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.
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
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RE: Constant external temperature heat transfer
RE: Constant external temperature heat transfer
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Constant external temperature heat transfer
RE: Constant external temperature heat transfer
RE: Constant external temperature heat transfer
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
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.
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RE: Constant external temperature heat transfer
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