The present invention is directed to an open coil electric resistance heater and method for use, and in particular to a heater configuration that uses offset insulators to support the open coils for improved heater performance.
The use of a single resistance wire formed into a helical coil for use in electric resistance heating either for heating moving air, for radiant heating or for convection heating is well known in the prior art. In one type of heater, the resistance coils are energized to heat air passing over the coils, the heated air then being directed in a particular manner for heating purposes. One application using such a heater is an electric clothes dryer.
Examples of open coil heaters are found in U.S. Pat. Nos. 5,329,098, 5,895,597, 5,925,273, 7,075,043, and 7,154,072, all owned by Tutco, Inc. of Cookeville, Tenn. Each of these patents is incorporated by reference in its entirety herein. One type of an open coil electric resistance heater is a two stage heater described in U.S. Pat. No. 7,075,043. A side view of this type of heater is shown in
The heater 10 includes opposing sidewalls (one shown as 6 in
Each of the electric heater elements, 10a and 10b, is arranged in series of electrically continuous coils which are mounted on the plate 14 in a spaced-apart substantially parallel arrangement. Each heater assembly 10a and 10b is arranged substantially equally and oppositely on both sides of the plate. Crossover portions 22a and 22b of each heater element 10a and 10b are provided wherein each crossover links one coil of each of the elements mounted on one side of the plate 14 with another coil of the same element found on the other side of the plate.
Electricity is supplied to the heater assembly through the terminal block 28. The heater elements, 10a and 10b, are arranged so that the terminal connector portions or wire leads 32 and 34 which extend from an end 38 of each of the mounted coil sections to the terminal block are as short as possible. This aids in eliminating or reducing the need for supporting the connector portions. For the longer runs, the wire leads, 32 and 34, are partially enclosed with an insulating member 36. The insulating member 36 may be formed from any type of insulating material suitable for this purpose, e.g., a ceramic type. The insulating member is generally tubular in shape and rigid.
The plate 14 also has a series of cutouts 30, which allow for mounting of the insulators 16. The cutouts 30 are elongated in shape to allow for insertion of the insulators in a first orientation and then twisting of the insulators for engagement with the plate for secure mounting. It should also be noticed that the cutouts are generally aligned in a longitudinal path along the plate 14. The resistance wire coil is supported along the path, designated as A in
The manufacturing of appliances and equipment, especially clothes dryer manufacturing, often requires that open coil electric resistance heaters be mounted in a heater duct. As the clothes dryer operates, the temperature experienced by the heater in a duct will increase and decrease over several hundreds of degrees. It is natural for the materials of which the heater and duct are made to undergo expansion and contraction during the thermal cycling of the dryer. Generally the metal plates supporting the coil, e.g., the plate 14 of
The prior art heaters made with heater coils supported by insulators as illustrated above are designed so as to accommodate differences in the thermal expansion rate of the metal plate as compared to the thermal expansion of the insulators which support the heater coils plus accommodate any build up of oxide on the metal plate. The method used in the prior art is to create the holes and tabs, i.e., the cutouts, in the metal plate and design the slots and arms of the insulators so there will be sufficient clearance to accommodate expansion, contraction and metal oxide buildup on the metal plate. Expansion, contraction and metal oxidation build-up can then occur without damage to either the insulator or to the metal plate. If such accommodations are not made, there may be conditions develop in which the insulators can actually break or the metal plate can crack or both conditions may occur.
A consequence of the necessary clearance between the cutouts in the metal plate and the mating slots in the insulators means that under certain conditions of operation vibration of the equipment may occur and the heater itself will vibrate which in turn results in the insulators vibrating against the metal plate. This means the necessary looseness of the insulators in the metal plate may create noise during operation. If the noise is high enough, the user of the equipment, e.g., the user of a clothes dryer in a home, may determine the noise level is objectionable. The objectionable noise level is considered a problem with the prior art.
Another problem with open coil electrical resistance heaters is the configuration of the open coil resistance wire heating element. That is, heaters are often designed with the heater coils configured so as to be made up of a number of straight sections, the axis of each section running parallel to the axis of the air duct. This is best seen when referring to
As with all open coil heaters, for a heater made with heater coils supported by insulators, which are in turn supported by a metal plate, it is desirable to have the heater coils arranged so that as much of the duct cross section as possible is filled or covered by heating element material so as to maximize heat transfer to the air stream. The straight coil pass method is not always the best for this because of the shadowing problem. The “figure 8” method of arranging the heater coil is one method used to increase the portion of the duct covered by heater coil. When the length of heater coil is sufficiently long or for multiple coils, as in multi-stage heaters, the “figure 8” arrangement is not a suitable choice as additional space to accommodate the arrangement may not be available.
Therefore, a need has developed to provide improved open coil electrical resistance heaters. The present invention responds to this need with an improved heater configuration that overcomes the problems noted above, especially minimizing noise due to vibration of insulators during heater operation, minimizing shadowing, and increasing the exposure of the coil to the air to be heated.
It is a first object of the invention to provide an improved open coil electrical resistance heater.
It is another object of the invention to provide an open coil electrical resistance heater that uses specially configured insulators to configure the open coil resistance wire heating element in a sinusoidal shape.
Another object of the invention is a method of heating air using an open coil electrical resistance heater having the specially configured insulators and resistance wire heating element.
Other objects and advantages will become apparent as a description of the invention proceeds.
In satisfaction of the foregoing objects and advantages of the invention, the invention is an improvement in open coil electrical resistance heaters that have a support, at least one resistance wire coil, and a plurality of insulators mounted to the support along a defined path, with each insulator configured to provide support to a portion of the resistance wire coil. The at least one resistance wire coil has a longitudinal axis generally parallel to an air flow path of the heater. According to the invention, at least a portion of the insulators are offset from the path. These offset insulators when combined with the insulators on the path cause at least a portion of the at least one resistance wire coil to have a sinusoidal shape. It is this sinusoidal shape that provides advantages in terms of noise reduction, reduction of the shadowing problem, minimizing vibration resonancy, and better filling the volume of the heater for maximized heat transfer.
While the support for the insulators can take on any number of configurations, a preferred configuration is a metal plate. Using a metal plate allows for the use of cutouts in the metal plate to mount the insulators therein. Offsetting the cutouts positions the insulators in the offset position to create the sinusoidal shaped resistance wire coil.
While the heater can be as simple as one resistance wire coil and the necessary components for energizing it, the support or metal plate can be configured to retain a number of resistance wire coils for increased heating capability. In this mode, the insulators can be configured to hold segments of a coil on each end, such that coils would travel along both sides of the support or metal plate.
The duct for use with the support, insulators, and resistance wire coils can be of any cross sectional shape, with a circular duct being one preference.
The sinusoidal shape of the resistance wire coil aids in vibration prevention by biasing sides of the insulators against sides of the cutouts.
The invention also entails the use of the improved heater assembly described above by passing air over the at least one resistance wire coil in a direction coincident with the longitudinal axis of the coil and energizing the heater for heating of the air passing therethrough.
Reference is now made to the drawings of the invention wherein:
The invention offers advantages in the field of open coil resistance heaters in that the problems in noise generation and premature failure of heater components are minimized. In addition, the inventive open coil electrical resistance heater is advantageous in reducing the amount of shadowing that occurs in prior art heaters and promoting a longer life operation of the heater. The invention is particularly adapted for heaters that employ resistance wire coils that are aligned with the flow of air through the heater. It is these coils that are susceptible to the problem of shadowing and the offsetting of the insulators to create the sinusoidal shape in the coil minimizes this problem. The offsetting that creates the sinusoidal coil configuration also contributes to filling the volume of the heater that air passes through for better heating efficiency.
Features of the invention also include the following:
1) An open coil electric heater for heating moving air with the heating element made up of sections of coils such that one end of a given coil section is located on the inlet air portion and the other end is at the exit air portion.
2) Insulators engage sufficient numbers of convolutions at points along each coil section supporting the coil thereby holding the heater coils section in place as each insulator is retained by a metal plate.
3) The insulators are retained in the metal plate by cutouts in the metal plate engaging slots and possibly arms in the insulators.
4) Each cutout in the metal plate is designed so as to engage the corresponding slots and possibly arms in the insulators retained yet allow for expansion and contraction resulting from the heating and cooling of the heater.
5) The insulators supporting a given coil section are arranged so as to create a sinuous path for the coil section. The sinuous coil path thereby creates sufficient tension so as to dampen vibration of the insulators against the metal plate.
6) The sinuous coil passes effectively expose a greater portion of each coil pass to the moving air stream for greater transfer of heat to the moving air stream being heated.
7) The sinuous coil passes effectively reduce the “shadowing” relative to a straight coil section arranged parallel to the air flow direction.
Referring now to
The plate 40 also includes tabs 43 and 45, which interface with a duct for attachment thereto. The other features of the plate are conventional and do not require further explanation for understanding of the invention.
Referring now to
The invention is ideally adapted for a heater that has the path of air aligned with the longitudinal orientation of the coils 47. This path of air is shown in
Second, by arranging the insulators 49 supporting a given coil section in an offset fashion, the resultant sinuous pattern of the heating coil reduces the tendency for vibration resonance to occur as compared to a straight coil pattern.
Third, by arranging certain of the insulators of a given coil in an offset fashion, shadowing of downstream heater coil convolutions in any given straight section by upstream heater coil convolutions from that same given straight coil section is reduced. Shadowing results when air heated by an upstream helix flows over and heats down stream helixes. By reducing shadowing, the operating temperature of the heater coil is reduced which is desirable. This is best seen in
Fourth, by arranging the insulators of a given coil section in an offset fashion, each subsection of the given heater coil will be angled relative to the axis of airflow through the duct and arranged so as much of the duct cross section as possible is filled or covered by heating element material to maximize heat transfer to the air stream.
The cutouts 41 and 42 are exemplary of ways in which the insulators can be mounted to the plate 40. Other modes of mounting could be employed if so desired. The important aspect is that a certain number of the insulators that support the resistance wire are offset from other insulators to create the sinusoidal shape of the coil and the advantages discussed above, e.g., noise reduction and minimizing shadowing.
Also, while a plate is employed to support the insulators, other types of supports could also be used. For example, a wire frame could be employed, with clips that hold the insulators as are found in some open coil electrical resistance heater configurations. Also, differently-configured insulators could also be employed with the support and resistance wire coil.
The degree of offset of certain of the insulators can also vary. The degree of offset can be gauged by the distance between the two paths X and Y of
While
As such, an invention has been disclosed in terms of preferred embodiments thereof which fulfills each and every one of the objects of the present invention as set forth above and provides a new and improved open coil resistance heater with a specially configured coil and a method of heating using the specially configured coil.
Of course, various changes, modifications and alterations from the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention only be limited by the terms of the appended claims.
Number | Name | Date | Kind |
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4268742 | Cottrell et al. | May 1981 | A |
5329098 | Howard et al. | Jul 1994 | A |
5895597 | Sherrill | Apr 1999 | A |
5925273 | Sherrill | Jul 1999 | A |
7075043 | Howard et al. | Jul 2006 | B2 |
7154072 | Sherrill et al. | Dec 2006 | B2 |
Number | Date | Country | |
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20090139984 A1 | Jun 2009 | US |