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Constant external temperature heat transfer

Constant external temperature heat transfer

Constant external temperature heat transfer

(OP)
Hello everyone.

My first time posting here so I apologize if I break any etiquette.

I used to be quite good at heat transfer but it has been 4 years since I took the course, and I am really starting to confuse myself.

Here is the problem.  I have attached a sheet to help visually, but basically:

I have a copper tube taking room temperature air and feeding into an oven.  The oven will be held at a constant temperature.  I want it to be as close to 850C as possible, but I also need the air exiting the pipe to be as close to 850C as possible.  This is because I am wanting the air out of the tube and the "valve" in the middle to be as close to 850 as possible.  I am specifying a diameter of 1 inch (ID) for the tubing, and I can control the flow rate.  

As I understand the problem, it should be quite easy.  Knowing the flow rate, I need to find a length of tubing that would allow enough time in the tube for the air to heat up.  It took me a while to realize I could not assume constant tube temperature, only constant external fluid temperature, and have not been able to make any real progress since then.

RE: Constant external temperature heat transfer

(OP)
The oven is not circulated.  Electric heating coils are built into the insulated walls.  There is no fan moving the air around on the inside or anything like that.

RE: Constant external temperature heat transfer

Then why run it through this "oven?"  Wouldn't it be faster and more economical to simply wrap the piping with heater tape?

TTFN

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RE: Constant external temperature heat transfer

(OP)
This is actually a test on a wastegate valve that has to be done at temperature.  The idea is that the air as it flows to the wastegate and the wastegate itself should be at a similar, common exhaust temperature to simulate time operating conditions.  

 

RE: Constant external temperature heat transfer

(OP)
I was hoping someone could jump in with a quick heat transfer equation/method that could relate all of these variables in an easy closed form solution.  Is this somehow more complicated in a way I am missing?

RE: Constant external temperature heat transfer

I'm with IRstuff on this one.

Then insulate your valve.  Run the air heater for a while and bring the valve up to operating temp within the insulation.  A lot easier than trying to build an air to air heat exchanger.
 

RE: Constant external temperature heat transfer

There is no closed form solution, there are only approximations, particularly at this temperature, which involves both convective and radiative transfer.

You can start with the basics:

(1.2 kg/m^3)*(1080 J/kg*K)*(850ºC-25ºC) = 505 W/cfm

You can download a free textbook: http://web.mit.edu/lienhard/www/ahtt.html  There is a section on heat exchangers.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Constant external temperature heat transfer

(OP)
I finally found a proper example and I was able to solve this using a couple lines of equations.  

I used Ra and Pr (and various other lookups) to solve for Nu.  That gave me my h value.

From there the only unknown was emissivity, which i simply set for its worst case scenario.  

I need almost precisely 3.25 meters of tubing at my specified flow rate and diameter.  I did not use the above calculation, although I suppose if I were to fix the length and vary the flowrate that would have been used again.

Thanks for the help,
-John

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