Air Piping Manual

7/27/2019 Air Piping Manual

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HandlingMachiningAssemblyAir preparation

PneumaticsElectronicsMechanicsSensoricsSoftware

ChineseEnglishFrenchGermanRussianSpanish

Blue Digeston Automation

052 912

HesseCompressed Air

as an Energy Carrier

M

M

M

Preparation and distribution


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Hesse

Compressed Air as an Energy Carrier


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Compressed Airas an Energy CarrierPreparation and distribution

Blue Digeston Automation

Air preparation

Pneumatics

Stefan Hesse


5/125

Blue Digest on Automation

2002 by Festo AG & Co.Ruiter Strae 82

D-73734 Esslingen

Tel. (0711) 347-0

Fax (0711) 347-2144

All texts, representations, illustrations and drawings included in this book are

the intellectual property of Festo AG & Co., and are protected by copyright law.

All rights reserved, including translation rights. No part of this publication

may be reproduced or transmitted in any form or by any means, electronic,

mechanical, photocopying or otherwise, without the prior written permissionof Festo AG & Co.


6/125

Today, there is hardly a factory that can function without the use of compressed

air. Pneumatic components generate movement and are important elements

of mechanization and automation systems. One traditional application of com-

pressed air is the operation of hand-held power tools. These range from pneu-

matic hammers to nail guns and from compressed air guns to screwdrivers.

There are other applications which make special demands on compressed air.

In a paint shop or for laser cutting of optical systems, for example, the air must

be clean, dry and oil-free.

As compressed air is not dangerous when it leaks out from the supply network,

many users do not take air economy seriously. But wasting compressed air is

wasting money!

There are therefore many reasons for addressing this problem, not only in the

case of intelligent valves, fast cylinders and practical handling devices, but also

with the preparation of compressed air in a pneumatic system. To this end, this

book provides detailed knowhow on the subject and deals with the routing of

compressed air including a number of physical fundamentals. The aim is to fillin any possible gabs regarding piping technology.

Frank Schnabel and Dipl.-Ing. Ditmar Bruder (Festo) assisted in the preparation

of this material with suggestions and knowledge.

Stefan Hesse

Preface


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Contents

Preface

1 Compressed air in industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2 Physical fundamentals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.1 Fluid dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2 Pressure and pressure units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3 Air humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3 Compressed air preparation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.1 Compressed air quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2 Drying methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.3 Filtering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.4 Compressed air lubricators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.5 Pressure regulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.6 Service unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

3.7 Pressure amplifiers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

4 Compressed air distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.1 Components of a compressed air line . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.2 Sizing of line systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.3 Pipes and connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.4 Tubing and connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

4.4.1 Types and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4.4.2 Types of tubing connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

4.4.3 Quick-coupling connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

4.4.4 Safety shut-off valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

4.4.5 Damage to tubing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 984.5 Reservoir. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

4.5.1 Design and application of reservoirs. . . . . . . . . . . . . . . . . . . . . . . . . 101

4.5.2 Sizing of reservoirs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

4.5.3 Safety guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

4.6 Threads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

5 Compressed air losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

5.1 Leakage and pressure drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

5.2 Locating and controlling leaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

6 Tips and checks for savings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Standards and guide lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

Index of technical terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122


9/125


10/125

After electricity, compressed air is the most important energy carrier for

industry, tradespeople and associated areas. Although the transmission of

force was discovered in ancient times, it was not until the 19th century that

the first functional pneumatic machines came on the market. A hundred years

ago, one could read about The application of compressed air in American

factories (1904) and the The compressed air system of the Imperial Shipyard

in Kiel (1904). During this period, there were many designs for pneumatic ham-

mers in which the percussion piston was self-controlling. Short-stroke devices

reached a velocity of 10,000 to 15,000 strokes per minute. Devices that opera-

ted at under 2,000 strokes per minute (Figure 1-1) were used for chiseling and

caulking.

It was not until after 1950 that the development of what we now call industrial

pneumatics started in the United States and Germany. The continued huge

acceptance of pneumatic machines results from several essential benefits.

These are:

Compressed air can be generated anywhere in unlimited quantities High energy density, low weight and simple energy transmission

Energy can be stored in containers and transported without difficulty

Non-combustible and non-flammable no explosion hazard

Low effort for planning, maintenance and care

Infinite variability of power characteristics within the permissible range

for pneumatics

Those are impressive benefits. Today, most industrial companies have a

compressed air system and use this to drive many devices and drives, where-

by the pneumatic cylinder is the most popular actuator. But in order for air toexpend energy, it must first be provided with energy. This is done by compres-

sing the air using compressors. There is a broad range of systems for doing this:

screw-type, piston-type, membrane, rotary, roots, spiral and turbo compressors,

both oil-lubricated and dry-running, water-injected, air- or water-cooled. But that

is only the first stage. Compressed air and suction air (the air drawn in to the

system) can be seen as a constant cycle, as shown in Figure 1-2.

1 Compressed air in industry 9

1

Compressed air

in industry

Figure 1-1

100 years ago, the

compressed air hammer

was the most widely used

pneumatic tool

a b c

g e g

h m i

k

kf

bf

d


11/125

1 Compressed air in industry10

Figure 1-2

Compressed air cycle

in industrial application

This book only covers compressed air more specifically, its preparation and

distribution. These are precisely the areas to which particularly close attention

must currently be paid, as they are the source of avoidable energy loss. The

distribution network is a soft spot that can result in enormous cost, particularlywhen incorrectly designed and/or poorly maintained. The following aspects play

a substantial role here:

The condition of the distribution network: Even small leaks are very costly

over time.

The sizing of the network: Inadequate cross-sections can result

in large pressure losses.

Consumption characteristics change: This requires modification of the

compressed air system to the new requirements.

Condensate draining and treatment is obsolete and requires a state-of-the-art

solution.

Each bar increase in pressure costs around 6 to 10 percent more energy. A well

maintained network should not have a leakage rate of more than 10 percent.

In practice, however, leakage ranges of 20 to 25 percent are not uncommon.

Vacuumconsumer

EjectorVacuumreservoir

Compressedair user

Exhaustair P = 0

Compressedair reservoir

Atmos-phericair

Compressed airpreparation

Compressed airdistribution

Compressedair

Vacuum

distribution

Evacuated

air

Vacuum

preparation

Energy

+P

P


12/125

2 Physical fundamentals 11

2

Physical fundamentals

2.1

Fluid dynamics

Figure 2-1

Friction-free flow

a) Secondary flow

b) Flow for changing

cross-section

Compressed air is compressed atmospheric air made up of 78% nitrogen,

21% oxygen and 1% other gases (primarily argon). The pressure of atmospheric

air depends on its geographical location. The following basic values are com-

monly used as reference variables for temperature and pressure of air:

po = 1.013 bar and to = 20 C or po = 1.013 bar and to = 0 C

The motion of liquids and gases is described as flow. The two types of media

are different in that liquids are practically incompressible, whereas the volume

of gas is a function of its pressure. For gas flows up to the speed of sound

(340 m/s), volume changes only play a minor role. Up to this threshold, air

can be regarded as having stable volume. In the temperature range between

0 and 200 C and at pressures up to 30 bar, air behaves as an ideal gas with

the exception of the internal friction. This means that fundamental fluidic

equations can be applied. The variables pressure (p), temperature (T) and

volume (Vsp) are then proportional to each other. This applies for the general

equation of gases:

When compressed air flows through a tube, the secondary flow volume

(as shown in Figure 2-1) results:

= A L in m3/s

whereA Inner tube diameter in m2; A = (D2 )/4

L Length of the secondary flow volume section in m/s

V

V

L

D

0 1 tA A

v v1

1

2

2

a) b)

p VspT

= constant


13/125

2 Physical fundamentals12

Figure 2-2

Types of flow

a) Laminar flow

b) Transition to turbulent

flow after an obstruction.

If one assumes that the air is in a closed system, it must also pass a constricted

section of tubing. The continuity equation (Figure 2-1b) applies to this situation:

v Velocity

To put this in words: The velocity of flow is inversely proportional to the cross

section for a constant flow volume.

In compressed air systems, the volumetric flow rate represents the consump-

tion of pneumatic drives or devices. This is normally given in litres per unit time.

The factors shown in Table 2-1 should be used for conversion. Normally, values

are given in litres per minute or cubic litres per unit time. The volumetric flow

rate is a characteristic value for the capacity or demand of a system.

The following have to be differentiated:

Volumetric flow rate of a compressor, measured on the suction

or pressure side Volumetric flow rate of consuming devices, as an absolute value or as a

requirement taking into consideration simultaneity factors.

If the volumetric flow rate is given in standard cubic metres per hour (N m3/h),

this applies to a pressure of p = 1.013 bar and a temperature t of 0 C.

In pipes that contain no obstructions, air flow is laminar, whereby the velocity

of flow is slightly lower near the pipe walls than in the middle of the pipe

(Figure 2-2). Bends in piping, branches, valves, fittings and measuring

devices, however, cause turbulence. The boundary between laminar flowand turbulent flow is characterized by the Reynolds number (O. Reynolds,

1842 1912). This number represents the influence of such friction forces.

A1 v1 = A2 v2 = V

V

a) b)


14/125

2

Physicalfundamentals

13

M

Conversion Conversion factors

to UK

from M l/s l/min l/h m3/s m3/min m3/h ft3/min ft3/hour gal/mi

l/s 1.0 60.0 3600.0 0.001 0.06 3.6 2.118882 127.133 13.198

l/min 0.016666 1.0 60.0 0.166104 0.001 0.06 0.0353147 2.118883 0.2199

l/h 0.278103 0.46105 1.0 0.2778106 0.166104 0.001 0.588103 0.035315 0.0036

m3/s 1000.0 60000.0 3600000 1.0 60.0 3600.0 2118.88 127133.0 13198

m3/min 16.6666 1000.0 60000.0 0.01667 1.0 60.0 35.31466 2118.8833 219.96

m3/h 0.277778 16.6666 1000.0 0.000278 0.01666 1.0 0.588578 35.3147 3.6661

ft3/min 0.471947 28.31682 1699.017 0.472103 0.0283169 1.699017 1.0 60.0 6.2288

ft3/hour 0.007866 0.471947 28.3168 0.78105 0.4719103 0.028317 0.016667 1.0 0.1038

UK gal/min 0.0757682 4.546092 272.766 0.758104 0.004548 0.272766 0.160544 9.63262 1.0

UK gal/hour 0.001263 0.075768 4.54609 0.12105 0.757104 0.004546 0.002676 0.160544 0.0166

US gal/min 0.063090 3.7854 227.125 0.631104 0.0037854 0.227125 0.133681 8.020832 0.8326

US gal/hour 0.0010515 0.06309 3.785411 0.1105 0.63104 0.003785 0.002228 0.133681 0.0138


15/125


The type of flow can be seen by the value of the Reynolds number Re.

If Re < 2320, the flow is laminar. If Re = 2320 to 3000, laminar or turbulent flowcan occur. If Re > 3000, the flow is turbulent. Blood flowing through arteries ofhumans, for example, exhibits laminar flow. In pneumatic systems, the average

flow velocity reaches values of 6 to 40 m/s. As a result, flow is generally turbu-

lent. Turbulence causes resistance to flow and thus causes pressure loss in the

system. Pressure loss is approximately proportional to the square of the flow

velocity. For this reason, the aim is to achieve smooth internal walls of tubing

and piping and to use fittings of optimum fluidic design. The average flow

velocity vm is derived from:

Mass flow per unit time (kg/s)

A Flow cross-section (m2)

Vspec Specific volume (m3/kg)

The average flow velocity vm is then put into the Reynolds Number:

d Pipe diameter in m

v Kinematic viscosity in m2/s

The volumetric flow rate (m3/s) is calculated by multiplying the flow

cross-section A (m2) by the average flow velocity vm (m/s).

What is the relationship to temperature?

The mutual dependencies of the status variables volume V (m3/kg), pressure p

(N/m2) and temperature T (K) are defined by the general equation for the

state of gases. This can be derived from Boyles Law (R. Boyle, 1627-1691)

and Mariottes Law (E. Mariotte, 1620-1684) and Gay-Lussac (L. J. Gay-Lussac,

1778-1850).

m

Avm = vspec (m/s)

m

vm dRe =

V


16/125


2.2

Pressure

and pressure units

Accordingly, the following applies if p, V and T change at the same time:

Pressure change from p1 to p2 at constant temperature T1(according to Boyle and Mariotte)

Vx specific volume as an intermediate value (for derivation)

Temperature change from T1 to T2 at constant pressure p2(according to Gay-Lussac)

This results in a general state change:

The special gas constant for air is Ri = 287 J/kgK, whereby 1 J (Joule) = 1 Nm.

Example: Given are 5 kg of air at an absolute pressure of 1.2 bar

and a temperature of 20 C. What is the volume?

The volume V of the air is determined by .

T = (t + 273.15)K = (20 + 273.15)K = 293.15 K

p = 1.2 bar = 1.2 105 N/m2

Pressure is normally understood to mean the force F acting on a surface A.

Pressure is expressed as a quotient by

Gases which includes air have the property of expanding under molecular

motion to uniformly fill the space available. This can be an enclosed container.

In this case the molecules strike the wall of the container, momentarily applying

a force. The sum of these motions results in a force that is detectable as the pres-

sure of the gas on the inside walls of the container. At constant temperature, thispressure is proportional to the number of molecules present per unit volume.

= or rather V2 = =Vx T2

T1

VxV2

T1T2

V1 p1 T2p2 T1

= = constant = Rip2 V2

T2

p1 V1T1

m Ri Tp

V = = = 3.5 m3m Ri T

p5 kg 287 Nm/kgK 293.15 K

1.2 105 N/m2

FA

p =

= or rather Vx=V1 p1

p2

V1Vx

p2p1


17/125


Figure 2-3

Diagrammatic

representation of pressures

Different types of pressure are differentiated:

Atmospheric pressure (barometric air pressure)

Absolute pressure (pressure compared to an absolute vacuum

as the zero value)

Differential pressure (pressure that represents the difference

between two absolute pressures)

Pressure above atmospheric (pressure greater than atmospheric pressure

and using atmospheric pressure as the zero value)

Pressure below atmospheric (pressure below atmospheric pressure and using

atmospheric pressure as the zero value)

Flow pressure (pressure in a consuming device at the time of air consumption)

Back pressure (pressure in an air supply line when not air is being consumed)

These pressures are shown diagrammatically in Figure 2-3.

In 1978, the International Standards system (SI) defined the Pascal (Pa) as the

unit of measure for pressure.

1 Pa = 1 N/m2 = 1 kg/ms2

105 Pa = 0.1 MPa = 1 bar

Conversion factors are shown in Table 2-2.

Atmospheric

Absolutepres

sure

Pressureabove

atmospheric1

pressure

Press

urebelow

atmospheric

Pressureabove

atmospheric2

Differential

pressure

100% vacuum


18/125

2

Physicalfundamentals

17

M

Conversion Conversion factors

to kp/cm2 mm Hg mm WS mbar bar MPa N/m2 kgf/cm2

from M (at) (Torr) (Pa)

kp/cm2 (at) 1.0 736 104 980.665 0.9807 9.807102 9.807104 1.03322

mm Hg (Torr) 1.36103 1.0 13.6 1.33322 1.333103 1.333104 133.3 0.0013591

mm WS 104 7.36102 1.0 0.09807 9.81105 9.81106 9.81 0.999104

mbar 1.02103 0.750062 10.197 1.0 0.001 104 100 0.0010197

bar 1.0197 750.06 1.02104 103 1.0 0.1 105 1.0197

MPa 1.02 7500 1.02105 104 10 1.0 106 10.1967

N/m2 (Pa) 1.02105 7.5103 0.102 102 105 106 1.0 1.019105

kgf/cm2 1.000278 735.559 10002.78 980.665 0.9807 9.807102 98066.5 1.0

in water 2.54103 1.868 25.4 2.49089 2.49103 2.49104 249 0.00254

in Hg 3.455102 25.4 345.4 33.8639 3.387102 3.387103 3387 0.034532

lbf/in2 (psi) 7.031102 51.71 703.1 68.9476 6.895102 6.895103 6895 0.070307


19/125


In compressed air systems, compressed air is generated by different types of

compressors. The following categories are differentiated:

Rotary compressors (screw compressors, vane compressors, liquid ring

compressor, roots compressor)

Piston compressors (plunger compressor, crosshead compressor, free piston

compressor, reciprocating piston compressor, diaphragm compressor)

Turbo compressors (radial and axial compressors)

One- and two-stage oil-lubricated piston compressors and single-stage oil-

injection screw compressors are primarily used for the generation of compressed

air in the low-pressure range (6 to 15 bar).

Gas expands uniformly in all directions. If pressure gauges are attached to

different locations of a pressurized container, they all show the same pressure.

This is known as the law of pressure transmission. In the case of flowing air, two

pressures are differentiated: static pressure pst and dynamic pressure pdynThe total pressure is

ptot = pst + pdyn

The pressure acts along the pipe axis in the opposite direction to the flow. The

static pressure pst acts against the wall of the pipe. The dynamic pressure pdyndepends on the kinetic energy of the fluid. At v = 0, there is only static pressure.

The sum of static and dynamic pressure is always equal to the static pressure

of the fluid at rest.

The dynamic pressure is the reference variable for all resistances actingon pure air flow. It can be measured with the Prandtl tube (pitostatic tube)

(Figure 2-4). It is a double-walled pipe with a central opening for the dynamic

pressure and an annular gap for the static pressure pst. Differential pressure

can be measured, for example, via a manometer.

The velocity of a flowing fluid can be calculated from the dynamic pressure (q):

q = in m/s

air density in kg m3v velocity of the fluid in m s1

v2

2


20/125

Example: What is the air velocity v, if the level difference h of the water columnin a manometer attached to a pitostatic tube is 13.3 mm and the water

temperature in the manometer is 20 C?

The value h represents the dynamic pressure, whereby 1mm water columncorresponds to a pressure of 9.81 Pa(= 9.81 Nm2, = 9.81 kgm/s2). This results

in a differential pressure (dynamic pressure) of

between the total pressure ptot and the static pressure pstat.

The air velocity v is calculated by:

How can the pressure be measured?

Possibilities for measuring pressure include a bourdon-tube pressure gauge

(see Figure 3-27, page 53). In many cases, however, a pressure switch or a

PE (pneumatic-electric) transducer is sufficient for pressure monitoring.

In the latter case, a pneumatic pressure signal switches an electrical change-

over switch. The switching force can be increased by an appropriately large

diaphragm surface area. If the switching range is adjustable, one speaks ofa pressure switch (Figure 2.5).


Figure 2-4

Measurement of pressures

using a pitostatic tube

(L. Prandtl; 1875-1953)

1 Pipe

2 Ptot-tube

3 U-tube manometer

pdyn

v

1

2

3

pstat

ptot

13.3 mmWS = 130.4 N m29.81 N m2

mmWS

v = 2 1

q = = 14.7 m/s

2 130.4 Nm2

1.199 kgm3


21/125


Figure 2.5

Pneumatic

switching elements

a) PE transducer

b) Pressure switch

1 Contact

2 Setting screw

3 Plunger 4 Compression spring

5 Diaphragm

6 Micro-stem pushbutton

x Pressure inlet

Figure 2-6

How switch signals result

a) Value exceeds or falls

below threshold value

b) Differential pressure

is exceeded

c) Pressure moves

out of window

H Hysteresis

S Set switching point

There are also devices that combine the sensor and the switch in one. Use

of such devices ensures safety in industrial compressed air networks. In thesimplest case, a signal Pressure present: Yes/No is returned. This case is

shown in Figure 2-6a. If a threshold is exceeded, a digital output switches.

If the actual pressure fluctuates around the threshold value, fluttering of the

switch signal results. For this reason, a switching hysteresis can be defined,

and only when the pressure falls below this value does the switch signal result.

A sensor for differential pressure is used to monitor filter condition. This com-

pares the pressure upstream and downstream of the compressed air filter

(Figure 2-6b). The result is only correct, however, if the flow rate is constant.

If the flow rate increases, the differential pressure increases, although the filter

has not necessarily become more contaminated.

1

2

3

45

5

6

x x

x

a) b)

1

2 4

Signal

1

0

H

Differential pressure pb)

Signal

1

0

H

S

Pressure pa)

Signal

1

0

H

Pressure pc)


22/125


2.3

Air humidity

In order to monitor the pressure in a network, the maximum and minimum

pressures are monitored. If the pressure moves out of this window (window

comparator) the pressure sensor responds. This function (Figure 2-6c) can be

used both for absolute pressure and for differential pressure measurements.

Example: The minimum pressure is set to 4 bar on the sensor switch.

The maximum pressure is set to 7 bar. Pressures outside this range can, for

example, lead to faulty operation of equipment or to endangering of personnel.

Pressure only remains applied within this window. If the pressure moves out of

this window, a switch-off signal is triggered.

Humid air is a mixture of dry air and water vapour. Air has a limited capacity

to absorb humidity. This limit depends on the barometric pressure and the air

temperature. If, for example, air cools against a cold pane of glass, the water

vapor condenses on the glass. Condensation has been known since ancient

times, as demonstrated by air wells. These are large domed stone buildings

which condense water in the cool of night. The limits at which condensationbegins are called the dew point and the pressure dew point.

Dew point

The dew point is the point on the temperature scale (dew point temperature)

at which air is saturated with water vapour. In other words, the humidity is

100%. As soon as the air temperature falls below this value, condensation

occurs. Ice forms at temperatures below freezing. This can have a substantial

impact on the flow characteristics and function of components in pneumatic cir-

cuits. The lower the dew point, the less water the air can hold. The dew point is

defined by the variables relative humidity, temperature and pressure, where-by:

The higher the temperature, the more water can be held.

The higher the pressure, the lower the amount of water that can be held.

Pressure dew point

The pressure dew point is used, for example, to facilitate comparison of various

air dryers. This is the dew point temperature to be applied to the appropriate

operating pressure. If the pressure is relieved to atmospheric pressure, the air

expands. For this reason, at a constant air temperature, the dew point for air at

atmospheric pressure is lower than the pressure dew point. If, for example, theair has a pressure dew point of +5 C, water cannot condense as long if the

ambient temperature is above +5 C. Condensation occurs if the temperature

falls below this value.


23/125


Figure 2-7

Basic structure of the

Mollier diagram (developed

by M. Zindl and T. Engelfried)

1 Unsaturated humid air

2 Mist

3 Frozen fog

T Medium temperature

X Water content per kg air

G Limit curve

Air humidity

The relative air humidity Wrel expresses the relationship between the actual

humidity and the maximum possible humidity in the air (saturation).

Please note: Temperature changes lead to changes in relative humidity,

even if the absolute humidity remains constant.

Maximum humidity (fmax in g/m3)

This is the maximum possible quantity of water vapour that can be held by a

cubic meter of air at a certain temperature (saturation quantity).

Absolute humidity (f in g/m3)

This is the actual amount of water vapour in one cubic metre of air.

How can the dew point be determined?

This can be done using the Mollier diagram. The basic structure of this diagram

is shown in Figure 2-7. The limit curve G separates the area of unsaturated

humid air from the area of fluid, ice or mist. Before using the diagram, one has

to know the water content of the air in grams per kilograms of air.

Wrel = 100 in per centAbsolute humidity (f )

Saturation quantity (fmax)

TemperatureinC

G

G

p = 6 bar

p = 1 bar1

2

3

Water content X in g/kg

20

0

20


24/125


Figure 2-8

Mollier diagram (excerpt)

T Dew point

p Total absolute pressure

in bar

The water content X can be calculated as follows:

p Total pressure absolute in bar

rel Relative humidity ( = 0 to 1.0)ps Saturation vapour pressure in bar

The pressure ps of the water vapour in the air is dependent only on the tempera-

ture. If the water content in the air is to be given in g/m3, the above equation

has to be multiplied by the air density pN. At Festo, this has been defined

as pN = 1.292 kg/m3. (Festo Info 980010. According to ISO, pN = 1.185 kg/m3).

The water content X can then be applied in the Mollier diagram (Figure 2-8).

Example: What is the dew point temperature if the relative humidity

Wrel is 0.5 (= 50%), the pressure p is 3 bar and the temperature T is 24 C.

X = 0.622 103 in g/kgrel ps

p rel ps

0 0.5 1.0 1.5 2.0 3.5 4.0 4.5 5 5.5 6.0 6.5 7.0 7.5 8.02.5

2018

16

14

12

10

8

6

4

2

0

2 4

6

8

10

12

14

16

18

20

10 7 6 54

3

2

1

T

p

X = 3.11 Water content X in g/kg

Dew

pointtemperatureinC

T = 13


25/125


The first step is to establish the saturation vapour pressure ps (24 C) at 24 C.

This can be read from the following vapour table.

Temperature ps Temperature ps Temperature psT in C in mbar T in C in mbar T in C in mbar

20 1.029 + 2 7.055 + 24 29.82

18 1.247 + 4 8.129 + 26 33.60

16 1.504 + 6 9.345 + 28 37.78

14 1.809 + 8 10.70 + 30 42.41

12 2.169 + 10 12.70 + 32 47.53

10 2.594 + 12 14.01 + 34 53.18

8 3.094 + 14 15.97 + 36 59.40

6 4.681 + 16 18.17 + 38 66.24

4 4.368 + 18 20.62

2 5.172 + 20 23.37

0 6.108 + 22 26.42

This results in ps (24 C) = 29.82 mbar = 0.2982 bar. The water content X is

calculated as follows:

The dew point temperature of 13 C can now be read off in the Mollier diagram.

It is the intersection of the saturation line with p = 3 bar and the line

for X = 3.11.

Although dry air is desirable, in practice air is seldom totally dry. Normally, rela-

tively dry air is adequate. The dew point temperature is the measure. Interna-

tional quality standards differentiate 6 humidity quality classes for compressed

air (see the compressed air quality table on page 31). Quality class 3, for

example, is required for machine tools, packaging equipment, and textile

machines.

How much humidity remains as water vapour in air after compression?

If, for example, 7 m3 of atmospheric air is compressed to 1 m3 at 6 bar,

for a constant temperature of atmospheric and compressed air there are 6 parts

of water vapour too many this condenses out. One cubic meter of compressed

air cannot hold more water vapour than 1 cubic meter of air under atmospheric

conditions. The amount of humidity actually remaining in the air depends on

the air temperature and the pressure. The maximum quantity of humidity can

be read off from the diagram in Figure 2-9. If the air cools during compression,

its capacity to hold water decreases. Water condenses. The remaining humidity

reaches all working elements of consuming devices. For this reason, water traps

should be installed upstream of these.

X = 0.622 103 = 3.11 g/kg0.5 0.02982

3 (0.5 0.02982


26/125


Figure 2-9

Water content in compressed

air as a function of air

temperature and pressure.

These can, for example, be cyclone-type filters. Here, air is set in rotation by

baffles, leading to cooling of the air. The centrifugal effect and cooling leads

to condensation.

An example for lowering of air temperature: One cubic metre of air at 6 bar and 40 C holds 7 g of water.

If the temperature is lowered to 10 C, it can only hold 1.3 g of water.

As a result, 7 1.3 = 5.7 g of water is condensed out.

Pressure in bar

Watervaporinsaturatedhumidairing/m3

TemperatureinC

+ 110

+ 100

+ 90

+ 80

+ 70

+ 60

+ 50

+ 45

+ 40

+ 35

+ 30

+ 25

+ 20

+ 15 25

20

15

10

5

0

+ 5

+ 10

50

40

30

20

0 1 2 3 4 5 6 7 8 9 10 15 20

0.6

0.8

1

1.5

2

3

4

5

7

10

15


27/125


If the control elements and actuators supplied with compressed are at room

temperature normally 20 C the remaining water content of 1.3 g does not

condense, but returns to the atmosphere with the exhaust air. If, however,

fittings and equipment are at a temperature of only 5 C for example in the

open further condensation will take place.

How can the dew point be measured?

The dew point can be measured using the dew point mirror method. It is based

on the physical relationship between the condensation temperature of the

water vapour and the water vapour content of a gas mixture. A stainless steel

mirror is cooled using a Peltier element to the point at which water vapour

condenses. An optoelectronic closed-loop control circuit detects the formation

of condensate through the reduction of the intensity of the light reflected by the

mirror surface. The control electronics regulates the current flow dependent on

the condensate formation. The dew point has been reached when condensation

and evaporation are in balance. This temperature is then measured with a highly

sensitive resistor, such as a Pt 100 sensor (platinum sensor with a resistanceof 100 Ohm at 0 C). The basic structure of the dew point sensor is shown in

Figure 2-10.

Figure 2-10

Dew point sensor

1 LED controller

2 Optical balance control

for reference beam

3 Dew point mirror

4 Temperature sensor

5 Cooling element

(Peltier element)

6 Air or gas mixture

7 Power supply 1 2

3

4

5

6

7

Control ofthermoelectriccooling element

Dew point temperature


28/125


More recently, further sensors have been developed for measuring humidity.

They measure the cooled surface electrically rather than optically. Figure 2-11a

shows the diagram of a polymer sensor. The mode of operation: water vapour

penetrates a dielectric and thus changes its dielectric constant. At low humidity,

the water evaporates from the dielectric layer.

With the sensor shown in Figure 2-11b a capacitor is embedded in silizium.

A force field is created if an alternating voltage is applied and the resulting

lines of the force field emerge from the silizium. The water condensate therefore

influences the frequency of the stray field. This results in a control signal for

the Peltier-current and as such for the surface temperature. The water does not

penetrate the sensor material as in the case of the polymer sensor, but adheres

to the surface. This results in a drift and hysteresis-free characteristic curve.

Figure 2-11

Humidity sensors

a) Polymer sensor

b) Condensation sensor

1 Water vapour

2 Dielectric

3 Capacitor

4 Leakage field

5 Condensed water

on the chip surface

6 Peltier element

7 Silicon chip

with embedded capacitor

1

a) b)

2 3 4

5

6

7


29/125

3 Compressed air preparation28

Compressed air preparation entails the conditioning of compressed air supplied

by the compressor station to the quality required by the compressed air con-

suming devices. Preparation can be divided into three areas: coarse filtering

(straining), drying and fine filtering. Coarse filtering is carried out immediately

after compression. Figure 3-1 shows the basic structure of a pneumatic system.

One basic principle of compressed air preparation is as much as necessary, as

little as possible. The compressed air has to be as clean as necessary and no

cleaner! The following points also have to be taken into account:

If compressed air of different quality levels is required, all compressed air

would have to be centrally prepared to the most stringent requirement.Economically, it makes more sense to prepare the better air at the

appropriate consuming device (fine preparation).

If compressed air is required at different pressures, it makes economic sense

to consider a decentralised pressure amplifier (pressure booster) in order to

avoid running the whole system at the higher pressure.

The air drawn in by the compressor should be cool, dry and largely

dust-free. Use of warm humid air results in greater condensation following

compression.

A small reservoir should be installed in the network upstream of the service

unit if there are large pressure fluctuations within the system. Devices for removal and collection of condensate in lines should be provided

at the lowest point in the network.

Compressed air preparation should not only be considered from the

production point-of-view. There are also health aspects: lubricated air

is harmful both to the health of employees at the workplace and to the

environment.

3

Compressed

air preparation

Figure 3-1

Basic structureof a pneumatic system

K Condensate

LF Filter

LOE Oil atomizer

LDF Dryer

LR Pressure reducing valve

M Motor

Me Measuring device,

pressure gauge

PEV Pressure switch

QH Shut-off valve

V CompressorWA Water separator

V M PEV

Reservoir

WA WA WA

Me

QH QH QH

LOELR

LR

LF

LF

LDF QH

QH

Machine

K K K K K

for larger

systems

1 to 2 degreesinclination

l

Separationpoint for projectplanning


30/125

3 Compressed air preparation 29

In order to make the energy carrier compressed air out of atmospheric air,

it has to be compressed to a fraction of its original volume. What are the

characteristics of the raw material air?

Air has the following physical characteristics:

Physical variable Value Unit

Density at 0 C 1.293 kg/m3

at 15 C 1.223 kg/m3

at 20 C 1.199 kg/m3

Gas constant R 287 J/kg K

Specific thermal capacity

at 0 C; p = constant cp = 1.005 kJ/kg K

at 0 C; V = constant cV = 0.716 kJ/kg K

Adiabatic exponent 1.4

Dynamic viscosity (normal pressure)

at 20 C 18.13 106 Pa s

Kinematic viscosity (normal pressure)

at 20 C (= viscosity/density ratio) 15.55 mm2/s

According to the ISO 6358 standard, air has a density of 1.185 kg/m3 under

normal conditions.

Compression of air is not without its problems. All airborne contamination such

as dust, soot, dirt, unburnt hydrocarbons, germs, and water vapour are also

compressed. These are joined by other particles from the compressor itself,

such as abraded material, carbonised oil, and aerosols. For this reason, com-

pressing atmospheric air to 8 bar increases the concentration of contaminants

by a factor of 9. But that is not all. There are also residues and sealants from

the pipe network such as rust, sinter, welding residues and sealants left over

from the installation of valves and fittings. Figure 3-2 gives an overview of the

particles that can be contained in compressed air and their size. City air hassome 140 million dust particles per cubic metre, with 80% of the particles

having a size of less than 5 m. Incidentally, a particle size of up to 0.01 mis permissible for clean breathing air.

3.1

Compressed air quality


31/125


So in its raw state, compressed air is by no means clean. Contaminants

can cause problems in pneumatic consuming devices and lead to damage in the

compressed air network. Contaminants can also have a mutual influence on eachother. Dust particles can join with water or oil to form larger particles, and oil

and water can combine to form an emulsion.

There are different recommended quality classes for different types of device.

The following table shows the required compressed air quality for each type of

contaminant. The quality classes are recommended in DIN ISO 8573-1.

Figure 3-2

Types and sizes

of typical air contaminants

(1 m = 0.001 mm)

Tobacco smoke

Oil vapour Oil mist

Atmospheric dust

Spray paint mist

Metallurgical dust

Cement dust

Road dustSoot

Coal dustSulphursmoke

Heavy

industry smoke

Foundry

sand

submicroscopic microscopic visible

Vapour, mist, smoke Dust Fog, mist Spray Rain

Particle size in m

0.01 0.1 1.0 5 10 40 100 1000

M

icrofilter

Finefilter

Strainer

(coarse

filter)

Fog


32/125


Applications Suspended Water dew Maximum Recom-

solids point oil content mended

(m) (0 C) (mg/m3) filter grade

Mining 40 25 40 m

Cleaning 40 +10 5 40 m

Welding machines 40 +10 25 40 m

Machine tools 40 +3 25 40 mCompressed air cylinders 40 +3 25 40 m

Compressed air valves 40 or 50 +3 25 40 or 50 m

Packaging areas 40 +3 1 5 m 1 m

Precision pressure regulators 5 +3 1 5m 1 m

Measuring air 1 +3 1 5m 1 m

Warehouse air 1 20 1 5m 1 m

Spray painting air 1 +3 0.1 5m 1 m

Sensors 1 20 or40 0.1 5m 1 m

Pure breathing air 0.01 0.01 m

This regulation also divides compressed air quality into 7 quality classes.

The following table shows the cubic metre specifications based on normal

conditions in accordance with ISO 554.

Class Particle size Particle density Pressure dew Residual oil content

max. in m max. in mg/m3 point max. in C max. in mg/m3

1 0.1 0.1 70 0.01

2 1 1 40 0.1

3 5 5 20 1.0

4 15 8 +3 5

5 40 10 +7 25

6 +10

7 not defined

Air heats-up during compression and then cools immediately after the compres-

sor. The heating results from the fact that the compressor drive energy for

increasing the pressure from p1 to p2 is associated with a temperature increase

from

T1 to T2. This can be calculated as follows:

where k can be in the range 1.38 to 1.4.

3.2

Drying methods

T2 = T1

p2p1

(k 1)

k


33/125


Air always contains some quantity of water vapour. It can only hold a limited

amount of water vapor, however, namely up to the saturation level. It is desirable

to condense out as much water as possible before the water reaches the con-

suming devices. If the air is lubricated, a compressed air/oil mixture results.

This oil must be separated out of the compressed air in an oil separator and

then recooled.

In order to ensure that the pneumatic control elements and actuators do not

become water hydraulic components, the air is dried. This drying is the most

important part of compressed air preparation. Good air preparation prevents

corrosion in the lines and pneumatic devices. The dew point temperature

(see Section 2.3) is the measure for air drying. The higher the temperature

of the compressed air, the greater the quantity of water that the air can hold

(saturation quantity). This is shown in the following table:

Temperature

in C 20 10 0 5 10 15 20 30 50 70 90 100

Water vapour

max. in g/m3 0.9 2.2 4.9 6.8 9.4 12.7 17.1 30.1 82.3 196.2 472 588

How can air be dried?

There are various methods for drying air. Figure 3-3 shows a schematic

overview.

Figure 3-3

Methods of drying air

Drying methods

Condensation Absorbtion Diffusion

Refrigerationdryer

Overcom-pression

Adsorption dryer(solid drying agent)

Absorptiondryer

Membranedryer

UnheatedHeateddrying agent

Heating ofregeneration air

Fluiddrying agent

Dissolvingdrying agent


34/125


In many cases, refrigeration dryingis sufficient. The compressed air is cooled by

a cooling agent. Water vapour then condenses out. As Figure 3-4 shows, the air

is cooled in reverse flow by a circulating cooling agent, normally in a multistage

process (precooling stage: air-air; main cooling stage: air-cooling agent). The

pressure dew point is around +1.5 C. If the operating temperature does not fall

below 3 C, there is no water in the compressed air network. Refrigeration drying

accounts for some 3% of the energy cost of the compressed air production. To

increase savings, there are now also dryers with a speed-controlled cooling

agent compressor. This adapts automatically to the quantity of air currently

requiring cooling.

Another drying method is overcompression (high-pressure compression).

In this method, the air is compressed to a much greater pressure than required

by the consuming device. The air is cooled, causing condensation. The air is thenallowed to expand again to the required pressure. This results in pressure dew

points way below 60 C. This process is, however, very expensive.

If ambient temperatures or applications make extremely low pressure dew

points of 0 to 70 C necessary, adsorption dryers and membrane dryers are

used. In this case, the proportion of the compressed air production cost

attributable to drying increases to some 20%:

Figure 3-4

The principle

of the refrigeration dryer

1 Compressed air entry

at 25 C

2 Coolant return line

3 Heat exchanger

4 Coolant entry

5 Compressed air outlet

at 15 C

6 Condensate separator

7 Water drain

8 Predryer

15

2

63

4

7

8


35/125


In the absorption dryer, water vapour is chemically absorbed by an agent.

This dissolves during drying. The chemical agent is a salt based on NaCl. The

structure of the dryer is simple and is shown in Figure 3-5. The chemical agent

is, however, consumed in the process. 1 kg of salt absorbs approx. 13 kg of

water condensate. This means that salt has to be regularly replenished. The

lowest pressure dew point achievable is 15 C. Other drying agents include

glycerine, sulphuric acid, dehydrated chalk and superacidic magnesium salt.

The operating costs are high, which means that application is limited in practice.

In the adsorption dryer gas or vapour molecules are attached by molecular

forces. The drying agent is a gel, such as silica gel. This is also used up during

the process, but can be regenerated. For this reason, two drying containers

(chambers) are required, allowing drying (A) and regeneration (B) to take place

simultaneously. Regeneration can be cold or hot. Cold regenerating dryers are

cheaper to purchase, but more expensive to operate. Figure 3-6 shows a dryer

with hot-air regeneration. The dryer is used in reciprocal flow mode. Dependingon the drying agent used, pressure dew points as low as 70 C can be achieved.

There are also adsorption dryers that use molecular sieves (crystalline metal

aluminosilicates or zeolites in spherical or granulated form as a drying agent.

Like all adsorptive agents, these have a large inner surface capillary action.

Here, too, the molecular sieves laden with water molecules can be regenerated

(desorption).

Figure 3-5

Principle

of the absorption dryer

1 Dried compressed air

2 Container

3 Salt

4 Condensate drain

5 Air from compressor

(humid)

6 Condensate trap

1

2

3

4

5

6


36/125


Membrane dryers consist of a bundle of hollow fibers that are permeable

to vapour. Dried air flows around these fibers. Drying is driven by the partial

pressure differential between the humid air inside the hollow fibres and the

inverse flow of dry air (Figure 3-7). The system attempts to achieve equilibrium

between the water vapour concentration on either side of the membrane.

Figure 3-6

Principle of the

adsorption dryer

1 Dried air

2 Drying tower

3 Heater

4 Fan

5 Hot air

6 Humid air 7 Valve

Figure 3-7

Principle of a membrane dryer

1 Hollow fibre

2 Scavenging air

3 Humid air intake

4 Membrane

1

A B

2

3

4

5

7

6

1

2

3

4 1


37/125


The hollow fibres consist of a silicone-free base material with a very thin coating

of the actual membrane surface. There are porous and homogenous membranes.

Homogenous membranes are only permeable to certain molecules, e.g. water

vapour. The oxygen and oil content of the air is not changed. The required dry

scavenging air is derived air that has already been treated. This constant con-

sumption of scavenging air lowers the efficiency of the dryer. For this reason,

there are many efforts to minimize this air consumption. The principle of opera-

tion means that this type of dryer is preferably used as a partial flow or point-

of-consumption dryer (Figure 3-8). No external electrical or auxiliary energy

source is required for control of scavenging air, allowing the dryer to be used

in explosion-hazard areas. The membrane dryer should be upstream of any

pressure regulator, as better drying efficiency is achieved at higher pressures.

It is also recommended that a combination of prefilter and microfilter be fitted

upstream of the membrane dryer, as this increases the service life of the hollow

fibers. One of the main differences to other dryers is the following:

Membrane dryers reduce the humidity by a certain proportion, while refrigera-

tion and adsorption dryers lower the pressure dew point.

Figure 3-8

Applications of dryer types

(based on Hoerbiger-Origa)

1 Adsorption dryer

2 Membrane dryer

3 Refrigeration dryer

up to 1000 m3/h

1

2

3

30

20

10

0

10

20

30

40

50

60

70

80

0 50 100 150 200

Volumetric flow in m3/h

Pressuredew

pointinC


38/125


The first air filters were built over a hundred years ago and their design has

undergone substantial development since. Originally, the filter medium was

woven. The selection of the correct filter has a decisive impact on compressed

air quality. High-quality compressed air requires several filter stages. A fine filter

alone is not a solution.

Filters can be divided into the following stages:

Filters: These capture particles greater than 40 m or 5 m dependingon the grade of the filter cartridge selected.

Fine filters: These capture particles larger than 0.1 m. Microfilters: These capture particles larger than 0.01 m. The air must,

however, have previously been filtered with a 5 m filter. Active carbon microfilters: These capture particles larger than 0.003 m,

such as flavouring materials and ordourous substances. Such filters are also

called submicrofilters.

In order to achieve better quality levels, suspended matter has to be filtered out

in stages, for example by connecting fine filters and microfilters in series.

What filter principles are used?

Inertial force filter

The air is set in rotation by a swirl vane, causing centrifugal forces to come into

play. Because of the similarity to the tropical cyclone, this also called the cyclone

filter (Figure 3-9).

3.3

Filtering

Figure 3-9

Principle for the cyclone filter

1 Air inlet

2 Air outlet

3 O-ring

4 Container

5 Cyclone insert

6 Separating cap

7 Securing screw

8 Filter element

9 Button for manual

condensate drain

10 Condensate

11 Condensate drain

1 2

3

45

67

8

910

11


39/125


Larger solid and, above all, liquid particles are thrown against the inside wall

of the filter bowl by centrifugal force. Up to 90% of condensate is separated.

The pre-cleaned air then passes through a filter insert with highly porous sinter

material. Condensate and contaminants are collected in the filter bowl. The drain

button has to be operated from time to time to drain the accumulated

condensate. The filter insert has to be removed and cleaned at longer intervals.

Surface filters

These filters consists of a metal or plastic braiding with a pore size of 5 or 40 m.

The braiding captures all contaminants larger than the defined pore size. The

surface filter is usually used as a prefilter to a centrifugal (cyclone) filter as

described in Figure 3-9 above.

Deep-bed filter

These are filters equipped with fine filters (1 m) or microfilters (0.01 m). The

filter material is a microfilter of non-woven fabric. This is a jumble of superfine

borosilicate fibres. The filter effect results from direct impact of the particles,

by absorption, sieving, diffusion, electrostatic charging and capture by meansof van der Waals force. Dust separation is shown in Figure 3-10. The particles

become entangled in the fibres. Liquid particles coalesce (join together) to form

larger drops, and can then be collected in the filter bowl.

Figure 3-10

Dust separation

using a non-woven textile

1 Filter medium

2 Embedded dust layer

3 Surface dust layer,to be cleaned off

4 Air intake

5 Air outlet


40/125


Deep-bed filters remove the smallest oil and dust particles from compressed

air. If active carbon filters are used, even undesirable oil vapours and odours are

filtered out. This is a requirement in highly sensitive areas such as the food and

packaging industry and pharmaceutical industry. The degree of filtering depends

on requirements. Permissible particle sizes in compressed air are, for example:

40 m to 5 m for vane motors, working cylinders, open-loop controllers

and percussion tools.

Smaller than 5 m for closed-loop controls, valves, measuring instruments

and spray guns.

Smaller than 1 m for applications in food and packaging, pharmaceuticals

and electrical and electronic engineering.

Active carbon filters

These contain a filter insert of largely amorphous carbon. It is porous. Active

carbon has an unusually high internal surface of between 500 and 1500 m2/g.

This results in great adsorption capacity for extremely small particles. The

adsorption takes place as the particularly active areas of the surface,

such as points, corners, edges and lattice imperfections.

The service life of active carbon filters is always extended by an upstream

prefilter and microfilter. Active carbon filter elements normally have to be

changed after around 1000 hours of operation or when oil can be smelt. The

residual oil content of air filtered in this way (with appropriate prefiltering) is

only 0.003 ppm (parts per million). This is not an SI unit, but still valid. More

easy to grasp is the expression as 0.003 mg/ m3). Such submicrofilters are

primarily recommended for use in compressed air applications in the food,

pharmaceutical and medical technology industries.

Note: Filters are always installed upstream of pressure reducing valves as the

pressure loss within these filters depends on the volumetric flow.

Figure 3-11 shows the symbols used in circuit diagrams.

Figure 3-11

Symbols for filters

and lubricators

1 Filter (removal of particles)

2 Water separator,manually operated

3 Water separator

(automatic drain)

4 Filter with water separator

(manually operated)

5 Filter with water separator

(automatic drain)

6 Air dryer

7 Lubricator

8 Filter combination

1 2 3 4

5 6 7 8


41/125


Some applications such as in the pharmaceutical and food industries require

compressed air that is free of oil. The residual oil compressor oil remaining in

the air has to be removed. Even air from non-lubricated compressors deliver

air that is contaminated with oil aerosols from the intake air. This oil can clog

sensitive components and wash out or damage basic lubrication of components.

The Pneurop classification (Pneurop Guideline 6611) provides for classification

according to the following standard values:

Class Oil content in mg/m3

1 0.01

2 0.1

3 1.0

4 5.0

5 25.0

In words, the oil content of compressed air can be expressed as follows:

Low oil-volume airThis is the normal case when air is passed through a 1 m to 20 m filter.This achieves the quality measuring air or normal breathing air, inasmuch

as environmental considerations are taken into account.

Technically oil-free air

The residual oil content is in the range 0.3 to 0.01 mg/ m3 and does not cause

problems in any technical application. This requires fine filters.

Absolutely oil-free air

During compressed air preparation, oil-free intake air has no contact with oil.

The oil content is less than 0.003 mg/ m3. This level can only be achieved

through active carbon filtering.

Three methods can be used to achieve low oil content:

Compressors for the production of oil-free air

Refrigeration dryer with simultaneous oil separation to approx. 80%

Oil separation filters

A combination of several methods is also possible, as is the series connection

of filters, for example two microfilters, whereby the second filter contains active

carbon and uses adsorption filtering. This results in the retention of oil odours

and other contaminants. By the way, most pneumatic actuators and controlelements work fine with non-lubricated air, as they have already been provided

with permanent lubrication in the factory. When using lubricated air, it must be

taken into account that once oil is used, it should be used continuously,


42/125


reverting back to unlubricated is impermissible. Whether it is better to produce

oil-free compressed air with non-lubricated compressors or to filter the oil out

after compression is still a matter of debate. Lubricated compressors are, howe-

ver, less expensive.

When compressed air is filtered, water is extracted. This is collected as conden-

sate and has to be drained from time to time. If large amounts of condensate

are frequently collected, automatic condensate draining should be provided.

This simplifies monitoring and checking of the filter. There are various solutions

for automatic draining:

Ball-float condensate drain

Drainage is controlled by the level of the condensate. A ball-float opens

a cock (see Figure 3-12). The condensate is forced into the drain pipe

by air pressure.

Electronically controlled condensate drain

A capacitive level control generates a signal when the condensate reaches

maximum level. A diaphragm valve is opened electrically. The condensate

is then forced into the drain pipe by air pressure. Time-controlled condensate drain with solenoid valve

Experience shows how often condensate has to be drained off.

A controller can be set to activate a solenoid valve at specific intervals,

then close it again.

Figure 3-12

Ball-float condensate drain

1 Housing

2 Float

3 Manually operated valve4 Cock (conical seat valve)

5 Condensate

6 Drain pipe

1

23

4 5

6


43/125


As the condensate consists not only of water, but also contains dirt and

carbonated oil, gumming of the drain cock (valve) may result. The drain does

not then open and close correctly, and compressed air is wasted.

Solenoid valves do not always work reliably. And during the time the cock is

open, compressed air is exhausted. Such losses are prevented by an electro-

nically controlled condensate drain, as the diaphragm valve is only open as

long as condensate is present.

Figure 3-14 shows a number of filter variants with comments in the next table

on page 43.

Figure 3-13:

Electronically controlled

condensate drain

1 Housing

2 Level sensor

3 Diaphragm valve

4 Riser pipe

5 Condensate

6 Electronics

Figure 3-14

Selection of filter types

1 Recooler

2 Reservoir

3 Main line filter

with automatic drain4 Standard filter

5 Microfilter

6 Refrigeration dryer

7 Submicrofilter

8 Active carbon filter

9 Adsorption dryer

1

2

4

5

3

6

1 2 3

4

5

55

6

77

7

98

5

A

B

C

D

E

FG


44/125


Filter selection in Figure 3-14 dependent on application

(based on Hofmann/Stein):

Filter type Application Main function

A Minor solid Operation of Removal of contami-

contaminants, humidity machine controls, nants, dust over 5 m;and oil are ok clamping mechanisms; liquid oil over 99%;

pneumatic hammers; supersaturatedblast air, workshop air humidity under 99%

B Primary concern is Industrial equipment; Removal of contami-

removal of dust and oil, pneumatic drives; nants, dust over 0.3 m;small amount of humidi- metal seals; oil mist over 99.9%;

ty is ok (resulting from machine tools; motors supersaturated

temperature gradient) humidity over 99%

C Primary concern is Similar for A, made more Removal of humidity,

removal of humidity, difficult by large tempera- dust over 5 m;small amounts of ture gradient in line or in liquid oil over 99%;

dust and oil are ok consuming device; spray- atmospheric dew point

and painting applications below 17 CD Removal of humidity, Process engineering, Removal of contami-

dust and oil required measuring devices; nants and humidity;

high-quality paint systems; dust over 0.3 m;cooling of molds and plastic oil mist over 99.9%;

injection molding machines atmospheric dew point

below 17 C

E Pure air is required Pneumatic measuring Removal of contami-

with almost total devices; fluidics; nants and humidity,

removal of humidity, electrostatic painting, dust over 0.01 m;dust and oil cleaning and drying of oil mist over 99.9999%,

electronic components atmospheric dew pointbelow 17 C

F Extremely pure air is Pharmaceutical industry, Removal of all

required with almost food industry, (packaging, contaminants, odours,

total removal of drying, conveying, brewing); dust over 0.01 m;humidity, dust, medical air treatments; oil mist over 99.9999%;

oil and odour sealing work atmospheric dew point

below 17 C, odour

removal over 99.5%

G Primary concern Drying (electronics, cargo Removal of all contami-

is low dew point tanks); pharmaceutical nants, humidity

and practically storage; marine measuring and vapours;no dust or oil devices; conveying dust over 0.01 m;powder materials oil mist over 99.9999%;

atmospheric dew point

below 30 C

Filter selection is carried out in the following steps:

What degree of purity is required?

What port size is required (dependent on pressure and flow rate)?

Type of evacuation (manual or automatic)


45/125


The table on page 31 can be used for establishing the degree of purity. The port

size is selected so that the pressure loss is not greater than 3% of the absolute

input pressure. At 6 bar working pressure, this is equivalent to p = 0.2 barpressure loss (Fig. 3-15). Naturally, even the best filter causes pressure loss.

Practical experience shows that filters should be selected such that the actual

flow rate at the appropriate operating pressure is below the straight line shown

in Figure 3-15.

Example: At a pressure of 6.3 bar, a pressure loss ofp = 0.2 bar resultsin a flow rate of 450 l/min.

It is important to observe the limits for maximum and minimum flow rate.

If the filters are operated at less than the minimum flow rate, the Waals forces

are often not sufficient to capture the particles. They are then not retained.

If the filter is operated at a flow rate greater than the maximum which occurs

frequently in practice the differential pressure increases rapidly. This impairs

efficiency and thus impacts economy. An even more dramatic effect is that

captured particles can also be loosened and thrust through the filter. The opera-tor is then surprised to find substantial quantities of particles in the application,

despite filtration.

Figure 3-15

Pressure loss in the filter

as a function of flow rate

a) Recommended maximum

flow rate

b) Operating pressure in bar

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Pressurelossinbar

0 200 400 600 800 1000 1200

Flow rate in l/min

3.2 6.3 10

b

a


46/125


One more point about compressed air condensate: It is a mixture of solid

particles, water and oil. The overall characteristics are aggressive. For this

reason, disposal of condensate is a serious matter. Thermochemical condensate

processors can turn condensate into water of drinking quality and filtered air

into air of breathing quality. Such eco-friendly filtration techniques avoid the pro-

blem of disposal.

Lubricated oil is required when the air is not only used as an energy carrier, but

also transports lubrication to moving parts of a system. The lubricator under-

takes automatic injection of oil mist. Oil mist prevents dry friction of moving

parts of pneumatic control elements and consumers or at least reduces wear.

However, it is not possible to simply leave the oil mist from the compressor

in the compressed air and regard it as a lubricant. The molecular structure

of this oil has been largely destroyed by pressure and heat, resulting in a

highly aggressive acidic medium. So compressor oil is entirely unsuitable for

lubrication.

The lubricator head of a standard lubricatorcontains a venturi nozzle through

which compressed air flows. The constriction in the tube results in a vacuum

at the suction opening. Oil is sucked out of the container via a riser pipe

(Figure 3.16). The oil drips into the flowing air and is atomized. The number of

oil drops entering the air flow can be set using a restrictor as a metering device.

3.4

Compressed

air lubricators

Figure 3-16

Compressed air lubricator

1 Lubricator head

2 Suction opening

3 Riser pipe

4 Container

5 Oil dropler chamber

6 Drain screw

2

3

4

5

1

6


47/125


The venturi principle is shown in Figure 3-17. The constriction of the tube causes

a pressure differential p, which draws out the oil.

In the microlubricator, oil droplets are finely atomized (less than 2 m)by a baffle plate. Only some 5 to 10% of oil droplets enter the air flow. Light

machine and hydraulic oils have proven suitable. The viscosity should be in

the range 17 to 25 mm2/s at 20 C. The flow characteristic is decisive for the

selection of a compressed air lubricator. The pressure loss should not exceedp 0.15 to 0.35 bar. Oil consumption depends on requirements and cannot beprecisely specified.

Figure 3-17

Venturi principle

Figure 3-18

Lubrication of the air flow

1 Standard lubricator

2 Proportional lubricator

p

1

2

Air flow

Oil/airflow


48/125


A rule of thumb is a rate of 2 to 5 drops per m3. The lower figure applies to

continual flow, the higher figure for intermittent flow. Microlubricators require

approx. 10 to 20 times as many drops. Some 4 to 6 drops per 1000 litres or

air is sufficient for the operation of compressed air motors, with one drop

corresponding to approx. 15 m m3. The number of drops is set by means of an

adjusting screw. During continuous and intermittent operation (running time

longer than 1 minute), the oil is added to the compressed air by means of a

lubricator. For intermittent operation with running time less than 1 minute,

injection lubrication near the consuming device is recommended, in order to

prevent inadequate lubrication resulting from loss of oil in the network. Cylinders

with heat-resistant seals should not be operated with lubricated air, as the speci-

al grease can be washed out by the oil. Mist lubricators (figure 3-19) must be

installed so that the air intake line points in the direction of flow.

Several lubricator installations are shown in Figure 3-20. The following table on

page 48 gives selection guidelines for the types A to E.

Figure 3-19

Mist lubricator

1 Adjusting screw2 Direction of flow

3 Drop dome

4 Housing

5 Bowl guard

Figure 3-20

Various lubricator

applications

1 Recooler

2 Reservoir

3 Differential

pressure lubricator

4 Standard lubricator

5 Multigrade lubricator

6 Pulse injection lubricator

5

4

3

2

1

1 2

3 4

5

6

7Sequencer

A

B

C

D

E


49/125


Filter type Applications Main function

(Examples)

A When homogenous Compressed air power Supply of many consuming

oil mist is required. tools on assembly devices over large distances,

For supply lines lines; pneumatic oil mist over 2 m; goodover 150 m long; controls; transfer, transportation characteristics

lubrication of a large welding and stamping over 150 m; installation

number of consuming lines and production above devices recommended;devices without units problem-free branching;

overlubrication continuous transport

of 7 to 12 mg/m3 oil

B For all standard applica- Tools; Supply-lubrication of individual

tions without special pneumatic drives; devices, oil mist 4 to 10 m;requirements. Basically controls satisfactory transportation up

supply-lubrication of to 6 m; installation above con-

individual devices, suming device required; oil

small distances transport 15 to 25 mm3/drop

C For applications with Tools with low air Low response threshold, large

a broad range of vol- requirement, control range of volumetric flow rates:umetric flow rates, high of compressed air oil mist over 10 m; transport-responsiveness, extrac- cylinders; controls characteristics satisfactory up

tion of non-lubricated air for extraction of non- to 6 m; installation above con-

upstream of lubricator lubricated air. suming device required; oil

transport 15 to 25 mm3/drop

D For single operation Short-stroke cylinders; Low lubrication at consumption

of a consuming device small compressed site: oil drops 1 to 30 mm3;

after long intervals, air tools; transport characteristics not

long distances between cutting tools applicable; installation at

lubricators and devices, point of consumption;

low flow rates oil transport 1 to 30 mm3

per piston stroke

E Wherever fine, uniform High-speed bearings; Transport of fine oil mist for

oil mist is required in grinding spindles; lubrication and cooling, oil

very small but well knitting machines; mist less than 2 m, transportmetered quantities gearboxes characteristics good over 30 m;

Installation above consuming

device recommended

Application variants A-E


50/125

3.5

Pressure regulators

Figure 3-21

Principle of

pressure regulators

a) Regulator

with exhaust hole

b) Regulator

without exhaust hole

1 Housing 2 Valve seat

3 Valve disk

4 Diaphragm with valve hole

5 Diaphragm, permanently

attached to valve piston

6 Exhaust hole

7 Pressure spring

8 Adjusting screw

for adjusting spring force

9 Pressure gauge

Pressure regulators have the role of providing a reliable constant pressure

(secondary pressure) despite all pressure fluctuations in the main compressed

air circuit (primary pressure). If such constant pressure is not ensured, unaccep-

table deviations in switching and motion times of control elements and actuators

result. Excessively high pressure increases wear and leads to unfavorable energy

efficiency. Excessively low pressure reduces efficiency and also may impair

the serviceability of consuming devices. Generally, the compressed air network

exhibits a pressure of 6 bar in the operating part and 4 bar in the control part.

Figure 3-21 shows two principles for the function of pressure regulators.

Mode of operation: If the primary pressure p1 is present, the valve disk (3)

is raised from the valve seat (2) against the spring force (7). An output pressure

p2 results. This pressure acts via an opening on the diaphragm (4) or (5).

In the case of a regulator with an exhaust hole (Figure 3-21a), the valve hole

in the diaphragm is released from a certain pressure, so that compressed air

can escape via the diaphragm (4) and the exhaust hole (6) to the environment

(= intrinsic consumption). The constant change in the cross-sectional area at the

valve seat (annular gap) and release of the valve hole in the diaphragm adjusts

the pressure on the secondary side to the current situation, for example whenthere is a change in the load of a working cylinder. The secondary pressure is

held almost constant.

In the case of a regulator without an exhaust hole (Figure 3-21b), the valve disk

and the diaphragm (5) act together as a twin-piston system. If the secondary

pressure p2 is too high, the pressure on the valve seat increases and presses

the diaphragm against the compression spring. This reduces the cross-sectional


a) b)

p1 p2

12 3

4 5

6

7

8

9

p1 p2


51/125


area for flow, possibly to zero. The air flow is then reduced or blocked.

Only when the operating pressure p2 again falls below the primary pressure

can compressed air flow again. Figure 3-22 shows a commonly used pressure

regulator with an exhaust hole.

Service units are compact combinations of devices located at the point of

consumption. They allow fine preparation of compressed air and normally

consist of an on-off valve, filter, pressure regular and lubricator. The components

also have to be installed in this order. The direction of flow as marked on the

outside of every device must be taken into account. Safety and monitoring

elements may also be integrated. In the case of larger machines, service units

are also integrated into the machine frame for basic supply of compressed air.

The space below the service unit must be large enough to allow the insertion of

a condensate collection vessel. A pressure regulator should keep the secondary

pressure as constant as possible even with fluctuating air consumption, ensu-ring that the desired operating pressure is maintained. The operating pressure

is set on the pressure regulator. Figure 3.23 shows the structure of a modular

service unit.

Figure 3-22

Structure of a

pressure regulator

1 Unregulated

compressed air

2 Regulated compressed air

3 Pin

4 Annular gap

5 Valve disk

6 Compression spring

7 Exhaust hole

8 Diaphragm

9 Relief hole

10 Spring disk

11 Adjusting screw

3.6

Service unit

1 2

345 6

6

7

89

10

11


52/125


Service units not only ensure optimum preparation of air, but also smooth

pressure fluctuations that can occur as a result of the compressor switching

on and off. Secondary and primary sides (network) are thus decoupled. Branch

modules allow air of varying qualities to be tapped, for example tapping non-

lubricated air upstream of the lubricator. Service units can also be configuredfor several independent pressure zones. The same can be achieved for different

levels of air quality by using modular filter combinations. Figure 3-24 shows the

diagram of a pressure regulator battery with several pressure zones and

through-connected primary pressure.

Figure 3-23

Main components

of a modular service unit

(example)

1 Pipe connector

2 Manual on/off valve

3 Filter and pressure

regulator

4 Filter5 Condensate drain

6 Branching module

7 Lubricator

8 Pressure regulator

9 Soft-start valve

10 Branching module

11 Pressure gauge

12 Pressure switch

Figure 3-24

Pressure regulator battery

with a service unit

1 Main on/off valve

2 Filter andpressure regulator

3 Branching module

4 Filter and pressure

regulator for battery

installation

1 2 3

4

5

6 7 8

9

10

11 12

1

2

3

4

5 bar 7 bar 4 bar

p1 p1


53/125


A soft-start valve can be installed upstream. This is a safety start-up valve and

ensures a gradual build-up of pressure in pneumatic systems when energy is

turned on. Downstream cylinders and operating elements then move slowly

rather than suddenly into their initial positions. Once 50% of the input

pressure has been reached, the danger of collision is past and the valve

opens fully (Figure 3-25).

In practice, several combinations typically occur. They are shown in Figure 3-26.

They can be combined as appropriate. There are already preassembled com-

binations for the most common applications. They are differentiated (from top

to bottom) by the following characteristics: Lubricated and non-lubricated oil is required. Non-lubricated oil is branched

off upstream of the lubricator. To ensure that compressed air from the lubri-

cated line cannot flow back, the manifold is equipped with a non-return valve.

The oil mist can be metered. The beginning of the service line must ensure

higher flow rates than sublines this has to be taken into account during

design.

Compressed air of different levels of quality is required. For example, multiple

filter stages allow air of different qualities to be tapped. The final stage

guarantees microfiltered compressed air (guaranteed oil- and dust-free).

Such microfiltering is required, for example, for low-pressure controllers.For reasons of economy, filtration is only undertaken to the degree necessary,

as each filter causes a pressure loss.

The service combination can also start with a start-up valve. This shut-off

allows pressurization and depressurization of pneumatic systems. The lever

can, by the way, be locked with a standard padlock.

Figure 3-25

Pressure curve

for a soft-start valve

p2

p1

p1

Time t

50%

Pressure


54/125


Pressure regulators have a pressure gauge to indicate pressure. Mechanical

pressure gauges have the advantage that they do not require an auxiliary energy

source. They use elastic deformation under pressure for measurement (Bourdon

tube pressure gauge, diaphragm pressure gauge, or capsule element pressure

gauge). Figure 3-27 shows two typical designs.

Figure 3-26

Practice-proven combinations

of service units

FRM Branching module

HE Manual on/off valve

LFR Filter and regulator

valve integrated into

one unit

LFMA MicrofilterLFMB Fine filter

LOE Compressed

air lubricator

P Compressed air source

Figure 3-27

Analog pressure gauges

a) Diaphragm pressure gauge

b) Bourdon tube pressure

gauge

1 Pressure chamber

2 Diaphragm

3 Toothed quadrant

4 Bourdon tube

5 Housing

6 Reversing lever

7 Connector M20 x 1.5

8 Scale

9 Connecting rod

2

1FRM-... LOE

P

HE

Pressuresource Basic filtering Improving quality Commentary

LFR

40 or 5 m

automatic drainingof water separator

PLOE

1

2

FRM-Hlubricated air (1)for fast power com-ponents and com-pressed air tools,non-lubricated air (2)for normal

applications

microfiltered air (3)(oil- and dust-free)through multistagefiltration

Switch-off andexhaust by meansof upstream mainon/off valve,

also lockable

3FRM-... LFMB LFMAFRM

1 m 0.01 m

a) b)1

2

3

4

5

6

7

7

8

1

2

3

0

9


55/125


In the case of the Bourdon tube pressure gauge, the elastic measuring element

is in the form of a tubular spring closed at the top. When it bends up under

pressure, this motion is translated into motion of the pointer by a the toothed

quadrant.

In the diaphragm pressure gauge, a pressure-proof diaphragm with a pressure

connection on one side is used as a measuring element. As a result of their

shape and mounti

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