Pneumatics is the teaching of all technical applications in which compressed air is used to do work. In contrast, hydraulics describes the use of a liquid as a working medium.
Compressed ambient air is called compressed air (outdated: compressed air). Compressed air can be used for many different purposes, for example as active air for conveying substances (e.g. conveying air or painting), as process air that is integrated into a process (e.g. drying) or as test air. Pneumatics therefore only make up a small proportion of all compressed air applications.
Conventional compressed air systems often work with 6 bar overpressure (relative pressure), which means that they are about seven times the atmospheric pressure. The pressure level in high pressure networks for pneumatic applications with high power requirements can be up to 18 bar, but then special components (hoses and connecting pieces) must be used that can withstand this high pressure. In special cases (e.g. when manufacturing PET bottles) the pressure level in the compressed air network can be up to 40 bar.
Every compressed air system consists of 4 sub-systems: compressed air generation, compressed air preparation, compressed air distribution and the actual application. Compressed air is generated by sucking in and compressing the ambient air in a compressor and, after processing (filtering, drying), is supplied to the application via a compressed air network (pipes and hoses) and used technically in it.
In pneumatic applications (controls and actuators ), the compressed air is used to perform work. It is usually directed to the desired location via valves. In a pneumatic cylinder, for example, the air is used to apply force to a cylinder piston and thus to move it in a certain direction.
Pneumatics is considered to be a simple technology that is inexpensive to purchase. Often, however, compressed air generation is said to be of low efficiency. In recent years this has led to discussions and the increased search for alternative technologies, e.g. due to the growing awareness of the topic of energy efficiency . B. electric drives . Practice shows, however, that depending on the application, a decision must be made as to which drive technology is most economical in terms of energy and economy. General statements are not possible in most cases.
Compressed air generation
The compressed air required to operate pneumatic systems is generated in a compressor . Usually an electrically driven motor generates a mechanical movement that is transmitted to reciprocating pistons or compressor screws. Atmospheric air is first compressed via suction and discharge valves and then pushed out into the compressed air network or an upstream air reservoir.
In systems with a high demand for compressed air, several compressors are often operated in a network. Unregulated large compressors are used to provide the basic requirement, the peak load is often covered by a speed-regulated compressor. A suitable control system coordinates the operation of the entire compressor system and ensures the most efficient possible operation.
Different types of compressors can be used depending on the pressure required and the desired delivery rate. For example, multi-stage reciprocating compressors are particularly suitable for generating high output pressures with rather low delivery rates. Screw compressors , on the other hand, tend to generate a lower outlet pressure with a larger flow rate.
Due to mechanical and thermodynamic processes, a large amount of heat is generated during the compression of the compressed air, which has to be removed from the compressed air. In many older systems, this waste heat remains unused. The overall efficiency of the pneumatic system can, however, be increased significantly if the generated heat is put to a sensible use, for example as heating, process heat (for hot water production) or, as required, to generate cold for room air conditioning ( adsorption chillers ).
Compressed air treatment
It is important to ensure the quality of the compressed air, as contamination of the compressed air in the application can influence the function of the pneumatic components or even lead to permanent damage. The compressed air can be prepared centrally or decentrally. Central processing takes place near the compressor station before the compressed air is fed into the distribution network. In contrast, the decentralized processing takes place directly before use in order to guarantee the compressed air quality required by the components.
Suitable filter systems are used to remove solid contamination. Refrigeration dryers , adsorption dryers or membrane dryers extract the water vapor from the compressed air and thus lower the dew point . This ensures that when the temperature drops in the components, no water vapor is deposited and the surfaces are damaged by corrosion.
A maintenance unit is usually placed in front of the pneumatic application , in which the locally required compressed air quality can be generated via various filter levels. Particles down to a size of 0.01 µm can be largely removed using fine filters.
Filters, dryers and pressure regulators represent a flow resistance in the pneumatic system . As a result, they generate a pressure drop when flowing through, which can be very high, especially if the filters are not cleaned regularly. A pressure loss always has a negative effect on the energy balance of the compressed air system and should therefore be avoided if possible. That is why the principle of "filtering only as much as necessary" applies.
Compressed air distribution
The compressed air distribution from the compressor to the consumers takes place via pipes and is comparable to an energy line such as B. a power cord. The quality of the compressed air should suffer as little as possible, i. H. Contamination from rust, weld scale, water or other substances should be kept to a minimum.
In addition, it must be ensured that the pipelines have a sufficient diameter so that the flow resistance can be kept as low as possible. If the diameter of a pipeline is halved, its flow resistance increases by a factor of approximately 32. This means that the resistance of a pipeline increases with the 5th power when the diameter is reduced.
Changes to the pipe direction must be considered separately, especially if narrow and non-rounded elbows are to be used. The flow resistance in such pipe elements can be much greater than in comparable straight pipe sections.
The compressed air is distributed via pipe networks with different topologies. Depending on the arrangement of the buildings and different requirements, the use of a ring structure or a meshed topology is recommended. In addition, the distribution should be secure (pressure equipment directive, industrial safety ordinance, technical rules for pipeline construction) and economical (documented dimensioning / documentation of the hazard analysis).
Particular attention is paid to locating and eliminating leaks when setting up and maintaining pipeline networks . Since leakage points in pneumatic systems only allow compressed air to flow into the environment, there is usually no safety or environmental risk from the leakage. Nevertheless, leaks should always be carefully removed, as they sometimes account for a large proportion of the total energy consumption.
When planning and dimensioning compressed air networks, specifically placed compressed air reservoirs can have a positive effect on the robustness of a compressed air network. This can be particularly useful if sporadic consumers with large amounts of air have an impact on the pressure stability in the entire network and thus also negatively affect the switching behavior of the compressor station. Compressed air storage can then briefly smooth out these high consumption levels and stabilize the network pressure.
Control system (valves)
- Pressure valves ,
- Special valves (e.g. proportional valves ),
- Shut-off valves ,
- Flow control valves and
- Directional control valves .
Number of switch positions
There are different numbers of switching positions: They range from 2 to 6. Mainly only 2 or 3 switching positions are used in industrial and automation technology because of the manufacturing costs, whereby valves with 2 switching positions are used in "normal" directional control valves for switching processes and such with 3 switching positions can be used as valves with stop function.
Number of connections
The number of connections varies between two and seven connections. With 2/2-way valves, there is only a normal passage from A to B (technically expressed from 1 (P) (= compressed air connection) to 2 (A) (= working connection)). So you can z. B. in painting or spinning machines on and off blowing functions. In the case of 3/2-way valves, in addition to the two connections mentioned above, there is also a bleeding connection that is able to bleed the hoses or the entire system. These 3/2-way valves are used z. B. in the control of single-acting cylinders, but also to "unlock" "new ways" of the pneumatic system.
With five connections there is a compressed air connection 1 (P), two working connections 4 and 2 (A and B) and two ventilation connections 5 and 3 (R and S). The 2 working connections are needed, for example, to control a double-acting cylinder, whereby one pressurizes the cylinder on one side with compressed air (so that it extends) and exhausts it on the other side (so that it can retract).
There are four connections on 4/2-way valves. The mode of operation is the same as with the 5/2-way valves, but the two ventilation connections were connected by an internal hole in the component (one compressed air connection + two working connections + one ventilation connection = four connections). Control connections are not counted as connections.
Note: The P for the compressed air connection stands for “Pressure” and the R for the ventilation connection stands for “Reset”. After the new DIN - standards of the compressed air port P "1", the working connections A / B "2" or "4", and the exhaust ports R and S "3" and "5" are marked. Control connections (required for pilot operated valves) are designated with X, Y or Z or 12, 14. “14” means that a signal at this connection enables the path from 1 to 4.
Type of actuation
Various types of actuation are used in pneumatics. These are divided into mechanical, electrical, pneumatic and manual actuations.
Mechanical actuations are plungers, springs, rollers, roller levers. Mechanical operations are operated by the machine itself. For example, if the piston of a cylinder hits the plunger of a valve, the valve is (mechanically) operated.
Electrical actuation takes place e.g. B. via a button that closes a circuit with an electromagnet in the electrically operated valve . The control slide in the valve - which closes and opens the path - is attracted and thus one path is opened for the air and, if necessary, another closed. Piezoelectrically operated valves use a piezo element instead of the electromagnet and are faster and more energy-efficient, but at the expense of the possible stroke.
Pneumatic actuation: The valve is actuated by the compressed air. For example, by manually actuating a valve, the working connection of the same is opened, and the pressure reaches another valve, which is actuated by compressed air. The valve slide just described is pressed into the desired position by compressed air. The example described is also referred to as " remote control ". Check valves can also be counted among the pneumatically operated valves.
Manual actuations are buttons, push buttons, levers and pedals. These are operated with muscle power. If a lever is moved, the valve spool addressed in "electrical actuation" is shifted in the desired direction and a different switching position is adopted.
In addition to the form of remote control already explained, valves can also be pilot controlled, which requires less primary switching energy. First the application example : A large volume flow is to be activated with a small switching force . When the power of z. B. pneumatic actuation is not sufficient to bring a valve to switch (as is the case, for example, with a pneumatic sensor ), this small switching force can in turn control a greater switching force in order to operate the valve. In electrically operated valves, the principle of pilot control is used particularly often because in this way u. a. large volume flows can be controlled with small, energy-efficient and inexpensive magnets. The main disadvantage of pilot operated valves is the greater switching delay that arises from the sequence of actuations. In addition, they are only functional from a certain pressure in the pilot control circuit (typically> 2 bar).
System for performing work (drives or actuators)
Compressed air can be used to drive air motors in tools such as B. Pneumatic hammers can be used for riveting and pneumatic screws. In control technology, linear drives in the form of cylinders are mainly used. These pneumatic cylinders are z. B. used for clamping and feeding workpieces in machining centers or for closing packaging. Compressed air can also be used directly to transport material using pneumatic tubes .
- Pneumatic motors for rotating movements
- Pneumatic muscle ,
- Cylinder for straight movements (e.g. for clamping ) and
- Cylinder with gear for swiveling movements.
In pneumatics, a distinction is made between cylinders that can be pressurized with compressed air on one and both sides (single-acting, double-acting cylinders). In the case of cylinders that can be acted upon on one side, the cylinder is returned to its original position by means of a spring integrated in the cylinder , while in the case of cylinders that can be acted upon on both sides, the forward and return strokes are carried out by appropriate control of the compressed air flow.
Compressed air and energy consumption
The energy consumption in pneumatic components is mainly determined by the air consumption. In most cases, the air consumption is given in standard liters or standard cubic meters per unit of time or per movement cycle. A standard liter describes the volume that a certain air mass occupies under normal conditions. Ambient pressure and ambient temperature according to ISO6358 are usually assumed as standard conditions.
The standard volume is proportional to the air mass and independent of the current pressure. In contrast, the operating volume indicates the real physical volume of the compressed air in the current pressure state. For example, if a pneumatic cylinder with a diameter of 32 mm and a length of 0.25 m at 6 bar rel. filled with compressed air, it then contains approx. 0.2 l of operating air. Under normal conditions, this corresponds to 1.4 normal liters.
If the air consumption of a system is known, the electrical energy consumption of the pneumatic components can be estimated using characteristic values of the compressor system. Depending on the size and effectiveness of the compressors used, an amount of energy of 0.1 kWh is usually required to generate one standard cubic meter of compressed air (at approx. 8 bar rel.).
Pneumatic energy has a general reputation for being a relatively expensive form of energy, the efficiency of which must be assessed critically in comparison to alternative drive technologies. The efficiency of pneumatic systems is often relatively low, so replacing pneumatic drives with electric drives is being considered.
The reason for this assessment is not (as is often assumed) due to the thermodynamic processes during compression in the compressor and the resulting waste heat. Inadequate design and maintenance of pneumatic systems is often responsible for low overall efficiency. The functionality of pneumatic components is usually still guaranteed even with incorrect design, oversizing, even with severe leakage and defects in the components, but the air consumption can increase sharply in such cases. Correct planning and design as well as fault monitoring (e.g. with leak detection) are therefore essential.
In most cases, the air consumption of pneumatic components can be calculated relatively easily using the geometry and the size of the volumes to be filled. If, for example, a pneumatic cylinder with a diameter of 32 mm is used to lift a load of 1 kg by 0.25 m, each double stroke generates an air consumption of approx. 2.8 Nl (the internal volume of the cylinder is 0.2 l, es is filled with 7 bar abs., so requires 1.4 Nl for one stroke). However, due to its area, the pneumatic cylinder could have lifted a load of approx. 45 kg, so it is greatly oversized and could, for example, be replaced by a drive with a diameter of 12 mm. The air consumption is reduced by approx. 85% to 0.4Nl, as the volume to be filled is also much smaller with a smaller drive diameter. An effective measure to reduce air consumption can therefore be to replace oversized cylinders with drives with a suitable diameter.
Practical application often shows that the level of the supply pressure can be adjusted within certain limits. If a system was oversized in the planning phase, the supply pressure can be reduced from 6 bar to 5 bar, for example. The general air consumption is reduced by approx. 15%.
An important aspect to improve the system efficiency is the elimination of existing leaks. Compressed air escaping at leakage points usually does not pose a safety risk and does not cause any environmental pollution. For this reason, leakages are often given little importance, and maintenance of the relevant system components is often delayed. A total leakage in a system, which corresponds to a nozzle diameter of 3 mm, can generate energy costs of over 5000 € per year in a system.
Consideration of the efficiency based on the "exergy"
Compressed air energy has a general reputation for being a relatively expensive form of energy, the efficiency of which must be assessed critically in comparison to alternative drive technologies. The reason given for this is usually the amount of heat generated during compression in the compressor , which is often dissipated unused as waste heat. In order to be able to show an exact picture of the energetic relationships in the individual processes in pneumatic systems, the thermodynamic processes must be analyzed and evaluated individually.
In the theoretical ideal case , the ambient air sucked in in the compressor is compressed isothermally , i.e. H. without temperature change. Any heat generated is dissipated immediately during the process. The compressor brings work into the system during the compression process, while heat is dissipated at the same time. For an ideal gas (a useful approximation for air) the amounts of work and heat are equal. The same amount of heat must be dissipated as the compressor generates during compression. The resulting heat is not an indication of a loss of energy or even poor efficiency, because it is only "pushed out" of the compressed air. In terms of energy, this means that the energy content of the air has not changed during compression, the energy efficiency of compressors is zero because compressed air contains as much energy as ambient air. This statement is confirmed in thermodynamics by the fact that for the ideal gas both the internal energy for closed systems and the enthalpy for open systems are only functions of temperature. The pressure has no influence on these two energy quantities. With isothermal processes, the energy content of a system does not change.
This finding shows that a benefit analysis of compressed air on the basis of the thermodynamic energy concept is not expedient, because although compressed air has the same amount of energy as ambient air, it can be used technically and can do work. A thermodynamic variable that better depicts this relationship is exergy . It indicates how much work a system can do when it is brought into equilibrium with its environment, ie what “workability” is present in a system. In contrast to energy, exergy is dependent on the temperature and pressure status as well as the environmental status.
An exergetic consideration of the idealized compression process shows that in the final state of compression, exactly that work ability is stored in the compressed air that was applied to work by the compressor. So there is no system-related reason that would explain the poor efficiency of pneumatic systems based on thermodynamic processes.
Real compressors, however, do not work isothermally, but are usually closer to adiabatic compression. The compressed air is hot after leaving the compressor and is only then cooled to room temperature. The adiabatic compression requires more energy and exergy is lost, or it is contained in the higher temperature of the waste heat.
Practical studies show, however, that in pneumatic systems a large proportion of the existing exergy is indeed lost during compression. In addition to the temperature increase, start-up and idling losses of the electric drive motors and mechanical losses due to friction are responsible for this. Further losses can result from a drop in pressure in processing, distribution and control. If a certain proportion of the compressed air is lost on the way to use in leaks, this also has a negative effect on the exergy balance. The actuators usually also have a low exergetic efficiency: In the case of pistons, the compressed air is usually simply discharged unused after the work cycle. Compressed air motors tend to work adiabatically, i.e. they cool down during operation and therefore perform less than isothermally possible. Overall, in reality, therefore, often relatively low degrees of efficiency must actually be expected.
Advantages and disadvantages of pneumatics
- The forces and speeds of the pneumatic cylinders can be adjusted continuously by selecting a suitable pressure level and using flow restrictors.
- Compared to hydraulic systems, compressed air systems have a lower energy density, but they are usually larger than with comparable electric drives. Comparatively high forces can be achieved in a small installation space.
- Pneumatic drives allow powerless holding with constant force.
- Pneumatic systems are robust against overload and insensitive to temperature fluctuations.
- Pneumatic actuators allow high operating speeds (standard cylinder up to 1.5 m / s, high-power cylinder 3.0 m / s, air motors to 100,000 min -1 )
- In most cases, the use of air as the drive medium ensures sufficient cooling of the drive components. Additional cooling is not necessary in the pneumatic application.
- Smaller leaks in the system do not cause any environmental pollution from escaping fluid (only energy loss).
- The viscosity of compressed air is relatively low compared to hydraulic oil. There are therefore only slight flow losses in pipes and hose lines.
- Pneumatic drives have a relatively simple structure and are therefore cheaper than electric drives with comparable performance data.
- No return lines are necessary.
- Pneumatic systems are explosion-proof (important in hazardous areas)
- Compared to hydraulic drives, the pneumatic forces and torques are significantly lower, as the operating pressure is usually below 10 bar (example: with a piston diameter of 200 mm and a standard operating pressure of 6 bar, a driving force of 18.8 kN can be achieved) .
- Pneumatic components can get cold during adiabatic expansion and even freeze up, e.g. B. air motors.
- For the required compression of the air, a certain amount of electrical energy is required on the compressor. A large amount of heat is generated here due to thermodynamic processes. Although this is not a direct indicator of energy losses (see section “Exergy”), studies nevertheless show that high losses occur due to the mechanical and thermal processes during compression. The overall efficiency of pneumatic systems is therefore often low, especially in old and poorly maintained systems.
- Conventional pneumatic cylinder movements are always point-to-point. The end position is defined by a fixed stop. Due to the compressibility of the air, exact approach to a certain position is only possible with complex servopneumatic systems.
- Escaping compressed air causes noise. As a countermeasure, the exhaust air can be transported away in a collected manner or released into the ambient air via a silencer.
- Depending on the application, complex air treatment is necessary in places to B. to ensure that the compressed air is free of oil, to limit the particle size contained in it to a minimum or to reduce the dew point (otherwise there is a risk of water formation and icing in valves).
- Air is compressible. If compressed air is expanded to atmospheric pressure, the volume increases many times over. A burst compressed air reservoir can therefore have a devastating effect in closed rooms. This is why pressure vessels of a certain size are subject to regular inspection (costs).
- Leakages in pneumatic systems cause a loss of compressed air. Unlike, for example, a fault in electrical systems (e.g. short circuit), this does not pose a safety risk in pneumatics. Escaping air does not cause any damage, no smoke is formed, and the temperature of the compressed air remains the same. While this is initially an advantage, it has a detrimental effect on troubleshooting. The need to eliminate leaks is often underestimated. In addition, leaks are difficult to locate. Therefore, especially in older systems, large leakage losses often occur, which can lead to a low degree of efficiency of the overall system.
- The correct planning and design of a pneumatic system can be relatively complex, but is still necessary to ensure efficient and energy-saving operation. Poorly designed systems often have a low level of efficiency.
Circuit symbols and circuit diagrams
An extensive list of symbols for accumulators, pumps and compressors, cylinders and valves in pneumatics can be found in the following list of symbols (fluid technology) .
A circuit diagram (also circuit diagram) is the plan of a pneumatic system. The components are represented by standardized circuit symbols (colloquially also called symbols). These plans are part of the documentation required for every system , particularly important for the creation and maintenance of systems.
Circuit diagrams can be created individually, company-specific or according to standards . You can use parts such as B. represent working and control circuits , the steps of the work flow , the components of the circuit with their identification as well as the lines and connections. The spatial arrangement of the components is not taken into account in a “simplified circuit”.
In flour mills , suction pneumatics are used for. B. used for ship unloading systems and pressure pneumatics for conveying passages or for conveying flour and by-products. These systems with small dimensions enable horizontal and vertical conveyance in one line.
In motor vehicles , work machines and trailers, compressed air is used in both braking and chassis systems. The chassis is adapted to the load and the terrain with the help of compressed air. This adjusts the height of the chassis and adjusts it to the payload.
In organ building in the late 19th and early 20th centuries, the pneumatic action was predominant. Self-playing musical instruments such as fairground organs, piano orchestrions and self-playing pianos such as B. the pianola were also pneumatically controlled, but the latter mainly with negative pressure, so-called suction wind.
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