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COMPRESSED AIR FUNDAMENTALS
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Fundamentals of Compressed Air Technology Compressed air is used so often in technological situations that any listing of applications must remain incomplete. No industrial or handicraft operation can forgo compressed air; no hospital, hotel, power plant or ship functions without it. It is needed in mining, in laboratories, in airports and in harbours. Compressed air is just as important in producing foodstuffs as in a cement plant and is as necessary in the production of glass, paper and textiles as in the lumber and pharmaceutical industries. Compressed air tools are used to tension, spray, polish and grind, as well as to stamp, blow, clean, drill and transport. Using compressed air, all types of machines and devices in countless applications are pneumatically started, driven and controlled. In addition, many chemical, technical and physical processes and procedures are regulated and monitored using compressed air.
But what is compressed air? Compressed air is compressed atmospheric air. Atmospheric air is the air that we breathe. It is a mixture of different gases: 78% nitrogen, 21% oxygen and 1% other gases. The state of a gas is described by three units: the pressure p the temperature T the specific volume Vspec
Not using compressed air as an energy source is Air behaves like an ideal gas over broad ranges in unthinkable in our highly technological world. pressure and temperature. Therefore, a linear correlation (ideal gas law) exists between the three units p, T and Vspec. This is described by the p× V spec = constant T General gas equation: Naturally, atmospheric air, as with all gases, is made up of molecules. If the air molecules are hindered in their heat motion, for example, by sealing it into a container, they collide with the container wall, resulting in a pressure p. The force exerted by the pressure p on a flat surface A is then: F=pxA
What is pressure? We are constantly surrounded by atmospheric l An additional unit of pressure is the bar: pressure, as a glance at a barometer easily shows. The various possible pressure ranges are diffe1 bar = 105 Pa = 0.1 MPa rentiated as follows: In compressed air technology, the operating - Atmospheric air pressure = Pamb pressure is usually given as the overpressu- Overpressure = P0 re in bar. Previously used units such as atm - Underpressure = -P0 (1 atm = 0.981 bar overpressure ) are no - Absolute pressure = Pabs longer permitted. (see Figure 1)
l The SI unit for temperature is degrees Kelvin (K). The relationship with degrees Celsius (degrees C), which is also permitted, is:
T(K) = t(degrees C) + 273.15 Overpressure
Barometric air pressure
l The volume V is used in compressed air technology to specify, for example, receiver sizes. To determine the delivered quantity of a machine that is generating or consuming compressed air, the air volume flow Veff (equals the air volume V per time unit) is used. If compressed air flows at a speed v through a tube with the cross-section A, volume flow Veff Veff = A x v
Figure 1: Representation of the pressure ranges
Units: l The recommended unit of pressure, which was bindingly introduced in 1978 with the international unit system (SI system), is the Pascal (Pa): N Pa = m²
l The air volumeflow represents the compressed air throughflow of a machine. The usual units for the air volume flow are: - l/min - m3/min - m3/h In practical applications with piston compressors, the unit l/min is used for specifying the air volume flow; with screw compressors, the unit m3/min is used.
Air volume flows can only be compared if l Another unit is of fundamental interest for comthey are at the same pressure and the paring compressors: same temperature. the specific power consumption Pspec This shows in kW (kilowatt) how much perforIn today's compressed air technology, the air mance is required to generate a volume flow of volume flow is nearly exclusively given for the 1 m3/min. air delivery amount of air compressors. In Power consumption (kW) addition to the measurement of other perforPspec. = mance data, the determination of the volume Volume flow (m³/min) flow is specified in the German DIN 1945 and in the ISO 1217 regulations. For example, if a compressor has a volume flow of 6.95 m3/min and consumes 42.9 kW of Standardised and oft-used reference units for power, the result is a specific power conpressure and temperature of the air are: sumption of p0 = 1.013 bar/t0 = 20 degrees C or p0 = 1.013bar/t0 = 0 degrees C l The volume flow is often given in norm cubic metres per hour (m3N/h). The norm cubic metre of air determined according to DIN equals an air volume of 1 m3 at p = 1.013 bar and t = 0 degrees C. During the comparison of volume flows as a measurement of the air delivery of compressors, the location of the measurement also has a significant influence on the result. It depends on whether the measurement is made on the suction side or on the pressure side of the compressor or, for example, at the exit of a complete compressor unit. Volume flows can only be compared if they are measured at the same pressure, temperature and location.
42.9 kW = 6.18 kW/(m³/min) 6.95 m³/min
The specific power consumption is probably the most important parameter for comparing different compressors in terms of their constructive quality. It provides information about the compressed air one obtains for the input energy. However, it is important that the comparisons are always made at the same operating pressure. For reasons of comparison, one must also pay attention - at which final pressure the values were measured, - whether the power consumption was measured on the compressor shaft or on the drive motor input side. Finally, the effectiveness of the drive motor and any existing belt drive or gear drive must also be taken into account.
Generation of Compressed Air What are compressors?
Compressors are machines for compressing gas and steam. In these machines, a compressing stage provides the compression itself.
The figure below provides an overview of the various compressor models. Compressor Piston Rotation
Maximum pressure in bar
Compressor models 100
Figure 2: Overview of the most important compressor models
Rotation and piston compressors form part of the displacement compressors. In this case, the air to be compressed placed in a space and compressed by decreasing the space. Turbo compressors, on the other hand, are dynamic compressors. The air to be compressed is subjected to energy and accelerated to a high speed. Pressure is increased by delaying the accelerated air.
Suction volume flow in m³/h
Figure 3: Areas of use of the most important compressor models
Figure 3 provides an overview of the areas of use of the most important compressor models. Compressed air technology uses mainly piston and screw compressors. In this chapter, there-fore, we will limit the discussion to these two models.
Piston compressors In the case of piston compressors, pistons are moved linearly to and fro in cylinders. The pistons are generally driven by a crank pinion with a crankshaft and connecting rods. Up to five connecting rods can be arrayed on one crank of the crankshaft.The inflowing and outflowing air is controlled by autonomously opening and closing valves.
Single stage model: The cylinders are equal in size. Both suction in air, compress it and transport it in a common pressure line.
Double stage model: In the first stage, the air is compressed to an intermediate pressure. After intermediate cooling, it is brought to the final pressure in a second cylinder. The relationship of the cylinder diameters to each other constructively There are piston compressors with one or several determines the value of the intermediate pressucylinders and with a fixed V- or W-shaped cylinder re. The piston displacement of the second stage is array. An additional differentiation criterion is the significantly less than that of the first stage because the pre-compressed air at the entrance of the number of compression stages. second stage has a significantly lower volume. Using the example of a twin-cylinder compressor Autonomous compact valves control the inflow with a V-shaped cylinder array (see Figure 4), the and outflow. The relationship of the stage pressure difference between single and double stage com- is determined in such a way that approximately pression, as well as the actual compression proce- the same amount of work is carried out in both stages. A V-shaped array of the cylinders and an dure, will be explained in detail: equal weight of the pistons for the first and second stages, aided by the counterweight on the crankshaft, permit a good mass balance.
Figure 4: Double stage compression in a piston compressor
1: Suction filter 2: Inlet valve 3: Outlet valve 4: First compression stage 5: Intermediary cooler 6: Second compressor stage 7: Crankshaft
Double stage piston compressors offer a lower drive performance per m³ of generated compressed air compared to single-stage machines. The intermediate cooling after the first stage results in a volume decrease of the compressed air and thus approaches isothermal compression. In practical terms, this savings in work compared to single stage compression at the same motor performance means that the volume flow at 10 bar is ca. 20% higher. An additional advantage is the lower temperature in the cy1inder space; for this reason, this model is extremely stable for larger units up to 15 bar, even if the unit is permanently operating. Piston compressors are usually driven by electric or combustion motors. The force between the drive unit and the compressor is transferred directly, using a clutch or, if a more flexible adaptation of the speed is required, using V-belt(s).
Compression occurs according to the following An increase in temperature is connected to the procedure (see figure 5): increase in pressure; this can be represented by the following equation: T2 = T1 x (P2/P1)(K-1)/K, where K = 1.38 to 1.4
Top dead centre
re mp Co
Bottom dead centre
4 Suction Piston movement
For oil-lubricated air compressors, the maximum possible pressure increase in a compression stage is limited by the maximum permitted final compression temperature. The upper limits are, for example, depending on operating conditions according to the German Accident Prevention Regulations (UVV, VBG 16), between 160 degrees C and 220 degrees C. The result of these temperature limits is the following rough approximation of the required number of compression stages for the desired final pressure (see Table 1):
Figure 5: Air compression stages
When the piston moves downward from the top dead centre, the pressure in the compression space decreases to below the suction-side pressure (point 4). The input valve opens and the air flows from the suction side into the compression space.
Final compression pressure
Number of compression stages
to 10 bar
6 - 40 bar
20 - 250 bar
120 - 350 bar
200 - 450 bar
The piston now moves upward and the pressure in Table 1: Number of compression stages depending on the the compression space increases. As soon as it is final operating pressure greater than the suction-side pressure, the input valve closes (point 1). The air heated during compression is cooled The pressure now continues to rise until it is hig- again in air coolers incorporated in the compresher than the outlet-side pressure (point 2). The sor and which follow after the compression stages. outlet valve opens and the compressed air is Due to physical factors, the entire driving energy released until the top dead centre is attained. required for operating a compressor is converted into heat, which must be removed. In the case of At the start of the concluding downward motion, piston compressors, this functions using air or the pressure in the cylinder space decreases very water cooling. Due to their simpler construction, quickly and the outlet valve closes again (point 3). air-cooled piston compressors are more common design.
Screw compressors Screw compressors form part of rotation compressors. These are compressors whose compression spaces are decreased by a rotary movement. Single and double stage rotation compressors have become popular on the market. A significant advantage of most compressors of this system is the total mass balance which allows a vibration-free installation.
1st phase: The air enters the compression housing through the inflow opening. The thread gaps of the rotors are then filled with air in a manner similar to the suction stroke of the piston compressor.
2nd and 3rd phases: When the rotors have turned past the inlet opening, they form a closed compression space betThe air end of a screw compressor of the con- ween the thread gaps and the housing. This struction mentioned above contains two rotors decreases in size due to the counter-rotating arrayed in parallel. One of these has a convex movement of the rotors; the enclosed air is comscrew profile while the other has a concave screw pressed. profile. The two profiles are in gear. In an opposite rotary direction, the air is compressed between Compression continues until the compression the profiles due to the different tooth numbers of space, which is steadily decreasing in size, reathe rotors according to the displacement princi- ches the outside edge of the outlet opening. ple. 4th phase: The procedure can basically be divided into four The compressed air flows out. phases (see figure 6):
Figure 6: Compression phases for screw compressors
Oil-injected screw compressors
In the case of oil-injected screw compressors, generally only one rotor is driven: the male rotor. Since the pair of rotors is interlocked, the female rotor turns automatically with every movement of the male rotor. The oil that is continuously injected into the air end prevents the metallic contact between the rotors. In addition to lubricating the compressor air end, the oil has two other important jobs: it seals the gap between the rotors as well as between the rotors and the housing, and it removes the heat produced during compression
Screw compressors are often used at a high base load at continuous operation (100% full load operation). Since ca. 80% of the installed motor performance is transferred to the oil in the case of oilinjected screw compressors (at an oil temperature of 85 degrees C), this energy can be used to heat household water and heating water (up to 70 degrees C).
The oil is injected during phase 2 into the compression spaces - ca. 1 litre per minute per kilowatt of drive power. It flows through the air end together with the air. That what leaves the air end is therefore a compressed air-oil mixture. Due to the very high oil content, the CE Machinery Directive does not permit the compression temperature to exceed 120 degrees C.
When a comparison of screw compressors is made, a significant amount of attention must be paid to the specific performance. Also pay attention which performances (main driver or ventilator motor) are included! In addition, make sure that the volume flow was measured at the outlet side of the system and not on the compressor block. Performance trials of compressors must be carried out according to the German DIN 1945 T1 or ISO 1217, where the required measurement conditions are determined.
The compressed air-oil mixture first flows into the compressed air/oil tank. There, the air and the oil separate mechanically. The oil, which has absorbed the main portion of the heat energy produced, is then cooled in an oil cooler and can then be re-injected.
Any remaining oil particles are then removed from the compressed air in a downstream oil separator before the air leaves the compressor. Oil-injected screw compressors work with final compression overpressures of 4 to 15 bar.Volume flows of 0.5 m3/min to 70 m3/min are attained with motor performances of 4 kW to 400 kW. The noise level with noise insulation is between some 63 and 80 dB(A). Due to their low-vibration running, screw compressors can be set up directly on a flat hall floor without a special foundation; due to their good noise insulation, they can also be set up in working rooms. When setting them up, heed the corresponding Accident Prevention Regulations.
Oil-free compressors There are many areas of application, especially in the chemical, pharmaceutical and foodstuff industries, in which oil-free compressed air is required. For this reason, various oil-free compressors were developped: oil-free piston compressors, drycompressing screw compressors, rotary tooth compressors and many others. An alternative in certain areas is paraffin oil-lubricated compressors, because paraffin oil, as opposed to mineral oil, is not toxic.
that is contained in the suction air condenses. In the case of oil-injected compressors, condensate would lead to damage - in the case of water-injected compressors, it even has a positive effect: it renews the water in the plant circulation (during continu-ous operation under normal ambient conditions) by itself within a few hours. This continuous re-generation practically eliminates the collection of dirt within the compression plant.
Water-injected screw compressors come close to ideal "isothermal" compression. Compared to the Dry-compressing usual oil-injected machines, this means a drastic energy savings - up to 20%! In addition, the temscrew compressors perature load of the components is minimised. In the case of dry-compressing screw compres- Therefore, the water-injected system guarantees sors, a synchronous drive that drives both rotors an especially high operational safety, particularly prevents metallic contact between the two rotors. in critical application areas. In addition, the remoHowever, this drive significantly increases the val of used oil, oil-containing condensate, oil filters price of the air end and the lack of cooling by the and oil removal cartridges are no longer necesoil permits compression in only one stage to 3.5 sary; the corresponding removal costs are also elibar. A intermediate cooler and a second stage are minated. required for further compression to 10 bar. Dry-compressing compressors have a significant- The technical realisation of water-injected screw ly lower effectiveness than the oil-injected ones. compressors occurred through the use of patented polymer-ceramic materials and newly developed, highly precise manufacturing procedures. A Water-injected screw compressors new injection system that has also been patented atomises and optimally distributes the water. This This is not the case for water-injected screw com- guarantees the practically complete transfer of the pressors. They are currently state of the art and produced compression heat with a low water combine the advantages of oil-lubricated and oil- throughput. free compressors: absolutely oil-free compression single-stage to 13 bar at an optimum effectiveness. The main characteristic of the new compressor generation is the replacement of compressor oil by the most natural, most environmentally friendly and, at the same time, least expensive fluid: water. Water is distinguished by its high specific heat capacity and heat conduction. Using specifically dosed injections into the compression space, the temperature increase during the compression procedure can be limited to ca. 12 degrees, regardless of the final pressure. Return cooling of the produced compressed air is thus no longer required. The circulating water can be cooled to near the ambient temperature. Then the moisture 10
Application Areas for Compressors in Compressed Air Technology
In addition, high pressure compressors are used in hydropower plants, other power plants, rolling mills, in the oil and gas industry, for pressure and sealing tests, in the airline and shipping indus-tries and for marine application.
Low pressure range (4-15 bar) In the largest application area for compressed air, in the low pressure range of 4-15 bar, single and double stage oil-lubricated piston compressors and single stage oil-injected screw compressors are chiefly used. All of the applications described in the introduction are covered by these compressors.
Mid-pressure range (16-40 bar) At mid-range operating pressures of 16 to 40 bar, generally 2- or 3-stage piston compressors are used; in the case of very large compressor ratings, double stage screw compressors are used. These compressors are mainly used to start diesel motors with higher ratings such as those in ship drives and stationary diesel power plants. In addition, they are used in industry for, for example, sealing tests and in the processing of plastics (i.e. Figure 7: Four-stage water-cooled marine high pressure comPET blowing). pressor of Sauer design Figure 7 shows a four-stage water-cooled piston compressor for the Navy with a air delivery of 160m3/h at a final pressure of 350 bar and a speed In this pressure range, only multi-stage piston of 1800 rpm. Due to the star-shaped ar-rangement compressors or the related membrane compres- of its four cylinders and the balance of its crank sors are used (ignoring turbocompressors for mechanism, this compressor is practically vibrativery high ratings). There are many different appli- on-free in operation. cation areas for high-pressure compressors. The high pressure is required in most application areas to store large amounts of air in as small as possible receivers. An example is the generation and storage of breathing air in bottles with 200 and 300 bar pressure, such as those used by divers and fire-fighters.
High pressure range (up to 400 bar)
Compressor Control Units - Principles
The job of the compressor control unit is to adapt the generation of compressed air to the consumption of compressed air. There are three types of control units for compressors: start/stop control,full load/unload control and the delayed idling control.
The compressor works between two pressure values set by pressure switches. The running compressor increases the working pressure until the cut-out pressure is attained. Then the pressure switch switches off the compressor. Due to the consumption of compressed air in the network, the network pressure decreases again. When it reaches the restart (cut-in) pressure, the pressure switch restarts the compressor to produce compressed air (see Figure 8).
t Dropout portions
Figure 8: Start/Stop control
Full load / Unload control
Delayed dropout controller
In this controller type, the compressor is switched to idling when the cut-out pressure is attained. It continues to run at idling without generating compressed air. The idling lasts until the restart pressure is reached, where the compressor enters a new production phase. The idling operation has two important functions: it limits the number of motor switchings, and the residual heat after compression is removed more efficiently (see Figure 9).
This type of control combines the advantages of the other two. If the cut-in pressure is not reached, the compressor switches off after a specified time. When the cut-in pressure is attained, the compressor starts up from the standstill (see Figure 10).
0 Idling portions
Figure 9:Full load / unload control
0 Idling portions
Figure 10: Delayed idling control
Control Units - Practice The following pages provide a representation of the range of functions of modern high-performance compressor control systems. The introduction of microprocessors in the last few years has increased the performance of the controllers enormously. This is not merely due to the number of possible functions that can now be put into practice; rather, it also indicates a significantly more intelligent method of operation using the input energy, in addition to a strong increase in the cost effectiveness of compressor systems.
Controllers for multiple systems If several compressors within a compressor system work on the same compressed air network, it is sensible and required that the compressors have the operating modes "basic load" and "peak load". In order to evenly load several compressors of the same size within a station, these compressors are alternatingly switched into the basic load mode and the peak load mode using a basic load switching system. Such a switching system works automatically (time-dependent) or can be operated manually.
Figure 11: Operating panel of a microprocessor controller for multiple systems
ALUP control unit
DISPLAYS - Network pressure - Operating state
Air delivery amount
Load operation of all compressors
Time and date
Total operating hours
Automatic functions Switching on/off the compressors by reaching the cut in/cut off pressure
Restart after power failure (programable)
Lead / Lag control system for several compressors
Group controller for up to 10 compressors
MONITORING Motor switching frequency
Network pressure too high
Protection from incorrect entries
DOCUMENTATION Load and operating hours
Basic values in case of interruptions
Annual switching calendar
LINK External malfunction alarm
Description of symbols: X = standard; O = option
Microprocessor controllers The greater the number of controller and monito- On the following pages, you will find an overview ring functions that are to be put into practice, the of the range of functions of modern powerful comearlier the use of an electronic microprocessor pressor controllers. controller for the compressor makes sense (see Figure 11a). Especially in the case of screw compressors, these controllers, which fulfil all the required controller and monitoring functions of a compressor system, are offered in series.
Figure 11a: Operating panel of a microprocessor controller
ALUP control units Suitable for ALUP compressors of series
CT - SCK - SCG
DISPLAYS - Network pressure - Final compression temperature - Operating state Maximum compression temperature
Cut-in and cut-off pressure
Time and date
Total operating hours
Remaining lifetime for - air filter - oil - oil filter - oil separation cartridge
Direction of motor rotation
Selection of the most cost effective operating mode
Switching off the system in case the limit values are exceeded
Multiple units control Maximum number of grouped compressors
Lead/lag control system for several compressors
Switching on the heater if oil temperature is too low
Restarting after power failure (programmable)
Description of symbols: X = standard; O = option
ALUP control units Suitable for ALUP compressors of series
CT - SCK - SCG
MONITORING Monitoring - Suction filter - Oil - Oil filter - Oil separation cardridge
- Oil change intervals
Temperatures - min. - max - compression end temperature
Motor switching frequency
Network pressure too high
Protection from incorrect entries
Load and operating hours
Annual switching calendar
Two additional selectable outputs
External malfunction alarm
Remote display using PC
Basic values in case of interruptions
Description of symbols: X = standard; O = option
Sound Insulation Sound power level Energy is required to generate every sound wave, and a portion of this energy carries each sound wave with it.The remainder is emitted into the air as friction heat. The power range is extremely large: Sound results from mechanical vibrations that are quiet whispering has a value of 0.00000001 watts transferred to the air. The vibrations can be cau- while starting a jet aeroplane has a value of 100,000 sed by very many sources: from a vibrating area watts. To simplify the use of these values, they are (for example, the covering of a machine) as well logarithmically represented as a "sound power as from flowing gases or liquids. Sound is therefo- level" with the unit "decibels" (dB). re pressure distorting the air, a wave that is super- (see Table 2) imposed on the atmospheric pressure. For human detection of sound, it is required that the frequency of the vibrations lies in the range between 16 Hz and 20 kHz. The detection of most sound consists of an overlapping of several sound sourSound power Sound power ces with different frequencies. Sound source (W)
Screw compressor with sound insulation
Screw compressor without sound insulation
Table 2: Relationship between sound power and soand power level
Sound pressure level
Although the sound power level defines the energy of a sound source, it provides no information regarding how it is detected by human hearing. This is defined by the sound pressure level, which is the term for the logarithmic sound pressure that is based on the human threshold of hearing at a frequency of 1000 Hz. The sound pressure level is represented in decibles (dB). Since it is dependent on the distance from the sound source, the distance between the sound source and the measurement point must always be specified.
During the operation of compressors, sound pressure level values of above 85 dB(A) can occur. According to, i.e. the German Worker's Protection Regulations, sound protection must be used starting at 85 dB(A). Therefore, it is often not only advantageous but also necessary to equip compressors with sound insulation.
Sound insulated compressors can be set up near the workplace, preventing the costs for long piping systems and for separate compressor rooms. In addition, the pressure loss in the pipeliDuring a measurement of the sound pressure nes is kept to a minimum. level according to DIN 45635, the measurement points are located on the surface of a "quadratic Certain requirements are made on the sound surface".This is a theoretical space with a height of insulation materials; they must be both non-com1.5 m and 1 m distance from the main surfaces of bustible and impervious against dust and oil. For the compressor (see Figure 12). this reason, mainly mineral wool and fluorocarbon-free self-extinguishing foams that are integrated into the covering plates are used.
Point 2 Point 4 Point 1
Figure 12: Arrangement of the measurement points on the quadratic surface.
Cooling and Room Aeration When you design a compressor station, take into consideration that the compressors convert the entire consumed power into heat. This requires that the room where the compressor is set up has adequate ventilation and aeration. This can be achieved with air inflow and exhaust openings and can be supported using ventilators. In some cases, the installation of air inflow or exhaust channels is required. Precise information regarding ventilation and aeration is given in the German VDMA unit sheet 4363.
100% electrical energy
While heat is removed directly at the point of origin by air or water cooling in the case of piston compressors, the heat from oil-injected screw compressors is first transported out of the compression block to the air or water cooler, where the heat exchange and removal then take place. Figures 14 and 15 show how the heat is distributed in different compressor types.
10% motor loss 5% compressed air residual heat 3% radiation 10% compressed air post cooling
72% Oilcooler Figure 14: Heat flow in oil-injected screw compressors
100% electrical energy
10% motor loss 5% compressed air residual heat 5% radiation 40% compressed air intermediate cooling
40% Oilcooler Figure 15: Heat flow in double stage piston compressors
Utilisation of Waste Heat
Hot air for heating
The heated cooling air is used for heating rooms using a airduct system. Using temperature-controlled flaps, a controlled, adjustable room tempeDuring operation of a compressor, the input ener- rature is attained. The length of the airducts is limigy is converted completely into heat and must be ted to some 4 to 8 metres. Additional ventilators removed by cooling. Heat can be covered and are required for longer channels. It is recommenused from the cooling media air, water and oil ded to contact the compressor manufacturer for either directly or using heat exchangers; it can getting detailed data. then be used. This makes it possible to reduce energy costs. In winter, the heat of the exhaust air is partially or completely used for heating; in summer, it is As can be seen in Figure 14, heat recycling from blown outside via an exhaust airduct. the 85 degree C hot oil is particularly attractive for (see Figure 16) screw compressors. This is very simple using oilwater heat exchangers. The three possibilities for utilising heat are represented in the following figures:
Heated air summer operation
Heated air winter operation
3 2 1
Figure 16: Heated air for room heating
1: 2: 3: 4:
Screw compressor Temperature controller Air distributor with flap controller Air duct
Hot water for heating
Heat for household water
Simple bundled tube heat exchangers are used to The heat recycling procedure is the same as that prepare hot water. for warming heating water (see the arrangement in Figure 18). Using safety heat exchangers or The heating water is fed through a bundled tube intermediate circuits prevents oil from entering in a closed sleeve. The hot compressor oil flows the household water, even if there are defects. This between the tubes and the sleeve, releasing heat is attained using a double tube in which two tubes energy into the heating water. are coupled. The water that is to be heated flows through the inner tube. A blocking medium is plaThe arrangement is not complicated ced in the space between the two tubes, whose (see Figure 17) and the extra investment is rather pressure is monitored. In case of a breakthrough, low. Due to the savings in heating costs, the system the monitor triggers an alarm. can be amortised in less than one year.
6 4 3
Figure 17: Hot water for heating
Figure 18: Heat for potable water
1: 2: 3: 4: 5: 6: 7: 8:
1: 2: 3: 4: 5: 6: 7:
Screw compressor Heat exchanger Circulating pump for heat recycling Expansion vessel for heat recycling Additional heating boiler Circulating pump for heating circulation Heater thermostat Heater
Screw compressor Safety heat exchanger Circulating pump Hot water tank Hot water consumer Water inlet Additional heater (electric)
What is the Use of Compressed Air Purification?
Therefore, parts of the humidity are removed as condensate due to the compression. In water, impurities are dissolved, and the whole thing forms an aggressive mixture that can attack the compressor and the pipes. Corrosion may result. And a disgusting mixture of condensate, rust parIn order for you to understand the material, here ticles and starter traces from the compressed air are a few basic remarks: pipes is transported to the connected machines In order to generate a cubic metre of compressed with the compressed air. It should be obvious that air with an overpressure of 10 bar, a compressor such machines wear significantly faster than must suction 11 cubic metres of ambient air. others that are operated with clean compressed Together with this air, it also suctions, just like a air; this has been proven by several on-site expevacuum cleaner, impurities contained in the air: riments. dust, steam, oil vapour and chemicals, to name just a few, not to mention the natural humidity. Despite high-quality suction filters, all of these components of the suction air are found in compressed air. The materials, which were distributed throughout the 11 cubic metres before compression, are now concentrated in a single cubic metre of compressed air.
Let us now analyse this process physically:
Humidity is nothing more than moisture in the (basically) dry air. The pressure of the humid air is a sum of PA (air pressure) and PM (moisture pressure). Dry air can take up moisture only until it reaches the dewpoint PD. If the moisture pressure Let us observe the subjects "Impurities" and increases beyond the dewpoint (PM > PD), the "Humidity" separately and first examine the sub- extra moisture condenses as fog. The capability of ject "Humidity". One can illustrate the subject as dry air to take up moisture varies depending on follows: its temperature, but is independent of the pressure. This results in the Picture the ambient air as a PM damp sponge. In a relative humidity of air ϕ rel = PD relaxed state, it can contain a certain For example, PD at 3 degrees C is 0.007576 bar, amount of but at 20 degrees C, it is 0.02337 bar. At a relative water without humidity of 70% and 20 degrees C, PM is 0.01636 dripping. bar. (See the following table, where the moisture pressure is already converted into mass proportions g/m3.) If one presses this sponge together, a portion of the water Since the capability of air to take up moisture runs out of it - but only a part. is independent of the pressure, these relationships do not change if the air is comEven if one wrings the sponge out, not pressed. all of the water will be removed.
Examples 8 m3 of moist air with a pressure of 1 barabs equals 1 m3 of moist air with a pressure of 8 bar, but the dewpoint pressure does not change. The moisture pressure Pd equals 0.02337 bar before compression and 0.18696 bar after compression (without a temperature change). Since the dewpoint pressure Ps remains constant at 0.02337 bar, 7/ of the contained moisture condense. 8
Here is an example for this: Starting conditions: - 8 m3 of air - 20 degrees C - 1 bar (abs) - ϕrel = 70% - Moisture content = 17.3 x 0.7 = 12.11 g/m3
Conditions after compression: - 1 m3 of air - 80 degrees C - ϕrel = 96.88/293.3 = 33% - 8 m3 of air - Maximum moisture content of the air - 20 degrees C at 80 degrees C = 293.3 g/m3 - 1 bar (abs) - Actual moisture content of the compressed air - ϕrel = 70% = 8 x 12.11 = 96.88 g/m3, which is less 3 - PD = 0.0234 bar = 17.3 g/m than 293.3 g/m3; 3 - PM = 0.0164 bar = 12.11 g/m therefore, moisture does not condense. Conditions after compression:
- 1 m3 of air - 20 degrees C - 8 bar (abs) - ϕrel = 100% - PD = 0.0234 bar = 17.3 g/m3 - PM = 8 x 0.0164 bar = 0.1312 bar - PM is thus greater than PD Result: moisture condenses Amount: 8 x 12.11 g/m3 - 17.3 g/m3 = 79.58 g/m3 Since you have worked with this before, you will correctly object that moisture does not condense in the compressor. This is correct. During compression, the air temperature and thus the dewpoint pressure PD increase significantly. Therefore, moisture generally can not condense in the compressor.
Therefore, the fact that moisture does not condense in the compressor is solely due to the temperature rise during compression. Compressor manufacturers make use of this fact and design their machines for an operating temperature of around 80 degrees C to prevent the formation of water pools. Compressor units, working under tropical conditions and high relative humidity, should work at even higher compression temperatures in order to prevent water condensate formation. Depending on a number of factors, including the compressor type, the final compression temperature and the final operation pressure, as well as the model of the intermediate and post-coolers, the dewpoint temperature is reached in precisely these coolers, and condensate begins to form.
Moisture content of the air at various temperatures
DEW P OI NT °C
MOI S TU RE g/m³
DEW P OI NT °C
MOI S TU RE g/m³
DEW P OI NT °C
MOI S TU RE g/m³
DEW P OI NT °C
MOI S TU RE g/m³
DEW P OI NT °C
MOI S TU RE g/m³
DEW P OI NT °C
MOI S TU RE g/m³
DEW P OI NT °C
MOI S TU RE g/m³
DEW P OI NT °C
MOI S TU RE g/m³
Example: At a dewpoint of 0 degrees C, one m3 of air contains 4.868 g of moisture.
Cyclone Separator When the compressed air leaves the compressor, it contains moisture in the form of tiny water droplets and steam. As the first stage in compressed air preparation, the droplets are to be mechanically removed using a cyclone separator at the compressed air outlet of the compressor. This functions as follows:
1 2 4 3 7 6
Compressed air and the included water droplets enter the cyclone. Due to the guide nozzle device (1), they are affected by a strong twisting motion so that they rotate in the cyclone space (2) at high speed around the cylinder axis. Due to the strong outwards-directed centrifugal forces, the water droplets are catapulted to the separator wall and then flow into the collection space (3). The collection space is separated from the cyclone space by a curved shield (4) so that the air flow cannot bring any condensate with it. The compressed air leaves the cyclone separator through a pipe and the pure gas outlet (5). The condensate is removed through an opening (6) in the floor of the collection space. It makes sense that there should be installed an electronically controlled condensate drain because the condensate level must not rise to the curved shield or beyond. Large cyclone separators have an additional inspection opening on the container floor (7); small ones must generally be disassembled for cleaning.
Drying of Compressed Air After the compressed air leaves the cyclone separator, it theoretically contains only residual moisture in the form of vapour because this is not mechanically separated and goes through the cyclone sepa-rator together with the compressed air. For further drying, various procedures can be used, depend-ing on the use of the compressed air. The following overview presents these procedures. Since we do not wish this presentation to be a purely academic list, we have marked the relevant procedures by large arrows and a black background. All other procedures require special applications (especially absorption) or are simply to uneconomic (over-compression; heat-regenerated adsorber with heating of the desiccant).
Sorption (removal of liquid)
Adsorption (chemical reaction of a desiccant)
Solid drying material
Viscous drying material
Adsorption (physical binding to a desiccant)
Fluid drying material
Over-compression and depression
Solid drying material
Cold regeneration with dried compressed air
Heating of a desiccant (internal heat regeneration)
Heating of the regeneration air (external heat generation)
Utilisation of the compression heat (oil-free compressor, full load and partial flow principle)
Regarding the individual procedure Refrigeration drying
Second phase: The compressed air flows through an refrigerant heat exchanger and cools down to the set pressure dewpoint. The remaining moisture cools to the pressure dewpoint, condenses and is automatically removed.
Figure 20: Functional schematic of a compressed air refrigeration dryer
Figure 19: Electronically controlled compressed air refrigeration dryer for volume flows up to 8000 m3/h.
Refrigeration drying is a procedure in which compressed air is cooled by a refrigerant in a heat exchanger. The moisture contained in the compressed air condenses and is removed. The larger the difference between the compressed air inlet and outlet temperatures, the larger the amount of con- Figure 20a: Funktional schematic of a TRISAB heat exchandensed moisture. The lower the cooling tempera- ger. ture of the compressed air, the lower the amount of remaining moisture. The refrigerant circulation is driven by a compressor. This compressor compresses the Drying takes place in two phases: gaseous cooling material and presses it into the condensator. There, it is liquefied and injected First phase: In an air/air heat exchanger, the warm, through an expansion valve into the refrigerant inflowing compressed air cools in the oppositely circuit, where it returns to gaseous form. This directed current of the already cold outflowing air. requires heat energy which is removed from the No additional energy must be consumed for this. compressed air. The compressed air cools until it Here, ca. 60% of the contained moisture already reaches the set pressure dewpoint pressure condenses. (see Figure 20). 30
Sorption Together with the condensate,a large portion of the oil that the compressed air contains in the case of oillubricated or oil-injected compressors is removed. It mixes with the water.This mixture must not be disposed of directly in the sewer system; rather, it must first be separated in a suitable separator into water and oil. Refrigeration dryers are generally fully equipped and wired; they must only be connected to the power supply. They are available in various sizes, differing in the volume flow, the permitted ambient temperature and the pressure dewpoint temperature. The performance range of freeze dryers is from ca. 15 to ca. 5400m3/h, where a dewpoint pressure of +2 degrees C or higher is attained at an ambient temperature of up to +50 degrees C. The required energy increases with the cooling performance that must be carried out; for example, to 14,5 kW at 8000m3/h. For 90% of all applications, refrigeration drying is the most economical way because both, the required energy and the operating costs, are significantly less than for other procedures.
Sorption means that fluid is removed from compressed air either chemically or physically. Since absorption drying plays a subordinate role in practice, we will concentrate solely on adsorption in the following sections. The principle of adsorption: The moisture that the compressed air contains is bound by adhesion force to the surface of a drying desiccant in granulated form (= adsorbate). In this process, a pressure dewpoint of up to -70 degrees C is attained. As opposed to refrigerant drying, the compressed air is not cooled. The process of adsorption itself requires no energy; energy is required only for the regeneration of the adsorbate, i.e. the removal of the deposited moisture. Since the regeneration procedure requires a certain amount of time, an adsorption dryer always consists of two vessels; one is in operation while the other is being regenerated. Until this stage, all dryers work according to the same principle; however, they differ in the type of regeneration. Basically, two procedures are available for this: cold and warm regeneration.
Cold regeneration: In cold regeneration, a part flow is branched off the dried main flow of compressed air; this is used as cleaning and regeneration air. This part air flow is first depressed. Thereby, it is strongly undersaturated with moisture. If it is then transported over the bed of desiccant to be regenerated, it absorbs the contained moisture and transports it to the outside. The regeneration air cannot be returned to the compressed air flow again and it leaves the dryer as loss air. If you lay out a compressed air plant, you must therefore calculate the regeneration air as an additional consumer!
Figure: Functional principle of adsorption drying 1: Adsorption Vessel 2: Desorption Vessel 3: Main valve 4: Upper valve block 5: Diaphragm 6: Outlet valve 7: Regeneration air exit with silencer
In warm regeneration, hot regeneration air is used for desorption. This is generated by an external heat source. Ambient air is sucked in, heated and transported through the bed of desiccant that is to be regenerated. In this procedure, temperatures between 150 and 300 degrees C are required, depending on the type of desiccant used. Regeneration is finished when the temperature at the exit of the regeneration air has reached ca. 100 degrees C. Then the bed of desiccant is cooled to the utilisation temperature with a cold cleaning air flow. As opposed to a cold-regenerated adsorption dryer, you do not need to calculate additional compressed air consumption for a warm-regenerated adsorption dryer. In a few cases only a small amount of compressed air for switching of the control valves is required (ca. 1%). The automatic switching cycles lie at ca. 4-8 hours, depending on the operating conditions. Life time of the desiccant: The life time duration of the desiccant is ca. 2000 to 4000 regeneration cycles. The following criteria can reduce the long-term ability of the desiccant to absorb water and thus the utilisation duration:
Since the balance between the remaining load of the desiccant with moisture and the partial pressure of the cleaning gas flow is attained relatively quickly, cold regeneration requires automatic swit- ching cycles of ca. 3-10 minutes. -
ageing and reduced affinity reduction of the granulate surface due to constant abrasion soiling due to oil particles in the compressed air
Filtration Several factors influence the generation of techni- the determination of the correct time to change a cally pure compressed air: filter. Generally, the time has been reached when the differential pressure is ca. 0.6 bar. A even hig• suction air that contains greater or lesser her economical operation, especially on bigger amounts of solid particles and/or chemicals, size (and more expansive) units, can be achieved depending on the local air pollution; with a microprocessor-equipped filter unit: the • the occurrence of condensate and the formati- actual pressure loss through the filter element is on of rust; permanentely monitored and its energy costs • oil-lubricated or oil-cooled compression spa- (through higher compression for equalizing the ces in compressors; pressure drop) are permanently compared with • inadequate maintenance. the costs for a new filter element. As soon as the energy costs will become more expensive than a In order to attain malfunction-free operation, dirt, new replacement filter element, a signal for filter water and oil must therefore be removed from the element change will be given. compressed air. The removal of water was described in detail in the chapter "Drying methods". After drying, the air contains very fine oil droplets The filters and separators used in compressed air and dirt only in very small quantities. Therefore, it technology can be categorised according to diffemakes sense to use filters at this point. Without rent viewpoints: pre-cooling and the preliminary removal of con- • the purpose (suction filter, pre-filter, sterile fildensate and dirt, the filter elements would becoter, oil steam adsorption filter, etc.); me soiled quickly. Due to the rapidly increasing • the working procedure (mass force separator, pressure loss that would result, the filter elements electrical separator, surface filter, membrane would have to be continuously replaced - a cost filter, depth filter); factor that should not be underestimated. • the degree of fineness (coarse filter, fine filter, microfilter); • the filter material (fabric filter, paper filter, fibre filter, sintered filters of metal, ceramic, plastic). In the filtration of compressed air, chiefly two filtration types are used: surface filtration and depth filtration.
Surface filtration In surface filtration, sieving is the main function of the separation mechanism. If the dirt particles are On the other hand, a small pressure loss cannot be larger than the defined pores, they are separated avoided. This loss can be measured with a diffe- on the surface of the filter material. rential pressure manometer and provides information about the degree of soiling of the filters. For this reason, manometers are attached to the filter head in the case of high-performance filters (see Figure 23). Using the manometer allows 33 Figure 23: Compressed air filter
In depth filtration, fleece fibres are used as the filter material; these are made up of an entanglement of very fine single fibres. These filter materials do not function only as a sieve; instead, dirt particles that are significantly smaller than the distance between the fibres are removed. A combination of several separation mechanisms is responsible for this: • the pushing action itself • adsorption • electrostatic discharge • diffusion • to a low degree, the function of the sieve • bonding due to van der Waal forces
In high-performance filtration of compressed air, the borium-silicate fibre material is the most popular of depth filter materials. Using these filters, the content of residual oil can be reduced to 0.01 mg/m3. If active charcoal layers are additionally used, a residual oil content of less than 0.005 mg/m3 can be attained.
On most pressure filter types, a combination of surface and depth filtration is effective.
Familiarity with operating conditions and compressed air quality requirements are necessary for the filter material and for chosing adequate filter types and systems. The main criteria for selecting the correct filter size are the temperature at the installation location, the delivered amount (volume flow) and the operating pressure of the compressed air . The volume flows for filters given in the manufacturers specifications are always based on a certain pressure. If your operating pressure changes, the maximum throughflow amount through the filters will also change. The amount that the throughflow is decreased or increased can be easily determined using conversion factors. These can usually be found in the manufacturers documents. At a temperature of +30 degrees C, five times as much oil passes through a filter than at +20 degrees C. If the temperature changes from +20 degrees C to +40 degrees C, the amount increases by 10 times. For this reason, you must pay attention that micro and submicro filters are installed in a location where the compressed air temperature is as low as possible.
Figure 24: High-performance filter set with 99.99% effect based on 0.01 µm particle size. 1: end cap of plastic or aluminium; 2: borium-silicate fibreglass layer; 3: steel outer support sleeve; 4: PVC-coated Styrofoam sleeve
In Germany, the Waste Removal Law (AbfG) is binding for the removal of soiled filter elements, which must be classified as hazardous waste.
Condensate drains Compressed air condensate from after-coolers, filters, dryers and pipe systems is aggressive, generally contains much oil and a large amount of dirt particles. Removing this condensate from the compressed air system poses certain problems for the user. Float-type condensate drains can stick; therefore they no longer remove the condensate. Under some circumstances, they permanently blow out the expensive compressed air. The functioning of time-controlled solenoid valves is not always reliable. Due to the removal, which is only set to certain times, not to the actual condensate deposition, they may cause significant losses of compressed air and thus high energy costs. For these reasons, electronic level-controlled condensate drains have become popular in compressed air technology. These condensate drains collect the condensate in containers with no moving parts and thus work without wear.
In electronic level-controlled condensate drains, the condensate drips through an inlet opening in a container. If the container is filled to the maximum, a capacitative level sensor sends an impulse to a solenoid valve. This opens the outlet pipe and the condensate exits. The valve only remains open as long as there is still condensate. Therefore, no compressed air can escape. In special models, condensate removers can also be used for aggressive condensates (such as those that occur in oil-free dry-running compressors), for operating pressures up to 63 bar and in explosion-endangered areas.
Oil-water separators The exiting condensate contains residual amounts of oil that can range from 1000 to 10,000 mg/l. Legal regulations prescribe preparation of oilcontaining water "according to the generally recognised technical rules" (Paragraph 7a of the German Water Supply Law). Accordingly, thinning to decrease the harmfulness is not permitted.Therefore, oil-containing condensate must be prepared in such a manner that the oil content of the water flowing out of the separation device does not exceed the permitted values. According to the German ATV (Technical Wastewater Association), Worksheet A115, the maximum value is 20 mg/l. However, the drainage statute of the responsible community is always authoritative; in certain cases, the maximum value may be lower (10 or 5 mg/l). So the local regulations will always have to be followed
Figure 25: Electronic level-controlled condensate drain.
There are two basic possibilities to satisfy the water supply statute: removal or preparation. It must be pointed out that removal of oil-containing condensate involves high costs.
Example: A compressor station with an airflow of 20 m3/min generates up to 60 m3 condensate as waste per year. The average disposal costs for this type of hazardous waste are currently (early 1995) ca. 1700 DM/m3 and will certainly increase. The operator of the compressor system will then have yearly disposal costs of hundreds of thousands of DM.
The utilisation duration of the active charcoal strongly depends on the degree of dispersion and emulsification of the oil in the water. In turn, these are determined mainly by the compressor design, the oil type used and the removal of condensate.
Normal oil-water separators are not able to separate stable emulsions. Stable emulsions can occur The less expensive alternative is condensate pre- due to high compression temperatures, poorly paration using an oil-water separator. Here, the demulsifying compressor oils as well as emulsioncondensate is fed into a separation container; the enhancing chemical substances in the suction air. dirt particles carried by the condensate are col- These stable emulsions require a special type of lected in removable containers. The condensate is preparation. separated in the separation container using gravity and transported through a filter combination of an oleophile pre-filter and active charcoal adsorption. The oil that slowly collects at the surface is transported via an oil overflow into an over-flowfree canister.
Figure 26: Function schematic of an oil-water separator.
Compressed Air Distribution They fulfil the following duties:
Compressed air receivers Compressed airoperated machines and tools require a continuous air flow for trouble-free operation. This is attained by using correctly proportioned compressed air receivers. The receivers are prime coated, laquered or internally and externally galvanised; they can be vertical or horizontal (see Figure 27).
• Compressed air storage The compressor builds up a storage volume in the receiver; this balances out varying compressed air consumption in the network, thus reducing the number of on/off cycles of the compressor. • Pulsation damping Displacement compressors, especially piston compressors, generate a pulsing compressed air flow that is damped by the volume of the receiver. • Condensate removal Due to cooling of the compressed air on the receiver walls, a portion of the condensate precipitates, collects on the receiver bottom and can then be removed from there without problems. The permitted number of times that the compressor can be switched on and off depends on the performance of the electromotor (see Table 3). Nominal performance in kW
Permitted motor frequency per hour
4 - 11
55 - 40
15 - 30
30 - 15
37 - 75
12 - 6
90 - 250
Table 3: Switching frequency of compressors depending on the performance of the drive motor.
Figure 27: Vertical compressed air receiver.
Determining the receiver size The following formula provides an approximation for determining the receiver size. In the case of multi compressor systems, this refers to the peak load compressor:
Voleff × Pa 4× Zs × ∆ p
where: V = compressed air container volume in m3 Voleff= volume flow in m3/h (ISO 1217) pa = ambient pressure in bar(a) zs = switching frequency (per hour) ∆p = switching pressure difference in bar Example: Voleff= 240 m3/h = 4 m3/min pa = 1 bar zs = 15h-1 ∆p = 2 bar 3
240 m × 1bar Vol eff × Pa h = 2,0m 3 V = = 4 × Z S × ∆ P 4 × 15h −1 × 2bar In case that a standard compressed air receiver in exactly the calculated size would not exist, one selects the next larger size.This formula applies for compressors whithout idling function, i.e. piston compressors. Compressor units with integrated idling function, like screw type units, usually can operate into a smaller receiver. It is advisable, however, to consider a certain compressed air buffer volume at fluctuating air consumption, which is the usual case in industrial air networks. The above mentioned formula can usally be applied also for screw compressor installations. IMPORTANT: The bigger fluctuations of compressed air consumption and the more such fluctuations differ to above from the compressed air flow of the compressor(s), the bigger the receiver volume should be.
Legal regulations for compressed air receivers (6th ordinance to the German Machine Safety Code, dated June 25, 1992; Pressure receiver Ordinance dated June 25, 1992) Pressure receivers of pressure volume product of 200 or more are to be inspected before commissioning; pressure receivers of pressure volume product of 1000 or more are also to be inspected repeatedly thereafter by professionals of the corresponding inspection authorities.
Constructing a pipeline network
In the case of a central compressed air supply, it is required that a pipeline system be installed; this supplies the individual consumers with compressed air. The job of a pipeline network is to provide compressed air to the consumers
It is recommended that a compressed air pipeline network be divided into individual segments (see Figure 28). The compressed air network begins in the compressor station with the main line, which connects the compressor with the dryer, compressed air receiver and filters. The pressure decrease via the main line should not exceed 0.04 bar, not including filters, armatures, etc.
• • • • • •
in a sufficient quantity, with the required pressure, at the required quality, with a smalllest possible pressure drop, safely and inexpensively.
The distributor line connects to the main line; this supplies the consumers with compressed air as a stub/ring line. Stubs are lines leading from a distributor line across the room/hall ending at a specific position. They have the advantage that they require less pipeline material than ring lines, based on the pipe length. On the other hand, individual segments of ring lines, which form a closed distributor ring, can be blocked off while still guaranteeing the supply of compressed air to other areas. A ring line requires lower nominal tube sizes. The pressure drop through the distributor line should not exceed 0.03 bar. In the case of moisture condensation, the pipeline should have a maximum tilt of 5 degrees to the lowest point so that the condensate can be collected and removed - this equals a drop of ca. 9 mm per 1 m pipe length.
2 1 3
Figure 28: Schematic of a compressed air pipeline network. 1: Ring line; 2: main line; 3: stub; 4: condensate drain; 5: consumer connection
The pipeline can be attached to the wall using pipe holders or by hanging it from the ceiling using a threaded control rod or loop. For repairs or reconstruction in segments, it is recommended that sufficient blocking equipment be planned in order to sequester pipe sections without affecting the entire system. Connection pipelines branch off from the distributor line; these provide the direct supply of the consumers. Armatures and connection accessories are used for this purpose (see page 32 ff).
The pressure decrease via the connection lines should not exceed 0.03 bar. In order to keep the connection lines as free from condensate as possible, it is recommended that the pipes from each connection line be bent upwards at 180 degrees ("goosenecks"). Although this is no longer required in the case of already dried compressed air, the pipelines should be installed this way anyway due to safety reasons.
A pressure decrease occurs due to • • • •
too low a pipeline cross-section flow obstacles in pipelines roughness of the walls leaks.
Leaks in the distributor line or on the connections to the consumer mean a high cost factor. The leaking positions act like nozzles from which air For industrial use, the German pipe size DN 25 escapes with enormous speed. Since outflowing (equal to one inch) or greater is always recom- air does not pose a direct danger, however, it is mended for the connection lines because there usually not treated with the same attention as, for are barely any cost advantages for material and example, a leak in a water pipe. assembly for smaller sizes. Therefore, consumers that require up to 1800 l/min compressed air at a The increasing required volume flow caused by nominal pipe line length of up to 10 m can be sup- leaks leads to higher energy costs in the generaplied without excessive pressure decrease. tion of compressed air. Table 5 provides an idea of the scope of energy costs caused by leaks. Table 4 shows the relationship of the pressure decrease and the volume flow at a pipe diameter of 25 mm and a nominal length of 10 m.
Volume flow in l/min Pressure drop in bar
Hole diameter (mm)
Max. leak flow at 7 bar (l/s)
Energy costs (DM/year)
Table 4: Relationship of pressure drop and volume flow
Table 5: Energy costs caused by leaks at 8000 operating hours/year and 0.2 DM/kWh.
Pipeline dimensions The amount of leaking in a compressed air system can be most easily measured by emptying the compressed air receiver. This shows in what amount of time the pressure drops by, for example, 1 bar. During the measurement, the receiver is no longer supplied with compressed air.
During construction of a new compressed air system, the pipeline dimensions are of primary importance.
Assuming that the compressed air flows out isothermally, the amount of leaking in a compressed air system can be approximated according to the following formula:
• set-up of the individual consumers
V R × ( PS − PF ) t
In order to obtain favourable dimensions, the following conditions are to be precisely determined:
• number of consumers • type of consumers • compressed air consumption of the various consumers
where VL VR PS PF t
= = = = =
Amount of leaking in l/min Pressure receiver contents in l Receiver starting pressure in bar Receiver final pressure in bar Measuring time in min
Example: VR PS PF t
= = = =
1000 l 8 bar 7 bar 2 min
1000l × (8 − 7) = 500l / min 2 min
Determination of the compressed air consumption Operation time Most compressed air machines and devices are not operated continuously. Therefore, it is important to determine the operation time as reference information for the total consumption. It is given as a factor or percentage.
Tool wear Tool wear consideres losses produced by wear caused by ageing, leaks and incorrect treatment of compressed air tools. Wear should be set to a maximum of 5% based on the total air consumption of a tool.
Table 6 in the Appendix of Chapter 2 provides examples of the operation time of certain compressed air consumers.
Pipeline diameter Pipeline diameters are either determined using a nomogram (see Figure 29) or calculated using an approximation formula:
Example: A mounting machine is in operation for 45 minutes per hour. The operation time is thus 45 = 0.75 = 75% 60
1.6 × 10 3 × Vol eff d= ∆P × P1 5
where d =internal diameter of the pipe in dm Voleff = total volume flow in m3/s Simultaneity factor L = nominal length of the pipeline in m The simultaneity factor is an empirical value. It is ∆p = pressure drop in bar based on the experience that not all consumers p1 = operating pressure in bar(g) are in operation simultaneously when compressed air consumers of the same type are in use. If the mounting machine mentioned in the above example has a switch-on duration of 75%,several independently working machines will not always run at the same time. Table 5 in the Appendix of Chapter 2 shows which simultaneity factors can be used in practice for a certain number of compressed air consumers. Example: Five mounting machines are operated in parallel. Taking into account the switch-on duration of 75%, each machine has a compressed air consumption of 200 l/min at 6 bar. If all machines always operated simultaneously, the total compressed air requirement would be 5x 200 l/min = 1000 l/min. However, since a simultaneity factor of 0.83 can be considered for five machines operated in parallel, an actual compressed air requirement of ca. 830 1/min results.
Determination of the internal diameter using a nomogram (example 1): Figure 29 shows a nomogram with which the Using a nomogram: internal diameter can also be determined. • Set the pipe length on line A and the volume flow on line B. •
Connect the points with a straight line and lengthen this to axis 1
Set the system pressure on line E and the permitted pressure drop on line G.
Connect the points with a straight line. This line crosses line D.
The pipe diameter that is to be determined lies at the intersection.
Pipe length (m)
Axis 1 Pipe width (mm) 500
System pressure (bar) 200 2 150 3
Figure 29: Nomogram for determining the pipeline diameter and pressure drop
Pressure drop (bar abs.) 0,03
Free air delivery (m³/h) 10.000
Determination of the internal diameter using a nomogram (example 2):
Using the nomogram:
Is the nomogram in Figure 29 too unclear for you • Set the air throughflow in the left column: mark or the work too difficult? Then see Figure 30. This the line with the required volume. nomogram takes only the most important para- • Determine the length of the pipeline and mark meters into account and is thus more clear. the corresponding column. • The intersection of the line and the column ends in an irregularly outlined area in which the correct diameter is located. Example: - Air flow - Length of pipeline - Required pipeline diameter
= 1000 l/min = 100 m = 1"
Air flow (l/min of free air)
Length of pipeline (m) 10 20 30 40 50 75 100 150 200 250 300 350 400 450 500
400 500 750 1000 1500
3/4" 1" 1 1/4"
3500 4000 4500
5000 6000 7000
8000 Pressure drop: ca. 0.1 bar at 8 bar Network pressure Figure 30: Nomogram for determining the pipeline diameter and pressure decrease
Additional armatures: All installed armatures (valves, brackets, elbows, Example: A G 3/4" shut-off valve has a factor of etc.) pose an additional resistance that must be 4.00; theoretically, the pipeline must thus be taken into account. The lengths that must be lengthened by 4 m. added to the length of the pipeline are found in the table.
Pipe and armature diameter G 3/8"
G 1 1/4" G 1 1/2"
Corresponding pipe length in metre
Pipe elbow r=d
Pipe elbow r=2d T piece
Materials for compressed air pipes
Criteria for selecting the material:
This section provides an overview of the advantaSeveral materials can be used within a compres- ges and disadvantages of the materials that are sed air network. The choice of the material is not most often used in compressed air pipes: only determined by the cost, but, as with all other factors in a compressed air system, depends on Steel several points. These are: • threaded tube: inexpensive, many shaped • compressed air quality parts • dimensions of the pipes • pressure • seamless: many nominal diameters; but: corro• influences from the surroundings sion and high flow resistance • installation effort • material costs • galvanised: resistant to corrosion; but: high flow • pressure drop resistance • resistance to ageing • stainless steel: resistant to corrosion, low flow resistance, sealed; but: limited number of shaped parts, expensive Copper • resistant to corrosion, low flow resistance; but: good technical knowledge required Plastic • polyamide (PA) • polyethylene (PE) • acrylnitril - butadieri - styrol copolymers (ABS) • the following applies to all: many shaped parts, no corrosion, generally simple to install; but: high length expansion, lower pressure resilience under increasing temperatures
Compressed air armatures are used in the entire supply chain, from the compressor to the consumer. They are thus very important. The characteristic of quality for compressed air armatures is the rela-tionship between the pressure drop and the air flow volume.
A maintenance unit is a three units combination of a filter, a pressure reducer and an oiler. Earlier, these were three independent components. Today, one also uses two units combinations where the filter and the pressure reducer are combined as a filter pressure reducer. So-called combi-devices also exist, consisting of one part: a filter on the bottom, a pressure reducer in the middle and an oiler on top.
The following armatures exist (see Figure 31 • • • •
maintenance units (1) cocks (2) couplings (3) hoses (4)
A compressed air oiler is absolutely required to operate compressed air tools and cylinders and to guarantee their lubrication. Installation should be carried out as close to the consumer as possible because oil vapour generated in the oiler combines over long distances to form oil droplets. Rule: normal vapour oilers should be installed at a maximum of 5 m from the consumer, proportional vapour oilers at a maximum of 10 m. Several consumers can also be connected to a correspondingly larger oiler.
Ball cocks Ball cocks are used to block compressed air pipes. They are characterised by a free throughflow without bottlenecks and cause almost no additional pressure decrease to existing distributor networks. A complete seal is attained by a brass ball turning in a Teflon seal.
Couplings Couplings are used to provide a sealed connection between two compressed air connections. Safety couplings form a special type of coupling. They can always block the flow from either side. Normal quick couplings have plugged beaks with return flow dampers to slowly aerate the decoupled hose.
Figure 31: Sampling of armatures
Hoses There are two main types of hose: spiral hoses and standard hoses. Hoses are generally made from PVC with a cloth inlay. Spiral hoses have a higher pressure drop than normal straight hoses. The longer a hose connection, the greater the cross-section that should be selected to ensure that the consumer is satisfactorily supplied with compressed air. Non-return valve with distributor prevents the return flow of compressed air into the pressure reducer in case of counter pressure that may occur.
Non-return valve with distributor
Cock with secondary aeration
prevents the return flow of compressed air into the pressure reducer in case of counter pressure that may occur.
as blocking device Cleaned, controlled and oiled compressed air
Compressed air inlet Compressed air oiler in compact block design Filter pressure reducer with manometer in compact design; return control possible; independent of pre-pressure; for temperatures of -10 to +50 degrees C (plastic) or -10 to +90 degrees C (metal).
Cleaned, controlled and unoiled compressed air
Figure 32: Example of a modern compressed air sampling station with block construction