5 replies to this topic

[Bal]

Hi all, 

 

 

How to calculate U in a heater? One fluid is flowing in the tubes and fuel gas is heating the tubes? 

 

Q=U*A*LMTD

 

In case the flow is laminar, the following correlation is valid:

 

 

Nu=h*d/k, i believe h=U (overall heat transfer coefficient). Also, Nu =f (Re,Pr,...)

 

==> U depends on Re and Pr.  

 

Re depends on the flow rate of the fluid. Which fluid is this?

 

 

 

Thanks, 

[breizh]

http://www.hcheattra...efficients.html

 

Hi ,

Hope this helps

 

Breizh

[Francisco Angel]

Dear Bal:

You can also check "Heat transfer equipment" chapter on Perry's Chemical Engineers' Hanbook. You can review the chapter in detail if you have the time, in case you are in a hurry, correlations for calculating heat transfer coefficient are tabulated for several configurations.

Best regards.

[srfish]

U does not depend on an individual Re and Pr but two separate ones. One for the shell side and one for the tube side.

[Bal]

U does not depend on an individual Re and Pr but two separate ones. One for the shell side and one for the tube side.

@srfish you mean we calculate U shell and U tube using the respective dimensionless numbers for each? 

 

Also, does Q in the shell = Q tube? If that's correct and logarithmic mean temperature difference is the same for each. 

 

Then using Q=U*A*LMTD:

 

Would that imply that (U*A) shell= (U*A) tube?

 

If Q in the shell = Q tube, how do we account for the losses?

 

Many Thanks,

 

Bal

[Francisco Angel]

Dear Bal:

 

 

 

@srfish you mean we calculate U shell and U tube using the respective dimensionless numbers for each?

 

That's correct, we generally call these the respective "h" values, be aware that the form of the correlation can also be different if the fluid runs inside the shell or tube side.

 

 

Also, does Q in the shell = Q tube? If that's correct and logarithmic mean temperature difference is the same for each. 

 

Then using Q=U*A*LMTD:

 

Would that imply that (U*A) shell= (U*A) tube?

 

Q Shell=Q tube is an assumption.

U is an overall resistance to heat transfer, so it doesn't belong solely to the tubes or side shell, it is calculated like this (some assumptions are involved in this formula, like neglecting curvature and the resistance of the tube wall):

 

U=(1/h_tube+1/h_shell)^(-1)

 

 

If Q in the shell = Q tube, how do we account for the losses?

 

As I said before, Q Shell=Q tube is an assumption. If you want to account for losses, you would need a model for them (which can be some simple like, 10% of QShell for example), and in that case, you will have:
 

QShell+QLoss+Qtube=0

 

Taking proper account of the signs of course.

 

Bal, you seem just starting with these topics, I recommend you to read an introductory chapter of some heat transfer book, where these concepts are explained in a simple manner.

 

 

Best regards.