Cooling of box and apparatus inside

[Miran Fernando]

Hello,

New user here, excited to draw on knowledge of others.

I have a small box to contain a science apparatus. The science apparatus needs to be kept cool and has inlet and outlets for fluid tubes. Sorry for this question if it is silly: do I need to cool the inside of the box as well? If I am actively cooling the apparatus, aren't I also cooling the inside air?

Some background. The box (which will be insulated) will be stored in a small shed outside that will be exposed to the ambient air. There are times of the day when the box may be in direct sunlight. The science apparatus will be cooled by a nearby chiller, so not too worried about it getting very warm. But I am unsure about the air inside the box. Doesn't the chiller need to "work harder" if the air begins warming up?

This is not my usual area of expertise and any guidance would be much appreciate.

Many thanks,
Miran

 

[1503-44]

For future reference, there is a heat transfer forum.

If your instruments are inside a box, then the box becomes your ambient environment of the instruments. Your instruments must be capable of functioning at any conditions that develop within the box. Your box will exist in its own ambient environment of the outside temperature.

The conditions that can develop within the box are affected by the conditions outside the box, the natural environment, but also the condition of the fluid within the instruments.

If the instrument fluid's temperature is cooler than the temperature outside the box, heat from the higher temperature outside the box will warm the interior of the box and also be transferred to whatever fluid is contained by the instruments. If the fluid is of a higher temperature than that outside the box, the fluid will warm the box, which in turn warms the outside environment. Either of those two conditions could define a steady state heat flow, which will develop after a sufficient time has passed.

There are transient heat flow conditions that can be present before either steady state condition is reached that could either cause the box interior temperature to rise or fall, depending on which way the heat is flowing. The temperature inside the box affects how fast heat comes across the box's surface as it moves to a cooler fluid, which in turn affects how fast that same heat is transferred to the fluid, or in the opposing case, the reverse flow of heat, if the fluid is warmer than the outside temperature.

The temperature at any time inside the box will be a function of how much heat/second is entering the box from the outside verses how much heat/second is being carried away by the fluid within the instruments. Or the reverse case. Each of those heat flows is measured in Btu/second, or Watts. Whichever heat flow is greater at any given time will determine if the temperature inside the box is rising or falling as it moves to a steady state where heat entering is equal to heat leaving. Mother nature will find that eventual balance point by changing the rate of heat transferred per second across each surface if the heat input and output remain constant for a sufficiently long time.

When only one heat transfer surface is involved, heat transferred is proportional to the temperature difference between one side and the other, which can be calculated by knowing the heat transfer coefficient of the material separating inside form outside. Is the box steel, copper, or Styrofoam.
When more than one heat transfer surface is involved, say you have steel and Styrofoam, heat transfer is a function of both materials. If you have air (like inside a box) we add another coefficient for that. Evaluating all coefficients, just like of electrical resistors, produces an "overall heat transfer coefficient" and we calculate the heat loss or gain into or out of the system as if all the materials were combined into just one.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Put shorter, your apparatus will cool the air in the box. So yes you are chilling the inside box air, but air is low density and hence it doesn't have much impact unless your box is big and your apparatus small.

Depending on the amount of insulation the impact of external temperature/sun could be very small or quite significant.

Diagram, sketch details would help....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Miran Fernando]

For future reference, there is a heat transfer forum.

Much apologies, I did not know. Should I ask moderator to move thread?

If your instruments are inside a box, then the box becomes your ambient environment of the instruments. Your instruments must be capable of functioning at any conditions that develop within the box. Your box will exist in its own ambient environment of the outside temperature.

Yes now that you say it, I think it is very obvious. Thank you for making it clear.

As you say, my box is steel and polystyrene so I will calculate heat transfer coefficient. I think next step is for me to calculate temperature of air inside box as the day goes on (maybe assume box is in direct sunlight for some time as well)? If temperature does not fluctuate too much, then apparatus cooling is sufficient? If it changes too much then maybe need heat exchanger inside box as well.

Diagram, sketch details would help....

Yes, it has been long day. So tomorrow I will provide sketch.

Thank you both for your advices so far.

Miran

 

[IRstuff]

More directly, your chiller will need to be sized to handle the heat load from the box interior air getting hot.

TTFN (ta ta for now)
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[1503-44]

If your box is exposed to ambient air, presumably the box is ventilated, so the box cannot reach any more than the ambient temperature outside the box. Note that the official high temperature readings in the USA are taken from the interior of a well ventilated white box placed 4 ft above ground level.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[IRstuff]

OP says box is insulated

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[Miran Fernando]

Hello,

Much apologies for the delay.

If your box is exposed to ambient air, presumably the box is ventilated

No, box is sealed and not ventilated.

Diagram, sketch details would help....

Here is sketch of box.

Box is 0.5 m in length, width and height. Apparatus will be attached to front of box but on little posts so not making directing contact.

I will assume all heat generated by apparatus will be carried away by apparatus cooling tubes. Only heat entering box is conducted through walls of box or from direct sunlight on box. Thickness of polystyrene can be made thicker, but steel probably 1.6 mm.

I zoom in on small section of box walls.

I assume inside temperature T3 is 25 °C. I assume outside temperature T1 is 40 °. Is this okay assumption, that surfaces are same temperatures as surrounding air?

I then use equation for steady rate heat transfer through multilayer wall:

Qdot = (T[sub]1[/sub] - T[sub]3[/sub]) / R[sub]total[/sub]

R[sub]total[/sub] = L[sub]1[/sub]/k[sub]1[/sub]A + L[sub]2[/sub]/k[sub]2[/sub]A

I careful to make sure areas (A) are the proper sides or tops of box. This tells me amount of heat getting through walls of box because of internal and outside temperature difference.

After this I calculate solar energy on box and how much is transferred inside (I use reflective foil on outside). I add this to previous amount of heat and that tells me how much total heat needs to be remove to keep inside of box at 25 °C.

Not sure if all correct, please tell me I make mistake.

Miran

 

[1503-44]

No. For example a black surfaces exposed to afternoon sun "on a 25°C day" can easily reach 70°C
I don't know what temperature a reflective surface might reach, but I'd guess it could be in the 40 to 50°C range in even high North latitudes.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

The side facing the sun will get hotter, even with reflective surface - maybe as high as 50C. Thermal input from peak solar is normally about 1kW/m2. With reflective maybe 600-700W?

The other sides will get simple air convection heating - much lower heat input, but if the box is freely suspended by say a bunch of wires then yeas the other sides could assume to be at ambient air temp.

The steel conductivity is so much higher than Polystyrene ( about 1,200 times) that it's almost not worth bothering about. Its only impact is that the inner surface of the box will be a constant temp as the steel will move any hot spots or sides.

Also as your inner box surface area is quite a bit smaller than the outer box surface area your simple flat plate analysis calculation formula doesn't work. Quite sure there's a formula somewhere, but don't know what it is....

But for a box this small nothing is going to be accurate and this really looks like a bit of a waste of time. With say 75mm / 3" of polystyrene insulation, your heat input is going to be really pretty small. How much heat are you taking out from the mysterious apparatus. Assuming this is not minute, then just add 10% for external heat. Or make the insulation 150mm and it will cease to be an issue with only 15C differential temperature.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Miran Fernando]

No. For example a black surfaces exposed to afternoon sun "on a 25°C day" can easily reach 70°C

Yes you are right, thank you. I will try to estimate somehow.

Also as your inner box surface area is quite a bit smaller than the outer box surface area your simple flat plate analysis calculation formula doesn't work. Quite sure there's a formula somewhere, but don't know what it is....

Yes I suppose it is no longer a wall and more like rectangular block with tapered ends.

But for a box this small nothing is going to be accurate

Why not accurate?

and this really looks like a bit of a waste of time. With say 75mm / 3" of polystyrene insulation, your heat input is going to be really pretty small

There might be opportunity soon to test internal temperature with thermometer. I am trying to estimate heat entering box as learning exercise. But I see your point.

How much heat are you taking out from the mysterious apparatus

Not much, probably less than 100 W. It is just small computer.

Thank you sirs for your continue help.

Miran

 

[LittleInch]

Based on 75mm polystyrene insulation with a 15 C temp difference I get a heat input of about 10W.

So my 10% wasn't too far off.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Miran Fernando]

Based on 75mm polystyrene insulation with a 15 C temp difference I get a heat input of about 10W.

So my 10% wasn't too far off.

Yes very close to my answer as well.

Thank you all for your contributions and advices.

Miran