If there is one constant in today's metal casting industry it is the never ending demand to do more with less. Increase production, improve efficiency, enhance quality and at the same time, of course, reduce lead time, trim maintenance expenses and lower the cost. It should be no surprise then, as production demand increases at metal casting facilities across North America and around the world that we receive frequent inquires for methods to increase melt rates in existing furnaces. In many cases the first question we hear is; 'can we simply apply more power to the furnace?'. While this may appear to be a quick fix, a little more thought is required.
As one of the leading manufacturers of aluminum melting and holding systems, Thermtronix® has the advantage of an exceptional depth of knowledge in the application of electric resistance technology for aluminum melting furnaces. To the benefit of our customers we have applied both our practical application experience and technical knowledge to produce superior quality furnaces that are found at leading foundries and die casting facilities worldwide.
In the application of electric resistance melting with silicon carbide crucibles there is a direct relationship between the physical size of the crucible and the amount electrical power that may be efficiently applied to the crucible. Thermtronix® considers the current practical maximum size for a silicon carbide crucible to be approximately 3,000 lb. of aluminum capacity. The principal reasons for this size limitation are handling and shipping restraints, physical integrity and reasonable availability from multiple sources. When higher kilowatt levels are applied to an electric resistance furnace, with no regard to crucible size, they often do more for utility company profits, via higher demand charges, than they do for the melt rate. Higher heating element temperatures, on the other hand, can have a dramatic effect on the melt rate while actually reducing demand and overall energy usage.
Think about it! Thermtronix® could easily apply five-hundred kilowatts on a one ton electric resistance crucible furnace if we chose to do so. What would this do for our customers? It would shorten the time it takes to raise the heating elements in the furnace to their maximum temperature. Big deal! With 100 kilowatts applied to a one ton furnace we can bring the chamber temperature on our furnace from a holding level of 1350°F to a high limit of 2000°F in about ninety seconds. This number of course, will vary a great deal depending upon how full the crucible is, the age of the crucible, the temperature of the metal inside the crucible, how clean the crucible is, etc.. With 500 kilowatts applied to the same furnace we can raise that temperature from 1350°F to a high limit of 2000°F in about twenty seconds. The result is a nearly negligible affect on the melt rate, a dramatic increase in utility company demand charges and a substantial reduction in heating element life.
The melt rate for any electric resistance crucible style furnace is limited by the surface area of the crucible, the age and cleanliness of the crucible and, most importantly, the temperature differential between the heating chamber and the molten metal temperature inside the crucible. Once you reach the limit of a crucible to efficiently absorb energy at a given temperature differential, no amount of additional energy will increase the melt rate. Adding more kilowatts will only increase your electric utility demand charge and shorten the useful life of the heating elements. A thorough analysis of these operating characteristics is what lead to the development of the Thermtronix® PowerGuard™ heating element protection system. By monitoring the actual surface temperature of the heating elements with the PowerGuard™ system our customers achieve the maximum practical melt rate while protecting the heating elements from over temperature conditions and increasing their useful life.
Additionally, our experience has shown, what most production supervisors have seen at their own facilities; there is a substantial loss of crucible conductivity during the first ten to thirty days of operation. This reduction in crucible conductivity continues through the useful life of the crucible, but at a much slower rate than during the initial use period. This phenomenon occurs, of course, due to the fact that both the inside and outside walls of a new crucible are completely free of any oxide buildup, slag, or other inhibitors to thermal conductivity. Further, a new crucible has not been exposed to the high operating temperatures of the furnace which naturally, over time, breakdown the carbon and other components of a crucible that affect both it's mechanical strength and thermal conductivity. Consequently, it is possible to demonstrate an increase in melt rate through the use of additional kilowatts during the initial startup of a new crucible. However, this advantage begins to diminish in a matter of hours and is completely lost in the first ten to thirty days of normal use.
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