Type W Silicon Carbide Heating Elements
Indicate which type of element you need.
Indicate the following dimensions identified in the element drawings above:
- Heated Length Diameter (d)
- Terminal Diameter (D)
- Heated Length (L1)
- Terminal Length (L2)
Note: Element diameters are normally specified in millimeters. Element dimensions may be specified in millimeters or inches.
Indicate any other critical dimensions or tolerances if needed.
Type W Elements are typically connected to a threephase line; therefore the resistance value for each leg is required. To determine the resistance on one leg, select the diameter from Table A, then:
1) Multiply the ohms/leg hot zone x hot zone length
2) Multiple the ohms/leg cold end x cold end length
3) Add the hot zone resistance to the cold end resistance.
As the Elements 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 substituted for the contactor.
For applications where close temperature control is desired and/or for temperatures above 1315°C a device for increasing the voltage to the Elements 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 Elements 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. Because of the poor power factor and other concerns regarding a large phase back, multiple tap transformers are recommended even when the control device is a SCR.
To compensate for the reduced output as the Elements increase in resistance, a voltage range is required that will compensate for a 100% increase in the Element 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 Elements, 1.5 = minimum margin to accommodate the doubling of the Element resistance and the plus 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 = √ (14,400) 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 to allow for the minus 20% resistance tolerance of the Elements and slow furnace heatup.
EASE OF REPLACEMENT
Elements can be replaced while the furnace is at operating temperature without danger of thermal shock damage. Because of the multiple leg configuration, these Elements are often installed through a refractory plug as shown in Figure 3, page 5. In this way, the entire plug and Element assembly can be removed from the outside of the furnace.
Elements 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 Element radiating surface
4. Type of operation (continuous or intermittent)
5. Operating and maintenance techniques
There are no restrictions on the mounting positions of Elements, although the horizontal and vertical positions are common. The most common is vertical with the Element being hung from the cold ends. When these Elements 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 Element should be mounted only vertically for the same reason. Extreme caution should be used when mounting to ensure that the Elements are not placed in tension. There should be adequate freedom to allow for the furnace and Elements 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 Element), as shown in Figure 3 and listed in Table B. Support hardware and support holes are not included for Elements to be mounted horizontally.
Elements 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.
Elements should not be placed closer than two Element diameters to each other or one and one half Element diameters to a wall or other reflecting body. If the Element 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 Element spacing to obtain an even temperature gradient on the product being heated is shown in Figure 4.
FURNACE HEATING CHAMBER
The furnace heated chamber dimension which the Element spans can be the same as the hot zone length of the Element as shown in Figure 4. Recommended terminal hole diameters for various refractory walls or plugs and Element sizes are shown in Table C, page 5.
SPECIFICATIONS AND MATCHING
Elements have a manufactured tolerance of plus or minus 20% on the nominal resistance. All Elements are calibrated at least twice prior to shipping to ensure their being within specifications. The calibrated amperage of each Element is marked on the carton and one cold end of each Element. When installing, arrange Elements with amperage values as close to each other as available. Longer service life will be obtained when series connected Elements are matched in resistance. Elements are shipped as closely matched as possible.
Elements can be shipped from stock, or two to three weeks after receipt of an order.