Handmade Albion

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Section 4. EXPERIMENTAL DETERMINATION OF THE PARAMETERS NECESSARY FOR CALCULATING THE AIR CONTAINERS

4.1. General questions of experimental methods

When using plastic and glass pipes, special devices are needed to remove statistical electricity from the surface of the pipes. Through every meter of the length of steel seamlessly drawn pipes, measuring washers are installed, which are figured casings welded to the pipes in places where 4 holes with a diameter of 1 mm are drilled to measure static pressures. On the surface of the aluminum and plastic pipes, a threaded hole was drilled through each meter, and static pressure fittings were installed. Holes were drilled on glass pipes in sections selected with certainty in the areas of steady motion (first with a 5 mm diameter drill with a soldered bit, and then the remaining part of the wall 1-1.5 mm thick with a needle with a broken eye). Static pressure fittings were glued to these holes on epoxy resin and connected to the battery micromanometer with rubber hoses.

    Pressure losses in product pipelines were determined based on static pressure drops or the distribution of static pressures along the length of the pipeline. To do this, measurements of static pressures were recorded using battery micro manometers. The found values ​​of static pressures (they were photographed three times) for each section in the same experiment were averaged

It is convenient to present the pressure loss in the pipeline through which the two-phase flow moves as the sum of the pressure losses during the motion of the carrier medium (clean air) - Н0 and pressure losses arising from the presence of material Nm:

                 Ncm = H0 + N m. [Pa] (4.1)

    The pressure loss during the movement of clean air is considered to be equal to the loss during the movement of the carrier medium (air) in two-phase flows of gas-solid particles.

     Before processing the data during the movement of air mixtures, it is necessary to perform such as applied to the movement of clean air and determine the dependence characteristic of the pipes of this installation for the coefficient of hydraulic resistance (friction).

     On the graph of changes in static pressures along the length of the material pipeline when moving clean air (Fig. 4.1) for any speed, their linear nature is visible. The equations of this line are written in the form:

               Hct = R0 ℓ + B sPasc (4.2)

where ℓ is the distance from the beginning of the pipe to the point of measurement of static pressure, m;

      R0 — pressure loss per 1 m of pipe length, Pa / m;

      B is a free member of the equation for the steady-state motion segment, numerically equal to the pressure loss in the material receiver, its aerodynamic drag coefficient ζпр is calculated by expression (1.5).

Fig. 4.1. 

Change in static pressure along the length of a horizontal steel material pipe with a roughness of the inner wall of 0.05 mm:

1 - when moving clean air at a speed of 22 m / s through a polished pipe with a diameter of 170 mm;

2 - when moving tobacco with a density of 55 kg / m3, through a polished pipe with a diameter of 130 mm with a productivity of 0.071 kg / s, the coefficient of external friction ftr = 0.75 at an air speed of 24 m / s.

Processing the experimental data for each straight pipe of the product according to these equations gives the values ​​of R0 when pure air moves in the straight line under consideration and the values ​​of the free terms of equation B. In cases where static pressures along the pipe length were measured at two points, then R0 is the pressure loss for such a straight were by the difference of the measured static pressures.

     Having R0 during the movement of clean air, it is possible to determine the values ​​of the coefficient of hydraulic friction of the product pipeline for a given mode of air movement:

               λ 0 = R0 D / 0.61 J 2 = R0 D / Nl, (4.3)

    Before processing the experimental data obtained when moving through clean air pipes, it is necessary to first clarify the question of which area of ​​hydraulic resistance should include the experiments conducted on the study of pneumatic transport. The Reynolds numbers in the experimental data for the pipes used were less than the value that determines the upper limit of the zone of hydraulically smooth pipes:

               Re = 2.7 (D / Δ) - 1.14, (4.4)

where Δ is the absolute roughness of the inner walls of the pipes in mm.

      The data obtained allow us to conclude that the experiments relate entirely to the zone of hydraulically smooth pipes. Therefore, the coefficients of hydraulic friction during the movement of clean air depends only on the Reynolds number and can be found by the expression:

                        λ0 = A / (Re) n. (4.5)

     Similar conclusions were made for glass pipes and for rolled steel sheet pipes through which fibrous materials were transported. As a rule, these pipes were pre-ground with sand for ≈ 100 hours, until the values ​​of λ0 are constant. If the product pipeline consisted of two straight lines connected by turns, then, as shown by carefully conducted experiments, the coefficient and in equation (4.5) for different straight lines of the same pipe can be different, in general, they did not differ much from each other.

   In experiments on straight-line steel material pipelines of different diameters, the values ​​ranged from 0.15 to 0.30, and the values ​​ranged from 0.18 to 0.88. The values ​​of the hydraulic friction coefficients calculated by these formulas do not differ much from each other. For glass pipes, they were 10-20% less than for steel.

     The nature of the change in static pressures along the length of the pipeline when a mixture of material with air moves along it is shown by the example of experiments on a pipe with a diameter of 130 mm (Fig. 4.1) at J = 24 m / s in the case of movement of cut tobacco with constant productivity. Examination of the graph shows that it is clearly divided along the abscissa into two characteristic sections:

     1- section of unsteady motion, the so-called acceleration section, where the product acquires some final speed of steady motion;

     2- section of steady motion, the points on which are located approximately in a straight line.

     Denoting by Rcm the pressure loss per 1 m of the product pipe length during steady-state movement when moving the air mixture in section 2, by analogy with the solution of this issue for clean air, we can write the equation for the direct pressure change in this section in the form:

                 Ncm = Rcm ℓ + B1, [Pa] (4.6)

where B1 is the free term of the equation for the steady-state motion section.

     If equation 4.6 is written for the straight line of the product following the receiver, then the free term in this equation is the pressure loss in the area of ​​uneven movement of the mixture, which is the sum of the pressure loss during acceleration and in the material receiver

                      B1 = Npr + Nrazg [Pa] (4.7)

     The magnitude of the pressure loss during acceleration (1.6), which, according to our researchers, depends on the concentration of the air mixture, the properties of the transported materials, and can be determined through the aerodynamic drag coefficient on acceleration, based on the criteria equations (1.8) and (1.9). Pressure losses in the material receiver are found from experiments in clean air (1.5, 4.2). The value of Rcm for inclined straight lines also includes losses for a rise of 1 m, which are calculated according to equation 1.19.

Just as in the case of moving clean air, having solved the system of equations 4.6 written for each section in which the static pressure was measured, we find the free term of the equation and the value, subtracting from which the pressure loss for lifting by 1 m (taking into account the slope of the pipe), we obtain Rcm, and by it also λcm by analogy with 4.3. For a particular mode of transporting one material at a constant speed, but with different consumption concentrations, we find that the function λcm = λcm (μ) (Fig. 4.2) can be considered linear, described by equation (1.11):       

                 λcm = λ0 + λm μ 4.8)

where λm is the coefficient of hydraulic friction caused by the movement of the transported material with concentration.

   Having solved the system of equations for any straight line under the same transportation conditions, but with different conditions, we find the values ​​of λm and λ0, which, as a rule, is not much different from λ0, found for this straight line of the product pipeline when pure air moves through it.

     In the case of measuring static pressure at two points of the product pipeline straight (these points were chosen with confidence in the areas of steady motion), pressure losses were determined per 1 m of the product pipe length R0 and Rcm by the difference of the static pressures in the measurement sections:

                   Rcm = (N cm 2 - Ncm 1) / ℓ, [Pa / m] (4.9)

where Ncm 2 and Ncm 1 - static pressure in the pipe sections, Pa;

      ℓ - the distance between measurement sections in m.

Fig. 4.2.

The dependence of the hydraulic resistance coefficient when moving the mixture λ cm on its concentration when moving the material along polished steel pipes with a roughness of 0.05 mm (external friction coefficient 0.75; bulk density of tobacco 55 kg / m3):

1 - with a diameter of 100 mm at an air speed of 24 m / s;

2 - pipe diameter of 170 mm at an air speed of 26 m / s.

The pressure drop in the branches during the movement of clean air is determined by the vortex zone at the outer wall at the inlet and the same zone at the inner wall at the outlet of the branch, and the appearance of a paired vortex in the cross-section of the pipe, as well as friction against the walls. In this case, the main role is played by vortex formation, and the paired vortex, and friction losses, according to the established procedure, are excluded in assessing the resistance of the outlet. They relate to losses along the length of the pipeline.

     The pressure loss in the turns and the sections behind them when moving the air mixture can be represented as the sum of the friction losses along its length during the movement of the air mixture Ntr and additional losses during the movement of air and material:

     Notv cm = Ndl + Ndop = Ntr + (H 0 + Nm) add, [Pa] (4.10)

     The pressure loss along the rotation length can be determined by an expression similar to (1.9).

 In the future, we will determine only additional pressure losses during the movement of air Н0 and material Nm caused by rotation, i.e. when calculating to determine the amount of pressure loss in any section of the pipeline, the sum of the straightened turns lengths included in the section should be added to the length of the rectilinear parts.

     Considering that the material-air medium is inextricable, that it is a single energy flow (not to mention the case of movement of clean air), based on the law of conservation of energy, we can imagine the pressure loss between sections I-I to the outlet and II-II after it:

         Notv cm = ΔНst 2-1 - ΔН d 2-1, [Pa] (4.11)

where ΔНst2-1 and ΔНд2-1 are the differences of the statistical and dynamic pressures between sections I1-I1 and I-I, respectively, located quite far from the turn.

     On the other hand, based on the Daria-Weisbach equation, the total pressure loss in the same section is:

              Npt 2-1 = R1 ℓ1 + R2ℓ2 + ζНд2, [Pa] (4.12)

where R1 ℓ1 and R2ℓ2 are pressure losses along the length of the section, respectively, from section I-I to the middle of the branch and from the middle of the branch to section II-II;

ζΔNd2 - additional pressure losses in the fittings (excluding losses along the length of the branch), referred to the dynamic pressure in section II-11 behind the branch.

     Having written an equation of form 4.6 for each point of measurement of static pressures in the areas of steady motion straight ahead and behind the branch,

            H st i = R1 ℓ1 + C1, (4.13)

             H st n = R2 ℓn + C2, (4.14)

and solving them, we obtain the values ​​of the specific pressure loss R1 and R2 and the free terms of the equations C1 and C2. Using these equations, it is possible to calculate the values ​​of static pressures at any point right before and after the turn, and the choice of the distance from the turn to the measurement section does not play any role. If the length from the beginning of the pipeline to the cross-section of the branch is selected, then the height difference of two points belonging to different straight lines (4.13 and 4.14) will be found, obtained by crossing them with a vertical line drawn on the graph through the middle of the branch. Equating the right-hand sides of equations 4.11 and 4.12, substituting equations 4.13 and 4.14 into them, after simple transformations, we obtain

     ζ = {[(R2 - R1) + (C2-C 1)] - ΔHд2-1} / Нд2 (4.15)

     To more accurately determine the air velocity, which changes with a change in its density with an increasing vacuum in the pipe, the calculated velocity established by the collector was corrected by the static pressure in the middle of the outlet. Using a similar equation (4.15), it is possible to calculate the coefficients of additional resistance of the outlets, both during the movement of clean air and aerosol mixtures.

     As the results of processing the ζ experiments for turns with the same geometric parameters showed, at Reynolds numbers in the range 9 * 104 - 4 * 105, it practically does not change.

     As a result of such data processing, several ζcm values ​​were obtained at the corresponding mass concentration values ​​for all materials at all types of turns. Consideration of a larger number of such dependencies shows that the function ζcm = ζcm (μ) is linear and can be described by an equation of type (1.16).

Solving a group of similar equations, we find numerical values ​​of the coefficients of additional resistance of the taps when moving the material with an aerosol mixture concentration equal to unity and with the movement of clean air ζ0, which are not much different from those found in experiments in clean air.

     An attempt to experimentally determine the speeds of tobacco in glass pipelines using video shooting did not give reliable results since conglomerates of transported materials do not have constant sizes and structures along the length of pipelines. Therefore, we can only talk about Orien additional hydraulic resistance ζcap, pull-off speed Jтр and weighing Jвв (winding Jвит for vertical pipes), the mass of individual particles.

(elements), their density (bulk and true), the size of individual particles - diameter, equivalent diameter, particle area in the cross-section of the pipeline, porosity, internal and external friction coefficients, shape coefficients, state (roughness) of the particle surface, etc.

    An equivalent particle diameter of irregular shape is defined as the diameter of a ball having the same mass and volume as the particle in question. For fibrous materials, determining the equivalent size of a lump of tobacco is very problematic, since it can be a fluffy conglomerate (lump in a loose state), the length of which is much larger than the diameter. Therefore, to somehow evaluate the geometric characteristics of tobacco lumps, we made balls of different sizes and masses from them (these are rather symbolic figures). It seems appropriate to evaluate the shape using a dynamic shape coefficient equal to the ratio of the aerodynamic drag coefficient of the test body (particle) and the aerodynamic drag coefficient of the ball:

                       K = С / Сш (4.16)

This method requires a special experiment to determine the speed of movement, and through it the coefficient of aerodynamic drag.

     Let us especially dwell on the values ​​of the coefficients of external friction of fibrous materials, due to several factors. The most significant of them is humidity w% and specific pressure q exerted on them, i.e.

                      ftr = ftr (w%, q). (4.17)

     The coefficients of mechanical external friction were determined on a special table with a lifting lid covered with sheet steel, aluminum, plastic, or glass. A protractor is attached to the tabletop. The specific pressure on the material changed due to the addition of weights of different weights to tobacco. They were placed on a plywood pad laid on a layer of fibrous material.

From the results of the experiments established:

1. With increasing specific pressure, the static coefficient of friction decreases (Fig. 4.3), tending asymptotically in the future to approach a constant value. This is especially pronounced with an increase in tobacco moisture from 15% to 25%.

2. With increasing tobacco moisture, the coefficient of external friction increases (Fig. 4.4). With an increase in the roughness of the sliding surface, the friction coefficient also increases (Fig. 4.5).

Under the conditions of tobacco pneumatic transport in a loose state, as our observations showed, the specific pressure values ​​are minimal and we can assume that the coefficient of external friction of tobacco depends only on its humidity. 

Fig. 4.3. 

The dependence of the coefficient of mechanical friction on steel cut tobacco with a moisture content of 15% from the specific pressure.

Fig. 4.4.

 The dependence of the coefficient of mechanical friction of tobacco on its moisture content at a specific pressure q = 0.5 Pa.

Fig. 4.5.
The dependence of the coefficient of mechanical friction of tobacco with a moisture content of 16% on the roughness of the steel surface at a specific pressure q = 0.5 Pa.

The coefficients of external friction of sheet and cut tobacco are slightly different from each other (by 4–8% for identical conditions) and can be described with the same accuracy for practice by one equation:

    ft = 0.2344w% 0.4162 (4.18)

when tobacco interacts with steel material,

    ft = 0.1702w% 0.4651 (4.19)

in the interaction of tobacco with glass material.

    Existing methods for determining the specific bulk and true density for solid or bulk materials, in this case, can not be applied. Therefore, we present only the estimated parameters. So we will assume that the bulk density of cut tobacco is ≈ 45 ÷ 50 kg / m3, (cotton ≈ 20 ÷ 25 kg / m3, wool ≈ 30 ÷ 35 kg / m3). The porosity of fibrous materials, which is the ratio of the volumes of interfiber space to the volume occupied by the entire embankment, is relatively difficult to determine since the slightest effect on the fibrous material leads to a change in its specific bulk density and porosity.

  The aerodynamic parameters of tobacco lumps were determined by two methods - the method of visual observation of the particle’s behavior in the pipe and the method of measuring the forces of resistance to the movement of particles associated with a halyard with a dynamometer (Repp K.R. method).

  The determination of these cargo parameters (including for pistons) was carried out mainly at special facilities, a diagram of one of which is shown in Fig. 4.6.

 The air velocity in the glass pipe 4 is set using the valve 11 and is controlled by a micro manometer 9 connected to the static pressure connection of the inlet measuring manifold 5. As the pipe 4, glass pipes of various diameters were used. To equalize the airflow in the upper part of the glass pipe 4, a rectifying grating 7 is installed with the plate length equal to the pipe diameter D and the distance between them - 0.15D. The whole device can change its position in space so that the glass pipe 4 is fixed at any angle from 900 to 00 tilt to the horizon (collector down). These angles are usually a multiple of 150 and in some experiments even 7.50. At low air velocities (less than 4 m / s) in the glass pipe 4, an inlet measuring manifold with a diameter smaller than the diameter of the pipe 4 was used.

Fig. 4.6.

The experimental setup for determining the aerodynamic characteristics of individual particles:

1 - hopper; 2 - fan; 3 - air duct; 4 - glass pipe; 5 - dimensional input collector; 6 - halyard (thread); 7 - rectifying lattice; 8 - blown particle (capsule); 9 - micromanometer; 10 - dynamometer; 11 - an adjusting valve; 12 - U - shaped micromanometer.

When visually determining the speed of soaring, the blown body (lump of tobacco) was laid on a mesh fixed between the measured input collector 5 and the glass pipe 4. After that, the fan 11 was turned on and the airspeed in the glass pipe 4 gradually increased by opening the valve 11 until the body did not rise above the mesh to a height of ≈0.7 ÷ 0.9 m. Visually, only the beginning of the body’s movement can be determined (i.e., for a vertical pipe — the speed of movement, for all other pipe positions in space — only the speed of moving away) and the speeds of wind, calculate aerodynamic drag coefficients C. The determination of the aerodynamic parameters of fibrous materials is complicated by the fact that an artificially created lump of fibrous material when blowing in a pipe changes its shape and the density of laying of fibers in the lump. It is practically impossible to repeat the experiment on the same lump of tobacco. Hence the large error in the experimental determination of the speeds of soaring. In fig. 4.7 ÷ 4.10 presents the results of the visual determination of the speed of soaring of lumps of tobacco.
 A method for determining the aerodynamic characteristics of transported cargo Repp K.R. provides for the connection of the experimental particle 8 with the halyard 6, fixed with its free end to the dynamo-meter 10 (Fig. 4.6). After introducing the particle 8, fixed on the halyard 6, into the pipe 4, the minimum air velocity was established in it, at which the dynamo-meter 10 recorded the tension of the thread 6. Then, the airspeed using valve 11 increased with a certain step (up to 1.0 m / s) and at each speed, the dynamo-meter readings were recorded. After reaching the maximum air velocity in the pipeline, it was reduced to the minimum with the same step.

By blowing a particle fixed on a thread at different air velocities, the measured drag force of a particle with thread S∑ is found from the dynamo-meter for a given pipe position. Subtracting from S∑ the resistance force of the clean (empty) thread Sн previously blown at the same airspeed, one can determine them
Fig. 4.7.
Dependence of the rates of soaring of lumps of tobacco weighing 30 g, humidity 17% of the ratio of the areas of vertical glass pipes and lumps of tobacco with a diameter of 198 mm
(according to visual observations).
body movement
Figure 4.8.
The dependence between the speed of the ball (d = 22.5 mm, M = 0.65 g) on the ratio of the cross-sectional areas of the vertical glass pipes and the blown body.
However, the application of this method in its pure form to fibrous materials is impossible, because the question arises of the place and method of attaching the lump of tobacco to the halyard. Our attempts to place the fibrous material in all kinds of nylon nets (connected via a halyard to a dynamometer, under the method of Repp K.R) did not lead to any positive results. The same thing is a distortion of the shape, laying of fibers, etc., as with the visual determination of the aerodynamic parameters of particles. We tried to simulate lumps of tobacco with foam particles of various shapes, packed in a bag made of nylon mesh, connected by a halyard with a dynamometer. But even in this case, the foam particles in the bag were deformed, changed their position, i.e. stacking porosity and geometric parameters of the foam bag varied. Moreover, when the purging of the bag was stopped and the bag was removed from the glass pipe 4 (Fig. 4.6), all of the above parameters again changed without repeating with the original ones. As a result, the experimental results were practically not repeated twice. Replacing the foam with different balls, nuts, etc., did not change the essence of the occurring phenomena and did not lead to stabilization of the experimental results.
 When studying pneumatic transport, it is customary to conduct experiments on particles of the correct form of artificial origin - balls and natural origin - grain materials, calibrated sand, gravel, etc. However, such
the approach to working with tobacco does not seem to work. Therefore, the question arises of modeling the lumps of tobacco themselves. It was decided to try experiments on plastic balls, cylindrical jars, which are now a great many - from medicine jars to bottles and bottles with various sizes. Moreover, these sizes and shapes remain almost constant regardless of the duration and conditions of the experiments. In the end (bow and stern) walls of the jars, holes were made, the total areas of which are approximately equal to each other. This allows you to simulate the porosity of tobacco. And the change in the mass of the jars was carried out by inserting metal rods of different weights into them.
 In the reference literature, as a rule, the aerodynamic properties of individual particles blown in the pipes are given for the ratio of the areas of the pipes F and particles f (their mid-section) not less than 100, since most researchers believe that at (F / f) <100 the laws changes in aerodynamic properties are different from those at (F / f)> 100. In Fig. 4.9 presented results obtained by applying the Repp method K.R. when blowing a plastic ball d = 22.5 mm, weighing 0.65 g in pipes with diameters from 25 to 750 mm, which slightly differ from the results obtained under the same conditions, but by visual observation.
 It can be seen from the graph that at (F / f)> 100, the speed of the whirlwind practically does not change (free whirling, flow around). But at (F / f) <100, even (F / f) <70 to ≈ (F / f)> 10, there is a sharp decrease in the speed of movement. Then, with a decrease (F / f) <10, there comes a zone in which the drop in the speeds of whirling take a landslide character - these speeds are rapidly approaching some constant value. When F≈f, the particle will move only under the influence of static pressure created by the fan. And in this case, the speed of the air J entering the pipe will be close to the speed of movement of the capsule ω and the particles should move without slipping (ω / J = i = 1). However, in practice, such a ratio is hardly feasible. There is a gap between the capsule and the tube, which, to reduce air leakage, is covered with rims made of soft materials, such as felt, leather, installed in the ends of the capsule. As for the pneumatic transport of tobacco, it is unlikely that it will be necessary to compact it to a state where they will be airtight.
 Comparing the laws of change in the speeds of hovering from constraint conditions [Jwit = Jwith (F / f)], obtained by directly blowing tobacco lumps (Fig. 4.7) and when blowing the simulator (model, layout) of tobacco lump (Fig. 4.8), we can note their certain similarities. When blowing such mock-ups and lumps of tobacco under other conditions, similar results were obtained. This suggests that the proposed method of working with simulators is quite acceptable for determining the aerodynamic properties of fibrous materials.
 In practice, the results of experiments are more often presented in the form of their function of porosity ε, which is the ratio of the volumes of voids to the volume of the part of the pipe occupied by the capsule (Fig. 4.9). In the case described, for porosity we will not take the ratio of volumes, but the ratio in the selected cross-section of the void areas f hollows and the pipe area F. The graph below demonstrates quite clearly one of the reasons for the decrease in air velocities when transporting tobacco in a compacted state. This is, above all, a decrease in speed the beginning of movement (soaking, pulling) of the cork as the porosity of the material in the pipe decreases (with a decrease in the F / f ratio - Fig. 4.9).
Figure 4.9.
Dependence
between the speeds of the ball soaring (d = 22.5 mm, weight 0.65 g) from porosity in vertical glass
pipes.
My Passport Folder 2012 Alex Dock Drive. Folder Experiments on the properties of fibers and soft materials 08/10/11. XL file Search for equations for the speed of soaring cotton and wool 08/10/11.
Fig. 4.10.
Dependence of the speeds of starting and soaking of lumps of sheet tobacco with a moisture content of 17% and a mass of 30 g, equivalent to a diameter of 60 mm, blown in an organic glass pipe with a diameter of 198 mm from the angle of inclination of the pipe to the horizontal: 1 - Jtr = Jtr (α); 2 - Jvit = Jvit (α).
Findings.
From a relatively large number of physicomechanical and aerodynamic characteristics of the transported materials, we have presented the laws of variation of the friction coefficients and the rate of tobacco depending on several factors:
1. The difference in the coefficients of external friction of the cut and leaf tobacco does not exceed 4–8%, and they, with sufficient accuracy for practice, can be considered equal to each other.
2. It was found that with increasing specific pressure, the external friction coefficient decreases, and with increasing humidity and roughness of the material along which the tobacco moves, it increases.
3. In the study of pneumatic transport, it is customary to conduct experiments on solid materials of the correct form. This is because the fibrous materials during the experiments change their shapes, sizes, and laying of fibers in the lump. Therefore, it is proposed to use mock-ups that mimic tobacco lumps. It is shown that the laws of change in the speeds of movement, defined on the models and the lumps of tobacco, have a similar form.
4. It was found that as the diameter of the pipeline decreases, the speed of the movement of the same particle decreases.

4.3. Experimental stands and research results of tobacco pneumatic transport

The first experiments on the study of pneumatic transport of fibrous materials in a loose state were carried out in the 80s of the 20th century with the control of air velocity in material pipelines along with an inlet collector and using a sacking machine as a gateway from the PZT line. Their use will limit the range of studies on the productivity of the transported load.
 The basic concepts described above (chapter 1. Pipeline pneumatic conveying systems) about the fundamental processes that occur during pneumatic conveying. The criteria equations given there (1.8, 1.9, 1.12, 1.13, 1.17) for the main characteristics required for calculating pneumatic conveyors made it possible to justify the creation of experimental plants, to plan experiments, methods for their implementation and processing of the results.
 Given the complexity of the processes of pneumatic transport, to obtain data to formulate recommendations for practice, studies were conducted mainly on installations of industrial scale and size. To study their hydraulic resistance during the movement of fibrous materials (tobacco), special installations were created, the schemes of which are shown in Fig. 4.11.
 The loading device of the installation, consisting of a storage hopper 1, a conveyor belt 2, a loading drum 3 with gripping combs, made based on the MKBF-56 L-P cigarette-packing machine, provides time-constant production of transported material at constant speeds of rotation of the conveyor 2 and the loading drum 3. The SHU-20 lock gate contributes to the same for supplying cargo to the material pipe 11. During the experiments, the fan 17 was turned on and the control valve 20 with a mechanical actuator opened. From the storage hopper 1, the amount of tobacco was maintained at approximately the same level, the belt conveyor 2 was fed into the funnel 4. The loading drum 3 with exciting combs installed under the drive drum of the conveyor 2 ensured a constant supply of time to the funnel 4 and through the lock gate 5 and transitional pipe 6 to the receiving device 7. The cargo transported through the material pipe 11 was separated from the conveyor air in the BP-1 unloader with a tangential air mixture inlet, inside of which a drum rotated, covered with a mesh with mesh 0.5x0.5 mm. Under the unloader 12, a locked gate of the ШУ-50 type is mounted. At both lock gates, the 5 and 13 ribs are cut 15 mm to the side, and strips of the rubber conveyor belt 10 mm thick are attached to the remaining edges on both sides. A plate with a brush was placed at the entrance to the airlock, which removed layers of material (tobacco) lying on the ribs.
Fig. 4.11.
The scheme of the experimental setup for the study of tobacco anemotransport:
1 - storage hopper; 2 - conveyor belt, 3 - loading drum with exciting combs; 4 - funnel; 5 and 13 - lock gates SHU-20; 6 - transition pipe with a vibrator; 7 - boot unit; 8 - input mercury manifold with a static pressure sensor 9 connected to a micromanometer 10; 11 - replaceable material pipe; 12 - gateway-separator BP-1, inside of which a drum rotates, covered with a mesh with mesh 0.5x0.5 mm; 14 - capacity mounted on the scales 15; 16 - tee with a cross over valve, interlocked with an electric stopwatch; 17 - fan with suction 18 and discharge 19 ducts; 20 - throttle valve; 21 - filter; 22 - battery micromanometer.

The drive of the locks and the conveyor belt with the loading drum is carried out from electric motors with adjustable speed. During the experiments, it was found that at a speed of rotation. a rotor of less than 25 rpm there is an unevenness in the supply of tobacco, as evidenced by the pulsation of the battery micromanometer 22. Control measurements showed that at rotational speeds of the rotor of the lock gate SHU-50 within 25 ÷ 50 rpm stable material performance is provided with an error no more than 5%. The presence of suction cups in the lock gate was judged by the testimony of a MacLeod homemade micromanometer connected to the static pressure fittings at the beginning and end of the transition pipe 6 with a vibrator (not shown in the diagram).
 To prevent arch formation, the transition pipe 6 is equipped with a vibrator and is connected to the lock gate 5 and the receiving pipe of the loading unit 7 through soft inserts.
 Conducting experiments at these facilities with air velocity control in the material pipe through the inlet manifold caused significant difficulties in supplying fibrous cargo (tobacco) to the material pipe, which is primarily associated with the need to use locks of large dimensions. As a rule, the larger the lock gate, the more air leaks through it into the material pipe. it
leads to an increase in the error of the speed controlled by the input collector and
airflow in the material pipeline, and, therefore, in the values ​​of the desired values ​​of the coefficients of hydraulic friction when moving not only air mixtures but also clean air.
 This forced us to develop a way to control the air velocity in the material pipeline and a device for its implementation using a special aerodynamic stand, which automatically takes into account air leaks in the loading device. Such a stand was thoroughly investigated in laboratory conditions and as a result, was accepted by us for use in the experiments. Necessary changes were made to the experimental setups. The new layout of the main nodes is presented in Fig. 4.12. The test material was selected from the production line of the preparatory workshop. It entered the storage hopper 1, from where it fell onto the conveyor belt 2, then it was sent through the transitional pipe 4 with a vibrator to the receiver 5 of material (cargo) with a loading drum with exciting combs. Variation of the plant’s capacity for the transported load was carried out by changing the rotation frequency of the drive of the conveyor belt 2 and the loading drum 3 with an exciting comb. From receiver 5, the load fell into the experimental material pipe 8, the length (height) of which was not less than 20 m. The product was transported to the separator 9 with a locked gate 10. For all diameters of the material pipelines, VRT-2 separators were used. In the separator 9, the transporting air was freed from the transported material. The allocated cargo was removed from the separator 9 through the lock gate 10 and along the side branch of the tee 13 was removed from the system (again sent to production). If necessary, measure the performance of the installation on the transported load, the flapper valve of the tee 13 was turned, connected with the stopwatch for a certain time so that the load entered the tank 11 mounted on the balance 12. Having determined the mass of the cargo in the tank 11, and, knowing the time of its supply, did not it is difficult to calculate the productivity of the installation mm by moving load in kg / s. The air freed from the material through the duct 20 entered the aerodynamic barrel 21, inside which there was a measuring manifold 22 and through the intake duct 15 with a valve 17 with a screw drive enters the fan 14. The exhaust air through the discharge duct 16 fell into the filter 18, from which it was discharged into the atmosphere. The aerodynamic barrel 21 together with the measured collector 22 forms an aerodynamic stand.

Fig. 4.12.

Simplified schemes of experimental stands for studying pneumatic transport processes: a - along horizontal pipes; b - along vertical pipes:

1 - storage hopper; 2 - conveyor belt, 3 - loading drum with exciting combs; 4 transition junction with a vibrator; 5 - receiver of material (cargo); 6 - input measuring manifold with a static pressure sensor connected to a micro manometer 7; 8 - removable material pipe; 9 - a separator of material from air VRT-2; 10 - lock gate SHU-50; 11 - capacity mounted on the scales 12; 13 - tee with a cross over valve, interlocked with an electric stopwatch; 14 - fan with suction 15 and discharge 16 air ducts; 17 - valve with screw drive; 18 - filter; 19 - battery micro manometer; 20 - duct; 21 - aerodynamic barrel with a fitting of static pressure on the shell; 22 - a measured collector with a fitting of static pressure inside the aerodynamic barrel 21; 23 - micro manometer for measuring static pressure in an aerodynamic barrel; 24 - micro manometer for measuring the static pressure drop in the aerodynamic barrel 21 and the measuring manifold 22.

If the horizontal material pipe is presented in the “pure” form (there are no fittings in the material pipe), then for the study of the vertical pipe. In the same paper it is shown that in the case of using steel pipes as material pipes, it takes some time to reduce their roughness of the pneumatic conveying, vertical straight material pipeline and after it, was horizontal straight. This is due to the lack of devices for supplying a controlled amount of air cargo directly to the vertical straight and separating it from the conveying air with a vertical inlet. In any case, the heights of the strands under study were at least 20 m, and the lengths were 30 m. New steel pipes have increased roughness and a relatively high coefficient of mechanical and hydraulic friction. Over time, the pipes are ground by the transported cargo and their resistance reaches constant values. In the experiments described, steel material pipelines were ground with sand for 100 hours with constant monitoring of the coefficient of hydraulic friction λ0 when moving clean air. Grinding of pipes stopped when λ0 ceased to change, which indicates the inappropriateness of further grinding of pipes. Without waiting for the roughness of the inner surface of the steel material pipelines to become constant, we conducted experiments at different stages of their grinding, which made it possible to determine with sufficient accuracy the effect of this parameter, and hence the coefficient of mechanical friction, on the hydraulic friction coefficients when moving tobacco.

 The static pressure fittings installed on the material pipelines are connected by flexible hoses to the battery micromanometer 19. This makes it possible to record the change in static pressures along the length of the material piping during the experiments, which ultimately allows us to obtain pressure losses over its sections.

 Before proceeding with the establishment of predetermined air velocities in the material pipe 8, the aerodynamic stand must be calibrated using a known reference device that measures the airspeed in the material pipe.

Such a device is an inlet measuring manifold 6, the static pressure fitting of which is connected to the micromanometer 7. In this case, to avoid suction of air into the material pipe 8, it is necessary to plug the inlet of the material (cargo) receiver 5. After examining and testing the installation mechanisms and devices, checking its tightness, measurements of temperature and humidity (i.e., determination of air density), the aerodynamic stand was tattooed.

Calibration of the aerodynamic bench is carried out at airspeeds in the material pipe 8 from 26 to 10 m / s in increments of 2 m / s in this sequence. The fan drive 14, the separator 9 and the lock gate 10 are started. A valve (17) with a screw drive sets a certain (set) airspeed  = 26 m / s in the material pipe 8 according to the readings of the micromanometer 7, which is connected to the static pressure connection of the inlet measuring manifold 6, and readings of micro manometers 23 — the static pressure on the nozzle on the side of the aerodynamic barrel 21 — Nab and 24 — the difference in static pressures — Nab and the manifold 22 — Nmk, i.e., ΔН (μ - ab) = Nmk — Nab.

After conducting experiments at all given speeds when moving clean air, a throttle is introduced into the material pipe 8 in the form of a metal blank of a certain section, which partially overlaps the transverse section of the material pipe, simulating the presence of the transported cargo. And again, according to the readings of the micromanometer 7, the valve 17 is installed in the material pipe 8, in turn, the same airspeeds that were when blowing clean air. At the same time, the readings of micro manometers 23 and 24 are also recorded. There were no less than six such simulations of capacities for the transported load in the entire expected interval of their change due to different chokes. The experimental data (Nmk - Nab) = f (Nab) obtained for each speed with different throttles in the material pipeline 8 are laid out on the graphs, which are the calibration graphs of the aerodynamic bench. These graphs are similar to those shown in Fig. 4.13, which are relatively convenient to use. Knowing what air velocity  must be installed in the material pipe 8, having a graph in the form of a change (Nmk - Nab) = f (Nab) using a valve 17, regulate the airflow so that the readings of devices 23 and 24 converge on the line of the graph constructed for this  (Fig. 4.13). For example, in material pipeline 8, it is necessary to set the air velocity  = 24 m / s. All possible combinations (Nmk-Nab) = f (Nab), regardless of the capacity of the installation for the transported cargo (or in its absence) are represented by the law depicted by line 1 (Fig. 4.13). Moving the valve 17, there is a point on the line of law (Nmk - Nab) = f (Nab) at which the values ​​(Nmk - Nab) converge and Nab. For example, the coordinates of such a point can be Nab = 5000 Pa and (Nmk - Nab) = 1170 Pa. The position of this point depends on the performance of the load being transported.

   Now, in the absence of a sealing device (for example, a locked gate) in front of the inlet of the cargo receiver 5, it is possible to set the air velocity in the material pipe 8 using the aerodynamic stand according to the obtained calibration graphs, regardless of the presence of air suction in the material receiver 5 and in the separator.

Having removed the plug from the inlet of the cargo receiver 5, a feeding mechanism is connected to it - a conveyor belt 2, a loading drum 3 with gripping combs, an adapter pipe 4 with a vibrator (Fig. 4.12). Then the experiments were carried out in the following order: - Initially, experiments were conducted when moving clean air through the pipelines;

- in the hopper 1 above the belt conveyor 2 is loaded pre-prepared test material, the presence of which is maintained at the same level; - turns on the fan drive 14, the separator 9 and the lock gate 10; - gate valve 17 according to the calibration schedule similar to that shown in fig. 4.13, the airspeed in the material pipe is set at 26 m / s (without moving the load); when the air velocity  is set in this way in the material pipe 8, the corresponding indexing is placed on the panel of the battery micromanometer 19 and its readings are photographed three times;

- then airspeeds 24, 22, 20, 18, 16, 14, 12, 10 m / s are set in turn and at each of them photographing the readings of the battery micromanometer is carried out;

- after experiments in clean air, the air velocity is again established in the material pipeline 26 m / s, the feeding mechanism is started — the belt conveyor 2, the loading drum 3 with gripping combs, the transition pipe vibrator 4, which feed the transported cargo into the material pipe 8 through the material receiver 5 (Fig. 4.12.); in all experiments, the measurement of the flow rate of transported tobacco was carried out using a rocker valve 13, installed after the lock gate 10 of the unloader 9. The valve drive is interlocked with an electric stopwatch, with which the time of feeding the transported material to a special container 11, mounted on the scales 12, was detected;

 -Some minimum productivity is set for the transported material. The air velocity in the material pipe 8 is set using the valve 17 according to the schedule (Fig. 4.13). Similarly, at the same productivity for the transported cargo, all provided experiments of air velocity in metrical pipeline 8, which are constantly monitored according to the calibration schedule. At each airspeed, the readings of the battery micromanometer are photographed three times, the indexing on it is changed and the corresponding time delay is given to stabilize each next mode of transport, which can be easily judged by the readings of the micromanometer. Throughout the entire time of experiments at one productivity for the transported load at each airspeed, its control is carried out.

 According to the laws of change of static pressures or their difference in sections, with confidence located in the areas of steady-state movement of the air mixture, pressure losses per unit length of the material pipeline are determined, and hydraulic friction coefficients λm ‘and λm are determined from them. Similarly, the specific pressure loss during the movement of clean air and the hydraulic friction coefficients during the movement of clean air are found. The specific pressure loss during the movement of the mixture for vertical pipelines also includes the pressure loss on the lift.

 The experiments conducted at different speeds on the same product on the same material pipe formed a series of experiments. Similar cycles of experiments, amounting to one sub-series on a given product, followed each other with an ever-increasing load until the onset of blockage. After this, the experiments were terminated. The number of subseries on this material was usually 4 ÷ 6. Then the transported material was replaced - (for example, cut tobacco on a sheet). The subseries of experiments on one pipe formed a series of experiments.

 After conducting experiments on all types of transported material, we switched to experiments on pipes of the same material, but of a different diameter. Series of experiments on pipes of different diameters from the same material formed subgroups of experiments.

 After conducting all experiments on the material pipelines of one material (for example, steel), we switched to material pipelines from another material (for example, glass). The volume of experimental studies on finding the characteristics of pneumatic transport was determined on the basis of the conditions for the need to obtain the amount of experimental data. These studies were carried out on the basis of the obtained criteria equations.

 Knowing the laws of changes in static pressures or their differential in sections, with confidence located in the areas of steady-state movement of air mixtures, we determined the pressure loss per unit length of the material pipeline. Similarly, specific pressure losses were found when moving clean air. The specific pressure loss during the movement of the mixture for vertical pipelines also includes pressure loss for acceleration. Having specific pressure losses Rcm straight when moving cargo with different concentrations at the same airspeed, the coefficients of hydraulic friction λcm were determined. The function λcm = λcm (μ) is a straight line described by an equation of the form (4.7)

Fig. 4.13. 

Calibration schedule of the aerodynamic stand, consisting of a hermetically sealed barrel, in which the collector is placed; air velocities in the equations are given for material pipelines with a diameter of 78 mm:

1-  = 24 m / s, (Nmk - Nab) = 1050 + 0.065 Nab; 

2- = 18 m / s, (Nmk - Nab) = 690 + 0.042 Nab;

3 - = 14 m / s, (Nmk - Nab) = 500 + 0.025 Nab

Having solved the system of equations for the same transportation conditions, but for different μ, we find the values ​​of λm and λo, which, as a rule, are not much different from the λo found for this product pipeline straight when moving clean air through it.

   The coefficients of hydraulic friction λcm, λm ‘, λm were determined according to the laws of change of static pressures or their difference in sections, with confidence located in the areas of steady-state movement of the air mixture. Similarly, the coefficients of hydraulic friction when moving clean air were determined. Talking about a section of steady motion is possible only conditionally. Nevertheless, we assume that at a certain length of the product pipeline, the speed of individual particles of the transported material will be almost constant.

  To more accurately determine the air velocity, which changes with a change in its density with an increasing vacuum in the pipe, the calculated speed established by the collector was corrected by the static pressure in the material pipe.

  Figure 4.14-4.16 shows the dependences of the coefficient of hydraulic friction for horizontal and vertical pipelines on the criteria that affect it.

     With the growth of the Re criterion, and hence the air velocity (at a constant pipe diameter), the hydraulic friction coefficient λm decreases for vertical and horizontal material pipelines (Fig. 4.14). If this function is exponential, then the exponent of the argument for horizontal pipes in absolute value is less than for vertical pipes.

Fig. 4.14.

The dependence of the coefficient of hydraulic friction λm - when moving tobacco with a concentration equal to unity on the Reynolds criterion:

1 - horizontal pipe, glass, wall roughness Δ = 0.007 mm, D = 144 mm, ft = 0.58, tobacco humidity 15%;

2 - a vertical pipe, bent from sheet steel, wall roughness Δ = 0.03 mm, D = 315 mm, ft = 0.74, tobacco humidity 15%.

With the increase of the conditional (dimensionless) diameter Dу of the material pipeline, the coefficient of hydraulic friction λm increases. The laws of such changes are to some extent identical to each other (Fig. 4.15).

Figure 4.15

The dependence of the coefficient of hydraulic friction λm - when moving tobacco with a concentration equal to unity on the nominal diameter of the material pipe:

1 – steel pipes, horizontal, seamless, air velocity  = 18 m / s;

2 - pipes bent from sheet steel, vertical,  = 21 m / s;

3 – pipes are glass, vertical,  = 23 m / s.

An increase in the coefficient of mechanical friction leads to an increase in the coefficient of hydraulic friction under the same transportation conditions for both vertical and horizontal material pipelines (Fig. 4.16). This dependence was obtained in experiments on glass (ft = 0.58), aluminum (ft = 0.66), steel (ft = 0.74), and steel (ft = 0.82) pipelines. As already noted, it is possible to change the mechanical coefficient of friction due to changes in the humidity of the transported cargo. To some extent, in these experiments, we obtained similar dependences λm = λm (ftr). But here, a change in the density of the transported cargo also begins to appear.

Fig. 4.16.

The dependence of the coefficient of hydraulic friction when moving tobacco with a moisture content of 15% in pipes with a diameter of 105 mm from the coefficient of mechanical friction:

1 - horizontal pipes, air velocity  = 18 m / s;

2 - vertical pipes, air velocity  = 19 m / s.

Processing the totality of the experimental results obtained at different facilities made it possible to obtain specific types of criteria equations for horizontal pipes
 m g = 4.4 * 10 3 Re - 0, 8 Du 1,5 ftr 0, 46; (4.20)
 and vertical pipes
 Вm in = 5.7 * 10 6 Re - 1, 33 Du 2 ftr, 0.2. (4.21)
No significant difference in m for cut and leaf tobacco was found.
 Numerous calculations of material pipelines showed that with an acceptable error for vertical and horizontal material pipelines, the acceleration coefficients can be taken equal to  2.3.
 As for the coefficients of additional aerodynamic drag of the branches when moving fibrous materials with an aerosol concentration of μ = 1, the experimental studies performed will not allow us to obtain empirical expressions that take into account all the factors affecting its value (1.15). For practical calculations, an option may be proposed according to which
 ζм ≈ аζ0, (4.22)
 where ζо is the coefficient of additional pressure losses in the bends when moving clean air, we can take ζо = 0.1 - 0.3;
m = aζо - coefficient of additional pressure losses in the bends when moving the material; for bends that change the direction of movement from horizontal to vertical a = 4, for all other a = 1.
For practical calculations, it is possible with an allowable error to recommend ζm ≈ 2 ζ0 (1.18) regardless of the position of the outlet in space.
 These equations are valid in the range of experiments:
Re = 66666D = 5 * 104 ÷ 6.5 * 10 5, D y = 0.075 ÷ 0.355, ft = 0.4 ÷ 0.9, i.e., at ω% = 12 ÷ 20%.
 The friction coefficient of tobacco increases with increasing humidity. So, at humidity ω% = 12%, the coefficient of friction on steel is 0.55, and on glass, it is 0.5; at humidity ω% = 20%, the coefficient of friction on steel is 0.9, and on glass, it is 0.7.

4.4. Section Conclusions

1. The criteria equations we obtained earlier for determining hydraulic friction coefficients have been greatly simplified. They made it possible to develop experimental facilities, experimental techniques, and data processing.
2. In the course of the experiments, pipelines of various materials — steel, aluminum, glass, and plastic — were used.
 3. Installed:
 - with the growth of the Reynolds criterion, the coefficient of hydraulic friction decreases for both horizontal and vertical pipes;
 - with the increase of the conditional (dimensionless) diameter of the pipeline, the coefficient of hydraulic friction increases;
 - as the coefficient of mechanical friction between the transported material and the surface of the pipeline increases, the coefficient of hydraulic friction increases. From these positions, glass pipelines are most preferred, and steel pipelines are the worst. The coefficients of hydraulic friction for glass material pipelines are 20–30% lower than for steel pipelines with other conditions being the same.

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