The most important parameters in adsorption column design are pressure drop, adsorbent capacity, reaction kinetics and adsorbent regeneration. However for those applications where flow losses have to be minimized, the pressure drop is the most important parameter. The Ergun equation is used for pressure drop calculations. For atmospheric operations, it does have good results, but it is inconsequential for sub atmospheric conditions and should be revised. The empirical data deviation from Ergun equation results for sodium fluoride adsorbents and medium vacuum conditions is studied in this paper. It shows that, the deviation risies up with vacuum increasing. As a result, using the Ergun equation in sub atmospheric conditions has too much error and causes useless adsorption column design.