Pneumatics

The word comes from the Greek πνευμα pneumatics - pneumatic and means something like " wind ", " breath" or "spirit." It refers to the use of compressed air in science and technology for the performance of mechanical work.

• 9.1 advantages
• 9.2 disadvantages
• 9.3 Conclusion

General

Pneumatics is the study of all technical applications where compressed air is used to do work. In contrast, describes the use of a hydraulic fluid as the working medium.

Compressed ambient air as compressed air ( obsolete: compressed air) respectively. Compressed air can be used for many different purposes, such as active air for the continuation of substances ( eg conveying air or paint), as process air, which is included in a process ( eg drying ) or as test air. Thus, the pneumatic forms only a small proportion of the total compressed air applications.

Conventional compressed air systems often work with 6 bar pressure ( gauge pressure ) in them, so there is about seven times the atmospheric pressure. The pressure level in high pressure pneumatic systems for applications with high power requirements can be up to 18 bar, but specific components (hoses and fittings ) must then be used already, can withstand this high pressure. In special cases ( eg in the production of PET bottles) may be in the compressed air system up to 40 bar, the pressure level.

Each compressed air system consists of four subsystems: compressed air, compressed air treatment, compressed air distribution and the actual application. Compressed air is generated by sucking and compressing ambient air in a compressor and supplied to the application after the processing (filtering, drying) with a compressed air system ( tube and hoses ), and used in this technology.

In pneumatic applications ( control and actuators ), the compressed air is used to do work. Usually it is passed via valves to the desired location. In a pneumatic cylinder, the air is for example used to be acting force on piston and a cylinder so as to move it in a certain direction.

Pneumatics is considered simple and inexpensive to purchase technology. Often the generation of compressed air but are said to have low efficiencies. This has led in recent years by the growing awareness of energy efficiency issues for discussion and the intensified search for alternative technologies, such as electric drives. Practice shows, however, that must be decided depending on the application, which drive technology is energetically and economically advantageous. Blanket statements are not possible in most cases.

Compressed air generation

Necessary for the operation of pneumatic systems, compressed air is generated in a compressor ( compressor). Usually an electrically driven motor produces a mechanical movement that is transmitted to the reciprocating piston compressor or screw. On intake and exhaust atmospheric air is first compressed and then discharged into the compressed air network or an upstream air reservoir.

In systems with a high air demand several compressors are often operated in combination. To provide the basic needs unregulated large compressors are used, the peak load is often covered by a variable-speed compressor. A suitable controller coordinates the operation of the complete compressor system and ensures efficient operation possible.

Depending on the required pressure and the required flow rate of different types of compressors can be used. For example, multi-stage reciprocating compressors are particularly suitable for the generation of high output pressures at rather low flow rates. However, screw compressors produce rather a lower output pressure at higher flow rate.

Due to mechanical and thermodynamic processes created during the compression of the compressed air, a large amount of heat which must be removed from the compressed air. In many older systems, this waste heat remains unused. The overall efficiency of the pneumatic system, however, can be strengthened if the produced heat to good use is supplied, for example, as heating, process heat ( for hot water ) or to generate refrigeration for air conditioning ( adsorption ) as needed.

Compressed air treatment

Ensuring air quality is important because impurities in the compressed air in the application affect the function of the pneumatic components or even cause permanent damage. The preparation of compressed air can be centralized or decentralized. The central processing occurs in the vicinity of the compressor station, before the compressed air is fed into the distribution network. In contrast, the distributed processing is carried out just prior to use, to ensure the quality of compressed air required in each of the components.

For removal of solid contaminants suitable filter systems are used. Refrigeration dryers, adsorption or membrane dryers, compressed air the steam out, thus lowering the dew point. This ensures that when the temperature drops at the components teeing not water vapor, and the surfaces are damaged by corrosion.

Prior to the application of pneumatic maintenance unit is usually placed in the various filter stages via the locally required compressed air quality can be produced. About fine filter and fine filter can be largely removes particles up to a size of 0.01 microns.

Filters, dryers and pressure regulator set in a pneumatic system flow resistance is due therefore you generate when it flows through a pressure drop, which can be very high, especially if filters are not cleaned regularly. A pressure loss affects always negative on the energy balance of the compressed air system and should therefore be avoided if possible. Therefore, the principle is "Filter only as much as necessary ".

Compressed air distribution

The distribution of compressed air from the compressor to the consumers via pipes and is similar to a power line, such as a power cable. The quality of the compressed air should thereby suffer as little as possible, ie Contamination by rust, welding scale, water or other substances should be kept to a minimum.

In addition, make sure that the pipes have a sufficient diameter so that the flow resistance can be kept as low as possible. If the diameter of a pipe cut in half, so their flow resistance increases by about a factor 32 That is, the resistance of a pipe increases with reduced diameter with the 5th power.

Changes the direction of the pipe must be considered separately, especially when close and not rounded angles to be used. The resistance to flow in such a tube elements can be much larger than comparable straight pipe pieces.

The distribution of the compressed air takes place via pipe networks with different topologies. Depending on the arrangement of the buildings and different demand profiles using a ring structure or a mesh topology is recommended. In addition, the distribution of safe (Pressure Equipment Directive, Ordinance on Industrial Safety, Technical Rules pipeline construction ) and cost should be ( documented sizing / documentation of the risk analysis).

Special attention in the construction and maintenance of transmission networks comes to locating and repairing leaks. Since leakage points only draining into the surrounding air pressure in pneumatic systems, composed by the leakage usually no safety or environmental risk. However, leaks should always be removed carefully, as they sometimes cause a large proportion of total energy consumption.

When planning and dimensioning of compressed air systems specifically placed pressure air storage can have a positive effect on the robustness of a compressed air system. This can be useful especially when sporadic consumers have high volumes of air pressure affect the stability of the entire network and thus adversely affect the switching behavior of the compressor station. Compressed air energy storage can then briefly straighten these high consumption and stabilize the network pressure.

Control system (valves )

In fluid power valves are commonly referred to as actuators, which take over the control of the working members. The following component groups are:

• Pressure valves,
• Special valves (for example, proportional valves),
• Check valves,
• Flow control valves and
• Way valves.

Number of switching positions

There are various numbers of switching positions, ranging 2 to 6 are mainly obtained in the industrial and automation technology because of the cost of only 2 or 3 positions used, and valves with 2 switching positions are used in "normal" way valves for switching processes and those be used with 3 switch positions as valves with stop function.

Number of connections

The number of connectors varying between two and seven terminals. In 2/2-way valves is just a normal passage from A to B. ( expertly expressed by 1 (P ) (= compressed air supply) after 2 (A ) (= working port ) ). So you can, for example, in painting or spinning machines blowing functions on and off. 3/2-way valves in addition to the two above-mentioned connections, a venting port is still available, which is able to vent the tubes or the entire system. This 3/2-way valves are used, for example in the control of single-acting cylinders, but also to " unlock " from " new ways " of the pneumatic system.

In five, the connectivity an air port 1 (P ), two working ports 4 and 2 (A and B) and two exhaust ports 5 and 3 (R and S). The two working ports are needed, for example to control a double-acting cylinder, a cylinder on one side filled with compressed air ( that he moves out ) and bled him on the other side ( that it can retract ).

Four ports are found in 4/2-way valves. The operation is the same as in the 5/2-Wegeventilen, however, the two exhaust ports were connected by a component- internal bore ( a compressed air connection two working ports a vent port = four ports ). Control connections are not counted as terminals.

Note: The P for the compressed air connection stands for " Pressure" ( = " print " ), and R at the vent connection is " reset " (= " reset "). According to new DIN standards are the compressed air connection P "1", the working connections in A / B with " 2" or " 4" and the exhaust ports R and S with " 3 " or "5". Control terminals (required for pilot-operated valves) are designated by X, Y or Z or 12, 14. " 14 " means that a signal opens the way from 1 to 4 in this connection.

Actuation

In pneumatics find different types of actuation application. Divide these are mechanical, electrical, pneumatic and manual operations.

Mechanical operations are tappet, spring, roll, roll lever. Mechanical actuators are actuated by the machine itself. Moves as the piston of a cylinder against the stem of a valve, the valve ( mechanically ) actuated.

Electrical actuation are eg button. A current pulse of a button transmitted, he must make an electromagnet in the electrically -operated valve. The spool in the valve - which way locks and opens - is closed attracted and thus opened a way for the air and another.

The pneumatic actuator: The valve is in this case actuated by the compressed air. For example, is opened by the manual operation of a valve of the working connection of the same, and the pressure reaches to a further valve which is actuated by compressed air. The valve spool has just been described is thereby pressed by the compressed air in the desired position. The described example is referred to as " remote control ". Check valves can also be counted among the pneumatically actuated valves.

Manual operations are buttons, push buttons, levers and pedals. These are operated by muscle power. If a lever is moved, in " electrical operations " mentioned valve spool is moved in the desired direction and thus taken another switch position.

In addition to the already stated the form of the remote control valves may be pilot operated. First, the application example: With a small shift force a large volume flow to be enabled. For example, if the force of the pneumatic actuator would not be sufficient to move a valve for switching ( as for example in a pneumatic sensor on the case ), this small force has to control switching a large shifting force, which is capable of the to control valve. For electrically actuated valves, the principle of feedforward control is very often used because large flows can thus be controlled with small, inexpensive magnets. At the same time in this manner, less electrical energy is needed, and the magnets heat up less. The main disadvantage of pilot operated valves is the greater time delay, caused by the sequence of operations. They are also functional bar until a pressure of 2.

System for the performance of work (drives or actuators )

Compressed air can be used to drive air motors in tools, such as compressed air, hammers and pneumatic riveting screws. In control engineering, linear drives are used mainly in the form of cylinders. These pneumatic cylinders are used eg for clamping and feeding workpieces in machining centers or to the closure of packaging. Compressed air can also be used directly for material transport by pneumatic tube.

In the Fluidics one speaks more generally of working members, as these systems perform mechanical work. The working members include:

• Air motors for rotary movements
• Pneumatic muscle,
• Cylinders for linear motions (eg for clamping) and
• Cylinder with gear for pivotal movement.

In pneumatics, a distinction ( single acting, double acting cylinders ) between one-sided and double-acting cylinders with compressed air. In single-acting cylinders, the provision of the cylinder is in its initial position by means of a built- in cylinder spring, while in double-acting cylinders forward and return stroke is made by appropriate control of the compressed air stream.

For more information about the different types of cylinders in pneumatics can be found in the article pneumatic cylinder. Examples of the use of compressed air motors can be found at air tools.

Compressed air and power consumption

The energy consumption in pneumatic components is determined mainly by the air consumption. In most cases, the air consumption in standard liters or standard cubic meters per time unit or per cycle of movement is specified. A standard liter here refers to the volume occupied by a given mass of air at standard conditions. As standard conditions ambient pressure and ambient temperature is usually accepted in accordance with ISO6358.

The standard volume is proportional to the air mass and independent of the actual pressure. In contrast, the operation volume is of the real physical volume of the compressed air in the current print condition. For example, a pneumatic cylinder relative to a diameter of 32 mm and a length of 0.25 m at 6 bar. filled with compressed air, it then contains about 0.2 l Operating air. In normal conditions, this corresponds to 1.4 standard liters.

If the air consumption of a system is known, it can be estimated from characteristics of the compressor system, the electrical energy consumption of the pneumatic components. Depending on the size and efficiency of the compressor, compressed air is used as a rule for the generation of a standard cubic meter ( about 8 bar rel. ) An amount of energy of 0.1 kWh is required.

Pneumatic energy is generally reputed to be a relatively expensive form of energy, their efficiency must be critically evaluated in comparison with alternative drive technologies. The efficiency of pneumatic systems is often relatively low, so the replacement of pneumatic actuators is being considered by electric drives.

However, the reason for this assumption is not (as is often assumed ) the thermodynamic processes in the compressor during compression and the resulting heat. Often, an inadequate design and maintenance of pneumatic systems for a low overall efficiency is responsible. The functionality of pneumatic components is still guaranteed even in the most erroneous interpretation, the excessive size, even with heavy leakage and defects in the components, but the air consumption may rise significantly in such cases. Therefore, both a correct planning and design, as well as an error monitoring (eg with leakage detection) are essential.

Air consumption of the pneumatic components can be relatively easily calculated from the geometry and the greater the volume to be filled for the most part. If, for example, a pneumatic cylinder with a diameter of 32 mm, in order to lift a load of 1 kg to 0.25 m, each double stroke produces an air consumption of about 2.8 Nl ( The internal volume of the cylinder is 0.2 liters, it is with 7 bar abs. filled, so needed for a Hub 1.4 Nl ). The pneumatic cylinder would for the large area, however, can raise a load of about 45 kg, it is therefore greatly oversized and could for example be replaced with a diameter of 12 mm by a drive. Air consumption is reduced by approximately 85 % to 0.4 Nl, since at lower drive diameter also the volume to be filled is much smaller. An effective measure for reducing the air consumption can therefore be to replace oversized cylinder by drives of suitable diameter.

It often turns out in practice that to a certain extent the level of the supply pressure may be adjusted. If a plant is over-dimensioned in the planning phase, the supply pressure can be reduced, for example, of 6 bar to 5 bar. The general air consumption is reduced by about 15 %.

An important aspect to improve the system efficiency is to eliminate existing leaks. Escaping pressurized air to leak points usually does not pose a security risk and it creates no pollution. Therefore, leakage is often attributed little importance, the maintenance of the corresponding parts of the plant is often delayed. A total leakage in a system that corresponds to a nozzle diameter of 3 mm, but can generate energy costs of over € 5,000 per year in an investment already. Therefore it is recommended to remove all known leaks as soon as possible.

Consideration of the efficiency on the basis of " exergy "

Compressed air energy generally has the reputation of being a relatively expensive form of energy, their efficiency must be critically evaluated in comparison with alternative drive technologies. The reason for this being produced during the compression in the compressor is usually given amount of heat, which is often wasted as dissipated heat. In order to show an accurate picture of the energetic relationships in the individual processes in pneumatic systems, however, the thermodynamic processes must be individually analyzed and evaluated.

The compression of the ambient air drawn in the compressor takes place in the theoretical ideal case isothermally, ie without temperature change. The arising heat is dissipated during the process immediately. The compressor brings so one during the compression process work in the system at the same time heat is dissipated. For an ideal gas ( a useful approximation for air) are equal to the amounts of work and heat. It has the same amount of heat to be dissipated as the compressor during the compression is applied. However, the heat generated is not indicative of a loss of energy or even a poor efficiency, because it is " pushed out " only from the compressed air. Energetically, this means the energy content of the air has not changed during the compression, the energy efficiency of compressors is zero, because compressed air contains as much energy as ambient air. This statement is confirmed in thermodynamics, characterized in that both the internal energy for closed systems and open systems, the enthalpy functions are only the temperature of the ideal gas. The pressure has no influence on these two energy terms. For isothermal processes, the energy content of a system does not change consequently.

This finding shows that a benefit analysis of compressed air is not effective on the basis of the thermodynamic energy term, because although compressed air as much energy content as having ambient air, it can be used for technical and do work. A thermodynamic quantity that is this relationship better reflects the exergy. It indicates how much work can perform a system when it is brought into equilibrium with its surroundings, so that 'working ability' in a system exists. In contrast to the exergy energy from the temperature and pressure condition and the environmental condition dependent.

An exergetic considering the idealized compression process shows that exactly that capacity for work is stored in the compressed air in the final stage of compression that was applied to work from the compressor. So there is no system-related reason that would explain a lack of efficiency of pneumatic systems already due to thermodynamic processes.

However, real compressors work not isothermal, but are usually closer to the adiabatic compression. The compressed air is hot after leaving the compressor and only thereafter cooled to room temperature. The adiabatic compression requires more energy and exergy is lost, or it is in the higher temperature of the waste heat.

Practical tests show that in pneumatic systems, a large proportion of the available exergy is lost in fact during compression. In addition to the rise in temperature for this start-up and no-load loss of the electric drive motors, and mechanical losses due to friction are responsible. Further losses can occur due to pressure drop in the preparation, distribution and control. Go on the road to application lost in leaks a certain amount of compressed air, so does this affect also have a negative effect on the exergy balance. The actuators are usually also a small exergetic efficiency: the case of bulbs after the stroke, the compressed air is usually discharged simply unused. Air motors rather work adiabatically, so cool off in the operation and thus make less than it would be possible isothermal. Overall, it must be actually likely to remain relatively low efficiencies in reality so often.

Advantages and disadvantages of the pneumatic

Benefits

• The forces and speeds of the pneumatic cylinder are adjustable via the selection of a suitable pressure level and the use of flow restrictors.
• Compressed air systems have, in comparison to hydraulic systems, namely a lower energy density, it is still generally greater than in comparable electric drives. In a small space comparatively high forces can be achieved.
• Pneumatic drives allow Powerless holding at constant force.
• Pneumatic systems are robust against overload and insensitive to temperature fluctuations.
• Pneumatic drives allow high operating speeds ( standard cylinder up to 1.5 m / s, high-performance cylinder 3.0 m / s, air motors to 100,000 min -1)
• Due to the compressibility of the compressed air to a flexible and resilient drive performance can be obtained. This may be particularly advantageous in applications in which mechanical impacts are to be expected from the outside.
• The use of air as the driving medium ensures in most cases for sufficient cooling of the drive components. Additional cooling is not necessary in the pneumatic application.
• Minor leaks in the system do not cause environmental pollution due to leaking fluid ( only energy loss).
• The viscosity of compressed air is relatively small in comparison to the hydraulic. It is therefore only small flow losses in pipes and hoses.
• Pneumatic actuators are relatively simple in construction and therefore less expensive than electric drives with comparable performance. In addition, they are usually easier, thus the physical stress of employees working in production workers diminishes.
• In contrast to electrical systems, the resulting heat is generated centrally - mostly during the generation of the compressed air in the compressor. Therefore, it is possible to apply central energy recovery process in the form of combined heat and power and thus to increase the efficiency of the overall system.
• As opposed to hydraulic systems, the fluid may be sucked from the environment and then released into the environment in pneumatics. A closed fluid circuit is not necessary in most cases, the topology of the compressed air line systems is relatively simple.
• Explosion safety is guaranteed ( with regular inspection of all pressure vessels). Pneumatic components can be used with regard to explosion protection therefore also in plant areas with high demands.
• Compressed air to magnetic pulses and atomic radiation resistant.
• The energy sources compressed air can be relatively easily stored in containers.

Disadvantages

• Compared to hydraulic drives the pneumatic forces and moments are much lower, since the operating pressure is usually below 10 bar (example: When a piston diameter of 200 mm and a standard operating pressure of 6 bar can be a driving force of 18800 N achieve ).
• During the movement phase of pneumatic drives a drive chamber is filled, the change in size during the movement. The additional volume must be filled while moving continuously. The dynamics of pneumatic actuators is therefore rather small in comparison with electrical drives.
• Pneumatic components, which are traversed quickly, can be cold and it even freeze. The same is true for components that perform a lot of work during the operation, eg Air Motors.
• For the required compression of the air a certain expenditure of electrical energy to the compressor is required. Due to thermodynamic processes to the large quantity of heat is produced. This is not a direct indicator of energy losses (see section " exergy " ), yet studies show that high losses occur due to the mechanical and thermal processes during compression. Especially in old and poorly maintained facilities, the overall efficiency of pneumatic systems is therefore often low.
• Conventional pneumatic cylinder movements are always point -to-point. The final position is each defined by a fixed stop. Precise selection of a particular position is only possible with expensive servo-pneumatic systems because of the compressibility of the air.
• Outflow pressure air causes noise. As a countermeasure, the exhaust air can be transported in focus or be fired through a silencer in the ambient air.
• Depending on the application places an elaborate air conditioning necessary to eg To ensure oil-free compressed air to limit the particle size contained to a minimum or to reduce the dew point ( or else risk the formation of water and icing in valves ).
• Air is compressible. Compressed air is let down to atmospheric pressure, the volume increases by a multiple. A bursting pressure air storage can therefore have a devastating effect in confined areas. Therefore, subject to pressure vessel above a certain size to a regular audit requirement (cost ).
• Leaks in compressed air systems cause a loss of compressed air. Unlike, for example, a fault in electrical systems ( eg short circuit) this represents in pneumatics but no security problem outflow air causes no harm, there is no smoke, the temperature of the compressed air even remains the same. While this is initially an advantage but that affects adversely on troubleshooting from. The need for repairing leaks is often underestimated. In addition, leaks are difficult to locate. Therefore, in particular occur in older plants often have large leakage losses, which may lead to a low efficiency of the overall system.
• The correct design and sizing of a pneumatic system can be quite complex, but necessary to ensure an efficient and energy-saving operation. Poorly designed systems often have a low efficiency.

Conclusion

Each drive technology, pneumatic, hydraulic and electric actuators, has specific advantages and disadvantages. It is not possible to make a blanket statement about the general best technology. The design of optimal application can normally be made only after a careful analysis and consideration of all aspects, both in terms of energy consumption, but also with regard to other aspects such as flexibility or cost.

Symbols and Schematics

An extensive collection of symbols for storage, pumps and compressors, cylinders and valves for pneumatics can be found in the following list of character ( fluid power ).

A circuit diagram (also diagram ) is the plan of a pneumatic system. The components are standardized by character ( colloquially also called symbols ) are shown. These plans are part of the required documentation to each facility, especially important for creating and maintaining equipment.

Circuit diagrams can be created according to standards individually, or company-specific. You can represent parts such as labor and control circuits, the steps of the workflow, the components of the circuit with its labeling as well as the lines and connections. The spatial arrangement of the components is not considered in a " simplified circuit ".

Applications

Industrial compressed air is used as an energy source in Germany since about the beginning of the 20th century to power hammers and drills.

Add flour mills suction pneumatic is used eg for ship unloading and pressure pneumatic passages for promotion, or the promotion of flour and Nachprodukten. These systems with small dimensions allow horizontal and vertical transport in the same direction.

In organ-building of the late 19th and early 20th century, the pneumatic action was predominant. Since about 1960, the pneumatics in the control and automation technology plays a significant role.

In the postal sector played the pneumatic tube, a pneumatically operated handling equipment, until the mid 20th century a significant role.

Self -playing musical instruments like the pianola were pneumatically controlled.

Lego Technic Pneumatic Also used as working with low pressure air is safe even for children.

Fluidic logic was realized about as DRELOBA.