Determining surface temperature

[gonj]

HI All,

This is the set up.

The room is at about 25CDB at 70%RH. I've worked out the due point temperature at these conditions is 21C.

There is an exposed concrete slab On top of this slab is a room with a temperature of 15C. The room is carpeted and I've worked the U value is approx 1.47W/m2.C

Could someone please help me determine the skin temperature on the underside of the exposed slab?

 

[IRstuff]

Somewhere between 15°C and the temperature 5 ft under the slab.

Is this for school?

TTFN

FAQ731-376

 

[gonj]

Thanks IR but that doesnt help me know if condensation will actually occur on the underside of the slab.

No its not for school.

Any other ideas on how to solve the problem?

 

[dvd]

The dew point is probably higher than 21C if the RH is 70% at 25C.

Which room is 25CDB and which room is 15C?

A little hard to follow.

 

[gonj]

Dew point is 19.2C at those conditions (link below is a good calc)

http://www.decatur.de/javascript/dew/about.html

level one is at 25C
level two is at 15C

level two slab consists of carpet, 200mm concrete and air films top and bottom - U valve works out to be 1.47W/m2.C

The problem im finding is, the surface temp of the underside of the concrete slab is not going to be as the level one ambient temperature because of this airfilm.

 

[IRstuff]

OK, it would have been really helpful if you had spent a few more words describing the problem; in cases like this, verbosity is the way to go.

So you have two rooms separated by a concrete slab and carpet whose combined thermal conductivity is 1.47 W/m^2-K, the bottom room is at 25°C, and the room above is at 15°C. The question is whether slab side of the bottom room can ever get below 21°C.

heat balance

htc1*(T1-T1s) = hslab*(T1s-T2s)
hslab*(T1s-T2s) = htc2*(T2s-T2)

T1=25°C, T2=15°C T1s,T2s=surface temps of flooring
hslab=1.47W/m^2-K htc1,htc2=convection coeffcient for each room

If htc1 is less than about 1.25W/m^2-K, then its possible for T1s to be less than 21°C.

I leave it to you to determine whether htc1 could be that low, or even lower.



TTFN

FAQ731-376