Compressed air

The KAESER Compressed Air Seminar Part 1 1

1. Fundamental Principles

1.1 What is Compressed Air?

1.2 Units and Symbols used in Compressed Air Technology

1.3 Physical Fundamentals

1.4 Gas Laws

1.5 Compressed Air in Motion

1.6 Graphical Symbols for Pneumatic Systems

1.7 Logical Symbols 1.8 P&I Diagram-Symbols


1.1 What is Compressed Air? Compressed air is pressurized (equal to 273.15 K) approximately 3 x 1023 atmospheric air. It is an energy carrier in molecular collisions per square the form of heat. It enables thermal centimetre and second can be expected. energy to be conducted across known distances and allows the expansion of The higher the temperature of the compressed air to be utilized as work. contained gas the higher the velocity and From a physical point of view, thus the energy of the molecules; the atmospheric air is a mixture of gases. The forces caused by collision increase and main component of this mixture is the pressure against the walls of the nitrogen of approximately 78% by volume container rises. and oxygen of approximately 21% by volume. Additionally, there is argon of approximately 1% by volume and traces of carbon dioxide and other gases.

1.2 Units and Symbols used in Compressed Air Technology

The „Units of Measurement Act“, passed on required, a numerical product. 2 July 1969, determined which mathematical quantities and units in „Official and Trade Before we approach the generation of compressed Usage“ are to be applied from 1 January 1978 air and its applications in further sections, it is onwards. In general it deals with the units used necessary to define some of these physical- in the international SI system (Système technical quantities and terms. Internationale d`Unités, French for International System of Units). This System comprises This is to ensure that a single, unambiguous seven base units and units derived from these language is used in compressed air techniques. base units through the use of exponents and, if

As in all matter, atmospheric air is formed from molecules that are bound to each other by molecular force. The molecules are in constant motion. In gaseous matter they are separated from each other by comparatively large distances and the molecular force is relatively low. For this reason, a gas spreads throughout the whole of the space available to it and mixes with other gases that may be present in that space. The total molecular mass contained in a certain quantity of gas is very low in relation to the volume of the gas. A quantity of gas can be compressed down to a very small proportion of its original volume. If a gas is contained within a space, the continuous motion and collision of the molecules against the walls of the container creates a force – the walls must withstand pressure. Pressure is defined as force per unit of area. It is measured in bar. At a pressure of 1 bar (atmospheric condition) and a temperature of 0 °C

Thus: If temperature rises with constant volume, pressure rises. If volume reduces with constant temperature, pressure rises. This physical law is utilized in compressed air technology: A compressor employs mechanical work to reduce the space within which air is contained in order to increase the pressure of atmospheric air. In a pneumatic machine the compressed air expands and provides work that can be utilized in many different applications.


If the volume of the container is reduced for the same quantity of gas, the molecular mass per unit of volume increases and thus the effect of force per unit of area increases; the pressure against the walls of the container rises. So pressure (p), temperature (T) and volume (V) are proportionally interrelated:

p ~ T/V or p x V ~ T


The base units are: A = ampere

m = metre K = kelvin kg = kilogram mol = molar mass s = second cd = candela The units and decimal multiples derived therefrom and used, in the selection affecting us, are as follows: N = newton W = watt Pa = pascal V = volt bar = bar Hz = hertz � = ohm °C = °Celsius J = joule t = ton For the purpose of an ABC of compressed air technology the use of the unit of pressure „bar“, which is practically the same as the old „at“ (one at = 0.981 bar) is preferred. A list of recommended units is available from the VDMA.

1.3 Physical Fundamentals

Composition of the air (dry):

Components Per cent volume Nitrogen 78.08

Oxygen 20.95

Argon 0.93

Carbon dioxide *) 0.03

Neon 0.018

Helium 0.00052

Methane 0.00015

Krypton 0.00011

Carbon monoxide *) 0.0001

Nitrogen monoxide *) 0.00005

Hydrogen *) 0.00005

Ozone *) 0.00004

Xenon 0.000008

Nitrogen dioxide 0.0000001

Iodine 2 x 10-11

Radon 6 x 10-18

0

10

20

30

40

50

60

70

80

90

100

Nitrogen

Oxygen

othergases

The components marked *) can vary



Atmospheric pressure... ... is caused by the weight of the atmosphere. It is related to the density of the air and to the height: the normal pressure at sea level is 1.013 bar, equal to 760 mm Hg (Torr).

Pressure Absolute pressure... Gauge pressure... ...is the pressure measured from abso- ...is the practical reference quantity and lute zero. It is required for all theoretical is measured from atmospheric pressure considerations both in vacuum and in upwards. blower technology. Compressed air

Compressed air is atmospheric air in a pressurized state. It is contained energy that can be utilized as work during expansion.

13 bar (ü)

14 bar

100

atmosphärischer


atmospheric pressure

vacuum100%

0%

pamb

absolute pressure

gauge pressurevacuum

(g) (g) (g) (g)

��

Definition of pressure

Generally: Dimensions:

1.1 Gas Laws Pressure, volume and temperature are directly related to each other. There is a proportional relationship:

Pressure and volume

Volume and temperature

Pressure and temperature


1 newton 1 pascal =

1 m2

1 N 1 Pa = 1 m2 105 Pa = 1 bar 1 MPa = 10 bar 1 bar = 14.5 psi 1 bar (g) = 14.5 psi (g) 1 bar = 10197 mm water column 1 bar = 750.062 Torr (mm Hg)

T p ~

V

p x V ~ T

If volume is reduced, at constant temperature, the pressure rises p0 x V0 = p1 x V1 = a const.

If heat is added, at constant pressure, the volume of gas behaves directly proportional to ist absolute temperature. V0 / V1 = T0 / T1

If heat is added, at constant Volume, the pressure is Directly proportional to absolute temperature p0 / p1 = T0 / T1

Force Pressure = Area

F p = A


V0 V1

removal of heat

isothermal T0 = T1

addition of heat

isobaric p0 = p1

addition of heat

isochoric V0 = V1


Pressure, volume and temperature:

cV is the quantity of heat required to heat up 1 kg of gas by 1 K at a constant volume (rise in pressure during heat addition, drop in pressure during heat removal). No change in the size of the space occurs here. cV = 0.72 kJ/kg K for air at ambient temperature. cp is the quantity of heat required to heat up 1 kg of gas by 1 K at a constant pressure (with heat addition: rise in temperature with increase in volume; with heat removal: drop in temperature and reduction in volume). cp = 1.01 kJ/kg K Accepted values for the polytropic exponent n occur during the generation of compressed air according to the applied principle of compression. Thus, the theoretical power required to compress a certain quantity of air per unit of time also changes depending on the compression ratio.

If the volume is reduced and removal of heat is not possible, temperature and pressure rise

T1 / T0 = (p1 / p0) (κ-1)/κ

T1 / T0 = (V0 / V1) κ-1

κ = cp / cV

All the changes of state named above can be regarded as theoretical cases of the practical change of state that is called POLYTROPIC. Thus:

T1 / T0 = (p1 / p0)(n-1)/n = (V0 / V1)

n-1

with n as empirical polytropic exponent.

n = 1.4 = κ (for air) n = 1.3 n = 1.1 n = 1.0 Theoretical specific power consumption for compression above 1 bar absolute


Absolute insulation, no exchange of heat

isentropic

isothermal

Adiabatic or isentropic p0 < p1 T0 < T1 V0 < V1

Compression ratio


Under ideal conditions the following mathematical relationship (general gas equation) results from the gas laws quoted previously:

where: p = pressure (bar(a)) V = volume (m3) T = temperature (K) R = special gas constants (kJ / kg K) R represents the amount of work done by 1 kg of gas per 1 K of temperature increase, at constant pressure. R is dependent of the type of gas. Rair = 0.287 kJ / kg K

Practical phenomena are easily explained with this formula, for example: if the volume is held constant and pressure is increased, the temperature must also increase. Or, with increased temperature, at constant pressure, the volume increases. Or, as in a tyre on a vehicle, with constant volume, the tyre pressure increases when it is heated up on the road.

Volume, weight and standardized cubic metre of air These three terms often dominate discussion when talking about compressed air. All too often confusion occurs that can lead to erroneous sizing of compressor installa- tions.

Temperature Pressure Relative humidity

Density

a) Volume according to DIN 1343 (normal physical state)

0 °C = 273,15 K

1,01325 bar

0 %

1,294 kg/m³

b) Volume according to DIN/ISO 2533

15 °C =

288,15 K

1,01325

bar

0 %

1,225

kg/m³

c) Volume related to atmosphere (normal state)

atmospheric temperature

atmospheric pressure

atmospheric humidity variable

d) Volume related to operating state

working temperature

working pressure

variable

variable

p0 x V0 / T0 = p1 x V1 / T1 = R = a constant



d) Volume referred to the working state (working volume „Working Cubic Metre“) Working volume is the volume of air in the pipework of the air main or stored in the air receiver that is compressed with the corresponding compression ratio. Its temperature normally corresponds to the ambient temperature, its relative humidity corresponds to the degree of air treatment. The weight of the air is thus variable as with standard volume.

In practice it is always useful to quote the actual conditions when volumetric flow rates are compared.

1) Reference conditions during measurements: • Reference conditions to DIN 1945, part 1, appendix F, or ISO 1217 – 1996, appendix E: If not otherwise defined, then 20 °C temperature, 1 bar (a) air pressure, 0 % air humidity • To PN2CPTC2 (PNEUROP / CAGI 1992): volume referred to actual conditions

Note !! The air delivery (capacity) of displacement compressors (screw compressors, reciprocating compressors, sliding vane compressors, rotary blowers) is always quoted in actual volume. 1)

eff. vol. of inlet air V0 x inlet air press. p0 Working vol. V1 = Working pressure p1


Umgebungsluftdruck1 bar (a)

Betriebsdruck7 bar (a)= 6 bar (ü)

1 Betriebs-m ³

7 m ³ atmosphärischesLuftvolumen

ambient air pressure 1 bar (a) (p0)

7 m³ atmospheric air volume (V0)

1 working m³ (V1)

working pressure 7 bar (a) = 6 bar (g) (p1)


Conversion of actual volume to volume according DIN 1343 Because actual volume does not suffice as a specification for air delivery in some applications, but instead, the precise weight of the air is required, the corresponding actual volume must be referred to the so-called „Standard Cubic Metre“ to DIN 1343. This represents the exact weight of air. The general gas equation serves as the basis for the conversion VN = V0 x TN x (pA-(Frel x pD)) / (pN x T0). where: VN = volume to DIN 1343 V0 = actual volume TN = temperature to DIN 1343, TN = 273.15 K T0 = temperature at place of installation in K pN = air pressure to DIN 1343, pN = 1.01325 bar pA = air pressure at place of installation in bar (a) Frel = relative air humidity at place of installation pD = saturation pressure of the water vapour contained in the air in bar, related to the air temperature (see table) The capacities of diplacement compressors are always referred to actual conditions. In the conversion to the physical standard condition the extreme conditions, i.e. highest ambient temperature, lowest air pressure and maximum humidity of the air at the place of installation must be used. Excerpt from the table of water vapour pressures for air: Saturation pressure pD (bar(a)) at air temperature t (Û&��

t pD t pD t pD -10 0.0026 +10 0.0123 +30 0.0424 -9 0.0028 +11 0.0131 +31 0.0449 -8 0.0031 +12 0.0140 +32 0.0473 -7 0.0034 +13 0.0150 +33 0.0503 -6 0.0037 +14 0.0160 +34 0.0532 -5 0.0040 +15 0.0170 +35 0.0562 -4 0.0044 +16 0.0182 +36 0.0594 -3 0.0048 +17 0.0184 +37 0.0627 -2 0.0052 +18 0.0206 +38 0.0662 -1 0.0056 +19 0.0220 +39 0.0699 0 0.0061 +20 0.0234 +40 0.0738

+1 0.0064 +21 0.0245 +41 0.0778 +2 0.0071 +22 0.0264 +42 0.0820 +3 0.0074 +23 0.0281 +43 0.0864 +4 0.0081 +24 0.0298 +44 0.0910 +5 0.0087 +25 0.0317 +45 0.0968 +6 0.0094 +26 0.0336 +46 0.1009 +7 0.0100 +27 0.0356 +47 0.1061 +8 0.0107 +28 0.0378 +48 0.1116 +9 0.0115 +29 0.0400 +49 0.1174

+50 0.1234 Conversion of weight of air to volume according to DIN 1343 If the air consumption is referred to the weight of the air, i.e. quoted in kg, then this weight of air is to be divided by the standard weight according to DIN 1343 (1.294 kg/m3). The result is the volume to DIN 1343 in „Standard Cubic Metres“. The conversion back to the actual volume is done as described previously.

1. Fundamental Principles Grundlagen der Drucklufttechnik


1.5 Compressed Air in Motion When compressed air is in motion, then other physical laws apply than those applying to stationary compressed air:

Flow characteristics: V = volumetric flow rate v = velocity A1, v1 A = area (cross-sectional)

Volumetric flow rate is calculated as follows (simplified formula for incompressible media):

V = A1 x v1 = A2 x v2

This shows that flow velocity is inversely proportional to the cross-sectional area.

A1 / A2 = v2 / v1 Flows may be laminar, i.e. aligned (ideal case) and turbulent (back flows and swirls). Types of flow:

Laminar flow Turbulent flow

Every pipe induces a certain resistance to the flowing air. This resistance is related to the inner surface of the pipe, to the cross-sectional area, the flow velocity and to the length of the pipe.

A2, v2

V

V


Pressure drop

pressure in bar

length in m


1.6 Graphical Symbols for Pneumatic Systems

as specified in ISO 1219

Conversion of energy

Compressor Vacuum pump Pneumatic motor with Pneumatic motor with Pneumatic oscillating one direction of flow two directions of flow motor

Single-acting cylin- Single-acting cylin- Double-acting cylin- Double-acting cylin- Differential cylinder der; returned by der; returned by der der with double - unspecified force spring ended piston rod

Double-acting cylin- Double-acting cylin- Multiple position Double-acting teles- Pressure intensifier der with single fixed der with double ad- cylinder, e.g. 3-posi- copic cylinder (transformer) cushion justable cushion tion cylinder Control and regulation of energy

2/2 directional control 2/2 directional con- 3/2 directional con- 3/2 directional con- 3/3 directional con- valve with locked off trol valve with trol valve with locked tro valve with trol valve with locked position through-flow off off position through-flow off centre position position position

4/2 directional con- 4/3 directional con- 5/2 directional con- 4/3 directional con- 5/3 directional control valve trol valve with locked trol valve trol valve with vented trol valve with locked centre position centre position centre position working line


Stop valves- Stop valves- Stop valves- Stop valves - shuttle Stop valves - rapid free non-return valve spring-loaded non- pilot controlled non- valve (double non- exhaust valve

return valve return valve return valve)

Flow control valves- Flow control valves- Flow control valves- throttle valve with adjustable throttle non-return valve with constant choke restriction

Pressure valves- Pressure valves- Pressure valves- Pressure valves- Pressure valves- diaphragm non-return manually adjustable adjustable throttle adjustable pressure adjustable safety valve throttle valve valve with mechani- relief valve with pilot valve with exhaust cal control against a control

spring

Pressure valves- Pressure valves- adjustable pressure adjustable pressure regulator without re- regulator with relief lief port port Transfer of energy

Pressure source Working line Pilot control line Drain, bleed or vent Flexible hose line

Electrical conductor Pipeline junction Crossed pipelines Air bleed Exhaust port with no (fixed) (not connected) provision for connection

Exhaust port threaded Power take-off point Power take-off point Quick coupling Quick coupling with for connection (plugged) (with take-off line) (connected) mechanically opened non-return valves (connected)

Quick coupling Quick coupling One-way rotary Two-way rotary Silencer (uncoupled, with (uncoupled, with connection connection closed end) open end)




Air receiver Filter or strainer Water trap, with Water trap, with Filter with automatic manual control automatic drain drain

Air dryer Lubricator Conditioning unit Cooler (simplified symbol) Control methods

Rotating shaft – in one Rotating shaft - in Detent Locking device - Over-centre device direction either direction (* = symbol for un- (prevents mechanism locking control) stopping at dead centre)

Simple pivoting Pivoting device with Pivoting device with device traversing lever fixed fulcrum

General symbol for Muscular control - Muscular control - Muscular control - muscular control by push button by lever by pedal

Mechanical control - Mechanical control - Mechanical control - Mechanical control - by plunger by spring by roller by roller operating in one direction only

Electrical control - Electrical control - Electrical control - Electrical control - by solenoid with one by solenoid with two by electric motor by electric stepping winding windings working in motor opposite directions

Direct acting control - Direct acting control - Indirect control - by Indirect control - Pressure control - by application of by release of pressure application of pres- by release of pressure by differential control pressure sure to the pilot at the pilot directional directional valve valve

Pressure control - Pressure control - with centring pressure with centring spring



Combined control - Combined control - Combined control - Combined control - by solenoid and pilot by solenoid or pilot by solenoid or general representa- directional valve directional valve manually operated tion ( * = explanatory reset spring symbol)

Special control - Special control - By pressure applica- by pressure application via fed amplifier tion, control creates (standards changeover suggestion) characteristic

(standards suggestion) Supplementary devices

Pressure gauge Differential pressure Temperature gauge Flow meter Integrating flow meter gauge (volume)

Pressure switch Pressure sensor Temperatur sensor Flow sensor (electric) Special symbols

Fluidic proximity Transmitting nozzle Supplied receiver Pilot tube sensor for air chamber nozzle for air chamber Abbreviations used for connections: A, B, C: working lines P: pressure connections R, S, T: drains, exhausts, bleeds, vents X, Y, Z: control lines


1.7 Logical Symbols


AND gate (AND-function, conjunction)

Exclusive OR gate (antivalence)

NAND gate (NOT-AND function)

Equivalence (IF-AND-ONLY-IF function)

NOT gate (NOT function, negation)

Implication (IF-THEN function, inclusion)

NOR gate (NOT-OR function)

Memory (flip-flop, signal ON priority)

Inhibit gate (NOT-IF-THEN function), exclusion

Memory (flip-flop, signal OFF priority)

OR gate (OR-Function, disjunction, inclusive OR)


inputs output inputs output

inputs output inputs output

inputs output

inputs output

inputs output

inputs output

inputs output

Engineering

Compressed air