It is often required to transfer a cryogen, such as liquid nitrogen or oxygen. This has been achieved by connecting a tube from the liquid nitrogen container to the destination for the liquid nitrogen. The tube dips into the liquid nitrogen and by pressurising the container the liquid is conveyed through the tube. This method has the disadvantages that the flow is rather sudden and unpredictable.

An improved apparatus and method for pumping is desirable for applications such as in the petrochemicals industry.

It was considered to use a pump immersed in the liquid. However, this has the problem that it is difficult to provide seals to prevent leakage of cryogen where the drive shaft passes from the drive motor to the pump.

Furthermore, there is thermal conduction along the drive shaft which causes cryogen to boil off. Problems can also occur when pumping cryogens due to differential thermal expansion coefficients of the components and possible seizure of bearings. For a given pumping power, the overall dimensions of the device cannot be made compact.

Accordingly, the present invention provides a pump comprising:

a fluid driver provided in a conduit for a fluid, wherein the fluid driver comprises a superconductive material such that the fluid driver can be rotated by an applied magnetic field to drive fluid through the conduit.

According to another aspect of the present invention, there is provided a method of pumping fluid comprising the steps of: providing in the fluid a fluid driver comprising a superconducting material; and applying a magnetic field to the fluid driver so as to rotate the fluid driver and drive the fluid.

The present invention enables a pump to be constructed without the need for connecting a rotating mechanical drive to the fluid driver and also normally without any electrical contacts to the fluid driver. The use of a superconductor enables the properties of the fluid being pumped, such as a cryogen, to be used advantageously.

The present invention enables a compact pump to be constructed. The use of a superconductive material enables the pump to have a high power density.

Preferably, the superconductive material is any superconductor with a flux trapping capability and/or levitating properties such as a type II superconductor.

This enables magnetic flux penetration and trapping for interaction between the applied magnetic field and the fluid driver.

Preferably, the fluid driver further comprises iron.

Iron enables the magnetic field to be concentrated at desired locations.

Preferably, the superconductive material is in the form of a cylinder. The advantage of this is that fluid can pass through the centre of the cylinder which preferably houses an impeller or archimedean screw.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a cross-section of a pump according tof the invention; Figure 2 shows a cross-section of part of a pump according to a second embodiment of the invention; and Figures 3 and 4 illustrate further embodiments of pump rotors in cross-section.

Figure 1 shows a conduit 10 in which the pump is housed. The conduit 10 can be an integral part of a tube or pipe for transferring fluid or can have suitable connectors to enable it to be joined to further pipework.

Normally, it will be of a generally cylindrical shape and, hence, will be described as cylinder 10.

In terms of an electrical machine, the pump has a stator 12, which comprises field generating coils mounted preferably on the inside of the cylinder 10. Rotatably mounted within the stator 12 is a fluid driver which may be considered as a rotor. The rotor comprises, in this example, a cylinder 14 formed of bulk superconductive

material rotatable about an axis 16. A cross-piece 18 of the rotor is rotatably supported on a shaft 20 at a needle bearing 22. The cross-piece 18 and shaft 20, which will be described in greater detail below, of course allow flow of fluid in the axial direction and may be perforate.

In this example, the pump is used for pumping liquid nitrogen. However, the pump of the present invention can be used to pump other suitable liquids or gases. The pump can also be a compressor.

The principle of operation of the pump is as follows.

One or more coils of the stator 12 are energised to produce a magnetic field which is principally transverse to the axis 16 within the cylinder 10. The superconductive rotor 14 is made of a material into which magnetic flux can penetrate when the magnetic field exceeds a critical value.

The magnetic flux penetrates the material in discrete amounts known as"fluxons". However, the fluxons cannot move freely within the material and become trapped, typically at defects in the material. When the applied magnetic field moves, the trapped flux produces a torque on the rotor 14 which provides the drive for the pump.

Examples of suitable materials for the rotor 14 are type II superconductors, such as the so-called high transition temperature ceramic superconductors for example BiSrCaCuO (referred to as BiSCCO in which the ratios of the bismuth, strontium, calcium and copper are 2: 2: 1: 2 or 2: 2: 2: 3), and ReBaCuO (referred to as ReBCO having a ratio of rare earth

or yttrium, barium and copper of 1: 2: 3).

When pumping liquid nitrogen, the pump, including the superconductive rotor 14 is immersed in the liquid nitrogen at a temperature of 77K. This is below the critical temperature of the above materials so they will be in a superconductive state. In the example shown in Figure 1, the field generating coils are also immersed in the liquid nitrogen. This enables them also to be made of a suitable superconductive material. However, even when copper coil windings are used, the low temperature reduces the resistivity of the copper by a factor of about 20 relative to room temperature and therefore enables much higher current densities to be used.

The small separation between the stator windings 12 and the superconductive cylinder 14 improves the efficiency of the pump. However, the windings could be incorporated into the wall of the cylinder 10, or if desired could be outside the cylinder 10 to avoid contact with the fluid being pumped.

The superconductive material forming the rotor 14 need not be in the form of a cylinder. However, this has advantages which include that it is as near as possible to the stator windings 12, it has axial symmetry which is convenient for it to be made rotatable and it has a bore in which the fluid driving means described below can be located to form a compact integral pump.

Another way of considering the operation of the

device is that a changing external magnetic field induces a current in the superconductive material which produces its own magnetic field that interacts with the applied magnetic field to generate torque. It would be possible to use a permanent magnet as the rotor. However, the maximum magnetic field available for a permanent magnet is about 1 Tesla, whereas the superconductive cylinder 14 can carry a high current density to maintain a field of around 2 Teslas. Since the power density in a motor is proportional to the square of the field, this gives a quadrupling of power density which is particularly advantageous in enabling compact and light pumps to be made. The present invention can operate with a relatively thin cylindrical shell of superconductive material, which enables an integral, internal pumping mechanism to be provided and avoids the use of a bulky permanent magnet. Cylinders of superconductive material are readily available as they are used for example as current lead tubes in cyrostats.

In the present example the stator coils 12 consist of three sets of insulated copper windings in a delta connection which are driven by a three phase power supply to produce a rotating magnetic field. Different winding configurations and numbers of phases, including single phase, can of course be used. A three phase arrangement is advantageous because the power delivered and hence the torque produced is constant. The pump is configured as a hysteresis motor which is also advantageous because it does

not need any special arrangements for starting up and the current drawn is fairly constant from start-up to maximum speed. This avoids the need to make provision for high start-up current which can in some motors be 5 to 8 times normal running current.

Finally, various fluid driving means may be provided for pumping the fluid. In one example, the cross-piece 18 in Figure 1 can comprise a number of blades forming an impeller. Alternatively, an archimedean screw can be provided within the superconductive cylinder 14. These both constitute axial pumps. It is of course also possible to configure the rotor as a centrifugal pump.

A specific example of a pump has a length of 20mm and an outside diameter of 25mm. The superconductive cylinder 14 is made of BiSCCO. Three-phase copper coils are energised at 50 Hertz with a supply of 2 volts and 13 amps rms. An archimedean screw with a pitch of 10mm and made of delrin is provided as the fluid driving means. This device achieves a pumping rate of up to 1 litre per minute and a maximum head of about 0.3m for liquid nitrogen.

Optimization of the pump design would be expected to improve significantly the performance.

With a three-phase 50 Hertz power supply, the magnetic field is rotating at 3,000rpm. A hysteresis motor usually operates synchronously, however, it was observed that the BiSCCO motor was only rotating at 1,500 to 2,000rpm. One reason for this is imperfect flux pinning.

A rotor of YBCO under these parameters would operate in a flux trapped region which would achieve synchronous rotation and a pumping rate of approximately 4 litres per minute for the same size pump. For improved efficiency, additional stator blades or vanes may be provided either or both upstream and downstream of the rotor and standard fluid dynamic analysis can be used to determine the optimum blade angles and profiles and/or the archimedean screw pitch length.

By way of further example, a pump according to this invention using YBCO as the superconductive material of the rotor, with a centrifugal water pump impeller, has achieved the following performance: maximum rotor efficiency of 65%; maximum motor efficiency including bearing friction of 62%; maximum flow rates for liquid nitrogen of 0.22 litres per second with a delivery head of 0.33m. The motor efficiency under these conditions is 47% and the overall pump efficiency is 2.7%. The maximum achieved delivered head is l. lm. These results were achieved with a demonstrator that had not been optimised to match the fluid properties of liquid nitrogen, for example the impeller was a water pump impeller. Similarly, the pipework was not optimised, so that, taking into account head losses, the effective head at the maximum flow rate is 2m. Also the bearings were non-optimised. The performance of the demonstrator was also limited by the current in the stator coils. As the stator current is increased, the efficiency and performance

increases. The predicted overall efficiency of the pump is at least 160.

The gap between the rotor and stator needs to be as small as possible for maximum efficiency. In the present example, the coefficient of linear thermal expansion of the copper coils is 17 x 10-6K-1 and that of the BiSCCO superconductor is 14 x 10-6K-1. An appropriate clearance must be allowed at room temperature so that, when the system is cooled down to liquid nitrogen temperature and the gap reduces, contact between the rotor and stator is avoided.

In use, the pump is immersed in the cryogen, such as liquid nitrogen, which cools the superconductive material below its superconducting transition temperature and no separate cooling system is required for the pump. Ideally the pump is located at a sufficient depth below the surface of the liquid nitrogen so that the absolute pressure of the liquid at the pump is above atmospheric pressure. This is to avoid the problems caused by cavitation in which the local pressure of the liquid being pumped falls below the vapour pressure of liquid at that temperature and results in vaporisation of the liquid and the formation and collapse of bubbles.

In the pump configuration in Figure 1, the pump is arranged to propel the fluid in the upward direction. This produces a downwards reaction force on the rotor. The rotor is vertically constrained by the prop 20 at the

needle bearing 22. An alternative arrangement is shown in Figure 2 which comprises a frictionless bearing. The upper end of the superconductive cylinder 14 is provided with a lip or flange 24. Repulsion of the flange 24 by the magnetic field of the coil windings 12 levitates the cylinder 14 and provides axial support. If necessary special magnetic materials, soft or hard, and/or the use of additional coils can be used to provide for a passive or an active bearing. Lateral constraint of the rotor can be achieved mechanically, eg. via the needle bearing, and/or by magnetic forces.

Although the arrangements shown in Figures 1 and 2 have a vertical pump axis, it is of course possible to provide bearing arrangement for the pump to operate in other orientations.

The pump has been described in terms of three-phase coil windings, but a number of different modes of operation are contemplated. For example, the device could be cooled below the critical temperature of the superconductor with zero applied magnetic field, alternatively the device could be"field cooled", by applying a magnetic field at room temperature which penetrates the material and then cooling it below its transition temperature. This may result in improved flux penetration. In operation, the magnetic field could be pulsed. Coils could also be used to write a particular magnetic structure on the rotor, known as a written pole motor.

The pump has been described above in terms of a rotor comprising a cylinder of bulk superconductive material, which could be made from individual pieces. Other constructions are possible which may enhance the performance. For example, pole-shoes made of iron could be included as part of the cylinder to concentrate the magnetic field.

Further possibilities are illustrated in Figures 3 and 4, which show the rotor cylinder in transverse cross- section and omit the other components of the pump. The arrangement shown in Figure 3 comprises axial rods of superconductive material 26 contained in an iron cylinder 28. The superconductive rods are joined at each end of the cylinder (not shown). This arrangement may be particularly suitable for large pumps. In Figure 4, the cylinder comprises alternate layers of iron 30 and superconductive material 32. This makes the cylinder highly anisotropic which constrains the orientation of the cylinder with respect to the penetrating magnetic flux and is a form of super reluctance motor. In both Figures 3 and 4 the bore through the cylinder can accommodate the desired integral pumping mechanism.

The rotor does not have to be cylindrical, but could be, for example, annular with radial iron impeller blades.

In all of the above examples that use iron, the iron is preferably in a soft magnetic form and could be replaced by another suitable soft magnetic material.