Vapor Pressure

Water Molecules in Liquid and Steam Vapor Pressure Water Molecules in Liquid and Steam FLUID POWER – I UNIT- I

Microscopic view of a liquid near its surface Vapor Pressure Microscopic view of a liquid near its surface The high energy molecules escape the surface. FLUID POWER – I UNIT- I

Behavior of a liquid in a closed container Vapor Pressure Behavior of a liquid in a closed container FLUID POWER – I UNIT- I

Vapor Pressure and Boiling Vapor Pressure – the pressure exerted by a vapor in equilibrium with its liquid state. Liquid molecules at the surface escape into the gas phase. These gas particles create pressure above the liquid in a closed container. FLUID POWER – I UNIT- I

Vapor Pressure increases with increasing temperature. 20oC 80oC As temperature increases, the amount of vapor generated by a liquid in a closed container increases.  This occurs because as the liquid gains kinetic energy, the molecules can overcome the intermolecular forces of attraction that are prevalent in the liquid phase. FLUID POWER – I UNIT- I

-boiling temperature when pressure is at 1atm or 760torr. Vapor Pressure Normal Boiling Point - -boiling temperature when pressure is at 1atm or 760torr. Boiling Point is a physical constant. It depends on the strength of the attraction between molecules. Strong attraction between molecules = higher boiling point. Weaker attraction between molecules = lower boiling point. FLUID POWER – I UNIT- I

Boiling Points Change with pressure changes. Vapor Pressure Boiling Points Change with pressure changes. Less pressure = lower boiling point (example = water boils at lower temperatures at high elevations- water boils at 76oC on Mt. Everest). Higher Pressure = higher boiling point (example = pressure cooker – cooks faster because you cook at a higher temperature.) FLUID POWER – I UNIT- I

Vapor Pressure FLUID POWER – I UNIT- I

Engineering significance of vapor pressure In a closed hydraulic system, Ex. in pipelines or pumps, water vaporizes rapidly in regions where the pressure drops below the vapor pressure. There will be local boiling and a cloud of vapor bubbles will form. This phenomenon is known as cavitations, and can cause serious problems, since the flow of fluid can sweep this cloud of bubbles on into an area of higher pressure where the bubbles will collapse suddenly. If this should occur in contact with a solid surface, very serious damage can result due to the very large force with which the liquid hits the surface. Cavitations can affect the performance of hydraulic machinery such as pumps, turbines and propellers, and the impact of collapsing bubbles can cause local erosion of metal surface. Cavitations in a closed hydraulic system can be avoided by maintaining the pressure above the vapor pressure everywhere in the system. FLUID POWER – I UNIT- I

Vapor Pressure Thank you! FLUID POWER – I UNIT- I