GB2280744A - Inverted heatpipes - Google Patents

Abstract

An inverted heatpipe using a mechanical lift pump, comprises a pipe shell to conduct heat away from a heat source, a working medium within the pipe and a pump section for pumping liquid from the heat-out section of the pipe to the heat-in section. The pump uses a movable element to pump the working medium via a riser tube to the heat-in section, with the movable element either forming part of the sealed envelope of the heatpipe or being located wholly within it, and being actuated by means which do not pierce the sealed envelope. <IMAGE>

Description

INVERTED HEATPIPES The present invention relates to heatpipes for conducting heat from one location to another.

Heatpipes can be used to provide a heat transfer capacity in excess of that of a solid heat conductor of comparable cross section. Heatpipes work by using the latent heat of evaporation, whereby the liquid phase of a working medium is vaporised in an evaporator region of the heatpipe by heat supplied from an external source and physically passes to a condenser region where it is cooled down, recondensing, and gives out the latent heat of evaporation, which is then conducted away externally.

The recondensed liquid must then be returned to the evaporator region. Traditionally, this is done by means of a porous wick, capillary action and/or gravity. Since gravity always acts on the medium, gravity assistance of the return of the liquid phase from the condenser section is greatest where the condenser section is vertically above the evaporator. However, it is not always possible to operate a heatpipe in that configuration and a number of applications require the evaporator to be above the condenser. A heatpipe in this configuration may be termed "inverted".

The power transport capability of an inverted heatpipe is limited in two major ways, both of which are governed by the wicking or capillary system. The wick is required to return the condensate at the bottom of the tube to the elevated evaporator heat input section. The lift height or wicking height of a wick i.e. the height through which the capillary action works, is a function of the open pore size the smaller the pore size the greater the potential "lift height". The permeability of a wick is also a function of the open pore size, the smaller the pore size the lower the permeability of the wick. Therefore when the pore size is smaller the potential height of the heat pipe is greater, but the rate of movement of liquid through the wick is reduced and thus the power transport capability of the heat pipe is lessened.

In practice a maximum lift height of about 400 mm is possible, using a pore size of between 50 and 100~pom, where the small open pore area restricts power handling to a few watts. For example: Heatpipe 6mm diameter x 170mm long = 30 watts maximum Heatpipe 6mm diameter x 300mm long = 10 watts maximum Heatpipe l9mm diameter x 350mm long = 80 watts maximum.

The practical maximum length is restricted by the reduced permeability associated with the smaller pore size that is required.

One proposed method for overcoming the disadvantages of using a capillary action solely is to use a system which works in a similar fashion to a coffee percolator and which is known as a vapour lift pump. A reservoir of the liquid phase of the working medium in the heatpipe is provided at the lower, condenser end and a riser column runs from the reservoir of liquid to the evaporator region. A heater positioned within the riser column, below the level of the reservoir of liquid, is then used to heat the liquid locally. This produces vapour which travels up the tube in bubbles and effectively reduces the gravity head in the column to below that of the external pressure head, thus causing the vapour/liquid mix to rise in the column and exit at the top into the evaporator region.

The problem associated with this type of inverted heatpipe is that the performance is found to be erratic and the auxiliary power required for the heater is approximately 30% of the forward (downward) capability of the heatpipe.

Repeated attempts to improve the efficiency of this type of heatpipe and to overcome the problems of the erratic results have not managed to produce a heatpipe which works on a similar basis which is satisfactory.

It is therefore an aim of the present invention to provide a heatpipe which overcomes or does not suffer from disadvantages associated with the prior art, to improve on the practical size limitations previously encountered and to provide a heatpipe which is as efficient or more so than conventional inverted heatpipes at the same or greater lengths.

The present invention proposes to use a mechanical pump to overcome the problems associated with the vapour lift pump or percolator system.

According to the present invention there is provided a heatpipe for conducting heat from a first location to a second location, comprising: a) an evaporator portion for positioning, in use, at said first location; b) a condenser portion for positioning, in use, at said second location; c) a reservoir containing, in use, the liquid phase of a working medium; d) a fluid return path from the evaporator portion to the condenser portion, and the reservoir; e) a riser tube whereby the working medium may pass from said reservoir to beyond said condenser portion; each of a) to e) inclusive being wholly within a sealed envelope of the heatpipe; and f) pumping means comprising a movable element for displacing liquid from said reservoir, through said tube, to said evaporator portion; wherein g) said movable element either 1) forms at least part of the wall of the envelope, or 2) is located wholly within the sealed envelope, and h) said movable element is actuated by means which do not pierce the sealed envelope of the heatpipe.

The present invention also provides a heatpipe comprising an evaporator portion and mechanical means for circulating working medium in liquid form to the evaporator portion; wherein said liquid circulation to said evaporator portion, irrespective of the orientation of use of said heatpipe, is maintained at a level to support practical power handling requirements. Preferably this heatpipe further comprises: a condenser portion, a reservoir, a fluid return path from the evaporator portion to the reservoir, and a riser tube from the reservoir; wherein said mechanical means includes pumping means with a movable element for displacing liquid from said reservoir.

The movable element preferably comprises a flexible part of the envelope wall for displacing said liquid from said reservoir. This flexible part may be all or part of the walls of the reservoir. For instance the reservoir may be a bellows.

The fluid return path may include an aperture between the reservoir part of the condenser and the liquid holding region, calibrated so that liquid may pass through to the liquid holding region, that the level of the liquid returned to the condenser portion does not rise above the condenser portion and that liquid flow back into the condenser, caused by the action of the movable element, is restricted.

The inner surface of the heatpipe may have no wick lining or may be partially or fully lined with a wick.

A non-return valve may be installed in the riser tube to prevent liquid passing back into the pumping section.

The pumping means may also include actuating means for actuating the movable member to expand and/or contract the volume available to the working medium within the reservoir.

The present invention will be further described by way of non-limitative example, with reference to the accompanying drawings, in which: Figure 1 shows a cross section along the length of a heatpipe according to an embodiment of the invention; and Figure 2 shows a cross section of the heatpipe of Figure 1 taken along the line AA of that drawing.

Figure 1 shows an embodiment of the heatpipe according to the invention and is a cross sectional view taken along its length. The central section of the heatpipe has been omitted, but is uniform with the section immediately above and below it.

The heatpipe (2) has two ends, a heat-in or evaporator end (4), to be installed at the location whence heat energy is to be absorbed and conducted away, and a heat-out or condenser end (6) from which the conducted heat energy is to be taken out. The extent of the heatpipe which serves as the evaporator (5) and the extent which serves as the condenser (7) will actually depend upon the use to which the heatpipe is put and how much of it is heated and how much is cooled. The heatpipe (2) itself comprises a cylindrical shell tube (10) of one or more suitable materials which may, according to the use to which the pipe is to be put, be metallic, ceramic or plastic or a combination thereof. The inner surface of the shell is lined with a wick capillary lining (12) similar to that found in the prior art.The liquid phase of the working medium (14) which, in this embodiment, is water, is held in a reservoir comprising a sump at the bottom of the tube, in the condenser end (6), and an area (26) within a set of bellows (8) below the base of the tube.

The bellows (8), also of any suitable material, for example phosphor bronze, are used to pump the water from the base of the heatpipe into and up a lift pipe or riser tube (16) running through the centre of the heatpipe to an area (18) at the evaporator end (4). The liquid passes into the riser tube via a hole (20) in its base and past a nonreturn valve (22), and from the sump in the base of the pipe at the condenser end (6) into the bellows (8) via a small diameter hole (24), which may be "open" or may have a nonreturn valve (not shown) to prevent water returning from the bellows (8) to the sump. The bellows (8) are powered by a low power linear solenoid. The stroke sequence of the pump and the rate of suction and pumping strokes influence the performance of the heatpipe.

The heatpipe is hermetically sealed around the tube and bellows to prevent the escape or entrance of any of the working medium from the unit.

Figure 2 shows a cross-section of the pipe, looking down along its length. The outer rim of the bellows (8), the tube shell (10), the wick (12) and the riser tube (16) are provided as concentric hollow cylinders.

In use, liquid (14) from the sump in the condenser end (6) of the heatpipe is fed to the bellows (8) via the hole (24) in the shell base. Upon compression of the bellows (8) the liquid (14) then has the choice of two possible escape paths. It is either forced through the entry hole (20) at the base of the lift pipe (16) or is pumped back through the small hole (24) into the sump from which it had been fed. Thus the small hole (24) must be calibrated to ensure that the liquid flow back into the sump is restricted.

In this embodiment the lift pipe (16) contains a non-return valve (22) to prevent liquid flowing back into the bellows. Whether or not the non-return valve is necessary depends upon the exact conditions of use. The cyclic rate, and the sucking and pumping proportions of the cycle provide differing heads and pumping rates from the riser tube. The necessity of the non-return valve may also depend upon the lift height and liquid pumping rate.

The pumping action of the bellows is achieved by means of the linear solenoid (28) which, when it is energised, extends the bellows (8), thereby sucking liquid from the sump in the lower part (6) of the heatpipe into the main area (26) of the bellows (8). When the solenoid is de energised the bellows contract because of inherent tension in the material. The sealed heatpipe unit is formed with a vacuum inside such that the internal pressure is solely a result of the water vapour and thus contraction may be aided by the sub-atmospheric pressure within the heatpipe which exists as long as the temperature of the working fluid is below its atmospheric boiling point, which in this case is 1000C.

The contraction of the bellows (8) provides the pumping stroke which lifts the seat of the non-return valve (22) and pumps liquid (14) into the riser tube (16). Since the diameter of the entry hole from the sump into the bellows (8) is calibrated to be of a minimum useful size, very little liquid escapes via this hole (24) back into the sump against the back pressure of the head of liquid (14) found therein. If desired a further non-return valve may also be provided in this feed hole to improve the pumping action.

At every stroke the liquid in the riser tube (16) is pumped up to a higher point until it is expelled at the top (18) of the tube (16) into the evaporator section (5) of the heatpipe. At this point the liquid passes into the wick lining (12) around the inner surface of the shell (10). The wicking ensures that the liquid emitted from the riser tube (16) is evenly spread over the inner surface of the heatpipe, so that heat input may be conducted away from anywhere, such as at the very top of the heatpipe and/or partway down the length of the unit. The liquid in the wick contacting the inner surface of the tube acquires heat from the external heat source (30), evaporates and passes from the hotter, evaporator section (5) to the cooler, condenser section (7) of the heatpipe. The liquid is aided by the action of gravity in saturating the entire wicked section.

Once the vapour has reached the lower, condenser section (7) of the heatpipe it condenses out, giving up its latent heat of evaporation, which heat passes out of the heatpipe to the coolant (32), and the recondensed liquid joins the rest of the liquid in the sump to repeat the cycle.

A heatpipe of the present invention is significantly more efficient than those encountered in the prior art. For instance a heatpipe with the following specifications: length (operational) 2000mm overall length 2250mm heatpipe diameter 12mm pumping frequency 1 complete cycle/ 2 seconds pumping rate 24cc per minute pumping energy input 12 watts temperature gradient 3ec maximum, provides heatpipe axial power 850 watts at 500C.

A conventional thermosyphon heatpipe (i.e. heated at the bottom, cooled at the top and working on capillary action and gravity) of this size will only handle 250 watts.

The power handling capability of a heatpipe according to the invention is a direct function of the liquid pumping rate.

A heatpipe according to the present invention can be used horizontally with the pump being provided lowermost to receive liquid from one end, to pump it to the other. If such a pump is used horizontally then it gives a performance increase over a conventional heatpipe, similarly used, of some 500%.

The heatpipe, on construction, is evacuated save for the working medium, and is hermetically sealed. The invention is not limited to the use of water as the working medium, but may use other fluids as the working medium, for example ammonia, methanol or ethanol; this will depend upon the temperatures which, in use, are to be encountered.

Depending upon the use to which the heatpipe is to be put the working medium does not even need to be a liquid at room temperature but may be a solid. For example, sulphur with a melting point just above the boiling point of water and with a boiling point of several hundred degrees celsius would be ideally suited for some situations and it is envisaged that it could well be used.

As an alternative to the internal vacuum a second fluid may be introduced into the heatpipe to provide an atmosphere and to produce a variable conductance heatpipe.

If a heatpipe is operated at above the atmospheric boiling point of the medium a double acting pump action will be required, that is providing a pumping stroke as well as the suction stroke.

The heatpipe is sealed with a vacuum tight seal, for at least the design life of the pipe. As mentioned above this does not necessarily mean that the heatpipe is wholly evacuated.

In the described embodiment the shell tube or main body of the heatpipe is cylindrical. The invention is, however, not limited to this shapes for instance, the main body could be square, rectangular, triangular or even irregular in cross-section. The cross-section can also vary along the length of the body. Further, whilst the riser tube is shown as centred within the heatpipe, this is not necessary to the working of the pipe. Instead it may be more toward one side or even external to the main pipe. Nor does it necessarily need to take fluid to the evaporator end of the heatpipe but can deliver it to any particular part other than the condenser.

Moreover, the wicking does not have to be over the entire length of the heatpipe but preferably it is wicked at least in those areas where the external surface is to be subjected to the external heat source (30). It is also possible that the heatpipe may not be lined at all. In which case especially, and also in most, if not all, other cases it is desirable that all covered or uncovered internal surfaces should be of such a material and/or finish that they are readily wetted by the working medium which forms a liquid film on them.

The pump drive source in the described embodiment is a linear action solenoid. Again, this does not have to be so. Any low power drive source could suffice, for instance a motor, series of cams, or air pressure or hydraulic pressure system. Also the form of the pump is unimportant provided the movable element which displaces the liquid is wholly within the sealed envelope of the heatpipe.

Thus, as in the present case the reservoir walls can be deformed, as a bellows or by moving a constriction along at least part of the length of the reservoir, or the envelope can be rigid with a movable pumping element such as a disc being moved remotely using means outside the envelope of the tube and reservoir, such as electromagnetic coils.

In any case the sealed envelope in which the working fluid is contained is not pierced by the means which actuates the movable element and thus there is no danger of any leakage.

Claims (21)

1. A heatpipe for conducting heat from a first location to a second location, comprising: a) an evaporator portion for positioning, in use, at said first location; b) a condenser portion for positioning, in use, at said second location; c) a reservoir containing, in use, the liquid phase of a working medium; d) a fluid return path from the evaporator portion to the condenser portion, and the reservoir; e) a riser tube whereby the working medium may pass from said reservoir to beyond said condenser portion; each of a) to e) inclusive being wholly within a sealed envelope of the heatpipe; and f) pumping means comprising a movable element for displacing liquid from said reservoir, through said tube, to said evaporator portion; wherein g) said movable element either 1) forms at least part of the wall of the envelope, or 2) is located wholly within the sealed envelope, and h) said movable element is actuated by means which do not pierce the sealed envelope of the heatpipe.

2. A heatpipe comprising an evaporator portion and mechanical means for circulating working medium in liquid form to the evaporator portion; wherein said liquid circulation to said evaporator portion, irrespective of the orientation of use of said heatpipe, is maintained at a level to support practical power handling requirements.

3. A heatpipe according to claim 2 further comprising: a condenser portion, a reservoir, a fluid return path from the evaporator portion to the reservoir, and a riser tube from the reservoir; wherein said mechanical means includes pumping means with a movable element for displacing liquid from said reservoir.

4. A heatpipe according to claim 1 or 3, wherein said movable element comprises a flexible part of the envelope wall for displacing said liquid into said riser tube.

5. A heatpipe according to claim 4, wherein said flexible part of the envelope walls comprises at least part of the walls of said reservoir.

6. A heatpipe according to any one of claims 1, 3, 4 and 5 wherein the reservoir comprises a part of the condenser and a liquid holding region in fluid connection thereto.

7. A heatpipe according to claim 6, wherein said liquid holding region comprises a bellows1 integral with the enveloping wall of the heatpipe.

8. A heatpipe according to claim 6 or 7, wherein the fluid return path includes an aperture between the reservoir part of the condenser and the liquid holding region, calibrated so that liquid may pass through to the liquid holding region, that the level of the liquid returned to the condenser portion does not rise above the condenser portion and that liquid flow back into the condenser, caused by the action of the movable element, is restricted.

9. A heatpipe according to claim 8, wherein said aperture is open.

10. A heatpipe according to any one of claims 1 and 3 to 9, further comprising a non-return valve in the liquid path between said reservoir and said riser tube whereby the liquid phase of the working medium may pass from the reservoir to the evaporator portion.

11. A heatpipe according to any one of claims 1 and 3 to 10, wherein the pumping means further comprises actuating means for actuating the movable element to expand and/or contract the volume available to liquid within the reservoir.

12. A heatpipe according to either claim 11, wherein said actuating means comprises a linear action solenoid.

13. A heatpipe according to any one of the preceding claims, further comprising a wicked lining on the inner surface of said main body at at least said evaporator portion.

14. A heatpipe according to any one of the preceding claims, wherein said working medium is one of water, sulphur, ammonia, methanol and ethanol.

15. A heatpipe according to any one of the preceding claims, wherein said evaporator and condenser portions are at opposite ends of the main body of the heatpipe.

16. A heatpipe according to claim 15, wherein said reservoir is positioned adjacent said condenser portion to the far side of said condenser portion from said evaporator portion.

17. A heatpipe according to claim 16, in use and positioned such that gravity acts along the length of the main body.

18. A heatpipe according to claim 15, wherein said pumping means is positioned to the side of the condenser portion nearest said evaporator portion.

19. A heatpipe according to claim 18, in use and positioned such that gravity acts in a direction substantially perpendicular to the axis of the main body.

20. A heatpipe according to any one of the preceding claims wherein the main body is cylindrical in shape.

21. A heatpipe substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.