06 Mem341
06 Mem341
06 Mem341
9.
Pneumatics
Introduction
Pneumatic systems use pressurized gases to transmit and control power (air). Air because it is a safe, low cost, and easily available fluid. Why we chose pneumatic systems? Gases have smaller inertia compared to liquids (acceleration, deceleration, sudden movement such as opening valves). Gases have lower viscosity (less friction forces). Do not require a return line. But air is compressible. Precision of control is less compared to hydraulic systems. Pneumatic is for low-power applications.
Properties of Air
Air is a mixture of gases (21% O2, 78% N, 1% other gases), moisture and other impurities. When making pneumatic circuit calculation, we used standard atmospheric conditions 20 0C, 101 kPa abs and 11.8 N/m3.
Boyles Law
States that if the temperature of a given amount of gas is held constant, the volume of the gas will change inversely with the absolute pressure of the gas.
V1 p2 = V2 p1
Charless Law
States that if the pressure on a given amount of gas is held constant, the volume of the gas will change in direct proportion to the absolute temperature.
V1 T1 = V2 T2
76
Gay-Lussacs Law
States that if the volume of a given gas is held constant, the pressure exerted by the gas is directly proportional to its absolute temperature.
p1 T1 = p 2 T2
p1V1 p 2V2 = T1 T2
PNEUMATIC COMPONENTS Compressors
Compressor is a machine that compresses air (gases) from a low inlet pressure to a higher desired pressure level. Generally used positive displacement pump (piston, rotary vane or rotary screw).
MEM341: FLUID POWER TECHNOLOGY Compressors operate by initially bring the pressure in the tank to the desire pressure. After that, will automatically control the pressure inside by starting and stopping pump when needed unloading control (pressure switch type or a centrifugal type).
Air Receivers
Main function is to store air from compressor and supply air to the system at constant pressure and as the storage unit for compressed air. It also functions to dampen pressure pulse/surge. Sizing of air receiver; 101 (time that the receiver can supply required amount of air) (system consumption rate compressor output flow rate) ( max receiver pressure min receiver pressure) Commonly air receivers are design to a larger size to accommodate future expansion.
Receiver size, Vr =
78
79
Air Lubricators
add lubricant(oil mist) to the air supply to pneumatic system. This is to ensure that proper lubrication to the internal moving parts of pneumatic components.
After-coolers
to remove moisture from compressed air before enter receiver. Basically a type of heat exchanger. It also functions to cool the air to operating temperature. Effectively remove ~85% of moisture in the air.
80
Air Dryer
Remove virtually all moisture from compressed air The air coming out of this component is basically free of moisture dry air. It is installed after air-cooler. 3 basic types available: o Chemical using chemical reaction to remove moisture (chalk, calcium chloride) o Adsorption using material such as silica gel. o Refrigeration using chiller dryers unit, just like in a air condition system.
Service Units
A system that combine filter, moisture, pressure regulator, and lubricator (FRL) into a single unit.
Figure: Service unit, from left to right, filter, pressure regulator and lubricator. Pneumatic Silencers used to control the noise level cause by the exhausting air stream to the environment.
81
Q = 0.0698 C v
Where,
( p1 p 2)( p 2)
T1
Q is air flow-rate in m3/min p1 is upstream pressure in absolute pressure kPa p2 is downstream pressure in absolute pressure kPa T1 is upstream air temperature Cv is flow capacity constant Only valid when, p2 > 0.53 p1 because beyond this the air flow is choked, where air speed reach the speed of sound.
Pressure Regulators
function to control the pressure enter each pneumatic circuit.
Check Valves
allow flow in one direction only it function to control flow direction.
Shuttle Valves
automatically selects the higher of two input pressures and connects that pressure to the output port. 82
Directional Control Valves (DCV) OUTPUT DEVICES ACTUATORS Pneumatic Cylinders working principal are just like hydraulic cylinder. Design are more simple and lightweight than hydraulic cylinder. many types of design. Control the direction of air travel. Different ways of actuating the valves; air pilot, roller, push button, plunger, detent, spring, level, solenoid etc.
Air Motor mostly based on vane type construction. Can provide infinite torque and speed. Can start and stop quickly. Can be stall with out concerned for heat buildup. Higher speed relate to lower torque, and vice-se-versa.
83
Service unit
Flow control valve 3-way, 2position spring offset push button DCV
Service unit
84
Pressure regulator
85
Shuttle valve will connect the inlet port with higher pressure to the outlet port.
Combination of limit valve and pilot operated DCV will divert the compressed air flow through either flow control valve V3 or V4.
6. Two handed safety control circuit. To extend the cylinder, both push button V1 and V2 must be press together.
86
7. Control of an air motor using two push buttons to start or stop the air motor.
87
Oil cylinder is used to control the speed of the air cylinder. Often called a hydraulic-check system. This will enable a slow, control, smooth control to be achieved on the air cylinder.
88
89
4. Air-oil intensifier
90
Appendices:
91
92