NZ188729A - Heat pipe with planar radiant heat collector - Google Patents

Description

1687 2 Patents Form No. 5 " "*--•* f''$*&'v*. m X,. 40cT*K) .... Jf PATENTS: ACT". T9 5 3 •' COMPLETE SPECIFICATION After Provisional No. 188729 dated 25 October 1978 b^k: . -i-n-nq ■ "A HEAT EXCHANGER" <2 WE, ARTHUR GORDON WILLIAMSON of 16.0 Hackthornj Road, Christchurch 2, New Zealand, NEVILLE WALTON FOOT of 1 Leeman Place, Christchurch 5, New Zealand, and KELVIN LESLIE WALLACE of 41 Fulton Street, Blenheim, New Zealand, all being British subjects and New Zealand citizens, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement 1 88729 This invention relates to a heat exchanger and more particularly the invention relates to a heat exchanger designed particularly for use as a solar heat » or energy collector but which can be used in other heat exchange situations.

Known heat exchangers used as collectors for solar heat or energy are generally constructed with a shaped collector, for example, a flat plate or cylindrical .collector which is directed towards the sun. The shaped collector has a fluid, generally water, therein which when heated flows to a storage tank or cylinder under the effect generally known as thermosyphon effect which causes the heated fluid to flow from the collector to the storage tank. The cooler water from the storage tank flowing to the shaped collector for heating.

Such known constructions of heat exchanger have a number of disadvantages in that they are generally inefficient and at night unless backflow preventers are fitted a reverse flow of. fluid can occur which reverse flow cools the heated fluid in the storage tank. In addition if the shaped collector uses water as the fluid therein there is a tendency in cold weather for the water to freeze in the shaped collector.

Another known construction of heat exchanger is a heat pipe and generally known heat pipes consist of an evacuated tube containing only a small volume of a volatile liquid within a relatively large vapour space (such a heat pipe is generally known as a thermosyphon type heat pipe) . When such a heat pipe is inclined to a 1 88729 horizontal plane the volatile liquid is generally at a lower end thereof and when the lower end is heated the liquid vapourises and immediately flows to the upper or cooler end where-it condenses out giving up its latent heat of evaporation. The liquid condensate then flows under gravity back to the hot end of the pipe and since the vapour pressure is a monotonic function of temperature, there is always a pressure gradient causing mass flow from the "hot" to the "cold" end of the pipe. The reasons for the exceptionally high thermal conductivity of heat pipes are as follows: (1) the energy is carried up the pipe via mass flow; (2) the-latent heat-of vaporisation is generally"" large; and (3) . the. heat- -transfer' -co-efficients-''for evaporating liquids and condensing vapours are large.

In some known constructions of heat pipe the return of liquid from the "cool" to the "hot" end of the pipe is achieved with the aid of a capillary wick. Such heat pipes have an advantage over the thermosyphon type of heat pipe in that they retain the capability of transmitting heat, though in somewhat attenuated form, even though the "hot" end is positioned above the "cool" end of the heat pipe. The thermosyphon type of heat pipe with no wick therein has a number of advantages in that it generally acts as a thermal rectifier. Such heat pipes have very high thermal conductivity when the "hot" i ao/z v end thereof is lowest and very poor thermal conductivity when the "hot" end is highest.

A heat pipe is considered by the applicants to V offer a number of desirable features for waste heat recovery and for solar energy recovery and these can be used in a number of ways. Examples of a number of the desirable features in relation to solar energy recovery are: (i) The very high conductivity which can be used: . (a) to simplify plumbing arrangements; and (b) to maintain a low temperature difference between the solar radiation collector surface (hot end of the heat pipe) and the receiver or storage tank (cold end of the heat pipe). (ii) The rectifying characteristics referred to above ensure a low heat loss from the storage tank when the collector is colder than the storage tank. That is in a heat pipe there is a natural prevention of "back circulation" characteristics of known single phase thermosyphon systems. (iii) Since the heat pipe contains only a small quantity of water or other volatile liquid and a large free space it has an inbuilt protection against frost.

An object of the present invention is to overcome at least in part the disadvantages inherent in known I88729 constructions of heat exchanger, A further object of the present invention is to provide a heat exchanger incorporating a heat pipe which heat exchanger is economic to manufacture and which achieves a relatively efficient rate of heat transfer.

Further objects and advantages of the present'invention which should be considered in all its novel aspects will become apparent from the following descriptions which are given by way of example only.

According to a first aspect of the present invention there is provided a heat exchanger for use as a solar and the like radiant heat source collector which includes a substantially planar flat sealed wickless heat pipe chamber from which substantially all non-working gases have been evacuated provided that the spacing between the planar surfaces of the chamber is less than 10mm and greater than 3.5mm, the chamber contains a small volume of a volatile working liquid within the relatively large vapour space of the chamber, most of the wall area of the chamber being flat and acting as a heat collector end (the "hot." end) of the heat pipe, the other and conductor end (the "cool" end) of the heat pipe acting as a conductor adjacent to which is a separate conduit in which a fluid to be heated is carried and which is in heat transfer contact therewith the arrangement being such that the volatile working liquid when heated vaporises and immediately flows to the other cooler conductor end of the heat pipe where it condenses giving up its latent heat of vaporization before it returns to the collector end, the latent heat of vaporization given up by the working liquid being conducted via the wall of the chamber to the wall of the conduit to heat the fluid therein.

The term "adjacent" as used herein with reference to the fluid being adjacent the other end of the heat pipe is to be construed to include references to the fluid passing around, through or in juxtaposition with the other end of the heat pipe.

The collector end of the heat pipe can be shaped as a flat plate connected to a transmission portion of the heat pipe which terminates in a conducting end within a storage situation in which the fluid to be heated 1887 2 9 surrounds the conducting end of the heat pipe.

The conducting end of "the heat pipe can have the fluid to be heated passing therethrough in a pipe or pipes which transfer the heated fluid to a storage situation spaced from the heat pipe.

The conducting end of the heat pipe can have the fluid to be heated passing through a pipe or pipes in juxtaposition therewith, the pipe or pipes being arranged to transfer the heated fluid to a storage situation spaced from the heat pipe.

A range of materials of construction and working fluids can be used depending on the conditions in which " the heat pipe is expected to operate.

The heat exchanger according to the present invention when used in relation to solar energy recovery can either be adapted to a reflector type of collector or to a flat plate type collector and in these configurations is subject to many of the same advantages and disadvantages in known constructions of solar heating systems.

The main advantages of a reflector type collector are that the collector or concentrator is designed so as to use a small heat pipe together with a mirror or mirrors focusing the solar radiation onto the relatively small collector. An advantage of this construction of collector is the relatively small mass of metal required for its manufacture and the main disadvantages are: 1. that a tracking system is generally required to maintain the focus of the sun's solar radiation on the heat pipe; and . 188729 2. the reflector type collector is not particularly effective for diffuse radiation.

An alternative construction of heat exchanger according to the present invention for use in solar energy recovery includes a fresijel lens or lenses which concentrates solar radiation onto a small collector rather than the use of mirrors. This construction suffers from a number of disadvantages and these are mainly similar to those in known reflector systems.

The heat exchanger according to the present invention when in the form of a flat plate type, collector requires no tracking system and is moderately.effective in both direct and diffused radiation. In addition it can be packaged in a relatively compact and convenient form.• A disadvantage is that it is limited to fairly low temperatures, as the..plate surf ace-temperature rises; the efficiency falls off significantly. This fall off can, however, be reduced somewhat by the use of special selective surface coatings. In addition further improvements in efficiency can be achieved by the use of double glazing and anti-reflective glass in the glazing which is used to protect the collector from wind and re-radiation.

The main advantage of a heat pipe as a plate collector is most fully realised when the whole of the collector surface is part of the heat pipe. The only resistance to heat transfer due to metallic conduction is 188729 then the thickness of the wall of the heat pipe.

The present invention 'attempts to overcome disadvantages inherent in known flat plate collectors % as it is considered that the heat pipe has a significant advantage in efficiency over conventional flat plate designs. This advantage is bound up with the minimising of temperature gradients in the overall transfer of heat from the collector surface to the storage tank or receiver .(water tank) by making the whole of the collector part of a heat pipe.

A number of problems were overcome in achieving this and these were: (1) . A hollow.flat plate■collector.must■ support itself from collapse arising from the fact that the internal pressure is generally less than atmospheric pressure, however, for some choices of working fluid and some temperature ranges the internal pressure may exceed atmospheric pressure and in such cases the surfaces of the flat plate collector are restrained at the same spacing to ensure th&t even redistribution of returning fluid is maintained; (2) The hollow flat plate.collector is designed to allow sufficient clearance between the surfaces thereof to allow free-flow of vapour from the hot face of the plate collector to the conduction end of the heat pipe; (3) The surfaces of the hollow flat plate. 188729 collector are spaced by any suitable means, for example, by indentations on one or both surfaces thereof which adequately redistribute returning fluid over the whole area of the plate collector; and (4) The hollow plate collector must be constructed of sheets of a material which is as thin as possible not only to ensure a good heat transfer to the working fluid but also to reduce cost in manufacturing.

Other aspects of the present invention which should be considered in all-its novel aspects.will become apparent from the following' descriptions which are given by way of example only.

Examples of the present invention will now be described with reference- to the accompanying drawings in which: Figure 1A is a schematic elevation of a heat exchanger according to one aspect of the present invention incorporated into a solar water, heating system; Figure IB is a schematic section through the solar water heating system shown, in Figure lA; Figure 2A is a plan of a pattern of indentations which can be used on either one or both of the surfaces of the hot end of a heat pipe collector plate usable in the heat exchanger shown in Figure 1; Figure 2B is a cross-section in the direction of arrows IIB-IIB through the collector plate shown in Figure 2A; 18872 Figure 2C is a cross-section in the direction of arrows IIC-IIC through the collector plate shown in Figure 2A; l Figure 3A is a vertical section through the cold 5 end of the !heat pipe which is positioned within a storage tank or receiving tank; Figure 3B is a cross-section through the cold end of the heat pipe shown in Figure 3A; Figure 4 is a plan of an alternative pattern of 10 indentations which can be used on either one or both of the surfaces of the hot end of a heat pipe collector plate; Figure 5 is a plan view of an alternative construction of heat exchanger according to the present 15 invention; Figure 6 is a section•through-the- heat exchanger shown in Figure 5; Figure 7 is a perspective view from one side of part of the front of a conduction end of another 2 0 construction of heat exchanger according to the present invention showing the manner in which a fluid to be heated is passed therethrough; Figure 8 is a section through a conduction end only of yet another construction of heat exchanger according 2 5 to the present invention showing the manner in which pipes with a fluid to be heated is positioned in juxtaposition therewith; Figure 9A is cross-section through yet a further alternative construction of heat pipe collector plate; - 10 183729 Figure 9B is a perspective view from the front of part of the collector plate shown in Figure 9A; Figure 9C is a perspective view from the rear of part of the collector plate' shown in Figure 9A; and Figure 10 is a perspective view partly in section of a heat exchanger according to the present invention in a waste heat recovery situation.

A range of construction materials and working fluids can be used in heat exchangers according to the present invention depending upon the temperature range in which the heat pipe is expected to operate.

The example-of heat pipe according" to' the' invention shown in Figures 1 to 3 of .the accompanying drawings :has ■ a collector plate 1 with an upper surface 2 in the form of a plain sheet of material which, in use, is generally directed towards the "sun and is positioned so that 'as * much sun falls directly thereon as is generally possible depending on the particular area in which the collector is used.

In the example shown the rear surface 3 (shown in detail in Figure 2) of the flat collector plate 1 has a pattern of indentations 4 thereon whose size and spacing is designed to keep the upper surface 2 and surface 3 apart. The plain sheet upper surface 2 being supported by the contact points 5 against the pressure difference between the atmospheric pressure outside and the vacuum therein. The indentations 4 are also positioned and designed so as to spread the returning liquid evenly over the surface of the collector, plate 1. 1 887 29 It is to be appreciated that alternative arrangements of spacing means can be used, and can be constructed so as to maintain the upper surface 2 and * surface 3 of the collector plate 1 apart.

An alternative example is shown in Figure 4 which shows a pattern of indentations 6 formed in a rear sheet 7 which is also generally constructed of a metal material. The indentations 6 which act as spacing members between the surfaces of the collector plate are shallow trough shaped indentations which are formed in substantially the same layout or configuration as the indentations 4 shown in Figures 2A, 2B and 2C. The shallow trough shaped indentations 6 (Figure 4) ensure that in a passive state (when no solar energy is falling on the collector plate) the working fluid 7' is retained and spread over the whole surface-of the collector plate rather than being collected at the bottom thereof. .

Alternatively the surfaces of the collector plate can be constructed from plain sheets of material which are spaced apart by suitably positioned spacing members.

It is to be appreciated that the indentations described hereinbefore as being formed on the rear surface 3 of a collector plate can be formed either on the front or upper surface or on both the upper surface and on the rear surface if this is preferred.

The collector plate 1 has a manifold section 8 at its upper edge 9 and this manifold section 8 is connected to an insulated transmission pipe or tube 10 which extends to a receiver or storage tank 11. 1 887 29 The end 12 of the transmission pipe 10 which extends through the bottom of the receiver tank 11 and to within the tank to a desired height is advantageously \ as shown in Figures 1, -.3A and 3B designed and constructed so that the perimeter of the pipe 10 has a large heat transfer area to compensate for the relatively low heat transfer co-efficient in this area.

The large heat transfer area on the end 12 of the .transmission pipe 10 can be as shown in Figures 1, 3A and 3B a plurality of fins 13 spaced around the perimeter of the pipe 10.

The overall system which is shown in Figure 1 can be dimensioned to suit any particular requirements.and is positioned in a suitable casing (not.shown) which has the rear and sides thereof insulated and can have a glazed face to minimise re-radiation.

The receiving or storage tank 11 is also.insulated with any suitable insulation material, for example, fibreglass or a similar insulation material and if the receiving tank 11 is situated on the outside of a building an additional"outer cover, for example a plastic bag or casing, can be included to protect same against the weather.

The working fluid within the heat pipe can be water or other suitable volatile liquids and the collector 1 and transmission pipe 10 can be evacuated by filling with rather more fluid than required and after boiling some out of the heat pipe the end of the pipe is pinched off and sealed while the vapour is flowing vigorously 1 887 2 through it. This procedure serves to remove the air from the system and to degas the fluid. Alternatively the collector 1, manifold section 8 and transmission \ pipe l'O can be evacuated with the aid of a mechanical or other vacuum pump connected to the end of the pipe 10 and after the end thereof is sealed it constitutes a heat pipe.

In use the heat exchanger according to the present .invention shown in Figures 1A to 3B when incorporated into a solar water heating system is sited so that it is directed toward the sun and the collector 1 has as much solar radiation falling thereon as possible. The fluid in the flat collector 1 is heated and the energy is carried by the vapour "up the flat collector 1.through the manifold section 8 to the transmission pipe 10 and the heat given off. during condensation in the storage or receiving tank 11 is conducted to the fluid or"liquid to be heated.

The example of heat exchanger shown in Figures 5 and 6 of the accompanying drawings is an alternative construction of flat plate collector in which plate collector portion 14 is constructed in a manner similar to that described hereinbefore with reference to the first example. The plate collector portion 14 has the upper edge 15 thereof sealed using known construction techniques. The upper edge 15 being formed with a bulbous portion 16 (seen more clearly in Figure 6) through which a heat exchange tube or pipe 17 passes.

The heat exchange pipe 17 has flowing therethrough 18 87 2 9 a fluid to be heated. The fluid flowing from an inlet end 18 of the heat exchange'pipe 17 which is connected to a cold output pipe of a storage system and an outlet end 19 is connected to-a hot inlet of the storage system so that a thermosyphon effect causes the fluid to be heated to pass through the heat exchange pipe 17 to be heated. Alternatively if necessary a thermostatically controlled circulating pump (not shown) can be connected .to the heat exchange pipe 17 to create a flow of fluid therethrough when the temperature of the fluid at the cold output from the storage tank is greater than the temperature of the fluid in the storage tank. : The heat ..exchange - pipe- -17 is advantageously constructed from copper.and can be a simple pipe or as can be seen in more detail in Figure 6 can be constructed from a custom manufactured tube manufactured in such a way as to have, internal fins as shown at 20 on Figure 6. A heat exchange pipe 17 of this shape has improved heat transfer properties.

In the alternative construction of heat exchanger shown in Figure 7 a flat plate collector generally the same as that shown in Figure 5 without the bulbous portion 16 has a small diameter copper tube 21 passing in a serpentine path through an upper portion 22 of the plate collector 14. In this example the copper tube 21 only passes through the end walls 23 of the collector plate 14 in two places. This ensures that the manufacture thereof is simple and such a small diameter tube 21 has similar heat transfer properties to that described 188729 hereinbefore with regard to the example shown in Figures 5 and 6. Similarly this small diameter tube 21 is connected to a storage system and fluid to be heated is preferably passed therethrough by a circulating pump in the manner described hereinbefore.

In the further, alternative construction of heat exchanger shown in Figure 8, the upper end 24 of the heat pipe which is in the form of a flat plate collector .is formed into the shape shown and is evacuated and then sealed.

The front surface 25 of the upper end 2 4 of the heat pipe has fixed thereto a small diameter pipe 26 which is laid thereon in a serpentine path similar to that shown in Figure 7.

The small diameter pipe 2 6 has the fluid to be heated flowing therethrough and the heated fluid is passed in a similar manner to that described hereinbefore with regard to the example shown in Figures 5 and 6. Advantageously the copper tube is 5/16 of an inch in diameter and is soldered or brazed to the front surface 25 adjacent the upper end 24 thereof.

Alternatively the copper tube can be held in place by brackets which are themselves soldered, brazed or welded to the heat pipe. An example of such an alternative is shown in Figures 9A - C where a pipe 27 is held adjacent to a flat plate collector 28 by elongate brackets 2 9 which are soldered, brazed or welded to the front surface 30 of the collector 28. The heat transfer between the shaped copper pipe 27 and the heat pipe being 1 v,-.. ' achieved via a thermally conductive mastic or cement 31. Figure 9 illustrates the use'of a single flattened copper pipe 27 instead of multiple passes of a small bore tube.

% Other arrangements are-clearly possible for attaching 5 the water pipe 27 to the plate collector 28, for example, the'elongate bracket 29 can be replaced by a plurality of spaced brackets.

It is to be appreciated that in all the embodiments of the present invention described herein it may be 10 necessary in certain circumstances to use within the heat pipe a non-corrosive liquid to reduce the possible corrosion within the heat pipe.

In accordance with an alternative aspect of the present invention, not shown in the'drawings, a heat pipe 15 can be sited within the roof space of a building beneath a perspex or the like transparent or translucent" "sheet' or cover. Alternatively the perspex or the like cover sheet can be replaced by a plastics or glass Fresnel lens arranged to direct and concentrate the solar radiation 20 onto the hot end of a heat pipe within the roof space of a building. It is envisaged.that the heat pipe according to this alternative aspect of the invention may not require a flat plate collector.

In addition to the flat plate heat pipe being used 2 5 as a solar energy collector the heat pipe can be used in waste heat recovery situations. In such a situation tubing for carrying a fluid to be heated is not always necessary as the heat recovery can be by direct heat exchange between two ducts in which fluids are flowing.

I 8 8729 An example of such a heat exchange situation is shown in Figure 10 and in this example fluids are flowing in superimposed ducts with the flat plate pipe 32 \ positioned vertically with about half thereof in the upper duct 33 and the remainder in the lower duct 34. The lower duct 34 being the heat source in which a hot fluid flows and the upper duct 33 being the heat sink in which cooler fluid flows. Such an arrangement can be used to recover heat from one process stream to the other process stream. For example, in heat recovery from hot air based drying processes where heat will be transferred from a hot humid air stream" to a cool dry air stream.

In accordance with yet an alternative aspect of the invention the heat exchanger can be used as a substitute wet-back for a fire with a hot eind of a heat pipe situated within the region of the fire and the cold end thereof substantially as hereinbefore described within a hot water storage tank. The arrangement being such that the heat is transferred from the hot end of the heat pipe within the fire to the water in the hot water storage tank so as to efficiently use the heat produced by the fire in heating the water.

In accordance with yet a further alternative aspect of the invention a heat pipe heat exchanger is positioned with the hot end thereof in a fireplace or directed to a heat source and the cold end thereof situated and shaped in the form of a panel sited in a room or space. The arrangement is such that the heat collected by the hot end of the heat pipe is transferred - 18 ~ 1887 2 9 to the cold end thereof which acts as a heat exchanger which can be sited in a room spaced from the fireplace so that as well as using the fire to heat by radiation . \ directly into the room-in which the fireplace is situated the heat exchange panel heats the other space by radiation or convection from the cold end of the heat pipe.

Thus by this invention there is provided a heat .exchanger which overcomes a number of the disadvantages inherent in known constructions of heat exchanger and which is simple to manufacture and economic to produce and-install.

Particular embodiments of the present-invention '. have been described by way of example and it is envisaged that modifications to and variations of the invention can take place without departing from the scope of the appended claims. 19872?

Claims (33)

WHAT WE CLAIM IS:-

1. A heat exchanger for use as a solar and the like radiant heat source collector which includes a substantially planar flat sealed wickless heat pipe chamber from which substantially all non-working gases have been evacuated provided that the spacing between the planar surfaces of the chamber is less than 10mm and greater than 3.5mm, the chamber contains a small volume of a volatile working liquid within the relatively large vapour space of the chamber, most of the wall area of the chamber being flat and acting as a heat collector end (the "hot" end) of the heat pipe, the other and conductor end (the "cool" end) of the heat pipe acting as a conductor adjacent to which is a separate conduit in which a fluid to be heated is carried and which is in heat transfer contact therewith the arrangement being such that the volatile working liquid when heated vaporises and immediately flows to the other cooler conductor end of the heat pipe where it condenses giving up its latent heat of vaporization before it returns to the collector end, the latent heat of vaporization given up by the working liquid being conducted via the wall of the chamber to the wall of the conduit to heat the fluid therein. S {

2. A heat exchanger as claimed in claim 1 wherein the flat collector end is connected to a transmission portion of the heat pipe which terminates in the other end of the heat pipe which is positioned within a storage situation in which the fluid to be heated surrounds the conducting end of the heat pipe.

3. A heat exchanger as claimed in claim 1 wherein the chamber .is inclined relative to a horizontal plane with the heat collection e\nd at a lower level than the other end and all the chamber is substantially flat so that the upper edge of the chamber 1 constitutes the other end of the heat pipe.

4. A heat exchanger as claimed in claim 3 wherein the other j i end of the heat pipe has the fluid to be heated j I 88729 passing therethrough in a pipe or pipes which transfer the fluid when heated to a storage situation spaced from the heat pipe.

5. A heat exchanger as claimed in claim 4 wherein the 5 other end of the heat pipe is formed as a bulbous portion through which a heat exchange pipe or pipes passes, the heat exchange pipe or pipes having the fluid to be heated flowing therethrough.

6. A heat exchanger as claimed in claim 3 wherein the 10 other end of the heat pipe has the fluid to be heated passing through a heat exchange pipe or pipes positioned, in juxtaposition to the other end of the heat pipe, the heat exchange pipe or pipes being arranged to transfer the fluid when heated to a storage situation spaced from 15 the heat pipe.

7. A heat exchanger as claimed in any one of the preceding claims wherein a fresnel lens or lenses concentrates solar radiation onto the collector end of the heat pipe.

8. A heat exchanger as claimed in any one of the 20 preceding claims wherein a selected surface coating is applied to the collector end of the heat pipe.

9. A heat exchanger as claimed in any one of the j preceding claims wherein ;the collector end of the heat pipe is surrounded by a casing which reduces re-radiation. 25 10. A heat exchanger as claimed in claim 9 wherein the casing reduces re-radiation by the use of double glazing or an anti-reflective glass. MZ. PATENT - 21 - -— ~T1 -2 MAY 1983 , becbved I 88729 5

10 15 20

11. A heat exchanger as claimed in any one of the preceding claims wherein the chamber of the heat pipe is formed as a flat plate from a pair of sheets of materia'l which are maintained spaced apart by spacing means.

12. A heat exchanger as claimed in claim 11 wherein the pair of sheets are maintained spaced apart by a pattern of indentations formed on one of the sheets.

13. A heat exchanger as claimed in claim 12 wherein the pattern of indentations are positioned so as to spread the returning volatile liquid evenly over the surface of the chamber.

14. A heat exchanger as claimed in claim 12 or claim 13 wherein each indentation is a shallow trough shaped indentation in which a portion of volatile liquid is caught thereon so that the volatile liquid is spread over the surface of the chamber when in its passive state.

15. A heat exchanger as claimed in claim 11 wherein the chamber of the heat pipe is constructed from a pair of plain sheets of material which are spaced apart by spacing members which are positioned so as to spread the returning volatile liquid evenly over the surface of the chamber.

16. A heat exchanger as claimed in any one of claims 1 to 3 wherein the upper edge of the collector end of the heat pipe is formed as a manifold section, the manifold section being connected to an insulated transmission pipe 22 - 2 MAY 1983 138729 which extends to a receiver or storage tank.

17. A heat exchanger as claimed in claim 16 wherein the transmission pipe extends into the receiver or storage tank and has a large heat transfer area comprising a plurality of fins spaced around the perimeter of the transmission pipe.

18. A heat exchanger as claimed in any one of the preceding claims wherein the volatile working liquid within the heat pipe is water. '

19. A heat exchanger as claimed in -claim 6 wherein the upper edge of the other .end of, the • heat pipe has the heat exchange pipe or pipes fixed thereto, the heat exchange, pipe having the fluid to be heated passing therethrough.

20. A heat exchanger as claimed in claim 19 wherein the heat exchange pipe is a small tube fixed to the front of the other end of the heat pipe, the small tube being fixed in a substantially serpentine path thereon.

21. A heat exchanger as claimed in claim 19 wherein a bracket or brackets are fitted to the upper edge of the other end of the heat pipe, the brackets supporting a heat exchange pipe so heat transfer occurs between the pipe and the conducting end by way of a thermally conductive mastic or cement.

22. A heat exchanger as claimed in any one of the preceding claims wherein the heat pipe is sited in the roof space of a building beneath a perspex or the like transparent or translucent cover through which solar radiation passes to contact the collector end of the - 23 - 188729 heat pipe which is positioned within the roof space of the building.

23. A heat exchanger as claimed in claim 1 when used in a waste heat recovery situation in which the heat 5 pipe is formed as a flat heat pipe which is positioned vertically between two ducts with part thereof in a first duct and the other part thereof in a second duct, the lower duct being a heat source in which a hot fluid flows, the upper duct being a heat sink in which cool 10 fluid flows so that heat is recovered by the collector end within the hot fluid in the lower duct before it is transferred to the cool fluid flowing past the other end of the heat pipe in the upper duct.

24. A heat exchanger as claimed in claim 23 when used 15 in waste heat recovery from a hot air based drying process where heat is to be transferred from a hot humid air stream in the lower duct to a cool dry air stream in the upper duct.

25. A heat exchanger as claimed in claim 1 wherein the 20 collector end of the heat pipe is situtated within the region of a fire and the other end is positioned within a hot water storage tank, the arrangement being such that heat is transfierred from the collector end of the 25 heat pipe within the fire to the other end in the hot water storage tank to heat the water therein.

26. A heat exchanger as claimed in claim 1 wherein the collector end of the heat pipe is placed in a fireplace or directed to a heat source and the cold end of the heat 24 - -2 MAY 1983 • RSC0VED I8S729 5 10 15 20 pipe is situated in the form of a panel sited in another room or space, the arrangement being such that the heat collected by the collector end of the heat pipe is transferred to the other end which acts as a heat exchanger to heat the other room or space by radiation direct from the other end of the heat pipe.

27. A heat exchanger substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

28. A heat exchanger substantially as hereinbefore described incorporating a pattern of indentations on the collector end of a heat pipe thereof substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.

29. A heat exchanger substantially as hereinbefore described with reference to Figures 5 and 6 of the accompanying drawings.

30. A heat exchanger substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.

31. A heat exchanger substantially as hereinbefore described with reference to Figure 8 of the accompanying drawings.

32. A heat exchanger substantially as hereinbefore described with reference to Figure 9 of the accompanying drawings. 25 2 MAY 1933 I&S729

33. A heat exchanger for use in a waste heat recovery situation substantially as hereinbefore described with reference to Figure 10 of the accompanying drawings. ARTHUR GORDON WILLIAMSON, NEVILLE WALTON FOOT and KELVIN LESLIE WALLACE By their Attorneys: BALDWIN, SON & CAREY 26