Fired heater features – what’s that on top of the stack for?
Finding out what the purpose of the cone featured on the top of some heater stacks is and what it actually does.
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What is the cone on top of some heater stacks for? The circled portions of the heater stacks in Figure 1 are commonly referred to as velocity cones (or tips). A velocity cone is a cone-shaped stack extension added to some fired heaters to increase stack exit velocity.
Here are three common reasons to increase the exit velocity:
υ It reduces the downwash of flue gas at the tip which causes stack metal corrosion. Some companies specify a minimum stack exit velocity (of 10-15 fps) at turndown operation for this reason. Using a stainless-steel tip instead of carbon steel can solve this issue.
ϖ A side benefit of higher stack exit velocity is that it provides some dampening of the effect that wind gusts have on heater draft variation.
ω Occasionally the stack flue gas dispersion analysis uses 50 feet per second as the exit velocity to determine the minimum stack height required. This is a common practice in the boiler industry.
=For boilers, flue gas dispersion analysis typically assumes 50 feet per second (fps) stack exit velocity as the basis. The stack height is then set to provide the desired flue gas dispersion. This does not have a significant impact on a boiler design or cost since boilers typically have fans that move flue gas. The stack pressure drop is typically insignificant compared to the entire boiler system pressure drop.
=On the other hand, natural draft heater stacks are sized differently. Natural draft stacks are typically most cost-effective with 20-30 fps stack exit velocity (compared to 50 fps for boilers). If designed for 50 fps exit velocity, the pressure drop (incremental vs an exit velocity of 20-30 fps) would be in the range of 0.15-0.25 in H₂O. This is a sizable portion of the draft generated by the stack. Unfortunately, personnel responsible for the flue gas dispersion analysis typically do not have this appreciation. Therefore, a minimum 50 fps exit velocity is specified and the dispersion analysis is carried out to establish a minimum stack height. Frequently, the height required from a draft perspective is considerably higher than the height for dispersion requirements.
Overcoming this extra pressure drop can add 30-60’ to the stack height of a natural draft heater. Not only does the stack cost more, but the heater structure and foundation become more substantial and costly. To minimise the cost and size impact, the stack designer typically only reduces the diameter of the very top of the stack to achieve the desired exit velocity instead of the entire length of the stack. This avoids additional pressure drop associated with higher stack frictional losses. A simplified dispersion calculation suggests that a 100 ft tall stack with 50 fps exit, has the same dispersion characteristics as a stack at 25 fps and 105-110 ft tall.
If you have a heater with a velocity cone sized for >40 fps and is draft limited, consider redoing the dispersion analysis with a lower exit velocity. You may discover you can remove the cone and gain 0.15-0.25 in H₂O of draft.
What’s that brick wall on the heater floor for?
There are four main reasons a brick wall (typically 3-5 bricks tall, with some space between bricks) is installed on the floor, shown in Figure 2, between the burners and tubes:
υ At heater turndown operation, the flue gas recirculation off the tubes can be cool enough to cause high CO emissions, especially with ultra-low NOx burners. A brick wall can help in two ways:
=By reducing the flow of relatively cool flue gas off the tubes
=By reradiating heat back to the burners
While not a complete solution, the bricks frequently help the heater run at a 25-50°F lower bridgewall temperature for the same CO emissions. Unfortunately, as a trade-off, NOx tends to be slightly higher at normal and design heat release if installed correctly. Also note that if the wall is too close to the burner, too tall, or does not have adequate brick spacing, it can block flue gas flow into the flame zone, which results in much higher NOx. If your heater has CO emission problems at turndown operation and has room to give on NOx emissions, consider adding a wall.
ϖ Occasionally, flue gas circulation currents in the radiant section are strong enough to push flames around in an undesirable manner, causing impingement. A common attempt to improve this condition is to install a brick wall to change the flue gas flow patterns. It does not always work but is still frequently tried. CFD modelling is improving at predicting these behaviours and is increasingly used to decide whether to install a wall.
ω For many years, some heater manufacturers installed the brick wall (Reed wall) by default as low-cost insurance against the above issues. Whether there would be any ill effect of removing the wall was hard to predict prior to CFD modelling.
ξ Occasionally a wall is installed to shield the radiant tube bottom guide pins from excessive radiant heat. If your heater has a brick wall but suffers from issues such as high NOx or poor flame patterns, consider XRG to analyse the situation and get your heater operating at peak performance.
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