HEAT EXCHANGER

Abstract

A heat exchanger. The heat exchanger comprises a plurality of primary fluid tubes configured to carry a primary fluid, a plurality of secondary fluid tubes configured to carry a secondary fluid, and a plurality of intervening layers, each intervening layer being thermally conductive and impermeable to both the primary and secondary fluids. Each intervening layer has one or more of the primary fluid tubes on a first side, and one or more of the secondary fluid tubes on a second side opposite the first side, such that the region between each pair of neighbouring intervening layers contains either primary fluid tubes or secondary fluid tubes, but not both primary and secondary fluid tubes.

Claims

1. A heat exchanger comprising: a plurality of primary fluid tubes configured to carry a primary fluid; a plurality of secondary fluid tubes configured to carry a secondary fluid; a plurality of intervening layers, each intervening layer being thermally conductive and impermeable to both the primary and secondary fluids; each intervening layer having one or more of the primary fluid tubes on a first side, and one or more of the secondary fluid tubes on a second side opposite the first side, such that the region between each pair of neighbouring intervening layers contains either primary fluid tubes or secondary fluid tubes, but not both primary and secondary fluid tubes, and such that wherever primary coolant tubes and secondary coolant tubes are in proximity, there is an intervening layer between them.

2. A heat exchanger according to claim 1, the heat exchanger further comprising a plurality of leakage channels, such that each of the primary and secondary fluid tubes forms the boundary to at least one leakage channel, the leakage channels being located between the neighbouring intervening layers.

3. A heat exchanger according to claim 2, and comprising a fault detection system configured to detect the primary fluid and the secondary fluid in the plurality of leakage channels, and to provide a signal indicating that a fault is present if either the primary fluid or the secondary fluid is detected in any of the leakage channels.

4. A heat exchanger according to claim 1, wherein the intervening layers are formed from one or more of: copper; aluminium; graphite; silicon carbide.

5. A heat exchanger according to claim 1, wherein each intervening layer comprises a plurality of channels, and the primary and secondary fluid tubes rest in the channels of the neighbouring intervening layers.

6. A heat exchanger according to claim 1, wherein inlets and outlets of the primary fluid tubes are located on a first set of external faces of the heat exchanger, and inlets and outlets of the secondary fluid tubes are located on a second set of external faces of the heat exchanger which does not overlap with the first set.

7. A heat exchanger according to claim 6, wherein: the plurality of primary fluid tubes or the plurality of secondary fluid tubes comprises a first set of fluid tubes; the other of the plurality of primary fluid tubes or the plurality of secondary fluid tubes comprises a second set of fluid tubes; the first set of fluid tubes are each straight and parallel to each other when passing through the heat exchanger; the second set of fluid tubes each have an elongate section which is straight and parallel to the first set of fluid tubes, and inlet and outlet sections which are perpendicular to the first set of fluid tubes.

8. A heat exchanger according to claim 1, wherein the primary fluid tubes and/or the secondary fluid tubes are formed from steel.

9. A heat exchanger according to claim 1, wherein the primary fluid tubes and/or the secondary fluid tubes are configured to carry a molten salt, and the intervening layers are formed from a material which is resistant to corrosion by the molten salt.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 shows a cross section of an exemplary heat exchanger;

[0012] FIG. 2 shows arrangements of primary and secondary fluid tubes within a heat exchanger

DETAILED DESCRIPTION

[0013] While the background has been described in the context of a nuclear reactor, it will be appreciated that there may be a need for heat exchangers to prevent mixing of the primary and secondary fluids in other applications, and the below description should not be taken as limited to nuclear reactors.

[0014] FIG. 1 illustrates an exemplary heat exchanger. Primary 101 and secondary 102 fluid tubes are arranged in alternating layers in the heat exchanger. Between each layer of tubes is an intervening layer 100 formed from a thermally conductive material that is impermeable to both fluids is positioned so that it is in close physical contact with both layers of tubes. The intervening material may be copper, aluminium, graphite, silicon carbide or other thermally conductive material depending on the nature of the fluids concerned. For molten salts at high temperatures, copper is a suitable material.

[0015] The shape of the intervening layer may be such that there is a small gap 103 between them bounded on two sides by two intervening layers and on at least one other side by the walls of one or more tubes within the same layer, which acts as a leakage channel as described in more detail below.

[0016] This arrangement functions as follows.

[0017] In normal operation the heat exchanger is highly efficient as the intervening layer is highly thermally conductive and heat transfer rate is dominated by the two fluid to tube heat transfer coefficients. Since heat transfer coefficient for the rapidly moving fluid in the tube is typically higher than that for the slower moving fluid in the shell of a tube in shell heat exchanger, this arrangement gives high thermal performance.

[0018] In the event of a failure of any tube, fluid from that tube leaks and the low resistance flow path may be through the channels formed by the gap between the intervening layers either side of the layer of tubes. These drain outside the heat exchanger so a failure is readily detected. Alternatively, such channels may be omitted and the fluid may be contained by the intervening layers. Most importantly however, whether or not such channels are provided, the leaking fluid does not come into contact with the alternating layer of tubes carrying the second fluid due to the impermeable layers separating them. For the two fluids to come into contact therefore requires a triple failure of both tubes carrying the two fluids and the intervening layer of material. The Dounreay heat exchanger in contrast had the intervening copper plates in a configuration where adjacent plates could be forced apart allowing direct contact between leaked fluid from one tube with the surface of adjacent tubes. Only two failures were necessary for fluids to mix.

[0019] The intervening layer of material can be manufactured by several process to have channels for the tubes but a particularly useful process is stamping of the material to create the channels. This is particularly useful where the intervening material is relatively soft, such as for copper and aluminium. Harder materials may have to have channels machined in their surfaces or be formed in the correct shape for ceramic type materials.

[0020] Multiple alternate layers of tubes and intervening material are clamped or otherwise pressed together to form a complete heat exchanger. Many arrangements of tube layers are possible as will be apparent to those of ordinary skill in the heat exchanger art, with the important factor being that where primary coolant tubes and secondary coolant tubes are in proximity, there is an intervening layer between them as described above, or equivalently that a region between each pair of neighbouring intervening layers comprises only primary coolant tubes, or only secondary coolant tubes, but not both.

[0021] A counterflow heat exchanger with end plenums 201 for fluid 2 and side plenums 202 for fluid 1 is illustrated in FIG. 2. Twenty alternating layers of tubes as shown are stacked with stamped copper intervening plates as illustrated in FIG. 1 between each layer of tubes. The whole assembly of tubes and intervening plates are clamped together with a top and bottom strong steel plate for mechanical strength. Any leaked fluid from any tube appears in the short length of tube between the intervening plates and the tube sheet allowing them to be collected without the chance of mixing with the other fluid.

[0022] Fluid 1 in FIG. 2 may be either the primary or secondary coolant fluid, with the other being fluid 2. In general, for such a structure, the primary fluid tubes or the secondary fluid tubes comprises a first set of fluid tubes 210, and the other of the primary fluid tubes or the secondary fluid tubes comprises a second set of fluid tubes 220. The first set of fluid tubes are each straight and parallel to each other when passing through the heat exchanger, i.e. they have end plenums which remain parallel up to the plenum. The second set of fluid tubes each have an elongate section which is straight and parallel to the first set of fluid tubes, and inlet and outlet sections which are perpendicular to the first set of fluid tubes, i.e. they have side plenums.

[0023] In an alternative construction, rather than an array of fluid tubes within each layer, a single wide fluid tube may be provided which spans across a substantial proportion of the layer, acting as a plate-type heat exchanger. The fluid tubes may have an aspect ratio (i.e. a ratio between two perpendicular dimensions of a cross section perpendicular to their length) of approximately 1 (e.g. round or square tubes), or may have higher aspect ratios, e.g. at least 2 or at least 5 (e.g. oval or plate-type tubes).

[0024] The fluid tubes may have internal features to aid in heat transfer to and from the fluid, such as fins, or may have additional features to impart desired flow characteristics to the fluid.

[0025] Suitable materials for the fluid tubes include steel. Where the primary and/or secondary coolant fluid is a molten salt, the respective fluid tubes may be configured to carry the molten salt, for example being made from a material which resists corrosion by the molten salt, or having an internal coating to resist corrosion, and being made from a material with a melting point higher than the operating temperature of the molten salt.