Pneumatic Conveying in Milling
04 March 20147 min reading
Certificated Milling Engineer
Paul Bruckmann Mühlenbautechnik - Paul Bruckmann Mühlenbautechnik
1. PNEUMATIC CONVEYING FROM
THEORY TO PRACTICE
1.1 Basic information for pneumatic conveying
Pneumatic conveying systems are fundamentally different in two variants:
a) Vacuum system
In a vacuum system air and solid particles are sucked into the pipeline. At the end of the pipeline the separation of solid and liquid flow is carried out with the help of a cyclone or filter. Driven by gravity, the solid particles fall down while the air flow is being sucked upwards into the direction of the vacuum pump.
b) Pressure system
In a pressure system atmospheric air is being continuously conveyed through a blower. Behind the blower, the solid particles are injected into the air flow. At the end of the pipeline the separation of solid and liquid flow is carried out with the help of a cyclone or filter. Driven by gravity, the solid particles fall down while the air phase is flowing upwards to return into the atmosphere.
Multiple feeding points to one single unloading possible/Single feeding point to multiple unloading points possible
In pneumatic conveying we find a multiphase flow usually consisting of a fluid flow (air) and a solids flow . To describe this two-phase flow the solid mass load μ defined as the ratio of solids mass flow and air mass flow is used. Very important in terms of the influence on the flow characteristics is the air flow speed v. The speed of the product particles is always lower. Depending on the values of μ and v and also on the kind of material to be conveyed we find different conveying methods.
In the following the dilute-phase flowing that is decisive in the flour milling industry is presented. To get a better understanding what happened with a solid particle, have a look at the following scheme.
That is just the velocity a vertical air flow has to provide to keep a solid particle floating. At a higher air velocity the particle is conveyed upwards, at a lower air velocity the particle falls down.
That means, that an air flow velocity of 9,9 m/s is required to keep the corn floating. A higher velocity would lead to vertical pneumatic conveying and lift the corn.
To design a pneumatic conveying plant, it is absolutely necessary to know the properties of the product. The following table shows the essential data. The air-velocity is based on an air density of 1,2 kg/m³ .
The next important value is the mass load μ.
The working condition of a pneumatic plant is limited by the maximal mass load μ 25-30.
The average value for μ = 10
1.2 Basic equations in pneumatic conveying pipe
In the pneumatic transport as for the clean air flow is the continuity equation which states that the air flow in cross-section 1 is equal to the cross section 2:
Is the pipe cross section constant, then applies:
It means that with the decreasing pressure along the pipeline, the air velocity is increased in proportion.
V2/v1 = p1/p2
This increase in air velocity, however, is undesirable because it increases energy consumption and equipment wear. So we expanded in certain places the pipe cross-section to reduce the increasing of the air velocity. This also means that due to the higher pressure, and the associated higher air weight at the beginning of the pneumatic line, the air velocity can be reduced accordingly.
1.3 System engineering
Let us now examine what this means with reference to a realized project. The task was to feed a pellet press with bran at the other end of the premises with a capacity of 8 t/h. It is an entire conveying line of 110m horizontal, 55m vertically and a total of 18 bends with different angles. The line passes through various buildings and underground tunnel connecting the milling building to the silo in which, because of space constraints, the pelletizing was installed.
1.3.1 Steps to design the pneumatic conveying plant With these data my own developed program suggests me the pipe diameter. Now it is the experience to choose the right pipe concerning the calculated pressure difference. The mass load can be changed accordingly. In this case, the internal pipe diameter at the beginning of the pipe line was chosen with 119 mm. According to DIN 2448 the selected pipe is 127 x 4 mm.
The calculated pressure difference without the extension of the pipeline is in the given conditions 840 mbar.
As seen under 2.2 the compressed air increases the specific weight up to 2,14 kg/m³. By applying the preceding formulas, it gives the following values:
The air speed at the beginning can be reduced from 22 m/s to 16,3 m/s. Then the pipeline will be extended as follows.
By these methods it is achieved to reduce the speed at the end of the pipe from 40 m/s to 24 m/s. Also the necessary pressure difference is reduced to 750 mbar. The required air quantity incl. the calculated air leakage for the airlocks is 22 m³/min.
The next step is to choose the right blower. Due to energy saving reasons the decision was to use a Delta Hybrid with 37 kW. More information will be found in 1.4.5
1.4 Variations for the generation of the air flow.
1.4.1 Radial fan:
The centrifugal fan uses the centrifugal power generated from the rotation of impellers to increase the pressure of air. When the impellers rotate, the air near the impellers is thrown-off from the impellers due to the centrifugal force and then moves into the fan casing. As a result the air pressure in the fan casing is increased. The air is then guided to the exit via outlet ducts. After the air is thrown-off, the air pressure in the middle region of the impellers decreases. The air from the impeller eye rushes in to normalize this pressure. This cycle repeats and therefore the air can be continuously transferred.
a) Forward-curved blades are in the direction of the fan wheel's rotation. Forward-curved blades are for high flow, low pressure applications.
b) Backward-curved blades are against the direction of the fan wheel's rotation. They are used mainly for high pressure and medium flow applications. Highest energy efficiency is provided.
c) Radial fan blades extend straight out from the hub. High speeds, low volumes, and high pressures are common with radial fans, but often characterized by greater noise output.
1.4.2 Side channel blower:
The small range of pressure and air volume limits the use of these machines.
The impeller is mounted directly on the motor shaft for contact-free compression. The air is taken in through the inlet. As it enters the side channel, the rotating impeller imparts velocity to the air in the direction of rotation. Centrifugal force in the impeller blades accelerates the air outward and the pressure increases. Every rotation adds kinetic energy, resulting in further increase of pressure along the side channel. The side channel narrows at the rotor, sweeping the air off the impeller blades and discharging it through the outlet silencer where it exits the pump.
1.4.3 Roots blower:
Beside the radial fan the roots blower is the most used in the milling industry. Recently, a distinction is made between 2-lobe and 3-lobe blowers.
Blowers function by positive displacement and without internal compression (merely isochor change of state), which means the existing counter pressure determines the level of compression and correspondingly the power consumption. Change of volume flow is only possible by change of speed. Vibrations and pulsations increase with the pressure.
Development of three lobe blowers with pre-inlet ducts
Lower vibration load of bearings, timing gears and housing
• The sound energy in the pressure pipe will be lower by approx. 25 dB
• Lower pulsations are decreased directly within the blower stage
• Invest cost for building and system components
• Increase of air capacity at identical size by higher speed
Analogue to 2-lobe blowers:
• Isochoric change of condition
• Actual counter pressure determines the amount of compression / power consumption
• Change of volume flow is only possible by speed regulation
1.2.4: screw compressor
Oil free screw compressors are sealed without oil. The air in the compression chamber does not get in contact with oil, the two rotors are connected by a synchronized transmission so the surface profiles do not touch.
1.4.5: Delta hybrid compressor
- The generation of high efficiency blowers
Over a ten year operating period, energy costs equate to about 90% of the total Life Cycle Costs of a compressor.
2.5 Comparing the blower selection for an example Project
The given data: 850 mbar, 18,3 m³/min
1.6.: Solution for the separation at the end of the pressure conveying line
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