EP0109843B1

EP0109843B1 - Resistance heater probe

Info

Publication number: EP0109843B1
Application number: EP83307064A
Authority: EP
Inventors: Robert George Howell; Alan Fletcher; Malcolm Stevens
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1982-11-23
Filing date: 1983-11-18
Publication date: 1987-08-26
Also published as: DE109843T1; DE3373255D1; EP0109843A1

Description

This invention relates to a resistance heater probe.
In particular, the invention relates to resistance heaters which have an important though not exclusive application to the heat treatment of metallic members which have undergone other operations which can affect metallurgical properties. One example of such a metallic member is a sleeve employed to repair a breach in a tube/tube plate weld of a heat exchanger by being secured in the relevant tube in a position to bridge the breach. The securing may be an explosive weld of one end of the sleeve to the bore of the tube plate, and a braze joint of the other end of the sleeve to the relevant tube. It is necessary to heat treat the sleeve after the making of the joints in order to restore the necessary properties to the braze-affected joints so as to ensure that design life can be expected. The braze is effected at a temperature of the order of1150°C for about four minutes and the braze bond is typically about 40 mm in length. Heat treatment however is at a lower temperature of the order of 750°C but for a longer period, typically one hour. A longer portion than the braze needs to be treated however, typically 150 mm.
The braze heating is performed in one preferred system by an induction probe inserted within the sleeve and accessed from the tube plate bore. However, such probes pose problems of adequate cooling and are expensive. Employment of such probes for heat treatment with its lower temperature and longer period would be wasteful.
British specification GB-A-2021369 shows a cartridge heater having a uniform helical conductor connected to inner and outer tubes which act as electrical conductors, a thermocouple extending along the inner tube.
According to the invention an electrical resistance heater probe comprises a tube providing a generally helical heating coil having convolutions of solid dross-section characterized in that the radial thickness of the convolufions varies lengthwise of the coil (16) so that the coil develops a temperature profile along its length which is dependent on such variation in radial thickness.
With this arrangement an effective temperature profile may be obtained over the length to which heat treatment is applied.
Preferably the axial width of the convolutions is substantially constant over substantially the entire axial length of the convolutions so that said temperature profile is determined solely by the variation in radial thickness of the coil convolutions. With this arrangement only the radial thickness need be varied.
An electrically insulating sleeve may be engaged over an elongate electrical conductor, the tube is disposed over said sleeve and with a non-heating part at one end region covered by an outer casing diametrically-opposed bimetallic strips secured to said casing and extending along part of the length of the heating part of said heating element, and at least one thermocouple hot junction in thermal conductive contact with said bimetallic strips being capable of moving outwardly under the influence of temperature so as to contact a tubular metallic member being heat treated and thereby monitor the temperature thereof. With this arrangement the temperature of the member being heated is effectively monitored.
In a typical example, to be applied to the heat treatment as aforesaid of a breach-bridging sleeve of a stainless steel over a length of said 115 mm at said 750°C ± 25°C for say one hour, a typical resistance heater is shown in the accompanying drawings, wherein:

Figure 1 is an elevation of the heater, and
Figure 2 is an enlarged view in section of part of Figure 1.

Referring to the drawings we provide a resistance heater 1 which is a composite of a number of parts which will now be described.
There is a cylindrical core member 2 of a high resistance Ni/Cr alloy such as KANTHAL or NICHROME (RTM) screw threaded at one end 3 and with a bush 4 also of KANTHAL or NICHROME welded to the other end at 5. Over the member 2 is fitted a ceramic (e.g. alumina) tube 6 which abuts the bush 4 at one end and is engaged at the other end by a nut 7 on the member 2. There is a longitudinal slot 8 (see Figure 2) in the tube 6 for reception of a thermocouple 9 which is cemented in position. In a modification, not shown, there is another longitudinal slot disposed in diametrically opposed relationship to the slot 8, enabling another thermocouple to be installed to give a check on the correct operation of the first one.
Over the ceramic tube 6 is fitted a heater tube 10 of KANTHAL or NICHROME which has over a portion 16 thereof a helical groove extending through the full thickness of the tube 10 and typically 1 mm wide on a pitch of 5 mm, the groove 11 after assembly with-the core member 2 and ceramic tube 6 being filled with cement of high electrical resistance. The profile of the tube 10, produced by machining with constant bore, is such that there is a part 16' of maximum diameter thickness situated -at the outer end of grooved portion 16 and welded to bush 4, and there is also a cylindrical, i.e. non-grooved, portion 13 which is of maximum constant diameter thickness and which is carried in a metallic (e.g. stainless steel) tubular part 14 via a heat conducting sleeve 15 secured by cementing to both parts (see Figure 2).
The grooved portion 16 of the heated tube 10 projects from the part 14 and is only a partially covered by two diametrically opposed part-annular_.bimetallic strips or wings 17 welded to the main body of part 14 and which terminate short of the outer end of portion 16. The grooved portion 16 of tube 10 diverges to a maximum diameter thickness, corresponding with that of cylindrical portion 13, at a position 18 in register with the outer ends of the strips or wings 17. It tapers in both directions from this position, in one direction to terminate at its outer end and in the other 'direction to a step where the portion 14 begins. There is a bracket 19 welded to the end of portion 13 of tube 10 which projects from the non-winged end of tubular part 14.-The thermocouple 9 leads to circuitry 20 for monitoring.
In operation, cemented groove 11 of the heater tube 10 functions to provide a helix of resistance heater material so that the tube 10 functions as a resistance heating element, the resulting heating effect being profiled along the grooved length of portion 16 by virtue of the varying thickness thereof as aforesaid.
Current is applied at bush 4 to the central core member 2 and returns via the grooved length of tube 10 to leave at the bush end thereof. The thermocouple 9 functions as the hot junction and is in good heat contact with the bimetallic strips of wings 17 since it is cemented in ceramic tube 6 which is in contact with heater tube 10 which is in contact with tubular part 14 via conducting sleeve 15 and the strips or wings 17 are welded to part 14. The strips or wings 17 are caused, when heat is generated in tube 10, to move outwardly to make firm contact with the breach-bridging sleeve aforesaid, thereby enabling the temperature of that sleeve to be constantly monitored by the thermocouple 9 so that the amount of heat generated by the tube 10 can be adjusted by varying the electric current supplied thereto so as to ensure that optimum conditions for the required heat treatment are provided.
The bimetallic strip is carried by a heater with a thermocouple hot junction, the bimetallic strip being caused, on heating, to move into contact with the wall of a tube into which the heater is inserted. The heater has two bimetallic strips at diametrically opposed positions so that temperatures are measured across a diameter of the tube, this being capable of being used to check uniformity of temperature or provide a check of correct operating of the hot junctions, a large difference of temperature measurement indicating either a faulting hot junction or a poor contact. Furthermore, the bi-metals can be fitted into slots cut into the heater.

Claims

1. An electrical resistance heater probe comprising a tube providing a generally helical heating coil having convolutions of solid cross-section characterized in that the radial thickness of the convolutions varies lengthwise of the coil (16) so that the coil develops a temperature profile along its length which is dependent on such variation in radial thickness.

2. A heater probe as claimed in claim 1, characterized in that the axial width of the convolutions is substantially constant over substantially the entire axial length of the convolutions so that said temperature profile is determined solely by the variation in radial thickness of the coil convolutions.

3. A resistance heater probe according to claim 2, characterized in that an electrically insulating sleeve (6) is engaged over an elongate electrical conductor (2), the tube (10) is disposed over said sleeve (6) and with a non-heating part (13) at one end region covered by an outer casing (14), diametrically-opposed bimetallic strips (17) secured to said casing (14)_and extending along part of the length (16) of the heating part of said heating element, and at least one thermocouple hot junction (9) in thermal conductive contact with said bimetallic strips (17) and located in a recess (8) in said sleeve, said bimetallic strips being capable of moving outwardly under the influence of temperature so as to contact a tubular metallic member being heat treated and thereby monitor the temperature thereof.