Watts required to raise air temp
Watts required to raise air temp
(OP)
Help!
I am in a huge pinch for time and thus throw myself on the mercy of the group.
I have a box that I am shipping off to a -45C environment. It houses to small PC Boards and is enclosed in another larger equipment box. The larger box is not sealed from the environment yet there will be no direct wind blowing on the little control box. The box is 320 cuin (5243 cu cm) internal and is made of stainless steel. It is vented top and bottom.
Is there a direct way to figure how many watts I will need in heating to raise the ambient inside the box to -40C? Or better still, how many watts per degree C.
I am in a huge pinch for time and thus throw myself on the mercy of the group.
I have a box that I am shipping off to a -45C environment. It houses to small PC Boards and is enclosed in another larger equipment box. The larger box is not sealed from the environment yet there will be no direct wind blowing on the little control box. The box is 320 cuin (5243 cu cm) internal and is made of stainless steel. It is vented top and bottom.
Is there a direct way to figure how many watts I will need in heating to raise the ambient inside the box to -40C? Or better still, how many watts per degree C.





RE: Watts required to raise air temp
Yes you can work out first the heat required to raise the temperature from -45 to -40C by using this formula:-
Q = m*Cp*(T1-T2) was m= mass of air in box
Cp= specific heat cap of
air
T1 and T2 are the final and
initial temps of air resp.
Now if you know your heat source power or how much time you want before the box gets to the reqiured temperature then the Q calculated above is related to power by:-
Q/(time) =power in watts
regards
desertfox
RE: Watts required to raise air temp
However, I don't think you asked the right question.
I suspect that you want to know the power that you need to MAINTAIN the box temperature at -40C.
RE: Watts required to raise air temp
Next, you are correct MJ. I need to maintain -40C.
How does that change things?
RE: Watts required to raise air temp
Well if its always in a -45 enviroment the heat calculated above is what you need initially to raise from -45 to -40, to maintain it you need to decide what tolerance on the -40 you can tolerate before a heat source kicks in to bring it back to -40, so if your heater cuts in at -42 you can rework the calculation to give you the heat required, the power then required is that heat energy calculated at -42 divided by the time required to bring it back to -40.
Basically you need some temperature control ie thermostat to turn heater on and off.
desertfox
RE: Watts required to raise air temp
So from the above calculations at -50C,
p of air =1.534,
C=1.005.
With a volume of .0052M^3, my air mass is then .0079Kg. Which gives a Q of .0039.
If I have 200W of heating, that means I should be able to change from -45 to 40 in .0017 hours?????
Something seems very wrong.
RE: Watts required to raise air temp
Assuming that your box dissipates more than 3.8W, it'll self-heat the box and account for the heat loss to the larger box in about 20 seconds.
Joule heating:
(320ci)*(1.2kg/m^2)*(1005J/kg-K)*(5K)/(10s) = 3.16W
Convective loss
5*[(320ci)^1/3]*(5W/m-K)*(5K) = 3.77W
TTFN
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RE: Watts required to raise air temp
There is also the heat loss from radiation and conduction from the outside of the box as you are heating it.
RE: Watts required to raise air temp
I really wish I had all the information to solve the problem, but as is often the case we are being forced to guess and fast.
The system is designed to turn on the 200W worth of heaters at 0C (Ohmite series 270 100W at 10 Ohm). As for how much heat the box will dissipate, you got me. It is stainless steel and in the volume I stated. The PC boards are extremely small so my gut tells me that they are negligible considering the surface area of the box.
The components in the box are rated to -30C. So I guess the better question may have been will 200W of heaters be sufficient to keep that volume of air above -30C when the ambient may drop (over time) to -45C?
RE: Watts required to raise air temp
Just check your figure because I get 0.012 hours,
Its a very small mass of air your dealing with and temperature range.
IRstuff and MrBTU also make good points.
desertfox
RE: Watts required to raise air temp
TTFN
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RE: Watts required to raise air temp
You state that it is vented top and bottom. As soon as you get a temperature differential you are going to start driving a convective flow of cold air in from the bottom.
RE: Watts required to raise air temp
You would do well to implement your 200W solution and then test it. Find someone with a deep freeze and pay them something to use it for a day.
I have worked in many deep freezes it's not a big problem.
MintJulep's warning is very true.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Watts required to raise air temp
Thank you so much for all your help.
I revising the design to have 4 100W heaters. Once complete, I am going to send it off to Baltimore for a cold room test. Though this looks like a simple problem to management, I know enough to know it is not simple.
Good suggestions and thank you for the teaching.
RE: Watts required to raise air temp