Heater calculation

[Gopalakrishnan C]

Hi,

Air amount: 8000 CFM
Burner temperature: 1800°F
Furnace capacity: 5 million BTU

Question:
I want to heat up the air from 70°F to 150°F, If the air flowing through the 2" OD X .15"wall-thickness of Stainless steel tube what will be the tube length I should use?

Thanks a lot!

 

[goutam_freelance]

You did not specify heating medium and temperature.
You can calculate the inside and outside heat transfer coefficients based on well known formula to arrive at overall heat transfer coefficient. Then based on temperature differences calculate LMTD as applicable and thus arrive at heat transfer surface area. Remember tube bundle formula is different.

 

[XL83NL]

21.354

Huub
- You never get what you expect, you only get what you inspect.

 

[Gopalakrishnan C]

@goutam_freelance
Thanks for your help! I used the below calculation and got tube length L=152 meter OR 5972 inches!

q=U A dT
q= 1465355 W m2(5 million BTU)
U= 28.15 watt/square meter/°C (Used std hi & ho for Heating air with saturated steam where as I use conventional burner)
dT= 2150 °C ( Used dT1=1800-150; dT2=1800-70 then converted into °C)
A= pi x .051 meter x L

I did this basic calculation and still wondering why is the Airflow(8000 CFM as I mentioned in the first place) is not even a part of this calculation? could you please give your comments?

 

[Gopalakrishnan C]

@XL83NL

Could you please provide me with the calculation you have used to get 21.354? Thanks for the help...

 

[goutam_freelance]

@Gopalakrishna_C

Regarding your last sentence, the 8000 CFM should appear in your calculation of ho as you need velocity, Reynolds number etc. to calculate ho.

Since you have given insufficient data I can not check your calculation but following are my confusions.

Is BTU actually BTU/h? Then the unit of q is W, not W m2.
Is dT LMTD? If so which formula did you use?
However, in general your calculation may be OK for a first estimate.