US2832875A

US2832875A - Electrical heating

Info

Publication number: US2832875A
Application number: US570976A
Authority: US
Inventors: William H Norton
Original assignee: Thermel Inc
Current assignee: Thermel Inc
Priority date: 1956-03-12
Filing date: 1956-03-12
Publication date: 1958-04-29
Anticipated expiration: 1975-04-29
Also published as: GB815559A

Description

April 9, 1958 w. H. NORTON 2,832,875

ELECTRICAL HEATING Filed March 12, 1956 ;With the resistor.

Unit tate P' '50a 2,832,875 ELECTRICAL HEATING Application March 12, 1956, Serial No. 570,976 8 Claims. (Ci. 201 -63) -Theinsta'nt invention relates to electrical heating, and

more particularly, to an improved electrical heating unit and method of making the same. Although the instant invention may have application in all fields of electrical heating, it is particularly adapted for use with relatively high temperatures. In units employed for developing high temperatures a suitable resistor element is employed in closely spaced. relation to the body to be heated, but electrically insulated therefrom. 'The material which maintains the spacing between'the 'resistor and the body to be heated must be an electrical insulator. Known electrical insulators are also quite resistant to heattransfer, so the object is to position the resistor as closely as possible to the body to be heated in order to minimize the heat insulated effect of the spacing material. vThe difliculty is that the heater may short out" by a breakdown of the electrical insulative properties of the spacing material, either through reduction of the thickness to too great an extent or as a result of high temperature conditions employed.

The instant invention is based in part on the discovery. of unusual electrical insulative properties coupled with relatively good heat conductive properties in a specific type of coating. The instant invention is further based on the discovery that such coating may be applied to a body to be heated and may be employed to mount a resistor in firm contact therewith in a heating unit. The instant coatings are capable of adhering to the body to be heated during temperature changes and also adhering to and/or covering a resistor element during such temperature changes. V In thepractice of the present invention a thin alumina ,coating is employed and this alumina coating is defposited bya flame spraying" process having certain specific procedural features which, per see, are not a part of the present invention. Instead, the present invention ref ventiQn to provide an improved heating unitand improved l ce Other and further objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed disclosure thereof and the drawings attached hereto and made a part hereof.

On the drawings: v

Figure .1 shows a coated article resulting from carrying out an initial step in the process of preparing the instant heating unit;

I Figure 2 shows the article of Figure 1 surrounded by a resistor, which is the assembly resulting'fromcarrying out the next step in the preparation of the heating unit;

Figure 3 shows the resistor tightly wrapped around'the coated element of Figures 1 and2,which results from the nextstep; and w Figure 4 shows a heating unit assembled, which embodies the'instantinvention.

As shown on the drawings:

' In Figure 1 there is shown an electrically and thermally conductive body, in the form of a stainless steel tube 10, which is an example of a body to be heated in the heating unit, indicated in its completion by the reference numeral 100 in Figure 4. The particular tube 10 is to be employed either as a pipe itself for conducting material to beheated or as a sleeve to cover a pipe (not shown) containing material to be heated. The tubular body 10, in essence, affords a supporting means or unit for the entire heating device 100. It will be appreciated that other shapes and sizes may be employed for the body 10 and that the body '10 may be made of different materials. Preferably the body 10 is made of metal so that it will be sufliciently thermally conductive to provide a heating surface, which in this case is the inside surface 10a of the-tube 10.

The terms insulator (or insulative) and conductor :(or conductive) with respec'tto both heat and electricity are well understood by those skilled in the art, notwithstanding the fact that all materials are generally considered to have at least limited heat and electrical conductivity and no material is considered to be a perfect ,square foot or 1 square centimeter) of unit thickness ('1 foot or 1 centimeter) having unit difference of temperature' (1 F. or 1 C.) between its faces.

The thermal conductivity of different materials varies greatly. For

metals and alloys, k is high, while for certain insulating materials such as asbestos, cork and kapoc, k is very low. Herein heat conductors will be considered to have a-thermal conductivity constant k of as much as about 10 (in the case of highly resistant alloys used as heating elements) and preferably of as much as about 30; "whereas the ordinary insulating materials have heat conductivity constants k of less than 1 and usually less than 0.1.

body and said resistor, said alumina coating being applied to said body by a process which comprises burning a combustible gas to provide a high temperature flame,

In the electrical arts, electrical conductors are usually understood to have resistivity which is defined as the resistance of a sample of the material having both a length and cross-section of unity. The resistance of a centimeter cubed and a circular-mil-foot are the two most common units of resistivity. Herein electrical conductors are un- Af ter the provision of the tubular body 10, the'initial step in the practice of the instant invention involves coat- 3 g ing the tube with a thin alumina coating 11. The specific tube 10 here provided has an inside diameter of 0.28 inch and a wall thickness of approximately 0.06 inch and is made of 410 stainless steel tube. The outside surface 101) of the tubeltl is preferably roughened fiistby etching, Parkerizing or sand blasting. *In this case the tube 10 is sand blasted to roughen thesurface.

Next, the alumina coating is appliedto the outside surface 1% by a process which comprises burning a combustible gas to provide a high temperature flame, in.- troducing alpha alumina particles into the flame, maintain ing the flame conditions in the flame sufficient to sinter the particles in the flame, and depositing a'crysta'lline coating 11 of the resulting sinteredproduct as an adherent coating on the body 10. As previously mentioned, the specific coating process'per se'is' not the instant invention, but"rather in the process sense the'instant invention involves steps which include the coating process per 'se in thefabric'ation of the'heating unit 100. The'alumina coating applied in the instant heating unit 100 cannotbe described other than by the process of applying the same, since this coating has certain peculiar properties. For example, thecoating has an X-ray diifraction pattern which indicates that it is predominantly gamma alumina, but it has a greater thermal conductivity than the heretofore known forms of gamma alumina. Also, it is in a unique crystalline though integrated form on the surface so as to resist thermal shock and to adhere extremely gamers The instant coating process is particularly eflicient -in-the utilization of the heat of the flame, since the positionin which the coating is being applied at the fast *vvell to the steel even though the coefficients of thermal be alpha alumina orany material such as aluminum acetate, alpha "alumina monohydrate, bauxite, etc. which yields alpha alumina (or which eflectively introduces alpha alumina into the flame).

The flame resulting from the burning of a combustible gas is preferably an oxyacetylene flame or an oxyhydrogen flame. The theoretical temperature of an oxyacetylene fiameis' about 3000" C., but the actual flame temperature is in the'order of 2200 C. In the case of the oxyhydrogen flame, the theoretical temperature and the actual temperature are about'300" C. lower than those of oxyacetylene flames. The sintering temperature of alumina is in the order of 1200 C. While the fiame'temperature may vary, depending upon the'type of fuel'being burned, it "is preferable to employ a flame which has atemperature of'at least 1500 C. in'ord'er to insure adequate sinterin'g of therefractory oxides. It 'is'desirable to employ substantially. "Particles as low as 2 or'3 microns in maximum dimension may be used successfully, and particles 'as large as 30 mesh have also been employed satisfactbrily. However, for-"most purposes, apa'rti'cle size be tween about 200' mesh to less than 500 mesh'willbe" found to bemost satisfactory. v

' As'p'reviously mentioned, the flame conditions rnnsfbe maintained sufiicient to sinter the particles in theflarne. In other words, the particles shouldbe sintered' rather tains its crystalline characteristics. The accomplishment of such sintering in the flamefmay be effectedby proper adjustment of the flame temperature, the' particle sizes, and the presence of additional ingredients as flame est rate is readily visible to the eye. As an example, in the coating of the outside surface 10b of the sand blasted tube 10, an oxyhydrogen torch is employed. A charge of alpha alumina monohydrate (commercial grade) admixed with 2% magnesium fluoride, having aparticle size of less than"325 meshand averaging'about 400 mesh, is injected into the oxygen stream of the torch (using unrestricted oxygen flow). The tip of the torch is held about'A inches from the surface 10b of the tube 10. A highly adherent and abrasion-resistant coating results, having white color. The coating is applied in a thickness of 4 mils and it is found that this coating has a dielectric'st'rength in excess of 1000 volts.

Substantially the same results are obtained using commercial gradeanhydrous alpha alumina, as well as other sources of alpha alumina hereinbefore mentioned. Thicknesses ranging from /2 mil to as much as 10 mils can be applied in the practice of the instantinvention depending upon the dielectric strength required for'the specific heating use.

Next, a slightly oversized coil of NichromeA 'Wire,

sh'own in' Figure2. The opposite ends 12a and 12b of the resistor coil 12 are then stretched axially of'the tube 10 so as to tightly wrap the resistor coil 12 against the coating 11, and the ends of the resistor 12a and 12b are clamped to the ends of the rod 10 by clamping rings 1321 and 13b, respectively. The resulting assembly is shown in Figure 3. The assembly shown in Figure 3 is then ready tobe enclosed in a unit, such as the unit 100, wherein the resistor 12 is further protected. Heretofore, assemblies were provided wherein the tube 10 and resistor 12 were maintained in spaced relation and a pow- "withthe magnesia particles and, even at greater 'more practical spacings for the compacted magnesia, breakthan'completely fused and in this way thecoa'ting recatalysts or fluxes. The fluxes which 'may be'includ'ed with the alumina particles in amounts ranging-"from about 2% to about 10% are preferably phosphates. Al-

A- particularly effective Y flame catalyst is magnesium --fluo'1ide,for-use with alpha alumina a l Irn'ay-"bemade-"of-reinforced packing or the like).

downs or shorting of the resistor over to the tube 10 resulted-in unit failures. In'addition, the instant alumina is asubstantially .betterheat conductor than magnesia. 1tisgenerally preferable to apply a second alumina coating ttothe unit of Figure 3 inorder to cover the resistor coil 12 and further protect the same. For the sake -of simplicity the resistor 12 is indicated merely a's 'a coil shown as a single line, but'the general resistor elerment sizes and shapes are well known in the art and need not be further described herein. The selection of the resistor is based upon the ohms required for the particular iz-heating job. As also shown in'Figure 3 partially, a second coating 11a (shown in section) of alumina may be applied to cover the Nichrome wire 12.

The assembly of Figure 3 (with or without thesecond alumina coating 11a) is then inserted in a cylindrical steel container 14 and mounted concentrically therein in spaced relation from the inner periphery of the container 14 by electrically insulative annular members 15 and 16' (which The

annular members

15 and 16 receive the resistor ex'tremities as a'r'esistor 12, is slipped over the coated tube 10, as.

or leads 12a and 12b, respectively, so as to insulate the same from the tube as well as the container 14, and these leads 12a and 12b are ultimately connected to a suitable source of electrical energy (not shown) for operation of the instant device.

In completing the unit 100 the space between the inner periphery of the container 14 and the resistor wrapped tube 10 is filled with particulate electrically insulative refractory material, such as magnesia particles, which are tightly packed within the unit 100. Actually, magnesia is considered to be a relatively highly thermally conductive refractory material which has very good electrically insulative properties. The instant alumina coatings 11 (and 11a), however, have superior electrical insulative properties and also better thermally conductive properties than the compacted magnesia M in the unit 100.

Preferably the container 14 is swaged to a reduced diameter after the magnesia has been vibration-packed therein. This results in a compacting of the magnesia particles M to improve the thermal conductivity thereof and to improve the electrical insulative properties thereof. For example, in the instant embodiment 100 the container 14 employed as an outside diameter of /8 inch (and is formed of stainless steel No. 304 tubing) and it is swaged down to an outside diameter of 0.75 inch. Such a reduction in outside diameter of 10 to effects a very thorough compacting of the magnesia particles M in the unit 100.

A unit such as the unit 100 specifically described herein has been in operation for a substantial period of time, much longer than any units prepared heretofore wherein an attempt was made to maintain the spacing between the resistor 12 and the tube 10 with magnesia. In addition, much greater heat energy can be developed per unit length of the instant device, particularly if the second alumina coating 11a is employed, because the individual wraps of the resistor 12 can be more closely spaced together, than in cases where magnesia is used to maintain electrical insulation therebetween.

It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.

I claim as my invention:

1. A heating unit comprising an electrically and then mally conductive body, a resistor, and a thin alumina coating maintaining spaced relation between said body and said resistor, said alumina coating being applied to said body by a process which comprises burning a combustible gas to provide a high temperature flame, introducing alpha alumina particles into said flame, maintaining flame conditions in said flame sufficient to sinter said particles in said flame, and depositing a crystalline coating of the resulting sintered product as an adherent coating on said body.

2. A heating unit comprising an electrically and thermally conductive body, a resistor, a thin alumina coating maintaining spaced relation between said body and said resistor, a container, and electrically insulative refractory material within the container mounting said body and resistor in spaced relation to the container, said alumina coating being applied to said body by a process which comprises burning a combustible gas to provide a high temperature flame, introducing alpha alumina particles into said flame, maintaining flame conditions in said flame sufiicient to sinter said particles in said flame, and depositing a crystalline coating of the resulting sintered product as an adherent coating on said body.

3. A heating unit comprising an electrically and ther mally conductive tubular body, a coiled resistor surrounding said body, and a thin alumina coating maintaining spaced relation between said body and said resistor, said alumina coating being applied to said body by a process which comprises burning a combustible gas to provide a high temperature flame, introducing alpha alumina particles into said flame, maintaining flame conditions in said flame suflicient to sinter said particles in said flame, and depositing a crystalline coating of the resulting sintered product as an adherent coating on said body.

4. A heating unit comprising an electrically and thermally conductive tubular body, a coiled resistor surrounding said body, and a thin alumina coating maintaining spaced relation between said body and said resistor, a container, and electrically insulative refractory material within the container mounting said body and resistor in spaced relation to the container, said alumina coating being applied to said body by a process which comprises burning a combustible gas to provide a high temperature flame, introducing alpha alumina particles into said flame, maintaining flame conditions in said flame suflicient to sinter said particles in said flame, and depositing a crystalline coating of the resulting sintered product as an adherent coating on said body.

5. A heating unit comprising an electrically and thermally conductive body, a resistor, and a thin alumina coating covering said resistor and maintaining spaced relation between said body and said resistor, said alumina coating being applied to said body by a process which comprises burning a combustible gas to provide a high temperature flame, introducing alpha alumina particles into said flame, maintaining flame conditions in said flame sufficient to sinter said particles in said flame, and depositing a crystalline coating of the resulting sintered product as an adherent coating on said body and on said resistor.

6. A method of preparing an electrical heating unit that comprises providing an electrically and thermally conductive body, burning a combustible gas to provide a high temperature flame, introducing alpha alumina particles into said flame, maintaining flame conditions in said flame suflicient to sinter said particles in said flame, and depositing a crystalline coating of the resulting sintered product as an adherent coating on said body, and mounting a resistor on said coating maintained by said coating in spaced relation to said body.

7. A method of preparing an electrical heating unit that comprises providing an electrically and thermally conductive tubular body, burning a combustible gas to provide a high temperature flame, introducing alpha alumina particles into said flame, maintaining flame conditions in said flame suflicient to sinter said particles in said flame, and depositing a crystalline coating of the resulting sintered product as an adherent coating on said body, tightly wrapping the coated body with a coiled resistor, and then coating the coiled resistor by the same process.

8. A method of preparing an electrical heating unit that comprises providingan electrically and thermally conductive tubular body, burning a combustible gas to provide a high temperature flame, introducing alpha alumina particles into said flame, maintaining flame conditions in said flame sufllcient to sinter said particles in said flame, and depositing a crystalline coating of the resulting sintered product as an adherent coating on said body and tightly wrapping the coated body with a coiled resistor.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain July 11, 1949