There is nothing that worries heat treaters quite like the energy bill, which is why The Center for Heat Treating Excellence (CHTE) at Worcester Polytechnic Institute (WPI) in Massachusetts conducted an analysis of major energy loss from commercial furnaces during heat treatment as one of its research projects. The CHTE collaborative is an alliance between the industrial sector and university researchers to address short-term and long-term needs of the heat-treating industry.
The goal of the study was twofold. Researchers looked at ways to help heat-treating companies understand how to estimate their energy usage and also how to effectively reduce it. The research was conducted on various furnace types, both new and old. It also assessed the flow of heat input and key loss areas such as flue gas, fixture and conveyers, wall, atmosphere and water cooling.
“We took on this project because nothing affects heat treaters more than the price of energy,” said Richard Sisson, WPI professor of mechanical engineering and technical director of CHTE. “The industry is energy-intensive, so we used the Department of Energy’s (DOE) Process Heating Assessment and Survey Tool (PHAST) to identify where furnaces are losing heat so that heat treaters could make informed operating decisions about how to reduce the loss.”
Since CHTE’s research was conducted, PHAST has been retired and replaced by the web-based Process Heating Modeler Tool (PHMT). See the sidebar for more information on this latest tool.
Project Overview: Assessing Energy Loss in Furnaces
The following is a summary of the steps involved in the research project.
- Conducted a literature review on industrial energy conservation and saving of heat-treating and furnace operations.
- Downloaded DOE’s free software and trained PhD students at WPI on the system.
- Analyzed various furnace types – gas carburizing, vacuum carburizing, batch, tempering and continuous furnaces – with the software.
- Conducted experiments on both old and new furnace types.
- Evaluated energy flow and loss in various scenarios – products and fixture preheating, combustion air preheating, air-fuel ratio control, comparison between old and insulation-enhanced furnace, comparison between batch furnace and tempering furnace.
- Developed a list of “best practices” for energy conservation and savings.
Where Furnaces are Losing Heat
CHTE research demonstrated that there are three significant areas of energy loss in natural gas furnaces during heat treating. They are flue-gas losses, wall losses and fixture losses (Fig. 1). Other areas of loss, although not as significant, incurred with atmosphere changes, furnace opening, water cooling and heat storage.
For vacuum furnaces (Fig. 2), if carbon-carbon composite materials are used to replace nickel alloy, the gross heat input will decrease by 18%.
Recommendations for Increasing Furnace Efficiency
The analysis showed that furnace efficiency will increase after applying improvements in the following three areas:
- To reduce flue-gas losses, which in CHTE’s experiments attributed to about 30% of total energy consumed, it is recommended that flue gas be used to preheat the load, the fixtures and combustion air. In so doing, energy consumption will be reduced by 26% as shown in Figure 3.
- To reduce fixture losses, which in the same experiments attributed to about 12% of total energy consumed, it is suggested that the weight of fixtures be minimized and that selecting fixture materials with low heat capacity will also boost performance.
- To reduce wall losses, which attributed to about 10% of energy consumption, it is recommended that a schedule for repair and maintenance be implemented. As illustrated in Fig. 4, this step will reduce energy savings by 18%.
Best Practices for Reducing Furnace Heat Loss
According to CHTE research findings, best-practice solutions include:
- Reducing excess air used for fuel combustion in burners. Suggested energy savings 5-25%.
- Using flue gas to preheat combustion air. Suggested energy savings 10-30%.
- Load preheating using flue gas. Suggested energy savings 5-30%.
- Minimize the weight of fixture. Suggested energy savings up to 35% (in vacuum furnaces).
- Reduce the discharge temperature of products. Suggested energy savings up to 5%.
- Atmosphere preheating using flue gas. Suggested energy savings up to 5%.
- Use proper type and thickness of insulation for furnace. Suggested energy savings up to 5%.
- Repair and maintain insulation and refractories used for the walls and doors. Suggested energy savings up to 5%.
- Select the proper fixture materials. Suggested energy savings up to 6%.
- Minimize the use of process atmosphere used in furnaces. Suggested energy savings up to 7%.
DOE’s Process Heating Modeler Tool
Recently, the U.S. Department of Energy (DOE) updated its software tool that helps users survey and assess process-heating equipment. It’s called Process Heating Modeler Tool (PHMT). The tool consists of 10 energy-flow calculators, which allow users to quickly assess the magnitude of losses within their system. Calculators included in the tool estimate losses in flue gas, atmosphere, walls, openings and cooling, among other areas. The tool also includes a Process Heating Modeler, which allows users to create an energy balance for their process heating systems and the equipment included in the systems. To learn more visit:
Other Furnace-Efficiency Software
FurnXpert is furnace software that simplifies the job of furnace sizing, furnace setup and thermal simulation inside industrial furnaces. The software has been developed specifically for furnace designers, furnace manufacturers, metallurgists and process engineers to simulate a heat-treatment process with a virtual furnace. More information can be found at: www.furnsol.com.
About the CHTE Collaborative
CHTE is an alliance between industry and university researchers that addresses short- and long-term needs of the heat-treating industry. Membership is unique because members have a voice in selecting quality research projects that help them solve today’s business challenges.
Member Research Process
- Research projects are member-driven. Each research project has a focus group composed of members who provide an industrial perspective.
- Members submit and vote on proposed projects.
- Three to four projects are funded yearly.
- Members have royalty-free intellectual-property rights to pre-competitive research.
- Members have the option of paying to sponsor proprietary projects.
- CHTE periodically does large-scale projects funded by the federal government or foundations. These projects keep members informed about leading-edge technology.
- Members are trained on all research technology and software updates.
Other projects that CHTE is currently working on include:
- Nondestructive testing for hardness and case depth
- Alloy life improvement
- Gas-quench steel hardenability standard
- Induction tempering
- Enhancements to CHTE software: CarbTool©, CarboNitrideTool© and NitrideTool©
- Cold-spray nanomaterials (supported by ARL)
- Additive manufacturing