Convection in isolated box (please help)

[rookie5]

Hi,

I do not have much knowledge about this. Please help me

I have a question. If cooling elements (e.g. ice) put inside an isolated box, is the surface area of the cooling element matter or not? With the same weight but different area, is matter or not?`

Thanks. I appreciate your help.

 

[Rspain]

Quick answer:

if box isolated, from a long-term thermodynamical equilibrium point of view, no difference, when time is infinite, result only depends on the total heat capacity (mass, Cp, and latent heat - if phase change involved) and the starting temperature of the cooling element.

from a kinetic point of view (time evolution of temperature profiles, etc.), many complex factors are involved and many other things matter.

Not sure to understand your question,

BR,

 

[rookie5]

If i put 2 kg of water divided into 2 bags and 2 kg of water divided into 4 bags, into the isolated box, is it going to have the same result?

 

[Rspain]

same end-result

 

[Rspain]

Think it this way:

If the box is isolated, the end result (infinite time) will arrive when the content reaches thermodynamical equilibrium.

From a heat transfer point of view, that means equal temperature.

since isolated, end result can only depend on starting condition.

Any two starting conditions that are equivalent will lead to the same end-result.

Now, do you think that splitting the cooling agent (with no variation in mass nor temperature) into two pieces changes in anyway the staring condition of the cooling agent, from the point of view of overall thermal energy in the cooling agent?

BR,

 

[rookie5]

Sort of. I thought with different size of cooling element like the thick ness (same weight), it will have different result.
Shouldn't be the thinner the cooling element, will melt faster than the thicker one?

 

[Rspain]

Please note I have said:
end-result
time infinite

Faster is a time-dependant concept.

Do not confuse the end-result with the time-dependant path to the end result.

Time-dependant path to the end result is a complete different story, since it has to do not with thermodynamics but rather with kinetics

 

[IRstuff]

Ice or water, which is it? The short-term answer is that everything has a dependence on surface area

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[rookie5]

Can I calculate how many cooling element do I need based on the surface area?

 

[IRstuff]

Perhaps you should explain the problem, rather than bits and pieces of your "solution," particularly as it appears that you don't have a thermo background.

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[rookie5]

So, I want to maintain the temperature product between 0°C-4°C inside an insulated box for 48 hours. The thickness of the insulation is 25mm and the thermal conductivity is 0,056 W/mK. Let's say the outside measurement of the box is 350 x 350 x 300 mm. Outside temperature 25°C. How many cooling elements do I need to maintain it?

Nope. I do not learn much about thermmodynamics. Sorry to bother you.

 

[Rspain]

isolated and insulated is not exactly the same thing...

My suggestion: you buy for less than 15$ a thermometre in Amazon (i.e. ThermoPro TP-50) that measures temperature and stores min and max temp. Place inside box + trial and error. Worst case scenario it will take you a couple weeks to get an acceptable solution.

BR;

 

[IRstuff]

OK, so now you've posted a realistic problem. How much volume in the box can be used for ice? What happens if the ice is colder that 0C?

Can you get more insulation, as it appears that you're describing a basic Styrofoam cooler, and a wimpy one, at that. You need a minimum of 12kg of ice to meet the requirements. Since your volume is is 37 liters, you could cram about 37 kg of ice in there, if your product was small.

You could put your cooler inside a bigger cooler.

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[Rspain]

I must be doing something wrong... I get 37W, how much do you get?

 

[IRstuff]

I got 25W, but that is a low end. My high end is around 30W. Both of those numbers are a bit pessimistic, by assuming that the inner surface is at 0C. Nevertheless, your 37W simply ups the minimum ice required to 19 kg.

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