Improving the efficiency and performance of heating furnaces
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Dear colleagues: Our facility is currently operating a naturally ventilated cylindrical furnace. This heating furnace is a refurbished existing unit; during the renovation, all the baffle tubes in the convection section were removed (as the convection section had no useful function during the renovation in 2005), and only the radiation section was used. As a result, the flue gas temperature of the furnace has remained relatively high since it started operating. The designed thermal load of the furnace is 2.68 MW, but in actual operation the flue gas temperature is always above 280°C, mainly due to the fact that the convection section is not being utilized. During the pandemic, our operational efficiency declined significantly. At present, we are working on improving efficiency by making better use of this furnace. I would like to ask my colleagues if they have any experience with renovations that could enable the use of the convection section. Our current idea is to use this furnace to heat deionized water and convert it into steam of a certain quality. Those with relevant experience are kindly asked to leave their comments. Thank you!
This modification plan is feasible; the specific parameters need to be calculated
The material is preheated in the convection section before entering the radiation section； Of course, generating steam is also an option, but the amount of heat that can be extracted from the steam is not significant, after all, there is no phase change process from saturated steam to superheated steam
1. What is the minimum exhaust temperature requirement for your furnace that you are considering? To prevent dew point corrosion, the exhaust temperature of ordinary industrial furnaces is generally kept at 136℃ ； 2. What is the temperature of your cold-deionized water when it enters? Edge wall effects are taken into account during design to prevent dew point corrosion ； 3. What is the total amount of flue gas? First, calculate how much heat is released in total as the flue gas cools from 280°C to the exhaust temperature you require If the amount is low, the heat recovered might not be sufficient to cover the cost of your equipment investment; it’s important to conduct proper economic calculations ； 4. I’m not sure if your fuel gas or air is preheated? It is better to preheat the air or fuel gas in order to improve the furnace’s thermal efficiency.
Our initial plan is to feed the steam condensate into the convection section; the heated vapor-liquid mixture is then discharged into a drum where the steam and condensate are separated. The condensate is sent back to generate more steam, while the steam is incorporated into a specific steam distribution network. It’s just an idea for now; I haven’t yet consulted the design department to check if it’s feasible.
Thank you very much for your suggestions; I will reply to each one of them. 1. We want to keep the flue gas temperature below 150°C. 2. It is not cold desalinated water, but steam condensate with a temperature of around 105°C. This condensate is generated by the reboilers in our facility and is collected in a boiler water tank; when there is a shortage of this condensate, cold desalinated water is used as a supplement. 3. We estimate the total flue gas volume to be 8500 m3/h; 4. Neither the fuel gas nor the air is preheated, being at ambient temperature.
A comprehensive technical and economic analysis is required to calculate the annual savings from the upgrades as well as the time it will take to recover the new investments; planning should be done based on the actual needs of the enterprise. The energy utilization in a typical heating furnace falls into two categories: one is the generation of steam for reuse by the enterprise, and the other is the improvement of the furnace’s efficiency through the coupling of the energy from the exhaust gases, the inlet gases, and the fuel gas.