EP0155572A2 - Device to prevent the formation of dew and frost on road signs - Google Patents

Abstract

The invention relates to a device for preventing the formation of dew and frost on road sign carriers (road sign boards), which are equipped for retroreflection, in particular to which retroreflecting foils are affixed. It consists in that there is arranged above and in front of the road sign carrier (road sign board) a natural thermal radiator or thermal reflector which operates without a supply of external energy and which for the midpoint of the board covers a solid angle of at least 0.8 sr (steradian), preferably a solid angle of 1.6 sr to 2.2 sr. The cooling, caused by radiation losses, of the surface of a road sign below the dew point or down to the temperature of the ambient air at which frost forms is thereby prevented. The thermal radiator (thermal reflector) (1) produced from coated metal or plastic is preferably of rectangular construction and can be joined to the road sign board (3) by bars (5) or by being mounted directly on the upper rim - in particular while leaving free an open area which enables the surface to be cleaned naturally by the action of rain. In this arrangement, additional thermal radiators (thermal reflectors) can be arranged on both sides of the road sign board (3). (2) denotes the retroreflecting road sign foil. <IMAGE>

Description

The invention relates to a device for preventing the formation of dew and frost on traffic sign carriers (traffic signs) which are equipped with retroreflective elements, in particular with retroreflective foils.

Retroreflective traffic signs have become increasingly important in view of the ever increasing traffic density in recent years. Compared to conventional traffic signs, they are characterized by their exceptional luminosity and clear recognition even over long distances, thereby contributing significantly to traffic safety.

The basic characteristic of a retroreflective material is its ability to return a luminous cone of light back to the source of an angularly arriving light beam, that is to say that light from a headlight striking such a material is reflected back to the driver's eyes and gives the impression of this as if the reflective material itself was illuminated.

- To be able to fulfill this task, it is necessary that the surface of such retroreflective traffic signs, which may consist in particular of a glued-on retroreflective film, is smooth and optically clear.

The drop-shaped, optically scattering dew and frost formation in the form of particles of 0.1 mm in size on these foils, which results on up to 250 days a year in the night hours by cooling the foil surface below the dew point temperature of the surrounding atmosphere, if due to effective radiation, the energy balance of the film surface is negative, has a similar effect to that of a diffuser attached in the beam path described above, as can be seen in Figure 1 for a retroreflective adhesive tape. The light reflected back to the vehicle is significantly weakened and the optical contrast between different colored areas is reduced. The result is that the information content of such a built-up or frosted traffic sign is recognized much more poorly by the vehicle driver.

It is known to prevent droplet-like condensation on optical surfaces, e.g. Apply hydrophilic coatings to the lens surfaces of glasses or the lens of a motor vehicle, as well as to prevent fogging of reflective surfaces, which are hardened to a water-insoluble, firmly adhering state (see e.g. DE-PS 1 928 409).

Such a hydrophilic coating, which should increase the free energy of the surface and make it easily wettable with water - in the patent literature also known as anti-fogging - or anti-foggant - causes a complete wetting of these surfaces. No optically scattering droplets form during the condensation, but a thin, optically clear water film on these coatings.

However, such hydrophilic coatings are relatively poorly resistant to mechanical and weather influences and often do not have the required light fastness. They are therefore easily removed when the surfaces are mechanically cleaned. In addition, they are unable to prevent the likewise light-scattering ice and frost formation.

To prevent fogging of retroreflective traffic signs, it has also been proposed to cover the back of the sign with a heat-insulating covering made of foam, e.g. Polyurethane foam to be covered and firmly connected to this covering (AT-PS 357 069). Practical tests have shown, however, that this measure cannot in all cases lead to a satisfactory result.

Another known solution to prevent the thawing of surfaces of objects is to attach an artificial, mostly electrically powered heater to them, which ensures that the temperature of the surfaces does not drop below that of the surroundings. This solution is used for example with meteorological rain detectors. It has the disadvantage that it can only be realized in the vicinity of electrical supply lines and causes high costs for larger surfaces due to the energy requirement.

The invention has for its object to overcome the disadvantages of the previous methods and to prevent both the thawing and the frost formation of retroreflective traffic signs by a permanent constructive measure without artificial energy supply.

This object is achieved in that the cooling of the surface of a traffic sign below the dew point temperature of the surrounding air caused by radiation losses in the sky is excluded by natural heat radiators which are placed above and in front of the traffic sign.

The invention thus relates to a device for preventing the formation of dew and frost on retroreflective traffic sign carriers, which is characterized in that a natural heat radiator or heat reflector which functions for and without the supply of external energy is arranged above and in front of the traffic sign carrier (traffic sign) Center of the shield covers a solid angle of at least 0.8 sr (steradian), preferably a solid angle of 1.6 sr to 2.2 sr.

(The solid angle unit 1 steradian (sr) is defined as the spatial angle that cuts out the area A = 1 m 2 from a sphere with a radius r = 1 m drawn around its apex).

According to the invention, the retroreflective material can consist of different material, in particular a retroreflective film, which is caused by the glass ball and mirror coating existing reflective layer has excellent reflectivity. In addition, the plastic layer (topcoat layer or topcoat layers) of such foils ensures a good bond between the foils and protects them adequately against wear, whereas the adhesive layer (self-adhesive layer) ensures long-lasting adhesion and firm attachment to the traffic sign carrier (traffic sign). In addition, the retroreflective material can consist of a material which is flat on its side facing the light to be reflected, but the other surfaces of which, according to a regular pattern, have evenly distributed and closely spaced triple mirrors.

The heat radiator (heat reflector), which is made in particular from anodized aluminum, is preferably rectangular and is advantageously attached directly to the upper edge of the traffic sign. Additional heat radiators (heat reflectors) can be arranged on both sides of the traffic sign. It is also advantageous to provide an open area (opening) between the location of the heat radiator (heat reflector) on the traffic sign and the heat radiator (heat reflector) itself, which enables the natural cleaning of the retroreflective film surface in the rain.

According to a further embodiment according to the invention, the heat radiator (heat reflector) can also consist of a plurality of inclined, lamellar heat radiators arranged parallel and at a distance from one another. This embodiment allows wind and snow loads on the traffic sign as well as the material consumption in the manufacture of the heat radiator be significantly reduced.

ergeben,

• mit _MT: Energieabgabe der Oberfläche durch die infrarote Ausstrahlung
• E : Energiezufuhr durch die Gegenstrahlung aus A dem Himmelshalbraum als Folge des Wasserdampf-und Kohlendioxidgehalts der Atmosphäre
• E_E: Energiezufuhr durch die Gegenstrahlung des Erdbodens, von Pflanzen und Gebäuden,

mit Hilfe von natürlichen Strahlungsquellen verhindert.The advantages achieved with the device according to the invention consist in particular in that neither a generally poorly resistant hydrophilic coating on the surface or a heat-insulating coating on the back, nor a cost-intensive artificial energy supply is required to prevent the light-scattering thawing and frosting. The formation of dew and frost on the surface of a traffic sign is reduced by reducing radiation losses due to the negative radiation balance

surrender

• with _MT : energy emission from the surface due to the infrared radiation
• E: Energy supplied by the counter-radiation from A in the sky as a result of the water vapor and carbon dioxide content of the atmosphere
• E _E : supply of energy by counter-radiation from the ground, plants and buildings,

prevented with the help of natural radiation sources.

mit L: Energiezufuhr durch Wärmeleitung aus dem Träger der FolieThe following energy balance can be drawn up for the film surface of a traffic sign, which cools down almost daily in the after hours due to radiation losses below the temperature t _{L of} the surrounding air:

with L: supply of energy by heat conduction from the carrier of the film

E: Energy supply through heat conduction from the surrounding air.

mit K: Energiezufuhr durch die Kondensationsenergie bei der Tau- und Reifbildung auf der Oberfläche.The radiation losses are compensated by the sizes L and E. Above all on nights with a clear sky (up to 250 nights a year), the above-mentioned radiation losses are so high that even the dew point temperature t _{d of} the surrounding air on the film surface is undershot, the energy balance of the surface then has the shape

with K: energy supply through the condensation energy during the formation of dew and frost on the surface.

As this energy balance shows, the thawing and frosting can be prevented if the energy supply is increased by the quantities E _A , E _E , L and E and the temperature t of the film surface is t _L ^{= tt} _d . The energy supply by E _E , L and E can only be increased in vertical traffic sign areas by measures that require an external energy supply, for example

• E by increasing the wind speed (heat transfer coefficient!) With the help of a fan
• L by an electrically powered heater for the character carrier
• E _E by electrically heated radiant heaters that are mounted on the ground.

Without an external energy supply, only the energy supply can be increased by the size E _A , in that, according to the invention, the counter-radiation of cold high layers of the atmosphere, which is weak at night, is increased by the radiation from natural sheet-like heat radiators of the temperature t _w t _L.

If the vertical arrangement of the traffic sign carrier surfaces is not absolutely necessary, then according to the invention the energy supply can also be increased by E _E by tilting the surface carrying the traffic sign forward by a small angle (up to approximately 20 °). Since with this measure the irradiance of E _E increases faster than E decreases, the sum of the energy supply E + E increases. L and E remain the same.

Flat heat radiators and inclined traffic sign surface can also be combined with each other, smaller heat radiator surfaces are then required. In view of this, it has proven to be advantageous if, in the case of a traffic sign inclined forward by 15 °, the heat radiator or heat reflector is arranged such that it covers a solid angle of 1.0 sr to 1.7 sr for the center of the sign.

Figure 2 shows the effective radiation E _z = M _{T, Z} - E _{A, Z} as a function of the zenith distance Z at two extreme water vapor pressures e _o on the ground (according to B. Lindenbein: "Meteorological treatises by the Institute for Meteorology and Geophysics of FU Berlin, Volume 72 (1967) Issue 3).

• In Fig. 1 ist ein durch Stege 5 gehaltener und am Verkehrszeichenträger 3 befestigter rechteckiger Wärmestrahler 1 dargestellt, der aus beschichtetem Metall, z.B. eloxiertem Aluminium, oder aus Kunststoff besteht. Mit 2 ist die retroreflektierende Verkehrszeichenfolie bezeichnet.
• Fig. 2 zeigt eine Ausführung, bei der ein rechteckiger Wärmestrahler 1 direkt an der Oberkante des Verkehrszeichenträgers 3 befestigt ist. Seitlich angebrachte Wärmestrahler 1a verhindern Strahlungsverluste an den Randzonen der Verkehrszeichenfolie 2.
• Fig. 3 stellt eine Ausführung dar, die der gemäß Figur 2 ähnelt, jedoch an der Oberfläche des Verkehrszeichens eine öffnung 4 besitzt, die die natürliche Reinigung der Folienoberfläche 2 bei Regeneinwirkung ermöglicht und aufgrund der geringen Strahlungsverluste in den zenitnahen Bereich des Himmelhalbraums eine mit dem in Fig. 2 gezeichneten Wärmestrahler 1 vergleichbare Wirkung erzielt.
• Fig. 4 zeigt eine Kombination aus mehreren, geneigten, parallel und im Abstand zueinander angeordneten lamellenförmigen Wärmestrahlern 1b, die als Einheit an der Oberkante des Verkehrszeichenträgers 3 befestigt sind. Diese Anordnung hat den besonderen Vorteil einer günstigen Auswirkung auf Wind- und Schneelast. Gleichzeitig kann bei der Herstellung eines solchen Wärmestrahlers der Materialverbrauch reduziert werden.

Figures 1 to 4 are perspective views of different embodiments of the device according to the invention for preventing thawing and frosting.

• 1 shows a rectangular heat radiator 1 which is held by webs 5 and fastened to the traffic sign carrier 3 and which consists of coated metal, for example anodized aluminum, or of plastic. The retroreflective traffic sign film is designated by 2.
• Fig. 2 shows an embodiment in which a rectangular heat radiator 1 is attached directly to the upper edge of the traffic sign carrier 3. Heat radiators 1a mounted on the side prevent radiation losses at the edge zones of the traffic sign film 2.
• Fig. 3 shows an embodiment that is similar to that of Figure 2, but has an opening 4 on the surface of the traffic sign, which enables the natural cleaning of the film surface 2 in the rain and due to the low radiation losses in the zenith area of the sky half with the in Fig. 2 drawn heat radiator 1 achieves a comparable effect.
• FIG. 4 shows a combination of a plurality of inclined, lamellar heat radiators 1b arranged parallel and at a distance from one another, which are attached as a unit to the upper edge of the traffic sign carrier 3. This arrangement has the particular advantage of a favorable effect on wind and snow loads. At the same time, the material consumption can be reduced in the manufacture of such a heat radiator.

In addition to the embodiments shown in FIGS. 1 to 4, the device according to the invention can be implemented by many other embodiments are without departing from the scope of the invention. However, when attaching the heat radiator to the traffic sign holder, it must always be ensured that the legibility of the sign must not be impaired in any way. Rather, the lettering must always remain fully visible.

Claims (11)

1. A device for preventing the formation of dew and frost on retroreflective traffic sign carriers, characterized in that a natural heat radiator or heat reflector, functioning without supply of external energy, is arranged above and in front of the traffic sign carrier, which has a solid angle for the center of the sign covers at least 0.8 sr (steradian). 2. Device according to claim 1, characterized in that the heat radiator (heat reflector) is arranged so that it covers a solid angle of 1.5 sr to 2.2 sr for the center of the shield. 3. Device according to claims 1 or 2, characterized in that the heat radiator (heat reflector) is rectangular. 4. Device according to claims 1 to 3, characterized in that the heat radiator (heat reflector) is attached directly to the upper edge of the traffic sign. 5. Device according to claims ₁ to 4, characterized in that an opening is provided between the attachment point of the heat radiator (heat reflector) on the traffic sign and the heat radiator (heat reflector). 6. Device according to claims 1 to 5, characterized in that in addition to the heat radiator (heat reflector) attached above and in front of the traffic sign, additional heat radiators (heat reflectors) are arranged at both points of the traffic sign. '7. Device according to claims 1 to 5, characterized in that the heat radiator (heat reflector) consists of several inclined, parallel and spaced apart lamellar heat radiators. 8. Device according to claims 1 to 7, characterized in that the retroreflective traffic sign is inclined forward by a small angle. 9. The device according to claim 8, characterized in that at a 15 ° inclined traffic sign the heat radiator (heat reflector) is arranged so that it covers a solid angle of 1.0 sr to 1.7 sr for the center of the sign. 10. Device according to claims 1 to 9, characterized in that the heat radiators (heat reflectors) consist of coated anodized aluminum. 11. The device according to claims 1 to 10, characterized in that the back of the traffic sign is additionally provided with convection slats.

Images