You are a fired heater designer and you are supposed to design a direct fired heater for a refinery process. You are clearly required by client specifications to design with an average heat flux density of 12,000 Btu/h.ft2.
Now the question is this:
How much margin is a good margin from 12,000?
6,000 or 11,950?
It is obvious that increasing the margin (designing with lower heat flux density) would increase the price of the heater and reducing the margin (designing with higher heat flux density) would make the heater more compact and save footprint for client.
Everything that I’m going to point out in this post will also hold true for a boiler as well as a process heater.
You have designed the required heater by client, and you have embedded a combustion air pre-heater to the design. But every time the operator lines up the preheater, the steam production in convection section drops significantly. It looks like preheating the combustion air reduces heater’s capacity of steam production.
The answer to why this happens lies within the decisions the designer has made at the very beginning of the design process.
When you design a heater with preheater, you should be aware that a preheater increases the firebox temperature and therefore if the design is limited by heat flux density, the system automatically reduces the fuel flow and consequently the combustion air because the temperature loop cascaded over fuel is overridden by TMT (tube metal temperature).
This happens to keep the tubes safe from overheating. This reduction in fuel therefore reduces the amount of generated flue gas and therefore the heat recovery duty.
This happens a lot in heaters that are retrofitted with an air preheater during revamp or capacity increase purposes.
Therefore, one can argue that in some cases, adding air preheater, although increases efficiency, might reduce the capacity of the heater at the same time. The issue needs to be investigated precisely before making decisions.
Now, what is the best margin for heat flux density?
Selection of the best heat flux density depends on the required efficiency, cleanliness of the fuel to be used, type of heater selected and possible plans of capacity increase.
The designer should only use the client heat flux density as the maximum amount, not the near exact ones!
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