Patent Application
20160338147
None
1. Field of the Invention
The present invention relates to an electrical resistance heater assembly for heating air. Such electrical resistance heater assemblies are used in clothes dryers.
2. Background of the Invention
An electrical resistance heater assembly disclosed in US 2012/0180334 A1 comprises a mica board supporting a first heater ribbon running in a looped configuration along rows that are substantially perpendicular to a longitudinal direction of the mica board, said rows being defined by series of mounting holes through which loops of the heater ribbon protrude. Adjacent rows are each connected by a single trapezoidal hole configured to turn the heater ribbon by 180°.
A problem of known electrical heater assemblies is that in use elevated temperatures can be reached causing mica boards to buckle. This problem can be avoided by using ceramic boards that can withstand higher temperatures. However, such ceramic boards are much more expensive.
An object of the present invention is to provide an electrical heater assembly comprising one or more mica boards that enables increased operating temperatures.
This object can be achieved by providing the mica board or boards with empty openings that are arranged between the rows of heater ribbon loops and are preferably larger than the mounting holes defining the rows. The mounting holes have opposing edges that are each touched by a section of the loop held in the respective mounting hole. Mica boards of prior art heater assemblies have numerous holes or openings that serve different purposes, e.g. rivet holes for rivets or mounting holes for holding heater ribbons. Such holes of prior art heater assemblies are not empty as they contain rivets, heater ribbons or other parts. The empty openings of the present invention serve to increase the elasticity of mica boards so that thermal expansions of the mica board are less likely to cause buckling. The empty openings thus increase the temperature tolerance of the heater assembly. In order to have a significant effect, the total area of all empty opening should be above 5% of the area of the mica board.
This object can also be achieved by connecting adjacent rows between terminal ends of the heater ribbon by arranging a plurality of substantially trapezoidal holes on a semicircle. Instead of turning the heater ribbon with a single trapezoidal hole between adjacent rows by 180°, this turning is achieved in a plurality of smaller steps. Thereby the mechanical load caused by bending and turning of the heater ribbon is reduced. Thus the risk of breaking is lower and heater ribbon of an increased width can be used. The larger the width of the heater ribbon the larger is the heating power that can be used. Thus larger operating temperatures can be achieved. The holes need not have a precisely trapezoidal shape because the bases of the trapezoid are not relevant, only the edges connecting the bases matter as they touch the heater ribbon. The bases of the trapezoid may therefore also be curved. Such a shape is considered to be substantially trapezoidal within the context of the application. The edges connecting the bases are arranged at an acute angle with respect to each other.
This object can also be achieved by providing the mica board at the terminal ends of the heater ribbon with an elongated hole which holds in addition to a loop of heater ribbon a section of heater ribbon that can be connected to a connecting part fixed to the first mica board. Thermal expansion of the heater ribbon causes severe stress, especially between the connecting part and the loop adjacent to it. An elongated hole fixes the loop adjacent to the connecting part only on the side facing away from the connecting part. Hence, the heater ribbon can move and thereby release mechanical stress much better. Thus the heater ribbon can stand higher temperatures and many heating-cooling cycles without breaking.
The above-mentioned aspects of the invention will become more apparent and will be better understood by reference to the following description of embodiments taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding or similar parts throughout all figures.
As can been seen, the heater ribbon 3a, 3b is bent into loops which form zigs and zags of a zigzag shape and protrude through mounting holes 4. The zigzag shape has lower apexes, which are connected to the respective mica board, and upper apexes, which are distanced from the respective mica board. By arranging the lower apexes at the mica boards, e.g. by clamping, sagging of the heater ribbon and, even in case of its breaking, shorts are prevented.
The mica boards 2a, 2b can be identical in shape or slightly different. An embodiment of the first mica board 2a is shown in
The rows of mounting holes 4 are connected by substantially symmetrical, trapezoidal holes 5 configured to each hold a loop of the heater ribbon 3a. These connecting holes 5 cause a U-turn between adjacent rows and are arranged between adjacent rows. The last hole of each row may have an asymmetrical shape, but is not counted as a connecting hole. Thus the heater ribbon 3a is turned by 180° for travel from one row to an adjacent row. Each connecting hole 5 causes a part of that U-turn. In the embodiment shown there are three connecting holes 5 between adjacent rows, although there may be more or less connecting holes 5. The lateral edges of the connecting holes 5 may, for example, enclose an angle of 45° or less. The first connecting hole has an edge that is parallel to the neighboring edge of the mounting hole 4 and an edge that is parallel to the neighboring edge of second connecting hole 5. Also each loop held in one of the substantially symmetrical connecting holes differs by an acute angle from the orientation of a loop held in an adjacent hole.
The mounting holes 4 have a length measured in a longitudinal direction of the rows and a width measured in a direction perpendicular to the longitudinal direction of the rows. The length of the mounting holes 4 is smaller than their width. The width of the heater ribbon 3a, 3b corresponds to the width of the mounting holes 4. Thus the heater ribbon 3a, 3b can have a significantly larger width than heater ribbon in conventional electrical resistance heater assemblies. The heater ribbon 3a, 3b used in the embodiment shown can have a width that is 30 times as large as its thickness. The width can be even larger relative to the thickness of the heater ribbon, e.g. 40 times as large as the thickness or more. A larger width of the heater ribbon 3a, 3b allows transferring a correspondingly larger amount of heat to an air flow. A large width of the heater ribbon 3a, 3b increases stiffness and makes turning the heater ribbon from one row of loops to the next row more problematic. This problem can be solved by increasing the number of trapezoidal holes 5 between adjacent rows.
As
One possibility illustrated in
Similar results can be achieved by electrically connecting the first heater ribbon 3a of the first mica board 2a and the second heater ribbon 3b of the second mica board 2b in series and to add another heater ribbon that likewise forms loops on the first mica board 2a and on the second mica board 2b as shown in the embodiments of
If there is no third mica board between the first mica board 2a and the second mica board 2b, as shown in the embodiments of
In the embodiments of
For a second power setting, a third series C of loops of heater ribbon that are supported by the first mica board 2a and a fourth series of loops D of heater ribbon that are supported by the second mica board 2b are electrically connected in series or in parallel. A second terminal 15 is provided for connecting the third and the fourth series of loops to a voltage source. If current flows through the third series C of loops of the first mica board 2a, it flows also through the fourth series D of loops of the second mica board 2b.
In order to increase the resistance of the mica boards 2a, 2b, 8 against buckling at elevated temperatures the mica boards 2a, 2b, 8 are provided with empty openings 9 that are larger than the mounting holes 4 and also larger than any rivet holes 7. The openings 9 are arranged between the rows of mounting holes 4 and can have a circular shape as shown in
As can be seen in
The mica board 2a shown in
The terminal section of a heater ribbon 3a, 3b is subjected to severe mechanical stress caused by repeated thermal expansion and contraction of the heater ribbon 3a, 3b. This mechanical stress can be reduced by arranging a terminal loop, i.e. a first or last loop of the heater ribbon 3a, 3b in an elongated terminal hole 10 such that the terminal loop touches only one edge of the terminal hole 10. The other side of the terminal loop extends into a substantially horizontal section of the heater ribbon 3a, 3b. This substantially horizontal section of the heater ribbon can be connected to a connecting part 11, e.g. by welding. In
The hook shaped section 13 need not be removed and may also be present in the finished heater. The hooked shaped section 13 is mechanically less strong than the heater ribbon. Thermal expansion of the heater ribbon causes the heater ribbon to exert a force on the hook shaped section. The hook shaped section 13 may then accommodate this force or, if the force is too high, break off. As the hook shaped section 13 is no longer needed in operation of the heater, breaking off of the hook shaped section is no problem.
