US3479490A - High temperature infrared radiant heating device - Google Patents

Description

Nov. 18, 1969 N. H. STARK 3,479,490

HIGH TEMPERATURE INFRARED RADIANT HEATING DEVICE Filed Feb. 6, 1969 2 Sheets-Sheet l INVENTOR.

NORMAN H. STARK RONALD E. BARRY Attorney Nov. 18, 1969 N. H. STARK 3,479,490

HIGH TEMPERATURE INFRARED RADIANT HEATING DEVICE 2 Sheets-Sheet 2 Filed Feb. 6, 1969 United States Patent US. Cl. 219-544 14 Claims ABSTRACT OF THE DISCLOSURE A heatingdevice including a dielectric high temperature insulating board formed from an amphibole asbestos and an inorganic silicate binder, an infrared radiating heating element embedded within the board and electric terminals connected to the heating element and extending through the board.

RELATED APPLICATION This application is a continuation in part of my copending application Ser. No. 562,352, filed July 1, 1966, now abandoned.

BACKGROUND OF THE INVENTION The increasing demand for higher temperatures in the manufacture of many of todays new products has created a demand for a high temperature dielectric insulating board. The materials presently being used for such a board have not been satisfactory either due to the inability to withstand high temperatures or to have the required dielectric characteristics.

SUMMARY OF THE PRESENT INVENTION The heating device of this invention provides a dielectric high tempearture board which is transparent to infrared radiation and provides heat radiation on one or both surfaces. The board is formed from an amphibole asbestos which has high tempearture and dielectric characteristics suitable for use with an infrared radiating heating element. The asbestos fiber is mixed with an inorganic silicate type binder having a high tempearture characteristic to form a mastic which is molded to a desired shape under heat and pressure. An infrared radiating heating element is placed in races provided in the molded board and connected to terminals provided in the board. The board is capable of withstanding both physical and thermal shock and can be scraped, cleaned with steam or washed in soap and water without ad.- versely affecting the heating element. In some instances the binder can be fused to form a ceramic. A finished surface can be provided on the insulating board by applying a ply of binder impregnated asbestos felt or coating the outer surface with a coating of the binder. The board can be made with heat radiating surfaces on both sides or on one side by varying the thickness of the board. In the latter instance the side opposite the heat radiating surface provides thermal insulation for the board. In this type of board, infrared radiation may be increased by providing a layer of reflective material adjacent the heating element.

Other objects and advantages will become apparent from the following detailed description when read in connection with the accompanying drawing in which:

FIG. 1 is a perspective view of a heating device with the heating element positioned in a groove provided in the board.

FIG. 2 is a perspective view of another form of the 7 3,479,490 Patented Nov. 18, 1969 ICE heating device with the heating element positioned in races provided in the board.

FIG. 3 is a perspective view of the device shown in FIG. 2.

FIG. 4 is a section vie-w taken on line 44 of FIG. 2 showing the layer of reflective material.

DESCRIPTION OF THE INVENTION (ZCaO, 4Mg0, FeO, SSiO H 0), (2) Tremolite (ZCaO,

SMgO, 8SiO H 0), (3) Anthophylite 7MgO, 8SiO H 0), (4) Amosite (5-5Fe0, 1-5Mg0, 8SiO H 0) and (5) Crocidolite (Na O, Fe O 3Fe0, 8SiQ H O).

The asbestos fibers are bonded by an inorganic silicate type binder which is capable of withstanding temperatures in excess of 1800 F. Although various types of silicate binders may be used to form the board, the following have provided the high temperature resistance required in this type of a board:

(1) Silicate of soda of the N type sold by Philadelphia Quartz Company, combined with a Ball Clay (Ky ON4-A/F);

(2) No. 36 refractory cement sold by A. P. Green Refractories Co. which is a wet, air setting, high alumina mortar;

(3) Loxol-65 sold by A. P. Green Refractories Co. which is a dry, air setting, high alumina mortar;

(4) Sairbond sold by A. P. Green Refractories Co.

Referring to FIG. 1 of the drawing, the heating device shown includes a high temperature insulating board 10 preformed from a mastic material as discussed hereinafter with an asbestos fiber felt ply 12 secured to the bottom surface of the board 10.

The board 10 is formed from one of the amphibole asbestos fibers, preferably Amosite or Crocidolite and an inorganic silicate binder. The asbestos fiber and the binder are mixed with a proportionate amount of water to form a plyable mastic which is pressed in a mold and heated to form a self-supporting insulating board. A board formed with these materials has been found to be capable of use in a temperature range of 1000 F. to 1800 F. without degradation. This is considered to be the maximum temperature range capability for the board. The following is an example of one method of forming the board:

(1) One part of Amosite fiber, two parts of Ball Clay, two parts of N grade silicate of soda, and two parts of water were placed together in a mixer and mixed thoroughly. This formed a dough-like mastic which was then placed into an extruder.

(2) A ply of Amosite felt, which had been impregnated with one part Ball Clay, one part of N grade sodium silicate and three parts of water was placed in a mold.

(3) Strips of mastic were then extruded over the ply of Amosite felt and a second ply of Amosite felt, also impregnated as described above, was placed on top of the mastic.

(4) The cover was placed on the mold and put into a press where the mastic was compressed to remove the excess liquid and to establish the final dimensions of the board.

(5) The mold was placed in an oven where it was baked until the moisture was completely removed. The oven temperature was held at approximately 500 F. and at this temperature the mastic was cured in about forty-five minutes.

The resulting product is a solid material which will withstand temperatures up to 1800 F., is dielectric to 15,000 volts at one quarter inch thickness, has rigidity and machine-ability to the extent that it can be sawed, drilled or fabricated by a punch press operation. The board also will not distort at 100% relative humidity.

In the above process the ply of Amosite felt could be eliminated from either surface if the finish of the surface is not considered necessary for the intended use of the final product. The mastic has been found to be sufliciently rigid standing along to act as an insulating board.

In the embodiment described above the binder used in the board contained substantially equal parts of Ball Clay and Silicate of Soda. This ratio can be varied by as much as however, the best results were achieved with equal amounts of Ball Clay and Silicate of Soda.

Again referring to FIG. 1 of the drawing, the heater board '10 is shown with a serpentine groove 14 either cut or formed in one surface of the insulating board 10. The thickness from the base of the groove 14 to the opposite side should be at least one-fourth inch thick to produce sufficient thermal insulation. An infrared radiating heating element 16 is postitioned in the groove 14 and secured at its ends to electric terminals or leads 18 provided in ports 19 in the base of the groove 14. The heating element 16 is formed from Nichrom type resistance wire or an Iron-Chromium-Aluminum Cobalt alloy. The latter is preferable because of its long life. If the heating element 16 must be held in position in the groove 14 because of the location of the board 10 in a furnace, that is, if the board is to be suspended at an angle where the heater element 16 may fall out, a glass fiber sheet 20 impregnated with a binder is mounted on the outer face of the board. This sheet is coated on one side prior to being positioned on the board with a binder and when heated to set the binder the wire heating element will adhere to both the coils of the heating element as well as the face of the board.

The fiberglass material must be capable of withstanding temperatures in excess of 1800 F. S-glass was used for the fiberglass woven to form a sheet. The sheet of fiberglass is then impregnated with a binder that is also capable of withstanding temperatures of 1800 F. A binder such as calcium aluminate cement or a sodium silicate composition which includes one part N-grade sodium silicate, one part of clay such as Ball Clay and three parts water are suggested for use as the binder. After impregnation the fiberglass sheet is heated to set the binder. A coating of the binder was applied to one surface of the fiberglass sheet prior to placing the sheet on the surface of the board 10. The board was then heated to cure the binder and to bond the wire heating element 16 to the sheet 20 of fiberglas.

The heating element 16 could also be applied to the surface of the board or fiberglass by spraying the heating element material onto the board or fiberglass before lamination. This is particular significant when the board thickness must be kept to a minimum. A layer of fiberglass as described above could also be secured to the outside surface of the board to protect a sprayed-on element.

Heater devices made as described above can be mounted in any position and at any angle. The sheet of woven fiberglass will hold the wire heating element in position as well as protect it from contact with other objects. The sheet of woven fiberglass is transparent to infrared radiation. The outer face of the board will remain substantially cool to the touch.

The heating device shown in the embodiment in FIGS. 2 and 3 includes a dielectric high temperature insulating board 40 having a heating element 42 positioned in races 44 molded in the board 40. The heating element 42 is connected to electric terminals 46 provided in the heater board. The heater board can be formed from an amphibole asbestos fiber and an inorganic silicate binder as described above. The races 44 are molded into the board 40. The heating element 42 is threaded through the races 44 and connected to the electric terminals 46.

If the heating device is used as a wall for a furnace, the heating element 42 should be located in close proximity, approximately to /2 inch, to the inner surface and spaced from the outside surface approximately 2 to 2 inches. The inner surface is then heat radiating and the outer surface is heat insulating.

Infrared radiation from the heater element 42 can be enhanced by providing a reflective layer or strata 50 of reflective material in the heater board in a position to reflect infrared toward the inner or heater surface. Mica flakes have been used as the reflective material. These flakes are distributed as a layer on the mastic material in the mold and the final layer of mastic extruded over the mica flakes.

In order to provide a heater board which is to be used for continuous operation at temperatures approaching 1800 F., a binder that can be fused to form a ceramic board should be used. The three binders set forth above, No. 36 Refractory Cement, Loxol and Sairbond, are all capable of being fused to form a ceramic insulating board. The following is an example of heating devices formed with a ceramic board:

1) One part of shredded Amosite fiber, two parts water (de-ionized) and nine parts No. 36 refractory cement were mixed in an electric mixer to form a mastic.

(2) The mastic is extruded into a mold having rods to form the races, compresses and heated at a temperature of 400 to 500 F. until hard enough to be self .supporting, approximately fifteen minutes. The board is removed from the mold and the rods withdrawn from the board. Heating is continued until the board is completely dry. The board is then trimmed to size.

(3) The board is wired by cutting slots along the top and bottom edges of the board to join the races and the heating element looped through the races and connected to the electric terminals. A fiberglass spacer is placed in the slot across the looped ends of the wire elements and the slot filled with mastic to seal the wire in the races. The board is reheated to 500 F. to dry the mastic in the ends of the board.

(4) If a smooth outer surface is required in the board, two coats of No. 36 refractory cement which has been ground in a rolling mill is applied to the board. The board is heated to 200 F. to remove most of the moisture and then to 300 F. to dry the cement.

(5) The binder is fused to produce a ceramic board by placing the board in a kiln and heating to 1800 F. for fifteen minutes. The temperature is then raised to 1950 F. and held at that temperature until the entire board is fused. The board is finally cooled to room temperature.

The board should have a thickness of to of an inch to provide adequate strength while allowing for maximum heat radiation from both surfaces. The board is capable of emitting the full spectrum of infrared radiation.

Any of the other fibers of the amphibole asbestos fibers can be substituted for the Amosite fiber in the above example as well as using either of the other two A. P. Green Refractories Co. binders, Loxol or Sairbond.

Various of the features of the invention are set forth in the following claims.

What is claimed is:

1. A heating device capable of high temperature operation, said device comprising a board formed from amphibole asbestos, and

an inorganic binder containing a silicate,

an infrared radiating heating element located within said board, and

a pair of electric terminals connected to said heating element.

2. A heating device according to claim 1 wherein said inorganic binder contains substantially equal parts of sodium silicate and Ball Clay.

3. A heating device according to claim 1 wherein said inorganic binder is fused at a temperature of 1950 F.

4. A heating device according to claim 3 wherein said board has a thickness sufficient to allow heat radiation on both surfaces.

5. A heating device according to claim 2 wherein said board has a thickness on one side of said heater element providing heat radiation and a thickness on the other side of said heater element providing heat radiation.

6. A heating device according to claim 5 including a strata of mica adjacent said heater element.

7. A high temperature heating device comprising a board containing an amphibole asbestos fiber and an inorganic silicate binder,

a heating element located in the board at a distance from one of the surfaces of the board to provide heat insulation,

and a pair of electrical terminals connected to said heating element.

8. A heating device according to claim 7 wherein said amphibole asbestos fiber and inorganic silicate binder are heated to form a ceramic.

9. A heating device according to claim 7 wherein said board includes a layer of infrared reflecting material.

10. A heating device according to claim 9 wherein said heating element is located a distance from one surface of the board to provide for heat radiation and from the other surface of the board to provide heat insulation.

11. A heating device capable of withstanding up to 1800 F. comprising a high temperature board formed from asbestos fiber material and an inorganic binder including equal parts of ball clay and sodium silicate mixed to form .a mastic and cured in a mold of predetermined dimensions,

a groove in one surface of said board, electric terminals at each end of said groove,

and an infrared radiating heating element positioned in said groove and connected to said terminals.

12. A heating device according to claim 11 including a sheet of woven fiberglass impregnated with an inorganic binder and bonded to the surface of said board and said heating element to hold it in position.

13. A heating device according to claim 11 wherein said board is formed from an asbestos fiber selected from the group consisting of amosite fiber and crocidilite fiber.

14. A heating device according to claim 13 including a ply of tamosite asbestos fiber impregnated with said inorganic binder,

said ply being mounted on the surface of said board and being capable of withstanding a temperature of 1800 F.

References Cited UNITED STATES PATENTS 1,742,159 12/1929 Hynes 219-213 2,138,217 11/1938 Sutter 219345 2,619,580 11/1952 Pontiere 219-528 2,710,909 6/1955 Logan et a1. 219--528 X 2,932,711 4/1960 Adams 219354 X 2,952,761 9/1960 Smith-Johannnsen 219-544 X 3,031,739 5/1962 Boggs 219345 X 3,155,814 11/1964 Appleman et al. 219--407 3,277,273 10/ 1966 Williams 219-345 VOLODYMYR Y. MAYEWSKY, Primary Examiner U.S. C1.X.R.

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