Transcript
Fundamentals of compressed air
1.2
Units and formula symbols
The SI-units ( Système International d'Unités ) were agreed at the 14th General Conference for Weights and Measures. They have been generally prescribed since 16.10.1971. The basic units are defined independent units of measure and form the basis of the SI-system.
1.2.1
Basic units
Basic unit Length Mass Time Strength of current Temperature Strength of light Qty of substance
Formula symbol l m t I T I n
Symbol [m] [ kg ] [s] [A] [K] [ cd ] [ mol ]
Name Metre Kilogramme Second Ampere Kelvin Candela Mol
1.2.2
Compressed air units
Engineering uses measures derived from the basic units. The following table shows the most frequently used units of measure for compressed air.
Unit Force Pressure Area Volume Speed Mass Density Temperature Work Energy Tension Frequency
Formula symbol F p A V v m ρ T W P U f
Symbol [N] [ Pa ] [ bar ] [ m2 ] [ m3 ] [l] [m/s] [ kg ] [t] [ kg / m3 ] [ °C ] [J] [W] [V] [ Hz ]
Name Newton Pascal Bar 1 bar = 100 000 Pa
Square metre Cubic metre Litre 1 m3 = 1 000 l Metre per Second Kilogramme Tonne 1 t = 1 000 kg Kilogramme per cubic metre Degree Celsius Joule Watt Volt Hertz
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Fundamentals of compressed air
1.3
1.3.1
What is compressed air ?
The composition of air The air in our environment, the atmosphere, consists of:
Nitrogen 78 %
78 % Nitrogen 21 % Oxygen 1 % other gases ( e.g.. carbon-dioxide and argon )
Oxygen 21 %
other gases 1%
Fig. 1.11: The composition of air
1.3.2
The properties of compressed air
Compressed air is compressed atmospheric air. Compressed air is a carrier of heat energy.
Compressed air Pressure energy Heat
Compressed air can bridge certain distances ( in pipelines ), be stored ( in compressed air receivers ) and perform work ( decompress ).
Fig. 1.12: Air compression
1.3.3
How does compressed air behave?
As with all gases, the air consists of molecules. The molecules are held together by molecular force. If the air is enclosed in a tank ( constant volume ), then these molecules bounce off the walls of the tank and generate pressure p. The higher the temperature, the greater the movement of air molecules, and the higher the pressure generated. Volume ( V ) = constant
p p p p
p p
p p p p
V
p p
Temperature ( T ) = is increased Pressure ( p ) = rises
T
Fig. 1.13: Air in a closed container
Boyle and Mariotte carried out experiments with enclosed volumes of gas independently of each other and found the following interrelationship: The volume of gas is inversely proportional to pressure. ( Boyle-Mariotte’s Law )
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Fundamentals of compressed air
1.4
Physical fundamentals
The condition of compressed air is determined by the 3 measures of thermal state: T V p = Temperature = Volume = Pressure
p × V ———— T
=
constant
This means:
Heat
Volume constant ( isochore ) Pressure and temperature variable When the temperature is increased and the volume remains constant, the pressure rises.
p0 , T0 p1 , T1
constant volume isochore compression
p0 —— p1
=
T0 —— T1
Temperature constant ( isotherm ) Pressure and volume variable
p0 , V0 p1 , V1
When the volume is reduced and the temperature remains constant, the pressure rises.
constant temperature isotherm compression
p0 × V 0 =
p1 × V1 =
constant
Heat
Pressure constant ( isobar ) Volume and temperature variable
V0 , T 0 V1 , T 1
When the temperature is increased and the pressure remains constant, the volume increases.
constant pressure isobar compression
V0 —— V1
=
T0 —— T1
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Fundamentals of compressed air
1.4.1
Temperature
The temperature indicates the heat of a body and is read in °C on thermometers or converted to Kelvin ( K ).
T
0°C Fig.1.14: Showing temperature
[K]
=
t [ °C ] + 273,15
1.4.2
Volume
Volume V [ l, m3 ]
Compressed air in expanded state, open air
The volume is determined, for example, by the size of a cylinder. It is measured in l or m 3 and relative to 20 ° C and 1 bar. The numbers in our documentation always refers to compressed air in its expanded state. d2 × π ———— × h 4 VCyl = Volume d = Diameter h = Height [m3] [m] [m]
VCyl =
Volume (V)
Normal volume VNorm [ Nl, Nm3 ]
Compressed air in expanded state under normal conditions
The normal volume refers to the physical normal state as specified in DIN 1343. It is 8 % less than the volume at 20 ° C. 760 Torr = 1,01325 barabs = 101 325 Pa 273,15 K = 0 °C
Norm volume 0°C + 8% = Volume 20 ° C
Operating volume Voperat [ Bl, Bm3 ]
Compressed air in compressed state
The volume in operating state refers to the actual condition. The temperature, air pressure and air humidity must be taken into account as reference points. When specifying the operating volume the pressure must always be given, e.g., 1 m 3 at 7 bar means that 1 m 3 expanded (relaxed) air at 7 bar = 8 bar abs. compressed and only occupies 1/8 of the original volume.
0 barabs
8 barabs
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Fundamentals of compressed air
1.4.3
Pressure
Atmospheric pressure pamb [ bar ] Atmospheric pressure is caused by the weight of the air that surrounds us. It is independent of the density and height of the atmosphere. At sea level, 1 013 mbar = 1,01325 bar = 760 mm/Hg [ Torr ] = 101 325 Pa
Under constant conditions atmospheric pressure decreases the higher the measuring location is.
Fig.1.15: Atmospheric pressure
Over-pressure pop [ barop ] Over-pressure is the pressure above atmospheric pressure. In compressed air technology, pressure is usually specified as over-pressure, and in bar without the index „ op“. Overpressure
pop
Absolute pressure pabs [ bar ] The absolute pressure pabs is the sum of the atmospheric pressure pamb and the over-pressure pop.
barometric air pressure
pabs
pabs
pvac
= pamb + pop
Partial vacuum
According to the SI-System pressure is given in Pascal [ Pa ]. In practice, however, it is still mostly given in „ bar “. The old measure atm ( 1 atm = 0,981 bar-op ) is no longer used.
pamb
Force
100 % Vacuum Pressure = ————
Area
F p = —— A
pamb pop pvac pabs
= = = =
Atmospheric pressure Over-pressure Partial vacuum Absolute pressure
1 Pascal =
1 Newton ———— 1 m2
1N 1 Pa = —— 1 m2 [ mm water head ]
Fig.1.16: Illlustration of different pressures
1 bar = 10195 mmWH
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Fundamentals of compressed air
• Volume flow V [ l/min, m³/min., m³/h ] The volume flow describes the volume ( l or m³ ) per unit of time ( minute or hour ). Working volume flow
Induction rate
1.4.3
Volume flow
Þ
A distinction is made between the working volume flow ( induction rate ) and the volume flow ( output rate ) of a compressor. • Working volume flow VWor [ l/min, m³/min., m³/h ]
Induction rate
Volume flow
Output rate
The working volume flow is a calculable quantity on piston compressors. It is the product of the cylinder size ( piston capacity ), compressor speed ( number of strokes ) and the number of cylinders working. The working volume flow is given in l/min, m³/min or m³/h. • VWor
Fig. 1.17: Working volume flow and volume flow
TDC
BDC
TDC = Top dead centre BDC = Bottom dead centre
Fig. 1.18: Cylinder movement
Û
=
A ×
s ×
n ×
c
• VWor A s n c
= = = =
Working volume flow [ l / min ] Cylinder area [ dm2] Stroke [ dm] Number of strokes [ 1/ min ] (compressor speed) = Number of working cylinders
• Volume flow V [ l/min, m³/min, m³/h ]
Output rate
The output rate of a compressor is normally declared as the volume flow. In contrast to the working volume flow, the volume flow is not a calculated value, but one measured at the pressure joint of a compressor and calculated back to the induction state. The volume flow is dependent on the final pressure relative to the induction conditions of pressure and temperature. This is why when calculating the induction state the measured volume flow to induction pressure must be „ relaxed“ and to induction temperature it must be „ re-cooled“. The volume flow is measured according to VDMA 4362, DIN 1945, ISO 1217 or PN2 CPTC2 and given in l/min, m3/min or m3/h. The effective volume flow, i.e., the output that can actually be used, is an important consideration for the design of a compressor. Volume flows can only usefully be compared when measured under the same conditions. This means that the induction temperature, pressure, relative air humidity and measured pressure must match.
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Fundamentals of compressed air
• Norm volume flow VNorm [ Nl/min, Nm3/min, Nm3/h ] As with the volume flow, the norm volume flow is also measured. However, it does not refer to the induction state, but to a theoretical comparative value. With the physical norm state the theoretical values are:
Volume flow 20°C Fig. 1.19: Norm volume flow + 8% = Norm volume flow 0°C
Temperature= 273,15 K Pressure = 1,01325 bar Air density = 1,294 kg/m3
( 0 °C ) ( 760 mm HG ) ( dry air )
• Operating volume flow VOperat [ Ol/min, Om3/min, Om3/h ] The operating volume flow gives the effective volume flow of compressed air. To be able to compare the operating volume flow with the other volume flows, the pressure of he compressed air must always be given in addition to the dimension Ol/min, Om3/min or Om3/h.
0 barabs Fig. 1.20: Operating volume flow
8 barabs
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Fundamentals of compressed air
1.5
Compressed air in motion
Different laws apply to compressed air in motion than to stationary compressed air.
1.5.1
Flow behaviour
The volume flow is calculated from area and speed.
A1
A2
• V
= A1 × v 1
= A2
× v2
A1 v2 —— = —— A2 v1 v1 v2 • V = A 1, A 2 = v 1, v 2 = The result of the formula is that: The speed of flow is inversely proportional to the cross section.
Volume flow Cross section Speed
Fig. 1.21: Flow behaviour
1.5.2
Types of flow
Flow can be laminar or even (Ideal), or turbulent ( with backflow and whirling ).
Laminar flow ( even flow ) low drop in pressure slight heat transition
Fig. 1.22: Laminar flow
Turbulent flow ( whirl flow ) high drop in pressure great heat transition
Fig. 1.23: Turbulent flow
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