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LECTURE &#;14
PNEUMATIC SYSTEMS. AIR TREATMENT AND PRESSURE REGULATION
14.1. AIR TREATMENT STAGES
For satisfactory operation of the pneumatic system the compressed air needs to be cleaned and dried. Atmospheric air is contaminated with dust, smoke and is humid. These particles can cause wear of the system components and presence of moisture may cause corrosion. Hence it is essential to treat the air to get rid of these impurities. The air treatment can be divided into three stages as shown in Figure 14.1.1.
Fig. 14.1.1. Stages of air treatment
In the first stage, the large sized particles are prevented from entering the compressor by an intake filter. The air leaving the compressor may be humid and may be at high temperature. The air from the compressor is treated in the second stage. In this stage temperature of the compressed air is lowered using a cooler and the air is dried using a dryer. Also an inline filter is provided to remove any contaminant particles present. This treatment is called primary air treatment. In the third stage which is the secondary air treatment process, further filtering is carried out. A lubricator introduces a fine mist of oil into the compressed air. This will help in lubrication of the moving components of the system to which the compressed air will be applied.
Filters
To prevent any damage to the compressor, the contaminants present in the air need to be filtered out. This is done by using inlet filters. These can be dry or wet filters. Dry filters use disposable cartridges. In the wet filter, the incoming air is passed through an oil bath and then through a fine wire mesh filter. Dirt particles cling to the oil drops during bubbling and are removed by wire mesh as they pass through it. In the dry filter the cartridges are replaced during servicing. The wet filters are cleaned using detergent solution.
Cooler
As the air is compressed, the temperature of the air increases. Therefore the air needs to be cooled. This is done by using a cooler. It is a type of heat exchanger. There are two types of coolers commonly employed viz. air cooled and water cooled. In the air cooled type, ambient air is used to cool the high temperature compressed air, whereas in the water cooled type, water is used as cooling medium. These are counter flow type coolers where the cooling medium flows in the direction opposite to the compressed air. During cooling, the water vapor present will condense which can be drained away later.
14.2. MAIN LINE FILTER
These filters are used to remove the water vapors or solid contaminants present in the pneumatic systems main lines. These filters are discussed in detail as follows.
Air filter and water trap
Air filter and water trap is used to:
· prevent any solid contaminants from entering in the system;
· condense and remove water vapor that is present in the compressed air.
Fig. 14.2.1. Air filter and water trap
The filter cartridge is made of sintered brass. The schematic of the filter is shown in Fig. 14.2.1. The thickness of sintered cartridge provides random zigzag passage for the air to flow-in which helps in arresting the solid particles. The air entering the filter swirls around due to the deflector cone. The centrifugal action causes the large contaminants and water vapor to be flung out, which hit the glass bowl and get collected at the bottom. A baffle plate is provided to prevent the turbulent air from splashing the water into the filter cartridge. At the bottom of the filter bowl there is a drain plug which can be opened manually to drain off the settled water and solid particles.
Refrigerated dryers
Fig. 14.2.2. Refrigerated dryers
It consists of two heat exchangers, refrigerant compressor and a separator. The system circuitry is shown in Figure 14.2.2. The dryer chills the air just above 0 °C which condenses the water vapor. The condensate is collected by the separator. However such low temperature air may not be needed at the application. Therefore this chilled air is used to cool the high temperature air coming out from the compressor at heat exchanger 2. The moderate temperature dry air coming out from the heat exchanger 2 is then used for actual application; whilst the reduced temperature air from compressor will further be cooled at heat exchanger 1. Thus, the efficiency of the system is increased by employing a second heat exchanger.
14.3. LUBRICATORS
Fig. 14.3.1. Air lubricator
The compressed air is first filtered and then passed through a lubricator in order to form a mist of oil and air to provide lubrication to the mating components. Figure 14.3.1 shows the schematic of a typical lubricator. The principle of working of venturimeter is followed in the operation of lubricator. The compressed air from the dryer enters in the lubricator. Its velocity increases due to a pressure differential between the upper and lower changer (oil reservoir). Due to the low pressure in the upper chamber the oil is pushed into the upper chamber from the oil reservoir through a siphon tube with check valve. The main function of the valve is to control the amount of oil passing through it. The oil drops inside the throttled zone where the velocity of air is much higher and this high velocity air breaks the oil drops into tiny particles. Thus a mist of air and oil is generated. The pressure differential across chambers is adjusted by a needle valve. It is difficult to hold an oil mixed air in the air receiver as oil may settle down. Thus air is lubricated during secondary air treatment process. Low viscosity oil forms better mist than high viscosity oil and hence ensures that oil is always present in the air.
14.4. PRESSURE REGULATION
In pneumatic systems, during high velocity compressed air flow, there is flow-dependent pressure drop between the receiver and load (application). Therefore the pressure in the receiver is always kept higher than the system pressure. At the application site, the pressure is regulated to keep it constant. There are three ways to control the local pressure, these are shown in Figure 14.4.1.
Fig. 14.4.1. Types of pressure regulation
· In the first method, load X vents the air into atmosphere continuously. The pressure regulator restricts the air flow to the load, thus controlling the air pressure. In this type of pressure regulation, some minimum flow is required to operate the regulator. If the load is a dead end type which draws no air, the pressure in the receiver will rise to the manifold pressure. These type of regulators are called as «non-relieving regulators», since the air must pass through the load.
· In the second type, load Y is a dead end load. However the regulator vents the air into atmosphere to reduce the pressure. This type of regulator is called as «relieving regulator».
· The third type of regulator has a very large load Z. Therefore its requirement of air volume is very high and can&#;t be fulfilled by using a simple regulator. In such cases, a control loop comprising of pressure transducer, controller and vent valve is used. Due to large load the system pressure may rise above its critical value. It is detected by a transducer. Then the signal will be processed by the controller which will direct the valve to be opened to vent out the air. This technique can be also be used when it is difficult to mount the pressure regulating valve close to the point where pressure regulation is needed.
14.5. RELIEF VALVE
Relief valve is the simplest type of pressure regulating device. The schematic of its construction and working is shown in the Figure 14.5.1. It is used as a backup device if the main pressure control fails. It consists of ball type valve held on to the valve seat by a spring in tension. The spring tension can be adjusted by using the adjusting cap. When the air pressure exceeds the spring tension pressure the ball is displaced from its seat, thus releasing the air and reducing the pressure. A relief is specified by its span of pressure between the cracking and full flow, pressure range and flow rate. Once the valve opens (cracking pressure), flow rate depends on the excess pressure. Once the pressure falls below the cracking pressure, the valve seals itself.
Fig. 14.5.1. Relief valve
Service units
During the preparation of compressed air, various processes such as filtration, regulation and lubrication are carried out by individual components. The individual components are: separator/filter, pressure regulator and lubricator. Preparatory functions can be combined into one unit which is called as «service unit». Figure 14.5.2 shows symbolic representation of various processes involved in air preparation and the service unit.
Fig. 14.5.2. Service unit components and service unit symbol
Impurities in the compressed air of a pneumatic system can result in the formation of condensate in the piping, greatly affecting the performance of a pneumatic system. To prevent that, pneumatic equipment requires clean, dry, and oil-free supply of air. We need to treat and filter the air passing through a pneumatic system to ensure that. In this article, we will examine the processes and components involved in that air treatment.
A compressed air treatment can be divided into three stages as illustrated below. In the first stage, we have a compressor inlet filter and aftercooler removing large particulate matter and water content respectively from the air. The air, still wet, is collected in the wet air receiver. From the wet air receiver, the air is passed through a refrigeration dryer and general-purpose filter for primary air treatment. The treated air is collected in the dry air receiver.
Figure 1. Overview Schema of Pneumatic Air Treatment
The air in dry air receiver, depending upon the primary treatment, is suitable for general industry use. For specific applications, like medical applications or automotive paint jobs, the air is treated further by passing through finer filters, which is the secondary treatment of air.
2. Preliminary Filtration
As the name gives away, preliminary filtration is performed before core air treatment. It is performed to prevent large particulate matter from damaging the compressor and fine filters and remove easily removable moisture from the air.
While air used in compressors is, usually, sourced from outside the plant to avoid contaminant-intense plant air, the outside air, still, has a dust concentration of 15-45 mg/m3. To prevent these dust particles from entering the compressor, coarse filters are used. These filters prevent more than 99 % of particles from entering the compressor. The usual filter types and their respective characteristics can be seen in the table below.
Filter Type
Characteristics
Paper Filters
Most common intake filter
Not recommended for reciprocating compressors.
Not suitable for air above 80°C
Fabric Filters
Stronger than paper filters
Link to Comair
Cleaned by the backflow of air or washing
Oilbath Filters
Used in high contamination areas.
Collect impurities equal to the mass of the oil
Labyrinth type oil wetted filters
Used for small compressors
Require regular cleaning
Inertial filters
Perform coarse filtering
Used as prefilters for other filter types
Table 1. Types of Pneumatic Filters
Intake filters are only effective at removing particles larger than 5 microns from entering the compressor. Smaller particles, water vapours, gases, and microbes are still allowed to pass through. Of these contaminants, the amount of water vapours is limited by the allowable amount at saturation. For example, 1 m3 of air can contain 12.7 g of water at atmospheric conditions at 15° C. This saturation limit increases with an increase in temperature and decreases with an increase in pressure.
While a compressor increases both pressure and temperature, the net effect is a reduction in the saturation limit. For example, when compressed to 4 bar and 35°C, the same air will retain only 7.87 g of water. Thus, 4.83 g (12.7-7.87) of water per m3 of air will have to be drained from the compressor.
As a reduction in the temperature can further reduce the saturation limit, an aftercooler is always used after the compressor. For example, if the air in the previous case is cooled to 25°C, an additional amount of 3.10 g (7.87-4.77) of water per m3 of air can be removed.
3. Primary Air Treatment
After the aftercooler, the air is stored in the wet air receiver (wet because air still contains moisture). Many pneumatic devices involve sudden expansion of air, therefore, the remaining moisture is still problematic. It can result in the production of ice in the passage and block exhaust, rendering a tool useless. Similarly, the moisture in the air causes corrosion and reduces the effectiveness of lubrication. Thus, these contaminants are to be removed. The usual devices for which are below.
3.1 Pre-filters
Pre-filters are usually installed right after the air receiver in cases where expected contamination is very high. It contains filter elements of sintered stainless steel, sintered bronze, or polypropylene with pore sizes of 5-25 microns. The air flows from inside to out, leaving the larger contaminants inside the filter and protecting the finer filter devices downstream.
3.2 Refrigerant Dryer
Dew point temperature of the air is the temperature at which the air is saturated with water vapours. Any decrease in temperature from that point results in the production of condensate. To prevent that from happening, the dew point temperature of the compressed air is to be maintained lower than the ambient air temperature.
Refrigerant dryers are commonly used for reducing the dew point in pneumatic systems. They facilitate heat exchange between a refrigerant and the compressed air, thus cooling the air. The air is cooled to 2-5 °C, resulting in the removal of excess water and lowering of dew point temperature. The cool air is warmed through the heat exchange with the inlet air to increase the efficiency of the system and to bring the air back to the system&#;s operating temperature.
3.3 General Purpose Coalescing Filter
Using pre-filter, we get rid of most of the particles above 3-5 microns. The smaller dust particles, oil droplets, and debris in the pipeline will, however, still be present in the air which can damage valves and cylinder downstream. To prevent that from happening, we use a general-purpose coalescing filter in most applications.
The construction and working of a general-purpose coalescing filter are illustrated below. At the air inlet, there are angled louvres, which cause the air to start spinning about the centre. Due to the centripetal force, the heavier particles are thrown against the wall. From where they fall toward the filter bowl, to be removed. The remaining air passes through the filter elements toward the air outlet. The coalescing type filter elements are successful in removing most of the aerosol particles (> 1 micron) and solid particles (> 0.4 microns).
Figure 2. Pneumatic Coalescing Filter Cross-Section
4. Secondary Air Treatment
After primary air treatment, the air is forwarded to the dry air receiver. The air treatment so far should be sufficient for most of the typical applications. Nonetheless, for sensitive applications like those in pharmaceutical industries and optical industries air require even stricter treatment. Therefore, air the collected in dry air receiver is passed through additional devices of secondary air treatment prior to its point of application. The usual devices involved in secondary air treatment are discussed below.
4.1 Absorption Dryer
In applications like air spraying, air bearings, and photographic film processing, any moisture content is extremely damaging. The dew point temperature requirements are usually below freezing point up to -70 °C, rendering refrigeration dryer inadequate. As a result, we require chemical drying to remove the moisture content and reduce the dew point temperature.
The usual absorption dryers are of the regenerative type and occur in twin tank construction, each of which contains desiccant material like silicon dioxide in the form of a gel to absorb moisture. During operation, one tank is absorbing the moisture, while the other is regenerating by hot-air flow as shown in the figure below.
Figure 3. Air Dryer
4.2 Activated Carbon Filter
In critical applications like those in hospitals and the pharmaceutical industry, we have stricter requirements for odour and oil vapours. For such cases, a filter with activated carbon in finely granulated form is used. It removes oil content up to 0.01 microns, which is considered oil-free.
4.3 Lubricator
The working of a lubricator is similar to that of a carburettor. The air passing through a lubricator is passed through a venturi ring, causing an increase in velocity and a decrease in pressure. As the pressure drops, the oil present in the linked chamber starts to move toward the venturi ring in the form of droplets due to pressure difference. These oil droplets mix with the incoming air to form a fine mist, which is sufficient to lubricate the downstream devices.
4.4 Which device should you include?
As hinted in the first figure at the top, the required air treatment depends upon the end application. At a given plant, one compressor may be supplying air for both spray painting and general-purpose applications. It would be expensive to use the air standards of spray painting on general-purpose applications. Therefore, it is recommended to design the air treatment system for general purpose applications and install secondary treatment for specific applications. In that case, the table below gives a general overview of when to use a specific device in your system.
Contaminant
Criterion
Requirements
Devices
Applications
Water
Dew Point
Above 10
No additional device
General Purpose
3 to 10°C
Refrigerant Dryer
Construction and mining
-7 to 3°C
Single Tower Desiccant Dryer
Brick and glass machines; Machine cleaning air
-70 to -7°C
Regenerative Desiccant Type Dryers
Spray painting, Food handling, and Photographic film processing
Oil
mg/m3
25
No additional device
Construction, mining, and welding
5
Pre Filter
Power circuits and machine cleaning air
1
Coalescing filter
Packaging and textile machines
0.1
High-efficiency coalescing filter
Conveying of power products
0.01
Activated Carbon Filter
Photographic film processing and food handling
Dirt
Particle Size (μm)
Above 40
No additional devive
Sandblasting
40
No additional device
Mining and machine tools
5
Pre Filter
Spray painting and pneumatic cylinders
1
General Purpose Coalsesching filter
Air bearings and food handling
0.1
High efficiency coalescing filter
Photographic film processing
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