SILICON CARBIDE HEATING

SIC RESISTORS FOR INDUSTRIAL ELECTRIC FURNACES

GENERAL DESCRIPTION

SIM S.r.l. SiC heating elements are silicon carbide
heating elements. SiC 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. There are two
types of SIM S.r.l. SiC:
1) The cold ends are impregnated with silicon
metal — referred to as one piece, and
2) low resistance cold ends which are furnace
welded to the hot zone — referred to as a three
piece or LRE (Low Resistance End) type.
This lower electrical resistance cold end causes
them to operate at a lower temperature. The extremities
of the elements are metallized with aluminum
to provide a low resistance contact surface to
which the electrical connections are made using
braided aluminum straps.
SIM S.r.l. SiC resistors are described by giving the
overall length, the heating section length, and the
diameter. As an example, RI 43 x 24 x 1 is a SIM
S.r.l. 43” overall with a 24” hot zone, and 1” in
diameter.

 

SUPERIOR PERFORMANCE

SIM S.r.l. SiC resistors will give you superior performance
due to their high density — approximately
2.4 gms/cc. This gives the SIM S.r.l. SiC
resistors very slow aging characteristics and additional
strength.

 

INTERCHANGEABILITY

SIM S.r.l. SiC resistors are interchangeable with all
silicon carbide heating elements manufactured in
Europe as well as higher resistance heating elements
manufactured for the Asian and United States
markets. It is important to provide the nominal
electrical resistance when ordering to SIM S.r.l.
Please contact us, before attempting to substitute.

 

SIZES AVAILABLE

SIM S.r.l. SiC resistors can be manufactured in any
length up to 228 inches (5800mm). The maximum
hot zone length is 166 inches (4216mm).

 

OPERATING TEMPERATURES

In an air or inert atmosphere of argon or helium
the one SiC piece can be operated at furnace control
temperatures up to 3100°F (1700°C), the three
piece SIM S.r.l. up to 2600°F (1425°C). There is a
protective coating of silicon dioxide on the silicon
carbide. Hydrogen reduces this coating and causes
the SiC resistor to deteriorate. Very dry or very
wet hydrogen is detrimental to long service life.
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
surface watt loading. Too high of a surface
temperature will result in a silicon nitride reaction.
A thermally insulative layer forms around the
SiC resistor resulting in very high surface temperatures
which damage the resistor.

 

ELECTRICAL FEATURES

The silicon carbide resistor of SIM S.r.l. 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 SiC resistor hot zone is a self bonded silicon
carbide. The lattice structure or bonds that hold
the silicon carbide grain together are formed by
recrystallizing the silicon carbide at very high
temperatures. SIM S.r.l. are manufactured of green
silicon carbide that is classed as an excess electron
type semiconductor.
The electrical resistance of a SiC resistor is difficult
to measure at room temperature due to minor impurities,
self heating, and contact resistance. Also
the green silicon carbide has a negative resistance
temperature characteristic from room temperature to approximately 1200°F (650ºC). It turns positive
at this point and remains so throughout its normal
operating temperature range.
The nominal SiC resistor resistance is measured
at the calibrating temperature of 1960ºF (1071°C).
The nominal resistance values of SiC resistors in
ohms per unit of length are shown in Table A.

 

ELECTRICAL LOADING

SiC resistors are not sized to a specific wattage
output like metallic heating elements. The amount
of energy that a SiC resistor is capable of converting
from electrical to heat energy depends on the
ambient furnace temperature and atmosphere in
which the SiC resistor is operating.
When calculating the wattage capabilities of a
SiC resistor, the unit of watts output per unit of
radiating surface area is used. Figure 1 shows the
recommended watt loading for a square inch or
square centimeter of radiating surface as a function
of furnace temperature.
To determine the recommended wattage capabilities
of the SiC resistor start with Figure 1, knowing
the furnace temperature and atmosphere in which
the SiC resistor will be operated. Follow the temperature
line until you intersect the heavy black
line (choosing the appropriate line according to
the atmosphere in which the SiC resistor will be
operating). Read the loading in watts per square
unit of radiating surface that can be applied to the
SiC resistor. To find the total amount of power one
SiC resistor could supply under these conditions,
multiply this value by the radiating surface of the
SiC resistor. The radiating surface area is calculated
by multiplying the diameter times the hot zone
length times pi (3.14).

Example:
At 2750°F the SiC resistor could be loaded to 35
watts per square inch. Therefore, a resistor with
10 square inches of radiating surface could supply
350 watts, whereas a resistor with 200 square inches
of radiating surface could supply 7000 watts.
(At 1500°C the SiC could be loaded to 6 watts
per square centimeter. Therefore, a SiC with 100
square centimeters of radiating surface could supply
600 watts, whereas a SiC with 2000 square
centimeters of radiating surface could supply
12,000 watts).

 

EXAMPLE OF RADIATING AREA:

The RI 43 x 24 x 1 has a hot zone length of 24 inches
on a diameter of 1 inch. The radiating surface
area is 24 x 1 x 3.14, or 75.4 square inches.
(The RI 1092 x 610 x 25 has a hot zone length of
610mm and a diameter of 25mm. The radiating
surface area is 610 x 25 x 3.14 or 47,885 square
mm, converted to centimeters is 478 square centimeters).

 

POWER SUPPLY

In the previous paragraph we explained how to
calculate the recommended wattage output of the
SiC resistors.
Now we shall explain how to compute the electrical
requirements to provide the recommended
power.
Knowing the wattage output and the resistance of
the SiC resistors, 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 SiC in watts, R = resistance
of the SIM SiC in ohms). The resistance of
the SiC. can be calculated using the values found
in Table A.
When solving for E, you would obtain the voltage
required on a nominal SiC resistor to provide the
wattage output desired.
Example: A SiC resistor RI 43 x 24 x 1 has a resistance
of 1.21 ohms and 75 square inches of
radiating surface. Loading to 40 watts per square
inch, this SiC could provide 3000 watts. To find
the nominal voltage solve for E.

E = √ (W x R)
E = √ (3000 x 1.21)
E = 60 volts

SiC resistors may be connected in parallel, series,
or combination thereof. Parallel connections are
preferred.
In a parallel arrangement the voltage across all the
SiC 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 SiC resistors in the parallel circuit
with the lowest resistance will supply more heat
energy and therefore operate at a higher temperature.
This higher SiC temperature will cause it to
gradually increase in resistance until all the SIM
S.r.l. have the same resistance. At this time the SiC
resistors 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
SiC resistors the following formula may be used:
Rn = R x S ÷ P (Rn = network resistance, R = resistance
of SiC, S = number of SiC connected in a
series, P = number of parallel circuits).
Example: Eight SiC RI 43 x 24 x 1 (R = 1.21 ohms)
connected 2 in series (S = 2) and 4 parallel groups (P = 4).

Rn = R x S ÷ P
Rn = 1.21 x 2 ÷ 4
Rn = 0.6 ohms

To compute the nominal voltage required to power
a set of SiC, we shall use a combination of the
formulas used in the two previous examples. En
= √ (Wt x Rn), (En = nominal network voltage, Rn
= network resistance, Wt = total wattage output).
Example: Eight SiC RI 43 x 24 x 1 (R= 1.21 ohms)
connected 2 in series, 4 parallel groups. Each SiC
provides 3000 watts. Wt = 8 x 3000 = 24,000
watts. Rn = 0.60 ohms.

En = √ (Wt x Rn)
En = √ (24,000 x 0.6)
En = 120 volts

The resistance of SiC resistors 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.
SiC resistors can be used directly on the line (fixed
voltages) at temperatures up to 2400°F (1315°C).
To compensate for the reduced output as the SiC
resistors 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
24,000 watts after all heat losses and load factors
have been considered. Increasing this 24,000
by 25% to 50% gives a wattage requirement of
between 30,000 and 36,000 watts.
By taking another look at the previous examples
it can be seen that 10 SiC RI 43 x 24 x 1 connected
two in series, five parallel groups on 120
volts would supply the 30,000 watts. If 12 SiC of
the same size were used, the output would be
36,000 watts. Twelve SiC connected four in series
per phase 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 SiC resistors
are new they will only be powered for 24/30 or 24/36 of an hour. As the SiC 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 24,000 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 2400ºF
(1315ºC) a device for increasing the voltage to the SIM S.r.l. 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 SiC resistors
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 SiC increase in resistance, a voltage range is required that
will compensate for a 100% increase in the SIM S.r.l. resistance. The following formula may be used
to calculate 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 SiC, 1.5 = minimum margin to accommodate the doubling of
the SiC resistance and the +20% resistance tolerance). A higher value will offer slightly longer usable
service life.
Example: The transformer is rated at 24 KVA and has a computed nominal full load voltage of 120 volts.

(Rn = 0.6, Wt = 24,000).
Emax = √ (Wt x Rn) x 1.5
Emax = √ (24,000 x 0.6) x 1.5
Emax = 180 volts

 

The nominal full load voltage and maximum voltage have been computed. When specifying the transformer
the nominal full load voltage is usually reduced by 5% to 10% to allow for the minus 20%
resistance tolerance of the SiC resistor. Also, lower voltage taps are usually provided for idling and slow
heatups.
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.

 

EASE OF REPLACEMENT

SiC resistors can be replaced while the furnace is at operating temperature. The power to the SiC
resistors being changed should be shut off, the spring clips and aluminum braid released, and the old
SiC removed. The new SiC 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 resistors 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 radiating surface)

3 – Atmosphere

4 – Type of operation (continuous or intermittent)

5 – Operating and maintenance techniques

 

MOUNTING

There are no restrictions on the mounting positions of SiC resistors, although the horizontal and vertical
positions are the more common. Extreme caution should be used when mounting to ensure that the SiC
are not placed in tension. There should be adequate freedom to allow for the furnace and SiC resistors
expand and contract independently.
When mounting SiC resistors vertically they must be supported on the lower end by electrically insulated supports.
SiC resistors 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 1/2 inch
(13mm) deep is sometimes located on each interior wall where the SiC passes through. This allows the
hot zone to radiate properly and helps maintain a uniform temperature in the kiln.

 

FURNACE HEATING CHAMBER

The furnace heated chamber dimension, which the SiC spans, can be the same as the hot zone length
of the SiC as shown by the SiC resistor under the load in Figure 3. Alternately the furnace heating chamber
dimension, which the SiC spans, can be one inch (25mm) less than the effective heating length of
the SiC. In this case there must be a 45° conical recess in the furnace wall as shown in Figure 3 for the
SiC resistor above the load. Recommended terminal hole diameters for various refractory walls and SiC
sizes are shown in Table B.
SIM S.r.l. should not be placed closer than two SIM S.r.l. diameters to each other or one and one half
SIM S.r.l. diameters to a wall or other reflecting body. If the SIM S.r.l. is not able to dissipate heat energy
equally in all directions, it may cause local overheating and possible failure.

 

SPECIFICHE E CORRISPONDENZE

 

SPECIFICATIONS AND MATCHING

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

 

AVAILABILITY

SiC resistors can be shipped from stock, or two to three weeks after receipt of an order.

 

CUSTOM CONFIGURATIONS
Special sizes and shapes are available. Cold ends can be different lengths. This, for example, would
be applicable for furnaces with arched roofs that require longer cold ends through the roof and shorter
through the floor.
Another modification is a two-temperature hot zone. This, for example, would be helpful to get additional heat energy into the lower, more densely loaded tunnel kiln. While this special modified hot
zone will not create a specific temperature differential, it does offer a convenient way to get more heat
energy into a specific area of a furnace.
The right angle (RA) shown here, has all the electrical characteristics of the RI. The cold ends are attached perpendicular to the hot zone. The RA is normally installed with the cold end through the roof of the furnace.

 

OUR COMPLETE RANGE OF HEATING ELEMENTS IN SILICON CARBIDE

Three Piece Straight Heating Elements SIC Rods

The Three Piece Straight heating elements SIC Rods have low resistance welded (LRE) cold ends.
These ends are cooler any other one piece cold ends. The maximum featured temperature of the rods
is 1550°C and for better energy efficiency, heat is concentrated in the furnace, not on the ends of the
rods. With the highest Hot: Cold ratio of 1:40, these rods make one of the most efficient Silicon Carbide
Heaters.

Reaction Bonded Single Spiraled Heating Elements

The Reaction Bonded Silicon Carbide is used in the fabrication of SPIRAL Silicon Carbide heating Elements. These are available in different sizes ranging from 12mm to 50mm in diameters and 2250mm
in length. They are spiraled heating elements, shaped in a thin wall and finely grained form of reaction bonded silicon carbide. These can withstand high electrical loads, rapid heating & cooling cycles and thermal shocks. To suit various heating processes, the elements are available in different forms.

 

Reaction Bonded Double Spiraled Heating Elements

The Reaction Bonded Double Spiraled Heating Elements possess all the terminal
connections at one end with the density of 3.3 Gms./cc ? 3.4 Gms./cc.
These elements are ideal in the conditions, where the furnace access is limited
to any one plane.

U-Shaped Heating Elements SIC Rods

The ‘U’ Shaped heating elements are
the SIC rods that are joined from both
the terminals in a form of thickened
bridge. These rods are ideal for the
conditions where one single rod is not
able to span the heating chamber.
Further, these rods are also good
for radiant tube systems and
drop through designs.

Drum (or dumbbell) shaped Silicon Carbide Heating Elements tamburo

The enlarged cold ends of the heating elements give them the name of Dumbbell Shaped Silicon
Carbide Heating Elements. The cold ends of the elements lower down the electrical resistance and
helps in increasing the cold end cross
section that is turn lowers the cold end
operating temperature.
In the modern dumbbell alpha rods,
the advanced technology is used for
keeping the ends of the terminals cool,
thus the over sized cold ends are no longer
required. Old style resistance ratio was 1:3.
New resistance ratio is 1:40 Maximum temperature is 1550 deg. C (For Dimensions, Resistance, etc. kindly contact us)