Heat Loss vs. Time from a Refrigerator

[H2Odrinker]

So I'm just starting work for a non-profit trying to improve vaccine refrigerators for developing countries among other things. This is the one project I've been given where I'm lacking any real world experience.

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The basic problem is determining how long it will take for a given regrigerator with some "R value" insulation to have an increase in the inside temperature from 3 C to 10 C in the event of a power loss.
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So I'm bench testing a small field refrigerator with thermocouples and varying the load in the refrigerators with water packs (no ice allowed because of vaccine freezing). What I would like to do is also develop a rough equation for evaluating the temperature vs time change while varying:
1: Insulation (R value = 1/k)
2. Thermal load inside the small refrigerator

I know of the basic equation for conductivity through the refrigerator insulation
1. Q = (k*A*dT)/x
and the equation for thermal capacitance of the load in the refrigerator.
2. Q = m*Cp*dT (for each given item of different material)

It would be great to know for a given refrigerator if its more worthwhile to say, increase the R value of the insulation by 1.5 (just an example) vs. adding, lets say, 5 liters of ice packs.

For the sake of simplicity, just pretending there is a box with an r value and "x" number of water packs inside would be great simple example to begin with.

Wow, this ended up long. Hopefully there is a good heat transfer saint out there.

thanks!

shawn

 

[IRstuff]

The missing equations are Q = A*htc*dT for convection, and Q = emiss*A*Boltz_const*(Tb^4-Tr^4) for radiation, including solar loading.

You can do a crude model assuming a constant dT to the ambient, and see how many watts are been gained.

TTFN

FAQ731-376