Fluid Mechanics: Hydraulic and Pneumatic Systems
Properties of Fluids
Liquids
In hydraulic circuits, an incompressible fluid like oil is used to transmit power. Other liquids, such as water, can also be used. Liquids have a definite volume and are not easily compressed.
Gases
Pneumatic circuits utilize compressible fluids, primarily air. Air, being a mixture of gases, behaves as a single gas with no definite shape or volume. It conforms to the shape of its container.
Pressure
Fluids exert force on the walls of their containers. This force per unit area is called pressure. Atmospheric pressure is the pressure exerted by the surrounding air. Relative pressure is the difference between the fluid pressure and atmospheric pressure. Manometers are used to measure fluid pressure.
Pascal’s Principle and Fluid Flow
Blaise Pascal’s principle states that pressure applied to a confined fluid is transmitted equally and undiminished in all directions. This principle is the basis for hydraulic presses, which use two interconnected containers with different cross-sectional areas.
For non-viscous fluids flowing through a pipe, the continuity equation and Bernoulli’s equation describe the relationship between fluid velocity, pressure, and pipe diameter.
Ideal and Real Gases
Ideal gases are theoretical gases where intermolecular forces and molecular volume are negligible compared to the container’s volume. Real gases deviate from ideal gas behavior at high pressures and low temperatures.
Hydraulic and Pneumatic Circuits
Differences
Hydraulic circuits use liquids, while pneumatic circuits use gases. In pneumatic systems, the air is exhausted into the atmosphere after use, whereas in hydraulic systems, the fluid is recirculated. Hydraulic systems require more complex return lines and filtration due to the use of oil.
Active Elements
Active elements provide energy to the fluid. In pneumatic systems, compressors increase air pressure by reducing its volume. Dynamic compressors increase air velocity and pressure by constricting the airflow. In hydraulic systems, pumps impart energy to the fluid by creating flow.
Accumulators
Accumulators store pressurized fluid. In pneumatic systems, accumulators (tanks) store compressed air to smooth out pressure fluctuations and allow intermittent compressor operation. In hydraulic systems, accumulators also store pressurized fluid, help separate air from the fluid, and provide a reservoir for the fluid.
Protection Elements
Protective elements maintain fluid quality and system safety. In pneumatic systems, filters remove particles and water from the air, lubricators introduce oil mist to reduce wear, pressure limiters prevent excessive pressure buildup, and silencers reduce noise from exhaust air. In hydraulic systems, filters and relief valves perform similar functions.
Transport Elements
Transport elements carry the fluid to consumption points. In both pneumatic and hydraulic systems, pipes are used to transport the fluid. Smooth internal surfaces minimize pressure loss. Flexible hoses are used in some applications, such as pneumatic hammers.
Control Elements
Control elements regulate fluid flow, pressure, and direction. Valves control the flow path, direction, and rate of fluid flow. Pressure relief valves maintain pressure below a set limit. Flow control valves adjust the speed of fluid circulation. Check valves allow flow in only one direction.