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## Finding tube surface temperature for HX tube

HI,

I am trying to find the T.s value in this file. Please  help me out. ThanksEnergy balance equation on surface of tube (Taking solar energy as source)

1 ACCEPTED SOLUTION

Accepted Solutions

## Re: Finding tube surface temperature for HX tube

You shouldn't be solving for Qabs as it's independent of any of the other Qs and Ts.  It can be fixed above the solve block.  Either (i) you should include an expression for Qcond, including values for conductivity, thickness etc., or (ii) replace Qcond in the first equality within the solve block by Qrad (I suspect Qrad should be included in this first expression anyway, as Luc noted).  See below for my suggested solution for (ii):

(I wasn't able to open your .rar file)

Alan

18 REPLIES 18

## Re: Finding tube surface temperature for HX tube

Hi,

You have unmatched terms in the Find statement, add Qfluid to right hand side of the equation for consistency.

The left hand side should be a column vector rather than row vector.

If you upload you worksheet by first making it a zip file, it is possible to show you.

Cheers

Terry

## Re: Finding tube surface temperature for HX tube

But i have tried it before as you said.Anyhow, i am uploading this file here. Please have a look and point out the error.

Thanks.

## Re: Finding tube surface temperature for HX tube

I should expect Qrad to be (also) part of the top equation in the solve block (the one with Qabs=Qfluid+...).

Success!
Luc

## Re: Finding tube surface temperature for HX tube

Thanks, Luc.

True. Qcond should absolutely be part of the Qabs equation. Actually i got stuck while sloving these equations. But eventually i need to add two resistance values in overall Qcond as in my design i want to add thin sheet of aluminum on tubes to make whole conduction process more effective. So R.tot = R.conv1+Rcond1+Rcond2+Rconv2.

And Aluminum sheet between tubes will also help to conduct the heat flux.

I share screenshot of design.

What do you suggest in this scanerioMultilayer

## Re: Finding tube surface temperature for HX tube

Thanks, Luc.

True. Qcond should absolutely be part of the Qabs equation. Actually i got stuck while sloving these equations. But eventually i need to add two resistance values in overall Qcond as in my design i want to add thin sheet of aluminum on tubes to make whole conduction process more effective. So R.tot = R.conv1+Rcond1+Rcond2+Rconv2.

And Aluminum sheet between tubes will also help to conduct the heat flux.

I share screenshot of design.

What do you suggest in this scanerio?Multilayer

## Re: Finding tube surface temperature for HX tube

You shouldn't be solving for Qabs as it's independent of any of the other Qs and Ts.  It can be fixed above the solve block.  Either (i) you should include an expression for Qcond, including values for conductivity, thickness etc., or (ii) replace Qcond in the first equality within the solve block by Qrad (I suspect Qrad should be included in this first expression anyway, as Luc noted).  See below for my suggested solution for (ii):

(I wasn't able to open your .rar file)

Alan

## Re: Finding tube surface temperature for HX tube

Thanks, Alan.

I tried to upload the file. Sorry i don't know what happend with it.

You highlighted such a good point about not using Qabs in find syntax. Sometime it's really hard to point out the errors.

True (Qcond). As i got stuck in these equations. I will absolutely add Qcond factor in these equation.

Initially, i started to solve these equations without units but now i check with units.

## Re: Finding tube surface temperature for HX tube

Hi Alan,

There is stil error while solving the equation with units. In case of conduction,I reviewed my calculation so in case of heat transfer to fluid inside tube i have added the conduction and convection resistance in it. So i think Qfluid is total heat transfer to fluid including conduction and convection factor.

Thanks

Farhan

## Re: Finding tube surface temperature for HX tube

@farhan07 wrote:

In case of conduction,I reviewed my calculation so in case of heat transfer to fluid inside tube i have added the conduction and convection resistance in it. So i think Qfluid is total heat transfer to fluid including conduction and convection factor.

This is the best approach as long as you aren't interested in the tube wall temperatures.

There is stil error while solving the equation with units.

Mathcad 15 doesn't like mixed units within a single vector or matrix.

I'm still unable to open your .rar file.  I can download it ok, but 7-Zip, which I use for .rar files, says it can't open it!

Alan

## Re: Finding tube surface temperature for HX tube

Alan,

Please find this file in ".zip" format in attachment.

## Re: Finding tube surface temperature for HX tube

Thanks Alan.

I am using Mathcad 14 so there might be problem to solve these equation in that version. Anyhow i am uploading again please point out the errors.

Thanks

## Re: Finding tube surface temperature for HX tube

Ok.  I can open both of these files.  See attached for solution.  I've only done one file; the same approach can be used with the other.

Alan

## Re: Finding tube surface temperature for HX tube

Thanks Alan.

By using your apporach, i got the same values without dividing and multiplying with units. Anyhow this also works now. Thanks for your help.

Well done!

Alan

## Re: Finding tube surface temperature for HX tube

Or like this (why do I get a different temperature?):

Obviously the 4th is the only one that can apply...

You can even get a fully symbolic solution, although that is not easy to handle due to it's size...

Success!
Luc

## Re: Finding tube surface temperature for HX tube

You have sigma with 10^8 instead of 10^-8 Luc!

Alan

## Re: Finding tube surface temperature for HX tube

You're right. How did I miss that?

Anyway, with the correct value I get the same answer as you.

Luc