For designing heat exchangers in Boiler economizers and other applications, overall heat transfer coefficient plays a determining role to find out the heating surface of economizers.What is the standard value from range of 20 to 300 for finding out the heating surface for low temperature zone of flue gas path.
Boiler design Engineers and Chemical Engineers with specialization in heat transfer can answer this question.
The overall heat transfer coefficient of a heat exchanger or an economizer combine the following elements:
1) Heat transfer coefficient on the tube side. For example, if the flue gas passes on the tube side, then it is the heat transfer coefficient of the flue gas.
2) Heat transfer coefficient on the shell-side of the heat exchanger / economizer. If air is passed on the shell side, then it is the air heat transfer coefficient.
3) The thermal conductivity of the tube material.
The economizer is used to pre-heat the ambient air by capturing heat from the flue gas leaving the boiler region. Pre-heating the air required for fuel combustion will improve the thermal efficiency of the boiler.
Hope this helps answer your question.
There are many heat exchanger software that can calculate the required heat transfer area based on the required temperature rise. Such software has electronic libraries of the gases and liquids heat transfer coefficients and thermal conductivity for a variety of materials of construction.
Professor Yehia Khalil
Yale University
USA
Thank you, sir, Yahia F.Khalil for your attention and answer. Actually, myself is designing a heat exchanger that will transfer heat from 2100 MT of flue gas in shell side(square size flue gas duct size 6 meters x 6 meters) at 1400C to cold water /condensate at 600C in tube side and this process will cool flue gas to 1000C, heating the condensate to 1020C. This process will help in reducing global warming. This is an innovation of mine and I have written the matter to American President Obama to implement this scheme in your country also after India and this is specifically meant for coal based thermal power plants. You please calculate correctly the heating surface of this heat exchanger in square meter and the fluids are flowing in counter current directions. Kindly utilize your best effort to have a precise value of this heating surface of the heat exchanger with appropriate value of overall heat transfer coefficient (in range 20 to 300) as the effort will check global warming. If any other data is required please feel free to communicate. Also inform me which value of overall heat transfer coefficient from 20 to 300 is suitable for this waste heat recovery process.
Thanking you, Radhashyam Muduli
Ex.Additional General Manager/ NTPC Limited, New Delhi,India
E mail: [email protected]
Thank you, Mr.Neeraj, the pdf you referred is highly informative. Can you please try an approximate value of heating surface in square metres for a case mentioned below?
1.Hot & cold flue gas temperatures 140 & 1000C flowing in shell side of a duct of rectangular cross section of 6 x 5 metres at pressure 40 mm of water column above atmosphere..
2.Cold & hot waters at 60 & 1000C at pressure 25 kg/cm2 flowing inside a mild steel pipe of 40 mm inside diameter and 2.5 mm thickness.
3. Flows are counter current flow.
4. Quantity of water and gas flows are 400 cubic meter(say 400 MT) & 2100 metric ton per hour.
5. The flows are continuous in duct and tubes.
6.The tubes are placed parallel to the duct.
You must follow the exchanger's manufacturer catalog data on performance when possible.
Dear Mr Muduli, Would you share your flue gas parameters (gas composition based on fuel). This would affect the CV value of the flue gas. Once this is available, using water as a medium, I could help you to calculate total heat transfer and heat transfer coefficient . Thanks.
One thing to consider may be the feasibility of this heat recovery proposal. Cooling the gas from 140C to 102 C while heating the condensate from 60 C to 100 C, . Getting the Flue gas to 102 C and the Condensate to 100 C at the same time may not be feasible in the proposed heat transfer unit.
Sir, what problem you are anticipating in the process kindly communicate and which factors are to be taken care of, please elaborate.
Based on the gas and condensate temperatures, there seems to be potential for a temperature cross in the exchanger. At minimum, as the gas and condensate temps approach each other the driving force for heat transfer will be much reduced and could imply significant area requirement if the heat transfer coefficient is not particularly large. The whole thing may depend on the type of unit that is used to accomplish the heat transfer, but a counter-current unit with out multiple-passes may pose a cost and a process challenge.
Sir, in steady state full load operation,temperature of flue gas will be maintained at its upper limit(140 degree Celsius) continuously and with low velocity of flue gas, it will provide a heat flow path to lower temperature condensate flowing counter currently in a heat exchanger. It will not hamper the heat transfer process as residence time is higher than it is expected. As per your predictions, heat transfer area will be definitely quite high.
What will be the ideal value of U in this case?.
Sir, thanks for the clarification. The high residence time will help with heat transfer, however the U ideal value is a bit of a challenge. "low velocity" may give a low heat transfer coefficient overall, but an absolute value may require more data and rigorous calculations. For preliminary guesstimates, U values between 60 - 120 W / sqm-K may provide an initial view of required area. U value is Assuming sensible transfer between vapor and liquid, with reasonable fluid velocities on both sides.
Dear Saravana Bavan Bala Krishnan, sorry to inform you that your update on 21/09/2016 was not available to me. I could see it today (27/10/2016) only. Well,the parameters are like this, flue gas will be cooled from 140 degree to 100 degree Celsius and water to be heated from 60 degree to 95 degree Celsius in the apparatus being discussed with heating surface more than 4000 square meter. The composition of flue gas is as follows:
Nitrogen :79%, Oxygen:03% and Carbon dioxide: 16% Rest is water vapor.
Thanks Radhashyam Muduli.
I , sincerely thank Mr.Dave Modeste and Sir Charles H.Beach,P.E. of US Defense for their valuable guidance in this research work.
Thanks & regards Radhashyam Muduli.
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