chapter 8 - pneumatic operation circuit and application

8/12/2019 Chapter 8 - Pneumatic Operation Circuit and Application

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MEM34 : FLUID POWER TECHNOLOGY

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9. PneumaticsIntroduction

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 Ai r

Air is a mixture of gases (21% O2, 78% N, 1% other gases), moisture and otherimpurities.When making pneumatic circuit calculation, we used standard atmospheric conditions 20 0C, 101 kPa abs and 11.8 N/m3.

The Perfect Gas Laws! PERFECT GAS DO NOT EXIST IN REALITY !But air behaves vary closely to that of a perfect gas.

Boyles Law

States that if the temperature of a given amount of gas is held constant, the volume ofthe gas will change inversely with the absolute pressure of the gas.

1

2

2

1

p

p

V

V=

Charless LawStates that if the pressure on a given amount of gas is held constant, the volume of thegas will change in direct proportion to the absolute temperature.

2

1

2

1

T

T

V

V=


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Gay-Lussacs LawStates that if the volume of a given gas is held constant, the pressure exerted by the gasis directly proportional to its absolute temperature.

2

1

2

1

T

T

p

p

=

General Gas LawIt is the combination of Boyles, Charles and Gay-Lussacs Laws into a single generalgas law.

2

22

1

11

T

Vp

T

Vp =

PNEUMATIC COMPONENTSCompressorsCompressor is a machine that compresses air (gases) from a low inlet pressure to ahigher desired pressure level. Generally used positive displacement pump (piston, rotaryvane or rotary screw).

Figure: Different types of compressor design.

When air is compressed, heat will be generatedbecause of the molecules of air come closertogether, excessive heat will damage thecomponents. Active cooling is used such as fan orwater cooling.

Single piston normally can efficiently achievecompressed pressure of 150 psi (10 bars). Toachieve higher pressure can used more than one

cylinder. Multi-stage compressors are more efficient.

Figure: Multi-stage compressor


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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 pumpwhen 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;

Receiver size, Vr=

101 (time that the receiver can supplyrequired amount of air) (system consumptionrate com ressor out ut flow rate

( max receiver pressure min receiver pressure)

Commonly air receivers are design to a larger size to accommodate future expansion.

Figure: Air receiver and cut-away diagram of a typical air receiver.


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FLUID CONDITIONING

Air Filters

- function of an air filter is to remove contaminants, 5 50 micron meter.

Air Pressure Regulators

- to regulate the pressure coming out from air receiver and ensure that a constantpressure is supply to the pneumatic system.


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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.

Pneumatic Pressure Indicators

- to give visual indication, whether inside the pneumatic system/hoses/components

are supply with compressed air or not.

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.


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Air Dryer

- Remove virtually all moisture fromcompressed 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 chemicalreaction to remove moisture (chalk,calcium chloride)

o Adsorption using material suchas silica gel.

o Refrigeration using chiller dryersunit, 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 bythe exhausting air stream to the

environment.


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PNEUMATIC CONTROL VALVES

Air Flow-rate Control with Orifice

- air flow-rate is control by creating flow restriction. Bernoulli Effect.

( )( )

=

1

2210698.0

T

pppCQ v

Where, Q is air flow-rate in m3/minp1is upstream pressure in absolute pressure kPap2is downstream pressure in absolute pressure kPa

T1is upstream air temperatureCvis flow capacity constant

Only valid when, p2 > 0.53 p1 because beyond this the air flow is choked, where airspeed 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 thatpressure to the output port.


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Directional Control Valves (DCV)

- 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.

-

OUTPUT DEVICES ACTUATORS

Pneumatic Cylinders

- working principal are just like hydraulic cylinder.- Design are more simple and lightweight than hydraulic cylinder.

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.


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Basic Pneumatic Circuits

1. Operation of a single acting cylinder

Service

unitSingle

acting

cylinder

with spring

return

3-way, 2-

position spring

offset push

button DCV

Flow control

valve

2. Operation of a double-acting cylinder

Double acting

cylinderService

unit

5-way, 2-position spring

offset push button DCV


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5. Two step speed control system

Shuttle valve will

connect the inlet port

with higher pressure to

the outlet port.

Combination of limitvalve 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 presstogether.


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Pneumatic and Hydraulic System

1. Air-oil system for speed control

Oil cylinder is used to control the

speed of the air cylinder. Oftencalled a hydraulic-check system.

This will enable a slow, control,

smooth control to be achieved on

the air cylinder.


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2. Air-oil reservoir system


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3. Two air-oil reservoirs used to control a double acting oil cylinder

4. Air-oil intensifier


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Appendices:


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Figure: Commonly used direction valves.

Figure: Examples of centre positions.

chapter 8 - pneumatic operation circuit and application

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