|
|
|
NEI's Heating Element Products
Coil Elements Molybdenum Plugheaters Rack Assembly Radiant Tubes Rod Elements Radiant Tube - Firebar® Elements Ribbon Heating Elements Tungsten Vacumm Furnace Elements
Coil Elements
By far the greatest number of heating element designs involve the use of round wire close-wound on an arbor or mandrel and then stretched to form a helix of at least twice the close-wound length. Having selected the proper NIKChrome® alloy, the next step in designing such elements is to choose a size of wire which will operate at the desired temperature and which can also be coiled to fit within the physical space limitations of the device. In most cases, some precedent will have been established as a basis for determining the proper wire size. Lacking prior experience, the data presented in the following tables will be helpful. To use them, proceed as follows:
- Calculate Wattage-The approximate wattage required to operate a given device at various temperatures can be determined from the Current-Temperature Relationship Tables which list current carrying capacities of NIKChrome® resistance alloys in most commonly used wire sizes. First, select a test wire size that "seems" suitable for the application. Then, knowing the voltage on which the device is to operate, calculate the wattage by multiplying volts times the amperes shown ( W = E X I ). These tables may also be used to estimate amperes required to heat coiled Chromed elements to different temperatures.
- Determine Wire Size-From the design established for the device, check the space dimensions allowed for the element to find out (a) the approximate coil diameter or size of arbor to use, and (b) maximum lineal distance available for the element. When these factors are known, refer to the Wattage-Wire Size Relationship table above for the voltage to be used-115 or 230 volts. From this, select a size of wire that will provide the wattage required and note the number of feet of straight wire that will be needed to make the coiled element.
- Determine Close-Wound Coil Length-Having selected a suitable wire size-and knowing the lineal feet of straight wire required to make a heating element of a given wattage, refer to the Coil Winding Graph at right that applies to the size of arbor to be used. Check the point at which the vertical coordinate representing the feet of straight wire intercepts the diagonal line shown for the wire size selected. Then follow the horizontal coordinate to the left hand column and read off the inches of close-wound coil obtainable. For example, using a 1/32" arbor, 40 lineal feet of 33 gauge wire will produce 30 inches of close-wound coil.
- Calculate Stretch Ratio-To determine the "Stretch Ratio" of the finished coil, divide the inches of close-wound coil into the maximum lineal inches of space allowed for the element in the device. For best results, the stretch ratio should be between 1.5-to-1 and 4-to-1. If the ratio does not fall within this recommended range, check the possibility of changing arbor size or selecting a different wire size-larger or smaller as required-and recalculate the coil element design data.
Rod Elements
Vary considerably in shape and size and are used extensively in Lindberg and Old Sauder type furnace apparatus. Life is usually very good, due to the greater cross sectional area. The rod sizes begin at 6 GA. (.162 dia.) and usually do not exceed 1/2" diameter. We stock all sizes in 80NI-20Cr known as NIKChrome® "A" Kanthal® APM and stock many sizes in 70/30 Alloy, NIKChrome® "AA", and Iron Chrome Aluminum Alloys, known as Kanthal® Alloys.
A typical set of Rod Heating Elements consists of 2 GA. (.258 DIA.) 80ni-20cr for use at 1850°F maximum mounted on sidewalls, floor, door and back wall. Most elements are kinked to raise the element off of the insulation and floor elements are often suspended on peers to allow for debris to collect underneath the elements. Most terminals on rod type elements are heavy, such as pictured below, 3/4 dia. 18" long, machined to accommodate electrical terminal clamps. Most of the heavy rod elements in small box furnaces are 240 volt, 3 phase. Blanket insulation used in modern furnaces today incorporate a variety of designs and support methods. Many box furnaces use this method for improved efficiencies of heat losses.
Radiant Tube - Firebar® Elements
US Patent No. 4,016,403 re 30838, plus Six (6) foreign countries.
In 1974 National Element, Inc. a world leader in the manufacture of industrial heating elements, undertook the challenge of developing an element, which would convert fuel-fired furnaces to electric. The result was Firebar®'? With Firebar®, conversion was easily accomplished by inserting the Firebar®, a round heating element into the old radiant tube without any other alteration to the furnace. Firebar® represented a tremendous break through in energy savings, economy of operation and longevity of equipment. Firebar® is capable of liberating 85 BTUs of energy per square inch of pipe surface and sustaining temperatures as high as 1850_F. Today over 10,000 Firebars® are in service throughout the world supplying heat to everything from carburizing furnaces to incinerators.
What is a Firebar®?
The Firebar® is an improved electrical heating element for high temperature furnaces, which includes a support structure, heating element and terminal connection rods. The support structure consists of a series of thin, flat, non-conductive plates or disks each having a plurality of spaced apertures, and, a series of non-conductive spacers interposed between each pair of adjacent disks. The electrical heating element assembly includes a plurality of elongated electrical conductors. U-shaped, each leg extending through one aperture in each disk radially outward of the spacers. The conductors are supported by and extended between the disks and the ends of the conductors and are interconnected so that the conductors define a single electrical path radiating outwardly of the spacers. It is these conductors which maintain the support structure in the assembly.
Firebar® Features
Firebar® requires no Transformer, Process Temperatures to 1850°F Horizontal or Vertical Installations Lengths from 18" to 10' Six Sizes of Heavy Round Rod in Two Rows for most Kilowatt Requirements.
Ten Standard Diameters Available: 2-3/4", 3-1/4", 3-5/8", 4-1/4", 4-7/8", 5-1/4", 6", 6-3/8", 6-1/8", 7-3/4".
Firebar® Applications
- Carburizing, Hardening, Draw Furnaces, Roller Hearth, Rotary Hearth Furnaces
- Typical Atmosphere Applications: Air, Inert, Strongly Reducing.
Firebar® Construction
The entire electrical path of the Firebar® radiates the complete electrical length outward of the central core, thus generating usable heat for the full length of the radiant tube or furnace. Present Firebar® construction includes the provision of electrical conductors of a sufficiently thick cross section to reduce oxidation and corrosion. Furthermore the construction technique permits the replacement of the conductors when they become pitted or melted without necessitating replacement of the entire electrical assembly. Finally, the support structure avoids sagging and resultant axial growth. The conductors are normally drawn from 80Ni-20Cr heavy round rod with many applications for endothermic generators designed with specialty alloys. Each circuit should contain a thermocouple for control start up. Units are normally shipped from stock complete with operating instructions, terminals, insulation, all internal connections, and, if necessary, a base plate for vertical operation. Sizes listed are standards, other diameters are available, data based on 480volt, Wye connected circuit, two (2) elements per phase, and various effective heater lengths and total design wattage.
Available Resistance Alloys
- NIKChrome® A ~ 80ni-20cr
- NIKChrome® C ~ 60ni-16cr
- NIKChrome® D ~ 35ni-19cr
- 70-30 Alloy ~ 70ni-30cr
- Iron-Chrome-aluminum Alloys
- Kanthal D Alloy
- Kanthal A Alloy
- Kanthal AF Alloy
- Kanthal A-1 Alloy
- Thermocouple Alloys
- Inconel 601 *
- Kanthal Alloys **
- Other Alloys Available
* Registered Trademark Of Huntington Alloys, Inc.
** Registered Trademark of Kanthal Corp.
Ribbon Heating Elements
Typical ribbon elements are often made consisting of two rows and are found suspended in refractory shapes in both ceiling, walls, and floors of small industrial high temperature furnaces. You will find the loops no longer than three to four inches, but the number of loops is unlimited. Some designs do have three layer terminals which are punched for terminal connections, while others have round terminal leads which are threaded for terminal connections, as per the picture. Every furnace manufacturer uses this style of element. The most popular electric heating element for a carburizing furnace for the last forty years has been the Leads & Northrup homocarb application. These elements are stock and on the shelf for quick delivery. There are typically three in each furnace; however, these furnaces may contain six elements. We stock material in all popular sizes, in both 35Ni -19Cr and the original equipment material 80Ni-20Cr. We stock all accessories, which include pins, hooks, and insulators.
Heavy ribbon elements are often found in Alloy Racks and porcelain insulators, this design permits changing an entire element assembly without cooling the furnace. National Element fabricates many types of Alloy Rack Assemblies and any part may be bought separate. Seco Warwick (Sunbeam Corp/Westinghouse) has thousands of generators and high temperatures brazing furnaces working throughout the country. Loops are usually very long, 18 inches in this case, supported by porcelain holders at the top of each loop and 2/3 of the way down. We stock the necessary insulators and hooks and we make all types of ribbon heating elements.