Lecture1 on Pneumatics

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LECTURE 1 ON PNEUMATICS


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INTRODUCTION

Pneumatics has long since played an important role as a technology in the

performance of mechanical work. It is also used in the development ofautomation solutions.

In the majority of applications compressed air is used for one or more of the

following functions:

To determine the status of processors (sensors) Information processing (processors)

Switching of actuators by means of final control elements

Carrying out work (actuators)

To be able to control machinery and installations necessitates the

construction of a generally complex logic interconnection of statuses and

switching conditions. This occurs as a result of the interaction of sensors,

processors, control elements and actuators in pneumatic or partly pneumatic

systems.


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Fundamentals of Pneumatics

Physical fundamentals

Air is an abundant gas mixture with the following composition:

Nitrogen approx. 78 vol. %

Oxygen approx. 21 vol. %

It also contains traces of carbon dioxide, argon, hydrogen, neon,

helium, krypton and xenon.

To assist in the understanding of the natural laws as well as thebehaviour of air, the physical dimensions which are employed. The datais taken from the International System of Units, SI for short.


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Atmospheric Pressure , (pamb)

The pressure prevailing directly on the earths surface is known asatmospheric pressure . This pressure is also referred to as reference

pressure.

Excess Pressure, ( pe > 0)

The range above the atmospheric pressure

Vacuum Range (pe < 0)

The range below the atmospheric pressure

Atmospheric Differential Pressure (pe) Formula


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Fundamentals of Pneumatics Atmospheric pressure does not have a constant value. It varies with the

geographical location and the weather.

The absolute pressure pabs is the value relative to pressure Zero - Vacuum.

It is equal to the sum of the atmospheric pressure and the excess pressure

or vacuum.

In practice, pressure gauges which only indicate the excess pressure are

generally used.

The absolute pressure value pabs is approximately 100 kPa (1 bar) higher.

Generally, in pneumatics all data concerning air quantity refers to the so

called standard state. According to DIN 1343, the standard state is the

status of a solid, fluid or gaseous substance defined by standard

temperature and pressure.

Standard temperature Tn = 273.15 K, tn = 0 C

Standard pressure pn = 101325 Pa = 1.01325 bar


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Fundamentals of PneumaticsCharacteristics of air

A characteristic of air is its minimal cohesion, i.e. the forces between theair molecules are to be disregarded for operating conditions usual in

pneumatics.

In common with all gases, air has no particular shape. Its shape changes

with the slightest resistance, i.e. it assumes the shape of its surroundings.

Air can be compressed and it endeavours to expand. The applicable

relationship is given in Boyle-Mariottes Law.

Boyle-Mariottes Law

At constant temperature, the volume of a given mass of gas is inverselyproportional to the absolute pressure, i.e. the product of absolute

pressure and volume is constant for a given mass of gas.


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Fundamentals of PneumaticsCharacteristics of air

Air expands by 1/273 of its volume at constant pressure, a temperature of

273 K and a temperature rise of 1 K.

Gay-Lussac Law

The volume of a given mass of gas is proportional to the absolute

temperature as long as the pressure does not change.


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The foregoing equations only apply if the temperatures in K are used. In order

to be able to calculate in0

C, the following formula is to be used:


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If the volume is kept constant during the temperature rise, this results in the

following formula for the pressure increase:

or

General Gas Equation

The general gas equation is a combination of all three:


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In the case of a given mass of gas, the product of pressure and volume

divided by the absolute temperature is constant.

This general gas equation results in the previously mentioned laws, if one of

the three factors p, V or T is kept constant in each case.


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Air distribution

In order to ensure reliable and trouble-free air distribution, a

number of points must be observed. This includes primarily:

the correct sizing of the pipe system

the pipe material

flow resistances

pipe layoutmaintenance


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Fundamentals of PneumaticsAir distribution

Sizing pipe systems

For new installations, allowance should be made in all cases for extension of the

compressed-air network. The main line size determined by current requirements

should therefore be increased to include an appropriate safety margin. Plugs and shut-

off valves allow extension to be carried out easily at a later time.

Losses occur in all pipes due to flow resistances. Flow resistances are represented by

restrictions, bends, branches and fittings. These losses must be made up by the

compressor. The pressure drop in the entire network should be as small as possible.

To be able to calculate the pressure drop the total pipe length must be known. Forfittings, branches and bends, equivalent pipe lengths are determined. The choice of

the correct internal diameter is also dependent on the operating pressure and delivery

of the compressor. Selection is best made with the aid of a nomograph.


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Reference

[1] Pneumatics Basic Level, Peter Croser, Frank Ebel, Festo

Didactic GmbH & Co., 73770 Denkendorf 2002

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Lecture1 on Pneumatics

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