Utilization Is Everything
Want to improve the energy efficiency of your melting furnace without spending any money? Improve your utilization! While you may want to say it is sales’ fault for low utilization or that you need state-of-the-art equipment to improve your energy efficiency, you can make many adjustments with your current sales and equipment.
Utilization is the percent of actual production versus the capacity of the furnace melting continuously (% utilization = actual / capacity). No one can melt all the time. We need to charge, cast, check chemistry, skim, and perform maintenance. Other downtimes include shift changes, lunch, and waiting for charges or for the next process. Finally, you may not have sales to fill your furnace. All this reduces utilization. In a study of 147 aluminum melting furnaces, the median utilization was a mere 39% (“Current State of Aluminum Melting and Holding Furnaces in Industry,” Cynthia Belt, JOM, Vol. 67, Number 11 (2015), pp. 2,690-2,695.)
Lower utilization directly affects the energy intensity (Btu/lb. or kWh/t) of a furnace. Figure 1 shows energy intensity versus percent utilization for cold-air aluminum melting furnaces. Higher utilization improves energy intensity. Ideally, utilization should be closer to 80%. At low levels of utilization, the effect is highly significant.
Every minute you are not melting reduces your utilization. You can improve your process in many ways, such as:
Reduce furnace downtime—Study your furnace to find out how to reduce non-melt time. For instance:
• Reduce charge time by preparing charge loads.
• Optimize the charge cycle. Instead of large charges, should you feed more frequently with smaller loads or continuously with a conveyor feed?
• Speed up the opening and closing time of the door. If you can change the time from 2 minutes to 20 seconds and you open and close your door twice an hour, you would save 2.7 hours per day.
• Increase casting speed with better troughs. Wider troughs or steeper angles can increase the pounds per minute.
Reduce process downtime—Examine process downtime and address the biggest issues. For instance:
• Reduce charging delays. We saw one instance where a tank for another process was installed in the normal traffic pattern to load the furnace. Operators could still load the furnace but it took over two minutes longer. This added 1 1/2 hours per day and was left this way for years.
• In one plant, casting stopped two hours before a shift change so operators “could get cleaned up.” That is four hours of downtime per day.
• Reduce the time needed to get chemistry results. Is the spectrometer room too far away? Is the spectrometer process optimized to start and complete quickly?
• Reduce chemistry rework with better alloying techniques or programming.
• Sometimes it is the next step in the process that causes problems. Delays in the casting department can feed back to the melting furnace.
Reduce maintenance downtime—Track maintenance problems and address the underlying causes. Examples:
• We’ve seen plants where burners flame out regularly. This shouldn’t be normal. Not only does this cause downtime but it may mean a poor air-to-fuel ratio. Tune the burners and replace defective parts.
• Unplanned refractory work can cause massive downtime when refractory materials and labor are not ready. The run-to-failure strategy only works if replacements can be made quickly and stock is on hand. In the case of furnaces, it causes production delays and may cause a delay in product delivery to your customer. Inspect refractory and plan maintenance.
• Excessive mobile equipment downtime can delay charging. Sometimes the answer is better maintenance, and sometimes you need additional spares.
• Metal pumps can be difficult to maintain. Make sure you have people trained to do this maintenance and spare parts are on hand.
Increase melt rate—During the time you are melting, make sure you are melting at the full capacity of the furnace. Watch for inaccurate burner tuning, excessive heat loss in furnace openings, and poor furnace pressure.
Shut down extra furnaces—If you have four furnaces but only need three furnaces for your production needs, shut down a furnace. Don’t use the excuse, “But we would have to work harder” to stop you. You have to work smarter. Four furnaces at 60% utilization versus three furnaces at 80% will save you approximately 12% in energy. Note: Don’t shock your furnace refractory by bringing a furnace up and down frequently but plan for your needs.
Optimize idling—If you aren’t producing metal 24/7 but your furnace needs to stay on, work on optimizing the idling process. You can run tests by measuring gas flow during downtime. Some things to try are closing all furnace openings, reducing high-fire settings, reducing temperature setpoints, and slowing down pumps or metal movement.
Work to increase utilization on your furnaces and you will see the improvement in energy efficiency.
The authors of this column can be contacted at: Cindy Belt (email@example.com); Robert Eppich (firstname.lastname@example.org); Brian Reinke (email@example.com).
Click here to see this story as it appears in the September 2018 issue of Modern Casting