The air supplied to a room of a building in winter is to be at 17°C and have a relative humidity of 60%. If the barometric pressure is 1.01325 bar, find :
(i) The specific humidity ;
(ii) The dew point under these conditions.
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The air supplied to a room of a building in winter is to be at 17°C and have a relative humidity of 60%. If the barometric pressure is 1.01325 bar, find :
(i) The specific humidity ;
(ii) The dew point under these conditions.
The air supplied to a room of a building in winter is to be at 17°C and have a relative humidity of 60%. If the barometric pressure is 1.01325 bar, find :
(i) The specific humidity ;
(ii) The dew point under these conditions.
Dry bulb temperature, tdb = 17ºC
Relative humidity, φ = 60%
Barometric or total pressure, pt = 1.01325 bar
Specific humidity, W :
Corresponding to 17ºC, from steam tables,
pvs = 0.0194 bar
φ = \(\cfrac{p_v}{p_{vs}}\)
0.6 = \(\cfrac{p_v}{0.0194}\)
pv = 0.6 × 0.0194 = 0.01164 bar.
Specific humidity,
W = \(\cfrac{0.622\,p_v}{p_i-p_v}\) = \(\cfrac{0.622\times0.01164}{1.01325-0.01164}\)
= 0.00723 kg/kg of dry air.
Dew point temperature, tdp :
If the air is cooled at constant pressure the vapour will begin to condense at the saturation temperature corresponding to 0.01164 bar. By interpolation from steam tables, the dew point temperature, tdp is then
tdp = 9 + (10 – 9) × \(\cfrac{0.011642-0.01150}{10.01230-0.01150}\)
= 9.18°C.