Hi all,(adsbygoogle = window.adsbygoogle || []).push({});

I am currently doing an engineering internship (still an undergrad), my field is electrical engineering so thermodynamics just confuses me!

My job has been to come up with a good Environmental Stress Screening porgram (ESS) plan, while doing this I have realised that the industrial oven that this company has is, well, crap. I am not the only one to realise this and the wheels are moving on getting a new oven, although I need to highlight how bad the current oven is for the head dudes to give the green light.

Right, to the actual problem.

I am trying to figure how long it should theoretically take for the rack of components in the oven to reach the desired temperature. There will be practical tests for this but I could do with the theory to back me up.

The plan is to set the oven to 80C (and -20C) and see how long it takes for the equipment to reach its steady state, now all the equipment is going to be running so getting it to it's hot steady state shouldn't be as hard as getting it to it's cold steady state.

From what I can gather from the internet I am basically looking at estimating the thermal time constant of the system. Then according to the paper I am reading ( which is attached, highlights all the steps I have skipped to get to the thermal time constant), 4 time constants should take it to around 98% to it's max temp which is close enough for me.

It gives the thermal time constant as:

Tau = (M X Cp)/ (h X A)

M=mass , Cp = specific heat capacity, h = convective heat transfer coefficient , A = surface area

well mass is easy enough to get (120kg), specific heat capacity isnt that easy because of the wealth of different materials in the components (I can attach a photo of the components tomorrow to show the set up, I am assuming a Cp of 0.162 Btu/lb F),I realise this is imperial units btw, I don't mind taking a rough approximate for this though. The surface area I am not so sure about because of what surface to take as the 'area' ( the dimensions being 765mmX600mmX390mm), and the heat transfer coefficient I a have no idea how to calculate.

I am assuming that the heat transfer coefficient is different for every oven? The power of the current oven is 3kw and the new oven is 51kw. I am guessing the power of the oven has to impact the transfer coefficient in some way because if it didn't then the time constant would be the same for both the garbagety oven and the new (hence no point getting a new oven). I have seen a few sources that give ranges for the coefficient of between 10-200(W/m2K) but that is quite a big range.

I would just like to know if I am on the right track with estimating how long it would take for this thing to get to its steady state temp? Is there more information needed?

Any help is greatly appreciated. I am very shaky with my thermodynamics but I am going to keep looking into this

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# Calculating time for object to reach ambient temperature in an oven

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