introduction to pneumatic systems

7/28/2019 Introduction to Pneumatic Systems

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INTRODUCTION TO PNEUMATIC

CONTROL SYSTEMS

INSTRUCTED BYMr. S.H.C.H. Kumara

NAME

COURSE

INDEX NO

GROUP

FEILD

DATE OF SUB

Hapuarachchi V.H.

B.Sc. Engineering

090170B

04

Mechanical

.01.2012

ME 3012


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INTRODUCTON

Pneumatics systems are being widely used in industrial automation mainly due to its

simplicity in installation and maintenance, fast operation, environmental friendliness even in

case of leakage, low operational and maintenance costetc.

In pneumatic systems compressed air is produced in a compressor and stored in a

receiver. From here it is routed to valves which control the direction of fluid flow, flow controlvalves which control the amount of power produced by the cylinders which converts the

potential energy of the compressed air into kinetic energy at the output.

EXPERIMENT

Introduction to pneumatic control systems

OBJECTIVES

To be familiar with basic pneumatic components commonly found in the industry To be familiar with the common pneumatic symbols To get a basic knowledge and experience on pneumatic circuit design and

simulation

To be familiar with real world control circuit design with pneumatics To get an overall idea about compressed air systems

PROCEDURE

A pneumatic control system was designed and simulated using FluidSIM software and made the

designed system using actual components. The design considerations of the system were as follow.

At the initial position, both cylinders were in the retracted state. Therefore, two limit switches,1S1 and 2S1 are activated.

To start the cycle, both 2S1 and push button valve must be activated. When the cycle starts, piston of C1 advanced slowly and when it came to the position of 1S2, the

piston of C2 started advancing.

When the piston of C2 reached 2S2, piston C1 retraced quickly back to the initial position of it. When the piston of C1 reaches back to 1S1 position, C2 also retraced back to its initial position. Then the cycle could be repeated with the press of the push button.

2S1 2S2

. C2

1S1 1S2

C1


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DISCUSSION

Different types of compressors and their features

Reciprocating compressors

Reciprocating air compressors are positive displacement compressors. This means they

are taking in successive volumes of air which is confined within a closed space and elevating thisair to a higher pressure. The reciprocating air compressor accomplishes this by using a piston

within a cylinder as the compressing and displacing element.

The reciprocating air compressor is considered single acting when the compressing is

accomplished using only one side of the piston. A compressor using both sides of the piston is

considered double acting.

The reciprocating air compressor uses a number of automatic spring loaded valves in

each cylinder that open only when the proper differential pressure exists across the valve. Inlet

valves open when the pressure in the cylinder is slightly below the intake pressure. Dischargevalves open when the pressure in the cylinder is slightly above the discharge pressure.

A compressor is considered to be single stage when the entire compression is

accomplished with a single cylinder or a group of cylinders in parallel. Many applications

involve conditions beyond the practical capability of a single compression stage.

Rotary Screw Compressors

Rotary air compressors are positive displacement compressors. The most common rotary

air compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These

compressors consist of two rotors within a casing where the rotors compress the air internally.

There are no valves. These units are basically oil cooled (with air cooled or water cooled oil

coolers) where the oil seals the internal clearances.

Since the cooling takes place right inside the compressor, the working parts never

experience extreme operating temperatures. The rotary compressor, therefore, is a continuous

duty, air cooled or water cooled compressor package.

Advantages Disadvantages

Simple Design Higher maintenance costLower initial cost Many moving parts

Easy to install Potential for vibration problems

Two stage models offer the highest efficiency Foundation may be required depending on size

No oil carryover Many are not designed to run at full capacity 100% ofthe timeLarge range of horsepower

Special machines can reach extremely highpressures


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Simple design Limited airend life

Low to medium initial cost Airends are not field serviceable

Low to medium maintenance cost High rotational speeds

Two-stage designs provide good efficiency Shorter life expectancy than other designs

Easy to install Oil injected designs have oil carryover

Few moving parts Single stage designs have lower efficiency

Most popular compressor design in plants Two-stage oil free designs have higher initial costDifficulty with dirty environments

Centrifugal Compressors

The centrifugal air compressor is a dynamic compressor which depends on transfer of

energy from a rotating impeller to the air. The rotor accomplishes this by changing the

momentum and pressure of the air. This momentum is converted to useful pressure by slowing

the air down in a stationary diffuser.

The centrifugal air compressor is an oil free compressor by design. The oil lubricated

running gear is separated from the air by shaft seals and atmospheric vents. The centrifugal is a

continuous duty compressor, with few moving parts, that is particularly suited to high volume

applications, especially where oil free air is required.

Centrifugal air compressors are water cooled and may be packaged; typically the package

includes the after-cooler and all controls.


High efficiencies approaching two-stage

reciprocating compressors

High initial cost

Can reach pressures up to 1200 psi Complicated monitoring and control systems

Completely packaged for plant orinstrument air up through 500 hp

Limited capacity control modulation, requiring unloadingfor reduce capacities

Relative first cost improves as sizeincreases

High rotational speeds require special bearings andsophisticated vibration and clearance monitoring

Designed to give lubricant free air Specialized maintenance considerations

Does not require special foundations

Rotary sliding vane compressor

A sliding (rotary) vane compressor has a solid rotor mounted inside a water jacketed

cylinder, similar to that of a jacketed water section of a reciprocating cylinder. The water jacket

around the cylinder is used for cooling. The rotor is filled with blades that are free to move in

and out of the longitudinal slots in the rotor. Blade configurations range from 8 to 12 blades,

depending upon manufacturer and pressure differentials. The blades are forced out against the

cylinder wall by centrifugal force, creating individual cells of gas which are compressed as the

rotor turns. As it approaches the discharge port, this area is reduced and the gas discharged.


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Simple design Oil injected designs have oil carryover

Easy to install Single stage designs have lower efficiency

Low to medium cost Difficulty with high pressures

Low maintenance cost Oil free designs are unavailable

Field serviceable airend

Long life airend

Low rotational speedsVery few moving parts

Forgiving to dirty environments

Operation of filters

A pneumatic filter is a type of filter specifically designed for use in a compressed air

stream. Filters are devices used to remove contaminants from a stream of liquid or gas.

Pneumatic filters remove contaminants, especially solid particulates, from streams of fluid in the

form of compressed air. They are frequently used in sequence with secondary filters that trapeven smaller particles that escape the primary filters.

Filtration for pneumatic systems is handled quite differently. In most industrial

pneumatics, compressed air is supplied from a single compressor to a large number of operating

systems, as a plant resource, much like light or electricity. Individual filters are used on the

separate systems. Sometimes more than one filters per system. Often the filters are found in

conjunction with regulators and sometimes lubricators comprising a filter-regulator-lubricator for

the system.

Operation of lubricators

A pneumatic lubricator is set up with the oil on one side of the machine, a narrow

chamber in the center and the injection valve on the other side. The movement of the oil from the

one side to the other (usually with the help of compressed air), creates the vacuum that pulls the

oil through. The speed that the oil is moving renders it into an aerosol, like a mist or a gas, which

is then pushed through the machine via pneumatic pressure. The finer the mist of oil, the further

it will go and the more of the machine will be oiled throughout the process.

Pneumatic lubricators differ in terms of specifications and features. A regulator lubricator can

operate over a range of pressures that are measured in pounds per square inch (psi). The ambient

operating temperature for a pneumatic lubrication system is also a range. Modular pneumatic

lubricators are designed to be incorporated into existing pneumatic systems. They differ in terms

of working pressure, operating temperature, and bowl capacity. Features for pneumatic

lubricators include automatic shut-down, warning lights, and dirt stoppers.


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Operation of pressure regulators

Regulators reduce the pressure of a gas or liquid from a source, such as a cylinder or

compressor, to a lower value needed by a device, such as an analyzer. A pressure regulator

provides better resolution and control when its inlet and control range pressures closely match

the pressure requirements of the fluid handling system. Resolution is the number of handle turns

needed to adjust a regulator from its lowest to highest outlet pressure setting. Control is the

ability of the regulator to hold a given outlet pressure set point.

A pressure regulator is a normally open valve. With a regulator positioned after a receiver

tank, air from the receiver can expand (flow) through the valve to a point downstream. As

pressure after the regulator rises, it is sensed in an internal pilot passage leading to the underside

of the piston.

Effect of moisture in compressed air

It is critical in the design of compressed air systems that safe moisture level limits be established

to protect the systems and processes. Appropriate purification components, such as coalescing

filters and desiccant dryers, can then be selected and installed to meet the system requirements.

Water droplets are a major cause of erosion. In a compressed air line, water is fluidized to an

aerosol mist by the turbulent air flow. The droplets are then propelled downstream at high

velocities until they impact on the first obstruction in their path, such as a piping elbow, a valve

disc, an orifice plate, or an air motor blade. The resulting repeated impulses produce destructive

forces on the solid surfaces that cause erosion.

Steps to take

Moisture must be reduced in compressed air lines by purification equipment. Liquid water andoil aerosols which cause erosion, oxygen corrosion, biological growth and product contamination

can be eliminated by high efficiency coalescing filters. Water vapor, which has similar effects,

can be reduced to safe levels by adsorption processes. The degree of dryness required must be

determined by an analysis of each individual compressed air system.

introduction to pneumatic systems

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