Gas-Fired Crucible Melting Improvements Highlight Cooperation Between Suppliers, Energy Providers
A case study at Eck Industries in Manitowoc, Wis., shows the fuel savings realized by the addition of a self-recuperative burner to crucible melting furnaces.
When looking at ways to streamline your business and make strides to cut costs and gain efficiency, sometimes it pays to ask for help. When Eck Industries of Manitowoc, Wis., asked for support, the result was a project that saved over 60% of the fuel required to melt and hold high-grade aluminum.
Eck Industries, which opened in 1948, is a long-established business. With World War II over, and the acceptance of aluminum as a viable metallic component in so many items of everyday life, Eck positioned themselves to take advantage of the needs for cast-aluminum parts. Early investors included not only the Eck family but also recognizable industrial partners from Harley-Davidson, Johnson Motors, West Bend outboard and Mercury Marine. They now produce many complex castings in almost all grades of aluminum. Over the years, the Eck family and management team have maintained control of the company, which now producea more than 8 million pounds of castings a year. In 2012, they were named Metal Caster of the Year by their peers and Modern Casting magazine.
In 2010, Eck was interested in exploring possibilities to save energy. They had seen information presented by Eclipse regarding a new type of burner that might have application in their plant. With interest generated in the new burner, Eck contacted Wisconsin Focus on Energy and their industry-specific arm, Cleantech Partners. Focus on Energy is the Wisconsin statewide energy-efficiency and renewable-resource program. It is funded by the state’s investor-owned energy utilities. They offer support through a network of partners, such as Cleantech, to analyze, test and ultimately fund research and improvement of energy-saving projects. DOE protocols for testing and verification are normally used.
Eck uses natural-gas-fired crucible furnaces to melt aluminum as the first step in the metal-casting process. Aluminum ingots and “returns” (scrap metal from earlier castings) are heated and then held in a molten state. The molten metal is poured into molds of various types and sizes to produce castings. Heat from the gas burner is transferred directly from the flame and flue products to a “crucible,” or holding pot. This crucible is contained in a refractory-lined furnace shell.
To transfer heat to the metal, the heat has to be contained. Heat is then driven into the crucible, which transfers that energy to the feed metal needing to be melted. Early designs of these furnaces are simply an injected gas burner firing into the chamber with exhaust gases exiting out of an open flue. Fuel efficiency in this style furnace was estimated at anywhere from 10-40%, depending on multiple factors including melt rate. The existing retrofitted units were open gas crucibles and are shown in Figure 1.
This project was designed to demonstrate the energy savings obtained through the separate and combined implementation of two measures:
1. Installing recuperative burners in furnaces that had been retrofitted with a new refractory (Fig. 2)
2. Placing covers on the furnaces (Fig. 3)
Testing was arranged to individually identify and quantify the reduction in gas use for each measure implemented during a typical melt process by metering melt process gas. The trials were conducted on a single gas-fired crucible furnace using the same operator for all tests. A new gas sub-meter was installed on this furnace prior to testing.
Working together with the customer, the project team collected and compared data on a retrofit furnace to determine the difference in gas usage attributable to the addition of new recuperative burners and covers for the crucible furnaces. The testing was conducted across a variety of melting processes, as they would typically be scheduled during normal operations. Gas usage was measured and recorded during a series of 16 test runs, some melting ingots only and some with both ingots and returns as the metal that was melted.
Eclipse TJSR Recuperative Burner
A recuperative burner uses the hot exhaust gases to preheat the incoming combustion air. In a crucible furnace, these exhaust gases are commonly about 2000°F (1093°C). The burner that was used in this demonstration is a 500,000-BTU/hour ThermJet Self-Recuperative (TJSR) burner, supplied by Eclipse, Inc. of Rockford, Ill. It is shown in Figure 2.
This burner is unique – instead of having a separate burner and recovery heat exchanger, they are combined into one unit. The design combines the burner and flue into the same unit as well. It extracts the flue gases and pulls them into the internal heat exchanger. Approximately half of the heat normally lost in exhaust gases is recovered into the combustion air. Instead of starting with approximately 80°F combustion air, now exhaust gases preheat the incoming air to approximately 1000°F (538°C). This saves the fuel normally needed to do this amount of combustion air heating.
In the Eclipse TJSR burner, the close proximity of the burner discharge and exhaust entrance encourage a portion of exhaust gases to be brought back into the flame path. This provides an additional opportunity to burn any carbon monoxide present that would normally be lost up the stack.
The retrofitted burner also reduces energy use by improving heat transfer between combustion gases and the furnace crucible. It does this by increasing the average temperature of the combustion gases and extending their time in the furnace. Despite a greater residence time, the burner velocity is about five times that of a normal burner. Combustion gas speeds of over 400 feet per second have been realized. The result is that the higher-velocity gases lead to improved heat transfer, which can reduce melt times. Also, crucible life can be improved because of more uniform heat delivery to the crucible, which minimizes thermal stress.
In the type of crucible furnace used in this demonstration, the air and fuel quantities are normally manually adjusted. This opens up the opportunity for error by either using too much or too little combustion air. The burner retrofit package includes recuperation as well as burner management, which automatically repeats the same air-fuel ratio at each firing rate. This feature reduces excess-air adjustment error that can lead to inefficiency.
Test Results and Conclusions
The overall testing procedure involved four trials with four test runs during each trial. The first set of trials was conducted on an original furnace to determine the baseline energy use prior to any implemented changes. After measured data was collected during a typical melt process, a cover was added to this furnace and testing was repeated under the same operating conditions. The energy savings provided by the addition of the cover was identified.
Next, the new refractory and recuperative burner were installed and testing commenced on the retrofitted furnace. The testing series was repeated to quantify the energy savings provided by the addition of the recuperative burner and the further value of placing a cover on the retrofitted furnace. All of the tests began with a hot crucible with 50 pounds of material in place and measured separate melts of ingots only and ingots plus returns. The same crucible was used for all testing, and the same individual performed all of the tests.
Meter readings were recorded at five intervals during the procedures. The readings were recorded in cubic feet of gas and converted to therms.
Assuming the $0.80 per therm cost for gas used in the grant-application calculations, the study concluded that Eck would save $258,250/year from implementing these measures to all 15 of their furnaces.
Many states have energy-advocacy groups tied to utility providers to provide technical help, testing and funding for projects like the one described in this article. It is the responsibility of the business owner to explore the help available in their geographic area. Furthermore, suppliers such as Eclipse are usually connected to these resources and stand ready to help bring energy savings to the manufacturing sector.
For more information: Contact Jim Roberts, central regional manager, Eclipse, 1665 Elmwood Road, Rockford, IL 61103; tel: 815-637-7217; e-mail: firstname.lastname@example.org; web: www.eclipsenet.com. Web links for Focus on Energy can be found here: https://focusonenergy.com.