Radiant tubes are prevalent in heat treating applications. They are very simple devices: basically, a pipe that enters and exits the work chamber. Geometrically simple — but the considerations of how they should be applied, the optimal materials for their construction, and the best burner to use present a myriad of challenges and opportunities for improvement. As all heat treaters know, radiant tubes represent a significant expense as well as an opportunity to save on maintenance costs and improve furnace performance.
This column is a Combustion Corner feature written by John Clarke, technical director at Helios Electric Corporation, and appeared in Heat Treat Today's November 2022 Vacuum print edition.
If you have suggestions for topics you’d like John to explore in future columns, please email Karen@heattreattoday.com.
Technical Director
Helios Electric Corporation
Source: Helios Electrical Corporation
In the coming months, I hope to challenge the reader to spend some time researching opportunities to improve their use of radiant tubes — that is to improve their performance, both heating rates and efficiency, as well as to extend their life and perhaps improve the uniformity of the furnace being heated.
I apologize in advance if I sound like an economist — “It is this way, but on the other hand . . .” There are a lot of factors to consider when planning to upgrade your radiant tubes, their associated burners, recuperators, mountings, and supports.
To start, let’s answer a simple question: Why do we use radiant tubes? Two reasons come to mind: to protect the furnace atmosphere from the products of combustion and/or to diffuse the release of heat within the furnace or oven chamber to maximize temperature uniformity. In many heat treating applications, even a very small leak will contaminate the furnace atmosphere, damaging the work being processed.
How do we size radiant tubes? Again, it is obvious that we need to have sufficient heated external surface area to transfer the heat to the furnace chamber. This heat transfer will occur through convection and radiation, with the latter mode being more significant as the furnace temperature rises. The rate of convective heat transfer will depend on mass and velocity of air or atmosphere passing over the tubes. The radiant heat transfer rate is a function of the difference between the tubes’ surface temperature and the temperature of the furnace and work being heated. The good news with radiant heat transfer in closed furnaces is that all surfaces in the furnace participate to a degree with the transfer of heat to the work.
There are many shapes for radiant tubes: U-shaped, W-shaped, three legged, as well as systems where the firing and exhaust occur at the same opening, including P-tubes and single-ended tubes. Each has its advantages and disadvantages, which we’ll discuss in future articles.
How about materials? Again, we have a lot of choices. The tubes can be centrifugally cast, fabricated from sheet, or made of some ceramic or composite material. [blocktext align="center"]The formulation of each material varies greatly, and it is important that the material is suitable for the use temperature and chemical composition of the furnace atmosphere as well as always being compatible with the common products of combustion.[/blocktext]
How are the radiant tubes installed? Are the ends welded to a mounting plate, or perhaps a packing gland is employed to seal the tube while allowing some expansion or contraction? Both methods are commonly applied successfully. Composite tubes may have a flange that is clamped at the mounting location, or they may use a packing gland. The tubes may have internal supports within the furnace to prevent sagging. The tubes can be hung vertically, located to the side of, or placed under and over the work being heated.
How long should my radiant tubes last? Simply answered, for as long as practical. As a young person, I was mortified when I dropped a hammer in a customer’s pusher carburizing furnace, and it broke an alloy tube. When I confessed to the plant metallurgist, he laughed and told me the tube I broke was over twenty years old. Other customers may be satisfied if their tubes last 18 months, so there is no simple answer. That said, there may well be opportunities to extend the life of the radiant tubes in your specific application.
We will revisit many of these discussions in later articles, but hopefully this column has whetted your appetite for the next discussion in December: What typically occurs inside the radiant tube? After all, this is the Combustion Corner.
