Field of the Invention The present invention relates to a heat exchanger and, in a preferred form, to a heat exchanger for use in a space heating system to heat a premises.

Background to the Invention

Conventional heat exchangers (HEXs) for hydronic central heating applications, commonly known as radiators, have changed very little over the past one hundred years or so. A typical space heating system comprises a boiler and pipes which distribute hot water, which has been heated by the boiler, to a set of heat exchanger units (radiators). At each heat exchanger unit hot water is channelled throughout the heat exchanger so that the hot water spreads over, and heats, a large enough internal surface area within the unit that sufficient heat can be dissipated into the room passively by buoyant natural convection. Newer devices operate on the same principle and may include external fins to decrease the overall size and weight of the units.

The main drawbacks of having the hot water flowing within a large internal volume of the heat exchanger are that: the water velocity and thus heat transfer capability are low so that the devices are unnecessarily large and/or must operate with high source water temperatures, typically above 7O ⁰ C, to compensate; the water cools as it crosses the HEX causing large temperature variations across the HEX (the cooler regions dissipate less heat, requiring the HEX to be physically longer to achieve the rated power); due to the very slow moving water within the HEX, the air which is dissolved within the water collects within them forming air pockets which have the effect of making the effective heat dissipating area smaller i.e. operating below design specification, requiring frequent bleeding to dispel the air. Finally, the combination of the fact that the surface area must be large enough to reach a given power dissipation and that the entire unit must have a built strength to withstand over 8 bar operating pressure results in very heavy HEXs that has the negative influence of taking a substantial time to reach a desired operating temperature/power level. The large mass of material combined with the large volume of water has a major adverse impact on start-up as well as the thermostatic control capability and room comfort. There have been some proposals to use heat pipes within heat exchanger units of heating systems. WO2005/054766 and WO2007/148149 Al describe radiators which use heat pipes to distribute heat from a hot water conduit to an area around the conduit. In WO2005/054766, a conduit carrying hot water has a plurality of hollow sleeves formed in it. The sleeves are open on one side and heat pipes can be slotted into the sleeves, as required. In WO2007/148149 Al a radiator comprising a plurality of heat pipes is mechanically separate from a conduit carrying hot water. The radiator can be clamped around the hot water conduit to provide a thermal link between the hot water conduit and the heat pipes in the radiator. Although these systems offer ease of mounting and dismounting of the radiator system, they will suffer from very poor thermal coupling between the water and the heat pipes which will severely limit the power output, and therefore usefulness, of the radiators.

Other radiators for central heating systems, which use heat pipes to distribute heat, are described in US 2004/0022529, WO00/70286 Al and WO03/048669 Al.

The present invention seeks to provide an improved heat exchanger.

Summary of the Invention

A first aspect of the invention provides a heat exchanger unit for a heating system comprising: a manifold having a fluid inlet for connection to a fluid conduit of the heating system and a fluid outlet, the manifold defining a flow path for fluid between the fluid inlet and the fluid outlet, the manifold having an outer wall with a plurality of apertures; a plurality of heat pipes, each heat pipe comprising a body having an outer sleeve, wherein each heat pipe is sealingly mounted in a respective aperture of the wall of the manifold, with a major part of the heat pipe extending outwardly from the manifold and one end of the heat pipe positioned in the flow path of the manifold, such that there is direct contact between the outer sleeve of the heat pipe and fluid in the flow path.

Each of the heat pipes absorbs thermal energy from fluid in the flow path (e.g. a hot water flow) and buses the energy, via a phase change of a working fluid within the heat pipe, to a heat dissipating region of the heat pipe. A further phase change of the working fluid within the heat pipe releases the heat energy. The use of phase change as the means of thermal spreading facilitates very compact, high power, lightweight and quickly-responding heat exchange units. Good thermal transfer occurs between hot fluid in the flow path of the heating system and the heat pipe by providing direct contact between fluid in the flow path and the outer sleeve of the heat pipe.

Advantageously, a plurality of fins are mounted to multiple ones of the heat pipes. This helps to increase the surface area of the heat dissipating region and helps to dissipate heat into the area surrounding the heat exchanger unit. Preferably, the fins are thermally bonded to the heat pipes, such as by soldering or welding. Alternatively, the fins can be removably mounted to the heat pipes.

Advantageously, the heat pipes each comprise an amount of a fluid, such as water, and a wick structure which is arranged to draw condensate along the heat pipe. This allows the heat pipes to be horizontally oriented, which offers better heat transfer between the heat pipes and the surrounding atmosphere. Advantageously, the plurality of heat pipes are configured as an array of heat pipes with at least two layers of heat pipes, to provide significant heat output with a compact physical size. The heat pipes in the layers are advantageously configured such that heat pipes are offset from one another in a direction of convection flow, such that a heat pipe of one layer does not overlie a heat pipe of another layer in the direction of convection flow.

Compared with conventional units which distribute hot fluid of the heating system throughout heat exchange units, the present heat exchanger unit is capable of approximately double the power density (i.e. thermal power output/volume of HEX). This significant increase in power density facilitates the following: for a desired thermal power rating, typically at hot water inlet of >70°C, the new HEX will occupy a much smaller space i.e. it is about twice as compact; the new HEX can dissipate significant thermal power at much lower source water temperatures (-5O ⁰ C). The lower source water temperatures is significant as renewable/sustainable energy technologies, such as geothermal heat pumps, condensing boilers and solar central heating, do not typically generate high temperatures and are best suited at ~50°C (without auxiliary heaters). As such, massively oversized radiators or under floor heating are required to achieve a desired power output. The heat pipe based HEXs can dissipate the required thermal load compactly and efficiently and are pluggable into existing central heating systems. In conventional fuel-fired boiler systems the water operating temperature is related to the rate of fuel consumption. The use of heat exchanger units according to the present invention can allow the boiler set point temperature to be reduced by 20-30 ⁰ C without compromising room comfort. This will result in significant fuel cost savings. Low Surface Temperature (LST) radiators (~50°C) are the required solution for those installations where there may be vulnerable people at risk of burning themselves or other locations where the maximum operating temperature must be controlled such as nurseries, schools, clinics and hospitals. Heat exchanger units according to embodiments of the present invention are a good fit for such applications. More generally, the system can operate efficiently with a wide range of water inlet temperatures through to pressurised hydronic systems with water or steam inlet temperature up to 110 ⁰ C.

The reduced material usage and low volume of water within the system is important on two fronts: the reduced weight has a positive impact on overall costing due to reduced material usage and shipping costs; tight thermal regulation is imperative from both an energy savings standpoint as well as from general room comfort point of view. The reduced mass of heat exchanger units according to embodiments of the present invention facilitates a quick thermal response system which offers superior and decentralized thermal regulation compared with the heavy and sluggish systems currently in use. A further disadvantage of conventional heating systems is that trapped air can require continual maintenance and can negatively impact room comfort (the primary purpose of the radiators).

The technology is particularly useful for hydronic central heating systems that utilize a heated flow of liquid, typically water, in conjunction with heat exchanger units (HEXs) mounted within a premises, to dissipate the heat into rooms of the premises. The premises can be a domestic, commercial or industrial premises. The technology can also be used in other heating applications, such as energy storage systems.

Brief Description of the Drawings Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings in which:

Figures IA- IE show a first embodiment of a heat exchanger unit; Figure 2 shows a heat pipe for use within a heat exchanger unit; Figure 3 shows a second embodiment of a heat exchanger unit;

Figure 4 shows an exploded view of the heat exchanger unit of Figure 3;

Figure 5 shows a cross-section through the heat exchanger unit of Figures 3 and 4; Figure 6A shows another cross-section through the heat exchanger unit of

Figures 3 and 4;

Figure 6B shows a variant of Figure 6A, with additional heat pipes;

Figure 7 shows a plot of temperature in the region surrounding the heat exchanger unit of Figure 3; Figure 8 shows a plot of flow velocity in the region surrounding the heat exchanger unit of Figure 3;

Figure 9 shows an embodiment of a heat exchanger unit with heat pipes extending from two perpendicular faces of a manifold;

Figure 10 shows an embodiment of a heat exchanger unit with fins mounted at a non-perpendicular angle with respect to the longitudinal axis of the heat pipes;

Figures 11 and 12 show a further embodiment of a heat exchanger unit with fins aligned with the longitudinal axis of the heat pipes.

Detailed Description of the Drawings Figures IA- IE show a first embodiment of a heat exchanger unit (HEX) for use in a heating system. The heat exchanger unit 10 comprises a manifold, or chamber, 11 with a fluid inlet 12 for connecting to a hot water conduit of the heating system, and a fluid outlet 13 for dispelling cooled fluid to a cooled water conduit of the heating system. The heat exchanger unit 10 also comprises a set of heat pipes 20 which, in this example, is a set of two heat pipes 20. One end 21 of each heat pipe, which will be called a "collector end", is mounted inside the manifold 11. The remainder of each heat pipe extends outwardly from the manifold 11. The region 22 of each heat pipe positioned outside of the manifold 11, will be called the "ejector region" as, in use, heat is ejected from this region. In this embodiment, the heat pipes 20 are configured such that they are aligned parallel with one another and are perpendicular to the direction of fluid flow through manifold 11. A side wall 14 of the manifold 11 has two apertures which are dimensioned to receive the heat pipes 20. The heat pipes extend through the apertures in wall 14. A fluid-tight seal is provided between the heat pipes 20 and the wall 14. The fluid-tight seal can be achieved by soldering, o-rings or any other suitable sealing technique. Advantageously, the heat pipes are permanently fixed in the wall 14 of the manifold, such as by soldering, as heat pipes have a long operating life and low failure rate, and because the provision of multiple heat pipes allows the HEX to operate at an acceptable output power even in the unlikely event that a heat pipe were to fail. A set of convector fins 23 are mounted to the heat pipes 20 along the full length of the ejector region 22 of the heat pipes between manifold wall 14 and the distal end of heat pipes 20. Each fin 23 is mounted perpendicularly to the longitudinal axis of the heat pipes 20. In a similar manner to wall 14 of the manifold 11, each fin has two apertures formed in it which are dimensioned to receive the heat pipes. Each fin 23 is thermally bonded to the heat pipes, such as by soldering, welding or any other suitable technique. The fins 23 have a number of advantages. Firstly, the fins help to distribute heat to the region surrounding the ejector region of the heat pipe. Considering the basic convection heat transfer equation: Q = h - A(T _hot - T _coM ) where: Q is the heat transfer rate; h is the heat transfer coefficient;

A is the area over which the heat is being transferred;

T _hot is the temperature of hot surface; T _cold is the temperature of the cold fluid.

Due to low heat transfer coefficients, h, of gases, when compared with liquids, the use of fins causes an increase in the area, A, over which heat is transferred and therefore compensates for the lower heat transfer coefficients. As well as this, because fins are generally made of very thin pieces of metal attached to the primary surface (a pipe in this case), a relatively large amount of additional surface area is achieved with a small amount of material.

A second advantage of the fins is that they define a convection path for the air. Heat exchange occurs in the regions defined by the heat pipe ejector section and the fins attached. As the air in such sections is heated, it naturally begins to rise. Since the air closest to the pipe receives the most heat and rises fastest, the fins act as 'channels', allowing only the air within the channel below the pipe to take the place of the upward rushing hot air. A third advantage of the fins is that they provide mechanical protection for the heat pipes. With the complex nature of the internal physics of a heat pipe, during both active and inactive phases of the life cycle, any motion of, or contact with, the heat pipe could reduce the life expectancy of the HEX. The fins act as a barrier between the heat pipe and the external environment and stabilise the structure.

Figures IB- IE show this embodiment of the heat exchanger from other views. Dimension labels which are defined below: H _m External height of the manifold (m)

W _m External width of the manifold (m) L _m External length of the manifold (m) t _m Thickness of the manifold (m)

D _{p o} Outer diameter of the heat pipe (m)

D _p _ _! Inner diameter of the heat pipe (m)

L _p Total length of the heat pipe (m) S _f i _n Spacing between fins (m) tfi _n Thickness of fins (m)

Le _j ector Length of the heat ejector section of the heat pipe (m)

Lcoiiector Length of the heat collector section of the heat pipe (m)

Pd Distance of the inset pipe away from the outer manifold wall (m)

Figure 2 shows a cut-away drawing of an example heat pipe 20 which can be used in the heat exchanger of Figure 1. The heat pipe 20 comprises a hermetically sealed tube 24 which contains an amount of fluid, typically water, under a partial vacuum. When heat is applied at one end 21 of a heat pipe the water within it evaporates. The vapour which is generated at the heated end spreads to the cooled end 22 of the heat pipe. Here the extraction of energy causes the vapour within the heat pipe to condense back to a liquid phase thus releasing the heat that was absorbed at the heated region albeit at a different location i.e. at some location remote from the heated end 21. A porous wick structure 25 is wrapped around the inner wall 24 of the heat pipe and, in use, draws liquid condensate back to the heated section where it is once again vaporized.

Operation of the apparatus of Figure 1 will now be described. In use, heated fluid (water) from a conduit of the heating system enters manifold 11 via the fluid inlet 12 and passes around the collector ends 21 of heat pipes 20. Heat energy in the heated fluid is absorbed by the collector ends of the heat pipes. Cooler water is expelled from fluid outlet 13. Within each heat pipe, water at the collector end 21 of the pipe is vaporised as heat is absorbed from the fluid flow through manifold 11. As the heat pipes are partially evacuated, the water within them boils at a temperature which is lower than the source hot water temperature. The vapour (steam) generated inside the heat pipes at the collector end flows away from the collector region 21 of the heat pipes to the heat ejector regions 22, where it condenses on the inner wall of the heat pipes thereby releasing the heat at a location which is remote from where it was absorbed. The metallic fins 23 in the heat ejector region help to achieve a required power dissipation for the given operating temperature. In the heat ejector region 22, which will be referred to as a finned tube bundle, the location of the heat pipes and the spacing between the fins is configured for optimal heat transfer by buoyant natural convection (i.e. hot air rises) and thermal radiation. Figures 3-6A show a second embodiment of a heat exchanger unit (HEX) for use in a heating system. The heat exchanger unit 110 comprises a manifold 111 with a fluid inlet 112 for connecting to a hot water conduit of the heating system, and a fluid outlet 113 for dispelling cooled fluid to a cooled water conduit of the heating system. The heat exchanger unit 110 also comprises a plurality of heat pipes 120. In this example, there is a total of six heat pipes 120 which are arranged in two sets of three heat pipes. A first set of heat pipes are positioned on one side of the manifold 111 and pass through apertures 119 in a plate 117 which defines a side wall of the manifold 111. A second set of heat pipes are positioned on the opposite side of the manifold 111 and pass through apertures in a plate 118 which defines a side wall of the manifold 111 on the opposite side of the manifold to plate 117. All of the heat pipes 120 have a collector end 121 mounted within the manifold 111. The heat pipes in each set are arranged on two different levels, denoted Layer 1 (Ll) and Layer 2 (L2). The two sets of heat pipes are arranged such that they interlock with one another in the manifold 111, i.e. all of the heat pipes in the first set occupy positions which are different to those of the second set. This is most clearly shown in Figure 6A, which shows a cross section along the line 'A- A' through the manifold 111. The first set of heat pipes is represented as "1" and the second set of heat pipes is represented as "2. Plates 115, 116 are mounted within the manifold 111 and define a non- linear, or serpentine, flow path 140 through the manifold between the fluid inlet 112 and fluid outlet 113. The manifold has three channels labelled Cl, C2 and C3 in Figure 6A. The temperature of the water decreases continually, due to heat being extracted, as the water flows along the flow path 140 from inlet 1 12 to outlet 113. Cl is thus the 'hottest' channel and C3 is the 'coolest' channel with C2 intermediate between the two. The heat pipes of the two sets "1", "2", are arranged such that the flow path 140 alternately passes a heat pipe from each set "1" and "2" in each of the channels Cl, C2 and C3. In this way, each set of heat pipes has a heat pipe immersed in the 'hot', 'intermediate', and 'cool' channels facilitating optimal temperature uniformity and heat transfer across all of the fins 123 by avoiding concentrated 'hot zones' and/or 'cold zones' on the fins. This also helps to evenly distribute heat from the fluid flow path 140 between the two sets of heat pipes, and hence the regions on each side of the heat exchanger unit.

The division of the manifold 111 in this way also helps to increase the flow velocity through the manifold 111. The maximum velocity of the water in the channel is proportional to the flow rate and inversely proportional to the area of the smallest gap:

V = *

A

Where V is the flow velocity;

^ is the volume flow rate of the fluid in the system; A is the cross sectional area.

A high velocity is desirable since it will provide a higher heat transfer coefficient and thus higher overall power throughput. In this example, wall 115 is mounted to, or formed integrally with, plate 118 and wall 116 is mounted to, or formed integrally with, plate 117. This eases assembly of the unit 110. Each plate 115, 116 has a length which is sufficient to span the width of manifold 111.

In Figure 6A the heat exchanger can be installed as shown in the drawing, with the layers being horizontally aligned. The serpentine flow path serves the heat pipes column-by-column, i.e. past a first column of heat pipes (heat pipe in layer 1 and then the heat pipe in layer 2) and then past the next column of heat pipes. An alternative would be to route fluid along a serpentine flow path on a row-by-row basis, i.e. past the pipes in layer 1 first and then past the heat pipes in layer 2. In both options the flow path alternately passes a heat pipe in the first set and then a heat pipe in the second set, which helps to evenly distribute heat on both sides of the HEX.

A set of convector fins 123 are mounted to the ejector regions of heat pipes 120 along the full length of the ejector region of the heat pipes between manifold wall 117, 118 and the distal end of heat pipes 120. Each fin has a set of apertures (in this example, three apertures) formed in it which are dimensioned to receive the heat pipes. Each fin 123 is thermally bonded to the heat pipes, such as by soldering or welding. Each fin 123 is mounted perpendicularly to the longitudinal axis of each heat pipe 120. The convector fins 123 define a convection path for air and help in transferring heat energy from each set of heat pipes 120 to the air in the convection flow path. In Figure 4 the optimal convection flow path is shown by arrow 150, and passes upwards through the finned heat pipe units. It can be seen that the heat pipes in each set are arranged such that heat pipes do not overlie one another in the direction of convection 150, i.e. a pipe "1" in layer L2 does not overlie a pipe "1" in layer Ll and, similarly, a pipe "2" in layer L2 does not overlie a pipe "2" in layer Ll . The thermal effects of this arrangement are shown in Figures 7 and 8.

The manifold 111 of the heat exchanger unit 110 is also provided with a bleed valve 131, as shown in Figure 4. This allows air to be bled from the manifold when the heat exchanger is first plumbed into a heating system and the system is filled with water, in a similar manner to conventional heat exchangers/radiators used in heating systems.

Figure 7 shows a cross-section through each of the first set and second set of heat pipes, and a plot of temperature in the region surrounding the heat pipes. The temperature profile is taken at a central position within a fin channel along the pipe. The majority of heat transfer can be seen to take place in the lighter grey regions to the sides of the pipes. The white areas around the pipes indicate the maximum temperature and initiation point for the heat. The darkest regions around the outside of the fin channel indicate the colder surrounding air. Figure 8 shows a plot of flow velocity in the region surrounding the heat pipes.

In Figure 8, a velocity plot of the same channel view as in Figure 7, the maximum velocities are seen to exist in the central, black, areas of the channels. This is caused by buoyancy forces which accelerate the air away from the respective tubes. In the left hand image of Figure 8, the interaction of the air flows (accelerating away from the individual pipes) with each other causes increased velocity and a lower pressure region in this area. This lower pressure region causes cold air to be dragged into this middle section, enhancing the heat transfer process. Comparing the left and right figures of Figure 8, it appears that the heat transfer process is enhanced slightly more by the left configuration, due to greater interaction between air flows leading to higher a higher velocity as indicated.

Figures 3-6A show a heat exchanger unit having a total of six heat pipes, arranged in two sets of three pipes. Other configurations of heat pipes can be used, depending on factors such as: the power output required from the heat exchanger unit; the operating temperature of the fluid circulating in the heating system. Other numbers of layers can be used, although it has been found that a two-layered structure with an alternation between pipes of the two sides in the direction of flow through the manifold is particularly advantageous as it distributes heat between the two sides of the apparatus, and avoids two heat pipes being located above one another, while also allowing a compact manifold. Figure 6A shows a serpentine path between an inlet 112, mounted on a lower side of the manifold 111, and an outlet 113 mounted on an upper side of the manifold 111, with three columns of heat pipes. If the manifold has an even number of columns (e.g. two rows or four rows) then the inlet 112 and outlet 113 can be positioned on the same side of the manifold, which can reduce the amount of pipe-work external to the heat exchanger unit 110.

Figure 6B shows a variant of the heat exchanger with a set of six heat pipes on each of a first side (pipes "1") and a second side (pipes "2") of the manifold 111. Each set of heat pipes comprises two layers of three pipes. The heat pipes on each side are offset from one another so that a pipe "1" in layer L2 does not directly overlie a pipe "1" in layer Ll. In each channel Cl, C2, C3 of the manifold 111, fluid flow alternately encounters a pipe of each set. In Fig.βB the sequence of heat pipes in the flow path 140 is repeated between channels (e.g. set "2" followed by another pipe in set "2") as this simplifies the mechanical layout of the heat pipes on each side, allowing them to be on the same level. However, in another variant the sequence could alternate between "1" and "2" pipes along the flow path 140.

The illustrated embodiments show heat exchanger units 10, 110 with fins mounted perpendicularly to the heat pipes. This has an advantage of simplifying the construction of the heat exchanger unit, as finned heat pipe bundles can be assembled by sliding an apertured fin 23, 123 along a set of heat pipes to the required position and fixing the fin in position, such as soldering or welding the fin to the heat pipe. This process is repeated for each fin. The simple construction reduces the cost of the heat exchanger unit. This arrangement also provides a good thermal design with a small overall size. In use, the heat exchange unit 10, 110 is mounted with the longitudinal axis of the heat pipes aligned horizontally with a floor surface. This defines a vertical convection path between the fins 23, 123.

The heat exchanger shown in Figures 3-6B has a set of heat pipes extending from opposite sides of a manifold 111. Other variants of the heat exchanger are possible. Heat pipes can extend from any two or more faces of the manifold. Figure 9 shows a top view of a heat exchanger unit which is particularly suitable for mounting in the corner of a room. The heat exchanger comprises a manifold 211 having heat pipes 220 which extend from a first face 217 and a second face 218, with the first and second faces being perpendicular to one another. Additional sets of heat pipes can be provided which extend from other faces 215, 216 of the manifold. In a further variant, not shown, the heat pipes radiate from the manifold in the form of an arc or circle. The manifold does not have to be cube or cuboid shaped. Such variants can be more aesthetically pleasing. It will be appreciated that the more elaborate variants described here, which depart from an array of parallel heat pipes, and a cuboid manifold, will increase manufacturing costs and may suffer from a poorer heat output.

Another variant of the heat exchanger has a set of heat pipes on a single side of the manifold 111, multiple layers of heat pipes, and a non-linear or serpentine flow path around the set of heat pipes. As described above, the heat pipes are advantageously offset from one another such that a heat pipe in one layer does not directly overlie a heat pipe in another layer. This variant of the heat exchanger will appear as previously shown: in Figures 3 to 5 (but with heat pipes on only one side of the manifold); in Figures 6A and 6B (but with the manifold containing only one of the sets of pipes "1", "2"); and it will have the same performance results as shown in Figures 7 and 8 (but the graph for only one of the sides will apply).

In another heat exchanger, shown in Figure 10 in side view, the fins 323 are not aligned perpendicularly to the longitudinal axis of each heat pipe. Figure 10 can represent a top view, or a side view, of the heat exchanger unit. It is preferred that the channels defined by the fins are vertically aligned when the heat exchanger is installed in a premises, as this is most thermally efficient. However, it may be desirable to mount the heat pipes at an angle to the horizontal. One technical advantage which can result from this inclined mounting is that the wick can be removed, or reduced, compared to embodiments where the heat pipes are horizontally mounted. In this variant, or any other variant, the heat pipes do not have to be straight along their length.

In another heat exchanger, the fins are mounted such that the longitudinal axis of a fin lies parallel to the longitudinal axis of a heat pipe. It is preferred that the channels defined by fins are vertically aligned when the heat exchanger is installed in a premises, as this is most thermally efficient. This variant requires fins to be secured to heat pipes along the longitudinal axis of the heat pipes, which is more difficult and expensive to manufacture. An example of this variant of the heat exchanger is shown in Figures 11 and 12. The heat exchanger with a manifold 411 and a set of heat pipes 420. Each heat pipe 420 has a square cross-section. Fins 424 are mounted to each heat pipe 420 along the forward- facing surface of the heat pipe, and fins 423 are mounted to each heat pipe 420 along each side surface of the heat pipe. The gap between adjacent heat pipes 420 is bridged by a set of fins 423 which define channels for air flow.

The collector end of each heat pipes within the manifold can be a bare heat pipe sleeve, or the sleeve can be provided with additional features, such as rings or ribs, which can help to maximise the surface area of the heat pipe which is brought into contact with the fluid flow path.

In the illustrated embodiments the fins mounted to the ejector region of the heat pipes are shown as planar structures. The fins can alternatively be formed as corrugated structures, which can, for a given volume of space, help to maximise the surface area of each fin which is brought into contact with air of the convection flow path.

It is preferred that the fins are thermally bonded, in a permanent manner, to the heat pipes. In an alternative embodiment, the fins can be provided as a fin bundle which can be removably mounted to the heat pipes. This has advantages of allowing cleaning or maintenance of the heat pipes, but will provide a less effective heat transfer. The manifold can be provided with a removable panel. The removable panel is provided with a seal which gives a fluid-tight seal against the manifold casing when the panel is fitted in normal use. The panel can be removed to allow access to the interior of the flow path, within the manifold, for cleaning. This can allow an efficient cleaning to be performed.

The heating can take place without the need for any hot fluid to enter the heated space. This is a particular advantage for applications such as clean rooms which cannot tolerate any water leak. The fluid conduits of the heating system and manifold 11, 111 are located outside of the space to be heated. The adiabatic section (see Figure 2) of the heat pipes 20, 120 are located in the boundary (e.g. wall or ceiling) of the space to be heated and the finned heat pipe bundle 22, 23; 122, 123 is located within the heated space. Although it is desirable that unassisted convection occurs around the heat exchanger, it is also possible to provide a fan with the heat exchanger to draw air into the channels, or force air out of the channels defined by the fins to enhance heat transfer.

The heat exchanger can also be used to implement cooling of an environment, such as during summer periods. Advantageously, a fan is provided to draw air from an environment across the finned bundle of heat pipes. Heat from the surrounding air is absorbed by the finned bundle end of the heat pipes and conveyed along the heat pipes to the manifold end of the heat pipes, where it is transferred to fluid flowing through the manifold. For cooling operation, the fluid flowing through the manifold is at a cooler temperature than the environment to be cooled. Thus, the heat exchanger can be used as part of a heating system or a cooling system.

Although not shown in the drawings, a casing, or "skin" can surround the heat exchanger unit. One purpose of the skin is to provide a more appealing physical appearance to the heat exchanger unit. The words "comprises/comprising" and the words "having/including" when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.