Question

Please Help.! Having a hard time trying to figure this problem out.!! The answer is 27%.!! Thanks

7. Air at ! 00 kPa, 20 and 50% relative humidity is heated in a control volume to so while keeping the pressure constan at 100 kPa. The relative humidity in the final state is

Answer 1

From thermodynamic tables, Saturation pressure at 20 deg C is p_g1 = 0.02339 bar

Therefore, p_v1 = $\phi$ ₁ p_g1 where $\phi$ denotes relative humidity

p_v1 = 0.5 * 0.02339 = 0.011695 bar

Both the composition of the moist air and the mixture pressure remain constant, so the partial pressure of the water vapor at the exit equals the partial pressure of the water vapor at the inlet: p_v2 = p_v1 = 0.011695 bar.

From thermodynamic tables, at 30 deg C, we get saturation pressure p_g2 = 0.04246 bar

Relative humidity at final state $\phi$ 2 = p_v2 / p_g2

= 0.011695 / 0.04246

= 0.27 or 27%