US 3777117 A
When an insulated conductor wire inside a steel tube carries alternating current as one leg of a circuit, and the tube itself carries the A.C. for the return leg, induction and magnetic effects develop which cause the A.C. flow to concentrate on the inner surface or skin of the tube, thus greatly increasing the resistance and the heat produced. No current is carried in the outer wall of tube; thus, there is no loss to ground or other surroundings. The heat-tube may be attached to or constructed so as to become an integral part of the wall of a pipe carrying a fluid, thus heating the pipe and the fluid. It may be the transport pipe itself, or it may heat an unenclosed body of fluid using heat supplied by direct contact of the fluid with both the conductor wire and the tube. The heat-tube may supply A.C. to other circuits either related or independent of the heating effect which may be quite unimportant in some of these cases.
Description (OCR text may contain errors)
United States atent [1 1 Othmer Dec. 4, 1973 1 ELECTRIC HEAT GENERATING SYSTEM 21 Appl. No.: 107,351
Related U.S. Application Data  Continuation-impart of Ser. No. 805,718, March 10,
1969, Pat. No. 3,617,699.
 U.S. Cl 219/300, 137/341, 138/33,
219/10.51, 219/306, 219/374  Int. Cl. H05!) 3/00  Field of Search 219/301, 300, 535,
2,224,403 12/1940 Lines 219/300 3,293,407 12/1966 -Ando 219/301 3,591,770 7/1971 Ando 219/300 3,617,699 11/1971 Othmer 219/300 FOREIGN PATENTS OR APPLICATIONS 019184 10/1954 Germany 219/300 756,945 9/1956 Great Britain 219/300 Primary Examiner-Anthony Bartis [5 7 ABSTRACT When an insulated conductor wire inside a steel tube carries alternating current as one leg of a circuit, and
the tube itself carries the AC. for the return leg, induction and magnetic effects develop which cause the AC. flow to concentrate on the inner surface or skin of the tube, thus greatly increasing the resistance and the heat produced. No current is carried in the outer wall of tube; thus, there is no loss to ground or other surroundings. The heat-tube may be attached to or constructed so as to become an integral part of the wall of a pipe carrying a fluid, thus heating the pipe and the fluid. It may be the transport pipe itself, or it may heat'an unenclosed body of fluid using heat supplied by direct contact of the fluid with both the conductor wire and the tube. The heat-tube may supply AC. to other circuits either related or independent of -the heating effect which may be quite unimportant in some of these cases.
52 Claims, 12 Drawinz Figures ELECTRIC HEAT GENERATING SYSTEM This is a Continuation in Part of my application Ser. No. 805,718, filed Mar. 10, 1969, now U.S. Pat. No. 3,617,699 of Nov. 2, 1971, entitled System for Electrically Heating a Fluid being Transported in a Pipe.
This invention relates to the use of the skin effect of alternating current (A.C.) flowing with an adjacent or concentric steel conductor supplying the return or back" leg of the circuit, thus causing induction and magnetic effects which greatly increase the effective resistance of the steel conductor, and substantially make its outer surface an insulator. The proxirriity relation of the two conductors may be changed to increase further these two effects.
Alternating current (A.C.) flows onlyalong the skin of a steel conductor under these conditions. In a tube having a minimum wall thickness of about U l 6 inch less for many steels and with A.C. carried out to the far end by an internal insulated wire, and back by the tube, due to what is called skin effect, all A.C. flows back on the inside surface or skin of the tube and its outside is completely insulated electrically. This considerable reduction of what is normally regarded as the effective cross-section of an electrical conductor greatly increases its effective resistance, so that steel tubes of substantial cross-section of metal, compared to the usual copper wire conductor, offer greatly increased resistanceyand hence can be used for resistance heating with A.C., for which they would be quite unsuitable with direct current (DC). It hasbeen found that these tubes can be designed to give off considerable heat; and they may be used as heaters in many in- .dustrial and domestic. operations. Since A.C. flows only on the inside surface of such a heat-tube, the outer wall of the steel pipe is perfectly insulated from the A.C. It usually is grounded and may be touched without shock. The tube may be used directly as a pipeline, even of considerable size, for liquids to be kept heated in transit.
Such heat-tubes may be made to advantage with extended surfaces of the same metal. Transverse or longitudinal fins or other protuberances on the outside increase the externaleffective heat transfer surface which is in contact with thegas or liquid to be heated. The extended surface effectively dissipates a much larger amountofheatgenerated by-the skin effect than has hitherto been used; and still other advancements have been found in the theory which improves the practical use of heat-tubes.
HEATING OF PIPELINES BY SKIN-EFFECT AS IN PRIOR ART .Long distance pipelines which require heat to obtain the lower viscosities of heavy oils at higher temperatures have used such heat-tubes, but with very low heat fluxes to date, from 10 to 15 watts per foot. Skin-effect heating of pipelines, even with the low performanceof the prior art, has large advantages over other systems which have been used, such as: v
a. heating the pipe wall electrically and directly by normal impedance or resistance to current flow, usually with low voltage D.C.;
b. heating of the pipe wall by a conventional electric resistor by attachment to its surface and suitably insulated therefrom;
c. heating of the pipe 'wall by a hot fluid, usually steam, in a small tube called a tracer tube" or trace running along and in contact with an element of the pipeline.
For long pipelines, the considerable distances from a central source of thermal energy (such as a boiler) makes the connecting steam system for a tracer installation extensive and expensive. Instead, tracer tubes can be heated economically throughout their length by the skin effect, using A.C., which has frequencies as low as the standard 50 and 60 cycles of practically all A.C. generation.
The several economies of the installation and use of the skin effect phenomenon has allowed its economical use in a tracer tube so heated even though the BTUs of heat from electricity are usually more expensive than from steam or heated liquids circulated in a trace tube. Compared with steam: electricity may be transported long distances, controlled much more readily, and utilized much more efficiently. Thus, this method of heating of pipelines is superseding other methods, which are relatively expensive and clumsy in installation and maintenance.
The industrial practice of using the skin effect of 50 to '60 cycle A.C. for resistance heating has so far been limited to the heating of oil-pipes to reduce the viscosity of the oil being transported. The teachings of U.S. Pat. No. 3,293,407 have been utilized;,and a relatively small steel heat-tube, inch standard pipe size, has been specified.
This heat-tube has always been substantially axially parallel to the oil-pipe and on its axis, with an internal insulated copper wire. This internal electric wire forms one leg out of the A.C. circuit; and its other terminal was at the far end of the heat-tube on the inside thereof. Electric current flows back on the return leg of the circuit on the inside wall of the heat-tube because of the skin-effect, with no current flowing on the outside wall, if the steel tube is more than about 0.04 inches thick. The other junction to the source of A.C. is a point at the near end of the inner surface of the heat-tube adjacent the entrance of the insulated copper wire into the heat-tube to carry the A.C. to its far end. The heat-tube has thus always pierced the oil-pipe and been in solid contact therewith.
The heat-tube hasalways been attached in substantially axially parallel relation with the oil-pipe and along one of the elementsof this much larger pipe which carries thefluid. Temperatures of the heat-tube more than a few degrees Fahrenheit higher than that of the oil-pipe could not be tolerated because expansion strains set up cracked off this type of attachment. The standard thermal insulation around the main pipeline also covered the small heat-tube to minimize heat losses; but this insulation had to be supplied in specially cut pieces, around the one to a half dozen heat-tubes built out as appendages on the surfaces of the oil-pipe.
In the construction of the prior art, the outer walls of the oil-pipe and of the heat-tube were two circles in contact and they were tangent, i.e., formed an angle of 0 at the point of contact. Thus, heat transfer has not been good from heat-tube to oil-pipe; heat fluxes have been low, particularly because of the low temperature difference allowable between heat-tube and oil-pipe. Also, thermal insulation has been difficult and expensive to install.
In an alternate construction, less desirable because of the inaccessibility, poorer heat distribution and greater pumping costs of the oil, the heat-tube, again in substantially axially parallel relation, has been installed inside of the oil-pipe supported along the axis by sets of 7 three braces at suitable distances. Besides these solid contacts, there have been also the contact or contacts of the heat-tube where it pierced the oil pipe.
However, only a small heat flux has been generated by skin-effect heating in either of these systems used to date, not more than 10 to watts per lineal foot of the heat-tubes. This has required a large number of heattubes applied to the outside surface of even a moderate size pipeline, even in very moderate winter temperatures of 32F., three heat-tubes for a 12-inch line, and six for a 30-inch line.
THE PRESENT INVENTION Methods have now been found to increase the heat flux supplied to oil-pipes and other adjacent materials through the use of heat-tubes by an order to magnitude, and to make possible the use of heat fluxes as high as 100 to 500 watts, or even 200 to 500 watts per lineal foot of heat-tube. This reduces the cost of application of the large number of heat-tubes, since only one instead of six or more may be necessary; also it reduces pumping costs of the oil. Furthermore, considerable savings in thermal insulation costs are made possible. While the improved systems of this invention singly or together do make possible these much greater heat fluxes, some of them also may be used to great advantage in those installations where low heat fluxes will suffice.
Improved designs of the heating tube have been devised by making the heat-tube conform closely to the external surface or to form an integral part of the oilpipe or its wall. These new designs allow much greater heat fluxes than formerly thought possible, with important advantages and economies. These new types of heat-tubes also decrease greatly the costs of pipe insulation.
It has been found that, by correct design, the A.C. may be made to travel several times the length of the pipe through spiral wound heat-tubes, rather than going through the straight line path of what would normally be quite the least resistance. These spiral wound heat-tubes thus made possible have now been found to be cheaper in both installation and operating costs, and to allow a very much greater heat flux per unit length of heat-tube than the longitudinal heat-tubes used to date, while substantially reducingpumping costs in the oil-pipe line due to more uniform heating of the pipe and the oil. While they may be used at any low heat flux, they are particularly advantageous when using the high heat flux of this invention.
It has also been found that an internal steel electric wire or tube for the out conductor leg can be used to give an additional skin-effect, the increased resistance of which generates still more heat. The use of steel instead of copper as the material for this conductor reduces greatly the cost of the installation, even if the skin-effect is not utilized on this internal conductor, and only the normal conductivity or resistance is effective and considered.
These new developments, and others, all a part of this invention, by creating heat fluxes, allow larger pipelines for transporting oil, particularly viscous oils such as crudes or residues, to operate in colder ambient conditions and to be heated more economically by the use of:
a. new designs of the heat-tube as an integral part of the wall of the pipe. The angle of the wall of the heat-tube section with that of the oil-pipe has been found best when it is at least and preferably more. By entirely burying the heat-tube in the wall of the oil-pipe, and sometimes partly in the internal fluid, this angle reaches the optimum, i.e.,
b. higher temperatures of the heated oil-pipes;
c. larger heat-tubes than heretofore thought possible,
i.e., as large as 3% inches standard iron pipe instead of inch standard pipe;
d. higher operating temperatures of the heat-tubes,
and particularly much higher differences of temperature between the heat-tube and the oil-pipe;
e. spirally-wound heat-tubes, even though the length of the A.C. path through the wall of the steel pipe is thus increased several times;
f. steel wire or tubes for the conductor of the A.C.,
which also reduces cost of installation;
g. improved forms of heat-proof insulation of the internal conductor;
h. use of the heat-tube itself as the transport pipeline.
' Because of the high heat fluxes now possible by the methods of this invention, the heat-tube may be used in many applications by itself and independent of its use as an appendate to an oil-pipe of the previous art, as in the invention of U.S. Pat. No. 3,293,407.
Such uses include heating of fluids in open space and in shells (heat exchangers) either with simple cylindrical heat-tubes, heat-tubes of other shapes, and particularly those with extended surfaces. Whereas the prior art allowed the heating of solids by imbedding heattube s, this takes no advantage of the mobility of fluids, their natural movement due to convection currents. By modifications of the structure of the heat-tube system, the direct transfer of the heat dissipated from the electrical conductor to the material being heated has been accomplished without first passing through the wall of the heat-tube.
Particularly, even very large steel pipes transporting oil may be heat-tubes themselves. Only an internal conductor is necessary, with greatly reduced costs of installation and maintenance. It has been found that, by moving the location of the internal conductor away from the axis of the tube, the effective resistance of the skin of theoil-pipe may be increased greatly. This may be important in large oil-pipes.
FUNDAMENTALS OF THE INVENTION Skin Effect Caused by Inducted Magnetic Flux Skin effect is a phenomenon of A.C., which restricts the flow of A.C. to the surfaces of some metal conductors exposed to electromagnetic fields. Iron and steel conductors-resistors are affected at commercial A.C. frequencies of 50 to 60 cycles per second, when another adjacent conductor carries A.C. so as to generate surface magnetic and induction effects with corresponding diffusional functions of the A.C. in these ferromagnetic materials. In some cases, all three phases of standard A.C. current generation may be used to advantage, as may frequencies from 10 to 1000 or more.
The electro magnetic flux surrounding a wire carrying an A.C. has been found to extend without practical dimunition of its influence on the skin effect for some distance, if not shielded by another metal. Thus, it has been found possible to use a larger individual heat-tube than before up to 3 inches or 4 inches iron pipe size and even to use a much larger tube, the transport pipe itself, as its own heat-tube.
The use of the larger individual heat-tubes allows a much greater effective conductor resistor, i.e., the inner skin of the larger heat-tube. Particularly it allows a heavier internal insulated electric wire, which larger size wire is necessary for carrying the high voltage and intensity A.C. required for heating long distance pipelines. Also, the larger heat-tube allows the use of a poorer conductor for the A.C., hence of larger diameter for the same intensity of A.C. than the usual copper wire. Steel wire or stranded steel cable may thus be used instead; and, if desired, only the skin effect of steel conductors may be utilized if wires larger than about $4; inch diameter are used. The increased resistance of the steel wire, whether or not the skin effect is utilized, gives more line loss and heat all of which adds to that supplied by the heat-tube itself. The use of steel gives the advantage of its high tensile strength in pulling the conductor through a pipeline of considerable length.
Where the terms skin. and skin effect are used, these are not absolute terms. There is a great tendency for A.C. flow to be near the inner surface of the wall of a tube having a nearby conductor of A.C., herein called a heat-tube; and the current density falls off in the tube wall according to an exponential function of the distance from the inner surface.
Under such conditions, it has been found that the effective thickness or depth of the skin through which A.C. is flowing, is directly proportional to the square root of the resistivity of the metal under the particular conditions and inversely proportional to the square root of the magnetic permeability, also inversely proportional to the square root of the frequency. However, a commerical A.C. with a frequency of 50 or 60 cycles per second gives very usable skin depths, as do those from to 1000 or more.
Variations in this skin effect are influenced by changes in the resistivity and the magnetic permeability which are caused by the temperature of the conductor. The gradation of current density against wall thickness from the inner surface of a small heat-tube is so large that, with the voltage drop experienced with steel tubes in heat-tube service (i.e., usually between 0.05 and 1.0 volts per lineal foot) and at temperatures up to about 400to 500F, the effective zero. of current flow or availability is reached within a depth somewhat less than approximately l/l6 inch from the inside tube surface when using 50-60 cycle A.C. For most mild steels worked with, this depth has been found to be between 0.025 inch and 0.075 inch. For any particular steel, the effective conductor area is thus the inner perimeter of the heat-tube times a value of the depth between 0.025 inch and 0.075 inch. V
In the outer part of the wall beyond this skin on the inner surface of the tube, there is practically no current flow; and that part of the tube near the outer wall may be regarded as practically insulated from this A.C. flow a small fraction of an inch away. To be sure that there is no current leakage or danger from the high voltage A.C. flowing on the inside skin, the thickness of the tube from the inner to the outer wall should be about two to three times the skin thickness, or A; inch under the usual intensities of A.C. flow used. There will then be no measurable voltage or power loss, even when the outside of the heat-tube is grounded or submerged in salt water. Unburied pipelines are grounded at reasonable distances, and pipelines in corrosive conditions may have the conventional sacrificial cathodic protection system with no interference with the skin effect heating.
It has been found that extended surfaces, e.g., fins, outside of the normal outer diameter and surface of the steel tube, help further to dissipate the high flux of the heat flow, due to the greater contact area of the metal in the extended surface with the adjacent materials.
Proximity Effect The proximity effect is a very important phenomenon of the electromagnetic field produced by an A.C. passing in one conductor in penetrating an adjacent conductor of a ferromagnetic material in this case, the inner skin of the heat-tube of whatever diameter it may be. As a relatively simple geometric example may be taken a steel heat-tube having a diameter which is large compared to that of the electrical conductor wire acting as one leg of the A.C. circuit. The inner skin of the heat-tube acts as the other leg of the A.C. circuit.
If the conductor wire is on the axis of the heat-tube, from the characteristics of such a magnetic field it follows that the concentration of the field, its degree of penetration of the wall of the heat-tube, and of the current flow in the heat-tube are uniform. There is a fixed resistance in the heat-tube leg of the circuit to the flow of the A.C., which has a definite number of cycles; and the resistance may be calculated from the permeability and resistiveity of the metal. The effective cross-section of this skin-effect-conductor is calculated and confirmed in practice by the assumption that the effective skin thickness is approximately 0.04 inch for usual mild steel, varying slightly with the properties of the particular steel used. This depth, multiplied by the inner perimeter of the tube, gives the effective area of the conductor, i.e., the skin which carries the A.C. The resistance may now be calculated using the tabular values of the properties for mild steel. This resistance may becalled R i.e., the resistance of the heat-tube leg of the circuit with the wire conductor along the axis or at the center, point 0. This position, with the uniform field and uniform penetration resulting, shows the lowest resistance of the skin of the heat-tube.
If now the center of the conductor be displaced some distance, e, from the center of the tube to an eccentric point, e, its proximity to one part of the skin of the inner perimeter of the tube and its distance from the opposite parts gives an entirely different degree of penetration of the magnetic field, and this non-uniform penetration gives a different resistance, and current flow for each different point of the skin on the inner perimeter of the tube. This non-uniformity and its effects depend on the distance of the eccentricity of the conductor. An integration of these effects for the different points of the perimeter will result in a value for the effective equivalent, R the resistance of the heattube leg of the circuit, when the wire conductor is parallel to the axis but at the distance, e, from the center.
This effective resistance of the skin of the heat-tube leg has been found to increase greatly as the eccentricity, e, increases; and it may be expressed as a ratio to the value R,,. Here, the radius, r, is taken an unity, and e is the distance from the axis of the heat-tube to the axis of the wire, which is assumed to be small in diameter compared to the diameter of the heat-tube.
R, R [(r/e e/r)/(r/e e/r)] Or, if R,, is the ratio of the value of R taken as unity:
e 1.0 1.02 1.08 1.2 1.38 1.67 2.12 2.91 4.56 9.57 19.6 25.0 infinity Thus, it is seen that the displacement of the wire to a point near the inner surface of the pipe increases very greatly the effective resistance of the return leg of the A.C. circuit. This eccentricity is limited by the size of the wire and thus the outer radius of the insulation enclosing the wire when it is touching the inner surface of the pipe. The center of the wire cannot have greater eccentricity than the radius of the pipe minus the radius of the outside of the insulated wire. The resistance of the conductor wire, if of copper, is substantially unaffected; and the total resistance which produces the heat in the heat-tube is the sum of that of the two legs. Changing the size of the copper conductor changes its resistance, but not that of the skin effect heat-tube surface. Proper design locates the wire to give the desired resistance; or eccentricity may often be varied in use to control the resistance and heat input.
Materials for Heat Tubes Other magnetic metals than carbon steel, usually other alloys of iron, also exhibit this skin effect and may be used in the systems of this invention. Usual conductors such as copper, brass, and aluminum also exhibit a slight skin effect but require frequencies of many more cycles persecond to reduce the effective permeability or skin depth. Such high frequency A.C. is considerably more expensive to generate than the standard 50 to 60 cycles.
A metal which has properties which giveeffectively only a thin skin for A.C. conductance gives the least effective conducting cross-sectional area, and hence the greatest resistance. Metals, of desired characteristics, may be selected from the above indicated relation of skin thickness as dependent on resistivity and magnetic permeability under the given conditions of temperature, A.C. flow, and the geometry of the system.
Such ferromagnetic metals, besides usual mild steel, which do have a relatively pronounced skin effect, i.e., a thin skin of conductance for A.C. under conditions of the present invention are: very pure iron; iron-nickel alloys, as Hipernik; those with a small amount of manganese, called Permalloys; and those with molybdenum, such as Superalloy. These have from four to six times the resistivity, and a thinner skin for conductances of A.C. in comparison with ordinary mild steel. Compared with ordinary steel, all of these are more expensive some are many times as costly. Thus, they are not to be considered except for very special applications.
While the term steel" is used herein to describe the material of construction of the heat-tube, it may be understood that this term for mild or ordinary carbon steel is used only as an example; and other metals, both ferrous and non-ferrous, may also be used. Usually they have a less desirable skin-effect, or are more expensive. In some particular instances, other physical or chemical properties of other metallic conductors make them worthy of consideration; but carbon steel is preferred for its cheapness, workability, and availability in many forms. Thus, the word steel is used as an example without being a limitation of the material of construction of the heat-tubes of this invention.
Most pipelines for commercial liquids are of steel or other metals, and the principal use of heat-tubes with the high heat flux and as described by this invention is in heating such pipes, which may thus be of substantially the same steel as that of the heat-tube. If the pipe is of other material, with a substantially lower resistivity, care should be taken that the steel heat-tubes have a wall thickness at least twice the depth of penetration of the skin effect, otherwise a current flow of some magnitude may result in the pipeline. In general, it has been found desirable to use a pipeline material with a resistivity not substantially lower than that of the heattube; if both are of ordinary mild steel, there is no question.
Pipelines are now also being made of plastics and of composites; and especially glass-reinforced polyesters or other resins are being used for some systems of larger size pipes. The heat-tube may be used with these pipes, and it may be exactly controlled against overheating which would burn the resin. The heat-tube would usually be made of steel; and it is preferred to be spirally wound around the plastic pipeline in one of the systems, suitable for such materials, as hereinafter described.
Fluids Many fluids have low viscosities at ambient tempertures. Besides petroleum oils (crudes, distillates, or residues), commercial fluids transported by pipe, include molasses, other food syrups or melts such as butter, I
other oils and fats, chocolate, etc. strong sulfuric acid, tar, bitumin, and many others.
Some materials are frozen solid at some ambient temperatures; in particular, there may be noted water and aqueous solutions, sulfur, also benzene, acetic acid, etc. These must be kept heated to prevent congealing or freezing if the ambient temperature is below the respective solidification temperature and sufficient heat above the freezing point cannot be added before the fluid enters the cold length of pipe. With acetic acid, the pipe may be of aluminum, stainless, steel, or copper because of corrosion of carbon steel. The heat-tube would be of steel and particular care would be taken that its thickness be at least twice that of the skin or penetration depth of the A.C. Most desirable would be a stainless steel. In other cases, it may be that ice and snow, with or without liquid being present, fall within the classification of fluids to be heated or melted, because of theirbeing adjacent to the heat-tube.
Also, in handling some vapors or gases, it is desired to keep them in the vapor state by heating to a temperature above the ambient and above their dew point, so that their condensation is prevented. For example, it has been found that hydrochloric acid in gaseous form with water vapor and air, if present, may be piped in a steel pipe if the pipe is always at a temperature above the condensing point of water, i.e., 212F, at atmospheric pressure. Gaseous HCl does not attack steel, only is it corrosive if it dissolved in the water which condenses on the inside of the pipe at temperatures below 212F. ln handling other vapors, it may be desired also to superheat them above their boiling point at the particular pressure of the pipeline.
All of these, and many other fluids, may be heated to a temperature above the ambient, by the methods of this invention, particularly through the high heat flux made possible. Temperatures as high or higher than 400 to 500 may be reached in an oil-pipe.
in those cases where the heat-tube is also the oil-pipe, and the fluid is an oil, which may be an excellent dielectric, no linear insulation is required for the internal conductor if there are insulated supports from the tube.
Heat-Tubes as Electrical Heaters Many uses of heaters, such as, for example, direct heaters open to the bulk of gas or liquids, allow the heat-tube to be used alone, without transfer of heat to another metallic section more or less permanently contacted, but with desirably an extended outer surface better to dissipate the large heat fluxes now possible with the improved heat-tubes of this invention.
The tube, through which the A.C. encounters greatly increased electrical resistance due to the skin-effect, generates heat thereby the develop and maintain a temperature higher than the ambient, up to 400 to 500F, or even more when desired, with the higher heat fluxes of the improved systems of this invention.
The heat-tube is particularly useful in heating fluids in which it may be directly immersed, particularly when advantage is taken of the convection currents of such fluids, and especially when extended surfaces, as fins, increase the rate of heat transfer. In many cases, as with gases and most non-aqueous liquids, where the fluid is a non-conductor of electricity, both the inside and the outside of the heat-tube and the surface of the electrical conductor may be partly or fully exposed to contact the liquid, thereby increasing greatly the rate of heat transfer and preventing the necessity of the heat given off due to the resistance of the conductor having to pass through-the walls of the heat tube to reach the fluid itself. This eliminates much thermal resistance in,- side the heat-tube and lowers the operating temperature of the electrical conductor as it transfers the heat more efficiently. This is particularly important in those cases where considerable heat may be developed in the electrical conductor; and it may be desirable to use an expanded surface to transfer it most efficiently to the fluid being heated.
Elements of the Heat-tube The heat-tube may be of a shape other than cylindrical and have varied configurations in respect to the oilpipe or other structure to be heated; or it may be an integral part thereof. However, it is referred to here simply as the heat-tube regardless of its cross-sectional shape or convolutions, whether it is made of several sections formed together longitudinally, or even laterally, or is a unitary tube, a strip, or other steel shape.
Also, the electric conductor for the one side of the A.C. circuit, if carried inside of the heat-tube in whatever configuration that may take, may be a copper wire in most cases or of other commercial metals or alloys. It may be of single or multiple strands of any desired arrangement or cross-section. It is referred to usually simply as the electric wire.
The electric wire may be made of other than usual metals. Metallic sodium may be used as the conductor. A lining tube of polyethylene, or more heat-resistant resin for the insulation, is drawn through the heat-tube and this may be filled with molten metallic sodium which solidifies and acts as the electric wire.
Whatever line loss of A.C. there may develop in the electric wire gives all of its heat to the heat-tube which surrounds it, and hence is utilized in heating the adjacent materials, e.g., if attached to an oil-pipe, to the oil therein. If the electric wire is of copper, steel, or other metal of greater resistivity, the additional heat which it gives up due to the larger line loss will all be utilized in the fundamental heating operation. Thus, a relatively inexpensive steel conductor or wire may be used in place of the more expensive but standard copper. However, the heat so generated within the wire will have to pass through the electrical insulation, thence through any air space between the insulation and the heat-tube before it adds to the heat from the heat-tube to be passed to the surrounding materials. The electric wire will usually have a somewhat higher temperature, i.e.,
-5F, than that of the heat-tube; and this must be considered in specifying insulation materials. As previously noted, if the fluid does not conduct electricity, it may be in contact with the surfaces of both the electrical conductor and the heat-tube, thus reducing the temperature rise in the electrical conductor.
If the internal conductor for the A.C. is of steel or one of those materials which develop the skin-effect possibly even more than does steel the resistance, and hence heat output, will be even greater. A large steel wire or even steel tube may be used inside the heat-tube; and the heat generated by its electrical resistance accentuated by its own skin effect passes through the electrical insulation and adds to that generated by the surrounding heat-tube. The skin effect of the inner steel wire or tube will be now on the outer surface, which is that part closest to the A.C. flow in the oppositeconductor.
.Insulation for the electric wire may be of any suitable material which will maintain its physical, electrical, and chemical characteristics as the temperatureof the heattube. It is specified depending on its operating temperature. Polyvinyl chloride is satisfactory up to about 180F, polyethylene up to about 215F, and specially cross-linked polyethylene up to about 260F. Silicon resin materials are available to be used from 350 400F. Higher temperatures up to 500F or above may use the insulation made of chloro-fluoro hydrocarbon resins commercially available at much higher prices.
The electric wire has not been more than a fraction of a degree warmer than the heat-tube in the prior art. Thus, the cheapest resins for insulation, PVC and Polyethylene give adequate heat transfer without increasing the cost by expensive insulation materials. Particularly is this true in most cases of the present invention because of the much better heat conduction of the novel heat-tube designs of this invention.
Essentially, the heat-tube is a steel shape which, because of its position in an A.C. electromagnetic field, changes its effective resistance to the conduction of the A.C. Particularly important is the steel tube around a conductor, but other steel shapes, strips, bars, will exert this same effect to a lesser degree, but may be more adaptable to taking advantage of the proximity effect,
as will be exemplified later.
Furthermore, while the temperature of the heat-tube itself was limited by the former art to a relatively few degrees above that of the oil-pipe and not much more than that above the ambient, it has been found that the new designs of the heat-tube allow it to be used up to at least 400F or 500F with special materials even higher. Thus, very large heat flux and heat input to the adjacent materials being heated are possible. By using different of the novel systems of this invention for the heat-tube, any desired difference up to 100F or even more, between the heat-tube and the oil-pipe, may be used satisfactorily. Higher'heat-tube temperatures are not usually necessary to secure the large advantages of the high heat fluxes of this invention, but an upper limit may be about 500F where the magnetic properties of the steel may change.
However, with thehigher heat fluxes made possible by the present invention, the electric wire may operate at higher temperatures; and ceramic and. other special inorganic insulations may be used in powder, cement, or bead form for these higher temperatures.
Special inorganic powders have been found to be useful for this purpose, particularly the oxides of the alkaline earth metals, such as Beryllium, Magnesium,
, and Calcium. These oxides, such as magnesia, may be incorporated as an insulator inside the heat-tube and around the electric wire if of copper or particularly if of iron or other material of greater resistivity then copper. This powder must be firmly packed with the wire correctly aligned in the center of the tube in a factory operation. To compact adequately the insulation powder so that its heat conductivity will be improved, the assembled heat-tube, insulation, and wire may be passed hot or cold through rolls to reduce the size of the heat-tube slightly. A notable one of these oxides in powder form is beryllium oxide, which has excellent electrical insulation properties, while being a good heat conductor. It, like magnesium oxide, or calcium oxide, may be used in the upper limit of effectiveness of heattubes of the high heat fluxes of the present invention, but is too expensive for most uses. The heat-tube preformed with the electric wire and its heat-proof insulation may then be brazed'or welded into the oil-pipe in the shop or in the field by one of the several designs of this invention.
When the heat-tube is also the oil-pipe, no insulation of the wire may be required if the oil, by itself, is a good dielectric, and the. same is true with other liquids and gases which are also good dielectrics by themselves.
If the so-called heat-tube does not completely surround the so-called wire, each of which carries one leg of the A.C., either or both, in cross-section, may be strips of steel, half tubular shapes or channels, or may take still other forms which develop concentrations of the magnetic field generated or influenced by the A.C. passing in the order. Thus, the skin effect may be less pronounced in one or the other, but may, nevertheless, in conjunction with the proximity effect, influence very markedly the resistance of either or both conductors, the heat-tube and the wire.
FIGURES In the equipment of this invention and in the figures as drawn, only A.C. is used; the conventional signs plus and minus indicate simply the two terminals for connection to the supply of A.C. by alternator or transformer. These symbols represent the direction of flow at only one instant of one cycle of the A.C., which is reversed many times per second.
Thermal losses are minimized by application of conventional insulation materials in the usual manner to pipelines heated by this or other methods. Such insulation is not shown in the Figures as it is not a part of this invention, by itself. However, one of the major objects of this invention as applied to the heating of pipelines is the improvement of the ease and economy of application of insulation, because the rough contours of a small heat-tube (or several or more such) on the periphery of the large oil-pipe have been largely eliminated. Insulation is a very substantial part of the cost of a large pipeline. By making the periphery of the heattube when combined with the oil-pipe deviate as little as possible from the general circular cross-section, or preferably not at all, as is done by the new systems of heat-tubes now possible, the costs of labor and material for application of insulation are reduced very considerably. Besides this reduction of costs, the firmness, strength and life of the insulation covering is greatly increased; and maintenance or replacement costs are reduced or eliminated.
All figures are to be regarded as diagramatic no scale is followed. In particular, for ease of representation, the ratio of the size of a heat-tube to its oil-pipe is usually somewhat larger as drawn than it would be in commercial pipelines, particularly if the oil-pipes are large. Also, the electrical wiring connections are merely basic circuit indications.
Especially it should be noted that those representations of sections of long heat-tubes might continue for tens of thousands of times the diameter, although conventional breaks are not indicated.
FIG. l'represents the cross-sectional view of an oilpipe which is, by itself, the heat-tube. The single electrical conductor is shown in three possible positions at the center, along the axis at some indefinite distance from the axis; and lying on the bottom or at other outer points.
FIG. 2A represents a longitudinal view of an oil-pipe as heat-tube, also with the conductor along axis.
FIG. 2B is a cross section showing insulating supporters for the conductor wire. In (a), (b) and (c) of FIG. 2A are diagrams of added wiring to the heat tube circuit; (a) and (b) show additional-resistance electrical loads connected in parallel between the conductor wire and the inside of the oil pipe, and (0) shows an additional resistance electrical load connected in series with the conductor wire.
FIG. 2C diagrams an additional electrical load as a resistance added in series between the end of the conductor wire and a nearby point of the inside of the pipe wall.
FIG. 3 diagrams an outer heat-tube and an inner coaxial tubular conductor, also operating as a source of heat.
FIG. 4A represents an air heater formed of an unenclosed heat-tube with extended surface and an internal conductor. In (a), (b), and (c) of FIG. 4 a longitudinal view is shown, in FIG. 48 an end view is shown. In (a) the heat-tube has fins; in (b) only the fins act as the heat-tube as the tube is removed, and in (c) the fins are on the tube which has perforations.
FIG. 5A is a longitudinal cross-sectional view at the cross-section A-A in 58 which is the horizontal crosssection taken at BB in SA. The fluid heater has both heat-tube and conductor with fins, all enclosed in a shell.
FIG. 6 is a horizontal cross-section of the electrical conductor with expanded surface as longitudinal fins inside a smooth bore heat-tube.
FIG. 7 is a longitudinal elevation of another electrical conductor with expanded external surface in the form of a screw thread inside a smooth bore heat-tube.
FIG. 8 is a cross-section of a heat-tube with an electrical conductor so arranged that it may be moved from the axis to very near the inner surface of the heat-tube.
HEAT-TUBES AS OIL'PIPES The heat-tube may actually be the oil-pipe itself, as diagrammed in FIGS. 1 and 2 of a pipeline carrying the oil or other liquid which must, in this case, be a nonconductor of the A.C., unless the internal electrical conductor is insulated. The electric-conductor is in direct contact with the fluid and thus supplies some heat directly to the oil or other liquid without the necessity of passing this heat through the wall of the heat-tube. The wall of the heat-tube, nowthe oil-pipe also, supplies more. The relative amounts are controlled by their relative resistances since they are in series and have the same intensity of A.C. flow. This design is preferred for long oil-pipes and may be used in any size of pipe used in present practice. There is thus no solid contact of a heat-tube in being attached to or piercing the wall of another transport pipe.
In one type of installation, as shown, lower 33, FIG. 1, the electric wire may be well insulated with material resistant to the oil or other fluid and simply dropped on the bottom, well anchored along its length, to prevent movement due to' friction of the liquid movement. There is a difference of the proximity relation of the A.C. conductor and of the penetration of the magnetic field which causes the current flow due to the skin effect to vary somewhat around the periphery of the heat-tube when the electric wire-is thus so much closer a point, 7, near the end where the electric wire enters. Here again, a branch, 9, allows a tubular insulator, 14, and a flange insulator, 44, to carry the wire outside the tube to the terminal.
na 8. immune m e n an jnamb temperatures of 50F, 175 watts of electric heat would have to be supplied per foot of length. In the use of an axial conductor, a copper cable, concentrically braided of 0.772 inch diameter, would be satisfactory. A 54 inch copper bar would also suffice. It maybe uninsulated if handling crude oil without water; or it may have a suitable insulation, not considered in the present examples. It would be supported at intervals of about 10 feet by three suspenders of suitable material, as polyethylene, carried from small eyes welded on the inner surface of the pipe. An aluminum bar may be used for the axial conductor. It would be slightly larger in cross-section in inverse proportion to resistivities, but would be lighter and less costly. Normally, this would be of circular cross-section, but it could be a flat strap or of other shape to give greater surface for heat transfer, for mechanical rigidity, or otherwise.
Approximately one-half of the heat input would be from the axial cable or bar and one-half would come from the resistance in the inner skin of the 48 inches conbined heat-tube and oil-pipe. A.C. is supplied for the 10 miles at 5,300 volts, or 0.1 volt per foot, and its intensity is 1750 amperes.
Both smaller and larger internal conductors may be used. If it is of copper wire, 0000 gauge, the weight of copper and its cost will be less. However, its resistance is higher, and therefore it will supply a larger percentage of the total heat supply. The A.C. for'the 10 mile pipeline will be 1445 amperes at 6,400 volts.
If an aluminum bar is used, it may be designed to have the same A.C. specification as for the 0000 copper wire. If circular, it will be 0.531 inch in diameter.
' lts 0.221 square inches of cross-section could be of to some part of the wall (here the bottom) than to the rest of the wall. However, this may be computed in designing the system and makes no substantial difference 'in operation of an oil-pipe which is also a heat-tube. For example, in a 48" pipe, the resistance to A.C. flow of the heat-tube might be expected to be as much as times as great for the inner skin effect of the pipe when the conductor is lying on the bottom at 33, as when it is in the center at 3. In some cases, two or three wires, as 33, may be in parallel near the inner perimeter in order to divide the current flow and reduce the proximity effect.
The alternative position 133 of the wire represents any other position between the axis and the wall of the oil pipe; and the effective resistances at such different positions will be discussed below.
Usually, however, the internal conductor is carried axially, and this will be considered first. Thus, in FIG. 2, the electric wire connected to one terminal of the A.C. is shown uninsulated except for the connection through the branch, 9, which has the tubular insulator, l4, and flange insulator, 44. Polyethylene or other suspenders, 4, support the wire, 3, along its length from eyes, 10, welded on the inside wall of the pipeline. The other connection to the A.C. is made at other shape. If the aluminum bar is "34 inch diameter, the current for 10 miles will be 1,800 amperes at 5,100 volts; if the bar is 1 inch diameter, the current for 10 miles. is 2,070 amperes at 4,450 volts. Increasing the size of the conductor makes its resistance less when compared to that of the skin of the 48 inch steel pipe; thus, its resistance will developless of: the total heat required.
The central conductor may also be of steel, but if it is a bar more than about Ms inch in diameter, the skin effect greatly reduces its effective cross-section area for A.C. flow. Also, any steel tube with wall thicker than the depth of penetration, or about l/25th inch, will have steel which is not conducting. A strip about twice as thick as the penetration depth is one preferred shape. A steel shape, tube, or cable may have advantages of tensile strength in pulling, or rigidly in support; or if of large surface per unit length, of increased rate of heat dissipation.
A 4 inch outside diameter tube of l/25th inch wall steel with a resistivity of 30 X 10" micro-ohms centimeters may be used. The A.C. supply for the 10 mile pipe of the example would be 12,300 volts at 750 amperes and the division of heat between the central conductor and the pipe wall (both of the same material and both having an effective thickness for current conductivity of 0.04 inch) would be inversely proportional to the 4 inch CD. of the conductor and the 48 inch ID. of the pipe. Relatively this is 12 for the axial conductor 15 and l for the pipe. This divides 1/l3 X 175 watts= 13.5 watts input per foot from the steel pipe itself, and l2/ 13 X 175 watts 161.5 watts input per foot from the axial steel conductor 4 inch O.D. X 1/25 inch.
If the central steel conductor is 2 inch OD. and 0.04 inch wall (or any thicker wall), 7 watts would be given up by the steel wall of the pipe and 168 watts by the axial steel conductor. The A.C. required would be 0.322 volts per foot or 17,000 volts for the 10 miles, with 540 amperes flowing.
In large pipelines, say of 48 inch diameter, the eccentricity may be made as much as 0.96 of the radius, with the center of the conductor wire about 1 inch from the inner wall of the pipe. The ratio of the resistance of the return leg of the A.C. the inner skin of the pipe is therefore about 25 times that when the conductor is on the axis. In large pipelines, this is important because the cross-sectional area of the skin becomes very large. This, for a 48 inch pipe, the skin with a thickness of 0.04 inch represents an effective electrical conductor of steel having a cross-section of about six square inches. This has a very low resistance indeed when the conductor wire is on the axis. However, this low resistance may be multiplied by a value of 25 or the effective cross-section of the heat-tube as a conductor may be divided .by 25 to equal an effective crosssection of only about 0.25 square inches of steel conductor, as is possible when the conductor wire is laid on the bottom of the pipe, inside.
It is thus apparent that the resistance increases very greatly from what would otherwise be obtained from the nominal skin as calculated above on the basis of the conductor on the axis. The use of the transport pipe as the heat-tube thus becomes quite practical despite the large apparent cross-section of the effective skin. Moreover, depending on the voltage drop 'per foot which is desired, the effective resistance of the wall of the pipeline may be varied within a range of about 25 times by moving the conductor wire from the center to the inner wall, e.g., the bottom of the pipe. This relation has been indicated above as a means of regulating or controlling the electrical input; and because of the increased resistance of the pipe wall itself, either the voltage supplied or the size of the conductor wire may have to be increased.
In placing the insulated internal conductor on the bottom of the oil-pipe, as compared to an axial position, while the resistance is greatly increased in the pipe, the percent of the current flow, and thus of the heat given off near the bottom, is very greatly increased, not only by the presence of the conductor and the heat which it produces, but also because of the much greater concentration of A.C. flow in the lower section of the skin of the pipe itself. Usually this will be of advantage, particularly at times of heat-up after a cooling-down. However, there will always be some concentration of current and hence of heat throughout the entire perimeter of the skin.
These examples show some relations of theseveral variables for large, long-distance pipelines, with an axial electrical conductor when the oil-pipe is also a heat-tube. In every case, the effective field penetration would average much less, and resistance of the skin of the pipe to A.C. flow would increase very much if the conductor was lying on the bottom of the pipe, rather than being along the axis.
One advantage of this system is that, by supplying heat directly to the oil-pipe wall, as well as to a conductor directly contacted by the oil, a more uniform distribution of the heat is possible. It is noted that particular advantages in design and operation are possible with the large heat fluxes made possible by the present invention; but this novel heat arrangement is also useful with the lower heat fluxes of the prior art.
Another advantage is that the conductor contacts directly the oil in the pipe to increase greatly heat transfer efficiency, since heat does not have to be transferred through a heat-tube to add to that which must be dissipated from its surface.
Here, as in other uses of the heat-tube principle, the oil pipes would be grounded frequently throughout their length; and they may also have the usual cathodic system for cathodic protection without reference in either case to the major A.C. current on the inner skin.
In many cases of long distance pipe transport, it may be necessary to install alternators, whose principal duty is the heating of the pipeline. If several or more are required so that a special design is warranted, these may be made to develop some other number of cycles than the usual 60. Since the thickness of the effective skin is inversely proportional to the square root of the number of cycles of the A.C., it follows that increasing this by nine times to 540 would reduce the thickness to /a; while reducing it to 20 would increase the thickness by the square root of 60/20 or 3, which is by 1.73 times. 10 to over 1000 cycles may be used.
The change in the eccentricity of the conductor and the number of the cycles of the current thus allows wide latitude in establishing the optimum resistance for the design of a major installation.
TWO COAXIAL HEAT-TUBES In FIG. 3, the outer thin cylindrical section, 2, is a heat-tube; the inner thin cylindrical section, 3, takes theplace of the usual electric wire; and it conducts the A.C. on the out leg of the circuit. The outside surface of tube, 3, connects with the inside surface of the outer heat-tube, 2, near the far end. At the near end, the inside-surface of 2 and the outside surface of 3 are connected to the respective terminals of the source of A.C. The inner tube, 3, is in the field of the electrical current passing through the outer heat-tube, 2. If 3 is also of steel or other material with a pronounced skin effect, this skin effect will be on the outer surface (closest to the flowing current in 2). The inner heat-tube, 3, will also generate heat which usually must pass through 2 to the surroundings. In the usual case, heat losses are all that are important; thus, this heat would merely be compensatory for these losses. However, 2 and 3 must be insulated from each other by a suitable tubular insulation, 4, which must retain its properties at the temperatures involved. In some cases, the space between 2 and 3 may be an air gap and there may be suitable spacers for maintaining this spacing.
This use of the skin effect resistance in 3 would have advantage if it was necessary to insulate the inside of 3 from the flow of A.C. on its outside. Otherwise, a lighter weight conductor would give the desired resistance and heating. This conductor of one leg of the A.C. would usually be a tube with a wall at least about .4; inch thick if, for some purpose in design, the inside of 3 was to be insulated from the A.C. flowing on its outer surface.
Because of the relatively large currents which will flow at the surfaces of even moderate size tubes, the coaxial tube system will havevery low voltage requirements per unit of length because of its low resistance. There are some means of increasing the resistance. However, this low voltage dropis a very large advantage in long-distance pipelines using the heat-tube as the oilspipe.
HEAT-TUBES AS ELECTRIC HEATERS A heat-tube may also be used for heating a fluid not only in apipeline, but if aliquid in an open vessel or other container, or if a gas in an open space. A very much higher heat flux may-be generated by the internal skin effect resistance of the heat-tube of the present invention than is used in heating an oil pipeline. This heat may be dissipated to the atmosphere, or into other surrounding liquids or gases. Especially if heat transfer is to air or gases, the heat transfer away from the heattube may be increased by means of expanded surfaces to dissipate the large heat input developed.
In these, as in every other application of heat-tubes, and with every other electrical heater, a careful design is necessary based on conductor size, and hence resistance, current carrying capacity at given voltage drops, and mechanical design as to configuration and particularly as to dissipation of the heat input.
Of major importance in such designs of electrical heaters is the relation now demonstrated that, whereas in ordinary heating resistors or conductors the resistance is proportional to the resistivity of the material, in the use of heat-tubes the effective resistance due to the skin effect is proportional only to the square root of the resistivity. The apparent resistance is greatly increased by the fact that the conductor only. conducts through its skin, and the balance of the normal crosssection may not be used. Precise designs are not given for these heaters but may be made by the usual calculations of electrical heat input balancing the calculations of heat transfer of. the surfaces involved to determine temperatures which will be developed in the heater. The permeability of the metal on surfaces of irregular shapes and distances from the conductor adds some complexity to design calculation which can only be made rigorously'for some idealized shapes. The operation is, however, quite definite but may require a slightly different voltage than that calculated to give the desired heat input and slight adjustments in the design to obtain the optimum.
Because of the large cross-section of the resistors, i.e., the surface or skin,.even though. it usually is not quite 1/16 inch thick, large intensities of A.C. are usually drawn at low voltages. This factor is particularly helpful in designs for many types of heaters; including those where long lengths use commercial voltages to give a drop per unit length which is reasonable with a heat-tube of a size which is satisfactory from other conside'rations. Other methods to be discussed allow higher resistances and hence voltage drops, thus reducingthe length of the'heaterrequired even with the high heat fluxes possiblewith this invention.
The amount of heat transferred from a heat-tube is a function of the area of metal contact with the cold fluid receiving the heat. Extended surfaces as anintegral part of the steel heat-tube may add greatly to these surfaces and they may be of steel or, in many cases, of other metals.
FIG. 4A and FIG. 4B diagram a heat-tube, 2, with one standard type of extended surface, .a spirally wound, helical fin, 20, which coils transversely to the flow of gases and may be attached to the heat-tube in any one of the several standard ways. A longitudinal view of the unit of fins and tube is in FIG. 4A, an end view in FIG. 45. Simple disk type fins or any one of the other type of extended surfaces: pegs, studs, stars, spines, etc. may be applied or made integral with the tube itself, as is conventional practice with tubes having extended surfaces, for increasing heat transfer to the surrounding fluid. The extended surface system is particularly useful in transmitting the large heat flux which may be generated in the internal skin of the heat-tube by the skin effect to air or other gas or liquid in flow across the extended surfaces on the outside. This cross flow of fluid at anangle to the axis quite often at is to be distinguished from the flow of the fluid being heated in parallel, or generally parallel flow to theheattube as used with an oil-pipe.
Such extended surface as that on the tube of FIG. 4 are commonly used in many heating services using a heating fluid passing through the center tube; e.g., steam, which, by condensing, gives up its heat to the tube, .thence to the fins, and thence to the surrounding fluid, often air in cross flow.
Practically the same designs of radiators, air heaters, etc. as the conventional ones heated by'steam may be used with heat-tubes and with either natural convective or with forced, movement of air across the tubes. The heat-tube, 2, in FIG. 4A is supplied with A.C., for which it acts as a conductor-resistor through the tube on one leg of the circuit due to the magnetic, inductive factors of the A.C. passing through an axial electric wire forming the other leg of the circuit.
Two differences from conventional construction with an internal heating fluid are: (i) therequirement of connections for A.C. supplied as above described to one or more heat-tubes, straight and parallel usually, or in any bent or other configuration, (ii) the heat-tube, using A.C. by the=skin effect with the higher heat flux input of this invention may require the fins to be designed somewhat larger and more effective to dissipate theheat input than when using a hot fluid as the heat source.
The heat flux which may be usefully developed by the skin effect on the inside of the heat-tubes of this invention are from 200 or even 300 to 500 watts per lineal foot; and the fin surface must be designed by standard methods, to dissipate this amount of heat to the fluid flowing usually at an angle to the axis. A very high performance'heater is this available where electric heat is to be used for heating a fluid.
It has been found that the normaldesign methods for extended surface heat exchangers for heating of air or other gas which are based upon the so-called fin effectiveness may be used immediately along with the same heat transfer coefficients for fins, as have been developed for these when using a heating fluid inside the tube. The rate of heat loss from the fins is immediately calculable with the heating done by the skin effect on the heat-tube, using the same methods and equations as are usedif the cylindrical core-tube is brought to the same temperature by a heating fluid.
F IG. 4A indicates simply the heat-tube, 2, with an extended surface, 20, which may be used, for example, as an air'heater. It may be heated by connecting one terminal for A.C. supply to the electric wire, 3, covered with insulation, 4, entering the left side, and as a dotted line passing from left to right through the center of the tube. Near the right end at the point, 6, the inside surface is connected to this wire, 3, thence A.C. flows back on the inside skin of the inner tube wall. Near the left end, it is connected to the wire, 13, at point 16, and to the other terminal right for A.C.
No housing or duct work is indicated around this heat-tube for supply of adequate movement of air across the surface by normal convection or by forced motion by a fan or blower. Such ducts, housings, fans, and blowers are familiar in the art and need not be described since they are not a part of this invention. The simple heat-tube with such expanded surfacemay furthermore be used without housing or ducts as a space or room heater, e.g., along the baseboard of a wall near the floor, or with a reverse or hairpin bend for direct immersion in liquids. Again, normal convection may suffice, but greatly increased heat transfer rates will come, as always, with positive induced velocity of the fluid over the surface.
The extended surface of 2, which is heated by the A.C. and skin effect resistance, heats the surrounding air which rises around the fins, as it is displaced by cooler air from below. A convective motion is set up. This increases as the surface becomes warmer until a balance or steady state is reached of heat input to inner skin of 2 by the A.C. resistance, metallic conduction to fins, and dissipation by convection to the surrounding air.
In the use described above of the heat-tube with an oil-pipe, only a relatively minor amount of heat is required to maintain the fluid at a temperature sufficiently above the ambient for whatever purpose is involved, e.g., to lower viscosity so that the viscous fluid may be pumped to keep the temperature above a solidification point, etc. Now it has also been found that with the large heat fluxes made possible by the present invention, the heat-tube may be used for relatively larger duties of heat transfer, and indeed in place of a conventional double pipe heat exchanger using the larger heat flux of this invention. I I
As an example, the extended surface on the heattube of FIG. 4 may be of the type adapted for longitudinal flo'w of fluid; one or many tubes as FIG. 4A may be encased in a shell with the two ends extended; and an inlet nozzle to the shell side may be added at one end for the cold fluid entering, and at the other end a nozzle for heated fluid leaving the shell side. This is a simple heater for fluids. Instead of a simple electric wire from the axial conductor of one leg of the circuit, the two coaxial heat-tubes of FIG. 3 may be used. Normal problems of electrical insulation willhave to be taken care of, as indicated elsewhere, if the fluid is a conductor.
The electrical conductor, 4, running the length of the heat-tube in FIG. 4 has resistance to A.C. and also gives off heat, particularly in the heavy duties possible with this electrical heating system. With the solid tube structure of (a) in FIG. 4, air could still circulate through the heat-tube to help cool 4. In (b) of FIG. 4, which would represent a construction used through most or all of the length, the tube is eliminated and only the helical fins remain with the helical opening between as the space between the coils of a helical spring. The gas can go through this opening to cool the insulated conductor wire, now completely exposed to its movement and heating. The electrical current passing in the inner cylindrical skin of the heat-tube now does not go end-toend, but must go through the much greater length of the coils of the helix, and the effective depth of penetration will be only about 0.04 inches of the inner cylindrical boundary or wall of the helical. Thus, the effective resistance of the heat-tube may be increased many times.
The removal of the heat generated by the inner conductor wire, 4, may also be accomplished by having many holes, '35, drilled in the tubular part of the heattube, 2, to allow air to pass through and contact 3 or its insulation 4. This is shown in (c) of FIG. 4.
An even simpler heater is that of FIG. 2 for oil or other liquid or gas which does not conduct the A.C. This heater was discussed above under Heat-Tubes as Oil-Pipes. The heat-tube, 2, is now the heated shell or oil-pipe; and it may be covered with thermal insulation if desired. Also, the ends of the tube, 2, are closed, although this is not shown in FIG. 2, which represents one advantageous use in oil transport operations wherein heat may be supplied both to increase the temperature of the oil and to balance that lost to the colder surroundings.
If the heater of FIG. 2 is used-as a space heater for ambient air, the heater may have extended surfaces attached to or an integral part of 2. A.C. is supplied as to the electric wire, 3, which is immune to the surrounding material inside the heat-tube and at its operating temperature. The electric wire is brought into the heat-tube, 2, through a branch connection, 9, with special insulation, 44, passes the length of 2, and is firmly connected electrically at 6 to a point near the end, and
on the inside surface of 2. The A.C. on its return leg heats the inside surface of 2, flows from a connection at point 7 to another wire, 13, with insulation, 14, passing through the special insulation, 44, inside the branch connection to the terminal of A.C.
This heat-tube of any desired length, and bent to any desired shape, may be closed atthe ends and filled with oilpConvective motions will be set up in the tubular vessel to give a uniform temperature throughout with considerable heat storage capacity for use as a space heater. The masses and respective specific heats of the oil and of the metal stores heat which gives a tempering effect between the on-and-off of the high heat flux supplied. Other materials than oil: gas, liquids, or soild, which are non-conductors of A.C. may be used. This electric wire may have usual insulation and simply be lying on the bottom of the outer pipe. Alternately, it may be carried axially by suspenders, 4, of polyethylene or other insulating materials, attached, as shown, to rings, 10, on the inside of the pipe.
A version of FIG.'2, relatively shorter than the heattube as oil-pipe already discussed, may be used primarily as a liquid heater rather than as a liquid transporter. It would have a much larger heat flux and would have open ends, as in FIG. 2, for liquid flow while being heated.
A further development of this use of heat-tubes with high thermal flux is shown in FIG. 58, a longitudinal method. The fins thus are formed as a substantially integral part of the tube. The heat-tube is inserted into a shell, 1, with a narrow clearance or none between the outside of the longitudinal fins and the inside of the shell which carries the fluid to be heated to a substantially higher temperature than its inlet temperature. The use of the heater of FIG. is as a heater for air, oil, or other fluid which does not conduct electricity; but other variations permit the heating of fluids which do conduct electricity. For this example, air is being heated.
FIG. 5 indicates a heat-tube, 2, with a large number of longitudinal fins, 20, extending substantially its entire length, in order to transfer the relatively much higher flux of heat used here from the supply of A.C., and the electric resistance, and the skin effect, to a fluid moving in the shell space outside the tube. In this use, the ratio of the internal diameter of the heat-tube to the internal diameter of the shell would be of the order of l to 5, to 3 to 5; or if more than one heat-tube is used in a shell, the ratio of the sum of their internal diameters to that of the shell might be 5 to 5, or even more. (A heat-tube has a relatively small internal diameter or sum of internal diameters if more than one when attached to an oil-pipe. The ratio may be 1 to 10, to l to thus the heat flux is low, i.e., small in amount per unit length.)
The two coaxial heat-tubes of FIG. 5 use the principle of those of FIG. 3. The inner one, 3, carries the A.C. from leftto right, here also fitted with extended surface fins, 30, in this case extending inwardly toward the axis. The skin effect of the inner tube is on its outer surface, close to the flow of A.C. in the outer tube. There could be an insulation surrounding 3 to protect it from shorting with the outer tube. Instead, this may be merely an air space through which additional air passes to be heated. Ceramic or other insulating spacers, such as the solid beads, 7, keep the two heat-tubes apart. The A.C. reaching the heat-tube, as usual, by connections shown here as .the short conductors, the open beads, 6, which may also act as spacers atthis far end.
In FIG. 5, the fluid being heated, e.g., air, moves along the axis of the heat-tube. The air supply indicated from the left inletnozzle, 8, may pass in contact with the outer fins, 20, of 2, with the inner fins, 30, of 3, and also with both cylindrical surfaces where conduction of A.C. gives'the respective skin effects, the inner surface of 2, and the outer surface of 3. A very compact utilization of surface is thus attained. By having separate inlets for the shell side and for the inner heat-tube, the space between the coaxial tubes does not needto be in contact with the flowing fluid (or two fluids may be used) if there is danger of shorting these two conductors of A.C.
While FIG. 5 shows a fluid heater with only a single pair of coaxial tubes, several or more such pairs may be incorporated as a bundle inside a shell. The outer fins of adjacent heat-tubes may come within the respective circles of the outer tips of the fins of other heat-tubes in order to give the maximum surface for dissipating the relatively large heat flux per unit length which is thus available.
In either case, the fluid would pass either left or right between the two flanged openings, 8 and 9. The arrows show a left-to-right movement of all gas.
The heaterof FIG. 5 has a resemblance to a standard heat exchanger except there is the same fluid flowing shell-side and tube-side, also in the space between the two heat-tubes. The electric current passing in the skins of the two tubes does supply heat uniformly throughout the length. The additional surfaces of the fins, 20 and 30, and their relative effectiveness in heat transfer may be calculated by the standard methods and equations, which have been derived for similar use wherein the heat is supplied by a hot fluid flowing inside the tube. Again, the square feet of effective extended surface per square foot of heated surface (i.e., the skin effect surface) may be higher than with steam heated tubes; for example, because of the greater heat flux which may safely be developed by theskin effect and the greater extended surface needed to dissipate it.
Heat exchangers using a heat-tube with an internal electric wire or with two coaxial heat-tubes, as in FIG. 5, may be used for heating gases or liquids; but, as always, in design of heat exchangers, the amount of extended surface should be increased for use with a gas as air because of the low volumetric heat capacity and low heat transfer coefficients of gases.
FOr either liquids or gases, the standard design practices may be used, within these-modifications.
The connection by the electric wire, 3, through the special insulation, 4, carries the A.C. as in the usual case via the internal conductor, 3, but on its outer surface; thence through the inner surface of the heat-tube, 2, and back out through the wire, 13, with special insulation, 14, to the other terminal of the A.C.
INCREASING SKIN EFFECT RESISTANCE Theproximity effect in determining the pentration of the magnetic field is important in those cases where the heat-tube may have a special configuration, i.e., internal fins in a tube, so that only the edges of some fins or teeth may come near the conductor itself. In this case, the calculation of the field penetration, the proximity effects, and hence the resistance, becomes difficult; and the resistance may most easily be actually determined experimentally for a given combination of heattube and conductor.
In the converse case, the internal conductor wire may actually be a steel axial conductor with an expanded or irregular surface. A skin effect on the conductor itself, will be experienced, as well as in the inner surface of the steel tube which is assumed to be close thereto.
The design of the geometry and arrangement of the two components of the heat-tube and the internal conductor, if of steel, may thus be made to take advantage of the skin effect and of the uneven pentration of the magnetic field due to the proximity effect. The eccentricity factor noted above is one means; and considerable change of the effective resistance of either the heat-tube or of the electrical conductor, or of both, and thus of power input to the combined system, may be made.
Thus FIGS. 6 and 7 illustrate a somehwat converse relation of the use of skin effect in heater design to the use of extended surfaces in FIGS. 4 and 5, although advantage may also be taken simultaneously of those extended surfaces for heat transfer to fluids contacting them and in motion.
FIG. 6 represents a design of a central conductor, 3, which might take the place of the one, 3, in FIG. 5; but
which has a very much higher electrical resistance because the effective cross-section of the skin effect has been greatly reduced. The skin effect of the electrical conductor, 3, is due to a proximity effect of the surrounding tube, not shown in FIG. 6, but as 2 in FIG. 5A. In FIG. 6, the dashed outside circle, 2, represents the inside surface of such a heat-tube. If the width of the outer fins, 30, of 3 are greater than about twice the depth of penetration of the skin effect, and if the depth of the teeth is also greater than about this much, the magnetic field cannot penetrate more deeply; therefore, substantially the only electrical conduction can be along the outer edges of the fins, 30. (For many steels the depth of penetration has been found to be about 0.04 inch.) This outer surface of the teeth alone may be only a small part of the total cross-section of the central conductor, and the resulting resistance is increased accordingly.
The same effect may also be obtained in reverse. The heat-tube may be assumed to have the cross-section of a shape something like that of 3, the internal conductor of FIG. 5A with the many fins, 30, projecting inwardly from a tube wall. At their center is an insulated electrical wire, 33, shown here as a dotted circle, because it does not refer to the other design of FIG. 5. What now becomes the heat-tube, 3, of FIG. SA has its effective perimeter of its inner surface reduced to the depth of penetration of the magnetic and induction effect back from the inner edges, thence into the teeth. The balance of the cross-section of the extended surface and of the tube itself is merely to conduct away and dissipate the heat flux to the fluid stream moving in the confines of what is now both the heat-tube and transportpipe.
Similarly, FIG. 7 diagrams another internal conductor, this time with an expanded outer surface in a form like that of a screw thread of a bolt. Here again the heat-tube outside of the conductor is not shown, but is indicated by the dashed lines, 2. r
If the width of the above groove does not exceed its depth, which is in turn not less than twice the depth of penetration, the only effective conductor is the periphery of the threads. The A.C. must then go around the outside of the threads and'follow a path very much longer and very much higher resistance than that conventionally, i.e., the axial length of the screw. This is a practical and very inexact experimental solution, but almost correct, of what is a rather complicated theoretical analysis.
While other types of screw threads may be used instead of the square ones of FIG. 7, e.g., those shaped as the fins of the heat-tube of FIG. 4, the estimation in advance of the proximity effect and field penetration and inter-shielding factors is much more difficult in evaluation of the effective penetration depth for A.C. in the skin. Other geometric modifications to take advantage of minimizing the effective conductor crosssection of either the heat-tube or of the internal conductor, may be made to advantage in the various embodiments of this invention.
The proximity effect on a conductor in a nonuniform field may change very considerably the extent of penetration of the magnetic forces, hence of the resistance of the heat-tube and/or of the conductor when one or both are of steel. As noted above, the system may be designed to best advantage to use this effect. Also, in those cases where practical, the proximity of the conductor to the heat-tube may be varied during service.
Thus, FIG. 8 diagrams the steel heat-tube, 1, which is a transport pipe. The conductor, 3, may or may not be insulated depending on the electrical conductivity of the fluid and it may or may not be of steel, depening on whether a skin effect in it is to be utilized, particularly in regard to whether a change in its resistance is desired due to the proximity effect.
The conductor, 3, is supported by one or more pivot arms, 32, made of an insulating material, which are in turn supported by a rotatable shaft mechanism, 31. This mechanism, 31, may be attached to the inner wall of the pipe; and it functions by allowing the rotation of 32, and hence of 3, so that 3 may occupy any position on the arc, 31, as shown by the several possible positions of the conductor varying from axial through a general position shown dotted as 33; and if insulated, the conductor, 3, may be rotated until its insulation touches the inner wall of 2.
As noted above, the increase of the resistance is limited by the closeness with which the conductor can approach the wall to increase this eccentricity. The thickness of the insulation and the radius of the conductor control this. To minimize the effective radius (i.e., the distance from the center to the wall side of the conductor) a strip of metal, 33, may be used, the thickness of which is less than twice the radius of an equivalent conductor;,thus, its effective distance, with the same insulation, will be less than that of a round conductor. This may also be rotated so 33 falls on the axis. If the conductor, 3 or 33, is of copper, there will be no measurable change in its own resistance in moving it from the axis to a position near the pipe wall. If 33 is of steel, and particularly if its effective diameter (or thickness) is greater than twice the depth of penetration or skin", its resistance will also increase markedly as the eccentricity increases. Other shapes and sizes of 3 or 33 may be used to change the effective resistance, as has been already described.
This device with a movable conductor allows the wide variation of its effective resistance and the amount of heat developed, as well as of the overall line loss.
HEAT-TUBES As TRANSMISSION LINES FOR A.C.
While the heat-tube as heretofore described has been primarily concerned with the heating of a steel surface by the skin effect, it is also essentially a Z-conductor circuit for A.C.; and in some cases it may be designed so that its production of heat is not much different than that of a standard 2-wire conductor.
Thus, in FIGS. 1, 2, or 3 as described above, the internal conductor, 3, usually insulated, is surrounded by a heat-tube, 2. In a pipeline of some length, it may be desirable to utilize electric current from this circuit by tapping off from 3 and from the heat-tube, 2, connector wires to supply A.C. power for some other purpose. A common use is the withdrawal of current for a branch connection of oil from the transport line; and a new heat-tube circuit would be set up using the taps or connections from 3 and 2 to connect to the corresponding conductor and heat-tube of the transport pipeline circuit. This connection could be in either series, or in parallel relation.
Such circuits including a resistor as an electrical load are diagrammed in FIG. 2A. A parallel connection between the skin of the heat tube and the conductor wire is diagrammed in (a) by taps to corresponding points. Diagram (b) also shows a parallel connection near the end; but if the conductor wire is connected to this auxiliary circuit at the end rather than to the inside of the heat-tube, circuit (b) is in series with the basic circuit of the heat-tube at its far extremity. Similarly, if the conductor wire is broken at any point along its length, and circuit (c) is connected to the two ends so formed, an auxiliary circuit in series is established.
FIG. 2C shows an added electrical resistance or load in series with the basic heat tube electrical circuit, and inserted between the extremity of the condcutor 3 and its connection with the far end of the transport pipe 2. Here the other side of the new resistance load in series is connected back to the transport pipe 2, preferably at an inside point on the wall as shown, and hence electrically insulated from the wall in piercing it. The connection to the wall of the pipe (indicating now the length of the heated part of the transport pipe) may actually be made to the outside of the pipe, the current flows through the pipe wall at this point, then through the skin to the other end as previously described. There is no noticeable electrical disturbance at the connection on the outside for more than a millimeter or two distance.
Many other reasons for utilizing the current so available have been found, particularly if there is no other source of electric current throughout the route of the pipeline. The tapping off of A.C. is usually in parallel. Contrary to a normal A.C. transmission line, in which it is attempted to maintain as nearly constant a voltage between the two conductors as is possible throughout their length, the available voltage of the A.C. between wire and heat-tube varies from a high point at the one end of the heat-tube circuit, to zero at the far end. There is a substantial voltage difference between 3 and 2 at the inlet of the heat-tube, wherein the corresponding connections are made to the alternator or to the transformer; while at the far end of the conductor and the heat-tube, the voltage drop has been reduced to zero. However, any new resistance or load inserted at the far end as in FIG. 2Cas an electrical load in series is supplied with A.C. through the entire length heattube plus conductor.
Thus, it is always necessary to determine-the voltage available on a point of tapping off A.C., depending on the voltage drop up to that particular point in the heattube circuit.
Nevertheless, in some cases it has also been found that the heat-tube-may thus be used along the length of the pipeline to provide an A.C. source, when proper attention is paid to the voltage available.
1. A heat generating system comprising:
a. at least one elongated hollow tubular shape made of a metal having magnetic properties and electrical conductivity;
b. a source of AC having a first terminal and a second terminal;
c. an electrical connection between said first AC terminal and one end of said tubular shape;
d. an electrical conductor means extending through the inside length of said hollow tubular shape and insulated electrically therefrom;
e. an electrical connection between said second AC terminal and the end of said electrical conductor means which is near the electrical connection of said tubular shape and said first AC terminal;
f. an electrical connection between said tubular shape and said electrical conductor means remote from its electrical connection with said second AC terminal;
g. an AC circuit established:
i. from said second terminal of said AC source through the substantial length of said electrical conductor means inside the said tubular shape, thus producing heat in said electrical conductor means;
ii. then back through the said tubular shape so as to produce a skin effect current concentrated in the inner skin of said tubular shape, said tubular shape having at least twice the thickness of said skin, said skin effect current thus producing heat in said tubular shape; and
iii. finally back to said first AC terminal, to complete said AC circuit;
h. means whereby a utilitarian fluid iscaused to flow at a substantial velocity in direct contact with a surface of said tubular shape and in direct contact with said electrical conductor means; whereby i. substantially all of said heat produced in said electrical conductor means is transferred to said fluid; and
j. at least some part of said heat produced in said tubular shape is transferred directly to said fluid from said surface of said tubular shape against which said fluid is directed.
2. In the system of claim 1, wherein said fluid is electrically insulative and said electrical conductor comprises a bare metallic member.
3. In the system of claim 1, wherein said inner surface of said tubular shape is expanded, whereby the rate of said heat transfer to said utilitarian fluid is increased.
4. In the system of claim 1, wherein the effective surface of said tubular shape for skin effect conduction of AC lies on a surface of an inscribing cylinder, but is less than the total area of said inscribing cylinder.
5. In the system ofclaim 1, wherein the effective surfaceof the'metal of said electrical conductor means for skin effect conduction of AC lies on a surface of a curcumscribing cylinder but is less than the total area of said 'circumscribing cylinder.
6. In the system of claim 1, wherein said electrical conductor means lies on the bottom of said tubular shape.
7. In the system of claim 1, wherein the metal of said electrical conductor means has magnetic properties whereby a substantial skin effect current is produced, flowing in the outer skin of the metal of said electrical conductor means.
8. In the system of claim 1, wherein at least some part of said elongated tubular shape is formed of an elongated strip wound as a helical coil with space between each turn of said coil, whereby said utilitarian fluid may be passed there through so as to directly contact said electrical conductor.
9. In the system of claim 1, wherein an external electrical circuit having a resistance representing an electrical load is connected in parallel from a point of said tubular shape and a nearby point of said electrical conductor means.
v10. In the system of claim 1, wherein an external electrical circuit with a resistance representing an electrical load is connected in series with said electrical conductor means by a pair of conductors penetrating said tubular shape and electrically insulated therefrom.
11. In the system of claim 1, wherein an external electrical circuit with a resistance representing an electrical load is connected in series as a part of said AC circuit and forms the said electrical connection between said tubular shape and said electrical conductor means, remote from its electrical connection with said source of the AC.
12. In the system of claim 1, wherein said electrical conductor means comprises a metal tube.
13. In the system of claim 1, wherein said electrical conductor means comprises a metal strip of rectangular cross section.
14. In the system of claim 1, wherein said AC has a frequency of from 10 to 1000 cycles per second.
15. In the system of claim 1, wherein said electrical conductor means is on the longitudinal axis of said tubular shape.
16. In the system of claim 1, wherein the length of at least one of: said tubular shape and said conductor, is expanded whereby the effective length of said AC circuit established is substantially greater than twice the axial length of said elongated hollow tubular shape.
17. In the system of claim 1, wherein said tubular shape is unenclosed.
18. In the system of claim 17, wherein the volume of said fluid is very large compared to the volume of said tubular shape.
19. In the system of claim 17, wherein the fluid flow is due, at least in part, to convection.
20. In the system of claim 1, wherein said fluid flow is substantially in one direction.
21. In the system of claim 20, wherein said fluid flow is substantially at right angles to the longitudinal axis of said tubular shape.
22. In the system of claim 1, wherein said surface of said tubular shape against which said utilitarian fluid is caused to flow is at least in part an external surface of said tubular shape.
23. In the system of claim 22, wherein the said utilitarian fluid is electrically non-conductive and is in direct contact with at least some part of the inner surface of said tubular shape and at least some part of the surface of the metal of said electrical conductor means.
24. In the system of claim 23, wherein at least some part of the surface of the metal of said electrical conductor means contacted by said moving fluid is expanded, whereby the rate of said heat transfer directly to said fluid is increased.
25. In the system of claim 22, wherein said outer surface of said tubular shape is expanded, whereby the rate of said heat transfer to said utilitarian fluid is increased.
26. In the system of claim 22, wherein the efiective surface of the metal of said electrical conductor means for skin effect conduction of AC lies on a surface of a circumscribing cylinder, but is less than the total area of said circumscribing cylinder.
27. In the system of claim 22, wherein said tubular shape has at least some part of its wall removed and open, whereby said utilitarian fluid passes through said wall at an angle of from to 90 to the longitudinal axis of said tubular shape and flows at substantially the same angle across the surface of said electrical conductor means extending inside said tubular shape.
28. In the system of claim 27, wherein said open part of said wall of said tubular shape has the shape of a helix continuing some'distance along and around said tubular shape.
29. In the system of claim 27, wherein said open part of said wall of said tubular shape is formed by a multiplicity of holes of a diameter less than the internal radius of said tubular shape.
30. In the system of claim 1, wherein said tubular shape has at least some part of its wall removed and open, whereby said utilitarian fluid passes through said wall at an angle of from 0 to 90 tothe longitudinal axis of said tubular shape and flows at substmally the same angle across the surface of said electrical conductor means extending inside said tubular shape.
31. In the'system of claim 30, wherein said open part of said wall of said tubular shape has the shape of a helix continuing some distance along and around said tubular shape.
32. In the system of claim 30, wherein said open part of said wall of said tubular shape is formed by a multiplicity of holes, each of a diameter less than the internal radius of said tubular shape.
33. In the system of claim 1, wherein said electrical conductor means is displaced at some distance from the axis of said tubular shape.
34. In the system of claim 33, wherein means is provided for varying the amount of said displacement of said electrical conductor means from the axis of said tubular shape during flow of said AC in said circuit.
35. In the system of claim 1, wherein said electrical conductor means is offset from the longitudinal axis of said tubular shape.
36. In the system of claim 35, wherein said electrical conductor means is closely adjacent to the inner periphery of said tubular shape for substantially its entire length.
37. The system for heating -a transport pipe carrying a fluid to be transported in forced flow, comprising:
a. at least one elongated transport pipe made of a metal'which has magnetic properties and conducts electricity;
b. a source of AC having a first terminal and a second terminal;
0. an electrical connection between the first AC terminal and one end of said transport'pipe;
d. an electrical conductor means extending through the inside length of said transport pipe and insulated electrically therefrom;
e. an electrical connection between said second AC terminal and the end of said electrical conductor means which is near the electrical connection of said transport pipe and said first AC terminal;
f. an electrical connection between said transport pipe and said electrical conductor means remote from its electrical connection with said second AC terminal;
g. an AC circuit established:
i. from said second terminal of said AC source through the substantial length of said electrical conductor means inside the transport pipe, thus,
skin efiect current thus producing heat in said transport pipe; and
iii. finally back to said first AC terminal to complete said AC circuit.
h. means whereby the transported fluid is forced through said transport pipe at a substantial velocity in contact with the inner surface thereof, and also in contact with said electrical conductor means; whereby substantially all of said heat produced in said electrical conductor means is transferred to said fluid being transported in forced flow through said transport pipe; and I j. at least some part of said heat produced in said wall of said transport pipe is transferred directly to said transported fluid from said inner surface of said transport pipe.
38. In the system of claim 37, wherein said fluid is a gas.
39. In the system of claim 37, wherein said fluid is a liquid.
40. In the system of claim 37, wherein said electrical conductor means lies in contact with the bottom of said transport pipe.
41. In the system of claim 37, wherein the metal of said electrical conductor means has magnetic properties, whereby a substantial skin effect current is produced, flowing in the outer skin of the metal of said electrical conductor means.
42. In the system of claim 37, wherein said inner surface of said transport pipe is expanded so as to increase the rate of said heat transfer to said fluid being transported.
43. In the system of claim 37, wherein an external electrical circuit having a resistance representing an electrical load is connected in parallel from a point of said transport pipe and an adjacent point of said electrical conductor means, whereby AC flows in said external electrical circuit.
44. In the system of claim 37, wherein an external electrical circuit having a resistance representing an electrical load is connected in series with said electrical conductor means by a pair of conductors penetrating said transport pipe and insulated therefrom.
45. In the system of claim 37, wherein an external electrical circuit having a resistance representing an electrical load is connected in series as a part of said AC circuit and forms the said electrical connection between said transport pipe and said electrical conductor means, remote from its electrical connection with said source of the AC.
46. In the system of claim 37, wherein said transported fluid forced through said transport pipe is substantially electrically non-conductive, whereby it serves to additionally insulate electrically said transport pipe from said electrical conductor means.
47. In the system of claim 46, wherein the surface of the metal of said electrical conductor means is in direct contact with said transported fluid, whereby heat produced in said electrical conductor means passes directly to said transported fluid without transfer through any other material.
48. In the system of claim 46, wherein at least some part of the surface of the metal of said electrical conductor means contacted by said transported fluid is expanded whereby the rate of said heat transfer directly to said fluid is increased.
49. In the system of claim 37, wherein said means for forcing said transported fluid at a substantial velocity produces a pressure at the inlet of said fluid, thereby forcing it from one end to the other of said transport pipe.
50. In the system of claim 49, wherein the total of said heat produced in said electrical conductor means and transferred to said fluid, and said heat produced in said transport pipe, is substantially equivalent to that lost from the surface of said transport pipe to its surroundings.
51. In the system of claim 37, wherein said electrical conductor means is displaced at some distance from the axis of said transport pipe.
52. In the system of claim 51 wherein means is provided for varying the amount of said displacement of said electrical conductor means from the axis of said transport pipe during flow of said AC in said circuit.