Heat Flux or Overall Heat Transfer

[takiyasamsama]

Hi all,

I wanted to know in heat transfer which of these two are the determining criteria for the optimum heat transfer? I usually heard that it would be the heat transfer coefficient and the overall heat transfer coefficient however recently it was brought to my attention that some party, determine their effectiveness of heat exchanger according to heat flux.

I understand that heat flux is referred as the amount of heat transfer over meter squared area (SI). It would be best if someone would describe to me the difference between these two factors? Also how do we find / calculate the heat flux? Any specific formula? I found one formula that states; Q/A = k * delta-T; is this the correct equation for heat flux?

Regards,
Sam

 

[davefitz]

the best definition is whatever definition helps to lower the cost of the HX and allows your bid to be the winning bid, and which provides reliable long term service to the owner. Normally the heat flux needs to be prefaced by a note as to its defining boundary; based on tube ID or tube OD or extended surface area.

If the governing heat flux that prevents tube failure is the cooing medium on the inside of the tube then the design process will need to accurately compute that flux at that ID and ensure that the worst conditions that cause overheat are addressed ( eg, furnace waterwall DNB or critical heat flux) . If you are simply filling out a form to compare your HX with a competitors, then outer surface area is the value that is likely compared by the client.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[IRstuff]

I suggest you read up on heat transfer:

https://en.wikipedia.org/wiki/Heat_transfer

https://en.wikipedia.org/wiki/Heat_transfer_coefficient

Ultimately, you are trying to move heat from one place to another, and typically, that's to cool one place by making some other place hotter. Therefore, what's important is the power that's being transferred, i.e., watts, which is equal to joules/sec, and the temperature difference that causes that heat flow. Therefore, there ultimately some heat transfer coefficient that has the form of watts/kelvin. Since most heat transfer devices are somewhat scaleable by area, they characteristically have a heat transfer coefficient of the form watts/meter^2-kelvin. But, that solely of concern to the heat exchanger design, per se, and not anything to do with the source nor the sink, which is typically ambient air or some form of coolant.


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