In an air or inert atmosphere of argon or helium the SER and TSR Starbars can be operated at furnace temperatures up to 3000ºF (1650ºC). In reducing atmospheres, the maximum operating temperature is 2500ºF (1370ºC). See Recommended Maximum Watt Loading Graph, Figure 1.

There is a protective coating of silicon dioxide on the silicon carbide. Hydrogen reduces this coating and causes the Starbar to deteriorate. Nitrogen atmosphere applications are limited to 2500ºF (1370ºC) and 20 to 30 watts per square inch (3.1 to 4.6 watts per square centimeter) maximum watt loading. Higher surface temperature will result in silicon nitride formation. A thermally insulative layer forms around the Starbar resulting in very high surface temperatures which damage the Starbars.

At 2.85 gms/cc, this high-density low-porosity element is able to survive severe environments. The high density prevents the internal lattice structure from being oxidized. This element therefore has an extremely slow aging characteristic.

• The working length of the SER and TSR aluminum braid is 10 inch (254 mm).
• The end clip hole diameter for 50 amp braid is 9/32 inch (7 mm), for 100 amp braid and higher is 9/16 inch (14 mm).
• The amperage rating of the aluminum braid according to diameter is 1/2 inch (13 mm) to 1 inch (25 mm) diameter – 50 amps, 1-1/4 inch (32 mm) to 2-1/8 inch (54 mm) diameter – 100 amps, 2-3/4 inch (70 mm) diameter – 400 amps, TSR elements – 1-3/4 inch (44 mm) to 2-1/2 inch (62 mm)

The silicon carbide Starbar is a linear type resistance heater that converts electrical energy to heat energy – Joule’s Law W = I2 x R, (W = power in watts, I = current in amperes, R = resistance in ohms). The SER Starbar has a negative resistance

Starbars are not sized to a specific wattage output like metallic heating elements. The amount of energy that a Starbar is capable of converting from electrical to heat energy depends on the ambient furnace temperature and atmosphere in which the Starbar

Knowing the wattage output and the resistance of the Starbar you have two parts of an equation with three unknowns. This equation is E= √(W x R), (E = nominal full load voltage, W = rating of the Starbar in watts, R = resistance of the Starbar in ohms). The resistance of the Starbar can be calculated using the values found in Table B. Solving for E determines the voltage required on a nominal-resistance Starbar to provide the wattage output desired. This assumes a nominal resistance.

Example: A Starbar SER 24 x 16 x 1.25 has a resistance of 5.76 ohms and 63 square inches of radiating surface. Loading to 40 watts per square inch this Starbar could provide 2500 watts. To find the nominal voltage solve for E.
E = √(W x R)
E = √(2500 x 5.76)
E = 120 volts
Starbars may be connected in parallel, series, or any combination thereof. Parallel connections are preferred because if the resistance of one or more Starbars increases, its portion of the load will be reduced and the group will remain in balance.

In a parallel arrangement the voltage across all the Starbars is the same. In the formula W = E²÷R (W = watts, E = voltage, R = resistance) it can be seen that, the greater the resistance, the lower the wattage output. The Starbars in the parallel circuit with the lowest resistance will supply more heat energy and therefore operate at a higher temperature.

This higher Starbar temperature will cause it to gradually increase in resistance until all the Starbars have the same resistance. At this time all the Starbars should have approximately the same resistance values and surface temperatures and therefore remain in balance.

To compute the network resistance of a group of Starbars the following formula may be used: Rn = R x S ÷ P (Rn = network resistance, R = resistance of Starbar, S = number of Starbars connected in a series, P = number of parallel circuits).

Example: Eight Starbars SER 24 x 16 x 1.25 (R = 5.76 ohms) connected 2 in series (S = 2) and 4 parallel groups (P = 4).
Rn = R x S ÷ P
Rn = 5.76 x 2 ÷ 4
Rn = 2.88 ohms
To compute the nominal network voltage required to power a set of Starbars, a combination of the previous two formulas is used as follows: En = √(Wt x Rn), (En = nominal network voltage, Rn = network resistance, Wt = total wattage output).

Example: Eight Starbars SER 24 x 16 x 1.25 (R = 5.76 ohms) connected 2 in series, 4 parallel groups. Each Starbar provides 2500 watts. Wt = 8 x 2500 = 20,000 watts. Rn = 2.88 ohms.
En = √ (Wt x Rn)
En = √ (20,000 x 2.88)
En = 240 volts
The resistance of Starbars increases gradually during their useful life. Therefore some means of keeping the power input to the kiln or furnace at a level sufficiently high to maintain the desired temperature is required.

Historically, expensive voltage varying equipment such as multiple tap transformers or saturable reactors were recommended for all but the very low temperature applications.

Spiral Starbars can be used directly on the line (fixed voltages) at temperatures up to 2500ºF (1370ºC). To compensate for the reduced output as the Starbars gradually age or increase in resistance, the furnace or kiln is initially overpowered by 25% to 50%. This type of arrangement eliminates the expensive voltage varying equipment and has proven very satisfactory in many applications. It is not recommended when fine process temperature control is required.

Assume a furnace will require approximately 20,000 watts after all heat losses and load factors have been considered. Increasing this 20,000 by 25% to 50% gives a wattage requirement of between 25,000 and 30,000 watts.

By taking another look at the previous examples it can be seen that 10 Starbars SER 24 x 16 x 1.25 connected two in series, five parallel groups on 240 volts would supply 25,000 watts. If 12 Starbars of the same size Starbars were used, the output would be 30,000 watts.

Twelve Starbars connected four in series, per phase, in a delta, on 240 volts would make a balanced three phase 240 volt network.

The temperature of the kiln or furnace is controlled by an off-on controller. When the Starbars are new, they will only be powered for 20/25 or 20/30 of the time (the ratio of the power needed to the power available). As the Starbars increase in resistance, they will be on for a greater percentage of the time. When they have increased in resistance to a point at which they supply 30,000 watts, they will be on 100% of the time. A SCR (silicon controlled rectifier) or thyrister can also be used.

For applications where close temperature control is desired and/or for temperatures above 2500ºF (1370ºC), a device for increasing the voltage to the Starbars is required. There are several methods of providing this variable voltage source.

1. The multiple tap transformer is the most common because it is usually the least expensive. The secondary of the transformer is provided with taps which usually vary in number from 10 to 36. By carefully selecting the voltage taps, the correct voltage output to match the resistance of the Starbars over their complete useful life can be made.
2. Saturable reactors and induction regulators are used to provide a stepless voltage control. They are also sometimes used with multiple tap transformers.
3. Capacitor controls are used infrequently. They, of course, will tend to improve a power factor which makes their use desirable in some areas.
4. Silicon controlled rectifiers (SCR) have become quite popular with the advances in solid state devices.

To compensate for the reduced output as the Starbars increase in resistance, a voltage range is required that will compensate for a 100% increase in the Starbar’s resistance. The following formula may be used to figure Emax: Emax = √(Wt x Rn) x 1.5, (Emax = recommended maximum voltage required to compensate for increase in resistance due to aging and resistance tolerance, Wt = rating of transformer in watts, Rn = network resistance of the Starbars, 1.5 = minimum margin to accommodate the doubling of the Starbar network resistance and the plus 20% resistance tolerance). A higher value will offer slightly longer usable service life.

Example: A transformer is rated at 24 KVA and has a computed nominal full load voltage of 240 volts. (Rn = 2.88, Wt = 20,000 for 8 Starbars)
Emax = √(Wt x Rn) x 1.5
Emax = √(20,000 x 2.88) x 1.5
Emax = √(57,600) x 1.5
Emax = 360 volts
The nominal full load voltage and maximum voltage have been computed.

When specifying the transformer, the nominal full load voltage is usually reduced to allow for the minus 20% resistance tolerance of the Starbars and slow furnace heatup.

To calculate the minimum voltage, take 70% of the nominal voltage. For periodic applications, take 30% of the nominal full load voltage.

Auto transformers may be used if primary voltage is 230 volts or less. They should not be used in a three phase arrangement. Accepted practice limits the secondary voltage on all transformers to 300 volts. Above this, refractory voltage leakage becomes a problem.

When computing the size of the voltage steps between taps, a value of 5% of the nominal full load voltage is often used. When SCR or thyrister controls are used on the primary, fewer taps are required. For example, if 6 taps are used, the idling tap can be 0.7 x nominal voltage, then each consecutive tap would be 14% higher. For 8 taps, the idling tap would again be 0.7 x nominal voltage, each consecutive tap at 9.1% higher than the preceding.

Starbars 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 Starbar radiating surface
3. Atmosphere
4. Type of operation (continuous or intermittent)
5. Operating and maintenance techniques

Starbars should not be placed closer than two Starbar diameters to each other or one and one half Starbar diameter to a wall or other reflecting body. If the Starbar is not able to dissipate heat energy equally in all directions, it may overheat and possibly fail.

The formula for computing the recommended Starbar spacing to obtain an even temperature gradient on the product being heated is shown in Figure 3.

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

Special higher electrical resistance values are available. This is accomplished by machining a smaller spiral width. Therefore, very high resistance and/or long hot zone lengths are not practical. Lower than standard resistances are not recommended.