DRAG REDUCER

DRAG REDUCER

Field of the invention

The present invention relates to liquid filled machinery, with parts rotating with so high velocity that hydrodynamic friction reduces the efficiency of the machine to a significant degree. The machine can be for example electric motors, pumps, compressors, torque converters, mechanical couplings, brakes, flywheels, mechanical couplings, magnetic couplings and magnetic gears. "Couplings" is here meant to include clutches, torque limiters, flexible joints, as well as rigid couplings. More specifically, the invention relates to reduction of hydrodynamic friction in equipment of the mentioned type, particularly embodiments thereof for use subsea.

Electric motors for use in hybrid-electric and fully electric vehicles use are often partially oil filled (e.g. as in Toyota's Prius and GM's Volt/Ampera) and they operate over a wide speed range. The invention might also be applicable to these.

Further application might be within the field of power generation, i.e. enclosed hydro power generators and turbines, generators in wave and tidal power plants.

Electric motors in cooling compressors (as in refrigerators, air conditioning and also heat pumps) are also applicable, so are also hydraulic power-packs, in particular when an electric motor and pump/compressor are placed together inside a fluid/liquid filled sealed compartment.

Background of the invention and prior art

For use subsea, electric motors are according to state of the art liquid filled, the liquid is typically oil or a mixture of glycol and water. The liquid filling reduces the risk for short circuiting (in case of being filled with oil), provides or contributes to cooling and lubrication and facilitates pressure compensation. Liquid filled motors for pumps of size, effect and pressure head feasible for pressure boosting of petroleum liquid and multiphase from petroleum wells and other sources are in practice limited to a rotational speed of about 4000 rpm. At higher speed, the frictional losses and heat become a severe and limiting problem. Accordingly, higher rotational speed, which is beneficial for

compressors and pumps, can be impossible or difficult to achieve. Mechanical and magnetic couplings, such as mechanical or magnetic gears, are also limited with respect to rotational speed, for similar reasons. Creation of vortices and turbulence in the liquid in the volume between rotating parts are assumed to be a significant contribution to the hydrodynamic losses.

As a result, for liquid filled equipment having rotational speed up to the practical limit, the cooling system must be comprehensive and of high capacity, and a comprehensive coolant supply can be required. At locations subsea, often tens or even hundreds of kilometers away from topside installations or shore, this represents severe challenges and high cost.

The objective of the invention is to provide a solution to the above mentioned limitation, in order to mitigate the problems and challenges and extend the practical speed limit for liquid filled motors, couplings and other units.

Summary of the invention

The invention provides a liquid filled, fast rotating unit, such as a motor, a mechanical or a magnetic coupling, distinctive in that the liquid filled unit comprises at least one rotating shield arranged between surfaces of different rotational speed. During operation of the unit, the shield rotates at an

intermediate speed compared to the adjacent machine elements. The shield may be free rotating (i.e. driven by the hydrodynamic drag), or there may be a kinematic link enforcing the intermediate speed.

The term rotational speed covers stationary parts to parts of maximum rotational speed, such as the gap between a stationary stator and a rotor, and the gap between a rotor and a stationary housing. A freely rotating shield will typically rotate at a speed in between the rotational speeds of the adjacent surfaces of the adjacent objects, for a shield arranged between a stator and a rotor the shield is expected to rotate at about half the rotational speed of the rotor. The hydrodynamic losses depend on the speed of the liquid by a power factor of approximately 3. The shield provides two volumes of half difference in relative speed, resulting in about 1/8 of the original loss in each of the two volumes .assuming that the gaps between the sleeve and the stator/rotor are equal to what is was before the sleeve was introduced. The sum of losses for both volumes (both sides of sleeve) will in such a case be reduced to

approximately 25%. The loss will however in a realistic application be reduced to 25 - 50% depending on the selected gap sizes. If using two or three layers of rotating shields, the sum of losses will be reduced to respectively approx. 1 1 % and 7% depending on selected gap sizes. Further tests have shown that the torque is reduced by approx. 50% and the pressure drop caused by the gap flow between the rotary and stationary parts is reduced with approx. 25% with introduction of such a sleeve in the gap compared to having no such sleeve.

The technical effect is that the dimensions of the full length or extent of the cooling system and supply chain can be reduced accordingly or the maximum rotational speed and pressure head can be increased, or energy for operation is reduced due to reduced hydrodynamic loss. The shield allows a higher maximum rotational speed for a specific demand for cooling, making liquid filled motors and couplings and gears more feasible for multiphase pump and compressor motors. In a preferable embodiment the shield is a tube section or sleeve arranged between the rotor and stator, rotor and housing, rotor and shaft, rotor and barrier wall or between any other rotational parts of cylindrical shape. The radial gap between said parts forms an annular volume into which the sleeve is arranged so as to rotate freely in the gap.

In another preferable embodiment, the shield is a disc or plate arranged between the rotor and stator, or rotor and housing, or in any other axial gap between rotating parts forming a disc shaped volume. This embodiment is relevant for axial couplings, axial motors and generators having rotors with high aspect ratio of diameter vs length.

Preferably the unit is an electric motor driving a subsea pump or compressor and the shield is a sleeve arranged between the stator and rotor and preferably also a sleeve arranged between the rotor and housing if the rotor is external to the stator. In other embodiments, the sleeve is arranged in the annular gap between any rotational parts of tubular shape, the unit can be a motor of different type than an electric motor, such as a hydraulic motor, or other units.

Preferably the shield is made of a non-ferromagnetic metal, polymer, composite material or ceramic material, if the shield is for arrangements in gaps with a rotating magnetic field. If there is no magnetic coupling between the rotating surfaces, the shield can be made of a ferromagnetic material (and with less resistivity) since there is no magnetic coupling for which magnetic losses can occur. When using a shield of materials with high electric conductivity in a rotating magnetic field, then the material can be laminated (with insulating layers) in order to reduce losses associated with induced electrical currents in the material.

Examples on some of the feasible metal materials are as follows: titanium, aluminium, austenitic stainless steel, hastelloy, stellite, talonite, monel, inconel or other nickel-based superalloys, massive or laminated in order to reduce electromagnetic losses. Monel can be completely non-magnetic, also after long term mechanical stress, contrary to many other metals, accordingly monel can be a good choice if minimal magnetism is crucial. Examples on some of the feasible non-metal materials are as follows: PEEK, carbon fibre reinforced PEEK, carbon fibre, carbon composites, polyimide, ceramic materials such as zirconiumoxide, and fiber reinforced (carbon, aramid, s-glass, boron fibers) polymers. However, fibers, prepregs or stitched fibres of carbon or other fibers can be wound with pretensioning on a metal or polymer underlayer which again can be arranged on a mandrel, providing a relatively dimensional stable high strength sleeve. The mandrel can be removed after production of the shield or the mandrel can remain as part of the shield. The mandrel can easily be removed after hardening the composite at elevated temperature if the mandrel is made of a material with high thermal expansion (e.g. aluminium), inside a sleeve of fibre reinforced composite.

Production of the shield can however take place by casting, milling, machining, extrusion and/or grinding using traditional methods, there are several options both with respect to choice of material, design and method of fabrication, all of which will be more or less readily available for the person skilled in the art from textbooks and from suppliers and fabricators.

The shield can comprise one or more grooves and/or rifling arranged so as to circulate, move or pump fluid adjacent the groove or rifling and to stabilise the shield by a hydrodynamically created bearing support of the sleeve (prevent oscillations), for a sleeve shaped shield, the groove is preferably shaped as a helix, for a disc shaped shield the groove is preferably shaped as a spiral, if spiral or helix is present on both sides of a shield, then the spiral or helix can be arranged in either opposing or parallel directions on the two sides.

The groove (rifling) can be similar in shape to the grooves used for journal bearings, or it can be similar to the grooves used oil cooled clutches (as in automatic transmissions for vehicles), or it can be similar to the rifling in a gun barrel. The grooves or rifling can be arranged on the inside and/or outside of a sleeve shaped shield and on either or both sides of a disc shaped shield. If spiral or helix is present on both sides of a shield, then the spiral or helix can be arranged in either opposing or parallel directions on the two sides. In addition or alternatively, slightly elliptical cross section shape or lemon bore shape can be used for the sleeve in relation to the annular volume between parts of different rotational speed. Said grooves or shapes are arranged in order to enhance circulation of fluid in order to improve cooling. However, depending on the shape and smoothness of the rotating surfaces on either side of the sleeve, grooves may not be required since fluid circulation may be promoted by the other rotating surfaces and from a fluid dynamic view it can be beneficial to have as smooth surfaces as possible.

The rotating shield relative to its mating parts (typically a rotor and a stator, possibly other rotating shields) can comprise bearing surfaces. The bearing surfaces can be there for several reasons; a) preventing the shield from rubbing against adjacent components, b) supporting the shield during stand-still, and c) facilitate freely rotation relative to the adjacent parts.

Preferably the sleeve is constrained from axial movement by the annular volume height or length or by machine elements in axial direction, providing a clearance gap sufficient to allow thermal expansion of the sleeve, and the sleeve is dimensioned so that on rest, at low ambient temperature, it has an internal clearance gap sufficient to allow rotation about the internal rotor or stator and at maximum operation speed and temperature it has an external clearance gap sufficient to allow rotation within the annular volume of the external rotor or stator or housing. The optimum clearance gap on either side of the sleeve depends on the viscosity of the fluid. 0,5 - 2,0 mm is considered suitable for water-filled machines, however, additional gap for differences in thermal expansion between the sleeve and the inside and outside equipment should be provided. Additional clearance or arrangements for installation can also be preferable.

Figures

The invention is illustrated with Figures 1 , 2 and 3, illustrating a unit of the invention, two shields for a unit of the invention and a further shield for a unit of the invention, respectively.

Detailed description

Figure 1 and figure 2 illustrate a unit of the invention with a during operation rotating rotor 1 , a during operation rotating shield 2 and a non-rotating stator 3. The shield and its top and bottom plates are one body with liquid circulation holes.

In figure 2 the shield consists of two concentrically arranged sleeves, typically rotating with approx. 2/3 and 1/3 of the rotor's speed.

Figure 3 illustrates a shield with helical rifling or groove pattern.