Energy Saver: Don’t Get Heated Up by a Hot Stack

Consider several projects to lower temperatures and recover energy.

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I hate hot stacks! It always bothers me when I check furnaces for an energy efficiency study and the first thing I notice is that the stacks are above 600ºF. It’s even worse when I’m asked to evaluate a unit’s maximum capacity and the furnace stack is hot. From a consulting standpoint, my job becomes easier, but I still hate to see a hot stack.

Why? Because it means someone designed the furnace to have a hot stack and no one bothered to correct it or, worse, ignored it. In addition, it means that through all the years the stack has been hot, the plant couldn’t justify saving that energy. So, as a hired consultant, I’ll have to point out that the plant can save money and increase capacity by reducing stack temperatures, which they probably already heard. In five years, I could show up again and likely say the same exact thing.

You can minimize shutdown time by installing a whole convection section at one time.

This isn’t an isolated phenomenon. It’s cheaper to build a less efficient furnace and project people like cheap. I’ve even been in meetings where the project engineer insists that it’s the only way to stay on budget. I’ve even been told that heat recovery could be tacked on as an additional project later and then watch as capital spins out of control and the furnace is installed with a hot stack. Then a consultant is brought in to tell them they need to reduce stack temperature to be more efficient.

So, before you spending money on a consultant, put a project together to lower stack temperature. Here are a few ideas.

Air Preheater – For large furnaces, air preheaters generally make the most sense. The amount of energy saved can be very large since stack temperature can be reduced to below 350ºF. Air preheaters require plot space, but energy savings can be as much as 30%. If you already have a Selective Catalyst Reduction (SCR) unit to reduce NOx, your design probably has room for an air preheater. If you don’t, it may be an opportunity to reduce NOx as well.

Process Heat – In some cases, you may have a process stream nearby that’s using steam or another heat source. Instead, heat that stream with stack flue gas. Either have a hot-oil circulating stream that can be used in multiple locations and you can have better control of the exchange with your process. Or use direct exchange by adding process coils to the furnace convection section. It doesn’t require another medium but gives less flexibility against over-firing.

Make Steam – Look at this as free steam. Adding an economizing and even heating boiler feed water is easy to do. The system can be set-up to make any type of steam you need. You can even superheat steam with added coils or by creating a higher-pressure steam and reducing pressure after it leaves the steam drum.

The Capacity Coil– So, you have hit your furnace limit and are looking to redesign the tubes to try to gain some capacity. However, your redesign has the box getting too hot, the pressure drop too large, or the project too expensive. Add a new coil to the convection section. Chances are there’s enough heat in your flue gas to heat some of your process. The added coil doesn’t increase pressure drop because it gives the process an additional path (if correctly designed). In most cases, you can add 5%-10% more flow through the new coil.

For any of these projects, the furnace must be in shutdown mode, but you can minimize shutdown time by installing a whole convection section at one time. It’s usually very difficult to remove or rearrange tubes in an existing convection section. Instead, just have a completely new section built and install it as one piece. You’ll save time and money versus tube replacement. Similarly, when you have convection sections designed, make sure they can be easily bolted and unbolted. In some processes, it’s easier to replace a convection section with a spare than to try to clean the section in place. Then you have years to clean the replaced bundle at ground level.

Gary Faagau is Chemical Processing's Energy Columnist. You can e-mail him at
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