FR2560362A1 - METHOD AND DEVICE FOR TRANSFERRING HEAT BETWEEN AT LEAST ONE HEATING ELECTRIC RESISTOR AND A FLUID - Google Patents

Abstract

1. Heat exchanger intended for the transfer of heat between at least one electrical heating resistor and a fluid, comprising at least one heating assembly (12) formed of an elongated heating element (13) containing at least one electrical resistor (20) and a guide tube (15) surrounding said heating element and defining with it an annular flow channel (16) for the fluid, the surface of the heating element (13) possessing, in the direction of flow of the fluid in said flow channel (16), a first non-heating zone (13a) forming with the surface of the tube (15) an upstream plenum zone for stabilizing the flow of the fluid and a second zone (13b) forming a heating surface for the fluid, said annular flow channel (16) being rectilinear and of constant thickness, characterized by the fact that it comprises, in addition, a hollow body (5) possessing a feed chamber (9) and a discharge chamber (8) for the fluid, these chambers being separated by a partition (7), and inside which body there is disposed said heating assembly (12) in such a manner that the heating element (13) is supported by a wall (4) of the feed chamber of said body (5) and that the guide tube (15) is supported by said internal partition (7), the ends of this guide tube (15) being open respectively in the feed chamber (9) and in the discharge chamber (8).

Description

1 2560362

  Method and device for transferring heat between at least one

  electric heating element and a fluid ".

  The present invention relates to a method of heat transfer between a heating surface and a fluid as well as a heat changer for the implementation of this method and intended for the transfer of heat between at least one electric heating resistance and a fluid in view increase its temperature or

change his physical state.

  In known devices of this kind, the electric heating resistors are immersed in a container filled with the fluid to be heated. This fluid flows with a low speed, which is not uniform and whose direction relative to the heating wall of the resistors varies from one point to the other thereof. As a result, the heat-exchange coefficient between the heating wall and the fluid is low and very variable from one point to another of this wall. Consequently, the heat flux which it is possible to transfer to the fluid to be heated is very low and particularly difficult to calculate with precision. In order to limit the temperature of the heating surface and to avoid the thermal degradation of fragile fluids which can undergo overheating in certain places of the heating surface, it is currently required to design electrical heating devices which include surfaces of

  significant heating, resulting in the production of devices including

brants, heavy and expensive.

  The present invention aims in particular to remedy and avoid the above drawbacks. The method of heat transfer between a heating surface and a fluid, according to the present invention, is such that it consists in stabilizing and standardizing a

  fluid flow so as to form a thick fluid blade

  constant sor and a rectilinear direction of flow and to bring into contact, perpendicular to the direction of its thickness, this blade of fluid with at least one heating surface forming a wall of a flow channel such that the flow remains constant thickness and its direction remains rectilinear, the Reynolds number associated with this flow being such that the flow of the fluid is turbulent. Preferably, this Reynolds number is chosen to be greater

2.0,000.

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  Thanks to the method according to the present invention, the speed of circulation of the fluid in the flow channel is uniform, parallel to the heating surface, and constant at all points of this heating surface so that the heat flux between the heating surface and the fluid is imposed. The present invention also relates to a heat exchanger for the implementation of the above method and intended more particularly for the transfer of heat between at least one electrical heating resistance and a fluid. According to the present invention, this heat exchanger comprises a heating element containing at least one heating resistor and having an external surface, and a

  guide element having a surface parallel to said surface

  external face of the heating element and delimiting with this external surface of the heating element a flow channel for the rectilinear fluid and of constant thickness, forming said fluid layer, the surface of the heating element having, in the direction of flow of the fluid between the surface of the heating element and the surface of the guide element, a first non-heating zone forming with the surface of the guide element a tranquilization zone upstream of the flow of the fluid, and a second area forming a surface '

heating for the fluid.

  According to the present invention, the surface of the heating element also preferably comprises, downstream of its heating surface, a third zone, non-heating, forming with the

  surface of the guide element a tranquilization zone downstream of the flow

fluid.

  According to the present invention, the length of the downstream tranquilization zone is preferably at least equal to five times the hydraulic diameter of the flow channel, defined as being equal to four times its section divided by the wet perimeter of the

flow channel.

  According to the present invention, the length of the upstream tranquilization zone is preferably equal to at least ten times

  the hydraulic diameter of the flow channel.

  The heat exchanger according to the present invention preferably comprises, upstream and downstream of said flow channel for the fluid, respectively, an inlet chamber and an outlet chamber having relatively large volumes

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  compared to that of the flow channel.

  In a particularly simple embodiment, said heating element is formed by a cylindrical element and said guide element is formed by a cylindrical guide tube coaxial with said cylindrical heating element and delimiting with the latter a flow channel of annular cross section and thickness constant surrounding

  said cylindrical heating element.

  According to the present invention, means are preferably

  They are provided for centering said cylindrical heating element in said cylindrical guide tube so as to keep the external surface of the cylindrical heating element and the external surface of said guide tube at a constant distance. In a variant, these centering means are formed by centering pins fixed to the guide tube and bearing on the external surface of said element

cylindrical heater.

  In another alternative embodiment of the present invention, these centering means can be formed by local deformations towards the inside of said guide tube which take

  pressing on the external surface of said cylindrical heating element.

  The heat exchanger according to the present invention also preferably comprises a cylindrical body closed at its ends, containing at least one assembly formed by said cylindrical heating element and said guide tube, this heating element extending in the direction of the axis of the cylindrical body and being carried by an end wall of the cylindrical body, the ends of said

  guide tube being spaced from the end walls of the cylinder body

  drique, the cylindrical body containing a radial partition crossed by said guide tube and delimiting in the cylindrical body two

  separate rooms which respectively constitute a feed room

  ment and an evacuation chamber for the fluid which are respected

  upstream and downstream of the annular flow channel formed

  between said cylindrical heating element and said guide tube.

  The present invention will be better understood from the study of heat exchangers described by way of nonlimiting examples and illustrated by the drawing in which: - the figure] represents, in longitudinal section, a

  electric heating device according to the present invention

  mant a heat exchanger comprising a heating element surrounded by a guide tube; - Figure 2 shows a radial section along II-II of the electric heating device shown in Figure 1; - Figure 3 shows a partial and enlarged longitudinal section of the heat exchanger of the electric heating device shown in Figure 1, showing a first means of centering the heating element inside the guide tube; - Figure 4 shows a radial section on IV-IV of the heat exchanger shown in Figure 3; - Figure 5 shows a partial longitudinal section of the heat exchanger shown in Figure 1, which

  shows a second embodiment of the means for centering the element

  heated inside the guide tube; - Figure 6 shows a radial section along VI-VI of the heat exchanger shown in Figure 5; - Figure 7 shows, in longitudinal section, a

  alternative embodiment of the electric heating device shown

  felt in Figure I and containing several heat exchangers, - Figure 8 shows a radial section along VIII-VIII

  of the electric heater shown in Figure 7.

  The electric heating device shown in Figure 1 and generally identified by the reference I comprises a cylindrical body 2 which has two end walls 3 and 4

  interconnected by a cylindrical wall in two cylindrical parts

  bricks 5 and 6 fixed end to end.

  Between the two cylindrical parts 5 and 6 extends radially a partition 7 which divides the interior of the cylindrical body 2 into two separate chambers 8 and 9 which communicate with the exterior

  respectively by radial pipes 10 and 11.

  Inside the cylindrical body 2 extends, axially,

  a heat exchanger generally identified by the reference

  rence 12. This heat exchanger 12 comprises a heating element 13 of circular section which passes through the wall 4 and which is carried by the latter, this heating element 13 extending axially inside the cylindrical body 2 and having a free end 14

  located at a distance from the end wall 3 of the cylindrical body 2.

  Inside the cylindrical body 2, the heating element 13 is

  straight and of constant circular section.

  The heat exchanger 12 also has a guide tube 15 which extends inside the cylindrical body 2 and which surrounds the cylindrical heating element 13 at a constant distance so as to form between the external surface of the cylindrical heating element 13 and the inner surface of the guide tube 15 an annular space 16 of constant thickness. The ends 13a and 13b of the guide tube 15 are located at a distance from the end walls 4 and 3 of the cylindrical body 2 and this guide tube 15 passes through a passage

  circular 17 of the radial partition 7 and is carried by the latter.

  In addition, the guide tube 15 is carried, on the side of its end 13b adjacent to the wall 3 of the cylindrical body 2 by a radial wall 18

having axial passages 19.

  The heating element 13 contains an electrical resistance.

  that heater 20 which extends over part of the length of the element

  menf cylindrical heater 13 so as to provide, on the external surface of this cylindrical heating element 13, facing the internal surface of the guide tube 15, from its end 15a to its end 15b, a first non-heating zone 13a , a second heating zone 13b along which the resistance extends

  electric heater 20 and a third non-heating zone 13c.

  The fluid to be heated which is introduced into the upstream chamber 9 via the inlet pipe 11 enters the annular space 16 through the end 15a of the guide tube 15, circulates in this annular space 16 up to 1 end 15b of the guide tube 15 and arrives in the downstream chamber 8 from which it is evacuated by the outlet pipe 10. The flow inside the annular space 16 separating the external surface of the cylindrical heating element 13 of the outer surface of the guide tube 15 has particular characteristics which are linked to the dimensions

of the heat exchanger 12.

  First of all, the axial dimension of the upstream non-heating zone 13a of the external surface of the cylindrical heating element 13 is preferably equal to at least ten fqis the hydraulic diameter of the flow channel 15 defined as being equal to four times its section divided by its wet perimeter

  which is, in this case, equal to the sum of the perimeters of the sur-

  heating face 13 and guide tube 15. The axial dimension of the non-heating downstream zone 13c

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  of the cylindrical heating element 13 is preferably equal to at least five times the hydraulic diameter of the annular channel 16. In addition, the Reynolds number of the fluid flow in the annular channel 16 is preferably at least 20,000 so that this flow is turbulent. ' Of course, the length of the

  heating surface depends in particular on the desired heat flow.

  The flow of the fluid in the annular flow channel 16, which enters the latter through the end 13a of the guide tube 13, is first of all stabilized and uniformized in a tranquilization zone situated between the upstream non-heating zone 13a from the outer surface of the cylindrical heating element 13 and the inner wall of the guide tube 15 so that the fluid reaches the heating zone situated between the heating surface 13b of the cylindrical heating element 13 and the inner wall of the guide tube 15 and

  crosses this heating zone with a circulating speed

  uniform relationship, parallel to the axis of the flow channel 16, parallel to the surfaces delimiting this channel and constant at all points of the heating surface 13b of the cylindrical heating element 13. Following this heating zone, the fluid passes through a second downstream tranquilization zone located between the downstream non-heating surface 13c of the cylindrical heating element 13 and the inner wall of the tube

  guide 15 so as to avoid any disturbance in the heating zone.

  Referring to FIGS. 3 and 4, it can be seen that so as to keep the outer surface of the cylindrical heating tube 13 and the inner surface of the guide tube at a constant distance so that the annular channel 16 has a constant thickness, pins can be provided which extend radially and which are fixed to the guide tube 15, these pins passing through the annular space 16 and bearing on the external surface of the heating element

cylindrical 13.

  In the variant shown in Figures 5 and 6, the centering of the cylindrical heating element 13 inside the

  guide tube 15 is obtained by local deformations 22, inwardly

  laughing, of the guide tube 15 which come to bear against the external surface

  of the cylindrical heating element 13.

  Referring to FIGS. 7 and 8, it can be seen that an electric heating device has been shown marked in a way

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  general by the reference 23 which has a cylindrical body 24

  equivalent to the cylindrical body 2 of the electric heater

  that I and which contains a central heat exchanger 25a and six heat exchangers 25b to 25, peripherals which are equivalent to the heat exchanger 12 of the electric heating device 1.

  Each heat exchanger 25a to 25. is mounted in the cylindrical body.

  drique 24 in an equivalent way to the mounting of the heat exchanger 12 in the cylindrical body 2 of the electric heating device I so that their flow channels are in parallel between an upstream chamber 26 supplied by an inlet pipe 27 and a downstream chamber 28 having an outlet pipe 29, these two chambers being separated by a radial partition 30 passed through and carrying the guide tubes of the heat exchangers 25a to 25g which are also carried by another partition 31 having axial passages 32 while their cylindrical heating elements are carried by the end partition 33 of the cylindrical cylindrical body 24 located on the side of the upstream chamber 26. The electric heating device 23 shown in Figures 7 and 8 can alo-rs constitute an exchanger

  very powerful heat.

  It can be noted that the electric heaters 1 and 23 are constructed of elements which can undergo deformations due to heat substantially independently

from each other.

  The present invention is not limited to the examples

  described above. Many other variant embodiments are pos-

  sibles without departing from the scope defined by the appended claims.

Claims (13)

  1. A method of heat transfer between a heating surface and a fluid, characterized in that it consists in stabilizing and standardizing a flow of the fluid so as to form a fluid blade of constant thickness and with a rectilinear flow direction and to bring into contact, orthogonally in the direction of its thickness, this blade of fluid with at least one heating surface forming a wall of a flow channel such that the direction of flow remains rectilinear and of constant thickness, the number of Reynolds associated with this flow being such that the flow of fluid is turbulent.   2. Method according to claim 1, characterized in that it consists in choosing for the flow of the fluid a number of Reynolds greater than 20,000.   3. Heat exchanger for implementing the process   according to one of claims 1 and 2, and intended for the transfer of   heat between at least one electrical heating resistance and a fluid, characterized in that it comprises a heating element (13) containing at least one heating resistance (20) and having an external surface and a guide element (15) having   a surface parallel to said external surface of the heating element   fant and delimiting with this outer surface of the heating element   fant a flow channel (16) for the straight and thick fluid   constant sor, forming said fluid layer, the surface of the element   heating element (13) having in the direction of flow between the surface of the heating element (13) and the surface of the guide element (15), a first non-heating zone (13a) forming with said surface the guide element (15) a tranquilization zone upstream of the flow of the fluid and a second zone (13b) forming a heating surface for the fluid.   4. Heat exchanger according to claim 2, charac-   terrified by the fact that said surface of the heating element (13) comprises, downstream of its heating surface (] 3b) a third non-heating zone (13c), forming, with said surface of the guide element   (15), a tranquilization zone downstream of the flow of the fluid.   5. Heat exchanger according to claim 4, charac-   terrified by the fact that the length of said tranquilization zone 9 2560362   downstream is at least five times the hydraulic diameter of the flow channel.   6. Heat exchanger according to any one of the res-   dications 3 to 5, characterized in that the length of the upstream tranquilization zone is at least ten times the diameter flow channel hydraulics.   7. Heat exchanger according to any one of the res-   dications 3 to 6, characterized in that it includes upstream and   downstream of said flow channel, respectively an inlet chamber   and a discharge chamber for the fluid.   8. Heat exchanger according to any one of the res-   dications 3 to 7, characterized in that said heating element is formed by a cylindrical element (13) and said guide element is formed by a cylindrical guide tube (15) coaxial with said cylindrical heating element (13) and delimiting therewith a flow channel (16) of annular section and constant thickness   surrounding said cylindrical heating element (13).   9. Heat exchanger according to claim 8, charac-   merited by the fact that means are provided for centering said cylindrical heating element (13) in said cylindrical guide tube (15) so as to keep the external surface of the cylindrical heating element and the internal surface of said distance at a constant distance guide tube (15).   10. Heat exchanger according to claim 9, charac-   terrified by the fact that said centering means are formed by centering pins (21) fixed to said guide tube (15) and bearing on the outer surface of said cylindrical heating element (13).   11. Heat exchanger according to claim 9, charac-   terrified by the fact that said centering means are formed by local deformations (22) towards the inside of said guide tube (15) which bear on the external surface of said heating element cylindrical (13).   12. Heat exchanger according to any one of   Claims 8 to 11, characterized in that it comprises a body   cylindrical (5) closed at its ends, containing at least one 2560362   assembly formed by said cylindrical heating element (13) and said guide tube (15), this heating element extending in the direction of   the axis of said cylindrical body (5) and being carried by a wall of ex-   end (4) of the cylindrical body (5), the ends of said guide tube (15) being at a distance from the end walls (3, 4) of said cylindrical body (5), the cylindrical body containing a radial partition (7) traversed by said guide tube (15) and delimiting in the body   cylindrical two separate chambers (8, 9) which respectively constitute   a supply chamber and an evacuation chamber for the   fluid which are respectively upstream and downstream of the flow channel-   annular ment (16) formed between said cylindrical heating element (13) and said guide tube (15).