Authors: Brian Pinto (Product Development Manager, North America) and Wenwu Shi (Senior Research Engineer), Foseco NAFTA
Abstract
Multivariate mathematical models were created to simulate crucibles being used in aluminium foundry applications with detailed materials characterisation data as inputs. The aim was to investigate the effects of crucible geometry and materials properties changes on the overall energy efficiency of the furnace toward melting and holding metal.
Effects of key thermal properties were also studied to understand their influence on energy efficiency and thermal stresses, another key factor in understanding crucible behaviour. Problems with evaluating these changes practically in foundries stems from the inability to separate out extrinsic factors that also affect furnace efficiency, such as unique configurations, furnace condition and, in some cases, poor operating practices.
Since melting and holding metal in crucibles accounts for a large portion of energy demand in the foundry industry, recent advancements in crucible technologies resulting from these studies could significantly impact cost-efficiency and carbon footprints across the industry. In case studies of applications such as aluminium melting and holding, considerable improvements in field performance have been reported.
Introduction
The energy used for melting and holding metal accounts for nearly 40% of the total energy costs in a typical foundry. Metal casting industries are known for high energy demands, low energy efficiency and high CO2 emissions. On average, the energy consumed by a foundry shop far exceeds that which it is predicted to use based on theoretical calculations. This is due to inefficiencies associated with the activities of metal melting and casting. Some are inherent to the process, while others are dependent on the types of equipment used as well as specific practices. There are opportunities to improve energy efficiency of a foundry operation, significantly reducing environmental impact while maintaining the sector’s competitiveness in the process.
One of the most common methods used to melt metals is with an electric-resistance or fuel-fired furnace. These furnaces contain molten metal at high temperatures within large refractory crucibles. To melt, energy from resistive elements or fuel combustion generated inside the furnace chamber against the outer crucible wall is directed to the metal charge inside and subsequently melts it at high temperatures. Literature studies reveal that recommended energy-saving measures are to optimise the furnace configuration and/or improve its melting rate with little or no focus on crucibles.
If metal is molten, a well-insulated furnace expends only nominal energy to keep it at a set temperature, compensating for heat losses to the environment. However, to get to this point requires a tremendous amount of heat energy, not only to bring the metal to its liquidus temperature and melt it, but also to transmit that heat through a thick, high emissivity ceramic material having high specific heat capacity, all the while opposing the thermodynamic forces that favour carrying heat away to the atmosphere. The crucible is a physical barrier between the heat source and the molten metal, so it plays a pivotal role in determining metal melting efficiency.
Thermal conductivity, specific heat capacity and geometry are the main factors, fixed quantities that govern heat transfer through a crucible. This appears to provide convenient solutions for improving furnace energy efficiency. However, if one considers the many aspects of crucible and furnace use across the industry, the solution becomes more complex. For melting, fast heat conduction through a crucible is very desirable, whereas for holding, slow heat conduction is best. When a crucible is used for both melting and holding applications within the same furnace the challenge of creating a universally efficient crucible becomes more apparent. To add to this complexity, customer practices across the industry are so variable that even correlating a furnace’s efficiency to its own crucible becomes extremely difficult.
For example, if a furnace has poor insulation, then the effect of changing to a high-thermal-efficiency crucible will be completely clouded by the gross inefficiency of the furnace itself. This has been observed in many field tests. Although laws of thermodynamics predict improved performance, it does not play out this way in practice, making it very difficult to demonstrate an energy-saving crucible to a customer. Therefore, a better way to study and, to an extent, prove the effects of a crucible on thermal efficiency is to completely normalise the environment. In practice this is not possible. However, using theoretical modelling based on finite element analysis methods it can be done. This paper explores how heat flow behaviour and energy efficiency can be studied based solely on changes made to the crucible material properties and design in 2D and 3D computer models, keeping the rest of the system constant. In doing so, the benefits of advanced crucible technologies start to become clear.
Experimental, results and discussion
The research, results and discussion are documented in Foseco’s well-known Foundry Practice magazine. References and graphs have been removed from this shortened version of the research paper because of space constraints. A full version of the paper is available BY CLICKING THIS LINK
Summary and conclusions
Using traditional evaluation methods, uncontrolled field trials, or simple energy comparisons, it has proven very difficult to justify changing to an energy-efficient crucible. Almost always the benefits are obscured in the presence of other foundry practice-related variables that detract from equipment efficiency. Were the foundry to eliminate or minimise these issues, when often it is something simple like replacing deteriorated insulation or keeping the furnace lid closed more the benefits of an energy-saving crucible would become more obvious.
With theoretical modelling it is possible to eliminate these variables from the equation to estimate differences in energy efficiency directly influenced by changes made to crucible geometry and composition, as well as gain insight as to the limits to which these features can be changed to support energy-saving initiatives. It is critically important not to neglect considering how changes to composition and/or geometry will affect the stress state of the crucible, particularly as a function of temperature. Fortunately, with a nominal amount of additional information, these conditions can be simulated in a computer model as well. With the ability to understand the characteristics and thermal behaviour of crucibles to a degree that is relatively unexplored, new materials were developed that not only showed high promise in the theoretical realm, but also showed definite improvements when applied to an actual crucible in a real foundry operation under close surveillance where actual data collected was able to validate the computer models.
Extrapolating this achievement across an entire foundry’s operation could have large implications with respect to increased energy savings, minimising carbon footprint and reducing overall costs of operation. These concepts are constantly being considered by foundry owners and managers. With the help of these and other evaluation tools they can begin to understand that something as unassuming as a crucible can have a significant impact on their bottom line.
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