the overall heat coefficient of an economizer was calculated to be 192.3 W/m2 oC. the overall heat coefficient of a shell and tube heat exchanger is supposed to be between 30-100 (Source: Coulson), for one in which there is flue gas and water.
The inlet temperature of water is 124oC and outlet 160oC. The inlet pressure is 120 bar and outlet 80 bar. It is a theoretical work. Please find attached my work that I have to submit very soon.
What are the reasons I can give in my project for an overall heat coefficient exceeding the limit?
Do you have phase change in the heat exchanger? Due to the latent heat involved in the phase change, the deltaT (°C) would be low and that might be giving you high value of heat transfer coefficient.
The range of" U" for water-Flue gas under normal conditions is O.K. But It is difficult to discuss without knowing the details of your project.*High value means ..50% higher or 100% or....* Is it a Theoretical work or Experimental work?*Tube fluid you might have chosen Water.What is its pressure?*What are the inlet temperatures?*What assumptions you have taken? etc..need to be known to identify the reasons for high value.If you can share those details ,you will get a satisfactory reply from the group.Good luck.
The value 480 W/m2oC seems quite high as there is gas on one side. Calculations and the validity of correlations used need to be checked.
I think gas side heat transfer coefficient has been calculated or assumed high. The overall coefficient will be controlled or nearer to gas side co efficient.
Borah has given the most useful answer here. The flue gas is naturally higher (perhaps much higher) than 160C. The water side heat transfer coefficient hardly counts here. The value you got is possible if you have high velocity, there are radiating soot particles along with radiating gas components (CO2, H2O). The walls will also get heated and reradiate to the tubes. Since you have not considered these effects, the result looks wrong.
Your choice of shell and tube exchanger is a bad choice for economizer of a power plant. I would use a simple crossflow heat exchanger.
Try this: Calculate the heat transfer coefficient on gas side in crossflow past one single tube, using the Zukauskas and Ulinskas correlation or even simple text book correlations. The value you get would be a good indicator of what you can expect.
Dear Nishtha! It seems that you are not familiar with the design of heat exchangers for power plants or boilers. I guess this is just a theoretical research.
As Mr. Raghavan said, the type of heat exchanger which you choosed (Shell and Tube) is not appropriate for an economizer.
Economizers are used as heat recovery equimpent in packaged boilers instead of air heaters because of NOx concerns. Economizers are generally of vertical gas flow and counter-flow configuration with horizontal tubes. The water side velocity rages from 3-7 ft/sec. There are many sources which include design of economizers.
Anyway, I found some wrong decisions in your design.
First, the range you specified for overall heat transfer coefficient is for heat transfer from flue gases to steam. But, you are working with sub-saturated water.
Second, you calculated the linear velocity of flue gases as 1257 m/sec! I guess something is wrong here.
If you have to design your economizer as a shell and tube heat exchanger, I suppose to use commercial softwares like HTRI or Aspen EDR.
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