Type w sic heating element

TYPE W ELEMENT

Made of high density recrystallized silicon carbide, these multiple leg SIC TECH® use the same hot zone and cold end material as the TYPE I SIC TECH® and offer a configuration that allows the electrical connections to be made at one end. The low resistance terminal ends allow for cool operation. The LRE (low resistance ends) have a high (15:1) electrical resistance ratio to the hot zone section. The first several inches of the cold ends are metallized with aluminum to provide a low electrical resistance contact surface. The electrical connections are made using flat aluminum braids held in compression to the circumference by a stainless steel spring clamp. W type SIC TECH® are described by giving the overall length, the heating section length, the cold end length and the diameter. As an example - W 23.5 x 12 x 9.5 x 1 is a SIC TECH® 23.5" overall with a 12" hot zone, 9.5" cold ends and 1" in diameter (W 597 x 305 x 241 x 25 is a SIC TECH® 597 mm overall with a 305 mm hot zone, 241 mm cold ends and 25 mm diameter).

MOUNTING

There are no restrictions on the mounting positions of SIC TECH®, although the horizontal and vertical positions are common. The most common is vertical with the SIC TECH® being hung from the cold ends. When these SIC TECH® are mounted horizontally the bridge should be supported. The legs should be positioned in the same horizontal plane, not on edge. If the legs were to be positioned in a vertical plane, it would be difficult to get proper support for each leg. The Y SIC TECH® should be mounted only vertically for the same reason. Extreme caution should be used when mounting to ensure that the SIC TECH® are not placed in tension. There should be adequate freedom to allow for the furnace and SIC TECH® to expand and contract independently.

The support hardware, included with each shipment, consists of a stainless steel washer for each leg and two stainless steel cotter pins (three for the Y SIC TECH®), as shown in Figure 3 and listed in Table B. Support hardware and support holes are not included for SIC TECH® to be mounted horizontally.

SIC TECH® should have their heating sections in the furnace chamber so that no portion of the heating section extends into the furnace wall or refractory plug.

SIC TECH® should not be placed closer than two SIC TECH® diameters to each other or one and one half SIC TECH® diameters to a wall or other reflecting body. If the SIC TECH® is not able to dissipate heat energy equally in all directions, it may cause local overheating and possible failure. The formula for computing the recommended SIC TECH® spacing to obtain an even temperature gradient on the product being heated is shown in Figure 4.

SPECIFICATIONS

OPERATING TEMPERATURES

All multiple leg SIC TECH® can operate at furnace temperatures up to 2600 °F (1425°C). The hot zone material is rated to furnace temperatures up to 3100°F (1700°C). The restriction on the temperature relates to the terminal or cold end portion of the SIC TECH®. If temperatures higher than 2600°F (1425°C) are required, the cold end section should be protected from the full furnace chamber temperatures.

The maximum operating temperature should be reduced for non-air applications. Only argon and helium can be used without any reduction of maximum operating temperatures. Reducing atmospheres – hydrogen or disassociated ammonia, particularly with low dew-points remove the protective silicon oxide protection that naturally forms on silicon carbide. In these atmospheres the chamber control temperature can not exceed 2500°F (1370°C).

Nitrogen atmosphere applications are limited to 2500°F (1370°C) and 20 to 30 watts per square inch (3.1 to 4.6 watts per square centimeter) maximum watt loading. Too high a surface temperature will result in a silicon nitride reaction. A thermally insulating layer forms around the SIC TECH® resulting in very high surface temperatures which damage the SIC TECH®.

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

W 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.

REFRACTORY PLUGS

If refractory plugs are used, multiple leg SIC TECH® can be installed from outside the furnace. A plug works the same as a large removable refractory brick. ECONOVA can manufacture cast refractory plugs. If both the SIC TECH® and the plugs are supplied, the fit between the two are guaranteed to be correct. Unless otherwise specified the material used is a 2800ºF (1535ºC) 145 pound density cast refractory. A one inch (25 mm) cross section flange is used to support the weight of the plug and SIC TECH®. The plugs are available in both rectangular and cylindrical cross sections, see Figure 5.

BRIDGE CONSTRUCTION

The bridge construction of the multiple leg SIC TECH® is unique. Unlike others available in the market, the SIC TECH® bridge is flush, not extending beyond the hot zone diameter. Custom bridge widths are available.

Mechanical failures have historically been most frequent at or just above the bridge. The SIC TECH® construction addresses this by eliminating any welds in this area. The hot zone material extends to the very end of the element. The bridge is two leg diameters high and is not self-heating.