In this installment of the Btu Buddy series, Bob stops at a diner for lunch and has something on his mind. He has been to school where all of the terms of pressure and vacuum have been discussed, but he does not have a firm grasp on the terms. He wants to better understand the evacuation of a system. About that time Btu Buddy appears.
He explains, “Vapor molecules in a vessel travel in a straight line and bounce around the vessel. If there is an opening in the vessel, the molecules will begin to move out of the opening in proportion to the pressure difference from the inside of the vessel to the outside (Figure 1). There are many more molecules at a higher pressure. If the vessel is under 20 psig of pressure and atmospheric pressure is 0 psig, there is a 20 psi incentive for the vapor to move out. There are a lot of vapor molecules at 20 psig. When the vessel pressure is at 0 psig, there is not much incentive for the molecules to move out of the vessel. The vacuum pump provides the incentive by creating a low pressure area for the molecules to move to.”