Question

If at 30F the air has 25% relative humidity, what is the relative humidity at 70F and how much moisture must be added to get it up to 50% relative humidity.

Answer 1

vapor pressure of water at 30F =0.6113 Kpa

relative himidity= partial pressure of water vapor/Vapor pressure of liquid =0.25

partial pressure of water vapor = 0.25*0.6113=0.1533 Kpa

at 70F, vapor pressure of water =31.176 Kpa

Relatvie humidity = 100*(partial pressure of water vapor/ Vapor pressure of liquid at 70deg.F )= 100*0.1533/31.1176=0.49%

assumig pressure of 1 atm

partial pressure of water vapor/ partial pressure of dry air = moles of water vapor/ moles of dry air

0,1533/(1-0.1533)= moles of water vapro/moles of dry air

moles of water vapor/ moles of dry air = 0.18

Moles of water vapor =0.18* moles of dry air

at 70% relative humidity , partial pressure of water vapor/ vapor pressure of liquid = 0.7

partial pressure of water vapor/ 0.6113= 0.7

partial pressure of water vapor= 0.7*0.6113 = 0.43 Kpa

partial pressure of water vapor/ partial pressure of dry air = 0.43/(1-0.43)= 0.75

moles of water vapor/moles of dry air =0.75

moles of water = 0.75* moles of dry air

Moles of water to be added = (0.75-0.18)* moles of dry air =0.57* moles of dry air