28 replies to this topic

[sang]

Steve

 

I fully agree with you - there's no reason to sum the two values - unfortunately, it's not possible to convince the big software developers.

 

Regards

Sanjay

[Chris Haslego]

Quick note from the admin: Steve's file has been uploaded to the file repository here:

http://www.cheresources.com/invision/files/file/298-pipe-heat-loss/

Any revisions or future additions will be managed in the repository.

[awais ilyas]

Dear Steeve and Friends 

We have a thermocouple consisting of two pipes . In the inner pipe there is the flue gas and the outer pipe we have a cooling water . Currently we are facing a problem that gases are being condensed below 100 C because of cooling water .We have to apply insulation between the inner pipe and the cooling water so that gas outlet temperature is limited to 100 C.

What approach can be used of insulation thickness so that condensation can be avoided .
Flue gas enters at 1500 C and should leave at above 100 C to avoid condensation.
Cooling water enter at 20 C .

 

Please help me as soon as 

 

Regards

Thanking you in anticipation

Muhammad Awais

m.awais29@hotmail.com

TUM Munich Germany

Edited by awais ilyas, 15 August 2013 - 03:08 PM.

[Steve Hall]

Muhammad,

 

You can apply the same formulas used in the spreadsheet in this thread to estimate the film coefficients for the gas and water sides of the pipe/insulation. In this case both sides are forced convection just like the formula used for the inside coefficient (NOT for the outside coefficient where heat transfer is to the environment).

 

From that, you can use the thermal conductivity of the insulation to calculate an overall heat transfer coefficient (U), and then calculate the surface temperatures.

 

I think you are mostly interested in the bulk temperature of the flue gas for which you can use the U to calculate in conjuction with the flow rate and inner pipe diameter. But if you are reducing the temperature from 1500 C to something near 100 C, you should divide the pipe length into segments and do all of the calculations step-wise down the pipe.

 

If you also want to avoid condensation on the inside wall due to the wall temperature ("sweating"), then you should also pay attention to the wall temperature which is why I mentioned it above. This is probably especially important if your Reynolds number is very low - like less than 100 - which would indicate that heat transfer into your gas is more conductive rather than through mixing. With very laminar flow conditions I envision significant thermal gradients from the pipe wall to the center.

 

I think you'll find that the minimum insulation thickness that you can practically install (typically 25 mm) will suffice. But your exit temperature might then be too high? Perhaps leave the first portion uninsulated to bring the temperature down to, say, 200 C and insulate the remainder? This would be a workable approach if you have steady-state conditions.