Type DUMBBELL sic element

TYPE DUMBBELL ELEMENT

One of the earliest heating element designs, the enlarged cold ends of the Dumbbell style element were originally made oversized to increase cold end cross section, lowering electrical resistance, thereby lowering cold end operating temperature. Modern Dumbbell SIC TECH®, by contrast, employ an advanced technology to keep the terminal ends cool by virtue of the decreased resistivity of the lower resistance cold end material used in the manufacturing process. Oversize cold ends are therefore no longer necessary. Non-dumbbell (TYPE I SIC TECH®) can be substituted for Dumbbell SIC TECH®. Improvements in the cold-end-to-hot-zone resistance ratio between the original DB and new DB SIC TECH® have been dramatic. The old style resistance ratio was 1:3, whereas the new DB and TYPE I resistance ratio is a minimum of 1:15.

The first several inches of the cold ends are metalized with aluminum to provide a low-resistance contact surface. Electrical connections are made using flat aluminum braids, held in compression to the cold end circumference by stainless steel spring clamps.

DB SIC TECH® are described by giving the overall length, the heating section length, the hot zone outer diameter and the cold end outer diameter.

As an example, DB 22 x 15 x .31/.50 is a Dumbbell SIC TECH® 22 inches long, 15 inch hot zone, .31 hot zone OD and .50 cold end OD. In millimeters the part number would be DB 560 x 380 x 8/14.

For sizes available, please refer to Table B, DB SIC TECH® Dimensions.

With a larger diameter, thinner wall, and clear inner diameter (ID) the TYPE DB are used as tube heaters. An electrically insulative load tube, usually made of mullite, can be used to isolate the product from the TYPE DB SIC TECH®. The surface of SIC TECH® are electrically live. See Table B, for the ID dimensions and recommended load tube diameters.

MOUNTING

Walls or in ceramic lead-in sleeves, if used, must be large enough to ensure no physical binding of the SIC TECH®. The alignment of the support holes through refractory walls should also guarantee that no physical binding occurs.

If ceramic fiber bulk is used to help reduce radiant energy losses around the SIC TECH®, it must be tucked gently around the diameter of the cold end for a distance of only 12 mm (1/2 inch). The ceramic fiber should never be packed so tightly that it restricts the thermal expansion differences between the SIC TECH® and furnace components. Aluminum braid should be kept slack so that there is no physical stress being applied to the SIC TECH®.

SPECIFICATIONS

OPERATING TEMPERATURES

DB SIC TECH® can operate to furnace temperatures up to 2730ºF (1500ºC). If the furnace temperature is to be 2600ºF (1425ºC) or above, the terminal end or cold end should be well protected within the refractory wall of the furnace. The hot zone material is rated to furnace temperatures up to 3100ºF (1700ºC).

The maximum operating temperature should be reduced for non-air applications with the exception of argon and helium that can be used without any reduction. Reducing atmospheres, such as hydrogen or disassociated ammonia, particularly with low dew-points, may remove the protective silicon oxide protection that forms on silicon carbide. In such atmospheres, temperature reductions are required as shown in Figure 1.

The maximum furnace temperature for a nitrogen atmosphere is 2500ºF (1370ºC) with a watt loading of 20 w/in2 to 30 w/in2 (3.1 w/cm2 to 4.6 w/cm2) maximum watt surface watt loading. Too high a surface temperature will result in formation of silicon nitride causing a thermally insulative layer to form around the SIC TECH® resulting in over-temperature damage.

ENGINEERING INFORMATION

INTERCHANGEABILITY

SIC TECH® are interchangeable with all dumbbell silicon carbide heating elements manufactured in the United States as well as higher resistance heating elements manufactured for the Asian and European markets. It is important to provide the nominal electrical resistance when ordering SIC TECH®.

EASE OF REPLACEMENT

DB SIC TECH® can be replaced while the furnace is at operating temperature. The power to the SIC TECH® being changed should be shut off, the spring clips and aluminum braid released, and the old SIC TECH® removed. The new SIC TECH® should be inserted smoothly through the hot furnace with sufficient speed to insure that the aluminum is not melted off the terminal end but not so fast as to cause thermal shock.

SERVICE LIFE

SIC TECH® increase gradually in resistance with use. This characteristic of increasing in resistance is called aging. Aging is a function of the following:

1. Operating temperature

2. Electrical loading – usually expressed in watts per square inch or watts per square centimeter of SIC TECH® radiating surface

3. Atmosphere

4. Type of operation (continuous or intermittent)

5. Operating and maintenance techniques

SPECIFICATIONS AND MATCHING

SIC TECH® have a manufactured tolerance of plus or minus 20% on the nominal resistance. All SIC TECH® are calibrated at least twice prior to shipping to ensure their being within specifications. The calibrated amperage of each SIC TECH® is marked on the carton and collar end of each SIC TECH®. When installing, arrange SIC TECH® with amperage values as close to each other as available. Longer service life will be obtained when series connected SIC TECH® are matched in resistance. SIC TECH® are shipped as closely matched as possible.

ELECTRICAL CONNECTIONS

Held in place by clamps, braided aluminum terminal straps in 25, 50, 100, and 200 ampere ratings, are available in two primary styles: Single Loop (for connecting binding post to element) and Double Loop (for connecting element to element). Post to Post straps are also available. Detailed descriptions and part numbers can be found in our accessory literature.

TERMINAL CLAMPS

There are two type of clamps: Type “M” (derives its name from its similarity to the letter M) and Type “T” (used with a tool). Please refer to our accessory literature for a detailed part number and description.