GB2203319A - Thermoinductive heater - Google Patents

Description

1 k 1 1 THERMOINDUCTIVE HEATER U cj 22n15319 The present invention

concerns a thermoinductive heater in which electrical energy is consumed to heat a heat-transfer fluid, such as water or air, for example.

The generators considered here are of the "electrical transformer" type, comprising a primary winding supplied with electrical current by the distibution network and coupled by a magnetic circuit with a secondary winding which is both tubular and short-circuited The secondary winding has a heat-transfer liquid running through it, which is heated on contact with the wall of the tube, itself being maintained at a high temperature by the induced currents which are produced by the variable magnetic flux generated in the magnetic circuit by the electrical current from the primary winding.

Known inductive heaters of this type are described for example in documents FR-B-527697 or EP-A-0193843.

In comparison with the other known electrical heating apparatus, these thermoi.nductive heaters have the advantage, particularly from the user security aspect, of totally dissociating the electrical circuit properly called (the primary winding) from the heating circuit represented by the secondary winding.

Elsewhere, the transformation relationship, peculiar to electrical transformers, can be put to good use in order to leave the secondary winding at low voltage while completely assuring a high power transmission by the primary, if one so wishes.

An essential aspect, not yet perfectly resolved it seems, remains however the control, which must permit the regulation of the heating of the fluid as necessary.

The first document cited above (FR-A-527697) suggests, to this end, the use of a rheostat placed in series with the secondary winding.

The second document mentioned (EP-A-0193843) proposes a solution more concerned with the energy efficiency of the apparatus consisting in providing the secondary winding with thyristors in cascade allowing the number of short-circuited turns to be varied. One thyristor per turn acts as an "all or nothing" gate with regard to that 2 turn. This relatively complex assembly is similar then to a multitiude of electronically driven switches, which provides for only a discreet regulation (non-continuous) of the heating by elementary leaps each ccrrresponding to the heating power of one turn of the secondary winding.

In this document, reference is equally made to an analogous apparatus, disclosed in published UK application number 2,105,159, in which control is effected by means of thyristors placed in the primary. Such an arrangement means that the voltage in the primary must be low enough to be compatible with the charge acceptable to the thyristors, which limits the power of these apparatus.

The aim of the present invention is to propose a simple and reliable solution for a continuous control of the heating, without presenting the inconveniences of the known solutions cited above.

The invention thus has as its objective the provision of a thermoinductive heater of the "electrical transformer" type comprising a primary winding destined to be connected to the distribution network and coupled, by a magnetic circuit, to a secondary winding made up of a short- circuited tubular serpentine through which a heat-transfer liquid to be heated flows, the heater being characterised in that means of continuously regulating the heating power are foreseen in the primary circuit, which means comprise a saturable inductance mounted in series with the primary winding and a generator of direct- or rectifiedcurrent driving said inductance in such a way as to alter the state of its magnetic circuit.

Preferably, the saturable magnetic circuit is a closed circuit. More preferably, the saturable inductance is comprised of a magnetic circuit comprising three legs, the outer legs each being equipped with a power winding, these two power windings being connected in parallel in the primary circuit, and the central leg being equipped with a control winding connected to the direct current generator. The power windings are wound in opposite directions so as to create in the central leg symmetrical magnetomotive forces which are opposed at all times.

4 0 A 3 As one will have gathered, the control according to the invention is based on the following principles: for a given voltage at the terminals of the heating serpentine, the current of the secondary depends on the voltage of the short-circuit of the apparatus. Now, the instantaneous heating power is directly related to the intensity of the current in the secondary. If the voltage at the terminals of the primary winding varies, the current in the secondary varies in proportion and, in consequence, the heating power also varies.

To achieve this an electromotive force is induced in the primary circuit by that of -the supply source itself, which is opposed to the latter and which is variable in such a way that, for a constant supply voltage, the voltage at the terminals of the primary winding varies proportionately.

This variable electromotive counter-force is produced by a saturable inductance, of which the magnetic permeability of the "unloaded" core (that is to say in the absence of current to the primary) depends on an applied magnetic field created to this end by a direct- or rectifiedcurrent whose intensity may be controlled.

Thus, the regulating of the heating power operates without any piece of the apparatus being set in motion. Moreover. the intensity of the direct (or rectified) current destined to drive the saturable induction can to advantage be controlled by a parameter related to the heating effect, for example the temperature of the fluid after heating, or the temperature of the place to be heated, by means of a controller which senses the parameter and compares the value sensed to the required value.

An embodiment of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a heater according to the invention.

Figure 2 is an enlarged view showing in detail the part of Figure 1 representing the means of regulating the heat.

Figure 3 is a s.cheme illustrating the grouping of three heaters according to Figure 1 in one single apparatus supplied by three phase current.

k 4 Figure 1 shows a conventional structure of an electrical transformer comprising a primary winding 1 coupled to a secondary winding 2 by a magnetic circuit 3 on which they are both wound.

The primary winding 1, preferably of copper or aluminium, is embedded in epoxy resin 4, according to an arrangement not obligatory here, but typical in transformers, called Mry", in which the means of cooling by the circulation of air have not been shown so as not to unnecessarily overcrowd the drawing.

The secondary winding 2 is formed by a tube., preferably metallic, to provide the requisite thermal and electrical conductivities. This tube 2 is connected at its ends into a heating circuit (or more usually to a user circuit) through which flows a heat transfer fluid, which may for example be water.

The tube 2 is short-circuited by the electrical connection 5 which joins its two ends. An earth 6 in the secondary circuit on the side of shortcircuit 5 remote from the secondary winding is provided for reasons of security.

The primary winding 1 is itself connected to the terminals under voltage from an alternating current supply 7. The A.C. supply is advantageously that of the mains network.

A saturable inductance 8, driven by a generator 9 of direct current Ic, is mounted in series with the winding 1 between the points A and B of the primary circuit thus realised. A capacitor 19 has been placed in parallel with the inductance 8 and the primary winding 1 to improve the cos. $ of the installation. A switch 10 is also provided to permit the rapid opening of this circuit in case of necessity.

The saturable inductance 8 is shown in detail in Figure 2, and comprises a closed magnetic circuit 11, formed by three parallel legs 12a, 12b, 12c, joined together at their ends by two linear joints, respectively the upper 13a and lower 13b.

The outer legs 12a and 12b serve to support the electical power windings 14a and 14b forming two parallel arms placed between points A and B in the primary circuit. These windings have equal numbers of turns and are wound in opposite directions in such a way that the magnetomotive forces which they each create in their 4 respective legs are, at any moment, of equal intensity but oriented in opposite directions so that their sum should be nil in the central leg for reasons of symmetry.

The central leg 12c comprises itself a control winding 15 connected to the generator 9 of direct current Ic. It is further equipped with a number of turns in short-circuit 16. These turns, by an effect of pure self-inductance, prevent the return to the DC generator 9 of the residual alternating magnetic flux resulting from the asymmetry of alternating magnetomotive forces opposed in the central leg when Ic is not nil.

The direct current winding 15 on the central leg allows the creation of a stationary magnetic field which changes the magnetic state of the outer legs 12a and 12b and, consequently, makes the power current more quickly saturating in one of the legs in a first halfcycle and more quickly saturating, in a symmetrical fashion, in the other leg at the following half-cycle. So as not to prejudice this greater rapidity in attaining the threshold of saturation of the magnetic circuit 11, the magnetic circuit is of a "closed" type structure.

In that way, when no direct current is circulating (Ic = 0) the effect of self induction in the inductance 8 is maximum ("self induction in iron") and then, the effective voltage at the terminals of the primary winding 1 is minimal. By contrast, when the intensity of the direct current Ic is raised sufficiently to saturate the magnetic circuit by itself, the effect of self induction becomes minimum ("self induction in the air"), and the effective voltage at the terminals of the primary winding 1 is the maximum. Between these two limits of functioning, the choice of intensity of the direct current Ic permits fine adjustment of the intensity of the current in the primary circuiti and thus of the amplitude of the alternating voltage at the terminals of the primary winding 1.

Returning now to Figure 1, to complete the description of the functioning principle of the heater, it is clear that due to the fact that the serpentine 2 of the secondary circuit is short-circuited an alternating electrical current circulates there and brings about a

6 heating of the tube through the Joule effect. This heat is transferred to the heat transfer fluid, at the time of its passage in contact with the internal wall of the serpentine. The amount of heat generated depends directly on the effective intensity of the current in the secondary (on the square of this intensity to be exact).

The current intensity is determined by the voltage induced at the terminals of the serpentine 2, which depends on the voltage maintained at the terminals of the primary winding 1. Clearly, since the primary winding voltage can be regulated through the direct current lc, then control of the heating power of the apparatus is achieved by the modification of the voltage at the terminals of the primary winding 1 by adjusting the current Ic to the saturable inductance 8, which current is controlled by the direct current generator 9.

The control may easily be automated if required, for example with the aid of a controller 18 driving the generator 9 in such a way as to maintain the difference in temperature with a certain desired range between a required value Vc and that which it recieves from a sensor 17 detecting the temperature of the water at the exit from the secondary winding.

Of course, the voltage induced at the terminals of the secondary winding 2 depends on the transformation relationship, on knowing the relationship between the number of turns making up the heating serpentine 2 and the inductive winding 1 respectively. It will then be advantageous, in order to obtain elevated heating powers, to make the apparatus work at low voltage by providing a larger number of turns on the primary 1 and connecting this to a supply source 7 at high or medium voltage.

Heaters with a very large range of powers may be constructed, from 100 Kw to about 10 Mw from a three phase supply of the medium voltage network, wherein each phase supplies a heating unit such as the apparatus shown in Figure 1. - With the arrangement according to the invention, control is possible between 100% of the nominal power of the apparatus and about 10 % of this power. Depending on the control setting, cos. $ varies between 0.85 "before" and 0.85 "after" thanks to the presence of the capacitors 19.

7 The metal from which the secondary winding 2 is formed can advantageously be of stainless steel, or another metal having a high resistivity, which can work with a low current density (in the order of 6 A/mm2). Moreover, in the case of stainless steel, control against heat corrosion is completely satisfactory. The usual arrangements equally advantageously are made to improve the heating output, such as thermal insulation of the serpentine 2 in particular.

As to the saturable inductance 8, several variations may be used to achieve this, other than that described with reference to Figure 2. However, whatever may be the structure employed for the saturable magnetic circuit 11, it is important that it can be saturated by the static field created by the direct current Ic, when this is adjusted to its maximum value. To this end, one will make provision for a sufficient number of turns for the winding 15, so as not to have to use high current values for lc, for example higher than 10 A.

Likewise, since the direct current of the regulator Ic is intended to modify the vertical position of the saturation on the curve of magnetisation giving the magnetic induction as a function of the field, as well as the location of the beginning of this plateau on the abscissa axis, the ideal dimensions of the magnetic circuit 11 are advantageously arranged in such a way that, for a given nominal power available to the primary, the field/induction curve is already in the neighbourhood of the beginning of the plateau when Ic = 0.

Beyond that. the circuit would be under-sized, because a fraction of the alternating magnetic field created by the windings 14a and 14b would spread in the air and one would then lose width in the range of regulation of the power transmitted to the secondary, a range which can go (as already indicated) from 10% to 100% of the nominal power of the apparatus. Below that poing the circuit would be over sized, which, in itself, would not only carry cost penalties but would also,moreover, restrict the range of secondary power regulation to narrower than 10-100%, and make a precise control of the power within that range more difficult.

8 One could, of course, arrive at the same result by adapting to a circuit 11 of a given size the number of ampere turns of the winding or windings 12a, 12b.

The invention is entirely compatible with a standard three phase supply from the medium or high voltage network of an electrical distribution system. In this case, the apparatus shown in Figure 1 becomes one unit in a more complex assembly which contains three of them.

Such an assembly is shown in an electrical schematic diagram in Figure 3. Each phase, marked U, V, W of the medium voltage network at 20.000 volts between phases, feeds, via a saturable inductance 8, 811 P1 a primary winding 1, 11, 10.

These three windings are mounted here in a star connection and each of them induces, by way of a magnetic circuit not shown, a voltage in a secondary winding in short-circuit formed by serpentine 2, 21, 211. Each serpentine is mounted on a branch of a hydraulic heating circuit which contains three of them in parallel. More generally, this type of apparatus can be multi-phased and consist of' as a result, a number of units corresponding to the number of phases of the supply.

It is obvious that the invention is not limited to the examples of reduction to practice which have just been described, but extends to multiple variations or equivalents, falling within the scope of the characteristics set out in the claims hereafter.

One will note that the field of application of the invention encompasses the production of hot water for the heating of buildings, or for inclusion in industrial processes. Likewise, the invention is applicable to the heating of heat transfer fluids other than water, example oil or even liquid sulphur or sodium, destined to be used as such, or to generate high temperature steam in the exchangers.

4 k V 9

Claims (9)

1. A thermoinductive heater of the "electrical transformer" type, comprising a primary winding (1) intended to be connected to the electricity supply network (7) and being coupled, by a magnetic circuit (3), to a secondary winding (2)_made up of a short-circuited tubular serpentine (5) in which circulates a heat transfer fluid to be heated, wherein means of regulating the heating power of the heater are provided in the primary, which means comprise a saturable inductance (8, 11) mounted in series with the primary winding and with a generator (9) or direct- or rectified-current driving said inductance in such a way as-to alter the magnetic state of its saturable magnetic circuit (11).

2. A thermoinductive heater according to Claim 1, wherein the saturable magnetic circuit (11) is a closed circuit.

3. A thermoinductive heater according to Claim 1 or Claim 2, wherein the saturable magnetic circuit (11) comprises three legs (12a, 12b, 12c), in that the outer legs (12a, 12b) each include a power winding (14a, 14b), these windings being connected in parallel in the electrical circuit joining the primary winding (1) to the supply (7), and wound in opposite directions in such a way that the alternating magnetic fluxes which they create in the saturable magnetic circuit (11) are opposed at any moment in the central leg (12c) and wherein the central leg (12c) includes a control winding (15) joined to the generator of direct- or rectified- current (9).

4. A thermoinductive heater according to Claim 1, further including means (17, 18) of automatically regulating the heating power.

5. A thermoinductive heater according to Claim 1, wherein means (17, 18) are provided for automatically controlling the heating power.

6. A thermoinductive heater according to Claim 1, wherein the tubular serpentine (2) is of stainless steel.

7. A thermoinductive heater according to Claim 1, wherein a capacitor (19) is mounted in parallel with the primary circuit.

8. A multi-phase electrical heating apparatus made up of as many elementary heating units as there are phases of the electrical supply to which it is intended to be connected, each elementary heating unit being a thermoinductive heater according to Claim 1.

9. A thermoinductive heater substantially as herein described with reference to the accompanying drawings.

J. U.

described herein A multi phase electrical heating apparatus substantially as Published 1988 at The Patent Office, State House, 6W 1 High Holborn, London WO 1R 4TP. Further copies maybe obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3BD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1187.

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