Temperature drop across refractory. 2

[its4harish]

Hello All.
This is my first post in this forum.
Please refer to the attached file. It’s an excel file for calculation which I tried to develop (formulas used are written) and when I substituted the Input values from vendor data it gives correct output (Outside metal shell temperature) as given by vendor. This is for a ground flare refractory.

http://files.engineering.com/getfil...93-4a34-b45a-ce0635a471e6&file=Calc_sheet.pdf

My initial doubts are:

a) Is this the right way to calculate? (I have an intuition that things are not as simple as this!)

b) Where does the outside wind velocity term fit in?

 

[ione]

It seems right to me, from the point of view of considering thermal resitances in series.......

Without measuring the outside metal shell temperature, you have to proceed by iteration, since the "Heat transfer coefficient for heat transfer outside" depends on temperature.

I suggest you to give a read to the thread below

http://www.eng-tips.com/viewthread.cfm?qid=268145

 

[its4harish]

Ione, Thank you for your reply. I had already read the topic you suggested before I posted this. In fact, I posted this because, to the best of my knowledge, only mention of effect of wind velocity in the above topic is in reply from "boilerone". As per the reply, it seems the change in ambient wind velocity will change the heat exchange coefficient at outer surface. In addition I couldn't find the velocity term in the informative file you attached with your first reply in the topic. Please correct me if I am wrong. So I guess, as "boilerone" pointed out, wind velocity changes the coefficient and once i get the coefficient right, result is right.

But I am curious about few more things:
I have seen some specs asking to limit outer casing temperature to 82 deg C (at ambient 27 degree C & Wind velocity = 0 ) and some other specs to 60~80 deg C (at ambient air temp & Wind velocity = 2.5m/s). It seems to show that wind velocity does play an important role in deciding the refractory thickness and overall heat transfer in the system.
a) Is there a formula or graph showing how the heat exchange coefficients change with wind velocity?
b) Please correct me if I am wrong. As per your reply, my calculation is theoretically right but practically i will have to consider the system after reaches the steady state and then i have to do the iteration as you explained in your earlier uploaded file. Right?

Thanks in advance. Sorry if I am wasting your time.

 

[IRstuff]

I suggest you do some more reading on the subject. Convection is the process by which heat is removed through collisions of gas molecules with the hot outer surface, imparting kinetic energy transferred to the gas. A higher wind velocity results in more collisions and faster removal of hotter gas molecules. There is no single, simple, equation.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[corus]

Take the worst case. Do you expect it to be always windy or would the worst case, in terms of shell temperature, be when there is no wind?

Tara

 

[its4harish]

IRstuff,
Thanks for replying. Yes, i do have basic understanding on the subject.
Referring to this thread,
thread391-69025
Last post:
"Assume the outside temperature is 25oC, wind and radiation effects result in an external htc=15W/(m2*K)"
"There are graphs that help in estimating the outside htc w/o many iterations."

If the equations are not simple as you said, i think the graphs will do for me. What I am basically looking for is:
How to find outside heat transfer coefficient if we know ambient wind velocity and ambient temperature.

Thank you. Have a nice day..

 

[ione]

You can consider the geometry is as that of a flat plate
The heat flow from the plate is calculated as:
q = h *A *(Tp - Ta)
Where
h = Nu k / L average heat transfer coefficient,
A= area of the plate,
Tp = plate temperature
Ta = ambient fluid temperature

Where Nu is the Nusselt Number

k = fluid thermal conductivity
L = length of the plate

You can calculate Nu using correlations at

http://www.egr.msu.edu/~somerton/Nusselt/index.html

Where Re is the Reynolds number and Pr is the Prandtl number and they are calculated using fluid properties at film temperature Tf = (Tp + Ta) / 2

The higher the fluid velocity the higher Re (this is how the heat transfer coefficient depends on fluid velocity).
Now, since Tp is unknown and the heat transfer coefficient h depends on it, you’ve to proceed by iteration.

 

[its4harish]

Thank you Ione and thank you all for the replies..
Have a nice day..

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