WO1992019864A1

WO1992019864A1 - Hydraulic circulatory systems

Info

Publication number: WO1992019864A1
Application number: PCT/GB1992/000796
Authority: WO
Inventors: Alan David Kenney
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-04-30
Filing date: 1992-04-30
Publication date: 1992-11-12
Anticipated expiration: 1993-10-30
Also published as: GB9109325D0; AU1653392A

Abstract

A hydraulic system comprises first and second columns (100, 102 respectively) containing a liquid, the liquid present in the second column containing a suspension of a finely-divided particulate material to increase its specific gravity relative to the liquid in the first column, means (104) for transferring liquid from the second column to the first column whilst retaining the particulate material in the second column, whereby liquid height in the first column is higher than in the second column, means (106, 110) for allowing liquid from the first column to overflow and means for maintaining liquid level in the second column.

Description

Title: Hydraulic circulatory systems

DESCRIPTION

This invention concerns hydraulic circulatory systems. Jean Bernoulli (1667-1784) described a thought experiment, in which a lighter liquid in a tube was balanced at a higher level than that of a heavier liquid in a vessel into which the tube was immersed at its lower end. The content of Bernoulli's vessel was a mixture of the lighter liquid and a heavier liquid. The lighter liquid in the tube was separated from the heavier liquid mixture in the vessel by a membrane across the lower end of- the tube, which membrane allowed passage of the lighter liquid but not of the heavier liquid in the vessel.

Bernoulli argued that the height of the liquid in the tube compared with the height of the liquid in the vessel, relative to the membrane would be inversely proportional to the densities of the liquids. He further argued that, if the tube were shortened below the height just defined, the lighter liquid would overflow into the vessel and, because of hydraulic pressure across the membrane, defined by the density differences, lighter liquid would continue ro pass into the tube and overflow back to the vessel.

Bernoulli's thought experiment took no account of the practicalities of obtaining a semi-permeable membrane which would behave in the required manner. Nor did it take account of osmotic pressure which would tend to oppose and even exceed the hydraulic pressure of the heavier liquid against the lighter.

Bernoulli's thought experiment may be defined as a potential energy pump, by which liquid is raised from a lower level to a higher level by the effect of gravity acting on a working liquid in such a way as to generate hydraulic pressure.

An object of this invention is to provide liquids of different densities in separate domains to produce a hydraulic circulatory system.

Another object of this invention is to provide apparatus, wherein a hydraulic circulatory system may be established for storage, recovery or continuous net production of energ . According to the invention there is provided a working fluid whose density may be varied from that of a single liquid to higher densities by suspension in the liquid of particles of even higher density with separation of fluid domains achieved by circulation of the liquid such that local flow rates at the plane of separation of adjacent domains are decreased to conrrcl the number of particles that traverse the plane against the action of a field which acts on the particles hence converting a higher density fluid in one domain to a lower density fluid in the adjacent domain while suspension of the particles in a particular domain is maintained by the input of mixing or local circulation energy but where overall circulation of the liquid between domains is maintained by deliberate imbalance of the hydraulic system defined by the fluid domains and where such overall circulation is not dependent on local mixing or circulation of the higher density fluids other than to maintain such densities and hence hydraulic pressures.

Further according to the invention there is provided apparatus to pump liquid from one level to a

•higher level, or for the storage and recovery of energy, or for the continuous net production of energy for providing motive power, through the circulation of fluids of different density in a hydraulic system comprising first and second columns of liquid, the liquid in the second column being the same as that in the first column but containing a suspension of a finely divided particulate material to increase the specific gravity of the resulting fluid relative to the liquid in the first column, means for transferring liquid only from the second column to the firsr column such that during the process of this transfer the particles are retained in the fluid of the second column and such that a ϋ-tube is effectively formed between the columns whereby liquid height in the first column is higher than in the second column, means for maintaining the particles in the second column in suspension, and means for allowing liquid from the first column to overflow thus causing an imbalance in the hydraulic system and establishing a circulation between the two columns which can be utilized for practical purposes.

For one such practical purpose there is provided means to replace liquid in the second column from a first reservoir such that if the liquid overflowing from the top of the first column is collected in a second reservoir at a higher level than the first reservoir the apparatus functions as a pump.

For other practical purposes there is provided means for allowing liquid overflowing from near the top of the first column to be returned to the top of the second column together with means for converting energy fro the resultant movement of liquid into motive power.

According to another aspect of the invention there is provided apparatus for providing motive power comprising first and second columns of liquid, the liquid in the second column being substantially the same as in the first column but containing a finely divided material to increase its specific gravity relative to the liquid in the first column, whereby liquid height in the respective columns is different, means for causing liquid from the first column to overflow into the second column, means for converting energy from resultant movement of liquid into motive power and means for returning the liquid to the first column.

The means for causing liquid from the first column to overflow into the second column preferably comprises means for disturbing balance of the liquid system.

Thus, the invention provides an open circulatory hydraulic system for pumping liquid from one level to a higher level for any suitable purpose including the storage of energy or a closed circulatory system for the storage and recovery of energy or for the continuous net production of energy for providing motive power.

The preferred means of maintaining the particles in suspension is by local circulation using a pump, for example to circulate suspension back to the top of the second column from a point close to the interface between the suspension at the bottom of the second column and the liquid as it is transferred to the bottom of the first column. Such means to maintain suspension may be augmented by the use of baffles, or replaced wholely or in part by other means of mixing as would be used by those skilled in the art.

A surprising feature of the invention is the low energy input required to pump the high density suspension back to the top of the second column as determined by the pressure drop across the circulation pump as measured by manometers. In part this is due to the balance of hydraulic pressures in the system, yet even when for operational reasons a head of liquid is allowed to develop above the point where the local suspension circulation returns to the second column together with the overflow from the first column in the case of a closed system no significant additional pressure difference is observed across the circulation pump, provided the pump is sited at or below the highest point of liquid pressure in the U-tube affectively formed between the columns.

A means of pumping the suspension which is particularly preferred is the use of one or more double action positive displacement pumps which are themselves driven hydraulicall , and which are mechanically sufficiently simple to be resistant to the abrasive effects of the suspension.

A preferred pump means suitable, but not exclusively, for use in the present invention comprises a pair of chambers into which liquid is feedable and then discharσeable alternately. Dischar inσ of liσuid from a chamber is preferably by means of applied pressure. Each chamber preferably has its own inlet for pressurised gas, typically air, and its own exhaust outlet for the gas. Filling of each chamber with liquid to be pumped is preferably via a non-return valve entry. Alternatively, other internal agitating means, such as an auger, or external means, such as magnetic means, may be used for agitating the finely divided material. The means for converting energy from liquid movement into motive power is preferably a turbine. The turbine may be located at any point in the apparatus where movement of the liquid only can be utilised. In one preferred embodiment the turbine may be located in the path of liquid falling from the first column to the second column to impinge directly on rotating blades thereof.

The preferred liquid in the system of the invention is water. The purity of the water is not critical, in that water with dissolved salts, for example sea water, may be used in place of fresh water." However, it is desirable that the water does not contain components which impede breaking of the particle suspension, or cause the particles to flocculate or stick to surfaces within the system.

Preferred particles are those which are easily wettable, are not degraded or corroded by water, and have high densities. Particle size is important: if the particles are too small they do not separate out of suspension readily while if they are too large they are difficult to maintain in suspension. Preferred particle sizes are those which will pass through a 20 mesh sieve but are retained by a 200 mesh sieve. Particularly preferred are particle sizes which will pass through a

60 mesh sieve but are retained by a 100 mesh sieve. Particularly preferred particles are ferromagnetic, for example, magnetite or ferrosilicon which are widely used in high density suspensions for processing coal and other minerals, because their use is well understood by those skilled in the art and they can be recovered and purified using magnetic techniques.

To remove particles from suspension mechanical separation means may be used such as filter means or a settling tank. The design of settling vessel needs careful consideration as the rate of suspension separation may determine the maximum system liquid flow, and hence energy available to the turbine. A preferred means of separating particles from the circulating liquid as it passes from the bottom of the first column to the bottom of the second column is a separating tank of sufficient dimension for the flow rate of suspension entering the lower section of the tank to fall below that required to keep the particles in suspension and the particles are retained by gravity in the denser fluid. In the case of magnetite particles of the preferred size, the flow rate should be reduced in the separating tank to below lft/minute. A cone shaped vessel may be adequate but settling rates in any settling vessel can be improved by installing suitable baffle plates or by use of tangential flow to induce longer flow paths for the suspension and hence longer residence time for the separation phase to take place. The use of magnetic material, such as magnetite or ferrosilicon, allows conventional magnetic recovery apparatus to be utilised. A further method that can be employed with magnetic material which is energy _„ efficient, is to direct the suspension flow within the

. settling ve,ssel through a weakly magnetic field, which has the effect of agglomerating the individual magnetic particles together, into heavier clusters, which causes them to sink faster, speeding up the separation process. In a preferred embodiment of the invention, the first column contains substantially water and discharges the water into a lower second column where it mixes with a suspension of finely divided material in water delivered by pump means. The diluted suspension is fed via a settling tank to the first column, where the finely divided material settles out tc be delivered to the pump means as a more concentrated suspension and the water returns to the first column.

The addition of liquid to the top of the second column results in a decrease in the density of the fluid flowing down the column in proportion to the relative rate of this addition to the rate at which suspension is also returned to the top of the column; this proportion is hereinafter defined as the operational ratio. In consequence, if no other action is taken, a head of liquid will build up to compensate for this decrease in density resulting in a limit to the net head of liquid available from which to obtain power. Typically the operational ratio may lie in the range 0.1:1 to 10:1, with a preferred range of 0.5:1 to 2:1. A further preferred means of operation of the second column is with a periodic variation in density achieved by sequencing the action of a double action positive displacement pump circulating the suspension such that an average operational ratio falls within the preferred range and utilising the resultant periodic tendency to form a head of liquid above the second column in order to compress air into a receiver. The compressed air may then be used to activate the displacement pump, hereinafter also referred to as a "force pump".

This invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows schematically a first embodiment of the invention; Figure 2 shows a variation on the embodiment of

Figure 1;

Figure 3 shows schematically a second embodiment of the invention including a force pump;

Figure 4 shows a variation on the embodiment of Figure 3; and

Figure 5 shows schematically a third embodiment of the invention.

Referring to Figure 1 of the accompanying drawings, hydraulic circulatory apparatus comprises a first column 10 and second column 12. First column 10 has an upper tank 14 and a lower tank 16 separated by pipe 18. From upper tank 14 extends a down pipe 20, which includes a valve 22 and a turbine 24, to feed into upper tank 26 of the column 12. Column 12 is connected at its lower end to column 10 at a T-junction 28 from which the other branch is connected to a column 30.

Column 30 has an upper tank 32 which contains a circulatory pump 34 for circulating medium via pipe 36 from the tank 32 to tank 26 of column 12. A dense medium circulatory system is established from tank 32 to tank 26 and back through column 12, T- junction 28 and column 30. The dense medium comprises a suspension of magnetite or ferrosilicon in water. The remainder of the apparatus contains substantially suspension free water, although there will be some mixing thereof at interfaces. The dense medium is initially circulated by the pump 34 at about 12" Wg pressure. The T-junction branches are wider at the base of tank 16 to allow space for the magnetite or ferrosilicon to settle to rejoin the flow without creating a blockage due to build up of the suspended material.

Due to the difference in specific gravity between the substantially pure water in column 10 and the suspension in column 12, the water rises higher in column 10 to a balance. By opening valve 22 that balance is disturbed so that water in tank 14 can be caused to fall into tank 26 via down pipe 20. The falling water passes through and drives turbine 24 to generate power. For a fuller discussion of the operation of this system, please refer to the description of the embodiment of Figure 3 of the accompanying drawings below, as operation thereof is similar.

In the variation of Figure 2, in which like parts have been given the same reference numerals an in Figure

1 for ease of reference, lower tank 16 has been replaced by a settling tank 16' and pump 34 has been lowered to reduce the energy taken up in maintaining the magnetite in suspension and thereby to increase the scope for net production of power. Turning now to Figure 3 of the accompanying drawings, a hydraulic circulatory apparatus is shown which is substantially similar to that of Figure 1 of the accompanying drawings except for the use of a force pump 50 in place of circulatory pump 34 and which will be described in detail first. Like parts of Figure 3 of those of Figure 1 have been given the same reference numerals for simplicity and will not be described in detail.

The force pump 50 has two tanks 52 and 54 which are fed from respective branches 56, 58 of column 30, each branch having a one-way valve 60 at its end. Each tank 52, 54 has an inlet valve 62 for air under pressure and a exhaust valve 64. Tanks 52 and 54 have pipes 65 and 66 respectively for feeding pumped medium to tank 26 each via a non-return valve 68, 70 respectively.

The two tanks 52 and 54 are alternately filled via the non-return valves 60 with the suspension under a small head pressure from tank 26. The tanks are alternately discharged by air pressure admitted sequentially to inlet valves 62. At the completion of a discharge cycle exhaust valves 64 are sequentially opened to allow air to escape and the filling cycle to commence. Non-return valves 68 and 70 prevent interflows between tanks 52 and 54. The air valves are normally closed solenoid valves and sequenced by an electronic timer.

The advantages of using this method of providing the necessary media circulation include:

1. The energy consumption of the suspension pumping circuit has a direct bearing on the net energy recoverable from the apparatus, so it is important that the most energy efficient means possible are employed.

The force pump is operated by air which is only compressed sufficiently to provide the required low head necessary to maintain an adequate flow and overcome friction losses.

2. Compressed air can be provided by modern equipment at between 85 and 90% mechanical efficiency when coupled directly to the turbine output shaft.

3. The only moving parts in the force pump in contact with the suspension are the non-return valves, which may be manufactured from abrasion resistant rubber. The tanks and pipe runs can be lined with abrasion resistant rubber also, resulting in an inexpensive and reliable pumping device with negligible operating costs.

4. The force pumps can have high mechanical efficiency relative to other commercially available pumps.

4.1 Very low head requirement.

4.2 Abrasion resistance. 4.3 Ability to handle dense media.

4.4 Very high mechanical efficiency.

4.5 Capable of being scaled up to virtually any size on commercial installations.

The apparatus of Figure 3 operates in the following manner. The suspension of magnetite or ferro¬ silicon in water is circulated by the force pump 50 along pipes 65 and 66 discharging into tank 26 at a head of only 12 to 18 inches Wg as between level 2 to level 3. This head pressure is transmitted back to the force pump 50 from tank 26 and pipe 12 and 30, opening the non-return'-valves 60 for recirculation.

Water is admitted to tanks 16 and 14 from an outside supply with tank 16 valved off. When the tanks are filled and the valve is opened a static balance is achieved. Alternatively the system may be filled from an independent suspension circuit and left to reach a balance as the magnetite settles out of tank 16 and the water rises to level 6. Fine adjustments to the static balance can be carried out by adjusting level 3 or by adding more suspension to raise the specific gravity.

When the desired balance has been achieved and the water is at level 6, then valve 22 is opened, allowing water to flow via the turbine 24 to tank 26, where it rejoins the suspension circuit to initiate overall circulation between the columns. This causes the suspension in tank 26 and column 12 to dilute, which results in a back up of this dilute suspension in tank 26 to regain the original balances between the columns. The suspension will settle out in tank 26 to the inlet point of the suspension circuit, causing a further back up clean water to regain the original balance and reducing the effective height of the free falling water from tank 14 via the turbine 24.

Additional water is added to tank 14 to replace that lost to tank 26 in establishing the backup, and to provide the necessary drive head to the apparatus. This ensures that the system remains unbalanced, with the water being constantly pressured up into tank 14 by the difference in specific gravities between the fluids in the two columns. The diluted flow in column 12 is directed to large cone shaped T-junction 28, which has the effect of reducing the velocity of flow of suspension at its junction with settling tank 16. Some of the suspension is carried upwards into the cone of tank 16 but the wider diameter of this tank causes the velocity of the flow to drop below the critical rate at which the suspension is maintained, allowing the magnetite to fail back into 28 and join the flow back to the force pump 50 for recirculation.

In the variation of Figure 4, in which like parts have been given the same reference numerals as in Figure 3 for ease of reference, lower tank 16 has been replaced by a settling tank 16" and force pump 50 has been lowered to reduce the energy taken up in maintaining the magnetite in suspension and thereby to increase the scope for net production of energy.

Turning to Figure 5 of the accompanying drawings,, a method for providing fluids of different densities together with domains of different fluid density together with an apparatus for the circulation of such fluids in a closed system is illustrated as follows. A first column 100 and a second column 102 form a U-tube connected through separation tank 104. The lower end of column 100 joins directly to the top of tank 104 and the bottom end of column 102 passes through the top or side of tank 104 to the bottom of the cone forming the base of the tank. The upper section of column 100 is formed by tank 104 and is situated higher than the upper section of column 102 which is formed by tank 108. A pipe 110 connects the bottom of tank 106 to the bottom of tank 108 and is fitted with a control valve 112. A pump 114 is fitted with its point of suction at the bottom of tank 104 and connected on its discharge side to one end of pipe 116 which in turn connects to the bottom of tank^' 108. Manometers 118 and 120 are connected to the suction and discharge sides of pump 114 respectively arranged to minimize venturi, turbulence and other flow effects.

The apparatus is filled with water to the top of tank 104 through a valved connection to the tank (not shown) . Tank 104 is also charged with a quantity of fine magnetite particles. An external pump and pipe circuit (also not shown) is first used to circulate the water and magnetite in tank 104 to give an even suspension of the magnetite in the water. The apparatus is further filled with water to the bottom of tank 108 before pump 114 is switched on to circulate magnetite suspension through pipe 116 to establish a return flow down column 102 to tank 104. At this point the external pump is turned off and its associated pipework is valved off from the rest of the apparatus. As magnetite suspension is pumped via pipe 116 to column 102 the liquid level in column 100 rises to balance the hydraulic pressure of the denser fluid in column 102. The liquid in column 100 at this stage is primarily water but may contain some magnetite until the external circulation pump is turned off and an effective phase separation occurs in tank 104. As this separation takes place, the density of the magnetite suspension flowing in pipe 116, hereinafter referred to as the dense medium circulation, rises to its maximum value for the flow capacity of pump 114. Additional water is added to tank 104 as the water level in column 100 rises to keep the magnetite suspension level up to the bottom of tank 108 and until a stable situation is achieved with the magnetite maintained in suspension in column 102 by the dense medium circulation and the level in column 100 balanced by a higher water level in column 100 reaching well into tank 106. This dynamically stable system can be maintained for extended periods, during which time manometer 120 shows a level about 12" of water higher than that in manometer 118. Control valve 112 is then opened to allow water to flow from tank 106 to column 102 disturbing the existing balance of the system and setting up an overall circulation of water which dilutes the magnetite circulation flowing down column 102 before passing through tank 104 and back up column 100. The overall circulation of water in the system is monitored by means of flowmeter 122 and adjusted by means of control valve 124 to obtain the desired operational ratio. For an operational ratio 1:1 the rate of overall circulation of water equals the rate of dense medium circulation and the specific gravity of the suspension returning down column 100 is reduced proportionately. As a consequence of this reduction in density water backs up in tank 108 and additional water must be added to the system to maintain the water level in tank 106. Stable operation over long periods of time can then be maintained without loss of overall circulation while retaining domains of different density though with some mixing at interfaces. Using a quantity of magnetite in tank 104 to achieve a specific gravity of about 2 and with an operational ratio of 1:1 the level in tank 106 is found typically to be 6ft above the level in tank 108, hereinafter defined as the working head, which is about 5 times the operational head of the dense medium circulation pump and characterises the typical commercial potential of the invention.

For a closed system operated over long periods the working head can be used to generate motive power, for example by incorporating a turbine to generate electricity in column 100 or in pipe 110. The power thus generated can be greater than that required to drive the dense medium system which would provide for the net production of power after discounting the energy required to establish the initial suspension of magnetite particles in water. Such a practical device would form an important component of any renewable energy programme, particularly where sunlight, wind and tides are unreliable, or for remote or underdeveloped areas because of the simplicity of the system and the well understood technology of the component parts. Large arrangements to provide the flows of water necessary to drive commercial turbines such as Kaplan turbines which can generate electricity with efficiencies greater than 90% may be formed by combined or linked multiple circulation units. Further applications of the invention are not limited to these examples, but include the storage of off peak energy for recovery at periods of peak demand. The closed system described above could be operated commercially in a discontinuous manner whereby operation ^" is commenced and the magnetite suspension established when off peak electricity is available, and overall circulation maintained through the turbine to generate electricity for only a limited period thereafter to satisfy peak demand. The invention may also be applied as a practical pump using an open system, similar in principle to the closed apparatus shown^* in Figure 5 but without pipe 110 connecting the bottom of tank 106 to the bottom of tank 108. In this case tank 106 is connected to or forms part of a water reservoir or means of using the water, for example, for irrigation, while a lower water reservoir is connected to tank 108 by means of some flow control device used to establish the operational ratio. The commercial value of such a system lies in its ability to amplify the operational head of the dense medium circulation pump to obtain overall pumping against a higher head. Such an open system may also be used for energy storage, this time through continuous operation as part of a hydroelectric scheme to pump water back to higher levels during periods of low demand for electricity.

The invention will now be further described by means of the following Examples:

EXAMPLE 1

An apparatus was constructed as shown schematically in Figure 5. A cylindrical steel separating tank (tank 104) 36 inches in diameter by 48 inches deep with a 48 inch 60 degree cone base giving a water capacity of 1,000 litres was supported in the lower half of a two tier steel framework 16.24 feet high. A telescopic extension to the framework was provided to support a 30 litre 11.5 inch diameter clear

PVC tank (tank 106) approximately 35 feet above ground level. Column 100 was formed from 2 inch steel pipe flanged to the top of the separating tank 104 and fitted with a calibrated 2 inch flowmeter 122 capable of measuring a maximum flow rate of 5,000 litres/hour with its upper section formed from 2 inch flexible hose by which means it was connected to tank 106. Column 102 was formed from 4 inch steel pipe mounted through the top of tank 104 and projecting 5 feet below the top of the tank to a deflector plate arranged to divert flow from the column into a horizontal plane to minimize swirling and to assist separation within the tank. At a height of 21.5 feet above ground level a manifold at the top of the 4 inch pipe forming column 108 was connected to 8 feet of 3 inch clear rigid PVC pipe forming tank 108. To this manifold was also connected 2 inch flexible tubing from the bottom of tank 106 to form the water circulation pipe 110 fitted with a 2 inch Saunders valve (flow control valve 112) and 2 inch flexible tubing carrying the dense medium circulation.

The dense medium circulation was achieved by locating a 1.25 inch low pressure submersible centrifugal pump (pump 114) of the type used for sump draining as near as possible to the bottom of tank 104 attached on its discharge side to a 4 inch manifold and hence with 2 inch flexible tubing passing through a seal in the top of tank 104 connected to the manifold at the top of column 102. Flexible pipe 1.25 inches in diameter was fastened to the side of pump 114 with its opening level with but not close to the inlet grill of the pump to form a manometer (manometer 118). A second 1.25 inch flexible tube manometer (manometer 120) was connected to the manifold on the discharge side of pump 114. Both manometers were led through seals in the top of tank 104 to a point 5 feet above the top of tank 106. Pump 114 was capable of delivering a full flow of 5,250 litre/hour limited to 2,220 litre/hour when part of its output was diverted through a bypass (not shown in Figure 5) attached to the manifold on the discharge side of pump 114 and leading back to the bottom of tank 104. Pump capacities were measured by disconnecting the flexible hose from column 102 and recording the time required to fill a 100 litre container to precalibrated levels.

Tank 104 was first charged with 750 kgs of 60 mesh magnetite powder and filled with tap water from a hose connected to a valve in the top of the tank. The magnetite was then fluidised to establish a suspension in tank 104 by means of an external circulation system

(not shown in Figure 5) consisting of a pump similar to pump 114 with its suction side fitted to the top of tank

104 and with a 1 inch valved pipework connection to the bottom of the cone forming the base of tank 104. It was found necessary to provide a pathway through the settled magnetite by leaving a steel bar lying down the inside of the bottom of tank 104.

The external circuit was then operated for 3 hours to establish a magnetite suspension in tank 104 before additional water was added to raise the level to the bottom of tank 108, the fluidisation pump turned off, and the external circuit isolated from the rest of the system. At the same time with flow control valve 112 fully closed pump 114 was started with its bypass open to establish a dense medium circulation through column 102 of 2,220 litres/hour. After 20 minutes further additional water was added to raise the level of water in column 100 until tank 106 was 3/4 full, the level of magnetite suspension in column 102 reached the bottom of tank 108 and a stable hydraulic balance had been reached.

Overall circulation was then established by opening flow control valve 112 to give a water flow of 1,500 litres/hour as measured by flowmeter 122 to give an operational ratio of 0.67:1 at the same time adding further water to the system to restore the level in tank 106 as a level of water backed up in tank 108. Once circulation was steady, the working head was found to be 8.09 feet with the water level in manometer 118 was 14 inches higher than the water level in tank 106. These conditions were maintained for 4 hours before flow control valve 112 was closed, pump 114 turned off and water columns 100 and 102 drained off to the level of the top of tank 104. EXAMPLE 2 To demonstrate that the bypass from pump 114 does not contribute to the local circulation requirement to keep magnetite in suspension in column 102, Example 1 was repeated with the bypass closed and an electronic phase shifter used to limit the speed of pump 114 to obtain a once through pumping velocity of 2,220 litres/hour of magnetite suspension. In this case the water flow through control valve 112 was also adjusted to 2,220 litres/hour, giving an operational ratio of 1:1. Once stable overall circulation had been achieved, a working head of 6.5 feet was obtained and maintained for a period of 1.5 hours. Within the experimental error involved in measuring magnetite suspension flow rates and densities, the lower working head compared with Example 1 corresponds with the larger operational ratio. For practical reasons this mode of operation was not used routinely, as the electrical control system of pump 114 burnt out rapidly when the pump was operated with the phase shifter. However this Example shows that the bypass system is not a necessary requirement of the invention and that a properly rated once through pump is adequate to maintain magnetite suspension.

EXAMPLE 3 To demonstrate that the kinetic energy of the water circulatinσ from tank 106 to tank 108 in Examole I is also not an essential requirement to keep magnetite in suspension in column 102 and to demonstrate the practicalities of installing a turbine to recover energy, the apparatus described in Example 1 was modified so that pipe 110 decanted into an intermediate 25 gallon tank placed adjacent to tank 108 to dissipate the greater proportion of this energy. The flow of water into this intermediate tank and hence overall circulation between column 100 and column 102 was still controlled by valve 112. The circuit was completed by pumping the water in the intermediate tank into column 102 by means of a 0.5 inch centrifugal pump of limited capacity. Magnetite suspension circulation was established at 2,220 litres/hour in the same manner as in Example 1, followed by water circulation at the

^' maximum allowed by the capacity of the centrifugal pump.

Once the latter flow was stable with a steady level in the intermediate tank, the water circulation from column

100 to column 102 was found to be 700 litres/hour as measured by flowmeter 122 giving an operational ratio of 0.32:1. In this case a working head of 11.2 feet was obtained with a pressure difference of 12 inches of water across pump 114. This working head was maintained for 2 hours and was if anything 10% higher than that expected by comparison with Example 1, after the lower operational ratio is taken into account though still comparable within experimental error. The discharge pressure of the centrifugal pump transferring water from the intermediate tank to the top of column 102 was 3 inches greater than the level of water in tank 108 as measured by a manometer, such that the water was overflowing from column 100 to column 102 with the energy equivalent to a fall of 0.25 feet compared with the energy equivalent of a fall of 11.2 feet if the intermediate tank had not been used and pipe 110 led directly to column 102. Thus the energy which may be extracted from the overall circulation of the system may be calculated to be 6.3 watts, compared with the energy absorbed by local circulation of the magnetite suspension by pump 114 of 3.4 watts.

Claims

1. A hydraulic system comprising first and second columns containing a liquid, the liquid present in the second column containing a suspension of a finely divided particulate material to increase its specific gravity relative to the liquid in the first column, means for transferring liquid from the second column to the first column whilst retaining the particulate material in the second column, whereby liquid height in the first column is higher than in the second column, means for allowing liquid from the first column to overflow and means for maintaining liquid level in the second column.

2. A system as claimed in claim 1, wherein liquid from the second column is replaced from a first reservoir and liquid overflowing from the first column is collected in a second reservoir at a higher level than the first reservoir.

3. A system as claimed in claim 1, wherein overflowing liquid from the first column is returned to the second column.

4. A System is claimed in claim 1, 2 or 3 having means for converting energy from the resultant movement of liquid into motive power.

5. A system as claimed in claim 4, wherein the means for converting energy comprises a turbine.

6. A system as claimed in any one of claims 1 to 5, comprising means for maintaining the particulate material is suspension in the second column.

7. A system as claimed in claim 6, wherein said means for maintaining the particulate material in the second column comprises a pump for local circulation of the particulate material.

8. A system as claimed in claim 7, wherein the pump is arranged to circulate suspension back to the top of the second column from a point close to the interface between suspension at the bottom of the second column and the liquid as it is transferred to the bottom of the first column.

9. A system as claimed in claim 7, wherein the means of pumping the suspension comprises one or more double action positive displacement pumps which are themselves driven hydraulically.

10. A system as claimed in claim 7 or 8, wherein the pump comprises a pair of chambers into which liquid is feedable and then dischargeable alternatively by means of applied pressure.

11. A system as claimed in any one of claim 1 to 10, wherein the liquid is water.

12. A system as claimed in any one of claims 1 tc 11, wherein the finely divided particulate material has particle sizes that will pass through a 20 mesh sieve but are retained by a 200 mesh sieve.

13. A system as claimed in claim 12, wherein the particles are of a size that will pass through a 60 mesh sieve but are retained by a 100 mesh sieve.

14. A system as claimed in any one of claims 1 to 13, wherein the particles are ferromagnetic.

15. A system as claimed in claim 14, wherein the particles are of magnetite or ferrosilicon.