Abstract:
A multicell heater is shown with typically round outer tubing, containing a plurality of resistive heating elements having a common inner sheath. The inner sheath has a preferably non-circular cross-section and is given a hairpin bend to form a “U” shape. The resulting configuration, which is much less expensive than conventional multicell heaters, provides efficient heating at the toe end—a usually unheated (“cold”) zone in conventional multicell heaters.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electrical heaters with resistive wire heating elements, and more particularly to an electric heater with multiple resistive wire heating elements in an inner sheath, said inner sheath being packed into an outer tubing. 
     BACKGROUND OF THE INVENTION 
     It has been known for some thirty years or more in the heater industry to make heaters having a single outer tubing in which is placed multiple sheathed heaters, each sheathed heater having a resistive coiled wire as a heating element. The resulting heater is known in the industry as a “multicell” because it has multiple heaters wrapped in a single tubing. In general the concept of multicell heaters are describe in U.S. Pat. No. 3,340,382 to Lennox, which is incorporated herein be reference. 
     The outer tubing (sometimes referred to as an “oversheath”) provides extra protection from the elements by further isolating the heating elements from possible contaminants. The outer tubing and can also provide the ability to place the heating power of several heaters within a body having a regular (cylindrical) contour, thus allowing it to be used like a cartridge heater, for example in heating large platens for use in molding aircraft parts. Having multiple heating elements within the same outer sheath allows for heating elements made of larger gauge wire without a reduction in total heat flux (sometimes referred to as “watt density”). This thicker wire typically has a longer life than heating elements made of a narrow gauge wire. 
     Another example of a heater with outer tubing can be found in U.S. Pat. No. 5,575,941 to Johnson, which is incorporated herein by reference. Johnson attempts to use outer tubing with a single sheathed heating element folded over several times within the outer tubing. The result is a cylindrical heater (like a cartridge heater) with a similar effect to having four heating elements within. Because the single heating element is folded over, it appears at any given perpendicular cross-section of the heater (except at the ends) that it has four heating elements. 
     “Like prior art, multicell heaters, Johnson has several limitations. The most important of which is that, like prior art multicell heaters, the toe (end) of the heater is left colder than the rest of the outer tubing. At the end of the outer tubing, the inner heater in Johnson only makes contact at a single point (arguably two single points, however in close proximity to one another). Furthermore, the construction of the heater in Johnson does not leave any additional space within the outer tubing for additional components, such as a thermocouple or other type of sensor.” 
     It is thus an object of the present invention to provide a multicell heater with only a single inner sheath within the outer tubing. 
     It is also an object of the present invention to provide a multicell heater with a toe end that heats to approximately the same temperature as the sides of the outer tubing. 
     It is yet a further object of the present invention to provide this multicell heater with a thermal well wherein optional sensors may be easily placed. 
     SUMMARY OF THE INVENTION 
     In keeping with the above, the present invention comprises outer tubing, wherein a plurality of resistive heating elements having a common sheath are placed. In the preferred embodiment, the resistive heating elements are parallel and coplanar within the inner before insertion into the outer tubing. The inner sheath is then bent into a “U” shape with the bend being made perpendicular to the direction of the heating elements. Thus, once inserted into the outer tubing, the inner sheath has a complete line of contact with the end disk of the outer tubing, rather than a single point of contact. The outer tubing will usually be round (cylindrical) to accommodate most practical applications, however the invention could work just as easily with outer tubing having other cross-sections should the need arise. 
     To produce this configuration, the inner sheath preferably begins with a non-circular cross-section. A typical cross-section for the inner sheath has flattened sides with semi-circular ends as will be made apparent below. 
     The bend in the inner sheath provides a gap within the outer tubing which may be filled with reinforcing bars, hollow tubes, or a combination thereof. The hollow tubes may be used as thermal wells, in which sensors such as thermocouples may be positioned. 
     A typical application for the multicell heaters of the present invention would be to provide a plurality thereof to be inserted together into a superplastic forming platen. Another application would be to place a plurality of the heaters in a duct to heat air or other gasses by forced convection. The number of different applications for the present heater is infinite. It can be used in any manner conventional multicell heaters are used, but are less expensive to manufacture and provides more efficient heating at the toe end. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above-recited features, advantages, and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
     It is noted however, that the appended drawings illustrate only several typical embodiments of this invention and is therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. Reference the appended drawings, wherein: 
     FIG. 1 is cross-sectional side view of a multicell heater embodying the present invention; 
     FIG. 2 is an end view of the heater of FIG. 1; and 
     FIG. 3 is an exploded view of the heater of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1 a multicell heater  10  embodying the present invention is shown generally. The main housing for the heater  10  is a section of outer tubing  12 . Although the composition of the outer tubing  12  is not important, the outer tubing  12  is known to be metallic, preferably being made of Inconel®, Incoloy®, or steel, depending upon the intended application. Within the outer tubing  12  is disposed a plurality of resistive heating elements  14  having a common inner sheath  16 . The inner sheath  16  is also known to be metallic. The inner sheath  16  and heating elements  14  without the outer tubing resemble the construction of a tubular heater known sold under the trademark Firebar® by the assignee hereof, Watlow Electric Manufacturing Company of St. Louis, Mo. 
     In the preferred embodiment, the inner sheath  16  has a non-circular cross-section. As best seen from FIG. 2, where both ends of the bent inner sheath  16  can be seen, the cross-section has two opposite flattened sides  18  connected by semi-circular edges  20 . The inner sheath is typically filled with an insulator  22 , most commonly consisting of magnesium oxide (MgO). 
     The heater may have several heating elements  14 , although the simplest case of two heating elements  14  is shown in the drawings. The heating elements are preferably, but not necessarily, parallel and coplanar. The resistive heating elements  14  can be made of any conventional material for such, but will most commonly consist of nickel chromium (NiCr) wire or NiCr wire with an outer treating. 
     The heating elements  14  and inner sheath  16  are bent to form a hairpin bend  24 . The resulting “U” shaped structure is then placed in the outer tubing  12 . The bend is preferably perpendicular to the heating elements  14  to provide symmetry of the inner structure (heating elements  14  and inner sheath  16 ). The space formed between the legs of the “U” can optionally be filled with a variety of components. The preferred embodiment, shown in the drawings, has a pair of reinforcing bars  26  and a thermal well  28 . The reinforcing bars  26  are typically solid metal rods that not only provide structure support within the outer tubing  12 , but also provide excellent heat transfer around the multicell heater  10 . The thermal well  28  is a hollow metal tube, in which a variety of sensor types (a thermocouple for example) may be placed. The thermal well  28  makes the sensor easily replaceable in comparison to heaters that have a temperature sensor integrated into the inner sheath. Any remaining dead space within the outer tubing  12  is filled with another insulating fill  36 , such as MgO powder. 
     The outer tubing has a lead end  30  and a toe end  32 . In a conventional prior art multicell heater the heating elements do not come close to the toe end, and therefore leave an unheated “cold” zone which remains at a significantly lower temperature than the remaining outer tubing. The present invention provides contact between the inner sheath and the end disk (or cap)  34 . Although in a prior art multicell heater the toe end typically gets too hot to touch, energizing the heater one can plainly see that the majority of the outer tubing becomes a glowing red, while a section near the toe end remains dark. During the same demonstration on a typical embodiment of the present invention, one can plainly see that the entire heater, including the toe end  32  and end disk  34  are glowing red. Operating temperatures for a typical application may range from 1300° to 1900° F. 
     After the heating elements  14  are placed in the outer tubing  12  with the sheath  16  and other components, power leads  38  are attached to the end of each of the heating elements  14 . The leads  38  may be made of a conventional lead material such as nickel and attached to the heating elements  14  by conventional means such as welding. In the preferred embodiment, ceramic insulators  42  are placed over connections between the leads  38  and the heating elements  14 . Additionally, an end disk  40  is placed over the lead end  30  of the outer tubing to keep out contaminants and keep in the insulating fill  36 . 
     Therefore it should be evident that the present invention possesses at least two, major advantages over prior art multicell heaters. The use of single inner sheath substantially decreases cost of manufacture, sometimes by as much as 56% depending on the number of heating elements (cells) used. The other result of using a single sheath in the manner described is that the cold zone usually found in conventional multicell heaters is eliminated. 
     While the foregoing is directed to the preferred embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims which follow.