Pneumatic Systems
Main Components
Unit 3
Dr. Hussein Sarhan
THE BASIC PNEUMATIC SYSTEM
Pneumatic cylinders, rotary actuators and air motors provide the
force and movement of most pneumatic control systems, to
hold, move, form and process material.
To operate and control these actuators, other pneumatic
components are required i.e. air service units to prepare
the compressed air and valves to control the pressure, flow and
direction of movement of the actuators.
A basic pneumatic system consists of two main sections:
• the Air Production and distribution System
• the Air Consuming System
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Air Production System
The component parts and their main functions are:
1 Compressor
Air taken in at atmospheric pressure is compressed and delivered at a higher
pressure to the pneumatic system. It thus transforms mechanical energy into
pneumatic energy.
2 Electric Motor
Supplies the mechanical power to the compressor. It transforms electrical energy
into mechanical energy.
3 Pressure Switch
Controls the electric motor by sensing the pressure in the tank. It is set to a
maximum pressure at which it stops the motor, and a minimum pressure at
which it restarts it.
4 Check Valve
Lets the compressed air from the compressor into the tank and prevents it
leaking back when the compressor is stopped.
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5 Tank
Stores the compressed air. Its size is defined by the capacity of the compressor.
The larger the volume, the longer the intervals between compressor runs.
6 Pressure Gauge
Indicates the Tank Pressure.
7 Auto Drain
Drains all the water condensing in the tank without supervision.
8 Safety Valve
Blows compressed air off if the pressure in the tank should rise above the allowed
pressure
9 Refrigerated Air Dryer
Cools the compressed air to a few degrees above freezing point and condenses
most of the air humidity. This avoids having water in the downstream system.
10 Line Filter
Being in the main pipe, this filter must have a minimal pressure drop and the
capability of oil mist removal. It helps to keep the line free from dust, water and oil .
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The Air Consuming System
1 Air Take-off
For consumers, air is taken off from the top of the main pipe to allow occasional
condensate to stay in the main pipe, when it reaches a low point a water take off from
beneath the pipe will flow into an Automatic Drain and the condensate will be
removed.
2 Auto Drain
Every descending tube should have a drain at its lowest point. The most efficient method is
an Auto Drain which prevents water from remaining in the tube should manual draining
be neglected.
3 Air Service Unit
Conditions the compressed air to provide clean air at optimum pressure, and occasionally
adds lubricant to extend the life of those pneumatic system components which need
lubrication.
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4 Directional Valve
Alternately pressurizes and exhausts the cylinder connections to control
the direction of movement.
5 Actuator
Transforms the potential energy of the compressed air into mechanical work.
Shown is a linear cylinder; it can also be a rotary actuator or an air tool etc.
6 Speed Controllers
Allow an easy and stepless speed adjustment of the actuator movement.
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Compressors
In order to supply pneumatic systems with compressed air we use a
machine called a compressor.
A pump that is driven by a motor, sucks in air from the room and
stores it in a tank called the receiver. You will be able to hear the
compressor when it is running. Sometimes though, it will stop
because the receiver is full.
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Compressors
A compressor is a machine that compresses air or another type
of gas from a low inlet pressure (usually atmospheric) to a
higher desired pressure level. This is accomplished by
reducing the volume of the gas.
A compressor converts the mechanical energy of an electric or
combustion motor into the potential energy of compressed
air.
A single central compressor can supply various pneumatic
components with compressed air, which is transported
through pipes from the cylinder to the pneumatic
components.
Air compressors fall into two main categories: Reciprocating and
Rotary .
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Reciprocating piston compressors
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Reciprocating Compressors
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Air taken in at atmospheric pressure is compressed to the
required pressure in a single stroke.
Downward movement of the piston increases volume to
create a lower pressure than that of the atmosphere,
causing air to enter the cylinder through the inlet valve.
At the end of the stroke, the piston moves upwards, the
inlet valve closes as the air is compressed, forcing the
outlet valve to open discharging air into a receiver tank.
This type of compressor is generally used in systems
requiring air in the 3-7 bar range
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In a single-stage compressor, when air is compressed
above 6 bar, the excessive heat created greatly reduces
the efficiency. Because of this, piston compressors used in
industrial compressed air systems are usually two stage .
Air taken in at atmospheric pressure is compressed in two
stages to the final pressure.
If the final pressure is 7 bar, the first stage normally
compresses the air to approximately 3 bar, after which it is
cooled. It is then fed into the second stage cylinder which
compresses it to 7 bar.
The compressed air enters the second stage cylinder at a
greatly reduced temperature after passing through the
inter-cooler, thus improving efficiency compared to that of a
single stage unit. The final delivery temperature may be
in the region of 120°C.
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Diaphragm compressors provide compressed air in the
3-5 bar range totally free of oil and are therefore widely
used by food, pharmaceutical and similar industries.
The diaphragm provides a change in chamber volume.
This allows air intake in the down stroke and
compression in the up stroke.
Smaller types, with an electric motor of ≤ 1 kW power,
make portable compressors ideal for spray painting.
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Rotary Compressors
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This has an eccentrically mounted rotor having a series
of vanes sliding in radial slots.
As the rotor rotates, centrifugal force holds the vanes in
contact with the stator wall and the space between the
adjacent blades decreases from air inlet to outlet, so
compressing the air.
Lubrication and sealing is achieved by injecting oil into
the air stream near the inlet. The oil also acts as a
coolant to limit the delivery temperature.
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Two meshing helical rotors rotate in opposite directions. The
free space between them decreases axially in
volume and this compresses the air trapped between the
rotors.
Oil flooding provides lubrication and sealing between the two
rotating screws. Oil separators remove this oil from the outlet
air.
Continuous high flow rates in excess of 400 m3/min are
obtainable from these machines at pressures up to 10 bar.
More so than the Vane Compressor, this type of compressor
offers a continuous pulse-free delivery.
The most common industrial type of air compressor is still the
reciprocating machine, although screw and vane
types are finding increasing favor.
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Flow compressor
These are particularly suitable for large delivery quantities. Flow
compressors are made in axial or radial form. The air is made
to flow by means of one or several turbine wheels. The kinetic
energy is converted into pressure energy.
In the case of an axial compressor, the air is accelerated in the
axial direction of flow by means of blades.
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Selection of Compressor
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Actual power required to drive a
compressor
To determine the actual power, theoretical
power is divided by the overall compressor
efficiency.
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Notes
Air compressors are generally rated in terms of cfm of free air,
defined as air at actual atmospheric conditions. Cfm of free air
is called scfm when the compressor inlet air is at the standard
atmospheric conditions of 14.7 psia and 68 F. The
abbreviation scfm means standard cubic feet per minute.
Therefore, a calculation is necessary to determine the
compressor capacity in terms of cfm of free air or scfm for a
given application.
In metric units a similar calculation is made using m^3/min of
free air or standard m^3/min, where standard atmospheric
conditions are 101,000 Pa abs (101 kPa abs) and 20 C.
*** In all equations, the absolute pressure and temperature
values must be used.
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Examples
1. Air is used at a rate of 30 cfm from a reciever at 90 F and 125
psi. If the atmospheric pressure is 14.7 psia and the
atmospheric temperature is 70 F, how many cfm of free air
must the compressor provide?
Solution:
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Examples
2. (a) Calculate the required size of a reciever that must supply air to a
pneumatic system consuming 20 scfm for 6 min between 100 and 80 psi
before the compressor resumes operation.
(b) What size is required if the compressor is running and delivering air at 5
scfm?
Solution:
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Example
3. Determine the actual power required to drive
a compressor that delivers 100 scfm of air at
100 psig. The overall efficiency of the
compressor is 75%.
Solution:
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FLUID CONDITIONERS
The purpose of fluid conditioners is to make air
more acceptable fluid medium for the
pneumatic system as well as operating
personnel. Fluid conditioners include filters,
regulators, lubricators, mufflers, and air
dryers.
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Compressed air filter
Condensed water, contamination and excess oil can lead to wear on
the moving parts and seals of pneumatic components. These substances can
escape as a result of leakage. Without the use of filters, for example, products
to be processed in the food, pharmaceutical or chemical industries could
become contaminated and therefore rendered useless.
The selection of the correct filter plays an important role in determining the
quality and performance of the working system which is to be supplied
with compressed air. One characteristic of compressed-air filters is the
pore size. The pore size of the filter element indicates the minimum
particle size which can be filtered out of the compressed air. The collected
condensate must be drained before the level exceeds the maximum
condensate mark otherwise it will be re-introduced in the air stream.
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If a large amount of condensate accumulates, it
is advisable to fit an automatic drain in place of
the manually operated drain cock. However, in
such cases, the cause of the accumulated
condensate is to be established. For example,
an unsuitable pipe layout may be the cause of
the condensate accumulation.
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Compressed air regulators
The system pressure which has proved in practice to be the best
economic and technical compromise between compressed-air
generation and the efficiency of the components is
approximately:
• 600 kPa (6 bar) in the power section and
• 300 to 400 kPa (4 bar) in the control section.
A higher operating pressure would lead to inefficient energy
utilization and increased wear, whereas a lower operating
pressure would lead to poor efficiency, particularly in the power
section.
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Principle of the Pressure Regulator
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Pressure regulators have a piston or diaphragm to balance the output
pressure against an adjustable spring force.
The secondary pressure is set by the adjusting screw loading the
setting spring to hold the main valve open, allowing flow from the
primary pressure p1 inlet port to the secondary pressure p2 outlet port.
Then the pressure in the circuit connected to the outlet rises and acts
on the diaphragm, creating a lifting force against the spring load.
When consumption starts, p2 will initially drop and the spring,
momentarily stronger than the lifting force from p2 on the diaphragm,
opens the valve.
If the consumption rate drops, p2 will slightly increase, this increases
the force on the diaphragm against the spring force, diaphragm and
valve will then lift until the spring force is equaled again. The air flow
through the valve will be reduced until it matches the consumption rate
and the output pressure is maintained.
If the consumption rate increases, p2 will slightly decrease. This
decreases the force on the diaphragm against the spring force,
diaphragm and valve drop until the spring force is equaled again. This
increases the air flow through the valve to match the consumption rate.
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Compressed Air Lubrication
Lubrication is no longer a necessity for the majority of modern Pneumatic
components, most are available prelubricated for life.
The life and performance of these components are fully up to the requirements of
modern high cycling process machinery.
The advantages of “non-lube” systems include:a) Savings in the cost of lubrication equipment, lubricating oil and maintaining oil
levels.
b) Cleaner more hygienic systems; of particular importance in food and
pharmaceutical industries.
c) Oil free atmosphere, for a healthier, safer working environment.
Certain equipment still requires lubrication. To ensure they are continually
lubricated, a certain quantity of oil is added to the compressed air by means of
a lubricator.
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Proportional Lubricators
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In a (proportional) lubricator a pressure drop between inlet
and outlet, directly proportional to the flow rate, is created and
lifts oil from the bowl into the sight feed dome .
With a fixed size of restriction, a greatly increased flow rate
would create an excessive pressure drop and produce
an air/oil mixture that had too much oil, flooding the pneumatic
system.
Conversely a decreased flow rate may not create sufficient
pressure drop resulting in a mixture which is too lean.
To overcome this problem, lubricators must have selfadjusting cross sections to produce a constant mixture.
Air entering at A (in Fig) follows two paths: it flows over the
damper vane to the outlet and also enters the lubricator bowl
via a check valve.
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When there is no flow, the same pressure exists above the surface of the oil
in the bowl, in the oil tube and the sight feed dome. Consequently there is
no movement of oil.
When air flows through the unit, the damper vane restrictor causes a
pressure drop between the inlet and outlet. The higher the flow, the greater
the pressure drop.
Since the sight feed dome is connected by the capillary hole to the low
pressure zone immediately after the damper vane, the pressure in the
dome is lower than that in the bowl. This pressure difference forces oil up
the tube, through the oil check valve and flow regulator into the dome.
Once in the dome, the oil seeps through the capillary hole into the main air
stream in the area of the highest air velocity. The oil is broken up into
minuscule particles, atomized and mixed homogeneously with the air by the
turbulence in the vortex created by the damper vane.
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Service Unit
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Modular filter, pressure regulator and
lubricator elements can be combined into a
service unit by joining with spacers and
clamps.
Mounting brackets and other accessories
can be easily fitted in more recent designs.
(see Unit 2)
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