Patent Application
20120243979
The present invention relates to a fan comprising a housing and a fan wheel arranged on a motor shaft and being drivable by a motor. The fan according to the invention is also designed specifically as a mixing fan for transporting and mixing combustible media with air, the combustible or explosive mixture usually being transported to a burner with this fan.
The invention also relates to the use of a fan wheel designed in an insulating manner in a mixing fan for transporting and mixing a combustible medium with air, gas being preferably used as the combustible medium.
The fans according to the invention are preferably used in the field of heating technology. In this context, the average person skilled in the art is taught from the outset that due to electrostatic charges from the housing components, especially the housing and the fan wheel, during the use of such fans and the transportation of explosive gas-air mixtures, there is a risk of sparks forming and a potential risk of ignition of the mixture inside the fan, which must be avoided at all costs. However, the precautions proposed in the prior art are cost-intensive, complex in design and difficult to recycle in some cases.
By way of example, proposals in the prior art suggest that earthing should be provided via the motor shaft, the shaft bearing and the shaft seal down to the self-earthed housing to dissipate any charges from the fan wheel. Dissipation in this manner is disclosed in DE 102 04 037 A1 for example.
DE 102 004 045 251 A1 instructs persons skilled in the art to provide the fan housing or the fan wheel with an antistatic, static dissipative or conductive surface coating to guarantee that all conductive fan components are thus earthed. However, applying such a coating is an additional manufacturing step (e.g. by painting) and is thus cost-intensive and is not generally performed in practice.
The provision of so-called compound materials for the fan wheel or housing of mixing fans is also known in the art, i.e. plastics materials which have been compounded with conductive materials such as carbon black or carbon to ensure that the component is conductive. However, such compound materials are more expensive and more difficult to manufacture. They are also difficult to recycle as the added conductive materials can no longer be dismantled as pure grade materials. The fan wheels made from compound materials used in the prior art are static dissipative, whereas components with applied anti-static surface coatings are static dissipative or conductive.
The object of the present invention is to provide a more reasonably-priced fan which is easier to recycle.
The object according to the invention is achieved in that a fan designed as a mixing fan for transporting and mixing a combustible medium with air comprises a fan wheel which is designed in an insulating manner.
According to the German Professional Association of the Chemical Industry, substances or materials are said to be “insulating” if they are neither conductive nor static dissipative. Accordingly, a substance or material with a specific resistance of p≦104 ohm metres (Ωm) is said to be conductive. A substance or material is also said to be conductive if its surface resistivity Ro is <104 Ω. Substances or materials with a specific resistance in excess of 104 Ωm and less than 109 Ωm or an object or device with a surface resistivity between 104 Ω and 109 Ω measured at 23° C. and 50% relative humidity or with a surface resistivity of between 104 Ω and 1011 Ω measured at 23° C. and with 30% relative humidity are said to be “static dissipative”. “Insulating” as per claim 1 means that the fan wheel is made from insulating material.
It is beneficial that the fan wheel can be mounted without earthing and can also be operated without additional earthing being provided during operation. It is particularly advantageous if the fan wheel is arranged on the motor shaft without earthing. In a preferred embodiment the fan wheel is made from pure grade plastics material, preferably polyamide, “pure grade” being understood here to mean a plastics material produced without compounding. However, it is possible to colour the plastics material without going beyond the protective scope of the term “pure grade”.
In order to guarantee the insulating properties of the fan wheel, this preferably has a surface resistivity Ro of at least 109 Ω. Materials with a surface resistivity Ro of 1010, 1011, 1012 Ω and all intermediate values are also protected accordingly. Surface resistivity can be measured by means of an LEM Unilab 5 kV tera ohmmeter for example, but the test methods described in DIN IEC 6093 are also feasible. In one embodiment of the fan according to the invention, the fan wheel may have an overall surface area, formed from the upper surface of the fan wheel and the lower surface of the fan wheel, of between 25 cm2 und 500 cm2, preferably between 25 and 400 cm2, particularly preferably between 25 und 300 cm2, especially preferably between 25 und 200 cm2, and more especially preferably between 25 und 150 cm2, a charging reaching voltage values which might cause ignition inside the fan not being reached. The overall surface area of the housing may be in the range between 25 cm2 and 1000 cm2.
It is also advantageous that the at least one shaft bearing and/or the motor shaft and/or the shaft seal can also be designed in an insulating manner to minimise dissipation of charges via the shaft bearing. The use of an insulating shaft seal in particular makes it possible to use a simpler, pure grade seal.
In the fan according to the invention which is designed as a mixing fan, it is also advantageous that the gas is supplied in the suction area. By mixing the gas in the suction area it is possible to achieve higher heat outputs as more gas can be added to the mixture due to the negative pressure in the fan. It is also beneficial that mixing takes place by means of the impeller, thus making it possible to achieve a higher mixing ratio per se and thus a higher fan modulation ratio as a result.
The above advantageous features listed for the impeller also apply mutatis mutandis to the fan housing which can be designed in an insulating manner, able to be operated without earthing and made from pure grade plastics material.
Both the housing and the impeller can be manufactured using the injection moulding process. It is also possible to use a traditional metal housing made from diecast metal, e.g. aluminium, and to use a fan wheel according to the invention. For this reason, the invention also covers the use of an insulating fan wheel in any mixing fan.
In the case of mixing fans used in the prior art, conductive components were always used to rule out the appliance itself as an ignition source for the explosive fuel-air mixture. When using hydrogen as the fuel, an ignition voltage of 1000-2000 V is sufficient to achieve ignition with an optimised electrode and optimum ignition sparks. When using the gases which are usually used in Europe, however, an ignition voltage in excess of 3000 V must be present to enable ignition. The fan wheel and/or fan housing used according to the invention and designed in an insulating manner may admittedly also become charged due to the flow inside the fan and, where applicable, in the dust particles contained in the suction air (up to 800 μg/m3 in extreme cases) at a high speed of over 6,500 revs/min, but the levels reached are significantly lower than the critical ignition values, which means that earthing can be dispensed with and unwanted ignition due to static electricity inside the fan can still be ruled out. The minimum charges which arise in some cases discharge of their own accord within a very short time without further action. In order to measure the charges or determine the surface potential, a PFM-711A electrostatic field meter may be used in this case, for example.
In an advantageous embodiment of the invention, a burner is connected down-stream of the fan to act as a flame arrester. The explosive atmosphere inside the fan is less than 10 litres and more than 0.05 litres in volume.
By way of example, the figure below is a schematic view of the previous earthing state of the impeller and dissipation of the charge using a fan in cross-section. Here:
The fan according to the invention overcomes the disadvantages of the prior art and makes it possible to achieve a reasonably-priced solution to requirements which have always existed by the fact that the fan wheel, at least, is designed in an insulating manner.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 056 837.9 | Dec 2009 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/067794 | 11/19/2010 | WO | 00 | 6/7/2012 |
