# Enclosure BTU calculation

1. Mar 29, 2017

### Acside

Hi all. I'm working on a small project and could use help.

I have an 3/8" thick aluminum box that is going to be placed outside. The dimensions of the box are 33"x33"x33" and inside the box there is a 3/4 air gap on all sides that is covered with 1/8" hdpe platic walls.

There will also be a 100w output within the box.

Lets say I want to maintain 70f degrees inside of the enclosure.

Using a DAQ setup I have been able to determine that the surface temperature in direct sun reaches ~130f and the internal hdpe walls reach ~100f. Using r3.5 foil/bubble insulation behind the hdpe gives me a lower internal surface temp of 90.

My questions are: 1) how could I calculate the actual R value of all the surfaces using my DAQ setup? This would be useful to mathematically test changes to the enclosure.

2) How could I determine the BTU's needed for cooling to maintain 70f? Would a simple conduction equation like "btu=UxAx(t1-t2)" be all thats required if I want to assume all external surfaces are the same temperature?

3) If I assume the internal air volume is 20cuft and 100 degrees could I use btu= (t1-t2)x1.08xcfm to determine cooling required to bring the air down to 70f when the box has been unconditioned for some time like a peak cooling load? " 30x1.08x20=btu?"

4) Finally, what would be the equation to determine actual temperature differences between one side of surface and the other. If I have an r4 insulation board that is 100F on one side how can i determine the temperature rise on the other?

Any input would be appreciated!

2. Apr 3, 2017

### PF_Help_Bot

Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.

3. Apr 3, 2017

### Staff: Mentor

Welcome to PF!
Add the R-value of a sheet of HDPE to 3.5.
Yes, though you need to add the heat that you are applying to the inside to that...
That's the energy required to cool the air inside down once...but doesn't include the heat capacity of what is inside or the walls.
The surface temperatures of each side are almost exactly the ambient temperatures in the two spaces.