Electrical resistance heating element for a heating device for heating a flowing gaseous medium

Abstract

An electrical resistance heating element for a heating device, with at least one flow canal through which the gaseous medium is able to flow from a canal inlet side to a canal outlet side of the resistance heating element, and with at least one heating resistor that extends essentially in the direction of the flow canal, with the medium flowing past the heating resistor and being heated in the process. The heating resistor is rod-shaped and produced from an electrically conductive ceramic material. The electrical resistance heating element is intended for installation in a heating tube into which air, for example, is blown at one end. The heated flow of air exits from the other end of the heating tube.

Description

CROSS-REFERENCE TO RELATED APPLICATION [0001]The present application claims priority under 35 USC §119 to European Patent Application No. 08 010 426.8, filed on Jun. 9, 2008, the entire disclosure of which is incorporated herein by reference.TECHNICAL FIELD OF THE INVENTION [0002]The invention concerns an electrical resistance heating element for a heating device for heating a gaseous medium, with at least one heating resistor that extends essentially in the longitudinal direction of the electrical resistance heating element, with the medium flowing past the heating resistor and being heated in the process. The resistance heating element has at least one flow canal that extends along the heating resistor and through which the gaseous medium is able to flow from a canal inlet side to a canal outlet side of the resistance heating element, with the heating resistor consisting of an electrically conductive ceramic material for conducting the current. The invention also relates to a heat...

AI Technical Summary

Benefits of technology

[0014]The heating resistor includes an electrically conductive ceramic material for conducting the current. The conductive ceramic material may itself serve as the ceramic heating resistor, or may be provided as the jacket of a ceramic rod consisting of electrically non-conductive ceramic material. Such a heating resistor has high mechanical as well as electrical wear and thermal strengths, thereby making a long service life possible. In addition, the ceramic materials used in this device also have excellent properties regarding thermal and electrical conductivity. The flow of current through the heating resistor can be influenced by the conductivity of the ceramic material as well as by the geometry of the ceramic heating rod. By changing the conductive and non-conductive portions of its components, the conductivity of the ceramic material can be varied over a wide range. An additional advantage is due to the fact that, compared with known heating elements, higher temperatures and faster temperature changes are possible.

[0017]Because of the achievable specific resistance values and the operating voltages, oblong heating elements must be used for the resistance heating element according to the invention. In order to avoid costly electrical connection technology at the side of the heating resistor that is hotter during operation, i.e. at the canal outlet side of the electrical resistance heating element, a U-shaped rod-shaped heating resistor offers advantages. This doubles the length of the ceramic heating rod and therefore its electrical resistance value. In order to further increase the electrical resistance of the resistance heating element and to enlarge the contact area with the flowing gaseous medium, several preferably U-shaped heating resistors may be provided. Depending on the values of the available voltage supply and the desired power, they may be connected parallel or in series.

[0023]The U-shaped ceramic heating resistor may have a flat or a structured surface. Preference is given to an embodiment of the invention where the heating resistor has indentations and / or raised areas in order to enlarge the surface area that is in contact with the gaseous medium, as compared with a flat surface. Besides providing as large a surface area as possible for the heat exchange with the flowing medium, this more complex geometry also has the effect of creating turbulence in the medium flowing past the heating resistor in the flow canal. This turbulence promotes the uniform and homogeneous heating of the entire flow of the medium.

[0025]The preferably positive thermal coefficient should not rise too steeply because this might lead to the formation of hot spots. Also, a steep flank of the specific resistance should not occur within the operating temperature up to approximately 1500° C., as is the case with some electrically conductive ceramic materials with positive thermal coefficients. The mixed ceramic material may also contain additional additives for improving the sintering characteristics or the stability. It is also possible to partially substitute other insulating ceramic materials such as oxide or nitride or silicate ceramic material for the aluminum oxide. The specific resistance of the material can be set within a certain range by the proportion of the conductive component. A proportion of 20 to 30% of molybdenum disilicide in the mixed ceramic material proved to be ideal. This makes it possible to achieve specific resistances between 0.01 and 1.0 Ω-cm at room temperature that increase by a factor of three or more up to 1000° C.

Problems solved by technology

If the resistance heating elements used there are operated without a sufficient flow of air for a certain amount of time, the heating wire will usually burn through, thereby rendering the heating element unusable.

An insufficient flow of air may be caused, for example, by a failure of the blower or a restriction of the air inlet or air outlet cross-section of the hot air device.

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