US20170030660A1

US20170030660A1 - Heat-exchanger module with improved heat exchange and compactness, use with liquid metal and gas

Info

Publication number: US20170030660A1
Application number: US15/303,756
Authority: US
Inventors: Lionel Cachon; Francesco Vitillo
Original assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2014-04-16
Filing date: 2015-04-14
Publication date: 2017-02-02
Also published as: EP3132222A1; FR3020135A1; TN2016000436A1; WO2015159213A1; KR20160145155A; EP3132222B1

Abstract

The invention relates to a heat-exchanger module with a longitudinal axis (X) comprising at least two fluid circuits, the first of which comprises at least one pair of channels (1, 2) for fluid circulation, each extending parallel to the longitudinal axis (X), wherein the two channels of a single pair are stacked on top of one another and are in communication with one another in a plurality of crossing areas (3) each defining an area for mixing the fluid with itself inside the first circuit.

Description

TECHNICAL FIELD

The present invention concerns a heat-exchanger module with at least one integrated fluid circuit.
The invention relates more particularly to the production of a new type of heat-exchanger module for improving the compactness and the thermal power exchanged at equivalent head losses.
Known heat exchangers comprise or at least two circuits with internal fluid circulation channels. In exchangers with a single circuit, heat is exchanged between the circuit and a surrounding fluid in which it is immersed. In heat exchangers with at least two fluid circuits, heat is exchanged between the two fluid circuits.
Chemical reactors are known that employ a continuous process in which a small quantity of co-reagents is injected simultaneously at the inlet of a first fluid circuit, preferably equipped with a mixer, and the chemical product obtained is recovered at the outlet of said first circuit. Some of these known chemical reactors comprise a second, so-called utility fluid circuit, the function of which is thermal control of the chemical reaction, either by providing the heat necessary for the reaction or to the contrary by evacuating the heat given off thereby. Such chemical reactors with two fluid circuits with utility are usually called exchanger-reactors.
The present invention concerns both the production of heat-exchanger modules with the sole function of exchanging heat and integrating one or two fluid circuits and the production of exchanger-reactors. Accordingly, it must be understood in the context of the invention that by “heat-exchanger module with at least two fluid circuits” is meant both a heat-exchanger module with the sole function of exchanging heat and an exchanger-reactor.
The main use of a heat-exchanger module for exchanging heat between two fluids according to the invention is its use with a gas as one of the two fluids. This may advantageously mean a liquid metal and a gas, for example liquid sodium and nitrogen.
The intended main application of a heat-exchanger module according to the invention is to exchange heat between a liquid metal, such as liquid sodium, from the secondary loop and nitrogen gas from the tertiary loop of a fast neutron reactor (FNR-Na) or a sodium fast reactor (SFR) cooled with liquid metal, such as liquid sodium, which is part of the family of reactors of the so-called fourth generation.
A heat-exchanger module according to the invention can also be used in any other application necessitating an exchange between two fluids, such as a liquid and a gas, preferably when it is necessary to have a compact heat exchanger of high thermal power.
In the context of the invention “primary fluid” has the usual meaning in a thermal context, namely the hot fluid that transfers its heat to the secondary fluid, which is the cold fluid.
A contrario, in the context of the invention “secondary fluid” has the usual meaning in a thermal context, namely the cold fluid to which heat is transferred from the primary fluid.
In the main application, the primary fluid is sodium and circulates in the so-called secondary loop of the thermal conversion cycle of an FNR-Na reactor and the secondary fluid is nitrogen and circulates in the tertiary loop of said cycle.

PRIOR ART

Tube heat exchangers are for example shell and tube heat exchangers, in which a bundle of tubes that are straight or curved into a U shape or helical shape is fixed to perforated plates and disposed inside a sealed enclosure called the shell. In these shell and tube heat exchangers, one of the fluids circulates inside the tubes and the other fluid circulates inside the shell. These shell and tube heat exchangers have a large volume and their compactness is therefore low.
Existing so-called plate heat exchangers have considerable advantages compared to existing so-called tube heat exchangers, in particular their thermal performance and their compactness thanks to a ratio of the area to the volume of heat exchange that is favorably high. Compact plate heat exchangers are used in many industrial fields.
In this field of compact plate heat exchangers, numerous elementary shapes defining thermal exchange patterns have been developed.
There may be cited first plate heat exchangers integrating fins in which a thermal exchange pattern is defined by a structure delimited by fins, the structures being mounted between two metal plates and having highly varied geometries. The exchange pattern may differ between one of the two fluid circuits of the exchanger and the other one. The metal plates are usually assembled by brazing or diffusion welding.
Also known are plate heat exchangers with undulations or corrugations. The undulations are created by pressing a plate separating the two fluid circuits. Because of this, the exchange pattern is identical for each of the two fluid circuits. The flow of fluids generated by this type of exchange pattern is three-dimensional and therefore offers high performance. The plates are either bolted together or assembled by welding their periphery (standard welding or diffusion welding).
Finally there are known plate heat exchangers with machined grooves, the machining being carried out by mechanical or electrochemical means. The channels defined by the machining are of millimeter section and are most often continuous with a regular zigzag profile. The plates are assembled by diffusion welding enabling welding at all points of contact between two adjacent plates. This type of plate exchanger with machined grooves is therefore intrinsically highly pressure resistant.
The inventors of the present invention have evaluated these various plate heat exchanger technologies to design an exchanger of heat between a gas and a liquid metal in the context of the production of a nuclear reactor of the so-called fourth generation family of reactors, that is to say in a configuration of heat exchange between an excellent heat conductor, the liquid metal, and a fluid with much lower thermal transportation properties, the gas. They have reached the following main conclusions:

- plate heat exchangers with integrated fins are a design that is difficult to make compatible with a nuclear component;
- pressed plate heat exchangers, although having a high compactness and a high unit thermal power, are not compatible with a pressure difference between gas and liquid metal in the context of the nuclear reactor cooled with the latter and therefore are not robust;
- plate heat exchangers with machined grooves are robust and make it possible to satisfy both the thermohydraulic and thermomechanical performance requirements but for an equivalent head loss are less compact than pressed plate exchangers.

There is therefore a need to improve further compact plate heat exchangers, notably with a view to conferring on them both a high unit thermal power and great compactness, whilst guaranteeing their robustness.
The object of the invention is to at least partly address this requirement.

SUMMARY OF THE INVENTION

To this end, the invention consists in a heat-exchanger module with a longitudinal axis (X) comprising at least two fluid circuits, the first of which comprises at least one pair of channels for fluid circulation, each extending parallel to the longitudinal axis (X), wherein the two channels of the same pair are stacked on top of one another and are in communication with one another in a plurality of crossing areas each defining an area for mixing the fluid with itself in the first circuit.
In other words, the invention essentially consists in proposing a fluid circuit in which the flow is three-dimensional by virtue of the presence of the crossing areas and that may be produced using the technology for manufacturing plate heat exchangers with machined grooves, of proven robustness.
Accordingly, for the same dimensions, a heat-exchanger module according to the invention has both heat exchange performance improved over a prior art heat exchanger with machined plates and a robustness improved over a prior art pressed plate heat exchanger.
In other words, for the same thermal performance a heat-exchanger module according to the invention has improved compactness compared to a prior art heat exchanger.
Now, this represents a key advantage in the main application of fourth-generation nuclear reactor heat exchangers because it is then possible to envisage limiting the number of heat exchangers, the size of the buildings containing them.
According to one advantageous embodiment, each channel has at least in part a curved zigzag profile, preferably regular over its length.
According to this embodiment, the regular curved zigzag profile advantageously includes bends and straight segments, a straight segment connecting two consecutive bends.
A regular curved zigzag profile of this kind for each of the channels that cross according to the invention enables great design flexibility through varying the geometrical parameters of each channel, notably the geometry of the section of each channel, the angle of the straight segments of the channel, the length between two bends, the radius of curvature of the bends, the distance between the channels.
Preliminary studies carried out by the inventors have shown that:

- the coefficient of thermal exchange and also the coefficient of friction increase with the angle of the straight segments;
- the coefficient of thermal exchange and the coefficient of friction decrease if the length between two bends increases;
- the coefficient of thermal exchange and the coefficient of friction increase when the radius of curvature decreases.

The metal constituting the heat-exchanger module according to the invention is chosen as a function of the conditions of its required use, namely the pressure of the fluids, the temperatures and natures of the fluids circulating through the module. This may for example be aluminum, copper, nickel, titanium or alloys of these elements as well steel, notably alloy steel a stainless steel, or a refractory metal chosen from alloys of niobium, molybdenum, tantalum or tungsten.
The fluid circulation channels have a width and a height that notably depend on the nature and the characteristics of the fluids conveyed and the required exchange of heat. The widths and heights may notably vary along the path of the channels.
Each channel preferably has at least in part a curved zigzag profile. The curved zigzag profile is preferably regular over its length.
According to one advantageous embodiment, the regular curved zigzag profile includes bends and straight segments, a straight segment connecting two consecutive bends.
A channel may have an oval, circular, rectangular or square section.
A section with a plane of symmetry (rectangular, square or circular) favors disruption of the flows and better mixing of the fluid with itself
Square or rectangular sections also make possible greater compactness.
The advantage of having a circular or oval section is to simplify the manufacture of the channels: in fact, an electrochemical erosion machining process, easy to implement, may be used.
Defining the channel by its hydraulic diameter (Dh), the preferred dimensions are as follows:

- the radius of curvature of the bends is between 0.5 and 3 Dh inclusive;
- the length of the straight segment is between 4 and 8 Dh;
- the angle between the straight segment and the longitudinal axis (X) is between 10 and 45° inclusive.

According to one advantageous embodiment, the curved zigzag profiles are identical for the two channels and symmetrical to one another with respect to the longitudinal axis (X) or a parallel axis.
According to a variant embodiment, the two channels of the same pair join at their longitudinal ends in the same rectilinear channel portion substantially parallel to the longitudinal axis (X).
According to a first embodiment, each of the two fluid circuits includes at least one pair of fluid circulation channels each extending parallel to the longitudinal axis (X), the two channels of the same pair being stacked on one another and in communication with one another in a plurality of crossing areas each defining an area for mixing of the fluid with itself in the first or second circuit.
According to a second embodiment, typically when one of the two fluids is a liquid metal (Na) and the other of the fluids is an inert gas (N2), the first fluid circuit includes at least one pair of fluid circulation channels each extending parallel to the longitudinal axis (X), the two channels of the same pair being stacked on one another and in communication with one another in a plurality of crossing areas each defining an area for mixing the fluid with itself in the first circuit, the second fluid circuit including at least one pair of channels of straight shape.
The invention also consists in a method of producing the heat exchange module as described above:

- machining at least one first groove in a first metal plate;
- machining at least one second groove in a second metal plate;
- positioning the machined second plate against the machined first plate so that the first and second grooves each delimit a fluid circulation channel each extending parallel to a longitudinal axis (X), the two channels being stacked on one another and in communication with one another in a plurality of crossing areas each defining an area for mixing the fluid with itself;
- assembling the first and second metal plates with one another, either by hot isostatic pressing (HIP) or by a process commonly called hot uniaxial diffusion welding, so as to produce diffusion welding between them, or by brazing.

The invention also concerns a heat exchanger comprising a plurality of heat-exchanger modules like that described above, each extending parallel to the central axis of the enclosure and each arranged inside the enclosure.
The invention also consists in the use of the heat exchanger described above, the first fluid, as the secondary fluid, being a gas or a mixture of gases and the second fluid, as the primary fluid, being a liquid metal.
The first fluid may primarily comprise nitrogen, the second fluid being liquid sodium. The first or second fluid may come from a nuclear reactor.
The invention finally consists in a nuclear installation comprising a fast neutron reactor cooled with liquid metal, notably liquid sodium (FNR-Na or SFR), and a heat exchanger comprising a plurality of heat-exchanger modules as described above.

DETAILED DESCRIPTION

Other advantages and features of the invention will emerge more clearly on reading the detailed description of embodiments of the invention given by way of nonlimiting illustration with reference to the following figures, in which:

FIG. 1 is a perspective view of a prior art heat-exchanger module made from two plates;

FIG. 2 is a transparency detail view showing the zigzag profile of a channel of a heat-exchanger module according to FIG. 1;

FIG. 3A is a perspective view of two plates with machined grooves before assembling them to constitute a heat-exchanger module according to the invention;

FIG. 3B is a perspective view of a heat-exchanger module according to the invention produced from the two plates according to FIG. 3A;

FIG. 4 is a transparency detail view showing the crossing areas between channels of a heat-exchanger module according to the invention;

FIG. 5 is a perspective view of three pairs of channels in a heat-exchanger module according to the invention;

FIG. 6 is a summary of points comparing the thermal power exchanged as a function of the Reynolds number (Re) between examples of fluid circulation channels according to the invention and according to the prior art.

For clarity, elements that are the same according to the prior art and according to the invention are designated by the same reference numbers.
In FIG. 1 there has been represented a prior art heat-exchanger module with longitudinal axis X and including at least one fluid circuit.
The module including a pair of fluid circulation channels 1, 2 each of which extends parallel to the longitudinal axis X.
The two channels 1, 2 are stacked one on the other with no crossing over between them.
To be more precise, each channel 1, 2 has a regular curved zigzag profile. The curved zigzag profiles of the two channels 1, 2 are identical and symmetrical to one another with respect to the longitudinal axis X or a parallel axis.
As can be seen better in FIG. 2, the curved zigzag channel profile features bends 14, 16 and straight segments, a straight segment 15 connecting two consecutive bends.
In FIG. 3B there has been represented a heat-exchanger module according to the invention with longitudinal axis X and including at least one fluid circuit.
The module including a pair of fluid circulation channels 1, 2 each of which extends parallel to the longitudinal axis X.
According to the invention, the two channels 1, 2 are stacked one on the other and in communication with one another in a plurality of crossing areas 3 each defining an area of mixing of the fluid with itself.
To be more precise, each channel 1, 2 has a regular curved zigzag profile. The curved zigzag profiles of the two channels 1, 2 are identical and symmetrical with each other with respect to the longitudinal axis X or a parallel axis.
As can be seen better in FIG. 4, the curved zigzag channel profile features bends 14, 16; 24, 26 and straight segments, a straight segment 15; 25 connecting two consecutive bends.
The following procedure is employed to produce a heat-exchanger module according to the invention as just described.
There are machined in each of two identical rectangular metal plates 10, 20 a respective open-ended groove along the regular curved zigzag profile 11, 12, 13 and an open-ended groove 20 along the same regular curved zigzag profile 21, 22, 23.
As shown in FIG. 3A, the machining profiles of the grooves of the two plates 10, 20 are produced according to two opposite patterns, i.e. the summit of one groove 11 faces the summit of the other groove 21 when the plates face one another.
The machined plate 20 is then positioned against the machined plate 10 so that each of the grooves 11, 21 delimits a fluid circulation channel 1, 2 each extending parallel to a longitudinal axis X and so that the two channels are stacked on one another and in communication with one another in a plurality of crossing areas 3 each defining an area for mixing the fluid with itself.
The two metal plates 10, 20 are then assembled with one another, either by hot isostatic pressing (HIP) or by a hot uniaxial diffusion welding process, so as to produce diffusion welding between them.
Studies have been carried out by the inventors in order to determine the thermal performance of the channels 1, 2 with crossing areas 3 according to the invention and to compare the latter with that of prior art machined plate heat exchangers with channels that do not cross.
It is specified here that a prior art channel as shown in FIGS. 1 and 2 has the same dimensions, i.e. width, length and height, as a channel according to the invention.
It is specified here that the thermal compactness is defined as the thermal power Pth exchanged per unit volume, which is proportional to the number N of channels multiplied by the overall length L of a heat exchanger.
All of the comparative tests are summarized in the table below and shown in the form of points in FIG. 6.
Examples 1 and 3 represent the invention, i.e. correspond to two channels 1, 2 with identical profiles that cross in a plurality of crossing areas 3.
Examples 2 and 4 represent the prior art, i.e. correspond to a channel of identical profile to the channels 1, 2 but not crossing another channel.
The geometrical data, namely the length L between bends 14, 16 or 24, 26, the angle between a straight segment 15, 25 and the axis X, the mean radius of curvature R of a bend, are shown in FIG. 4.
It is specified that the total length of a channel corresponds to that L of the curved profile plus that of the rectilinear end parts, referenced 4, 5 in FIG. 3B.

TABLE

					Length L
		Length L			of curved	Head	Coefficient
Geometry		between	Radius R	Total	zigzag	loss	of thermal
(cross section	Angle α	bends	of curvature	length	profile		1, 2	ΔP	exchange
in mm*mm)	(degrees)	(mm)	(mm)	(mm)	(mm)	(Pa)	(W/m²K)

Example 1	45	9	4.3	124	149	29710	3769
(2 x (2x1))
Example 2	45	9	4.3	124	149	8931	2843
(2x2)
Example 3	20	14	6	164	172	15590	3308
(2 x (2x1))
Example 4	20	14	6	164	172	5353	2493
(2x2)

From this table it is clear that, for the two reference geometries (examples 1 and 2 for one geometry, examples 3 and 4 for the other), the coefficient of thermal exchange is higher for the two channels 1, 2 with crossings 3 according to the invention than for a channel according to the prior art with no crossing.
However, higher head losses are seen for the two channels 1, 2 with crossings 3 according to the invention. However, these higher head losses are compensated by the improvement in terms of the thermal power exchanged: comparing examples 1 and 3 according to the invention and those 2 and 4 according to the prior art in terms of thermal compactness, it is seen that a pattern with two channels 1, 2 with crossings according to the invention enables better thermal performance than a prior art single channel pattern. It is even clear from the points plotted in FIG. 6 that this improved thermal performance can be quantified as a 20% increase in the thermal power exchanged.
There has been represented in FIG. 5 a heat-exchanger module according to the invention with three pairs of channels 1.1, 2.1; 1.2, 2.2, 1.3, 3.3 according to the invention arranged parallel to one another and all with identical regular curved profiles.
FIG. 5 shows the arrangement of the channels according to the invention at the scale of one plate: if the distance between channels is sufficiently small, other crossing areas and therefore other areas for mixing the fluid with itself are created, notably at the level of the bends.
Other variants and improvements may be provided without this departing from the scope of the invention.
Thus in all of the embodiments of the invention shown only one fluid circuit with the zigzag channel profile and the crossing of the channels is shown and explained.
In a heat-exchanger module according to the invention with two fluid circuits the other fluid circuit may be envisaged with channels identical to those of the invention, i.e. with the channels crossing.
Alternatively, the other fluid circuit may equally well be envisaged with rectilinear profile, i.e. straight and not crossing, channels.
For example, in a heat-exchanger module between a liquid metal, such as liquid sodium, and a gas, such as nitrogen, the gas circuit may therefore and advantageously be envisaged with the channels crossing according to the invention and a liquid metal circuit with straight channels, preferably of larger section than the channels of the gas circuit in order to limit the risks of blocking them.
It goes without saying that a liquid metal/gas heat exchanger is one example of application and having the pattern with crossing profile according to the invention may very well be envisaged for both fluid circuits in the same heat exchanger.

Claims

1. A heat-exchanger module with a longitudinal axis comprising at least two fluid circuits, the first fluid circuit comprising, at least one pair of channels for fluid circulation, each extending parallel to the longitudinal axis, wherein the two channels of the same pair are stacked on top of one another and are in communication with one another in a plurality of crossing areas, each defining an area for mixing the fluid with itself in the first circuit.

2. The heat-exchanger module as claimed in claim 1, wherein each channel has at least in part a curved zigzag profile.

3. The heat-exchanger module as claimed in claim 2, wherein the curved zigzag profile is regular along its length.

4. The heat-exchanger module as claimed in claim 3, wherein the regular curved zigzag profile includes bends and straight segments, a straight segment connecting two consecutive bends.

5. The heat-exchanger module as claimed in claim 4, wherein the radius of curvature of the bends being is between 0.5 and 3 Dh inclusive, Dh being the hydraulic diameter of the channel.

6. The heat-exchanger module as claimed in claim 4, wherein the length of the straight segment is between 4 and 8 Dh inclusive, Dh being the hydraulic diameter of the channel.

7. The heat-exchanger module as claimed in claim 4, wherein the angle between the straight segment and the longitudinal axis is between 10 and 45° inclusive.

8. The heat-exchanger module as claimed in of claim 2, wherein the curved zigzag profiles is identical for the two channels and symmetrical to one another with respect to the longitudinal axis or a parallel axis.

9. The heat-exchanger module as claimed in claim 1, wherein the channels have an oval, circular, rectangular or square section.

10. The heat-exchanger module as claimed in claim 1, wherein the two channels of the same pair join at their longitudinal ends in the same rectilinear channel portion substantially parallel to the longitudinal axis.

11. The heat-exchanger module as claimed in claim 1 for two fluids, wherein each of the two fluid circuits includes at least one pair of fluid circulation channels each extending parallel to the longitudinal axis, the two channels of the same pair being stacked on one another and in communication with one another in a plurality of crossing areas each defining an area for mixing the fluid with itself in the first or second circuit.

12. The heat-exchanger module as claimed in claim 1 for two fluids, such as a liquid metal (Na) and an inert gas (N2), wherein the first fluid circuit includes at least one pair of fluid circulation channels each extending parallel to the longitudinal axis, the two channels of the same pair being stacked on one another and in communication with one another in a plurality of crossing areas each defining an area for mixing the fluid with itself in the first circuit, the second fluid circuit including at least one pair of channels of straight shape.

13. A method of producing the heat exchange module as claimed in claim 1, comprising the following steps:

machining at least one first groove in a first metal plate;

machining at least one second groove in a second metal plate;

positioning the machined second plate against the machined first plate so that the first and second grooves each delimit a fluid circulation channel each extending parallel to a longitudinal axis, the two channels being stacked on one another and in communication with one another in a plurality of crossing areas each defining an area for mixing the fluid with itself;

assembling the first and second metal plates with one another, either by hot isostatic pressing (HIP) or by a process commonly called hot uniaxial diffusion welding, so as to produce diffusion welding between them, or by brazing.

14. A heat exchanger comprising a plurality of heat-exchanger modules as claimed in claim 1 each extending parallel to the central axis of the enclosure and each arranged inside the enclosure.

15. The use of the heat exchanger as claimed in claim 14, the first fluid, as the secondary fluid, being a gas or a mixture of gases and the second fluid, by way of primary fluid, being a liquid metal.

16. The use of the heat exchanger as claimed in claim 15, the first fluid primarily comprising nitrogen and the second fluid being liquid sodium.

17. The use as claimed in claim 15, the first or second fluid coming from a nuclear reactor.

18. A nuclear installation comprising a fast neutron reactor cooled with liquid metal, notably liquid sodium (FNR-Na or SFR), and a heat exchanger comprising a plurality of heat-exchanger modules as claimed in claim 1.