Pobierz pełny numer 1/2010 S&E - Structure and Environment - Kielce

Ewa Zender-Świercz, Jerzy Zbigniew Piotrowskiwhere: U i– heat transfer coefficient of the ceiling,U e– heat transfer coefficient of the roofing surface,t i– temperature of internal air, t e– temperature ofexternal air.Fig. 3. The internal air humidityIn spite the low humidity of external air, the humidityof internal air was high. Its value was almostthe same when the humidity of external air was highbut the air grates were covered. We can concludethat when the humidity of external air is high and theair grates are uncovered, the humidity of internal airincreases. As a result of this condensation can occur.The above formula does not take into account thesurface of inflow – grate. The new formula, wherethis parameter is considered, is proposed.The air velocity inside roof space is much lowerthan wind velocity. So we can presume that pressureof internal air is the same barometric pressure (statical).Therefore pressure loss is equal to:(2)where: K 1– aerodynamic coefficient, v – external airvelocity, r N– air density.On the basis of above-mentioned the modified formulais proposed:(3)where: m – spreading of inflow air coefficient, F N–inflow – grate surface, K 1– aerodynamic coefficient,v – external air velocity, r N– air density.Now the calculation of air temperature in roof spaceis given by:Fig. 4. Temperature of internal airThe average value of the temperature in the roofspace was 3.8 o C. After the air grates were uncoveredthe temperature increased to 5.9 o C.4. Calculation and analysisThe calculation of the temperature in the roof spacewas made using the formulas from the literature [2].Steady – state temperature in some distance frominflow – grate is given by:(1)(4)where: U i– heat transfer coefficient of the ceiling, U e– heat transfer coefficient of the roofing, t i– internalair temperature, t e– external air temperature, F i–ceiling surface, F e– roofing surface, C p– specific heatof air, G N– air stream.When the temperature is calculated and the humidityis measured the dew point may be determined(Mollier graph) t R= -0,1 o C.The next step is calculation of the roofing surfacetemperature:60