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re: wet genset building (continued)
5 jun 1999
steve cothran  wrote:

>this is a 10'x10' all concrete-block building with a 4" poured roof.
>about 50% of the building is below grade. there is no soil on the roof.

so the building has a thermal capacity of about 3500 btu/f. digging it up
and insulating with foamboard all round (including the roof) should help.

>the building houses a 20kw onan deisel...

which probably makes at least 50 kw of heat, ie 170k btu/h...

>and is equipped with a 2500 cfm blower, and 2x2' vent opening with a door...

you might start by sealing up the vent opening, and adding a 120 f cooling
thermostat to the blower. 

>when the temperature changed drastically... the building would "sweat" to
>the point that it would "rain" inside and rust things and damage controls.

specifically, this might happen when humid air leaks into the building and
condenses on the inside of the walls, which have a temperature below the
dew point of the air. sealing the building against air leaks should help. 

>i could stop this from happening by running the blower continuously...

seems like you want the building warmer, not cooler, to keep the walls
above the dew point of any air that leaks in. 

>aside: this set is used for... about 4 hrs./week on average.

hmmm. about 680k btu/week of heat, enough to heat 10k btu/f of thermal
capacitance 68 f.

>here's my next move. i've installed steel studs ("2x4's") like they
>use in offices for partitions. i plan to add r-whatever insulation,
>and vapor barrier/drywall...

seems like it's better to insulate outside, or insulate inside and add
a water tank on the roof, with insulation outside of that. the record
maximum temp in january in nashville is 78 f, so keeping the inside of
the building above that should prevent condensation. suppose you ran the
generator 4 hours a week, heating the building to 120 f, and it cooled
for the next 164 hours, when the average temperature is 36 f in january,
after which 78 = 36 + (120-36)exp(-164/tau). that makes tau = rc = 237
hours. with r15 insulation (3" of styrofoam), the building's thermal
resistance r = 1/40, so c = 40x237 = 9500k btu/f, which requires adding
about 6k btu/f above the roof, eg 6,000 pounds or 750 gallons of water,
eg an 8'x6'x2' deep plywood box full of water lined with a single piece
of 10' wide epdm rubber roofing material or a $200 8' diameter x 2' tall
plastic stock tank heated with generator cooling water. you could also
use that water for showers or house heating... 


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