The main disadvantage of pneumatic actuators compared with electrical and hydraulic actuators is that the motive fluid (air) is compressible and hence accurate speed control and position control is difficult and often requires ancillary systems..

The advantages of pneumatic systems are

Operating Conditions.
Normal compressed air systems operate at a pressure of approximately 7 barg...
The compressor would need to be rated for some margin above the operating pressure e.g. 10-12 barg.
The equipment would be rated for use at pressures 4 barg to 6 barg.

A compressed air system generally includes the following components..

Air compressor..
This is either electric driven or driven by an internal combustion engine.. The compressor output has to be balanced with the demand and can be operated on a variable displacement basis or, more normally, it is operated intermittently under pressure control..

Air Receiver..
This is a vessel so sized that the pressure is maintained within set limits as the flow to the users varies.

Air Preparation
On a large system there may be a main air treatment system on the main header and a individual air treatment system for each group of users.  An air treatment system will include an Filter, Lubricator, Isolation Valve, and Pressure Regulator (with gauge) as a minimum.

Air piping system
On normal systems the main air pipes will be suitably rated steel /galvanised iron /Wrought iron piping. The local piping to users will be copper and plastic piping can be used for small bore connections.  The piping should include the necessary slopes down to separators for removing any moisture in the compressed air supply which forms in the piping over time.  The velocity of air within the air supply header should be limited to about 15m/s.

Air Consumption Data
Note: The following information represents crude first estimate data. Actual information from suppliers literature should be used for detailed estimates..

Note: Table allows rough estimation of pressure drop along

For other diameters

Δ P _other = Δ P. (ID/ID_other ) ⁵

For other pressures than 7 barg and other temperatures then 15 ^oC..

Δ P _other = Δ P . (7 +1,013) / ( P_other +1,013) . (273+ t _other) / 288

For other Lengths..Pressure drop is in proportion to length ..

Hydraulics are used for providing high torques and high forces with a high level of control of the motion.  Hydrualic fluid is virtually incompressible so controlling the flow of fluid provides accurate control of the motion of the relevant actuator..    Typical uses of Hydraulic drives include..

The primary advantage of hydraulic systems compared to pneumatic and electric systems is that high forces and torques can be developed with comparatively compact motors without the need for gearboxes.  Very accurate motion controls are possible using sophisticated servo valves.

The disadvantages of hydraulic systems include the relatively high cost of components and the need to condition and contain the hydraulic fluid.

Hydraulic systems are generally confined to local areas e.g. a vehicle, a machine tool. They are not designed as distributed systems as are pneumatic systems.   However large distributed systems have been employed. In times past, in cities, very extensive systems have been engineered to serve diverse hydraulic units elevators - bridges etc.

Hydraulic systems ...

Operating Conditions.
Hydraulic power systems systems use hydraulic fluids at pressures between 35 barg and 350 barg.
The hydraulic fluid must be maintained within safe temperature regime by use of coolers.
The hydraulic components have close clearances so the fluid must be filtered.

Suggested velocities in hydraulic systems are as follows.

Power Pack
The hydraulic power pack impart the essential potential energy to the hydraulic fluid using appropriate pumps e.g. Gear, Vane, Piston pumps are used. The power pack normally includes the following features.

Typical power pack duties obtained from a suppliers literature
10 kW ....36 l/s......120 bar...3 kW Oil cooler
50 kW ....160 l/s.... 200 bar...15 kW Oil cooler


Interconnecting Pipework
The interconnecting piping on hydraulic systems feeds should include for pressurised piping from the power pack to the equipment and also return piping from the equipment to the reservoir through the cooler.   The interconnecting pipework is very important in hydraulic systems because it is essential to have no fluid leakage against an environment with a high degree of continuous vibration

Connections systems are available using compression fitting, o-ring fittings, and at the highest level welded fittings.   It is often necessary to include flexible hoses connecting to the actuators . These are generally the most unreliable part of the system and must be carefully designed in strict accordance with the suppliers instructions

Hydraulic Valves
Hydraulic valves are generally highly engineered components.  Valve types include

The control valves are often controlled using electronic /computer control systems...

Hydraulic Actuators
Hydraulic actuators generally based on single or double acting cylinders and motors.   Small cylinders of less than 10mm dia are available.  At the other extreme hydraulic press tools are available capable of forces in excess of 5000 te.

The motors can be low speed (less than 0.5 rpm ) through to high speed (more than 10,000 rpm). Motors can be provided with torque ranges for 1Nm (high speed)to 20000 Nm.(Low Speed)..