Project ForgeA building has to be maintained at 21°C (dry bulb) and 14.5°C (wet bulb). The dew point temperature under these conditions is 10.17°C. The outside temperature is −23°C (dry bulb) and the internal and external surface heat transfer coefficients are 8 W/m²K and 23 W/m²K respectively. If the building wall has a thermal conductivity of 1.2 W/mK, the minimum thickness (in m) of the wall required to prevent condensation is
- A0.471
- 0.407
- C0.321
- D0.125
Explanation
Condensation is prevented as long as the inner wall surface stays at or above the dew point (10.17°C); the critical (minimum-thickness) case sets it exactly there. Heat flux from inside air to the inner surface: q=h_in(Tin−Tdp)=8×(21−10.17)=86.64 W/m². Required total resistance: R=(Tin−Tout)/q=44/86.64=0.5079 m²K/W. R=1/h_in+L/k+1/h_out=0.125+L/1.2+0.0435. Solving: L/1.2=0.339, L≈0.407m.
Source: GATE ME 2007 (2007)
This question is part of a 15-day revision cycle — sign in and Project Forge will bring it back the day before you'd forget it.