LECTURE № 13

PNEUMATIC SYSTEMS

 

13.1. PNEUMATIC SYSTEM

 

Pneumatic technology deals with the study of behavior and applications  of compressed air in our daily life in general and manufacturing automation in particular. Pneumatic systems use air as the medium which is abundantly available and can be exhausted into the atmosphere after completion of the assigned task.

 

Basic Components of Pneumatic System

 

Fig. 13.1.1. Components of a pneumatic system

 

Important components of a pneumatic system are shown in fig. 13.1.1.:

1.       air filters: these are used to filter out the contaminants from the air;

2.       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;

3.       air cooler:  during compression operation, air temperature increases. Therefore coolers are used to reduce the temperature of the compressed air;

4.       dryer: the water vapor or moisture in the air is separated from the air by using a dryer;

5.       control valves: control valves are used to regulate, control and monitor for control of direction flow, pressure etc;

6.       air actuator:  air cylinders and motors are used to  obtain the required movements of mechanical elements of pneumatic system;

7.       electric motor: transforms electrical energy into mechanical energy. It is used to drive the compressor;

8.       receiver tank: the compressed air coming from the compressor is stored in the air receiver.

 

Receiver tank

The air is compressed slowly in the compressor. But since the pneumatic system needs continuous supply of air, this compressed air has to be stored. The compressed air is stored in an air receiver as shown in Figure 13.1.2. The air receiver smoothens the pulsating flow from the compressor. It also helps the air to cool and condense the moisture present. The air receiver should be large enough to hold all the air delivered by the compressor. The pressure in the receiver is held higher than the system operating pressure to compensate pressure loss in the pipes. Also the large surface area of the receiver helps in dissipating the heat from the compressed air. Generally the size of receiver depends on:

·          delivery volume of compressor;

·          air consumption;

·          pipeline network;

·          type and nature of on-off regulation;

·          permissible pressure difference in the pipelines.

 

 

Fig. 13.1.2. Air receiver

 

Compressor

It is a mechanical device which converts mechanical energy into fluid energy. The compressor increases the air pressure by reducing its volume which also increases the temperature of the compressed air. The compressor is selected based on the pressure it needs to operate and the delivery volume. The compressor can be classified into two main types:

1.       positive displacement compressors;

2.       dynamic displacement compressor.

Positive displacement compressors include piston type, vane type, diaphragm type and screw type.

 

Piston compressors

Piston compressors are commonly used in pneumatic systems. The simplest form is single cylinder compressor (Fig. 13.1.3). It produces one pulse of air per piston stroke. As the piston moves down during the inlet stroke the inlet valve opens and air is drawn into the cylinder. As the piston moves up the inlet valve closes and the exhaust valve opens which allows the air to be expelled. The valves are spring loaded. The single cylinder compressor gives significant amount of pressure pulses at the outlet port. The pressure developed is about 3 – 40 bar.

 

 

Fig. 13.1.3. Single acting piston compressor

 

Double acting compressor

The pulsation of air can be reduced by using double acting compressor as shown in Figure 13.1.4. 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.

 

 

Fig. 13.1.4. Double acting piston compressor

 

13. 2. COMPRESSORS

 

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. Figure 13.2.1 shows the construction of Diaphragm compressor.

 

Fig. 13.2.1. Diaphragm compressor

 

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

 

 

Fig. 13.2.2. 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.

 

13.3. ROTARY VANE COMPRESSORS

 

 

Fig. 13.3.1. Rotary vane compressor

 

The principle of operation of vane compressor is similar to the hydraulic vane pump. Figure 13.3.1 shows the working principle of Rotary vane compressor. 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.

The casing is filled with liquid up to rotor center. The air enters the compressor through the distributor fixed to the compressor. During the impeller rotation, the liquid will be centrifuged along the inner ring of the casing to form the liquid ring. There are two suction and discharge ports provided in the distributor. During the first quarter of cycle, the air is sucked in both suction chambers of the  casing and during the second quarter of the cycle, the air is compressed and pushed out through the two discharge ports. During the third and fourth quarters of the cycle, the process is repeated. This type of compressor has no leakage and has minimal friction. For smooth operation, the rotation speed should be about 3000 rpm. The delivery pressure is low (about 5 bar).

 

13.4. LOBE COMPRESSOR

 

Fig. 13.4.1. 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. Figure 13.4.1. shows the construction and working of Lobe compressor. 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.

 

13.5. DYNAMIC COMPRESSORS

 

When very large volume of compressed air is required in applications such as ventilators, combustion system and pneumatic powder blower conveyors, the dynamic compressor can be used. The pressure needed is very low in such applications. Figure 13.5.1 shows a typical Centrifugal type blower.

 

Fig. 13.5.1. Blower (Centrifugal type)

 

The impeller rotates at a high speed. Large volume of low pressure air can be provided by blowers. The blowers draw the air in and the impeller flings it out due to centrifugal force. Positive displacement compressors need oil to lubricate the moving parts, whereas the dynamic compressors have no such need. The efficiency of these compressors is better than that of reciprocating types.