A large manufacturer of consumer glassware products in the North East sought a solution for injecting cold compressed air into its refractory furnace. Doing so would minimize the internal corrosion thereby extending the life of the furnace lining and their annual maintenance interval. The manufacturer opted for a unique solution from Aggreko Engineering featuring a rental, oil-free rotary screw air compressor combined with a heat exchanger and chiller. Installed in 2019, the solution is expected to save the company \$9 million monthly given the ability to maintain extend furnace maintenance from one year to two years – and boost plant uptime.
A large manufacturer of consumer glassware products has been able to save \$9 million monthly in operational costs by extending the maintenance intervals of its refractory furnace, thanks to a compressed air system featuring an oil-free air compressor, heat exchanger and chiller.
Any Extension of Furnace Repair Intervals a Plus
During a visit to the facility, the Aggreko engineering team noted the plant was using oil-free diesel air compressors to blow compressed air on the exterior of the furnace. This air was applied to cool the furnace walls as a way to keep the interior temperature of the furnace down. The inefficient use of air pointed to an opportunity to improve the situation, in turn, allowing the plant to achieve its operating goals.
Temperature Control Crucial to Furnace Uptime
Refractory furnaces are often used to contain a high temperature corrosive environment for a particular process. Yet the walls of these same furnaces can lose their thickness and overall mass due to the heat and its environment.
While there are many types of refractory systems the thin lining thickness is usually between 25 millimeters for small systems and 75-plus millimeters for larger systems. Regardless of the wall thickness corrosion is still a common problem that all furnaces face. Managing the heat of the furnace is critical to having less maintenance and repairs over time.
According to a University of Missouri-Ralla study, the corrosion of the refractories (ceramic materials used in the furnace) begin in the melter due to batch carryover (lime, soda, fluorides, lead oxide, borax, silica, other glass constituents), volatile fluxes (i.e., volatile alkali oxides penetrating pores, where solid, liquid, and gas coexist), and melt attack, mainly at the metal line[i].
Erosion and wall degradation often follow this initial phase of corrosion. A study by Clemson University emphasizes the importance of temperature control. According to the study, “It is found that the hot face temperature primarily affects the rate of corrosion reactions. If the hot face temperature is held just below the point that the products of corrosion become liquid (melt), corrosion will be very slow or nonexistent[ii].”
The Clemson study concluded the temperature gradient on thick furnace wall linings should not exceed 50oF per inch (10oC per centimeter). While this criteria is typically hard to maintain considering the furnace’s application it is critical to achieve in order to maintain its productivity.
It was also noted in the Endell, Fehling and Kley model, “there is a very strong dependence of refractory corrosion rates on hot face, temperature. In fact, temperature is the most important process variable that can be considered in a furnace design or process control[iii].”
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Engineering a Solution
The Aggreko engineering team is no stranger to that fact that compressed air is often used very inefficiently at industrial facilities and delivered to the production floor at suboptimal pressures and temperatures. Also crucial is air compressor reliability. If the air compressor goes down there is no production.
In this situation, temperature was of the utmost importance. The compressed air temperature leaving a typical rotary air compressor is 15oF to 20oF (-9.4oC to -6.7oC) above the ambient. While compressed air temperature is normally not an issue with standard production equipment, this particular application required cold air at less than 55oF (12.8oC) air.
The team designed a solution featuring an oil-free air compressor rated to deliver 1,500 scfm at 125 psig. The oil-free air compressor prevents the spread of oil vapor, while providing the amount of air needed at the right flow and pressure at all times. Additionally, the rental compressed air system includes a heat exchanger and chiller to cool the compressed air to a temperature of 45oF (7.2oC). By lowering the temperature of the air water vapor is condensed into liquid and removed from the air stream.
The compressed air system at the glassware products facility uses a heat exchanger in combination with a chiller to deliver compressed air to the company’s refractory furnace at a temperature of 45oF (7.2oC).
The team also calculated how heat affects the furnace’s lining by taking into account its wall thickness, processing temperature, age and other critical factors. The solution was engineered to ensure compressed air was directed to the external locations within the furnace experiencing the most corrosion. The facility opted for a rental solution since it required its existing air compressors for other areas of plant production.
Cool air delivered by the unique compressed air system gives the glassware manufacturer the ability to extend furnace maintenance from one year to two years, significantly increasing plant uptime.
Production Gains Equal \$9 Million Per Month
With the compressed air system up and running, the team noted the furnace’s corrosion curve significantly slowed due to the cooler temperatures applied. The cooler temperature also enabled the plant to prolong the life of the furnace lining – in turn – allowing it to achieve its production target and delay the need to shut down their furnace for another year. The company estimates it saves \$9 million per month due to the production gain and extended service interval.
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There are several industries that rely on refractory furnaces for production. Yet lining corrosion on the furnaces is one of the most critical challenges companies face given the consistently high temperatures involved. More frequent lining replacements also directly contribute to higher operational costs. Fortunately, more and more solutions are being introduced, such as the oil-free air compressor solution installed by Aggreko Engineering at the glassware manufacturing operation. The result is the ability to mitigate issues with a furnace’s temperature for positive outcomes.
About the Author
Fernando Arce-Larreta is Aggreko’s OFA (Oil Free Air) Leader, email: Fernando.Arce-Larreta@aggreko.com, tel: 713-882-9314.
We provide power, heating, cooling, oil free air and energy services to make a difference for people, industries and communities, globally. We know that as the world demands cleaner energy, we can fulfil that with our expert people, our dedication and investment in new technology and keeping our customers at the forefront of everything we do. Together we make a difference. For more information, visit www.aggreko.com.
All photos courtesy of Aggreko.
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[i] Refractory Degradation in Glass tank Melters. A Survey of Testing Methods M. Velez, J. Smith, R. E. Moore. University of Missouri-Rolla, Department of Ceramic Engineering, Rolla, Missouri.
[ii] A Chapter in the Refractories Handbook, edited by Charles A. Schacht, and to be published by Marcel Dekker, Inc., New York, NY 10016 Corrosion of Refractories by Denis A. Brosnan, PhD, PE Clemson University Clemson, South Carolina.
[iii] “Influence of Fluidity, Hydrodynamic Characteristics and Solvent Action of Slag on the Destruction of Refractories at High Temperatures.” Published by The Journal of the American Ceramic Society, December 1939, K. Endell R. Fehling R. Kley.