Maximizing Power Utilization
The ideal induction melting system pours the most metal at the lowest level of kilowatt demand.
Michael Nutt, Inductotherm Corp., Rancocas, N.J.
(Click here to see the story as it appears in the February issue of Modern Casting.)
Energy consumption in a metalcasting facility’s melt department isn’t the only consideration when it comes to cutting costs. Still, in the large majority of facilities, energy offers the greatest potential for savings when compared to labor, melt loss and maintenance. The melting department consumes more energy than any other operation, so it is a natural starting point when looking to reduce consumption and minimize demand costs.
The key factors when examining production versus demand include efficiency, performance, power supply technology and power utilization, which includes equipment configuration and type, operational practices, melt preparation and charging and pouring processes.
Efficiency: In recent years, significant improvements have been made in induction power supply efficiency. Today’s solid state power supplies are achieving efficiencies around 98%, which is substantially higher than earlier solid state or hard state technology. Metalcasters should review the efficiency of an existing melt system to see if substantial gains can be achieved via newer technologies.
Batch melting, where the furnace is emptied after the metal has reached the proper temperature, is typically 7% more efficient than “heel” melting, where a certain amount of metal is kept in the furnace after melting. This difference results from improved coil efficiency during the early stages of the batch melt cycle.
Melt System Performance: To maximize production for paid demand, the melt system must be able to pull full power throughout the entire melt cycle. True batch melting systems should be designed to pull full power from the initial charge until the target temperature is reached. Some melt systems may not be able to reach full power in the early stage of the melt, meaning consumption lags below the demand level.
Power Supply Utilization: Power supply utilization is the ratio of melt time to the overall total melt cycle time, which includes the melt time and the time required for tasks such as temperature measurements, slag removal, pouring and initial recharging of the furnace. (This is also referred to as the off time or hold time portion of the melt cycle.) In Fig. 1a, the melt time of one system is 40 minutes and the time required for the off-time functions is 22 minutes. The overall melt cycle time is 62 minutes, meaning the power supply utilization is 65% (40 divided by 62). In Fig. 1b, the melt time of the second system is 60 minutes and the off time is only 14 minutes, meaning the overall melt cycle time is 74 minutes. This example has a power supply utilization of 81%. Being more efficient with off-time functions can increase utilization.
Five Hypothetical Cases
Five hypothetical melt systems, all with varying power demands and capabilities, are shown in Fig.s 2a-2e to illustrate how the highest rate of production versus demand can be achieved by maximizing efficiency, melt system performance and utilization.
The first three examples are heel melting operations with differing configurations and capabilities. System 1 is a one-on-one mains frequency system pouring 6.5 tons per hour with a utilization rate of 65%. System 2 also is a mains frequency heel melting system, but it butterflies between two furnaces allowing it to increase utilization and decrease demand.
System 3 is a medium frequency heel melting operation. Medium frequency systems allow for smaller capacity furnaces, which reduce holding energy and direct more energy into the melt. System 3 operates at a total kW demand of 4,000. With a melt rate of 8.6 tons per hour, using a power utilization of 75%, it is able to achieve a pour rate of 6.5 tons per hour. The power density is 400 kW per ton, substantially higher than Systems 1 and 2.
System 4 is a medium frequency batch melting system. The inherent efficiencies in batch melting reduces demand while maintaining similar production. The higher efficiency combined with the higher utilization offered by the multi-output power system reduces demand to 3,000 kW. Medium frequency increases the power density and reduces the size of the furnaces. This is a batch butterfly operation, where one furnace is melting while the other is either off or at hold for melt preparation and pouring. Once the first furnace has completed its melt cycle, the melt process begins on the second furnace resulting in a power supply utilization of 95%. Compared to the previous examples, System 4 substantially reduces demand while still maintaining a similar pour rate.
System 5 is another medium frequency batch melting operation, but it features a multi-output system powering three furnaces simultaneously. Such a system can be used with one furnace melting, the second in melt preparation, and the third furnace pouring. This increases the time allowed for pouring and other melt cycle functions such as slagging, chemistry analysis and charging. Similar to System 4, this system maintains demand at 3,000 kW, but has a power supply utilization of 100%. It can pour 6.5 tons per hour with an increased power density up to 750 kW per ton.
The five different melting systems all produce between six and seven tons per hour. However, as seen in Fig. 3, System 1’s kW demand was 40% higher than System 5 because of decreased utilization, efficiency and performance. As power utilization increases, the kW demand required for the same amount of production decreases, leading to cost savings. In this case, because System 5 uses 40% less demand, costs will decrease by 40%.
Another way to view this is by examining the cost per ton poured. Fig. 4 shows the paid demand divided by the amount of metal poured. With a hypothetical value of $10/kW of demand, System 1 paid $22.89 per ton while Systems 4 and 5 paid near $14/ton. It’s clear the batch melting systems have a substantial competitive advantage over the other systems using older or less efficient technology.
Other areas within a melt system can be considered to reduce demand and optimize an operation. All of these areas can help increase the utilization of your melt system:
Routine Maintenance: Check and recheck your system to optimize performance.
Power Transmission Controls: Maintain power transmission systems, including clean and secure terminations and connections and properly configured furnace power leads.
Furnace Covers: Keep the furnace lid closed whenever possible throughout the melt cycle to reduce radiant heat loss.
Clean & Dry Scrap: Use clean and dry scrap to reduce energy consumption. Materials such as dust, dirt, rust and slag absorb energy and decrease melting efficiency.
Proper Charging/Filling: Don’t over charge or overflow the furnace. Furnaces are most efficient when operated at their design capacity.
Proper Lining Dimensions: Maintain lining dimensions per OEM specifications. Although the thick lining may reduce the lining cost per ton, it may not outweigh the cost of operating a less efficient furnace.
Control Fume Collection Flow: Ensure the fume collection system is effective without taking energy from the furnace and increasing thermal heat loss.
Heat Recovery Systems: Utilize heat recovery systems, such as air handlers, to put energy back into the facility to recirculate warm air.
Maximizing the melt system’s production at its lowest level of demand is an integral part of an overall energy management program. Optimizing melt equipment utilization will minimize demand and maximize tons poured. New induction melting technology offers increased performance, higher efficiency and enhanced flexibility to achieve higher power utilization and lower cost of operation.
This article is based on Michael Nutt’s presentation, “Maximizing Melt System Production at the Lowest Power Demand,” from the 2014 AFS Metalcasting Congress.