Pneumatic Circuit Components by Er. Abhisek Bhattacharjee

Introduction

Pneumatic systems are power systems using compressed air as a work-
ing medium for the power transmission. Their principle of operation is
similar to that of the hydraulic power systems. An air compressor
 converts the mechanical energy of the prime mover into, mainly, pres-
sure energy of the compressed air. This transformation facilitates the
transmission, storage, and control of energy. After compression, the
compressed air should be prepared for use. The air preparation includes
filtration, cooling, water separation, drying, and adding lubricating oil
mist. The compressed air is stored in compressed air reservoirs and
transmitted through transmission lines: pipes and hoses. The pneumatic
power is controlled by means of a set of valves such as the pressure, flow,
and directional control valves. Then, the pressure energy is converted to
the required mechanical energy by means of the pneumatic cylinders
and motors.

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Peculiarities of Pneumatic Systems

The static and dynamic characteristics of the pneumatic systems differ
from those of the hydraulic systems due to the difference in the
 physical properties of the energy transmitting fluid, mainly the high
compressibility, low density, and low viscosity of air.

BASIC COMPONENTS OF PNEUMATIC SYSTEM

Important components of a pneumatic system are shown in figure 1.
a) Air filters: These are used to filter out the contaminants from the air.

b) Compressor: Compressed air is generated by using air compressors. Air compressors are either diesel or
electrically operated. Based on the requirement of compressed air, suitable capacity compressors may be
used.

c) Air cooler: During compression operation, air temperature increases. Therefore coolers are used to
reduce the temperature of the compressed air.
d) Dryer: The water vapor or moisture in the air is separated from the air by using a dryer.

e) Control Valves: Control valves are used to regulate, control and monitor for control of direction flow,pressure etc.

f) Air Actuator: Air cylinders and motors are used to obtain the required movements of mechanical elements of pneumatic system.

g) Electric Motor: Transforms electrical energy into mechanical energy. It is used to drive the compressor.

h) Receiver tank: The compressed air coming from the compressor is stored in the air receiver.

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Advantages of Pneumatic system

− Low inertia effect of pneumatic components due to low density of air.
− Pneumatic Systems are light in weight.
− Operating elements are cheaper and easy to operate
− Power losses are less due to low viscosity of air
− High output to weight ratio
− Pneumatic systems offers a safe power source in explosive environment
− Leakage is less and does not influence the systems. Moreover, leakage is not harmful

Disadvantages of Pneumatic systems 

− Suitable only for low pressure and hence low force applications
− Compressed air actuators are economical up to 50 kN only.
− Generation of the compressed air is expensive compared to electricity
− Exhaust air noise is unpleasant and silence has to be used.

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Actuator/Pneumatic Cylinder:

The actuator is a pneumatic motor that generates outward force through unequal pressure in a pneumatic cylinder, often connected to a piston rod mechanism. Pneumatic cylinders come in many types. Those that use piston rods are divided into single acting and double acting cylinders.

• A single acting pneumatic cylinder exerts a unidirectional outward force when the cylinder is pressurised. When the full outward stroke is reached, the compressed gas is expelled, creating a vacuum and allowing the piston rod to return to its start position.
• In a double acting cylinder, compressed gas is required for both outward and retract movements: the piston end is pressurised on the outward stroke and the rod end is pressurised on the return stroke. This has the advantage of increasing the stroke length and maintaining constant force on alternating movements.

So called ‘rodless cylinders’ do not use piston rods, but have an externally mounted carriage to carry out linear movements along a bearing. Also known as linear drives, rodless cylinders can be mechanically operated through compressed air, or magnetically powered.

Air Treatment System

Pneumatic cylinders are dependent on compressed air or inert gas, which enters the cylinder barrel at both ends via a control valve. Before the air reaches this valve it is channelled through a treatment system that composes the following parts:

Intake Filter

The intake filter channels either atmospheric air or an inert gas into the pneumatic system, filtering it of dust, VOCs and other unwanted particles. At this stage the air has a low pressure to volume ratio. This will change as it progresses through the treatment system.

Compressor

The compressor takes air and compresses it to reduce its volume and increase its temperature. There are several types of compressor, which operate under the principle of either positive displacement or dynamic displacement.

• Piston compressors – use a single acting or double acting piston to compress air and direct it through an outlet pipe into the cooler/separator/dryer unit.
• Diaphragm compressors – use a flexible rubber diaphragm in place of a piston to compress small quantities of air. These are used in sterile environments where contaminants from lubricating oil or piston surfaces may affect food, cosmetics or pharmaceuticals.
• Screw compressors – use a helical shaft to progressively compress air to high pressures. Useful when low to medium volumes of extremely compressed air are required. These compressors provide a continuous flow of compressed air, rather than the pulsating current characteristic of piston and diaphragm compressors.

Cooler, Separator & Dryer

Compression increases the temperature of the air, which must be cooled through a heat exchanger to reach operating temperature. Cooling units use a counter flow of air or water to extract and remove surplus heat from the compressed air.

The Separator, or drying unit, purges the compressed air of excess water vapour and gaseous contaminants. Dryers may use heat to drive off volatile components from the compressed gas, or chemical absorption (e.g. phosphoric pentoxide, calcium chloride or silicon dioxide). Some systems use refrigerant compressors to remove moisture from the compressed air.

The pure, dry compressed air is channelled into the receiver tank, sometimes after further filtration, while the moist, contaminated air is condensed to liquid form and expelled through a drain.

Receiver Tank

As pneumatic systems depend on a continuous supply of compressed air/gas, this fluid must be stored in a receiver tank. The large internal surface area of the receiver tank further helps to dissipate excess heat while maintaining the required pressure. This is measured by one or more pressure gauges. Compressed air is fed into the control valve by an exit pipe linked to a shut-off valve. As the pressure drops in the receiver tank the change is detected by pressure sensors at the inlet valve, which opens to refresh the tank.

Control Valve

The compressed gas as it enters the control valve to the actuator has a high pressure to volume ratio. The control valve, or regulator, feeds the gas into the actuator to control its speed and movements. Movement is achieved through motor displacement, so the purpose of the control valve is to moderate the flow rate and pump volume as the compressed air leaves the receiver tank. This is achieved through a series of stop valves, controlled by flow transducers, proximity sensors and pressure gauges.

Details of Compressor

Piston Compressors

In the piston class of compressors, the process phases of expansion, suction, compression, and discharge are accomplished by the recip-rocating motion of a piston. The compression process is based on displacing the gas by the piston. A functional schematic of a piston-type compressor and the associated theoretical p-V diagram are presented in Fig. 11.3, where V is the volume trapped by the piston in the cylinder.

Double acting compressor 

− The pulsation of air can be reduced by using double acting compressor as shown in Figure below. It has two sets of valves and a crosshead. As the piston moves, the air is compressed on one side whilst on the other side of the piston, the air is sucked in. Due to the reciprocating action of the piston, the air is compressed and delivered twice in one piston stroke. Pressure higher than 30bar can be produced.

Multistage compressor 

− As the pressure of the air increases, its temperature rises. It is essential to reduce the air temperature to avoid damage of compressor and other mechanical elements. The multistage compressor with intercooler in-between is shown in Figure.  It is used to reduce the temperature of compressed air during the compression stages. The inter-cooling reduces the volume of air which used to increase due to heat. The compressed air from the first stage enters the intercooler where it is cooled. This air is given as input to the second stage where it is compressed again.  The multistage compressor can develop a pressure of around 50 bar.

Combined two stage compressors 

− In this type, two-stage compression is carried out by using the same piston.Initially when the piston moves down, air is sucked in through the inlet valve. During the compression process, the air moves out of the exhaust valve into the intercooler. As the piston moves further the stepped head provided on the piston moves into the cavity thus causing the compression of air. Then, this is let out by the exhaust port.

Diaphragm compressor

These are small capacity compressors. In piston compressors the lubricating oil from the pistons walls may contaminate the compressed air. The contamination is undesirable in food, pharmaceutical and chemical industries. For such applications diaphragm type compressor can be used. − The piston reciprocates by a motor driven crankshaft. As the piston moves down it pulls the hydraulic fluid down causing the diaphragm to move along and the air is sucked in.  When the piston moves up the fluid pushes the diaphragm up causing the ejection of air from the outlet port. Since the flexible diaphragm is placed in between the piston and the air no contamination takes place.

Screw compressor 

Piston compressors are used when high pressures and relatively low volume of air is needed. The system is complex as it has many moving parts. For medium flow and pressure applications, screw compressor can be used. It is simple in construction with less number of moving parts. The air delivered is steady with no pressure pulsation. It has two meshing screws. The air from the inlet is trapped between the meshing screws and is compressed. The contact between the two meshing surface is minimum, hence no cooling is required. These systems are quite in operation compared to piston type. The screws are synchronized by using external timing gears.

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Rotary vane compressors 

 The principle of operation of vane compressor is similar to the hydraulic vane pump. The unbalanced vane compressor consists of spring loaded vanes seating in the slots of the rotor. The pumping action occurs due to movement of the vanes along a cam ring. The rotor is eccentric to the cam ring. As the rotor rotates, the vanes follow the inner surface of the cam ring. The space between the vanes decreases near the outlet due to the eccentricity. This causes compression of the air. These compressors are free from pulsation. If the eccentricity is zero no flow takes place.

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Lobe compressor 

− The lobe compressor is used when high delivery volume but low pressure is needed. It consists of two lobes with one being driven and the other driving. It is similar to the Lobe pump used in hydraulic systems. The operating pressure is limited by leakage between rotors and housing. As the wear increases during the operation, the efficiency falls rapidly.