It Was Shown in Recent Experiments and Molecular Dynamics (MD) Simulations that Schrage Equation Predicts Evaporation and Condensation Rates of Water in the Absence of a Non-Condensable Gas with Good Accuracy. However, It is Not Clear Whether Schrage Equation is Still Accurate or Even Valid for Quantifying Water Evaporation and Condensation Rates in Air. in This Work, We Carry Out MD Simulations to Study Steady-State Evaporation and Condensation of Water at a Planar Water-Air Interface. the Simulation Results Show that the Evaporation and Condensation Fluxes of Water in the Presence of Air Are Still in a Good Agreement with the Predictions from Schrage Equation. from Schrage Equation and Stefan's Law of Mass Diffusion, We Derive an Analytical Expression for the Effective Thermal Conductivity of a Planar Heat Pipe. the Analytical Prediction of the Dependence of Effective Thermal Conductivity on Heat Pipe Length and Density of Non-Condensable Gas is Corroborated by Our MD Simulation Results and Recent Experimental Data.