Introduction
● As discussed in that Blog, the fluid power is divided into two groups:
(a) Oil hydraulic system and
● Oil hydraulic system as we know and studied, is related with use of pressurized oil for various industrial applications. While pneumatic system which is related with use of compressed air or gas for various applications.
● Pneumatics" is the technology deals with study and applications of compressed air/gas for transmission and control of motions and forces.
● Pneumatic system utilities the power of compressed air/gas to operate the actuator to perform the given application.
● Air from atmosphere is get compressed to high pressure and enters in the actuator to generate force which is used for given application (In this case to cut sheet metal work piece using punch and die).
● The various important components of pneumatic system are :
(a) Compressor : It is the source to supply compressed air to the pneumatic system.
(b) Control valves : They are used for pressure, flow and direction control of compressed air.
(c) Actuators : They are used to obtain motion or force by using compressed air.
(d) Accessories : They are auxiliary devices for improving performance of the pneumatic system.
● In hydraulic system, we use pump to increase pressure of oil similarly we use compressor to increase pressure of atmospheric air, in pneumatic system.
● Compressor unit consists of compressor with large capacity air receiver to store compressed air which is supplied to number of pneumatic devices and equipment located nearby.
● Compressor with drive motor and air receiver requires more space and it also makes noise and vibrations during working, hence it is kept separately.
Definition of Air compressor:
● It is device which converts mechanical energy into pressure energy in the form of compressed air at high pressure.
● It is the device which sucks atmospheric air and compresses it by suitable mechanism to increase it pressure and deliver to the air receiver for storage, which is further utilised for operation of pneumatic system.
Examples:
1) Compressor used for spray painting.
2) Compressor used to fill air in the automobile tubes tyres.
Types of Air Compressors
These are two basic type of compressors:
(i) Positive displacement compressors.
(ii) Dynamic or turbo compressors.
(i) Positive displacement compressors :
● They work on the principle of increasing the pressure of a definite volume of air by reducing it in an enclosed chamber.
● They are subdivided into two groups.
(a) Reciprocating type:
● The reciprocating motion of piston or diaphragm is used to decrease volume of air for increasing its pressure.
● It includes piston, labyrinth, and diaphragm compressors.
(b) Rotary type:
● It utilises rotary motion of the mechanical elements like vanes, gear, lobes, screws for increasing pressure of air.
● It includes vane, screw, lobe compressors.
(ii) Dynamic (turbo) compressors:
● It employs rotating vanes or impellers to give velocity and pressure to the flow of air being handled.
● The dynamic effects such as centrifugal force helps to achieve pressure of air.
● It includes centrifugal, axial, ejector compressors.
Compressor classification :
(a) According to compression by piston:
(i) Single acting compressor :
Compression takes place in the space on one side of the piston.
(ii) Double acting compressor :
Compression takes place on both the faces of the piston.
(b) According to number of stages of compression :
(i) Single stage compressor:
Only one stage of compression.
(ii) Two stage compressor :
Two stages are provided for compression to give more pressure
(iii) Multiple stage compressor:
More than two stages are provided to obtain large pressure.
(c) According to position of cylinder:
(i) Vertical : Cylinder is mounted vertically.
(ii) Horizontal : Cylinder is mounted horizontally.
(iii) V-type : Cylinders are mounted along V shape.
(d) According to drive/prime mover used:
(i) Electric motor driven
(ii) Diesel engine driven
(iii) Gas/turbine driven
(e) According to condition of compressed air :
(i) Oil free air
(ii) Lubricating oil contaminated air
(f) According to mounting/portability condition:
(i) Portable compressor : It can be moved easily.
(ii) Stationary compressor : It is fixed and mounted firmly.
(g) According to cooling medium used:
(i) Air cooled compressor
(ii) Water cooled compressor
(iii) Liquid injected compressor
Reciprocating Compressor:
Definition : When reciprocating motion of piston or diaphragm is used to compress the air to increase its pressure, the device is known as reciprocating compressor
The reciprocating compressors are classified as:
Piston type :
It uses cylinder and piston mechanism for its operation. It is further grouped as:
Single Acting Compressor:
● When suction and compression is carried out only from face side of the piston, it is known as single acting compressor.
● There is one compression stroke per stage for each revolution of crank shaft.
Important features of single acting compressor:
1. They are used for small capacity only.
2. The machine is more compact and less costly.
3. They are portable.
4. They are commonly air cooled.
5. They are suitable when demand is infrequent or compressor is to be installed at the worksite.
Double Acting Compressor:
● The compression takes place on both the faces of the piston giving two compression strokes for each revolution of the crank and crankshaft.
● It can be used in multistage compressor system.
Construction of two stage reciprocating compressor :
● The most commonly used air compressor in the industry is air cooled two stage reciprocating compressor with pressure up to 7 to 12 kg/cm sq.
● Outer construction of reciprocating compressor. It includes following main parts:
(a) Air receiver or tank : It is large horizontal cylinder (pressure vessel) mounted on base stand. It stores compressed air. It is fitted with various accessories.
(b) First stage cylinder : The atmospheric air enters in this cylinder. The cylinder is mounted along V type of arrangement with inlet valve for suction and outlet valve for supply of compressed air to the second stage cylinder.
(c) Second stage cylinder : This cylinder is mounted along with first stage cylinder in radial manner. The inlet is connected to outlet of first stage cylinder and outlet is connected to the air receiver tank.
(d) Intercooler : It is fitted between first stage cylinder and second stage cylinder. It is provided to reduce the temperature of compressed air nearly equal to suction temperature. It has fins for cooling purpose.
(e) Pressure gauge : It is fitted on air receiver tank to indicate the pressure of compressed air stored in the tank.
(f) Electric motor: It is the prime mover to run the compressor. The motor shaft is connected by belt drive to compressor shaft.
(g) Air Inlet filter : It is provided for filtration of atmospheric air entering in the first
(h) Air outlet valve: It is provided to supply compressed air to the pneumatic system.
(i) Oil reservoir : It is used to store oil for lubrication of crank shaft.
(j) Safety valve : It is provided as a safety measure against high pressure developed in the system.
(k) Pressure switch: It is used for automatic trip-off when air receiver is filled with desired set pressure.
Working:
● When the starting of the electric motor, the crank rotates and the piston in the first stage cylinder sucks the atmospheric air through the suction filter and inlet valve.
● On further rotation of the crank, the piston reverses and it compresses the air. The compressed air forces the outlet valve to open.
● Then it enters in the intercooler to the second stage cylinder forcing open its inlet valve. The air is further compressed in this cylinder to the desired pressure.
● It is then fed to the air receiver tank through the condenser on compression through the outlet valve.
● The first-stage cylinder is called Low Pressure Cylinder(LP). It can develop pressure up to 5 bar. The second stage cylinder is called as High Pressure Cylinder (HP). It can develop further pressure up to 10-12 bar.
● Multistage compressor can produce discharge pressure up to 300 kg/cm sq.
Important features of double acting compressor:
1. Air delivery is double that of a single acting compressor.
2. They are mostly suitable for large capacity.
3. They are mostly water cooled.
4. They are suitable for continuous and heavy duty work.
Function of intercooler :
● When air is compressed in the first stage, its temperature will increase due to heat generated during compression. Heat generated may increase the volume of air by expansion and increases work done on the compressor.
● Hence the compressed air is passed through intercooler located between stages of compression. Intercooler is a heat exchanger device designed with air cooling or water cooling arrangement.
● Intercooler with cooling fins provided on outer surface of the body for air cooling.
● Intercooler cools the compressed air by taking away the heat and reduces its volume. This helps to reduce work done and power in the second stage of compression.
Diaphragm Type Compressor :
● In this compressor, the piston is separated from the friction chamber by a diaphragm. The air does not come in contact with reciprocating parts. Hence air is kept free of oil.
● They are limited to a very small capacity, low initial cost, limited life and need maintenance frequently.
● They are most suitable for food stuff, chemical and textile industries, pharmaceutical industries etc.
Specification of Reciprocating Compressor :
1. Pressure of compressed air in kg/cm sq.
2. Volume of air receiver in cm cu/min.
3. HP of driving motor.
4. Volumetric efficiency.
5. Number of stages available for compression.
Rotary Compressors :
Definition of rotary compressors:
When rotary motion of vanes, screw, lobes, etc is used for compression of air, it is called as rotary compressor.
Definition of rotary vane compressor:
When sliding vanes are used with rotor for compression of air, it is known as rotary vane compressor.
Construction :
It consists of:
(a) Rotor:
It has rectangular slots provided along radius accommodate number of vanes. It is fitted eccentrically inside the cylinder block.
(b) Sliding vanes:
They are inserted in longitudinal slots in the rotor and moves to and fro into the slots.
(c) Cylinder block:
It is circular block in which rotor along with vanes is mounted.
(d) Inlet and outlet ports :
They are provided with cylinder block for suction of air and discharge of compressed air.
Working :
● The shaft of compressor starts rotation and causes rotor along with vanes to rotate. Initially vacuum is created at inlet port for entry of atmospheric air.
● This air is carried out and get compressed as radius of rotation goes on decreasing towards the discharge port.
● The high pressure compressed air is delivered to the pneumatic system or stored in a air receiver.
● It gives pulse free/continuous discharge of compressed air.
Advantages :
1. Smaller in size.
2. Lighter in weight.
3. It can work at high speed.
4. Maintenance cost is less.
5. It may not require expensive foundation.
Liquid Ring Compressor :
Definition : When water or other low viscosity liquid is used in addition with casing and vanes to compress the air, it is called as liquid ring compressor.
Construction:
It consists of following components :
1. Rotor with sliding vanes : It is similar to rotary vane compressor, mounted eccentrically with respect to casing.
2. Liquid ring : Liquid ring is formed by water or low viscosity liquid in between casing and sliding vanes. It avoids metal contact between blades and wall.
3. Casing with suction and discharge port: It is circular housing with suction and discharge ports at opposite ends.
Working :
● It is modification of vane compressor. Its working is similar to vane compressor. Initially partial vacuum is created for suction of atmospheric air.
● The air is trapped between liquid ring and rotating vanes. The radius of rotation of vanes decreases and compress the air near the discharge port.
● Due to liquid ring, physical metal contact of vanes is eliminated and there is no wear of vanes and casing at contact surface.
● It is necessary to maintain a constant liquid level for which special arrangement is made.
● It is available in single stage with wide range of flow capacities with compression ratio up to five. The pressure range is about 6 bar.
Lobe Compressor :
Definition : When mechanical lobes are used to increase pressure of air with the impeller force. It is known as lobe compressor.
Construction :
It consists of following main parts:
1. Casing :
It is the housing in which pair of lobes is mounted it has inlet and discharge ports for entry and exit of air respectively.
2. Pair of lobes :
The lobes are meshing elements with small clearance between them which is fixed by timing gears.
Working :
● The air is transferred from suction side to the delivery side with continuous rotation of the two lobes.
● There is practically no change in the volume, the air pressure is increased due to impeller forces the air through the discharge port against back pressure of the system.
● The pressure generated is generally very low and mostly used for low pressure applications.
● The compression ratio for single stage is limited to 1.7.
Screw Compressor:
Definition : When pair at screw is used to compress the air with the help of screw movement in the casing, it is called as screw compressor.
Construction :
It consists of following main components:
1. Pair of screws :
● In screw compressor two screws one with convex and the other with concave contour are used and mostly called as male and female rotor respectively.
● They are made to rotate by means of gear train or other source outside the compressor casing.
2.Timing gears :
● They are fitted on the shaft of each screw and help to maintain clearance between mating screws.
3. Casing with inlet and outlet ports:
● It is a closed chamber with inlet and outlet ports for entry and exit of compressed air respectively.
Working :
● When screw pair rotates, the helical groove of one screw will open with another screw and air enters through inlet port. When female rotor screw is filled over its entire length, the intake phase is complete.
● With further rotation, the male rotor screw starts to mesh at the inlet and with the female rotor, helical ridges at mating screws trap the air during its movement. The male rotor screw starts to squeeze the trapped air towards the discharge end cover.
● On reaching the maximum discharge pressure, the rotor pair uncovers the discharge port and expels the trapped compressed air volume to the discharge port.
● There is no need of inlet and discharge valves as in case of reciprocating compressor.
● It gives pulsation free discharge of compressed air.
Dynamic or Turbo Compressor
● It works on the principle of dynamic/turbo effect of rotating impeller which gives pressure to the air.
● The most common dynamic compressor is centrifugal compressors which work on the principle of centrifugal force produced by the rotating impeller.
Construction :
It consists of:
1. Casing : It is spiral shaped casing in which impeller is mounted.
2. Impeller: It is rotating disc with number of vane which imparts kinetic energy to the air. It rotates at very high speeds.
3. Suction port: It is provided for suction of air from atmosphere.
4. Discharge port : It is provided for delivery of high pressure compressed air.
Working :
● When compressor is started with the help of electric motor, the impeller starts rotating mounted on the driving shaft. The vacuum pressure is created initially as air in the casing is forced out.
● The air from atmosphere is sucked through inlet at the centre of the impeller. As impeller has high speed, it imparts kinetic energy to the air.
● High velocity air is passed through the passages between impeller and the scroll casing where diverging passages convert kinetic energy of air dynamically into pressure head.
● There is continuous passage for air from inlet point to discharge point of compressor the compressed air from these compressors is absolutely oil free and very high discharge pressure and capacity are achieved.
● Turbo compressors are low pressure compressors producing pressure lower than 2 kg/cm².
● They are mostly used for air conditioning and cooling operations.
