If your furnace is a bottleneck in your operation, it may be time to consider adding a new one. This article reviews the basics of furnace structure and style, refractory selection and combustion design. It concludes with a recent case study.
It’s no secret that manufacturers are having a hard time keeping up with demand. That’s especially true for steel forgers, who are seeing backlogs of up to 18 months. But there are a few secrets to increasing the power of your process through smart furnace design, whether retrofitting an old furnace or building new. These secrets lie in your combustion design, burner placement, BTU calculations and refractory lining specifications.
American forges are adapting to a new normal with mass reshoring, material shortages, endless advancements in technology, and new rules and regulations for operational safety. It’s never been more important for your industrial furnace to operate as efficiently as possible in order to keep up with in-creased demand and meet the needs of each of your clients. That means your furnace configuration needs to be optimized based on your production requirements.
If your furnace is the bottleneck of your operation, it may be time to consider adding a new furnace that is specifically designed to maximize throughput for your product line. There are a ton of considerations to weigh when you embark on a new furnace build, so it’s important to have a 360-degree view of your requirements and constraints. A smartly designed furnace can increase your profitability while helping you reduce production backlogs.
Designing a Furnace Based on Your Process
When designing a new furnace, start by looking at your current operation and your long-term production goals. The more detailed you can get the better. Define the number of parts you want to process in what amount of time, the type of material you’re heating up, the shape and weight of your biggest and heaviest pieces, and the highest temperature you need to achieve. Answering these questions in advance will help determine the best option to heat up your parts quickly and uniformly.
Furnace Structure and Style
Once you review your production goals, you can begin evaluating different methods for opening and closing the furnace. Does a tip-up furnace, where the roof hinges open (Fig. 1), make sense or a lift-off hood, which requires a crane to pick up the top shell of the furnace and set it aside to put the steel in and take it back out? Would a box-style furnace with a traditional door be better?
3D model of a tip-up forge furnace with car out
Determine the inner and outer dimensions of the furnace. It is important to know how much space you have to work with, its proximity to other equipment and the configuration of your production flow.
Everything that goes into the furnace needs careful consideration as well, such as the car that carries the parts and the device used to load and unload it. These systems need to be carefully engineered to ensure they can withstand the heat and handle the weight of both the parts you’re processing and the furnace structure itself.
The overall life of a furnace is largely determined by the ability of the refractory lining to hold up to its harsh operating environment. The high temperatures inside the furnace and variation in temperature when it opens and closes can wreak havoc on refractory lining. So too can chemical exposures from slag and sometimes even mechanical abrasion, depending on the purpose of the furnace. Unfortunately, it’s nearly impossible to find a single refractory composition that’s capable of holding up to all of these variables at a reasonable price. It is possible to replace the refractory lining, but you want to put that expense off as long as possible.
Fig. 2. Refractory fiber combination module
A high-strength refractory (Fig. 2) with abrasion resistance is more likely to crack under extreme temperature variations due to thermal shock. However, refractory material that doesn’t crack under thermal shock might still corrode from chemical exposure. You often have to balance material properties for different zones to ensure the longest life of the refractory. The refractory used near the door where you see drastic temperature fluctuations will be different from the type used on the roof and sidewalls. Finding the right combination at the right price can be very tricky.
Now that you know the size, shape and style of the furnace – in addition to your production parameters – you can calculate the size, number and placement of your burners so that the furnace will heat parts to the right temperature without damaging them.
Heat loss is another crucial factor. Calculate how much heat you are losing when the door opens and closes or that escapes through the sidewalls or the flue, so you can come up with a BTU number and divide that by the number of burners. The wider the furnace, the bigger the burners (typically) so that they push the heat harder to reach the other side of the furnace quicker.
Other considerations include the level of control you want over what’s happening inside the furnace, what potential problems may arise and maintenance needs you may have down the road. It is possible to design the furnace in a way that makes any issues a lot quicker, cheaper and easier to address. For example, you can split burners into different zones, each with its own thermocouple, which also allows for more adjustment and stability.
Specialized furnaces require technical expertise and engineering for optimal outcomes. Just as there is a wide range of variables in the steel production process, there are even more variables involved in the design of a furnace. Ranging from the combustion system to the car that carries the steel, there are hundreds of considerations if you want a system that increases profitability. Not only that, but the system should be designed and built to last for decades, with minimal maintenance, allowing you to sup-port the demands of a growing customer base.
A new furnace (or even a retrofit) is a huge capital expense, so choose a supplier that understands your business, has the technical expertise required to support your equipment and production needs, and can ensure maximum return on your investment.
Furnace Design for Heavy Steel Plate Processing
A steel processing plant in the Midwest trusted Onex to design and build a new furnace to expand their production capacity. They needed to add a new furnace that would heat up steel ingots to an internal temperature of 2250°F. The ingots then go into a mill for processing. Their single existing furnace can take up to 16 hours to heat up eight ingots.
During our initial consultation, we learned this furnace feeds a mill that will still have available capacity even after adding the second furnace. This fact told Onex’s design team that if we could find a way to reduce the time to heat up the four additional ingots it would increase the customer’s throughput even more than they were planning. Considering they only have one furnace to feed the mill now, this was an exciting prospect.
Onex reviewed several different designs and ultimately proposed a tip-up furnace. The top section of the shell hinges along the back edge and opens up, allowing the car carrying the hot steel ingots to drive in and out.
The key to heating those ingots faster is to fire them from the top and bottom, which creates a more uniform heat. By propping the ingots up on piers made of specialized refractory material, the bottom heats at the same rate as the top, creating an even temperature distribution. In order to prevent flame impingement, Onex added small burners that fired in a strategic flame path below the parts in between the piers with larger burners firing above the parts. This burner design aids in airflow and facilitates better part uniformity.
Fig. 4. Furnace assembly in progress
Fig. 5. Furnace located in place
The furnace for this steel processing plant (Fig. 4) is designed to have four zones of control – essentially one for each ingot – that offer a greater level of heat regulation as ingots are added and removed from the furnace. This requires that the refractory withstand the thermal shock in addition to the ultra-high heat. We designed a refractory system comprised of engineered shapes, including the precast car deck and piers, along with ceramic-fiber combination modules for the roof with a refractory-grade polycrystalline hotface. Precasting the shapes allows for quicker on-site installation and provides better mechanical properties.
Our calculations estimate that this new furnace (Fig. 5) will be more efficient than the existing furnace and require only routine maintenance. New motors and fans every couple of decades can be planned as a capital expense. The new furnace will take up about half the space as the existing furnace and run nearly as much steel, increasing the customer’s overall throughput.
For more information: Patrick Laskey is Business Development Manager at Onex Inc. in Erie, Pa. He can be reached at email@example.com or 814-440-1494. For additional information, visit www.onexinc.com.
All graphics provided by the author.