CA1080291A - Heating apparatus partly equipped with skin effect heat-generating pipe - Google Patents

Abstract

Abstract of the Disclosure For heating a lengthy to-be-heated material which requires different heat quantities in the longitudinal direction thereof, there is provided a heat-generating pipe of induction type skin effect current along the first parts of said to-be-heated material which require relatively large heat quantities and in which parts the insulated wires are placed in the inside of said heat-generating pipes,and along the second parts of said to-be-heated material which require relatively small heat quantities, the portions of the insulated wires are disposed outside said heat-generating pipes as extension from the inside of said heat-generating pipes of the above-mentioned first parts.

Description

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Detailed Description of the Invention This invention relates to an apparatus for economically varying heat quantities when different heat quantities are required along the longitudinal direction of a lengthy to-be-heated material such as a pipeline which is to-be-heated for ¦~
maintaining a temperature or a material surrounding a lengthy pipeline. -~` This invention can be applied to generally a lengthy to-be-heated material, but as examples in order to give ; 10 better understanding of the nature of the present invention to persons skilled in the art, description will be made hereinafter with regard to two cases: i.e. a pipeline for transporting benzene and the other pipeline which passes through the area of ground wherein freezing and melting of the ground are repeated all the year round.
Since the freezing point of benzene is about 5C, it is ; necessary not only to insulate a pipeline carrying benzene 7 but also to heat it to prevent benzene from freezing in a cold area where atmospheric temperature becomes lower than 5C. I , For example, if a pipeline is to be buried under the ground in the southern districts of Japan, to an extent of ; about 1.2 ~1.8 m deep from the surface which is a usual depth in case of a buried pipeline, then the ground temper-ature does not drop far below 5C even in the winter season.
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: ' 1080;~91 Accordingly only a few heating makes it possible to prevent a liquid in the pipeline from freezing even if the pipeline is not covered with any other insulating layer than the surrounding ground, because the ground performs the role of an insulating layer. It is, however, difficult to bury such a pipeline all through its entire length. It is often unavoidable that some parts of the pipeline are laid above the ground. In such a case, since such parts above the ground may sometimes be exposed to atmospheric temperature of about -10C, it is impossible to prevent the liquid from freezing only by the heat quantities applied to the buried parts of the pipeline without the insulating layer. Therefore, the parts of the pipeline above the ground require any insulation means and a larger amount of heating than that in the buried - parts.
An object of this invention is to provide a heating element which produces different heat quantities in the 3 longitudinal direction in response to the requirement of a to-be-heated material requiring different heat quantities in the longitudinal direction.
According to the present invention, a heat-generating ' pipe of induction type skin effect current is installed for- the parts where the to-be-heated material requires a heat ~ quantity larger than for the other parts, and it is so -i~ arranged for the other parts requiring a smaller heat .
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-`- lO~OZ9~ 1, quantity or no heat that at least one extension part of at least two rows of the wire passin~ through said heat-generating pipe, is passed through the inside of another ferromagnetic pipe whereby heat generation is suppressed due to cancellation of induction effect. Alternatively, said extension part is passed through a non-ferromagnetic pipe or a plastics pipe to avoid induction heating for the latter parts, where heat is generated only by the electric resistance of the wire.
The heat-generating pipe of induction type skin effect current herein referred to is the one the principle of which ; is described in U.S. Pat., 3,515,837 or Canada Pat. 844,088 ~; and which consists of one or a plurarity of ferromagnetic pipes and an insulated electric wire passed through these pipes and connected with an alternating current source to form a primary circuit wherein both the ends of said ferro-magnetic pipe are connected with each other so that a secondary current, induced in said pipes by the alternating current flowing in the insulated electric wire forming a primary circuit, concentrates on and flows only through the inner skin portion of said pipes, and the thickness of said pipe walls is several times larger than the skin depth of said secondary current so that the induced voltage formed by said secondary current does not appear on the outer surface of said pipe.

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The connections of both the ends of said ferromagnetic pipe will be concretely illustrated hereinafter. It is assumed that there are two ferromagne-tic pipes desiynated as A and B which are disposed mutually in the outside of the other, i.e. one is not disposed in the inside of the other. An insulated wire extending from one of the two ter-minals of a single phase current source, passes through the inside of A from the first end to the second end thereof, and after leaving the second end~further passes through the s from the second end of B to the first end thereof, and after leaving the first end of B, it is connected with the other end of said single phase current source. Both the first ends of A and B and both the second ends of A and B
are connected with each other so as to give impedance as !small as possible. In order to make the impedance as small ~-as possible, both the ends of A and B to be connected with each other are disposed as close as possible.
According to another embodiment of the heat-generating i pipe of induction type, the second ends of said pipes A and B are connected with the ends of a pipe bent to form a half circle, through which said insulated wire is laid. There is a case where both the pipes A and B form one piece U-form pipe. In either case, the remaining ends of pipes A and B
are connected with each other.

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- lO~OZ91 If another example is to be illustrated, there is a case where three ferromagnetic pipes C, D and E are in the relationship mutually disposed outside the others, three insulating wires extending from the three terminals of a three phase electric source, pass from the respective first ends of the pipes C, D and E through the inside of the respective same pipes,and after leaving the respective second ends, they are connected with each other, the first ends of C, D and Eare connected with each other so as to give impedance as small as possible and the second ends ~; thereof are also connected with each other so as to give impedance as small as possible.
In the above-mentioned heat-generating pipe of induction type skin effect current, commonly and practically ~ steel pipes are used as ferromagnetic pipe and electric 'J wires of copper or aluminum are used as insulating electric wires. In such a case, a greater part of generated heat -, (normally 80 ~90% of the total generated heat) is produced in the steel pipe but there is also heat generation (remain-ing 10 ~20%) in the electric wire. Accordingly, in the above-mentioned examples, it is possible to utilize the insulating wires extending from the first ends of the pipes A and B or the pipes C, D and E to the electric source as well as the insulating wires extending outward from the second ends of these pipes, for the heating of the parts :' . I
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requiring a smaller hea-t quantity of a to-be-heated material.
Thus, in the apparatus of the present invention, for the parts re~uiring a larger heat quantity, heating is effected by the heat-generating pipe of induction type skin effect current that comprises the ferromagnetic pipe and the insulated wire disposed within said pipe, and for the parts requiring a smaller heat quantity, heating is effected by the electric wires extending outside said ferromagnetic pipe (including the part of the wire extending from said ferromagnetic pipe in the directions to an electric source and opposite to the electric source.
In the above-mentioned concrete examples, there may be a case where the extension parts of the two electric wires from inside the pipes A and B pass through another set of pipes A' and B'. This arrangement becomes necessary in the case where a part of a to-be-heated material requiring ., ~
a small heat quantity is interposed between the parts 31 requiring a larger heat quantity in the to-be-heated material.
It goes without saying that this may be applied to the third, the fourth, and the subsequent sets of A", B"; A"', B"';
..., etc.
Furthermore, there may be also cases where the diameter, length, etc. of pipes are different from each other in the combinations of A, B; A', B'; ..., etc. It is also naturally possible, as the cases dictate, to vary the kind of the material of , I

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lO~OZ91 the insulating layer of the electric wire to be passed through these pipes, particularly the material thereof in terms of its heat-resisting property, and the kind of the material and the size in the cross-sectional area, of the conductor of the electric wire, depending upon the requirements of the to-be-heated material. Particularly between the part of the above-mentioned insulating electric wire lying within the above-mentioned ferromagnetic pipe and the part extending outside the pipe, the above-mentioned variations may be often preferable.
The apparatus of the present invention will be described ` more concretely by referring to the drawings. Figure l is a schematic plane view of one example of the heating element of the present invention stretched linearly in the direction }
of length. Figure 2 1S a schematic side view of a pipeline ` equipped with the heating element of Figure l. Figure 3 is an enlarged view of the cross-section taken along the line III-III of Figure 2. Figure 4 is an enlarged view of the cross-section taken along the line IV-IV of Figure 2.
Figure 5 is a schematic cross-sectional view of a pipeline ~t and two heat-generating pipes of the present invention laid -~ along said pipeline so as to warm the ground close to said pipeline.
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i08029~ i In Figures 1 and 2, a pipe 6 corresponds to the part buried in an underground 10, a heat-generating pipe 3 corresponds to an overground part 11. Similarly, there are correspondences between 7 to 12; 4 to 13; 8 to 14; and 5 to 15, respectively.
In Figure 1, feeders 2 and 2' from a single phase electric source 1 pass through the inside of the pipe 6 as a go-and-return circuit, and the heat in this part is generated by the feeders without any induction current on 10the pipe. This holds true even when the pipe 6 is of ferromagnetic steel pipe because the induction electro-magnetic fluxes are cancelled within the pipe 6 by the go-and-return currents which flow in the electric wires

2 and 2'. Thus there will be only the heat generation due to the electric resistance of the wires 2 and 2'. On the other hand, 3 forms a heat-generating pipe of induction t type skin effect current. Namely, the feeders 2 and 2' pass through the respective insides of two ferromagnetic steel pipes 31 and 32 which form secondary circuits consist-20ing of these pipes and the connection parts 3' and 3"
relative to the primary circuits 2 and 2', and if only necessary conditions are satisfied, the secondary current concentrates on and flows through only the inner skin portions of the two steel pipes. Accordingly, even when any other metal is placed in contact with the outer surface '' , : , ,' .

108l)~91 of these two st~el pipes, no current appears on this metal and thus these two steel pipes can be used as a safe heat-generating pipe.
In Figure 2, the aboveground part 11 of a pipeline represents the part where there is a larger heat loss and lower ambient temperature. It is further assumed that only the insulating layer 20 shown in Figure 3 is insufficient for temperature maintenance and the heat generation in the ; inside of the steel pipes 31 and 32 must be added to the heat generation due to the electric resistance of wires 2 and 2'.
In Figure 3, if a transportation pipe 11 is of a metal, it is a common practice to wela two heat-generating pipes 31 and 32 to the transportation pipe 11. In such a case, the role of the connection parts 3' and 3" on both the ends in Figure 1 is taken by a part of the transportation pipe 11.
It goes without saying that if said transportation pipe 11 is a plastics pipe, a special connection material is necessary.
The above-mentioned fact will be explained again by referring to Figure 1. The electric wires 2 and 2' carrying the electric current from the single phase source 1 do not cause the heat generation due to the induction current of pipes in the parts of pipes 6, 7 and 8, and the heat generation due to skin effect current is added to the heat .
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1080Z9l generation due to the resistance of wires in the parts of the heat-generating pipe of skin effect current. The primary circuit of these heat-generating pipe of skin effect current is constructed usually by connecting the electric wires 2 and 2' on the side 9 remote from the electric source 1.
In Figure 2, there is shown a case where a liquid from a tank 17 is sent through transportation pipes 10, 11, 12, 13, 14 and 15 to a tank 18 by a pump 16. Since 19 shows the surface of the ground, 10, 12 and 14 are the buried underground parts and 11, 13 and 15 are overground parts.
In Figure 4, there are shown two pipes indicated as 8 and 8' in which electric wires 2 or 2' are passed. Pipe 8 is situated in the inside of the transportation pipe 14, and ' pipe 8' is installed on the outside of the transportation pipe 14. Since it is a customary practice to apply an anti-corrosion coating around the outer surface of a transportation pipe when it has no thermal insulation layer, inner tracing as shown in 8 is common.
The foregoing description is directed to the examples of application for preventing the transporting liquid flowing through a pipeline from freezing but the heating apparatus is also useful for the prevention of freezing of the soil surrounding buried pipelines.

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- i ~' 1080291 Near the north pole, there is an area where the ground is frozen all the year round. There is also an area where the ground repeats freezing and melting all the year round in the southern part of these areas and there is also an area where the ground is not frozen all the year round in the southern part of the last mentioned areas. Let us assume a case of transportation of, for example, natural gas through a pipeline spanning all the above-mentioned areas. Even in a permafrost area, the temperature of gas can rise above 0C due to the friction loss of flowing gas relative to the wall of pipe and melt the frozen soil surrounding a pipeline. The melting of the frozen soil often makesthe fixing of the pipeline difficult, and, in the worst case, it would work havoc with the pipeline. It 1S
is possible to prevent the melting of the frozen soil by precooling the gas to keep its temperature lower than 0C
throughout such an area and to make the fixing of the pipe-line stable.
However, it is often insufficient for preventing the melting only by precooling the gas in such an area where melting of soil and freezing thereof occur all the year ; round. In such an area, if the soil surrounding the pipeline is electrically heated and prevention of freezing is possible all the year round,the fixing of pipeline becomes rather easier. However, the boundary line between the zone where .

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` 1080291 .1 such melting and freezing are repeated and the zone of permafrost soil is not always drawn clearly, and it is rather common that area of permafrost soil and areas of repeated melting and freezing are existing in non-orderly state.
According to the apparatus of the present invention, the above-mentioned problem can be solved economically.
Namely, in the area requiring heating i.e. the area which repeats the freezing and melting as mentioned above~a heat-generating pipe of induction type skin effect current isinstalled in the lower part ground surrounding a pipeline, and in the area of permafrost land, heat generation is restricted only to the heat generation caused by the electric resistance of wire, in order to minimize heat generation.
Figure 5 shows the cross-section of a pipeline laid in the region requiring heating. In Figure 5, 11 is a pipeline for transporting natural gas; 20 is its insulation layer;
31' and 32' are heat-generating pipes of induction type skin effect current containing insulated wires 2 and 2' therein (not shown in the drawing); 19 is a ground surface and 21 is a sand layer.
With the above-mentioned arrangement, the soil surround-ing the pipeline 11 will not be froæen all the year round by the heat generation of the heat-generating pipe of skin effect current, and the unstability in the fixing of a pipeline, !

-- 1080Z9l , which results from frost heave of the surrounding soil can be avoided. Moreover, wasteful heat generation of permafrost land can be minimized.
Further when even a small quantity of heat generation is detrimental to the permafrost landj it is possible to prevent heating by keeping the parts of the wire extending to the outside of the ferromagnetic pipe of a heat-generating , pipe of skin effect current (the parts 6, 7 and 8 of Figure 1) slightly away from the pipeline (it will be sufficient to keep about a distance (center to center) of 5 m away therefrom).
The electric power necessary for the prevention of freezing is unexpectedly small. It is usually about 10 to several tens watts per meter though it varies depending upon the diameter of pipe, the flow quantity of a gas, change of 1, atmospheric temperature, etc. Accordingly,it hardly affects the cost of transportation cost of the gas.
In Figure 1, it is preferable that the heat-generating pipes of skin effect current 3, 4, 5, etc. are electrically connected with each other. The reason for the necessity of such connections is due to the fact that if the insulation of wires 2 and 2' laid through the heat-generating pipes

3, 4, 5, etc. should be broken, such a failure would be detected easily by a leakage current detector as provided on a switch panel, with the mutual connection of the heat- .
generating pipes, tho~gh such a connection is not shown in ,, I
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10~0;Z:9~ 1 the drawinc~s. Such a connec~ion can utilize the transportation pipe itself when the pipe is made of a metal but if the trans- ¦
portation pipe ll in Figure 3 is made of a plastics, the purpose can be attained by using metal pipes for the pipes of 6, 7 and 8 of Figure l and connecting them together with the heat-generating pipes 3, 4 and 5.
As above-mentioned, according to the apparatus of the present invention, the heating or temperature maintenance of a pipeline requiring a wide range of variation of heat generation in the direction of the lengthy pipeline or prevention of local freezing of the soil surrounding a pipeline becomes feasible with a single electric source apparatus without requiring any special feeder lines and thus without loss occurring in these lines.

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Claims (5)

What is claimed is:

1. A heating apparatus for a to-be-heated long material requiring different heat quantities along the longitudinal direction comprising (a) at least one ferromagnetic pipe or at least one set of ferromagnetic pipes or both disposed in the longitudinal direction of at least one part of said to-be-heated material requiring a comparatively large heat quantity to form a secondary induction circuit and (b) at least two rows of insulated electric wire passed through said ferromagnetic pipe or respective ferromagnetic pipes and connected with an alternating current source to form a primary circuit, (c) said at least one ferromagnetic pipe or at least one set of ferromagnetic pipes, respectively forming a heat-generating pipe of an induction type skin effect current with said insulated wire by having a means for connecting both the ends of said pipe or pipes in such a way that a secondary current induced in said pipe or pipes by the primary current flowing in said insulated wire concentrates on and flows through only the inner skin portion of said pipe or pipes and further by having such a thickness that the induced voltage forming said secondary current substantially does not appear on the outer surface thereof, and (d) said at least two rows of insulated wires having at least one part extending outside said pipe or pipes which part is disposed along the longitudinal direction of at least one of the parts of said to-be-heated material requiring a comparatively small heat quantity or requiring no heat at all and is passed through one ferromagnetic pipe or at least one non-magnetic pipe.

2. A heating apparatus described in Claim 1 wherein said to-be-heated material is a pipeline at least one part of which is buried under the ground, the other part or parts are installed on the ground, and a heat-generating pipe of an induction type skin effect current is provided on at least said one part on the ground.

3. A heating apparatus described in Claim 1 wherein said to-be-heated material is the environmental ground area surrounding a pipeline, said pipeline is buried under the ground passing through two kinds of ground areas one of which is a permafrost ground area and the other repeats freezing and melting all the year round and an induction type skin effect current heating pipe is provided on said area which repeats freezing and melting.

4. A heating apparatus described in Claim 1, 2 or 3 wherein the insulation layer and/or the material and/or cross-sectional area of conductor of the insulated wires are different between the parts disposed inside the ferro-magnetic pipe of said skin effect current heating pipe and the other parts extending from the inside parts and disposed outside said pipe.

5. A heating apparatus described in Claim 1, 2, 3 or 4 having at least two said skin effect current heating pipes being electrically connected with each other to form one body.