Heavy Metal

[HVACMech]

I have a very interesting wall construction I am dealing with, and wanted to see if the group agrees with my assessment. I have a wall construction that is 3/8" steel on the outside, with 6x6x1/4" tubes every 24" on center with a 14 ga liner on the inside, in-between each column is R-13 insulation. At each one of those tubes I can't see any reason that during the winter the inside wall condition wouldn't be the same as the outside air, and during the summer the outside wall would be the same as the inside condition. This issue, given the coastal climate these buildings would be in, would cause condensation on the inside during the winter, and condensation on the outside in the summer.

 

[LittleInch]

A drawing would help a bit here - those "tubes" do you mean 6" square section boxes? What's in the box?

14 ga liner?

There will certainly be a fair bit of heat transfer here going on, but the temp won't be identical - 3" of metal temp needs to go via a 1/4" steel plate 6" deep.

what is this - a container?

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

 

[HVACMech]

It is actually a blast proof electrical building.
By tubes I mean 6x6x1/4" steel square tubing.
In the box is electrical switchgear.
It is 14 gauge steel liner.
I agree it seems that way, but it would actually be 1/2" of steel plate (because there are two sides to the box) and then the other 5.5" is dead air space, so no insulating value there either...

 

[dbill74]

For the wall construct you described, I'm thinking the R-13 between the steel tubes is going to be nearly useless.
It really brings to question what is driving the heat transfer through the wall.
My thoughts and experience tells me the mass and temperature of the ambient air.
In which case with cold walls in winter; condensation on the inside during winter would be a potential issue.
In summer however, the wall will tend to be warmer and condensation on the outside would not be an issue.

 

[HVACMech]

Well the R-13 between the tubes is the only thing that provides any U value to the assembly, so I would have to disagree with you on that point.

 

[berkshire]

Is there enough room in the building to put insulation on the wall between the 6x6 square tubes and the 14ga skin, if there is not , for practical purposes you are pretty much dealing with an un insulated building. What temperature and humidity levels do you have to hold in there ?
B.E.

You are judged not by what you know, but by what you can do.

 

[HVACMech]

You brought up exactly what I am trying to propose. Glad you agree with me!

 

[dbill74]

Point I was trying to make is that with 25% of the core wall being metal, the contribution of the R-13 insulation will be so small you might as well have an un-insulated building like Berkshire said.

I did some calcs once for a wall with R-19 insulation between metal studs, the overall U-value of the wall went down to about 0.091 (R-11). Unless you add insulation like Berkshire said, you'll be lucky if the overall U-value of the wall you described is 0.2 (R-5).

 

[HVACMech]

I came out at R-8 after my calc.

 

[dbill74]

R-8? Wow! I am honestly surprised it is that good.

 

[HVACMech]

I might have made a mistake

 

[DRWeig]

You could get a free copy of THERM 7.4 from LBNL and draw (pretty easily) a cross-section of each portion of your wall. It'll calculate U-values, and you can compute a weighted average based on area of each cross-section type... It could be done in a couple of hours. Nice program.

Best to you,

Goober Dave

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[lukaiENG]

Am I missing something? if this is an electrical switchgear enclosure, surely you are only heating it (not humidification).
In this case, assuming any ventilation comes from outdoors the dew point of the air in the space will match the dew point of the OA. Therefore the air will not condense on either side in the winter.

 

[HVACMech]

Yes you are missing something. The load in the switchgear building is over 10x the heating requirement for the building, and the building is almost IN the sea. So the building will be in cooling mode during the winter, and the humidity will still be high. My concern was that the outside air will be colder than the dew point of the building. After doing a very iterative calculation I think that the coil will keep up with the proper amount of dehumidification to eliminate the concern I had (similar to your point the space will follow the OA).

 

[lukaiENG]

It still seems like there is no humidity source - therefore the space dew point will equal OA dew point as the vapor pressure will find an equilibrium.

 

[HVACMech]

You are correct. There is no humidity source in the building. The concern during the winter was if it is 60 degrees outside, and then goes to 30 degrees in a couple hours, would my interior dew point keep up with outside air dew point.

 

[RossABQ]

The load in the switchgear building is over 10x the heating requirement for the building

I would expect electrical gear only needs to be maintained at a reasonably hot condition, perhaps suitable for techs working inside, high above any ambient wet bulbs? I guess I'm not understanding why you are insulating this at all. What temperature are you maintaining in the room? Most electrical rooms are left to rise to 80-90 degF or more. If you are at similar temps inside, heat flow thru the walls reduces your cooling loads, except perhaps in summer. I don't see how your inside wall condition is ever going to be the same as the outside ambient condition in the winter.

 

[HVACMech]

Building needs to be at 77 because there are large battery racks in the space. 77 is the preferred temp by battery manufacturers. Above that they de-rate them.

 

[RossABQ]

So is there also a large ventilation requirement? or are these sealed batteries?

 

[HVACMech]

Yes we do have to ventilate the spaces. It didn't turn out to be terribly large, about 1 ACH is what it shook out to.