The present invention relates to a system, its use and to an electric heating lattice. In particular such an electric heating lattice, comprises first and second mutually spaced wires which are arranged mainly transverse/cross to each other.

Such a system and heating lattice as known usually comprises additional construction means for installing and holding the lattice in particular its crossing isolated electric wires in a wanted mutually spaced relation relative to each another. Electric currents running through the wires generate heat to be supplied to the environment wherein the lattice is installed. The system serves to heat the surrounding material, such as an underground in order to prevent in particular its upper surface from freezing or cooling too much. If applied in for example a runway, a floor or a sporting ground this ground has to be heated uniformly in order to prevent the local occurrence of frozen, snowy or icy spots. Also some spots may become overheated resulting in a bad growing of grass, or as the case may be plants, crops or the like growing on the upper surface .

It is an object of the present invention to provide a system and an electric heating lattice wherein the above problems are counteracted, which are light weighted, and nevertheless capable of uniformly and in a sufficient amount spreading heat over the electrically powered lattice in particular within the boundaries of officially prescribed maximum allowable power values.

Thereto the system according to the invention has the features of claim 1 comprising: a) an electric heating lattice comprising first mutually spaced wires having wire ends and second mutually spaced thermally conductive wires, said first and second wires which are both bare wires are arranged transversely relative to each other, while leaving between said first and second wires openings dimensioned to allow water and/or air to pass; whereby at least said first wires which are composed of an electric resistance alloy and are arranged side by side in a wire density of 2-4 wires/cm -preferably on average 3 wires/cm- make thermal contact with the second wires at nodes, such that heat to be developed in the first wires spreads out transversally via the nodes through the second wires, and b) an electric power supply having power distributors to be connected between the electric power supply and the wire ends of the first wires of the electric heating lattice .

It is an advantage of the system according to the present invention that its lattice comprises first and second bare wires which can easily be connected at the nodes for example in a continuous welding fabrication process. Not only do the first and second contacting wires mechanically stabilise the structure of the lattice for easy handling, but they also provide a web of contacting wires. Via the web heat generated in the first bare wires will spread out in all directions along the plane of the lattice web. In particular the making of at least thermal contact between first and second wires at the nodes results in that heat generated in said first wires spreads out via the nodes in a transverse plane also along the second wires which are not electrically powered. In addition a spreading of heat along the transverse web plane promotes a uniform distribution of heat from the directly electrically heated first bare wires and from the indirectly, through thermal conduction, heated second bare wires to the surrounding of the web. In principle this surrounding may be air or water to be heated which is flowing through the openings of some air or water treatment system. If the lattice is buried in an underground it will be the underground which is uniformly and homogenously heated. In general the occurrence of significant temperature gradients in the lattice is prevented in an easy and cost effective way, also because bare wires are applied.

At least said first bare wires which are composed of an electric resistance alloy are arranged side by side in an average wire density between 2-4 wires per centimetre. An average wire density which is less than 2 wires/cm on average results in currents through the first wires which are generally too high and consequently the temperature gradients are too high, thus negatively influencing the application area of the system because the growth of roots and/or plants and/or grasses in and on the surface as well as bottom life in the underground will be influenced negatively. More than an average of 4 wires per centimetre makes the lattice expensive, heavy to transport and not easy to position and handle in the field. An average first wire density of 3 wires/cm is often preferred in practise.

The choice of the average first wire density is also important in relation to the particular boundaries of officially prescribed maximum allowable power, voltages as well as currents and the way wherein these requirements can be met in optimum embodiments of the system and lattice concerned. Keeping these power parameters in the lattice within these prescribed boundary conditions and nevertheless generating enough but not too much uniform heat in said plane is proven a real challenge solved by the system and lattice according to the present invention.

Embodiments of the use of the system and its electric heating lattice according to the invention are outlined in the independent claims 11 and 14 respectively. Associated merits of the claimed inventions are outlined hereinafter.

At present the system, its use and lattice according to the invention will be elucidated further while reference is made to the appended drawings, wherein similar components are being referred to by means of the same reference numerals. In the drawing:

Fig. 1 schematically shows a system and lattice according to the invention -together with a detail of a lattice mesh- in a meandering underground configuration of lattice strips; and

Fig. 2 shows a system having an electric AC or DC power supply for control of the electric power supplied to the lattice of fig. 1.

Fig. 1 shows an electric heating lattice 1 which is embodied here as a web or weave comprising a first type of mutually spaced longitudinal wires 2 and a second type of transverse/cross wires 3, being arranged transverse to the first type of wires 2. Both types of wires 2, 3 are made of a generally metallic alloy, possibly the same alloy for reasons of cost-price or for easy of production. The first type of wires 2 are made of an electric resistance alloy chosen to have a certain specific resistance, and the second type of wires 3 at least thermally conductive. The first type of resistance alloy wires 2 comprises at least one and generally a composition out of the mainly metallic group containing: iron, steel, chrome, nickel, copper and carbon. The group constituents may be chosen to result for example in constantan or stainless steel which are among the many other alloys easy to manufacture and readily available. Stainless steel is preferred due to its durability and rust resistance. Stainless steel 316 has the appropriate specific resistance. It is preferred that the wires 2, 3 are made of an alloy which is easy to weave to form a lattice or web. And at least the first to be electrically powered wires 2 should have the specific resistance and diameter that a wanted sufficient amount of heat per unit area can be developed. In general the amount of heat generated in said first wires meant for heating 1 m ² ground area is between 50 and 150 Watt on average. In particular 100 Watt/m ² on average is a maximum for application of lattices in agricultural or sports grounds, which is due to the fact that more heat would be detrimental to underground microbiological life and insect life in the underground and the growth of plants, crops, grasses and the like on the surface of the underground. The lattice 1 can be buried in an underground 7 of for example a playing ground, an agricultural ground or a sporting ground such as a football field, or the lattice is provided in or under a road, pavement, floor, wall, ceiling, runway, airfield, approach, drive or ramp or the like. By burying the lattice 1 approximately 25-50 centimetre underground the surface of the underground is -depending of the specifically kind of application of the lattice- kept above wanted minimum temperatures, such as freezing temperatures.

Preferably the electric resistance alloy of at least the first type of wires 2 is a raw, that is not isolated alloy. In particular the outer surface of the raw alloy is bare or untreated that is unprocessed or uncoated which promotes heat delivery by the wires 2 to a surrounding with a high efficiency. If the lattice 1 is applied in an underground such as soil, openings 6 therein -to be elucidated hereinafter- serve to allow water and/or air to pass. If such an application of the lattice in soil is intended then it is preferred to use stainless steel 316L as alloy for the first and second wires 2, 3 as this alloy given its lower amount of carbon provides an even better resistance against rust and other mostly chemical substances present in soil. Apart from the aforementioned application the lattice 1 can be used to heat water or air passing through the openings 6 in the lattice such as in water and/or air systems for heating or treatment.

Given the length of a field or ground wherein the lattice 1 has to be provided, the manufacturability of the lattice web, and the required flexibility of the rolls of lattice here called strips 4, the diameter and total weight of the lattice 1 has to lie within manageable boundaries in order to position the strips easily and to develop a sufficient amount of heat in the electrically powered wires 2. In general several consecutive individual lattice strip configurations are necessary to cover a whole field or underground. For example for a European football field the use of some 55-65 lattice systems as shown in fig. 1 are required .

The wires 2, 3 elucidated in the lattice mesh detail of fig. 1 make at least thermal contact at nodes 5 which are made by generally welding the metallic alloys for example in a continuous electric welding process. Also spot welding is an alternative. The process allows the manufacturing of a normally regular web of longitudinal wires 2 and transverse wires 3 which are welded at the nodes 5. If the lattice web is regular four neighbouring spots on 2x2 neighbouring wires 2, 3 surround an opening 6 which is rectangular or preferably square in shape. If squared as shown in the detail of fig. 1 the lattice 1 has a fully regular pattern resulting in a very uniform heat distribution in both the lattice web plane and in height and thus also in ground, soil and underground material.

The first type of mutually spaced wires 2 and/or the second type of transverse wires 3 may simply cross each other at nodes 5 or they may alternatively cross over and under each other at consecutive and neighbouring nodes 5 as shown in the detail of fig. 1. In the latter case the mechanical stability of the woven lattice 1 improves at the cost of a slightly more complex manufacturing and handling thereof. Both types of wires 2, 3 may have equal cross sections, which in practice will be sufficient to allow heat to be spread out more evenly and homogeneously via the nodes 5 along the at least thermally conductive cross wires 3 too. E.g. such equal wire type cross sections doubles the total heat radiating outer surfaces of the wires 2, 3 while only the wires 2 have to be electrically powered.

The amount of heat developed in the first wires 2 i.e. also depends on the diameter of the first wires 2 which is chosen between 0.5 mm and 1 mm, in particular 0.6 mm and 0.8 mm. In a layout of a 3 wire/cm density the preferred diameter of at least the first wires 2 is 0.7 mm if stainless steel 316L is used for these wires 2 to provide the wanted generally controllable amount of electrical heat.

Given the amount of heat to be developed and the official respective requirements and boundaries concerning maximum allowable open AC or DC voltages it is necessary to fine tune in practical applications of the system and lattice the various above mentioned parameters. Examples of further such parameters are: specific resistance of the chosen alloy or alloy composition, first and second type wire diameters, number of side by side arranged first wires and second wires per cm. cf. wire density, the thermal properties of types of underground 7 concerned as well as the water content of the soil concerned.

The system as further shown in fig. 2 comprises an AC or DC electric power supply 8 which is connected to wire ends 2-1 and 2-2 of the longitudinal wires 2. Given the mentioned official constraints posed on in particular the maximum voltages and/or currents in an underground bare wired -open- electric system 1, 8 the power supply 8 will deliver at wish a controllable electric power. Thereto the power supply 8 has a power control input 8-1 for controlling the AC or DC power supplied to the respective wires 2 of the underground lattice 1. The amount of power per unit area generated in the lattice 1 can be controlled with the help of a properly programmed general microcontroller p included in the system. Automated control of the power if dependent on temperature will be achieved by means of measuring the open air temperature and the temperature at various places in the underground 7 by means of several temperature measuring means T provided in and/or above the soil 7, and/or close to the lattice 1. These temperature measuring means T are through the microcontroller p coupled to the control input 8-1 of a Norton current source for controlling the soil temperature based on measured temperature data, expected outside weather temperature and conditions and the wanted temperature of the underground 7 and its surface. The microcontroller is properly programmed to that effect.

A complete system 1, 8 for heating for example a football field may have several power supplies whose provided powers are each galvanically separated from earth. Stabile and accurate control of current to the lattice-strips can take place by measuring the total current delivered to power distributors 9 and by feeding back a total current related control signal via the microcontroller p to the associated control input 8-1 of the power supply 8 concerned.

The wire ends 2-1, 2-2 of the wires 2 are connected electrically to the power distributors 9. This may be achieved by means of arranging these distributors as parts which clamp the wire ends or as an alternative the power distributors may have longitudinal and/or transverse hollows 10 meant to clamp or cast therein the wire ends 2-1 and 2-2 respectively. This minimises contact resistances and heat generated locally therein, as substantial currents will flow through each lattice 1. This also results in precise lengths and resistance values along the lattice strips 4, which promotes an even and balanced heat generation over the length and width of the strips.

Figs. 1 and 2 also shows how the power distributors 9 are via electric terminals A and B connected to the power supply 8 of fig. 2. Fig. 1 also shows how lattice strips meander from a left distributor 9 at terminal A to the right to a prolonged distributor 9 and then back to a further left distributor 9 to terminal B in order to make contact at terminals A and B which are preferably powered on one side of the underground only. The distributors are outlined to conduct currents of hundreds of amperes.

In case the lattice system is DC powered the electric power supply 8 will have a rectifier circuit which may be arranged with microprocessor controlled semiconductors to control the DC current for the first type wires 2 in a way known per se. DC control is relatively simple in terms of required hardware if duty cycle and/or amplitude of the then block shaped current is controlled.

The power supply 8 may be connected to solar panels and/or electric wind mills for providing auxiliary electric power. Solar panels advantageously generate DC power in which case convertors become superfluous if connected to a

DC system 1.

As also shown in fig. 1 there may be some space 11 in the form of ground area not covered by said first wires 2 between meandering neighbouring strips 4. This saves lattice material and electric power on the assumption that if that space 11 is not too wide the heat developed in the lattice is spread out, such that seen on the outer surface of the underground 7 the heat will nevertheless be sufficiently uniformly distributed there over. In particular lattice widths and spacing between neighbouring strips of lattice may be chosen to save material costs, without jeopardising the requirements concerning the minimum and maximum wanted power per square metre.