Patent Application
20100237059
The present disclosure relates to electrical heaters, fluid moving applications, and appliances. In particular, the present disclosure relates to open coil heaters that include resistive heating elements with improved structures to reduce material costs.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An open coil heater generally includes a resistive heating element to generate heat. The resistive heating element is generally in the form of a coiled wire and generates heat as electrical current passes therethrough. The resistive heating element is in direct contact with a surrounding fluid, such as air or water, for example. Heat exchange between the resistive heating element and the surrounding fluid is efficient. Therefore, a quick response time can be achieved.
The length, material, and diameter (e.g., the wire gauge) of the coiled wire of the resistive heating element need to be properly selected to generate a desired heat output. The selection of an appropriate wire type, wire gauge and length requires experience. While standard coiled resistive wires may be used, the coiled resistive wires are generally custom-made for a specific application.
A resistive heating element for a resistance heater is provided. In one form, a resistive heating element for a resistance heater includes a first heating section and a second heating section. The first and second heating sections are configured to jointly generate a power output that is equal to that generated by a single reference resistive element under a same applied voltage. The single reference resistive element has a reference length, a reference mass, and a reference surface area. The first and second heating sections are configured to transfer an amount of heat equal to that transferred by the single reference resistive element. A total mass of the first heating section and the second heating section is less than a reference mass of the single reference resistive element.
In another form, a resistive heating element for a resistance heater includes a plurality of heating sections connected in parallel. The plurality of heating sections are configured to generate a total power output that is equal to a reference power output generated by a single reference resistive element. A resultant resistance of the plurality of heating sections is equal to a reference resistance of the single reference resistive element. The plurality of heating sections each have a cross-sectional area less than a reference cross-sectional area of the single reference resistive element. A total length of the plurality of heating sections is greater than a reference length of the single reference resistive wire. A total surface area of the plurality of the heating sections is at least equal to a reference surface area of the reference single resistive heating element to provide a heat transfer efficiency at least equal to that of the single reference resistive element. A total mass of the plurality of the heating sections is less than a reference mass of the single resistive heating element.
In still another form, a method of manufacturing an electrical heater includes: determining a desired power output; determining a single reference resistive wire that generates the desired power output, the single reference resistive wire defining a reference resistance, a reference length, a reference diameter, and a reference surface area; and selecting a plurality of heating sections that has a resultant resistance equal to a reference resistance of the single reference resistive wire. A total surface area of the plurality of heating sections is equal to the reference surface area of the single resistive wire. At least one of the plurality of heating sections has a diameter less than a reference diameter of the single reference resistive wire.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.
The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.
Referring to
The resistive heating element 12 is a single, coiled resistive wire having a cross-sectional area A0 and a diameter D0 and extending an (uncoiled) length L0 between the terminals 16. The diameter D0 may be indicated by a gauge number under American Wire Gauge (AWG) System. In the American Wire Gauge System, a (AWG) gauge number represents a standard diameter of a round, solid electrically conducting wire. The larger the gauge number, the smaller the wire diameter. The surface area of the resistive heating element 12 exposed to the surrounding environment is approximately π-D0L0. The cross-sectional area A0 of the resistive wire is approximately π-D02/4.
Referring to
wherein V0 is a voltage of the power source 18 and R0 is an electrical resistance of the resistive heating element 12.
The heat generated by the resistive heating element 12 through ohmic losses is equal to the power output P0. The actual heat transfer from the resistive heating element 12 to the surrounding environment, however, is generally less than the power output P0 and depends on efficiency of heat transfer.
By way of example, it is known to produce a 5000 W heater by employing a single 5000 W heating element, comprising a coiled 16 gauge wire having a resistance of about 10.5 ohms and weighing about 0.290 lb., in a single circuit in the heater. To produce a 10 kW heater, then two (2) 5000 W heating elements are employed in two circuits in the heater (see, e.g.,
Referring to
More specifically, the electrical heater 30 is an open coil heater and is shown to include a resistive heating element 32 comprising a coiled resistance wire and a housing 34. A support 35 extends outwardly from the housing 34 and secures the resistive heating element 32 to the housing 34. The resistive heating element 32 is exposed to the surrounding environment and in direct contact with the surrounding medium, such as air. The coils of the resistive heating element 32 (though represented in
The resistive heating element 32 may include coiled resistive wires made from a metal or alloy, such as, but not limited to, a Nickel-Chromium alloy, an Iron-Chromium-Aluminum alloy, and a Nickel-Chromium-Iron alloy. Any suitable alloy may be utilized without departing from the scope of the present disclosure. Alternatively, the resistive heating element 32 may comprise a resistive ribbon element(s) instead of a wire(s), as described below. The resistive heating element 32 includes a first heating section 36 and a second heating section 38. Dimensions of the resistive wire (or other resistive element) for the first heating section 36 and the second heating section 38 are expressed relative to the dimensions for the resistive element of the single resistive heating element 12 of
The first heating section 36 and the second heating section 38 are configured to have a resultant resistance equal to the reference resistance R0. Therefore, the first and second heating sections 36 and 38 generate a total power output equal to the reference power output P0. The heat generated by the resistive heating element 32 through ohmic or resistive loss is equal to the heat generated by the reference resistive wire 12 of
The first and second heating sections 36 and 38 are connected to a power source 39 through a plurality of terminals 40. The terminals 40 include any suitable electrical conductor for conducting electrical current from the power source 39 to the first and second heating sections 36 and 38. The terminals 40 can be manufactured from a metal (such as steel or copper), or from a bimetallic construction (such as a copper core steel pin).
The first and second heating sections 36 and 38 extend between their respective terminals 40 to define a serpentine shape. The first heating section 36 has a first length L1, a first surface area AS1, and a first cross-sectional area A1. When the first heating section 36 is a round wire, the first surface area AS1 is approximately equal to πD1L1 and the first cross-sectional area a1 is approximately equal to πD12/4, wherein D1 is the diameter of the round wire. When the first heating section 36 is in the form of a ribbon element, the first surface area AS1 is approximately equal to 2(b1+t1)L1 and the first cross-sectional area a1 is approximately equal to b1·t1, wherein b1 is the width of the ribbon element, t1 is the thickness of the ribbon element.
Similarly, the second heating section 38 has a second length L2, a second surface area AS2, and a second cross-sectional area A2. When the second heating section 38 is a round wire, the second surface area AS2 is approximately equal to πD2L2, and the second cross-sectional area A2 is approximately equal to πD22/4, wherein D2 is the diameter of the round wire. When the second heating section 38 is in the form of a ribbon element, the second surface area AS2 is approximately equal to 2(b2+t2)L2 and the second cross-sectional area A2 is approximately equal to b2·t2, wherein b2 is the width of the ribbon element and t2 is the thickness of the second ribbon element.
The first and second heating sections 36 and 38 are configured to have a total surface area (AS1+AS2) approximately equal to the reference surface area AS0. At least one of first cross-sectional area A1 and the second cross-sectional area A2 is less than the reference cross-sectional area A0. When the reference resistive heating element and the first and the second heating sections 36 and 38 are in the form of round wires, at least one of the first diameter D1 and the second diameter D2 is less than the reference diameter D0. Therefore, a total length (L1+L2) is greater than the reference length L0 to maintain the same surface area. By making the total length (L1+L2) larger than the reference length L0 and by making the first cross-sectional area A1 and/or the second cross-sectional area A2 less than the reference cross-sectional area A0, a total mass (or weight) of the first and second heating sections 36 and 38 is less than a reference mass (or weight) of the single reference resistive element 12.
A resistive heating element of an open coil heater may be arranged in a number of ways to generate the same power output (i.e., P0). The actual heat transfer from the resistive heating element to the surrounding environment, however, may not be the same, depending on efficiency of heat transfer. In an open coil heater, the resistive heating element is exposed to the surrounding environment, such as open air, and heat is transferred to the surrounding environment mostly through convection. Heat transferred from a relatively hot source (for example, the resistive heating element) to the surrounding environment is proportional to the surface area of the hot source. The efficiency of heat transfer from the resistive heating element 32 and consequently the actual heat output to the environment remain the same when the total surface area is not changed. Therefore, by maintaining the same power output, the resistive heating element 32 of this embodiment generates the same theoretical heat output due to ohmic loss. By maintaining the same surface area, the resistive heating element 32 achieves the same efficiency of heat transfer and outputs the same amount of heat to the environment, taking into account the heat transfer efficiency.
By reducing the cross-sectional area of the first and/or second resistive heating section 36, 38, a total mass of material needed for constructing the first and second heating sections 36 and 38 can be reduced when a same material is used. Mass of a resistive wire is proportional to the cross-sectional area (and square of radius in the case or a round wire) and is directly proportional to the length. While the total length is increased, the reduced cross-sectional area of at least one of the first and second heating sections 36 and 38 results in a mass/weight reduction. Therefore, less material is needed to form the resistive heating element 32, thereby reducing the material use and corresponding costs.
Referring to
wherein R is the electrical resistance of a conductor, ρ is resistivity of a conductor, L is the length of the conductor, and A is the cross-sectional area of the conductor.
According to Joule's law, resistance R may also be represented as follows:
wherein V is the potential (in volts) and P is the power or energy rate of transfer across the resistor. Substituting for R of eq. 3 into eq. 2, then
Therefore, when the first and second heating sections 36 and 38 are longer and thinner than the single reference resistive heating element 12, the resistance R1 and R2 of the first and second heating sections 36 and 38 become greater than the reference resistance R0. The first and second heating sections 36 and 38 may be connected in parallel to have a resultant resistance equal to the reference resistance R0.
To simplify the determination of dimensions of the resistive heating element 32, the first heating section 36 and the second heating section 38 may be made of the same material and be configured to have the same dimensions, i.e., L1=L2, A1=A2 (or D1=D2). The first and second heating sections 36 and 38 may each have an electric resistance of 2R0 and are connected in parallel. The resultant resistance, therefore, is equal to the reference resistance R0 and the power output P0 remains the same.
The following example is illustrative. First, consider a single, 5000 W resistive heating element operating at 240 Volts (V). The heating element is made from 16 AWG Kanthal® NIKROTHAL® 60 wire material. Kanthal® NIKROTHAL® 60 is a known nickel-chromium-iron alloy suitable for heating elements for household appliances and the like that is available from Kanthal A B, Hallstahammar, Sweden. The 16 AWG wire has a diameter (D)=0.0508 in. The resistivity (ρ) of NIKROTHAL® 60=1.11Ω·mm2·m−1[4.37×10−5Ω·in2·in−1] and it has a temperature factor of resistivity (Ct) @600-700° C. of about 1.09. In a 16 AWG wire, the NIKROTHAL® 60 weighs 0.0072 lb./ft.
From the foregoing, it can be calculated that a 16 gauge NIKROTHAL® 60 heating wire of approximately 490.18 in. (40.8 ft.) in length and weighing approximately 0.294 lb. is needed to construct the 5000 Watt resistance heater. Such a heating element would produce about 63.9 Watts/in.2 of the heating element's surface, consistent with HVAC heater design guidelines which generally suggest about 65 Watts/in.2
Now consider the following. In the electrical heater 30 of
As previously described, additional circuits may be added to produce heaters of 10 kW, 15 kW, and so on.
Referring to
The cross-sectional areas A3, A4, A5, and A6 (or diameters D3, D4, D5, D6 in case of round wires) of the first, second, third and fourth heating portion 54, 56, 58 and 60 are smaller than cross-sectional areas A1 and A2 (or diameters D1 and D2) of the first and second heating sections 36 and 38. The total length (L3+L4+L5+L6) of the first, second, third and fourth heating portions 54, 56, 58 and 60 is greater than the total length of the first and second heating sections 36 and 38. The heating portion 54, 56, 58 and 60 are configured to be longer and thinner than the heating sections 36 and 38 of
Referring to
The concept of weight reduction is not limited to a heating element having a circular or rectangular cross section. The concept of weight reduction can be applied to any shape including, but not limited to, circular, oval, rectangular, square, and triangular, for example, without departing from the scope of the present disclosure. In addition, the concept of weight reduction can be applied to resistive heating elements wherein the reference resistive heating element and the desired resistive heating element have different shapes. For example, a reference resistive element may be a coil wire, whereas the desired resistive heating element that has a reduced mass may be a ribbon element.
More specifically, to build a 10 kW heater, two 16 gauge nickel-chromium coil wires connected in parallel may be used, as previously described in connection with
It is understood and appreciated that while the resistive heating element 32 or 52 has been described to include two or four heating sections, the resistive heating element can include any number of heating sections without departing from the scope of the present disclosure. Moreover, the plurality of heating sections may be connected in a number of ways to achieve the desired, reference power output (i.e., same theoretical heat output by ohmic loss) and to maintain the same surface area (i.e., same efficiency of heat transfer to the surrounding environment). For example, some of the heating sections may be connected in series and some of the heating sections may be connected in parallel.
It is also understood and appreciated that, in some situations, the total surface area of the plurality of heating sections may become different from the reference surface area of a reference resistive element if the power output, the applied voltage, the resultant resistance remain the same. For example, a first material for constructing the reference element may be different from a second material for constructing the desired resistive element having multiple heating sections. In this situation, the dimensions of the multiple heating sections may be properly selected so that the multiple heating sections have the same resultant resistance to generate the same power output under the same applied voltage but with a reduced mass of the second material.
The disclosure of the present disclosure can be applied to an electrical heater that heats an adjacent object or fluid by conduction, radiation, or convection. The amount of heat transfer from the resistive heating element to an adjacent object or fluid by conduction, radiation, or convection is proportional to the exposed surface area. Therefore, the concept of material reduction by reducing the diameter of the resistive heating element while maintaining the same exposed surface area is equally applicable to a resistive heating element that transfers heat by conduction, radiation, or convection.
This description is merely exemplary in nature and, thus, variations that do not depart from the gist of the disclosure are intended to be included within the scope of the disclosure. Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of this disclosure.
This application claims the benefit of U.S. Provisional Application No. 61/161,701, filed on Mar. 19, 2009. The entire disclosure of the above application is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61161701 | Mar 2009 | US |
