US 3904851 A
An electric heater for heating flowing air comprises a sheet support for a heater filament of non-circular cross-section and twisted about its longitudinal axis. The heater filament is wound about or attached in meander fashion to the support sheet in such a manner that successive segments of it are parallel and extend between opposed edges of the support sheet in contact with a face of the latter. An insulating cover layer disposed in contact with the heater filament segments defines together with these segments and the support sheet a multiplicity of relatively narrow, open-ended passages for the air to be heated. The support sheet may be flat and rigid or flexible and convoluted on itself together with the cover layer, or it may be tubular.
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[ Sept. 9, 1975 United States Patent [191 Gustafson et a1.
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Lightfoot 932 Van DaamHW... 956 Constantinesco Primary Examiner-A. Bartis Attorney, Agent, or FirmHill, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson [5 7] ABSTRACT An electric heater for heating flowing air comprises a sheet support for a heater filament of non-circular crosssection and twisted about its longitudinal axis. The heater filament is wound about or attached in meander fashion to the support sheet in such a manner that successive segments of it are parallel and extend between opposed edges of the support sheet in contact with a face of the latter. An insulating cover layer disposed in contact with the heater filament segments defines together with these segments and the support sheet a multiplicity of relatively narrow, open-ended passages for the air to be heated. The support sheet may be flat and rigid or flexible and convoluted on itself together with the cover layer, or it may be tubular. 8 Claims, 9 Drawing Figures  Assignee: Separex SA, Switzerland  Filed: Oct. 2, 1973  Appl. No.1 402,804
 Foreign Application Priority Data Oct. 5, 1972 12823/72  11.8. C1. 219/374; 219/307; 219/367; 219/376; 219/542; 219/546; 338/58; 338/320  Int. Cl........ H051) 3/10; F2411 3/04; H010 3/00  Field of Search 219/374-376, 219/381, 382, 373, 523, 531, 544, 542, 546, 551, 296299, 305-307, 319, 366368, 369, 370; 338/57, 58, 282, 286, 297, 287, 320, 315, 321
 References Cited UNITED STATES PATENTS 776,491 12/1904 Bristol... 338/320 X v H! II a. I.- F ul 5 SHEET 1 o 2 muhq ELECTRIC HEATER FOR HEATING A FLUID MEDIUM This invention relates to electric heaters, and more particularly to electric heaters of the kind in which air or some other flowing fluid medium contacts an elec tric heating element to be heated thereby.
A common type of electric air heater comprises a heat-resistant insulating support which carries a filamentary heating element and is disposed in a flow passage for the air to be heated. In heaters of this type, the transfer of the heat from the heating element is often rather nonuniform and inefficient, because only a small portion of the air flow contacts the heating element. In addition, the electric insulation of the heating element from the walls of the flow passage often renders the heater bulky and structurally complicated.
A general object of this invention, therefore, is to provide an electric heater for air or other fluid mediumswhich is simple and inexpensive and ensures an efficient and uniform heating. A more specific object is to provide an electric heater in accordance with the foregoing general object which is compact so that it can be used in applications where the available space is limited.
The above and other objects, features and advantages of the present invention will be understood from the following detailed descriptions of exemplary embodiments shown in the accompanying drawings.
FIG. 1 is a longitudinal cross-sectional view along line l-I of FIG. 2 of an air heater constructed in accordance with the invention;
FIG. 2 is a cross-sectional view along line II-II of FIG. I;
FIGS. 3 and 4 are cross-sectional views along line III- III of FIG. 4 and line lV-IV of FIG. 3, respectively, showing a modification of the heater of FIGS. 1 and 2;
FIG. 5 is a developed view of an insulating support strip which is provided with a helically wound heater filament and placed on an insulating cover strip and which, in the finished heater, is convoluted together with the cover strip to form an annular heater body;
FIG. 6 is an end view showing the finished heater with the convoluted heater body disposed in the annular space between two concentric casing members;
FIG. 7 is a longitudinal cross-sectional view along line VllVII of FIG. 6;
FIG. 8 is a longitudinal cross-sectional view of a heater comprised of three tubular, concentric heater bodies disposed in a tubular outer casing;
FIG. 9 is a perspective view of one of the heater bodies in FIG. 8
In the drawings the flow of air to be heated is indicated by arrows, but for simplicity of illustration, the means producing the forced air flow through the heater are omitted.
As shown in FIGS. 1 and 2, the heater 10 includes a casing 11 of rectangular cross-section (FIG. 2) and having a through opening 12 adapted to receive the air to be heated and discharge the heated air. The two sidewalls of the opening 12 are each provided with an electrically insulating, heat-resistant side plate 13 each of which serves as a cover layer. Between the two side plates 13 and spaced therefrom there is disposed another electrically insulating and heat-resistant rigid plate 14 on which a filamentary resistance heating element E5 of non-circular cross-section is supported. The
heating element 15 consists of two resistance wires 15 of circular cross-section which are twisted together about their length and have their ends connected to a pair of terminals 16 on the support plate 14 the wires thus defining helical surfaces.
As best seen from FIG. 1, the heating element 15 is helically no und about the support plate 14 in such a manner that the substantially straight segments 17 of its convolutions are generally parallel to the longitudinal axis of the opening 12 and substantially uniformly dis tributed over both faces of the support plate 14. As seen from FIG. 2 the segments 17 are covered and engaged by the side plates 13, whereby each side plate 13 defines together with the adjacent face of the support 14 and the segments 17 a group of parallel and substantially identical, open-ended air passages 18 extending longitudinally of the opening 12. The side plates 13 sealingly engage the side walls of the opening 12, and all air traversing the opening 12 accordingly flows through the passages 18. A suitable choice of the spacing of the segments 17 and the diameter of the wires 15' (and thus the dimension of the air passages 18 transversely of the side and support plates 13,14,13, this dimension being twice the wire diameter) ensures an efficient and uniform heating of all air passing through the opening 12.
In assembling the heater 10 of FIGS. 1 and 2, the support plate 14 with the heating element 15 and the terminals 16 are slid as a unit into the space between the side plates 13 in the opening 12. A number of terminal posts 19 are then inserted through bores (not shown) in the casing 11 and secured to the terminals 16. Alternatively, the terminal posts 19 may be secured to the terminals 16 before the unit is slid into the casing; notches in the casing then receive the terminal posts.
In the heater 102 shown in FIGS. 3 and 4, the side plates 13 belong to the aforesaid heater unit, which is adapted to be slid into the opening 12 of the casing 11. To this end, the plates 13 and the plate 14 are rigid and held together by a helically twisted and helically wound wire 20 extending transversely of the heating element 15a as shown in FIG. 3. The diameter of the wire 20 is chosen such that the heater unit fits snugly in the casing opening 12. The wire 20 spaces the side plates 13 from the sidewalls of the opening 12 but prevents the air from bypassing the air passages 18 between adjacent segments 17 of the heating element 15.
The resistance heating element 15a in FIG. 3 is a narrow strip twisted about its longitudinal axis, and the dimension of the air passages 18 transversely of the plates 13 and 14 thus is approximately equal to the width of the strip. By varying the width of the strip while maintaining a constant cross-sectional area, different transverse dimensions of the air passages can be obtained without changing the resistance value of the heating element.
In FIGS. 5 to '7, a resistance heating element 21 formed by a helically twisted narrow strip is helically wound about an elongated, electrically insulating support strip 22 with the parallel, uniformly spaced segments 23 extending transversely of and between the longitudinal support strip edges and in engagement with the support strip faces. The ends of the heating element 21 are connected to a pair of terminals 24 adjacent the ends of the support strip. The support strip 22 with the heating element 21 is placed on an electrically insulating, flexible cover strip 25 slightly wider than the support strip 22 and having its end portions 25A and 25B projecting from the ends of the support strip.
FIGS. 6 and 7 show a complete heater 26, in which the support strip 22 and the cover strip 25 are convoluted longitudinally on themselves to an annular heater body, the inner end of the convoluted body being the right-hand end in FIG. 5. As seen in FIG. 6, the cover strip 25 engages and covers all of the heating element segments 23.
The annular heater body is inserted and snugly fits in the annular space defined between two concentric casing members, an inner tubular member 27 and an outer tubular member 28 having an end wall 29. The air to be heated is passed through the several relatively narrow air passages 30 defined by the strips 22 and 25 and the heating element segments 23 and leaves the heater 26 through the inner casing member 27 as shown in FIG. 7.
The heater 31 shown in FIGS. 8 and 9 comprises three tubular concentric heater bodies 32, 33, 34 and a tubular casing body 35 concentrically surrounding the heater bodies. Apart from their dimensions the three heater bodies are substantially identical, and the outermost heater body 32 is shown in FIG. 9.
As shown in FIG. 9, the heater body 32 has a multiplicity of notches 36 and intervening lips 37 at both ends. The lips serve to hold a resistance heating element 38 formed by a twisted strip which runs along a meandering path around the circumferential outer surface of the body and has its ends connected to a pair of terminals 39. The parallel segments 40 of the heating element extending between the notched ends are slightly inclined relative to the axis of the body 32 and define between them a large number of correspondingly inclined open-ended air passages 41.
As shown in FIG. 8, the spacing between adjacent heater bodies and between the outermost heater body 32 and the casing body 35 is equal to the maximum cross-sectional dimension of the heating element 38. That is, the circumferential inner surfaces of the casing body 35 and the heater bodies 32 and 33 engage the heating element segments 40 on, respectively, the outermost heater body 32, the intermediate heater body 33, and the innermost heater body 34 and thus fulfil the function of the covering layers 13 and 25 in FIGS. 1 to 4 and to 7.
The three heating elements 38 of the heater 31 may be connected in series or parallel or may be provided with with heating current independently of each other. In order that unwanted short-circuiting of the heating elements may be avoided, the three heater bodies are of different lengths and staggered shown in FIG. 8.
The air to be heated is directed into and discharged from the heater 31 in the same direction, and bypassing of the air passages 41 is prevented by a transverse wall 42 in the innermost heater body 34.
Since the air passages 41 are slightly inclined, the tubular stream of heated air leaving the heater 31 has a rotational component of motion, the magnitude of which of course depends on the degree of inclination of the air passages.
The heater bodies 32, 33, 34 may be made from an elongated flexible strip the ends of which are connected together after the heating element 38 and the terminals 40 have been applied to the strip.
longitudinally as As shown in FIGS. 1, 3, 5, 9, the ratio of the length L of the air passages to their width W is relatively high, namely, of the order of ten to one. Although a high value of this ratio often is advantageous, the value is not very critical and it may be substantially smaller but should always be higher than about two to one; if smaller it might be difficult to obtain'the desired high efficiency of the heat transfer.
Any suitable material may be selected for the heating element support and the covering layers as long as it provides the desired electrical insulation and is heatresistant enough to withstand the temperature of the heating element. As a nonlimiting example of a suitable material for the heating element support, anodized sheet aluminum may be taken.
What is claimed is:
1. An electrical heater for heating a fluid medium flowing therethrough comprising:
a. an electrically insulating heating element support;
b. a filamentary heating element, twisted about the length of the element, mounted on the heating element support and including a plurality of parallel heating-current conducting segments extending in spaced relations between opposed edges of the support and in engagement with at least one face of the support, the length of said segments being more than twice the spacing of adjacent segments; and
e. an electrical insulating cover layer having one face thereof in engagement with said segments and uniformly spaced from said face of the heating ele ment support;
the heating element segments defining between the heating element support and the cover layer a plurality of helically walled parallel passages for the passing of the fluid medium to be heated, the passing being parallel to said walls from one of said edges of the heating element support to the other, said passages having a dimension transverse of said faces substantially equal to the maximum crosssectional dimension of the heating element segments.
2. A heater according to claim 1 including:
a casing having a first opening defining a fluid inlet,
a second opening defining a fluid outlet, and an elongated flow path extending between the inlet and the outlet in which said heating element, said support, and said cover layer are disposed with said passages extending substantially parallel to the longitudinal axis of the flow path.
3. A heater according to claim 2 in which the heating element support and the cover layer are held together as a unit fitting snugly in the casing.
4. A heater according to claim 1 in which the filamentary heating element comprises two wires twisted together.
5. A heater according to claim 1 in which the filamentary heating element is a flat strip twisted about its longitudinal axis.
6. A heater according to claim 1 in which the heating element support is a flexible strip,
the cover layer is flexible,
the heating element support and the cover, layer are convoluted longitudinally on themselves to an annular structure, and
inner and outer concentric annular casings are provided, which define between them an annular space in which said annular structure is disposed with a snug fit.
7. A heater according to claim 1 in which the heating element support is a first tubular body, the filamentary heating element segments are disposed on the outer surface of the tubular body and extend between its ends, and the cover layer is a second tubular body concentric with and enclosing the first tubular body.
8. A heater according to claim 7, including a second the inner surface of the first tubular body.
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