Starbar RR

STARBAR TYPE RR
SILICON CARBIDE HEATING ELEMENTS

GENERAL DESCRIPTION

The RR STARBAR is a resistance type silicon carbide heating element.
STARBARs are rod shaped or tubular depending on the diameter.
They have a central heating section referred to as a hot zone and two terminal sections called cold ends.

RR STARBARs are described by giving the overall length, the heating section length, and the diameter.
As an example, RR 43*24*1 is a STARBAR 43" overall with a 24" hot zone,and 1" in diameter.

SUPERIOR PERFORMANCE

STARBARs will give you superior performance due to their high density - approximately 2.4gms/cc. This gives the STARBAR very slow aging characteristics and additional strength.

SIZES AVAILABLE

STARBARs can be manufactured in any length up to 3300mm. The maximum hot zone length is 2500mm.

FURNACE HEATING CHAMBER

The furnace heated chamber dimension which the STARBAR spans can be the same as the hot zone length of the STARBAR as shown by the STARBARs under the load in figure 1. Alternately the furnace heating chamber dimension which the STARBAR spans can be 25mm less than the effective heating length of the STARBAR. In this case there must be a 45 degrees conical recess in the furnace wall as shown in figure 1 for the STARBARs above the load.

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 cause local overheating and possible failure. The formula for computing the recommended STARBAR spacing to obtain an even temperature gradient on the product being heated is shown in figure 1.

Figure 1, Recommended STARBAR Spacing

OPERATING TEMPERATURES

In an air or inert atmosphere of argon or helium the one piece STARBARs can be operated at furnace control temperatures up to 1700 °C, the three piece STARBAR up to 1425 °C.
In reducing atmospheres the maximum operating temperature is 1370 °C, see figure 2.

There is a protective coating of silicon dioxide on the silicon carbide. Hydrogen reduces this coating and causes the STARBAR to deteriorate. Very dry or very wet hydrogen is detrimental to long service life.

Figure 2, Recommended Maximum Watt Loading

ELECTRICAL CHARACTERISTICS

The silicon carbide STARBAR is a linear type resistance heater that converts electrical energy (Joule's Law W = I2R). W = power in watts, I = current in amperes, R = resistance in ohms.
The heating elements are often referred to as self-bonded, for the lattice structure or bonds that hold the elements together are formed by recrystallizing the silicon carbide at very high temperatures. The elements are manufactured of green silicon carbide which is classed as an excess electron type semiconductor.

The electrical resistance of a STARBAR is difficult to measure at room temperature due to minor impurities, self heating, and contact resistance.
Also the green silicon carbide has negative resistance temperature characteristic from room temperature to approximately 650 °C. It turns positive at this point and remains so throughout its normal operating temperature range. A typical resistance temperature characteristic of a STARBAR is shown in figure 2.

Nominal STARBAR resistance is measured at the calibrating temperature of 1071 °C.
Nominal resistance value of STARBARs in ohms per unit of length are shown in Table A.

TABLE A
RR STARBAR Dimensions				^**RR Electrical Resistance
Diameter		^*Maximum Overall Length		Ohms Hot Zone		Ohms Cold Ends
MM	Inch	MM	Inch	Ohms/MM	Ohms/Inch	Ohms/MM	Ohms/Inch
10	3/8	660	26	0.01372	0.3486	0.000686	0.01743
11	7/16	915	36	0.01009	0.2563	0.000505	0.01282
13	1/2	1090	42	0.00773	0.1963	0.000387	0.00982
16	5/8	1250	50	0.00497	0.1262	0.000248	0.00631
19	3/4	1575	62	0.00341	0.0865	0.000170	0.00433
25	1	1900	74	0.00197	0.0500	0.000098	0.00250
32	1-1/4	2210	86	0.00134	0.0343	0.000067	0.00171
35	1-3/8	2290	90	0.00106	0.0270	0.000053	0.00135
38	1-1/2	2340	92	0.00092	0.0234	0.000046	0.00117
44	1-3/4	2670	104	0.00065	0.0165	0.000032	0.00082
54	2-1/8	3300	130	0.00059	0.0150	0.000030	0.00075
^*Recommended overall length = 79% of listed maximum length
^**All resistance values are +/-20%

ELECTRICAL LOADING

STARBARs are not sized to a specific wattage output like many of the more common metallic heating elements. The amount of energy that a silicon carbide element is capable of converting from electrical to heat energy depends on the ambient furnace temperature and atmosphere in which the element is operating.

When calculating the wattage capabilities of STARBAR,the unit of watts output of one square millimeter of radiating element surface area is used. figure 2 shows the recommended watt loading for a square millimeter of radiating surface as a function of furnace temperature.

SERVICE LIFE

The I Squiared R heating elements increase gradually in resistance with use.
This characteristic of increasing in resistance is called aging.(figure 3) Aging is a function of the following :

Operating temperature
Electrical loading (usually expressed in watts per square millimeter of STARBAR radiating surface)
Atmosphere
Type of operation (continuous or intermittent)
Operating and maintenance techniques

Figure 3,

MOUNTING

There are no restrictions on the mounting positions of STARBARs, although the horizontal and vertical positions are the more common. Extreme caution should be used when mounting to ensure that the STARBARs are not placed in tension. There should be adequate freedom to allow for the furnace and STARBARsto expand and contract independently.

When mounting STARBARs vertically they must be supported on the lower end by electrically insulated supports.

STARBARs should have their heating sections centered in the furnace chamber so that no portion of the heating section extends into the furnace wall. A conical or truncated cone shaped recess 13mm deep is sometimes located on each interior wall where the STARBAR passes through. This allows the hot zone to radiate properly and helps maintain a uniform temperature in the kiln.

POWER SUPPLY

In the previous paragraph we explained how to calculate the recommended wattage output of the STARBAR. Now we shall explain how to compute the electrical requirements to provide the recommended power.

Knowing the wattage output and the resistance of the STARBAR you have two parts of an equation with three unknowns. This equation is
E = root of (W*R), (E = nominal full load voltage, W = rating of the STARBAR in watts, R = resistance of the STARBAR can be calculated using the values found in Table A.

When solving for E, you would obtain the voltage required on an nominal resistance STARBAR to provide the wattage output desired.

Example : A STARBAR RR 1100*600*25 has a resistance of 1.24 ohms and 478 square centimeters of radiating surface. Loading to 6.2 watts per square centimeters, this STARBAR could provide 2970 watts. To find the nominal voltage solve for E.

E = root of (W*R)

E = root of (2970*1.24)

E = 60 volts

STARBARs may be connected in parallel, series, or combination thereof. Parallel connections are preferred.

In a parallel arrangement the voltage across all the STARBARas 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 the STARBARs should all 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*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 RR 1100*600*25 (R=1.24 ohms) connected 2 in series (S=2) and 4 parallel groups(P=4).

Rn=R*S/P

Rn=1.24*2/4

Rn=0.62ohms

To compute the nominal voltage required to power a set of STARBARs,we shall use a combination of the formulas used in the two previous examples.
En=root of (Wt*Rn), (En=nominal network voltage, Rn=network resistance, Wt=total wattage output).

Example : Eight STARBARs RR 1100*600*25 (R=1.24 ohms) connected 2 in series, 4 parallel groups.
Each STARBAR provides 2970 watts. Wt=8*2970=23,760 watts. Rn=0.62 ohms.

En=root of (Wt*Rn)

En=root of (23,760*0.62)

En=121volts

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 temperatures is required.