Vapor Pressure of Fluid

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. When liquid is placed in a closed, evacuated container, molecules escape from the liquid surface into the space. As more molecules escape, pressure increases until equilibrium is reached, where the escape rate equals the return rate. This equilibrium pressure is the vapor pressure.

Vapor pressure increases with temperature because molecules gain kinetic energy, more easily overcoming intermolecular forces and escaping into the vapor phase. For example, water at 100°C has a vapor pressure of 101.3 kPa, matching atmospheric pressure at sea level and causing it to boil. Higher temperatures result in higher vapor pressures.

At higher elevations, atmospheric pressure is lower. Water boils when its vapor pressure equals the surrounding atmospheric pressure. Since atmospheric pressure decreases with altitude, water's vapor pressure reaches this lower pressure at a reduced temperature, causing boiling to occur earlier than at sea level.

Cavitation occurs when local fluid pressure drops below the vapor pressure, forming vapor bubbles that collapse violently and damage equipment. Monitoring vapor pressure is crucial in designing and operating pumps and turbines to prevent cavitation. Understanding vapor pressure helps engineers maintain adequate pressure to avoid this destructive phenomenon.

Vapor pressure indicates the tendency of molecules to escape from the liquid surface into the vapor phase. It represents the balance between molecules leaving the liquid and returning to it. Higher vapor pressure means molecules more readily escape, reflecting their increased kinetic energy and reduced intermolecular force strength.

A liquid boils when its vapor pressure equals the atmospheric pressure surrounding it. At standard atmospheric pressure of 101.3 kPa, water boils at 100°C because its vapor pressure reaches this value. The boiling point is the temperature at which vapor pressure matches the external pressure, allowing bubbles to form throughout the liquid.

Vapor pressure is one of several critical fluid properties engineers must consider. Like density, specific weight, specific gravity and compressibility of fluid, vapor pressure affects fluid behavior and system performance. Understanding how vapor pressure interacts with other fluid characteristics helps engineers design safer, more efficient hydraulic and pneumatic systems.