump. The low temperature zone may correspond to a low-pressure zone and / or an inlet area of the vacuum pump. The high temperature zone can experience additional heat energy generated from gas loading during operation of the vacuum pump when compared to the low temperature zone. Without intervention, this can result in temperature differences between the high and low temperature zones. The inventors have found this temperature differential can be mitigated by the vacuum pumps and methods described herein, more specifically by the use of heat pipes. More specifically, the temperature differential tends to be reduced to less than or equal to 10°C, more preferably less than or equal to 5°C, even more preferably less than or equal to 1°C. The at least one heat pipe may be a passive device. Thus, no electric energy tends to be required to transfer heat. The heat transfer device, particularly the at least one heat pipe, may have a thermal conductivity of 1,000 to 100,000 W / (m*K), preferably of 1,500 to 50,000 W / (m*K) The thermal conductivity tends to be largely a result of the heat transfer by the working fluid within the at least one heat pipe. The use of a high thermal conductivity material such as copper for the heat pipe can further improve the thermal conductivity. The low temperature zone and the high temperature zone may be in, the at least one pumping chamber of the vacuum pump. The low temperature zone and the high temperature zone may be in the same pumping chamber of the vacuum pump, for example in the chamber of one stage of the of the vacuum pump. The high temperature zone and low temperature zone may correspond to parts of the stator and / or rotor. The vacuum pump comprises a stator defining the at least one pumping chamber of the vacuum pump. The pumping chamber particularly corresponds to the interior of the stator. The at least one rotor is arranged within the at least one pumping chamber. The at least one rotor may be at least partly arranged inside the chamber. The at least one heat pipe may comprise at least one first heat pipe. The first side of the at least one first heat pipe may be arranged to receive the heat from the high temperature zone of the stator. The second side of the at least one first heat pipe may be arranged to provide the heat to the low temperature zone of the stator. The at least one first heat pipe may be arranged to extend at least partially through at least one first conduit. The at least one first conduit may be arranged in the stator and / or in at least one heat spreader that is attached to the stator. The heat spreader may be attached to the stator using screws. The thermal conductivity between the heat spreader and the stator may be aided by the use of a thermal gasket located between the heat spreader and the stator. The at least one heat spreader may connect the at least one first heat pipe to the outside of the stator for transferring heat between the at least one first heat pipe and the stator. In view of this, it is preferred that the heat transfer device is connected, particularly attached, to the stator, for example the outside of the stator. "Connected" in the present disclosure may correspond at least to a thermal conducting connection, for example such that heat is transferred between the heat transfer device and the stator body. A thermal conducting connection may be achieved using a thermal gasket or heat transfer mat, for instance. The first side (i.e., the evaporator section) of the at least one first heat pipe, particularly of each first heat pipe, may be connected to the stator in the vicinity of the high temperature zone and the second side (i.e., the condenser section) of the first heat pipe / s is connected to the stator in the vicinity of the low temperature zone. The first and second side of the at least one first heat pipe may be opposite longitudinal or axial sides, particularly longitudinal or axial opposite ends, of the at least one first heat pipe. The connection between the at least one first heat pipe and the stator may be a direct connection for instance using a soldered or welded connection. An intermediate section between the first and second ends of the at least one first heat pipe may not be connected to the stator. The region of the stator of the vacuum pump adjacent the intermediate section of the at least one first heat pipe may be referred to as an intermediate zone of the stator / vacuum pump. It will be appreciated that the intermediate zone may not be directly connected to the at least one first heat pipe. By way of example, the intermediate zone of the stator may be thermally insulated from the at least one first heat pipe by, for example, an air gap and / or a thermal insulator medium. The heat spreader may be a heat spreader bar or block of material. The heat spreader preferably comprises aluminum and / or copper which offer relatively good thermal conduction properties. The first conduit / s through which the at least one first heat pipes extend may be arranged in the stator only, in the heat spreader / s only, or in a combination thereof. For instance, the first heat pipes may extend externally around the stator and be received at the first end and second end of the first heat pipes into respective conduits in heat spreaders attached to the stator. In such examples, the stator itself does not require specific adaptation and the heat transfer device may be retrofitted to the stator. In a further example, the first conduit / s extend only through the stator and no heat spreaders are used. This tends to avoid additional components such as heat spreaders and further mitigates increases in the overall spatial size of the vacuum pump. In a further example, the first conduit / s extend through the stator and one or more heat spreaders that are countersunk into the stator. This tends to combine the enhanced heat spreading effect of the heat spreaders whilst mitigating increases in spatial size of the vacuum pump. Furthermore, locating the heat spreaders within the stator also allows for easier fitment of other components such as o-rings and thermal grease. The at least one first heat pipe may provide a clearance fit with an interior surface of the at least one first conduit. During use, when the at least one first heat pipe expands, the outer diameter of the at least one first heat pipe may therefore come into contact with the interior surface of the at least one first conduit. This tends to hold the at least one first heat pipe in place. To aid with thermal conduction between the at least one first heat pipe and the interior surface of the at least one first conduit, a thermal paste may be applied to the exterior surface of the at least one first heat pipe before locating the at least one first heat pipe within the at least one first conduit. Alternatively, a first gap may be provided in a radial direction of the at least one first heat pipe between the at least one first heat pipe and the interior surface of the at least one first conduit. The first gap extends at least partially, or fully, along the at least one first heat pipe between the first side and the second side. The first gap may provide a clearance between the at least one first heat pipe and the interior surface of the conduit adjacent an adiabatic region of the heat pipe, such that the adiabatic region does not come into direct contact with the interior surface of the conduit. The first gap may extend to the first side and / or the second side of the at least one first heat pipe. A thermal grease may be arranged at one or more of the first side and the second side between the first heat pipe and the interior surface of the first conduit. This tends to provide a thermal contact between the first heat pipe and interior surface of the conduit at the first side and / or second side. Furthermore, the thermal grease tends to act as a lubricant, minimizing wear during thermal expansion and contraction. A reservoir may be provided in fluid connection with the first conduit at one or both of the first side and the second side. The fluid reservoir contains a supply or thermal grease for supplying thermal grease to the first conduit. The reservoir may be arranged internally to the stator or heat spreader. The reservoir may be arranged externally to the stator or heat spreader. The vacuum pump may further comprise a piston seal arranged between the at least one first heat pipe and the at least one first conduit. The piston seal tends to contain the thermal grease and mitigates the thermal grease from drying out. The piston seal tends to be arranged where the at least one first heat pipe interfaces with the at least one first conduit. A second gap may be provided in an axial direction of the at least one first heat pipe. The second gap may be arranged between either the first side and a respective end of the first conduit, the second side and a respective end of the first conduit, or both. The second gap allows for axial expansion of the at least one first heat pipe during use. Whilst the at least one first heat pipe may be fixed to the stator and / or the at least one heat spreader at either of both of the first side and second side (i.e., through soldering), alternatively, the at least one first heat pipe is slidably received into the at least one first conduit. The sliding arrangement allows for tolerances in expansion from heating and cooling of the heat pipes, the heat spreader and the stator, to be accommodated. This tends to mitigate stress buildup and buckling of the at least one first heat pipes. One of the first side and the second side of the at least one first heat pipe may be fixedly attached to the at least one heat spreader. The fixedly attaching of the first heat pipe may be achieved through soldering or welding providing improved conductivity over, for instance, a slidable connection using thermal grease. When the at least one first heat pipe is not fixedly attached to the heat spreader, the second gap may be arranged between the first side and / or the second side, and the respective end of the at least one first conduit. By providing also the second gap, the axial expansion of the first heat pipe is accommodated. Hence these particular examples balance the various features described herein into an optimum solution. A heat pipe generally comprises a wick structure. The wick structure comprises capillaries to move the working fluid as a liquid from the condenser section to the evaporator section. The wick structure may comprise arteries, bi-dispersed sintered powder and / or composite wick structures. The one of the first side and the second side fixedly attached to the at least one heat spreader may further extend through the heat spreader so as to protrude externally to the stator and heat spreader. The protruding part may be the non-wicked part of the first heat pipe. The protruding part may comprise a section of the first heat pipe that is pressed flat for ease of attachment. This tends to enable the wicked part of the first heat pipe to be arranged within the heat spreader which tends to provide more efficient heat transfer. The vacuum pump may comprise a cooler, particularly a water cooler, connected to the heat transfer device. The cooler may comprise one or more cooling blocks. Preferably, the cooler provides external cooling. In conventional vacuum pumps, the cooling blocks are attached to the heat spreader of the heat transfer device at the hotter region i.e. the heat spreader bar at the evaporator end of the heat transfer device. When the thermistor situated near the exhaust region of the stator detects a temperature rise above the set point temperature, it sends data to the controller which then signals a solenoid valve to open allowing the flow of cooling water into the cooling blocks. This tends to result in large temperature fluctuations in one or more cooling pipes of the cooler, and a consequential risk of thermal cracking of cooling pipes. The inventors have found that by using the vacuum pump and methods described herein, heating elements on the stator or heat spreaders adjacent the low temperature zone (i.e., adjacent the inlet region) can be removed. The wattage of the heating elements on the stator or heat spreaders adjacent the high temperature zone may be increased (i.e., doubled) if required. Increasing the heating element wattage will result in even higher heat spreader temperatures at the hotter region. To mitigate the risk of thermal cracking and calcification, the cooling blocks may be attached either directly to the stator adjacent the low temperature zone and / or onto / with the spreader bar on the stator adjacent the low temperature zone. The vacuum pump may comprise a heater connected to the heat transfer device. The heater may be an active heater, for example an electric heater. The heater preferably provides external heating. Particularly, the heater is arranged on the stator adjacent the high temperature zone. The heater may be connected to the first side of the heat transfer device on the stator, particularly of the at least one first heat pipe. The heater may be directly connected to the heat transfer device. For instance, the heater may be received into a groove or channel of the heat transfer device. The diameter of the channel or groove may allow for radial expansion of the heater. This tends to allow for the heater, when operating at a working temperature, to have good thermal contact with the channel or groove. Improved thermal conductivity can be achieved by using a thermal paste in the channel or groove. The vacuum pump may comprise heaters only adjacent the high temperature zone. The one or more heaters may be adapted to be able to reach the desired temperature for the whole stator. The heater may have a power in the range of 100 - 600 W, or in excess of 600W. In particular, the heater may be a cartridge heater. Being able to locate heaters on the stator or heat spreaders only adjacent the high temperature zone tends to be achievable owing to the use of the first heat pipes to transfer heat. This tends to reduce the overall number of heaters, saving energy and cost. The heater and / or the cooler may be operated, for instance activated and / or deactivated, using one or more sensors, for example temperature sensors, of the vacuum pump. Operation of the heater and / or the cooler can be achieved using a thermistor and / or a control unit of the vacuum pump. The heat transfer device may comprise at least one thermal insulator, such as an air gap. The thermal insulator may be arranged between the at least one first heat pipe and the stator adjacent an intermediate zone of the vacuum pump, wherein the intermediate zone is a zone between the low temperature zone and the high temperature zone. The thermal insulator preferably provides the thermal insulation. During use of a vacuum pump, particularly multi-stage vacuum pumps, the rotor tends to provide greater compression to gases at one end (i.e., in the outlet pump chamber) of the rotor when compared to an opposite end (i.e., in the inlet pump chamber). This tends to create respective high temperature and low temperature zones owing to the different levels of gas compression. Accordingly, even though the rotor conducts some heat along the rotation axis between the zones, a temperature gradient still tends to prevail. Accordingly, the low temperature zone and the high temperature zone may comprise respective zones separated along a rotation axis of the rotor. The at least one heat pipe may comprise at least one second heat pipe arranged concentrically within the rotor and extending along the rotation axis for rotation with the rotor, wherein the at least one second heat pipe comprises an internal bore that tapers from the first side to the second side. By providing the at least one second heat pipe, heat tends to be transferred along the rotation axis between the high and low temperature zones. During use, the working fluid of the at least one second heat pipe tends to be evaporated at the first side and propagate to the second side where the vapor condenses to liquid. Owing to the rotation of the at least one second heat pipe, the liquid tends to be thrown radially outwards (under the centrifugal force) to the internal sides of the bore of the at least one second heat pipe. The tapering of the bore tends to ensure that a component of the centrifugal force acts to force the working fluid back towards the first side where the evaporation process can repeat. Accordingly, differential heating of the rotor can be mitigated. This allows for tighter control of the clearances of the rotor which improves pumping performance. The at least one second heat pipe may extend through at least one second conduit in the rotor. A thermal epoxy may be provided into the at least one second conduit prior to insertion of the at least one second heat pipe. The at least one second heat pipe may be pushed into the thermal epoxy in the second conduit. The thermal epoxy tends to settle, harden and hold the at least one second heat pipe in place. The thermal epoxy also tends to provide good thermal conductivity between the at least one second heat pipe and the at least one second conduit. Such a design tends to provide a simpler approach without requiring the use of sleeves, pinch points, or interference fits as will be later described. Alternatively, a third gap may be provided in a radial direction of the at least one second heat pipe between the at least one second heat pipe and the interior surface of the at least one second conduit. The third gap may extend at least partially along the at least one second heat pipe between the first side and the second side of the at least one second heat pipe. The third gap tends to ensure the adiabatic region of the at least one second heat pipe does not come into contact with the rotor. At least one metal sleeve may be arranged within the at least one second conduit for receiving the at least one second heat pipe. The respective metal sleeves may be arranged at the first side and / or the second side of the at least one second heat pipe. The metal sleeves provide a thermal connection between the second heat pipe and the rotor at the first and / or second sides of the second heat pipe. The metal sleeves may be split metal sleeves to accommodate thermal expansion during use. The metal sleeves may comprise of copper for thermal conductivity. When the rotor of the vacuum pump rotates, the at least one second heat pipe may have a tendancy to bend owing to the centrifugal force acting on the at least one second heat pipe in the unsupported region between the first side and the second side of the second heat pipe / s. This may be particularly the case if the at least one second heat pipe is not perfectly concentric with the rotation axis of the rotor. Accordingly, one or more supports may be arranged between the at least one second heat pipe and the second conduit, wherein the one or more supports are arranged at one or more locations between the first side and the second side of the at least one second heat pipe. The one or more supports mitigate the distortion or bending of the second heat pipe / s owing to the centrifugal forces. The one or more supports may comprise a ring-like protrusion encircling the at least one second heat pipe. The radially outer surface of the ring-like protrusion may be in contact with the inner surface of the second conduit thereby provided support for the at least one second heat pipe. The width of the ring-like protrusion may be specified to balance the physical support to the at least one second heat pipe with any additional heat conduction in the adiabatic region through the ring-like protrusion. The one or more supports may be attached to the at least one second heat pipe through an interference fit or welding, for example. The method of temperature control of a vacuum pump is preferably a method for controlling a vacuum pump with one or more features of the vacuum pump according to the first aspect. According to the method, the heat is transferred between a high temperature zone and a low temperature zone using thermal conductivity, for example, using a thermal conductivity means. The heat transfer is particularly implemented to balance the temperature between the high temperature zone and the low temperature zone, and / or to reduce the temperature gradient between the high temperature zone and the low temperature zone. Particularly, the thermal conductivity is adapted such that the low temperature zone is implemented as a heat sink, preferably for the high temperature zone. The thermal conductivity preferably is a high thermal conductivity. Particularly the thermal conductivity provides 1,000 to 100,000 W / (m*K), preferably 1,500 to 50,000 W / (m*K). The method may comprise introducing heat into the high temperature zone. The heat may be introduced using a heater. The heat may be introduced to achieve a set point temperature, preferably a minimum operating temperature in the pumping chamber, at least in the high temperature zone. The heat may be introduced as a result of gas compression during operation of the vacuum pump as the gas has to be compressed to over atmospheric pressure so it can exit the vacuum pump via the exhaust. This tends to result in higher temperatures in the high temperature zone relative the low temperature zone. The heat may be introduced owing to a combination of heating using heaters and the gas loading. The heating may be transferred from the high temperature zone to the low temperature zone using the at least one heat pipe. Optionally, the method may further comprise introducing external cooling into the low temperature zone. Particularly, the external cooling may be introduced using a cooler, such as a water cooler. Heat may be transferred to the cooler. The use of the heaters and / or coolers may depend on the conditions at a given time for the vacuum pump. For instance, a vacuum pump may have a set point temperature for operation. The set point temperature may be reached by operation of the heaters alone, the heat generated from gas loading alone, or a combination thereof. A set degrees before the set point is reached, the heaters are turned off. The natural heat loss to the environment via conduction and convection through or from the stator may be less than the heat energy generated during vacuum pump operation that enters the stator. In this case, temperature of the stator will continue to rise, and accordingly, cooling will be required to mitigate the temperatures of the vacuum pump exceeding the set point temperature. In some environments however, natural heat loss to the environment via conduction and convection through or from the stator may exceed the heat energy generated during vacuum pump operation that enters the stator. In this case, the temperature of the stator will drop below set point and heaters will be activated again. In such a case, temperature control can be maintained without utilizing the cooling blocks. It will be appreciated that the vacuum pump according to the first aspect may comprise one or more features described in connection with the method of the second aspect; and vice versa. The vacuum pump and / or the method, tend to provide one or more of the following advantages: - reduction of heater components, particularly active heater components, such as of heater cartridges; - reduction of the temperature in the high temperature zone, and / or increase of the temperature in the low temperature zone, and / or reduction of the temperature gradient between the high temperature and low temperature zone; - reduction of cables and / or control units, particularly for active heating - reduction, preferably elimination of the risk of cooler calcification and / or thermal cracking; - increase of performance, particularly pumping performance; - increase of heat control efficiency, particularly in moving heat; - increased efficiency owing to reduction of energy and / or water usage for cooling and / or heating, improved heat recovery and reduced wastage. Increased performance tends to be achieved if back leakages in the rotor are reduced by reducing the rotor-stator clearances, especially in the exhaust stages. The rotor-stator clearances on each stage of the rotor are provided based on the temperature of the rotor and stator in that stage. Higher temperature will require larger clearances at start up to allow for greater thermal expansion. The heat pipes improving the heat transfer in the rotor from exhaust stages to inlet stages tends to enable a reduction in these clearances on the exhaust stages, thereby reducing the back leakages and consequently improving performance of the vacuum pump. In the following the present invention is described in more detail with reference to the accompanying drawings. The figures show: Figure 1 a schematic sectional view of part of a vacuum pump of the state of the art. Figure 2 diagrams of heat distribution in a stator of a vacuum pump. Figure 3 a schematic sectional view of an embodiment of a vacuum pump according to the invention. Figure 4 diagrams of heat distribution in the stator of the vacuum pump of figure 3. Figure 5 a schematic side view of a further embodiment of a vacuum pump according to the invention. Figure 6 a schematic side view of a section of a further embodiment of a vacuum pump according to the invention. Figure 7 a schematic side view of a section of a further embodiment of a vacuum pump according to the invention. Figure 8 schematic side views of a section of further embodiments of a vacuum pump according to the invention, showing a first heat pipe slidably received into a heat spreader. Figure 9 a schematic side view of a section of a further embodiment of a vacuum pump according to the invention, showing a first heat pipe extending through a stator. Figure 10 schematic side views of a section of further embodiments of a vacuum pump according to the invention, showing a first heat pipe extending through a stator and a thermal grease reservoir. Figure 11 a schematic side view of a section of a further embodiment of a vacuum pump according to the invention, showing a first heat pipe extending through a stator between two countersunk heat spreaders. Figure 12 a schematic side view of a section of a further embodiment of a vacuum pump according to the invention, showing a first heat pipe extending through a stator with only one countersunk heat spreader. Figure 13A a schematic side view of a section of a further embodiment of a vacuum pump according to the invention, showing a second heat pipe arranged concentrically within a rotor. Figure 13B a schematic partial side view of a section the embodiment of Figure 13A showing a sleeve. Figure 13C a schematic partial side view of a larger section of the embodiment of Figure 13A. Figure 14 a flow diagram of a method of temperature control of a vacuum pump according to the invention. Figure 1 shows a configuration of a vacuum pump, particularly a dry vacuum 10' pump of the state of the art. The vacuum pump 10' comprises a stator 11 defining a pumping chamber 13, particularly for receiving at least one rotor (not shown) inside. The vacuum pump 10' has a high temperature zone 12 particularly corresponding to an outlet area and / or to a high-pressure zone of the pumping chamber 13. Further, the vacuum pump 10' has a low temperature zone 14 particularly corresponding to an inlet area and / or to a low-pressure zone of the pumping chamber 13. Heaters 20 are attached to the outside of the stator 11 to introduce heating to the stator 11. The heaters 20 are heating blocks into which one or more heating filaments are inserted. The heaters 20 are attached to areas of the stator 11 corresponding to / adjacent the high temperature zone 12 and to areas corresponding to / adjacent the low temperature zone 14. The heaters 20 may be switched on or off depending on operating conditions of the vacuum pump 10'. For example during gas loading, the compression of gases tends to rapidly heat the at least one rotor (now shown), however the stator is not as rapidly heated. If the stator 11 is not heated using external heat i.e. via heaters 20, the stator will not thermally expand at the rate that is required to prevent rotor stator contact. However, the heaters 20 tend not to be operated constantly. Indeed, the heaters 20 may be turned off a set degrees below the set point temperature, and remain off particularly if the compression of gases generate sufficient heat to cause the temperature of the stator 11 to exceed the set point temperature. Furthermore, coolers 22, for example cooling blocks, are attached to the heater blocks 20 that are attached to the stator 11 in areas of the stator 11 corresponding to / adjacent the high temperature zone 12 to provide cooling to the stator 11. A condition can arise in the vacuum pump 10' whereby there is a temperature gradient across the stator 11. This tends to arise because of differential heating of the high temperature zone 12 and the low temperature zone 14. By way of example, more gas compression near the exhaust region or the high temperature zone 12 during vacuum pumping can cause more heat to be generated in the high temperature zone 12, whereas due to less compression in the inlet region or in the low temperature zone 14, the temperature in the low temperature zone 14 can be less compared to the high temperature zone 12. As previously discussed herein, temperature gradients across the stator 11 can have undesirable consequences for vacuum pump operation. In figure 2, diagrams showing the heat distribution in the vacuum pump of figure 1 are provided. The darker zones indicate areas with higher temperatures. The shading in Figure 2 to illustrate heat distribution does not apply to the cooling blocks. Where the heaters in Figure 2 are shaded darker than the stator, the heaters are to be understood as being switched on. Where the heaters in Figure 2 are shaded lighter than or the same color as the stator, the heaters are to be understood as being switched off. Figure 2 a) shows the thermal profile of a stator when all heaters are off, for example after the stator has been brought to set point temperature. Owing to the gas loading, a higher pressure and higher amount of heat is generated in the high temperature zone 12 relative to the lower pressure and lesser amount of heat generated in the low temperature zone 14. Accordingly, a temperature gradient exists between the high temperature zone 12 and the low temperature zone 14. The coolers 22 may be switched on in the example of Figure 2a, when the stator temperature gets above the set point Figure 2 b) Shows the thermal profile of a stator when all heaters are on, for example during warm up stage, to bring the stator up to the set point temperature. The greater compression in the high temperature zone 12 leads to a high temperature in the high temperature zone 12. However, the temperature gradient across the stator 11 is reduced as indicated by the shading across the stator 11. This is because the heaters 20 in Figure 2b have been switched on to raise the temperature of the stator 11 adjacent the low temperature zone 14 whereas the heaters 20 in Figure 2a adjacent the low temperature zone 14 have not been switched on. It should be noted that the exhaust in Figure 2a is generally hotter than the exhaust in Figure 2b. This is because Figure 2a is representative of a scenario where heaters are not required i.e., there is enough heat being generated from compression to not warrant use of heaters. It should be noted that figures 2a-2b are provided for illustrative purposes. During vacuum pump operation the heaters 20 and coolers 22 may cycle on and off in an attempt to maintain a set point temperature within the vacuum pump. Figure 3 schematically shows an embodiment of a vacuum pump 10 according to the invention. The arrangement shows a stator 11 defining a pumping chamber 13 having a high temperature zone 12, particularly in an outlet area corresponding to a high-pressure zone, and having a low temperature zone 14 particularly in an inlet area and corresponding to a low-pressure zone. The arrangement is similar to figure 1 unless otherwise stated. In contrast to figure 1, in the vacuum pump 10 of figure 3, the coolers 22 are located on the stator 11 in the vicinity of / adjacent the area of the low temperature zone 14 and the heaters 20 are, preferably only, located on the stator 11 in the vicinity of / adjacent the high temperature zone 12. Coolers 22 may comprise cooling blocks. First heat pipes 18 of a heat transfer device 16 are connected, preferably attached, with their first side 17 (the evaporator section side), to the stator 11 in the vicinity of / adjacent the high temperature zone 12 and to the heaters 20. The second side 19, particularly the condenser section side, of the first heat pipes 18 are connected, preferably attached, to the stator 11 in the vicinity of / adjacent the low temperature zone 14 and to the coolers 22. The first heat pipes 18 contain a working fluid, water in this example, that can be evaporated and condensed at the first side 17 and second side 19, respectively. Thus, heat can be transferred between the regions of the stator 11 that are in the vicinity of / adjacent the low temperature zone 14 and the high temperature zone 12 by means of the heat transfer device 16, particularly the first heat pipes 18. The heat transfer device 16 may be adapted to even out a temperature gradient across the stator 11. Particularly, the heat transfer device 16, i.e., the first heat pipes 18, are adapted and / or arranged such that the region of stator 11 in the vicinity / adjacent the low temperature zone 14 is implemented as a heat sink for the region of the stator 11 in the vicinity / adjacent the high temperature zone 12. Figure 3 shows bent first heat pipes 18. Nevertheless, is also possible that at least one first heat pipe 18 is linear. The heaters 20 may have a higher power compared to the heaters 20 of figure 1. The heaters 20 may be adapted to reach the desired temperature for the whole stator 11. Figure 4 shows heat distribution diagrams similar to those of figure 2 but for the vacuum pump of figure 3. Figure 4 a) shows a thermal profile of a stator when heaters are not required for example after the set point has been reached. The heaters 20 may be activated, nevertheless, it is preferred that the heaters 20 are turned off, at least if the compression gas heat power that enters the stator is greater than the heat loss from the stator to the surrounding environment. Furthermore, the coolers 22 may be activated to remove excess heat from the stator. The greater compression in the high temperature zone 12 leads to a higher temperature in the high temperature zone 12 compared to the low temperature zone 14. This temperature difference results in a temperature gradient between regions of stator 11 in the vicinity of / adjacent the zones 12, 14. This temperature difference is balanced by the first heat pipes 18, whereby heating is transferred by the first heat pipes 18 from the region of stator 11 in the vicinity of / adjacent the high temperature zone 12 to the region of stator 11 in the vicinity of / adjacent the low temperature zone 14. Thus, particularly in comparison to the heat distribution as shown in figure 2 a), the temperature gradient between the high temperature zone 12 and the low temperature zone 14 is reduced. Figure 4 b) Shows a thermal profile of a stator when heaters are needed for example, when the temperature of the stator 11 is below set point. At startup of the vacuum pump 10, the heaters 20 may be activated to bring the stator 11 up to the set point temperature. Indeed, in Figure 4a the exhaust is hotter than that of Figure 4b and the heaters 20 in Figure 4a are switched off, whereas the exhaust in Figure 4b is cooler than that of Figure 4a and the heaters 20 in Figure 4b are switched on. Furthermore, the greater compression in the high temperature zone 12 leads to a high temperature in the high temperature zone 12 relative to the low temperature zone 14. This heat is transferred by the first heat pipes 18 from the region of stator 11 in the vicinity of the high temperature zone 12 to the region of the stator 11 in the vicinity of the low temperature zone 14 (and consequently, assists in balancing the temperature gradient across the stator 11 and across the zones 12, 14). The coolers 22 tend not to be required during the operation of the vacuum pump as shown in Figure 4b. The location of the coolers 22 on the stator 11 mitigates the risk of cooler calcification. It is evident that in Figure 4b the heater 20 is shaded darker and hence is hotter than the heater 20 of Figure 2b. This is because Figure 4b employs two heaters instead of the four heaters as shown in 2b, and so, the heater 20 of Figure 4b has a higher wattage than the heater 20 of Figure 2b. It is also evident that the temperature difference across the stator 11 in Figure 4b is less than the temperature difference across the stator 11 in Figure 2b. This is owing to the use of the heat pipes 18 in Figure 4b. Figure 5 schematically shows a further embodiment of a section of a vacuum pump 10 according to the invention. A heat transfer device 16 comprising three first heat pipes 18 is connected to a stator 11 of the vacuum pump 10. It should be noted that greater than or less than three heat pipes 18 may be used. The first side 17 of a heat transfer device 16 is attached via a heat spreader 24, for example a heat spreader bar, to the high temperature zone 12 via the stator 11. Further, the second side 19 of a heat transfer device 16 is attached via a heat spreader 24, for example a heat spreader bar, to the low temperature zone 14 via the stator 11. The heat spreader 24 may have a thickness of, for instance 30-40mm, preferably 34mm. The first heat pipes 18 may have a length of 150-250mm, more preferably 200-250mm, even more preferably 250mm. A portion of the length of the first heat pipes 18 may be embedded within the heat spreader 24. For instance, 5-20mm may be embedded, more preferably 6-18mm, more preferably 6mm or 18mm. The first heat pipes 18 may have a diameter of 10mm. The first heat pipes 18 may comprise copper with a standard thickness of less than or equal to 1mm, more preferably less than or equal to 0.5mm. The sintered material thickness may be 0.4mm. The working fluid within the first heat pipes 18 may be water. An example of a first heat pipe 18 is the Spreadfast SF-10-250-S (sintered, straight). An intermediate zone 23 of the vacuum pump 10 is thermally insulated from the first heat pipes 18, for example by an air gap between the intermediate zone 23 and the heat pipes 18. Figure 6 schematically shows a section of a section of a further embodiment of a vacuum pump 10 according to the invention. A heater 20 is attached to the stator 11 in the vicinity of the high temperature zone 12 via a heat spreader 24. Thermal break plate 26 acts as a thermal break (insulation) between the stator of the vacuum pump and the head plate (not shown). In conventional vacuum pumps, active heating elements are inserted in the thermal break plate to heat the thermal break plate to ensure that it expands at the same rate as the stator and does not distort. Figure 6 shows an arrangement in which the heat pipe can be employed within the thermal break plate instead of an active heating element. A linear, i.e. straight, first heat pipe 18 is inserted in the thermal break plate 26 and attached to the heat spreader 24 to provide heat transfer between the high temperature zone 12 and the low temperature zone 14. A cable 28 provides electricity for heater 20. Figure 7 schematically shows a section of a further embodiment of a vacuum pump 10 according to the invention. Two, preferably linear, first heat pipes 18 are attached to a heat spreader 24, which is connected to the stator 11 in the high temperature zone 12. A heater 20 is connected to the heat spreader 24, whereby the heater 20 may be arranged, at least partly, within the heat spreader 24. Figure 7 shows thermal break plate 26 on each end of the stator, acting as an insulation between stator and head plate (not shown). Figure 7 shows heat pipe 18 employed in each thermal break plate 26, however connected to the same spreader bar 24. The first heat pipes 18 provide heat transfer between regions of the stator 11 that are in the vicinity of / adjacent the high temperature zone 12 and the low temperature zone 14. Figure 8A shows a schematic side view in cross-section of a section of a further embodiment of a vacuum pump according to the invention. A first heat pipe 18 is shown. A heat spreader 24 is also shown connecting the first heat pipe 18 to the outside of the stator 11. The heat spreader 24 enables the transfer of heat between the stator 11 and first heat pipe 18. The arrangement shown in Figure 8 may be used in any of the examples disclosed herein. The heat spreader 24 comprises a first conduit (i.e., a tubular recess) 81 into which the first heat pipe 18 is received. The first conduit 81 is a cylindrical bore into the heat spreader 24. The first conduit 81 has an internal diameter that is larger than the external diameter of the first heat pipe 18 such that the first heat pipe 18 can be received into the first conduit 81. The larger internal diameter of the first conduit 81 provides a first gap 81a between the outer surface of heat pipe 18 and inner surface of first conduit 81. The first gap 81a extends radially from the exterior of the first heat pipe 81 to the interior surface to the first conduit 81. Whilst the first heat pipe 18 is shown as being received into the heat spreader 24 at a first side 17 of the first heat pipe 18, alternatively the second side 19 of the first heat pipe 18 may be received. Alternatively, both the first side 17 and second side 19 of the first heat pipe 18 may be received into respective heat spreaders 24 in the manner shown in Figure 8. The Figure 8 shows an example of a connection between an end of the first heat pipe 18 and the heat spreader 24 and is not intended to be limiting. Related is that whilst the other end of the first heat pipe 18 is not shown connected to a heat spreader 24, this is purely for simplicity of illustration and again does not imply such a first heat pipe 18 is not connected at both ends to separate heat spreaders 24. The connection between the first heat pipe 18 and the heat spreader 24 is a sliding / slidable connection. The first heat pipe 18 can thus move longitudinally along the first conduit 81. This tends to allow for thermal expansion of the first heat pipe 18, the heat spreader 24, the stator 11, to be accommodated. This tends to assist in mitigating buckling of the first heat pipe 18, for instance, as may be experienced in examples where both sides of the first heat pipe 18 are fixed i.e., soldered to respective heat spreaders 24 . Also shown is a thermal grease 82 arranged between the first heat pipe 18 and the heat spreader 24. The thermal grease 82 provides a thermal connection between the first heat pipe 18 and the heat spreader 24, even when the first heat pipe 18 moves longitudinally in the first conduit 81. The thermal grease 82 may also be referred to as a thermal interface material. The thermal grease 82 is a thermally conductive compound and acts to minimize gaps or spaces between the first heat pipe 18 and the heat spreader 24 to enhance heat transfer. The thermal grease 82 may comprise a matrix material and a conductive filler material. The conductive filler material may comprise a metal oxide for example (i.e., Aluminium or Zinc oxide). The thermal grease 82 is arranged between the first heat pipe 18 and an interior surface of the first conduit 81. The thermal grease 82 can be fed into the first conduit 81 via a feeding channel 84. A screw cap 85 may be provided to seal the feeding channel 84. The screw cap 85 may comprise an o-ring for sealing purposes, such an o-ring not being visible in the figure. Also shown are two piston seals 83. The two piston seals 83 extend around the first heat pipe 18. The two piston seals 83 are disposed between the first heat pipe 18 and the heat spreader 24 within the first conduit 81. The piston seals 83 seal the first heat pipe 18 to the heat spreader 24 whilst allowing for a longitudinal sliding motion of the first heat pipe 18 in the first conduit 81. The thermal grease 82 is disposed between the two piston seals 83. The thermal grease 82 is thus contained and sealed between the two piston seals 83. This tends to mitigate loss of the thermal grease 82. This also tends to mitigate the thermal grease 82 from drying out (i.e., being exposed to the exterior / ambient environment / air). In other examples, only one piston seal 83 may be present in order to seal the thermal grease 82 from the exterior. Such an arrangement is shown in Figure 8B where like reference numerals indicate like features of Figure 8A. In other examples, more than two piston seals 83 may be present. The first heat pipe 18 may comprise copper with the heat spreader 24 comprising Aluminium. The sizes and shapes of the first heat pipe 18 and heat spreader 24 are not intended to be limiting. Figure 9 shows a schematic side view of a section of a further embodiment of a vacuum pump according to the invention. A first heat pipe 18 is shown extending through a first conduit 91. The first conduit 91 extends entirely through the stator 11. A first gap 91a exists between the exterior of the first heat pipe 18 and the interior surface of the first conduit 91. The first gap 91a extends radially from the exterior of the first heat pipe 18 to the interior surface of the first conduit 91. At both of the first side 17 and the second side 19 of the first heat pipe 18, a thermal connection with the stator 11 is achieved. The thermal connection is achieved by a thermal grease 92 provided in the first conduit 91 at the first side 17 and second side 19 of the first heat pipe 18. A piston seal 93 comprising a plurality of o-rings is also provided to contain the thermal grease. The first conduit 91 is sealed using plugs / screws 94 arranged at each end of the first conduit 91. Between each plug 94 and the respective first side 17 and second side 19 of the first heat pipe 18 is a second gap 91b. The second gap 91b may be considered to be arranged axially with the first heat pipe 18. The second gap 91b may be arranged at only one end of the first heat pipe 18. Figure 10A shows a schematic side view of a section of a further embodiment of a vacuum pump according to the invention. This particular example tends to allow for easier maintenance of thermal grease. A first heat pipe 18 is shown extending through a first conduit 101. The first conduit 101 extends entirely through the stator 11. A first gap 101a exists between the exterior of the first heat pipe 18 and the interior surface of the first conduit 101. The first gap 101a extends radially from the exterior of the first heat pipe 18 to the interior surface of the first conduit 101. This gap ensures the intermediate region of the heat pipe is insulated from the stator. At both of the first side 17 and the second side 19 of the first heat pipe 18, a thermal connection with the stator 11 is achieved. The thermal connection is achieved by a thermal grease 102a provided in the first conduit 101 at the first side 17 and second side 19 of the first heat pipe 18. A piston seal 103 comprising a plurality of o-rings is also provided to contain the thermal grease 102a. The first conduit 101 is sealed using plugs / screws 104 arranged at each end of the first conduit 101. Between each plug 104 and the respective first side 17 and second side 19 of the first heat pipe 18 is a second gap 101b. The second gap 101b may be considered to be arranged axially with the first heat pipe 18. This allows for the axial expansion of heat pipes. This gap may be provided on side 17 and / or side 19. The thermal grease 102a is supplied to the first conduit 101 through a reservoir 102b that is in fluid connection with the first conduit 101 between the piston seal 103. The reservoir 102b is shown as being within the stator 11 but may alternatively be mounted externally to stator 11. Hence the thermal grease 102a can be maintained or topped up via reservoir 102b. The piston seal arrangement 103 resembles that of Figure 8A, however alternatively the piston seal arrangement of Figure 8B could be used. If using the piston seal arrangement of Figure 8B the thermal grease 102a could be provided into the conduit 101 via the plugs / screws 104 or via a thermal grease reservoir 102b as shown in Figure 10A. This latter arrangement is shown in Figure 10B. Figure 11 shows a schematic side view of a section of a further embodiment of a vacuum pump according to the invention. This particular example tends to allow for easier insertion of the sealing arrangement. A first heat pipe 18 is shown extending through a first conduit 111. The first conduit 111 extends entirely through the stator 11 and through heat spreaders 115. The heat spreaders 115 are countersunk into the stator 11 at either end of the first conduit 111. The heat spreaders 115 may provide a clearance fit with the stator 11 and may be held / retained in the stator using screws or similar means. When the stator 11 gets hot, the heat spreader 115 will expand, the clearance will be eliminated, and the heat spreader 115 will come into contact with the stator 11. To ensure good thermal contact with the stator 11, thermal grease 112a can be employed on the outside surface of heat spreader 115 before inserting it into the stator. A first gap Illa exists between the exterior of the first heat pipe 18 and the interior surface of the first conduit 111. The first gap Illa extends radially from the exterior of the first heat pipe 18 to the interior surface of the first conduit 111. At both of the first side 17 and the second side 19 of the first heat pipe 18, a thermal connection with the respective heat spreader 115 is achieved. The thermal connection is achieved by a thermal grease 112a provided in the first conduit 111 at the first side 17 and second side 19 of the first heat pipe 18. A piston seal 113 comprising a plurality of o-rings is also provided to contain the thermal grease 112a. The first conduit 111 is sealed using plugs / screws 114 arranged at each end of the first conduit 111. Between each plug 114 and the respective first side 17 and second side 19 of the first heat pipe 18 is a second gap 111b. The second gap 111b may be considered to be arranged axially with the first heat pipe 18. at both ends or just one. The thermal grease 112a is supplied to the first conduit 111 through a reservoir 112b that is in fluid connection with the first conduit 111 between the piston seal 113. The reservoir 112b is shown as being within the heat spreaders 115 but may alternatively be mounted externally to heat spreaders 115. Hence the thermal grease 112a can be maintained or topped up via reservoir 112b. The piston seal arrangement 113 resembles that of Figure 8A, however alternatively the piston seal arrangement of Figure 8B could be used. If using the piston seal arrangement of Figure 8B the thermal grease 112a could be provided into the conduit 111 via the plugs / screws 114 or via a thermal grease reservoir 112b as shown in Figure 11. Figure 12 provides a schematic side view of a section of a further embodiment of a vacuum pump according to the invention. This particular example offers fewer components making assembly and disassembly easier. Furthermore, this particular examples tends to offer improved heat conductivity. A first heat pipe 18 is shown extending through a first conduit 121. The first conduit 121 extends nearly entirely through the stator 11 and may be considered a blind hole in the stator 11 that is blocked at one end. The first conduit 121 also extends through heat spreader 125 at the open end of the first conduit 121. The heat spreader 125 is countersunk into the stator 11. The heat spreader 125 may provide a clearance fit with the stator 11 or may be held / retained in the stator using screws or similar means. When the stator 11 gets hot, the heat spreader 125 will expand, the clearance will be eliminated, and the heat spreader 125 will come into contact with the stator 11. To ensure good thermal contact with the stator 11, thermal grease 112a can be employed on the outside surface of heat spreader 115 before inserting it into the stator 11. A first gap 121a exists between the exterior of the first heat pipe 18 and the interior surface of the first conduit 121. The first gap 121a extends radially from the exterior of the first heat pipe 18 to the interior surface of the first conduit 121. At the first side 17 of the first heat pipe 18 a thermal connection with the stator 11 is achieved. The thermal connection is achieved by a thermal grease 122a provided in the first conduit 121 at the first side 17 of the first heat pipe 18. The thermal grease 122a is supplied to the first conduit 121 through a reservoir 122b that is in fluid connection with the first conduit 121. The reservoir 122b is shown as being within the stator 11 but may alternatively be mounted externally to the stator 11. Hence the thermal grease 122a can be maintained or topped up via a feeding channel 122b. Note that the instead of thermal grease 122a, a thermal epoxy may be used. Lubrication is not necessarily required in this region, so a thermal epoxy can be used instead of thermal grease 122a for heat conductivity. At the second side 19 of the first heat pipe at thermal connection with the heat spreader 125 is achieved. The thermal connection is achieved through welding or soldering. This tends to provide improved thermal conductivity between the heat spreader 125 and the first heat pipe 18. Details of soldering techniques are not shown in the Figure ( for instance, a solder feeding channel may be required). The first conduit 121 is sealed by the stator 11 at one end and the welding / soldering to the heat spreader 125 at the other end. Hence additional plugs / screws 114 are not required. Furthermore, between the stator 11 and the first side 17 is a second gap 121b. The second gap 121b may be considered to be arranged axially with the first heat pipe 18 and allows for axial expansion of the first heat pipe 18. The first heat pipe 18 also protrudes beyond the heat spreader 125 to an exterior of the heat spreader 125 and stator 11. The protruding part 18a comprises the non-wicked or flattened part of the first heat pipe 18. This tends to allow the wicked part of the first heat pipe 18 to be in contact with the heat spreader 125 improving further the efficiency of heat conduction from the first heat pipe 18 to the heat spreader 125. Figure 13a shows a schematic side view in cross-section of a section of a further embodiment of a vacuum pump according to the invention, showing a second heat pipe 18' arranged concentrically within a rotor 131. The rotor 131 comprises a plurality of rotor blades 131a corresponding to different pump stages or chambers (as for instance defined by a stator, not shown). The number of rotor blades 131a is not intended to be limiting. Whilst a single rotor 131 is shown, examples of vacuum pumps may comprise two or more of the rotors 131. The rotor 131 may be used in any of the examples disclosed herein. During use of the rotor 131 in a vacuum pump, a low temperature zone 14' and a high temperature zone 12' may arise. The two zones 12', 14' may arise owing to the different compression / pressure experienced along the rotation axis 0 of the rotor 131 (i.e., in the different pump stages / chambers). This can lead to different stresses and strains experienced along the rotor 131 and differential thermal expansion along the axis of the rotor 131. As described herein, it is preferred to minimize thermal gradients within a vacuum pump. Particularly for a rotor 131, thermal gradients arising can result in clearances between the rotor 131 and stator changing. Mitigating thermal gradients tends to enable tighter control of clearances, which minimizes back leakages, and improve pumping performance. The second heat pipe 18' is arranged concentrically within the rotor 131. The second heat pipe 18' extends along the axis 0 of the rotor 131. The second heat pipe 18' is fixed within the rotor 131 such that the second heat pipe 18' rotates with the rotor 131 about axis 0. The fixing may be through welding, adhesive, soldering, or interference, for example. The second heat pipe 18' may be integrally formed with the rotor 131 during manufacture or inserted into the rotor during manufacture . The second heat pipe 18'comprises an internal bore 18a'. The internal bore 18a' tapers from a first side 17' of the second heat pipe 18' to a second side 19' of the second heat pipe 18'. Expressed differently, an internal diameter of the second heat pipe 18' is greater at the first side 17' than at the second side 19'. The internal bore 18a' comprises therein a working fluid such as water. During use, the rotor 131 and the second heat pipe 18' rotate together about axis 0. Heat from the high temperature zone 12' conducts through the rotor 131 to the first side 17' of the second heat pipe 18'. The heat conducts through the first side 17' to evaporate the working fluid within the second heat pipe 18' at the first side 17'. The vapor propagates to the second side 19' of the second heat pipe 18' located in thermal contact with the low temperature zone 14' and where it is cooler. The vapor condenses, giving thermal energy to the second 19' of the second heat pipe 18' and hence through thermal conduction to the rotor 131 and low temperature zone 14'. Owing to the rotation of the second heat pipe 18' with rotor 131, the condensed working fluid is thrown radially outwards from axis 0 under the influence of centrifugal force. The working fluid impacts the interior surface of internal bore 18a'. The tapering of the internal bore 18a' results in a force component forcing the working fluid away from the second side 19' and towards the first side 17'. The evaporation-condensing process can then repeat to assist in balancing the temperature differential between the first side 17' and second side 19'. Figure 13B shows a section of the embodiment of Figure 13A. More specifically, the section can be considered a zoomed view of the second side 19' of the second heat pipe 18'. The second heat pipe 18' extends through a second conduit 132 of rotor 131. A third gap 132a is provided extending radially from the exterior surface of the second heat pipe 18' to an interior surface of the second conduit 132. At the second side 19', a sleeve 133 (optionally formed of metal) is provided within the second conduit 132 to provide thermal conduction between the heat pipe 18' and the rotor 131. The sleeve 133 could be of split ring structure for accommodating thermal expansion of the sleeve. It will be appreciated that the sleeve 133 arrangement may also be provided at the first side of the second heat pipe 18'. One or more support structures may also be provided to support the second heat pipe 18' between the first side 17' and the second side 19' of the second heat pipe 18'. The sleeve 133 if formed of metal may be welded to the heat pipe 18'. Alternatively, the sleeve 133 may be fitted to the heat pipe 18' using an interference fit. Such an interference fit will be greater than the interference between the heat pipe 18' and the second conduit 132, such that the sleeve 133 does not slide relative the heat pipe 18' when the heat pipe 18' and sleeve 133 are fitted into the second conduit 132. Figure 13C shows a larger section of the embodiment of Figure 13A. Two metal sleeves 133 are shown as metal rings welded onto the heat pipe 18'. By welding the metal rings onto the heat pipe 18' tends to ensure the heat pipe 18' and metal rings can be removed at the end of their useful life. The metal rings may be the same material as the heat pipe 18' such that there is similar thermal expansion and high thermal conductivity. A relatively soft metal such as copper may be used to allow for deformation. The metal rings may provide an interference fit to the second conduit 132 which tends to ensure that even when the heat pipe 18' is cold it is retained in the rotor 131. A thermal grease may be provided to assist in pushing the heat pipe 18' and metal sleeve / rings 133 into the conduit 132. Support structures 134 are shown supporting the heat pipe 18' between the first side and second side 19' of the heat pipe 18'. At the first side 17' of the heat pipe 18' a screw hole 135 is provided to allow a screw to be inserted to pull the heat pipe 18' out of the rotor 131 at the end of its useful life. A stub shaft 136 is shown as being screwed into the second conduit 132 and a clearance 137 is provided between the stub shaft 136 and the heat pipe 18' to allow for axial expansion of the heat pipe 18'. Figure 14 shows a flow diagram of a method 140 of temperature control of a vacuum pump according to the invention. The method comprises transferring heat between the high temperature zone 12, 12' and the low temperature zone 14, 14' using thermal conductivity using the examples of vacuum pumps described herein. Step 141 comprises introducing heat into the high temperature zone 12, 12'. Step 142 comprises evaporating the working fluid at the first side 17, 17' of the at least one heat pipe 18, 18' using heat from the high temperature zone 12, 12'. Step 143 comprises condensing the working fluid at the second side 19, 19' of the at least one heat pipe 18, 18' thereby transferring heat to the low temperature zone 14, 14'. Apparatus for implementing the above arrangements and performing the method steps to be described above / below, may be provided by configuring or adapting any suitable apparatus, for example apparatus including one or more computers or other processing apparatus or processors, and / or providing additional modules. The apparatus may comprise a computer, a network of computers, or one or more processors, for implementing instructions and using data, including instructions and data in the form of a computer program or plurality of computer programs stored in or on a machine-readable storage medium such as computer memory, a computer disk, ROM, PROM etc., or any combination of these or other storage media. It should be noted that certain of the process steps depicted in the flowchart of Figure 14 and described above / below may be omitted or such process steps may be performed in differing order to that presented above / below and shown in Figure 14. Furthermore, although all the process steps have, for convenience and ease of understanding, been depicted as discrete temporally-sequential steps, nevertheless some of the process steps may in fact be performed simultaneously or at least overlapping to some extent temporally. List of reference numerals 10, 10' vacuum pump 11 stator 12, 12' high temperature zone 13 pumping chamber 14, 14' low temperature zone 16 heat transfer device 17, 17' first side 18 first heat pipe 18' second heat pipe 18a' internal bore 19, 19' second side 20 heaters 22 cooler 23 intermediate zone 24 heat spreader 26 thermal break plate 28 cable 81 tubular recess 82 thermal grease 83 piston seal 91 first conduit 91a first gap 91b second gap 92 thermal grease 93 piston seal 101 first conduit 101a first gap 101b second gap 102a thermal grease 102b reservoir 103 piston seal 111 first conduit Illa first gap 111b second gap 112a thermal grease 112b reservoir 113 piston seal 114 Plug 115 heat spreader 121 first conduit 121a first gap 121b second gap 122a thermal grease 122b feeding channel 125 heat spreader 131 rotor 131a rotor blade 132 second conduit 132a third gap 133 sleeve 134 support structure 135 screw hole 136 stub shaft 137 clearance 140 method 141 method step 142 method step 143 method step

Claims

1. A vacuum pump having a high temperature zone and a low temperature zone, the vacuum pump comprising:a stator defining at least one pumping chamber;at least one rotor arranged within the at least one pumping chamber; anda heat transfer device comprising at least one heat pipe for transferring heat from the high temperature zone to the low temperature zone, wherein the at least one heat pipe comprises:a first side arranged to receive heat from the high temperature zone via the stator and / or via the at least one rotor;a second side arranged to provide said heat to the low temperature zone via the stator and / or via the at least one rotor; anda working fluid sealed within the at least one heat pipe;such that in-use the heat can be transferred from the high temperature zone to the low temperature zone by evaporation of the working fluid at the first side and condensation of the working fluid at the second side of the at least one heat pipe.

2. The vacuum pump according to claim 1, wherein the at least one heat pipe comprises at least one first heat pipe, wherein:the first side of the at least one first heat pipe is arranged to receive the heat from the high temperature zone via the stator; andthe second side of the at least one first heat pipe is arranged to provide the heat to the low temperature zone via the stator.

3. The vacuum pump of claim 2, wherein the at least one first heat pipe is arranged to extend at least partially through at least one first conduit, wherein the at least one first conduit is arranged:in the stator; and / orin at least one heat spreader attached to the stator.

4. The vacuum pump of claim 3, further comprising a first gap provided in a radial direction of the at least one first heat pipe between the at least one first heat pipe and an interior surface of the at least one first conduit, the first gap extending at least partially along the at least one first heat pipe between the first side and the second side.

5. The vacuum pump of claim 4, further comprising a thermal grease arranged in the first conduit at one or more of the first side and the second side of the at least one first heat pipe.

6. The vacuum pump of claim 5, further comprising a reservoir in fluid connection with the first conduit and for supplying the thermal grease to the first conduit.

7. The vacuum pump of any one of claims 5-6, further comprising a piston seal arranged between the at least one first heat pipe and the at least one first conduit.

8. The vacuum pump of any one of claims 3-7, further comprising a second gap provided in an axial direction of the at least one first heat pipe, the second gap being arranged between either the first side and / or second side of the at least one first heat pipe and a respective end of the at least one first conduit.

9. The vacuum pump according to any one of claims 3-8, wherein at least one of the first side and second side of the at least one first heat pipe is slidably received into the at least one first conduit.

10. The vacuum pump according to claim 8, wherein:one of the first side and the second side of the at least one first heat pipe is fixedly attached to the at least one heat spreader; andthe second gap is arranged between the other one of the first side and second side of the at least one first heat pipe and the respective end of the at least one first conduit.

11. The vacuum pump according to any preceding claim, wherein the low temperature zone and the high temperature zone comprise respective zones separated along a rotation axis of the rotor, wherein the at least one heat pipe comprises at least one second heat pipe arranged concentrically within the rotor and extending along the rotation axis for rotation with the rotor, wherein the at least one second heat pipe comprises an internal bore that tapers from the first side to the second side of the at least one second heat pipe.

12. The vacuum pump of claim 11, wherein the at least one second heat pipe extends through a second conduit in the rotor.

13. The vacuum pump of claim 12, wherein a third gap is provided in a radial direction of the at least one second heat pipe between the at least one second heat pipe and an interior surface of the second conduit, the third gap extending at least partially along the at least one second heat pipe between the first side and the second side.

14. The vacuum pump of claim 13, further comprising:at least one metal sleeve arranged within the second conduit for receiving the at least one second heat pipe, the at least one metal sleeve being arranged at the first side and / or the second side of the at least one second heat pipe, wherein the at least one metal sleeve optionally comprises a split for accommodating a thermal expansion of the at least one metal sleeve; and / orone or more supports arranged between the at least one second heat pipe and the second conduit, wherein the one or more supports are arranged atone or more locations between the first side and the second side of the at least one second heat pipe.

15. A method for temperature control of a vacuum pump according to any one of claims 1-14, by transferring heat between the high temperature zone and the low temperature zone using thermal conductivity, wherein the method comprises:introducing heat into the high temperature zone;evaporating the working fluid at the first side of the at least one heat pipe using heat from the high temperature zone; andcondensing the working fluid at the second side of the at least one heat pipe thereby transferring heat to the low temperature zone.

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