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One of our Unit has four furnaces viz. F-1, F-2, F-3 & F-4. There are two fire boxes: one for F-1/F-2 and one for F-3/F-4. Both fire boxes have two parallel convention banks and a common stack. The furnaces have the issue of low steam generation from convection banks, high stack temperature, BFW flow mal-distribution in convection banks and hence a lower efficiency than design. Sketch of the furnaces with current & design flows are provided below for reference. What could be the possible reasons and remedial solutions.

Jan-2024

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Mary Winter, TTC, george@ttclabs.com

We have seen this problem several times. Usually, the performance of the CFE looks OK based on Q/MTD or other simplified measurements. ON doing a rigorous rating of the exchangers, however, in every case we found that the CFE was fouled, Cleaning the exchanger saved millions in cost to modify or expand the fired heaters. Based on this experience, it might be appropriate to do a rigorous rating before spending any money on modifications.

Apr-2024
Raghu Nee, HPCL, neeraghu@gmail.com

Some more inputs needed to analyse this problem further.

What is the waste heat available in flue gasses from each of these furnaces? If the subject furnaces are of CCR, then that is not equal from each of the heaters.

What is the design pass flow distribution? is it 40-60 or 50-50? or something else?

Are all or some of these tubes in convection bank finned? I came across one design where finned tubes are specified in datasheet but bare tubes are installed. This lead to low steam generation and higher stack temperature.

Mar-2024
VIKRAM BABA, INDIAN OIL CORPORATION LIMITED, vikram.baba@gmail.com

Thanks for the replies. CFD of external BFW piping's were done and found to be symmetrical. One more, input I'd like to provide is that each parallel convection bank has two pass flow configuration with 9 tubes per pass & 4 rows in F-1& F-2, which seems to be unconventional. Can this configuration be one of the reason for maldistribution in BFW flow? Current BFW flow to F-1& F-2 is around 40% of design flow. Stack temp are in range of 225-230 degC.

Mar-2024
RUPAM MUKHERJEE, ENGINEERS INDIA LIMITED, rupammukherjee86@gmail.com

This needs to be reviewed in detail. In many cases, even simple configuration changes can do wonders. The draft profile is also required to be studied.

Feb-2024
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

The question does not specify what kind of processing unit provides the flue gases (Catalytic Cracking, Distillation, etc.), but I believe that it's possible to consider some verifications generally.

It's important to consider if the furnaces have soot blowers, if the response is yes, how is the availability of these devices? The deposition of soot or ash in the furnace wall and specially in the convection tubes (which are generally finned) can significantly reduce the thermal exchange in the furnaces, leading to a poor thermal efficiency and the main side effect is a high stack temperature like mentioned in the question. This is specially true in furnaces using FCC flue gases due to the carryover of catalyst fines in the flue gas which will be deposited in the furnaces wall and convection tubes.

Another key point is the distribution of BFW (Boiler Feed Water), which seems to suffer with a poor distribution. This is a key factor to the performance of the furnace and the flow and symmetry criteria needs to be accomplished in order to reach the expected design. A good strategy to identify poor flow performance of gases and BFW is a CFD (Computational Fluid Dynamics) study using a simulated model of the furnace.

A general review of the design with verifications of fin distribution on the convection tubes, their area, and the diameter of the convection tubes facing the eventual operating changes of the process conditions of the furnace over the years (capacity raising) should help to understand the poor performance of your furnace system.

Jan-2024