PNEUMATICS SYSTEMChapter 7

Properties of Compressed AirAvailabilityEasily stored in large volumesSimplicity in design and controlProvides linear and rotary movementLow system cost due to low component cost

Components have long working life resulting in longer system reliabilityEnvironmentally friendlySafety issues are minimized e.g.. Fire hazards; unaffected by overloads (actuators stall or slip)Pneumatic actuators in a system do not produce heat (except for friction)Properties of Compressed Air

Pneumatic systemA pneumatic system uses compressed air, and this usually stored in a pressure vessel called an air receiver. Air, from atmosphere, is delivered to the air receiver by a motor-driven compressor. Unlike the hydraulic pump, the compressor is controlled by a pressure switch on the air receiver and either starts/stop on demand or vents to atmosphere when the receiver is charged.

The cylinder movement is again controlled by a directional control valve, but air returned from the cylinder vented to atmosphere. A pneumatic system is an open system, the air being obtained from and returned to the atmosphere.Pneumatic systems also require clean dry air. A practical pneumatic system has additional air treatment elements not present in a hydraulic system.

Pneumatic system

Pneumatic power supply

Electric motor drives an air compressor.Air inlet to the compressor is likely to be filtered and via a silencer to reduce the noise level.Pressure relief valve provide protection against the pressure in the system rising above the safe level.

Pneumatic power supply

Since the air compressor increases the temperature of the air there is likely to be cooling system and to remove contamination and water from the air via a filter and water trap.An air receiver increases the volume of air in the system and smoothes out any short term pressure fluctuations.

Pneumatic power supply

Pneumatic CompressorThe compressors compress the air from atmospheric pressure to the desired working pressure.The compressor converts air at atmospheric pressure into high pressure by changing it volume.Compressed air serves as a source of pneumatic energy by producing pressurized air.

Two types of pneumatic compressor:Positive Displacement CompressorDynamic Compressor (Blowers)

Positive Displacement CompressorConfine the air within a closed space and compress it by decreasing the volume of the space.Basically producing a low volume high pressure air supply.Two types:Reciprocating piston compressorRotary piston compressor

Reciprocating piston compressorMost widely used.When higher pressure is required, a multistage compressor is used.

Reciprocating compressorsTwo stage reciprocating compressorsPositive Displacement Compressor

Air CompressionReciprocating:Single stageDouble Stage

Rotary piston compressorCompressor with rotating pistonAdvantage over reciprocating compressor: Compact dimensions, quiet running and smooth, steady air delivery.Disadvantage: slightly reduced efficiency and lower operating pressuresTwo types:Vane compressorScrew compressor

Positive Displacement Compressor

Vane compressorSame operation as Vane pump.Screw compressorUses two intermeshing counter-rotating screws.These mesh with a clearance of a few thousandths of an inch, and are driven by timing gears. As the screw rotate, pocket of air are carried from the inlet port to the outlet port.

Positive Displacement Compressor

Vane compressorPositive Displacement Compressor

Screw compressorPositive Displacement Compressor

Air CompressionRotary:VaneScrew

Dynamic Compressor (Blowers)Draw air in one side and compress it by mass acceleration.Dynamic compressor accelerates the air by rapidly turning the rotor blade.Basically they are large volume low pressure device.

Two types of blowerCentrifugal typeAxial type

Centrifugal typeAxial type Dynamic Compressor (Blowers)

Pressure SwitchCompressor DrainCheck ValveA Basic Pneumatic SystemAir Production and Distribution SystemMotorTankPressure GaugeRefrigerated Air DryerLine Filter

Directional ValveA Basic Pneumatic SystemAir Consumption SystemAir Line Take-offDrainConditioning UnitActuator

Compressor

Air ReceiverA pressure Vessel (Tank) normally made of metal and installed either horizontally or vertically. This vessel provides a volume of airto be utilized by the system and reduced the loading and unloadingof the compressor.Air Receiver (Tank)

Air TreatmentAll atmospheric air carries both moisture and dustInlet FilterUsed to remove large particle dirt from entering the system. Used to prevent excessive wear of cylinder, piston rings and other moving parts in the initial stage of the system

Air DryerCompressed air cannot hold the same amount of moisture as non compressed air. Once the air is compressed, some of the moisture condenses in the tank and is removed through a drain valve. The remaining moisture will rust internal components of the system and must be removed. One way this is done by sending the air through an air conditioned chamber which will condense more of the moisture which is present.

Regulating and ConditioningThe compressed air to be used in a system needs to be treated because of contamination inside the system. An example of this could be the scaling of piped or worn seal material. A line filter is used to collect water and solid impurities. A Lubricator is used to vaporize oil into the air stream to lubricate moving parts in the system such as air tools A regulator is used to adjust the line pressure to a lower usable pressure needed for some tool applications.