REFRIGERATORS

Field of the invention

The present invention relates to an electric heating cable for the defrosting of domestic refrigerators.

State of the art

A refrigerator comprises a main housing that is carefully insulated from the external environment, which can have one or more areas at different temperatures.

The refrigeration apparatus generally comprises a compressor, which compresses a refrigerant gas bringing it to high pressure and high temperature; a condenser, which condenses the gas previously treated by the compressor and transforms it into a liquid phase; a laminating element, that transforms the refrigerant liquid treated by the condenser into a mixed liquid/gaseous phase characterised by a very low temperature and low pressure; an evaporator, which transforms the refrigerant treated by the laminating member into a phase that is again gaseous, absorbing heat from the refrigeration zones.

The surfaces of the evaporator operate at temperatures that are lower than the temperatures desired in the areas of refrigeration. In "no-frost" refrigerators, air is used as a means of transferring heat, skimming, by means of a fan, the exchange surfaces of the evaporator and absorbing the heat in the storage environment/s. The air contains a certain amount of moisture, that is, water in the form of particles; this fact, means that contact with very cold surfaces, i.e. the exchange surfaces of the evaporator, causes the change of state of the water that passes from the the liquid state to the solid state, frost formation, with the formation of ice on the heat exchange surfaces of the evaporator. Cyclically it becomes necessary to remove the layer of ice, layer, which, if not removed, causes a marked degradation of the thermal performance as well as possible damage. The removal of is implemented, after preventive shutdown of the aforementioned refrigeration cycle, through the use of a heat source that, commonly, consists of the operation of an electric heater based on the Joule effect, which exercises the heat exchange by means of a mechanism that is primarily of the conductive, secondarily radiant, and lastly of the convective type.

The electric heater commonly employed in this context has a typically tubular shape, is generally shaped with a "coil" design, so as to maximize intimate contact with the exchange surfaces of the evaporator; in order to achieve this, it is preferable to have as small as possible cross-dimensions of the heating element to allow as narrow as possible a bend radius. The heater commonly, but not exclusively, consists of an electric heating cable protected by a shell that is typically, but not exclusively, made of metal, which exercises a mechanical protection and maintenance function of the desired geometric shape.

In order to reduce the cross-dimensions of the electric heater so as to consequently be able to Increase the bend radius thereof, the prior art has taught us to arrange a central core made of insulating electrical material (typically fibre glass, rayon, polyester, etc.), on which there is coil-wound a resistive metal alloy wire, which when wound on the central core has a radial section with respect to the lower heater with respect to the respective orthogonal section.

It is indeed known that by reducing the cross-section of the cable, it is possible to reduce the bend radius of the heating cable.

The central core with the resistive wire wound thereon, is covered by at least one electric insulator layer.

Examples of such heaters are given in US 2846560 and US 5081341.

The heating cable obtained is therefore sufficiently flexible to be able to be used in various applications, among which primarily for the heating and defrosting of refrigeration equipment, installations and industrial apparatus.

Depending on the more or less heavy uses there can be provided more than one electric insulating layer. On increasing the cross-section of the heating cable, the flexibility of the cable is reduced when using the same materials. Consequence thereof is a limitation of use of the heating cable.

It was noted that with the passage of time, the heating cable, albeit there is present a good deformability, tends to deteriorate its characteristics in proximity of the bends; this is because a resistive wire having a circular section, with a small section, that is pre-established on the basis of the required powers, behaves like a pointed object on the insulator, also in that it mainly concentrates the heat in a very small zone. Consequently, the electrical insulation tends to yield.

Summary of the invention

Object of the present invention is to provide an electric heating cable adapted to resolve the above-mentioned problem, i.e. adapted to ensure an appropriate electrical insulation and at the same time a good unalterability thereof.

In order to fully understand the phenomenon, heating and cooling cycles were carried out at temperatures and with powers that were extreme with respect to those required in the "normal" use of the heater, for different embodiments of the heating cable, in particular with different sections of the central core, of the resistive wire and of the insulating sheath.

It was obtained that the insulating sheath tends to be cut from resistive wire at the bends.

An obvious attempt to solve the problem has led to increasing the thickness and/or the mechanical strength of the insulating sheath. This determined an overall rigidification of the heater, indeed at the same time worsening the behaviour thereof. On the other hand, it is not possible to reduce the thickness of the sheath by virtue of technical regulations that impose a certain degree of electrical insulation that is function of the thickness of the sheath.

Another obvious attempt led to producing a ribbon-like resistive wire, that is, having a much larger section than the other, that is, with a ratio greater than 30, like the one shown in the above-mentioned documents of the prior art. This so as to reduce the radial rigidity of the wire and therefore the damage to the sheath. It was not possible to utilise a ribbon-like wire of the same metal alloy as this would have determined a substantial reduction of the heating power per unit length of the device, due to a significant increase in the resistance of the wire itself.

The object of the present invention was achieved by preparing a resistive wire having a substantially flattened section with rounded edges 20, i.e. with the side intended to come into contact with the sheath having rounded edges.

More specifically, the expression "substantially flattened shape" means that there is a ratio of the perpendicular dimensions of the section of the resistive wire that is greater than two. More precisely it is preferred for this dimensional ratio to be of between 7 and 14. In addition, this dimensional ratio is particularly functional of the metal alloy that the resistive wire is made of, which is preferably FeCrAI. The substantially flattened shape is to be understood as "squat shape" and not ribbon- like.

Advantageously, this dimensional ratio together with the use of said metal alloy is an optimal combination for average powers in the order of 30W/m which finds particular application in the defrosting of domestic refrigerators.

According to one optimal variant of the invention, a thickness associated to a section of the resistive wire is less than 0.1 mm and/or a corresponding width of the resistive wire is of between 0.5 mm and 1 .1 mm.

According to a further, more preferred variant, the resistive wire consisting of a FeCrAI alloy with Cr 12+1 5%, Fe (balanced) Al 4÷6% having a resistivity of 125 μΩ-cm and wherein a specific power of the heating cable is of about 30W/m, with, preferably, a winding pitch of said resistive wire of between 0.5 mm and 2 mm. Indeed, before establishing said combination, tens of costly laboratory tests were carried out. These tests also entailed long heating cycles with controlled overpowers in order to estimate the behaviour of the heating cable over time.

A form of section that is particularly suitable for achieving the object of the present invention is rectangular with rounded edges.

This section also allows, with respect to the circular section of the other known resistive wires, an increase of the contact surface available to the mechanical crimpers, for the electricity supply. A greater contact surface determines a greater passage section of the electric current and consequently a lower heat level and a lesser voltage drop.

This and further objects are achieved by the present invention, described in detail by means of the claims which are an integral part of the present description.

Brief description of the drawings

Further characteristics and advantages of the invention will become more evident in the light of the detailed description of preferred, but non-exclusive, embodiments of an electric heating cable, illustrated by way of a non-limiting example, with the assistance of the accompanying drawings, in which; - Fig. 1 represents a cutaway, longitudinal section of a heating cable compliant with the present invention and a respective cross-section,

- Fig. 2 represents a cross-section of a component of the cable of figure 1 ;

- Fig. 3 represents an axonometrlc view of the heating cable of figure 1 ,

- Fig. 4 represents a perspective view of one variant of the cable of figure 1 ,

- Fig. 5a represents a section of a part of the device of the preceding figures;

- Fig. 5b represents a possible variant of the section of Figure 5a;

- Fig. 6 shows a heating cable according to the present invention, in respect of which a 90° bend is set;

- Fig. 7 represents an enlarged view of the detail VII of Fig. 6;

- Fig. 8 represents a further enlarged view of the particular VIII of Fig. 7;

- Fig. 9 represents a view corresponding to that of Fig. 8, of a detail of a known heating cable.

The same reference numbers and letters in the drawings identify the same elements or components.

Detailed description of a preferred embodiment of the invention

With reference to the drawings, a heating cable compliant with the present invention comprises a central core 1 typically having a circular section, but not exclusively made of fibre glass, that is electrically insulating and that defines a common axis of longitudinal development Y common to the heating cable. An electrical resistive wire 2 having a squat section with rounded edges 20 is wound on the central core 1 , to form a coil along the Y-axis. At least one insulating sheath 3, typically, but not exclusively, made of PVC or silicone or fluorine carbon resin, covers the central core 1 with the relative electrical resistive wire 2.

The present invention envisages that the section of the electrical resistive wire 2 has a flattened, preferably rectangular shape with rounded edges. It is provided that one long side of the section is in contact with the central core 1 , i.e. tangent to the circumference defined by the section of the central core 1.

This implies that the short side of the section of the electrical resistive wire 2 is radially aligned with respect to the section of the central core 1 , thus limiting the overall section of the central core 1 - electrical resistive wire 2. So that the electrical properties of the electrical resistive wire remain unchanged it can envisaged that the area of its section remains unchanged with respect to the use of a resistive wire with circular section. As will become clearer below, even with the same section surface, the thermal efficiency of the present invention is improved.

This implies that with the same electrical characteristics and size of the section of the heating cable, there is the possibility of predisposing an insulating sheath 3 of greater thickness or two sheaths, resulting in a greater degree of insulation and electrical safety of the obtained cable and a satisfactory flexibility thereof.

According to another variant of the present invention, a heating cable, with particular reference to figure 4, comprises a central core 1 , on which is coil-wound a first electrical resistive wire 2 with flattened section, i.e. approximately rectangular, on which there is arranged a first insulating sheath 3 on which there is arranged a second wire 4 that is coil-wound or longitudinally extended with or without resistive characteristics. To insulate the heating cable obtained there is arranged a second insulating sheath 5, which is wound around the cable thus ensuring a pre-defined degree of insulation.

Advantageously, in the construction of Figure 4, the two coils of resistive wire 2 and 4 have discordant winding directions so as to eliminate the torsion effects generated on the heating cable by the manufacturing process, thus making it possible to have heating cable that is as straight as possible and free of residual stresses. In addition, in the case of resistive wires that are simultaneously powered, the opposing winding directions of the two coils of resistive wire 2, 4 involves an abatement of the magnetic fields generated by the passage of the electric current.

Advantageously, the fact of having reduced the radial thickness occupied by the electrical resistive wire 2, allows

- the end cross-section of the cable thus constructed to be decreased so as to allow an increased possibility of decreasing the bend radius of the heating cable,

- the thickness of the insulating sheath 3, or 3 and 5, to be increased while the section of the cable remains unchanged with respect to a known cable, - the insulating cable of a further sheath to be covered while the overall section of the heating cable remains unchanged with respect to a heating known cable,

- the insulating sheath 3 to be covered with a further electrical resistive wire 2 and with a further insulating sheath 5 while the section of the cable with respect to a known cable having single electrical resistive wire can remain unchanged, with certain advantages in terms of section/power ratio,

- a second electrical conductor wire to be arranged superposed to the insulating sheath 3 and with an outer insulating sheath 5 that is superposed to the electrical conductor wire,

- the flexibility of the heating cable to be increased by means of a reduced overall section, while the thickness of the insulating parts, i.e. the central core 1 and insulating sheath 3, remain unchanged,

- the flexibility of the heating cable to be increased by means of an increased distance between the coils of resistive wire.

On the other hand, advantageously, the fact of having eliminated sharp edges guarantees a greater duration of the insulating sheath.

In addition, advantageously, the fact of indicating a ratio of the orthogonal dimensions of the section of the resistive wire of between 7 and 14 allows the specific power for the defrosting of domestic refrigerators to be optimised.

Therefore, a good technical result is achieved if the section of the electrical resistive wire assumes a generally flattened rather than rectangular shape with rounded edges, at least those facing the insulating sheath 3.

With reference to figures 5a, 5b, a preferred section of resistive wire has a rectangular shape with a greater side 2a that is intended to come into contact with the insulating sheath 3, having rounded edges 20. Therefore, the section of the electrical resistive wire has a larger dimension D1 with respect to a respective coordinated direction and a lesser dimension S1 , perpendicular to D1. The greater dimension D1 coincides, in the sectional view of figures 5a, 5b, with the extension of the greater sides 2a, 2b. The lesser dimension S1 is radially arranged with respect to the section of the heating cable and it results that the contact surface with the sheath 3 is free of sharp edges. The values of S1 are preferably less than 0.1 mm and better if equal to 0.08 mm; D1 can vary range from 0.5 mm up to 1.1 mm. The choice of dimensions and of the ratio of the dimensions of the sections are determined by the need to guarantee a predetermined thickness of insulator without changing the overall outer diameter of the cable and a desired flexibility of the cable without having variations of the electric power values.

Preferably the resistive wire is a FeCrAI alloy with a hardness of 200-260 HB. Unlike the common DSD alloys, the FeCrAI alloys used in the following invention has an optimal chemical composition, facilitating tight winding around an insulating core 1 , consisting of a greater content of Fe and a lesser content of Cr.

Preferably the resistive cable is made of a FeCrAI alloy with 12÷15% Cr, Fe (balanced), 4÷6% Al having resistivity of 125 μΩ/cm and wherein a specific power of the heating cable is of about 30 W/m. This FeCrAI alloy has, with respect to other known alloys, for example, with respect to the CuNi alloys commonly used, a greater mechanical strength, a lower linear thermal expansion coefficient, a lower thermal conductivity. The. latter translates into a greater thermal inertia, i.e. into reduced thermal stresses that determine a greater reliability of the heating cable. The tensioning, in the production phase for coil-winding the above-mentioned resistive wire having a flattened section around the central core, must be set to a higher value than the value utilised with the use of resistive wires having a circular section and at an equal ohmic value, in order to guarantee a perfect adhesion between resistive wire and central core and prevent the slipping of the coils. In compliance with the present invention, the winding pitch varies in a range of values of between 0.5 and 2 mm with a ratio of the coordinated dimensions of the flattened section of between 7 and 14. In addition to the tensioning, the speed at which the resistive wire having a flat or similar section is wound is also set to a different value from the case of wire having a circular section.

The electrical insulating layer (typically, but not exclusively, a plastic PVC or silicone or polyolefin resin or fluorine carbon resin or other resin with high electrical insulation power) placed to coat the above-mentioned cable consisting of central electrical insulating core central electrical Insulating and the resistive wire made of metal alloy has a minimum thickness of 0.7 mm. The foliowing table records the characteristic heats of a heater compliant with the present invention.

One variant of the invention, envisages a further outer electrical insulation, i.e. a "double insulation", achieved with a further insulating layer that is superposed to the primary electrical insulation.

Another variant of the invention, envisages a further winding achieved with a resistive wire that is superposed to the primary electrical insulation. The second winding of resistive wire, having a flat or similar section, can have an ohmic value that is identical to the first winding or an ohmic value that is different from the first. By choosing to supply, by means of an external switching system, only the first winding, only the second winding or both the windings connected in series or parallel, there can be obtained various modes of operation of the cable in relation to the power supplied.

A further embodiment of the invention envisages, superposed to the electrical insulation of the first winding of resistive wire, the presence of an electrical conductor with a function other than heating (e.g. sensor cable), which can be positioned by coil winding or simply extended in longitudinal direction; superposed thereto there is the presence of an electric insulator typically made of plastic resin with function of final outer insulation.

Figure 6 shows a bend induced in a heating cable according to the present invention. We realise, with the magnification of a portion of the cable at the curve (Figures 7 and 8), that in this outer section of the curve, the resistive wire tends to wedge against he insulating sheath. With respect to a traditional cable with rectangular section and sharp edges (Figure 9), with the same normal force F1 at the section of the resistive wire, the resultant normal Fy at the surface of the resistive wire at the edge 20 is less than the resulting Fz on the sharp edge of the section of Figure 9. The force Fy is less than Fz since it is distributed over a larger rounded surface (Figure 8) with respect to the sharp edge of Figure 9. Due to the principle of action and reaction, this translates into a less strain on the insulating sheaths 3, 5 that are respectively in contact with the coil of resistive wire 2, 4. The insulating sheath 3, 5 is consequently less easily subject to rupture, to the advantage of the duration and overall reliability of the heating cable.