Modeling Convection Oven Flow

[tsantn]

This may be a double post...sorry if it is.

Im trying to predict the amount of time to heat a sample to a certain temperature using a forced convection oven.
Its a small sample sitting in the oven. Can i neglect internal energy of the sample and set the Heat input via Conv and Cond equal to the work supplied by the oven?

I contacted the manufacturer of the oven and they gave me a wattage value supplied to the heating element (3kW) Typically how much of this is actually supplied to the oven? Also, can i assume this input is constant?

Thanks,
This is a pretty stupid question, but im just starting out my engineering career and i've got lots to learn.

 

[Transient1]

A good heat transfer reference is "A Heat Transfer Textbook" by Lienhard and Lienhard. You can download the textbook for free, just google the title.

For heating, a lumped capacitance method (i.e. Biot Number<<1):

T-T_infinity =( T_initial-T_infinity)*e^(-time/(R*C))]

Where
C: is thermal capacitance it is equal to: Density*Volume*Specific Heat of material
R: is thermal resistance and it is equal to heat transfer coefficient * Area exposed to heat transfer

Determing the Heat Transfer Coefficient is not a simple task, but you can find some ways to estimates it in a heat transfer textbook. It may be simpler to calibrate and use a thermal couple and determine the time to heat your material experimentally.


By the way, Your problem is not a super simple problem at all and its never a sign of stupidity to ask a question.

 

[IRstuff]

A convection oven is usually not very well insulated, so there's a bit of heat loss. You can probably assume a worst-case of the oven's outer surface being at 60ºC, which can cause skin burns with prolonged contact.

A lumped model will be pretty close, so long as the sample is relatively small and relatively thermally conductive.

TTFN

FAQ731-376

 

[MintJulep]

can i assume this input is constant?

Probably not.

Assuming that the controls work to maintain a (near) constant temperature, once the contents of the oven reach near steady state the power input has to equal the box losses. The controls with cycle or modulate the power to do that.

 

[tsantn]

Thanks for all the suggestions.
-Nick