Question

A weather forecaster reports that on a summer day the air temperature is 86°F, the pressure is 30 inHg and the dew point is 63°F. Find the following for the air

a) Mole fraction of water in the air

b) Relative humidity of the air

c) Degrees of superheat of the air in °F

d) Absolute humidity of the air

Answer 1

Dry air temperature T=86 oF =30 oC

Dew point Td =63 oF =17.22 oC

P =30 inHg

From Humidity chart using T & Td , Humidity H =0.0025kg H2O/kg dry air

=> Mol fraction of H2O= (0.0025/18) / { (0.0025/18)+1/29)} =4.0116*10^-3

b) H = 18/28.97 *{P_A/(P-P_A)} =>0.0025 =0.6213*P_A/(30-P_A)

=>P_A/(30-P_A) =4.0236*10^-3

=>P_A =0.1207-4.0236*10^-3*PA =>P_A =0.1207/(1+4.0236*10^-3) =0.1202in Hg

Vapor pressure of H2O at 86 oF =31.8439torr =1.2536 in Hg

b) HR =100*PA/PAS = 100*0.1202/1.2536 =9.59%

c) Degree of superheat of a gas is the deference between the temperature of the gas and the dew point.

=> Degree =86-63 =23 oF

d) Absolute humity from a) is 0.0025kg H2O/kg dry air