Type SPIRAL SINGLE element

TYPE SPIRAL SINGLE ELEMENT

The spiral SIC TECH® are made of special high-density reaction-bonded silicon carbide. A spiral slot in the hot zone reduces the cross sectional area. This provides the electrical resistance ratio to make the ends cool and the hot zone hot. Special cold ends may be welded to further enhance the resistance ratio. To provide a low resistance contact surface the extremities are metallized with aluminum. The electrical connections are made with flat aluminum braids and spring clamps. The terminal ends of the SE element are normally plugged.

Type SE SIC TECH® are described by giving the overall length, the heating section length, and the diameter. As an example, SE 36 x 14 x 1 is a spiral element 36" long with a 14" hot zone and 1" in diameter. In millimeters, a SE 914 x 356 x 25 is a spiral element 914 mm long with a 356 mm hot zone and 25 mm in 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

In an air or inert atmosphere of argon or helium the SE SIC TECH® can be operated at furnace temperatures up to 3000ºF (1650ºC). In reducing atmospheres the maximum operating temperature is 2500ºF (1371ºC). See Watt Loading Graph, Figure 1.

There is a protective coating of silicon dioxide on the silicon carbide. Hydrogen reduces this coating and causes the SIC TECH® to deteriorate. Very dry or very wet hydrogen is detrimental to long service life. Nitrogen atmosphere applications are limited to 2500ºF (1370ºC) and a maximum watt loading of 20 to 30 watts per square inch (3.1 to 4.6 watts per square centimeter). Too high of a surface temperature will result in silicon nitride formation. A thermally insulative layer forms around the SIC TECH® resulting in very high surface temperatures which damage the SIC TECH®.

ENGINEERING INFORMATION

HOT ZONE / COLD ZONE (ELECTRICAL RESISTANCE)

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

SE 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

SPECIFICATION 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 CONNECTORS

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

TERMINAL CLAMPS

There are two type of clamps: Type “M”, manual, (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.