Heat Generating Pipe

Abstract

A heat-generating pipe arrangement employs at least one pipe of ferromagnetic metal, an insulated electric conductor line connected to a source of AC supply and inserted within the pipe throughout the entire length thereof and a good heat-conductive material such as water, sea water or the like which exists in the clearance space between the conductor line and the pipe. The pipe is heated by the alternating current flowing through the inner wall portion thereof on account of the skin effect, which is a return current from the conductor line to the source of AC. The heat-conductive material is effective in preventing the temperature rise of insulating-material covering the conductor line and reduces the cost of heat-generating pipe per unit of heat generation.

Description

CROSS REFERENCE

This invention is concerned with an improvement on the apparatus disclosed by the present inventor in the Japanese Pat. No. 460,224 corresponding to U.S. Pat. No. 3,293,407.

BRIEF SUMMARY OF THE INVENTION

This invention relates to a heat-generating pipe. More particularly this invention relates to a heat-generating pipe comprising an arrangement including a pipe of ferromagnetic metal, an insulated electric conductor line connected to a source of AC supply and inserted within said pipe throughout the entire length thereof and a good heat-conductive material existing in the clearance space between said pipe and said conductor line, said pipe being heated by the alternating current flowing through the inner wall portion thereof on account of the skin effect.

DETAILED EXPLANATION

It is well-known that when an alternating current flows through a conductor, the current concentrates on the surface of the conductor and shows a so-called skin effect. When the skin effect is pronounced, the depth of the skin S (cm.) in which a current flows can be expressed by the following formula

S = 5030 .delta./.mu.f (1)

Wherein .delta. (.OMEGA. cm.) is a resistivity of a conductor, .mu. is a permeability and f (cycle/sec.) is a frequency.

When an alternating current is supplied to a conductor line inserted within a pipe of ferromagnetic metal throughout the entire length thereof and the current which flows through said pipe is caused to concentrate on the inner wall portion of said pipe by the skin effect, there will be substantially no current flow on the outer surface portion of said pipe so long as the relations expressed by the following formulas

t> 2S

d>> s (2)

1>> d

wherein D (cm.) is an inside diameter of said pipe, t (cm.) is its thickness and 1 (cm.) is its length, are satisfied. When a commercial steel pipe is used as a pipe of ferromagnetic metal and an alternating current of commercial frequency (50 or 60 cycles) is applied, the depth of the outer skin calculated by the formulas is about 1 mm. Accordingly, there will be substantially no current flow on the surface of a pipe so long as its thickness is more than 2 mm. The alternating current is supplied to the circuit consisting of an insulated conductor line inserted within a pipe of ferromagnetic metal throughout the entire length thereof and a return path, i.e., the metal pipe formed by connecting the end of the conductor to the end of the metal pipe, as disclosed in the specification of the established Japanese Pat. No. 460224, the current flowing through the metal pipe is concentrated on the inner wall of the metal pipe on account of the skin effect. When a suitable thickness is selected for the pipe, substantially no electric potential appears on the outer surface of the pipe. If a heat-generating pipe of this kind is used, there is no need of insulation between the heat-generating pipe and supporting materials or material to be heated. There is substantially no leakage loss of current and no electric injury to human beings and animals even when they touch the pipe. The current which is concentrated on the inner surface of the pipe flows therethrough and generates heat by the resistance of the metallic material.

The heat-generating pipes disclosed in the above-mentioned patent can be used for heating various materials. Particularly they are suitable for heating transportation pipes or maintaining the temperature of such pipes as those used for transporting materials or solutions which solidify or increase their viscosities, e.g. crude oil, heavy oil, solid paraffin, acetic acid, naphthalene, aqueous solution of caustic soda of high concentration, etc. They are also applied advantageously to high-speed traffic ways in which snowdrifts and freezing are extremely harmful or to runways for aircraft because of simplicity, low construction cost, high reliability and easy maintenance.

An object of the present invention is to provide a heat-generating pipe which is improved over the ones disclosed in the above-mentioned prior patent in durability, unit cost and readiness of installation. This object can be attained by spacing the conductor line apart from the inner wall of the pipe throughout its length to form a clearance space therebetween and filling the space with a heat conductive material having a conductivity higher than that of air. Another object of the present invention is to provide a heat-generating pipe which allows a liquid surrounding said pipe to enter its clearance space and utilizes such a liquid as a heat-conducting medium. These objects and other advantages can be attained by the heat-generating pipe of the present invention.

One feature of the present heat-generating pipe consists in causing a substance having a good heat conductivity to fill the clearance space between the insulated conductor and a ferromagnetic pipe surrounding the said insulated conductor in the arrangement that alternating current flowing through the said insulated conductor generates heat upon the inner surface of the said ferromagnetic pipe.

Another feature of the present heat-generating pipe consists in causing a liquid surrounding said pipe to enter the clearance space between the insulated conductor and a ferromagnetic pipe surrounding the said insulated conductor in the same arrangement as mentioned above thereby to improve the heat conductivity between the conductor and the pipe.

The heat-generating pipe of the present invention is illustrated more fully by referring to the accompanying drawings.

FIG. 1 is a cross-sectional view of the present heat-generating pipe.

FIG. 2 is a schematic wiring diagram for illustrating the principle of the present heat-generating pipe.

In FIG. 1, 1 is a ferromagnetic pipe, 2 is an insulating material covering a conductor 3 and 4 is the clearance space between the ferromagnetic pipe and the insulated conductor and filled with a material having a good thermal conductivity.

FIG. 2 is the connecting diagram disclosed in U.S. Pat. No. 3,293,407. In this FIG., 33 is an alternating current source and 34 is a ferromagnetic metal pipe. An insulated conductor line connected to the terminal of AC source and inserted within said pipe throughout the entire length thereof, is connected to the end 36 of said pipe 34 remote from the AC source to form a circuit. When an alternating current is supplied to this circuit, the current flowing through the pipe 34 is concentrated on the inner wall 39 of the pipe 34 and there is substantially no potential appearing on the outer surface of the pipe 34. Accordingly, even when such a heat-generating pipe is brought into direct contact with a supporter thereof or a material to be heated therewith, the leakage of current from the pipe 34 into such a material does not occur practically.

The foregoing is an illustration of the heat-generating pipe of the present invention with a single power source. It goes without saying that a multiphase alternating current as an AC source, and various kinds of wiring systems can be used in the practical application of the present heat-generating pipe.

The cost of the heat-generating pipe of the present invention depends upon the cost of the insulating material. The allowable current in the insulated conductor is decided by the allowable temperature of the insulating material. When an alternating current is supplied to the above-mentioned heat-generating pipe in order to generate heat, the heat is mostly generated on the inner surface of the ferromagnetic pipe. The heat is transmitted through the pipe wall and dissipated from the outer wall of the pipe. Even with the assumption that heat is not generated in the conductor, the inside of the pipe shows the highest temperature. Besides this, conductors such as copper and aluminum which have been widely used on account of their low electric resistance, generate heat on account of their inherent electric resistance according to the strength of the current. Accordingly, among the construction elements of the above-mentioned heat-generating pipe, the insulated conductor 3 encircled by a ferromagnetic pipe 1 shows the highest temperature. The heat generated in the conductor 3, conducts through the insulating material 2, reaches the outer surface thereof and further conducts through the clearance space 4 and reaches the ferromagnetic pipe 1.

The allowable current in the insulated conductor of the heat-generating pipe is determined according to the allowable temperature of the insulating material. Needless to say, the temperature of the insulating material depends upon the temperature of the inner wall of the ferromagnetic pipe 1 and the heat conductivity of the material existing in the clearance space 4.

The temperature distribution in the heat-generating pipe of this kind is determined by measuring the temperatures in the conductor, in the insulating material and on the inner surface of the metal pipe. In accordance with measurements carried out on the present invention, it was found that when the clearance space 4 is occupied by air, about 90 percent of the overall temperature difference between the conductor and the inner wall of the pipe occurs at the clearance space and the remaining 10 percent, as little as it is, occurs in the insulating material of the conductor; therefore, provided that the clearance space had no thermal resistance, the overall temperature difference could be greatly reduced so as to be equal to the temperature difference between the conductor and the surface of the insulating material. As a result, compared with the former case where the clearance space was occupied by air, the latter case could be 10 times as large and the transmission from the conductor to the inner wall of the pipe, and about three times as large in the allowable current going in the conductor when the inner wall of the pipe is kept at a constant temperature difference is proportional to the heat quantity generated in the conductor, while the heat quantity is proportional to the square of the current flowing through the conductor.

Operation of the conductor at high currents is not practical. From the economical view point on account of voltage drop and power loss in the common power distribution, but as is readily understood from the construction of the heat-generating pipe, even the heat generated in the conductor can be effectively used without any trouble. In other words, the heat generated in the conductor provides for the saving of the amount of conductor necessary to generate a unit calorie and reduces the cost of the heat generating pipe.

Since there is no material having zero heat resistance, it is impossible to make the resistance zero, but if the clearance space is filled with a material having a good heat-conductivity, the temperature difference between the conductor and the wall of the metal pipe can be reduced. For example, by comparing the heat transmission rate and heat conductivity of water, 1,000 Kcal./m..sup.2, hr., .degree. C. and 0.54 Kcal./m., hr., .degree. C. respectively, with those of air, ca. 10 Kcal./m..sup.2 , hr., .degree. C. and 0.023 Kcal./m., hr., .degree. C., respectively, it is apparent that the former affords higher rate of heat transmission than the latter.

As the materials which can satisfy the above-mentioned purpose, water, sea water, aqueous solutions of salts, fats and oils, mineral oils, glycols, aqueous solution of alcohol or glycol can be illustrated. When the heat-generating pipes are used at a relatively low temperature e.g. lower than 100.degree. C., an inexpensive material such as water or aqueous solution of slat is preferable.

Further the filling of the clearance space with a material having a good heat-conductivity affords another advantage that when a high voltage current is supplied to the insulated conductor 3, the electric field strength on the outer surface of the conductor can be made uniform and the life of the insulating material can be prolonged.

As for the insulating material covering the conductor line, conventional materials such as various kinds of rubber, polyvinyl chloride, polyisobutylene, Teflon or the like may be selected according to the service temperature. When the temperature is higher than 100.degree. C., Teflon is a suitable material for the insulator.

When the present heat-generating pipe is used with a transportation pipe installed in water including sea water or in the underground water, further notable advantage can be attained by the use of the heat-generating pipe having an open construction. By such a construction, it is possible to utilize as a heat-conductive medium the liquid surrounding said pipe (the environment liquid), entering the inside of the heat-generating pipe and filling the clearance space. However it is necessary to pay attention to the construction of the heat-generating pipe so as not to allow the environmental liquid come in or go out of the pipe too easily thereby to cause the heat loss by such movement. On the contrary, when the heat-generating pipe is so long that the entering of the environment liquid from the pipe end is insufficient, or when, due to the uneven configuration of the terrain over which the transportation pipe passes, a gas trap may be formed thus preventing the environment liquid from filling the heat-generating pipe. It is necessary to use a means so as to fill the liquid in the entire length of the pipe. For example, it is possible to provide small openings such as holes or slits in the metal pipe at suitable intervals or to use a number of short independent metal pipes which are arranged in series, electrically connected and both the ends of which are open.

When the present heat-generating pipe is installed in the air or in the place where environmental liquid is not present, it goes without saying that the construction must be of the type which does not cause leakage of the liquid, i.e. the heat conducting medium, and vent parts must be provided so as not to cause the gas to be trapped in the pipe.

When the present heat-generating pipe is connected to the material to be heated, by contacting or welding, a temperature difference is naturally established between the contact part of the heat-generating pipe and other parts. However such a temperature difference is slight in the present heat-generating pipe and hence the life of the heat-generating pipe can be prolonged and the heat insulation can be made simpler.

Claims

I claim:

1. A heat-generating pipe arrangement comprising at least one pipe of ferromagnetic metal, a conductor line electrically connecting one end of said pipe to an AC supply source, an insulated conductor line connected to said AC supply source and extending through the inside of said pipe and being electrically connected to the other end of said pipe remote from said one end of said pipe to form an AC circuit and being spaced-apart from the inner wall of said pipe throughout its length to form a clearance space therebetween, the clearance space being completely filled with a material having a higher thermal conductivity than air, said pipe having a wall thickness greater than twice the penetration depth of the alternating current flowing therethrough, whereby the current flowing through said pipe is caused to concentrate on the inner wall portion thereof by the skin effect and generates heat thereon without any practical voltage appearing on the outer surface of said pipe.

2. The heat-generating pipe arrangement according to claim 1, wherein the material having a higher thermal conductivity than air is a liquid.

3. The heat-generating pipe arrangement according to claim 2, wherein said liquid is water.

4. The heat-generating pipe arrangement according to claim 2, wherein said liquid is salt water.