23 Pneumatic Conveying Test Facilities
1
INTRODUCTION
Since the use of test facilities for obtaining data for pneumatic conveying system design is so important, consideration is given here to the requirements of such a test facility. A detailed specification for all the major components required is given, for a facility capable of testing materials over a wide range of conveying conditions. The specification covers a range of pipeline bores from two to six inches, and the use of such a test facility in determining test data is considered. Consideration is also given to service facilities and material characterization equipment requirements. The starting point here is to assume that no such facility exists and that there is little experience in working with such a system. In the fist instance, therefore, a system is required that will be capable of achieving as much as possible with a single system. With experience derived from operating the system the facilities could be extended with additional and more specialist equipment. 1.1
Conveying Requirements
It is suggested that any system should be capable of both dilute and dense phase conveying, at a reasonably high material flow rate, and over a reasonably long conveying distance. The conveying system should have a wide range of controls,
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
652
Chapter 23
and be suitable for test, development and research work, and conveying trial demonstrations to potential clients. 1.2
Accommodation
Consideration must be given to the space required, and this includes areas for the test rig and associated plant and equipment. A separate area for the compressor would be recommended, because of the noise, and a separate room for any computational and electronic equipment, because of the potential for dust generation with this type of test facility. 1.3
Preliminary Decisions
A number of initial decisions need to be made which will affect the scope of the work that it will be possible to undertake, and the capital investment on the test facility. In some cases a definite recommendation will be made, where it is felt that the given parameter should be adopted for an initial facility. In other cases a range of values will be given so that the cost implications can be considered. 1.3.1 System Type If a single pneumatic conveying system is to be installed initially, it would be recommended that this should be a positive pressure conveying system. This is probably the most useful and versatile of all conveying systems for test work. At a later date a vacuum conveying system could be considered. 1.3.2
Material Capability
If there is no previous experience of pneumatic conveying it would be suggested that testing should initially be limited to non explosive materials. At a later stage a suppressant system could be fitted, or provision could be made for explosion venting or making the system into a closed loop and employing nitrogen. The conveying system should be capable of handling most powders and granular materials. 2
CONVEYING PARAMETERS
The conveying plant needs to be built to a scale that will be seen by clients to be sufficiently large to provide reliable scale up of test data, and to achieve the widest possible range of conveying parameters. 2.1
Conveying Air Pressure
For dense phase conveying, and dilute phase conveying over a long distance, to a certain extent, air must be available at a high pressure. It is recommended that the compressor for providing the air should be capable of about 100 psig. This will also be useful for clearing pipeline blockages.
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Conveying Test Facilities
2.2
653
Conveying Air Velocity
The test facility must be capable of conveying materials in both dilute and dense phase. For dilute phase conveying a minimum conveying air velocity of at least 3000 ft/min is often required. It is not envisaged, however, that air at a pressure of 100 psig would be used for such materials. For dense phase conveying it would be expected that tests would be conducted with conveying line inlet air velocities down to about 600 ft/min. With a low velocity, and hence lower air flow rate requirement, it would be expected that tests could be carried out with high air supply pressures with materials capable of being conveyed in dense phase. A compromise clearly needs to be made here. The appropriate model relating these parameters was presented in Chapter 3 with Equation 1 and is: ,2 f~i
V0 = 0-1925
P]
'
ftVmin
(1)
*1
where V0 = volumetric flow rate of free air Pi d C; and Tt
= = = =
conveying line inlet air pressure pipeline bore conveying line inlet air velocity conveying line inlet air temperature
- ft7min - lbf/in 2 abs - in - ft/min -R
An alternative form of this model, in terms of conveying air velocity, was presented in Chapter 5 with Equation 11 and is: C
=
T V
5-19 —5-2d~ p
ft/min
- - - - - - -
(2)
It is recommended, therefore, that the conveying system should be capable of achieving a maximum value of conveying line exit air velocity, C2, of about 9000 ft/min. This means that for dense phase conveying, with the maximum possible air supply pressure of 100 psig, it will be possible to undertake tests with conveying line inlet air velocities up to a maximum of about 1150 ft/min. For dilute phase conveying, requiring a minimum conveying air velocity of 3000 ft/min, it will be possible to undertake tests with conveying line inlet air pressures up to a maximum value of about 30 psig. It is suggested that this is probably the best compromise in terms of selecting a single compressor for such a test facility. The problems here can best be illustrated by reference to Figure 4.10. Conveying characteristics for two different materials are presented, each conveyed through the 165 ft long Figure 4.2 pipeline of two inch nominal bore. They are reproduced here in Figure 23.1 for reference.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
654
Chapter 23
Solids Loading / Ratio 60
60
300200 120 100 80
NO 50
_50
8 o
o o o
GO
40
AREA
Solids Loading Ratio Conveying Limit
B 30 oi
> o
E 20
Conveying Line Pressure Drop \-lbt7in 2 15
10
10 0 50
(a)
100
150
(
200
Free Air Flow Rate - frVmin
(b)
50
100
150
200
Free Air Flow Rate - fVYmin
Figure 23.1 Conveying characteristics for (a) A fine grade of pulverized fuel ash and (b) a fine granular grade of silica sand conveyed through the pipeline shown in figure 4.2.
Figure 23.1 a is for a fine grade of pulverized fuel ash and Figure 23.1 b is for a fine granular grade of silica sand. These are typical of materials that might be tested. For the cases illustrated it will be seen that within a pressure capability of 100 psig, conveying is limited by a combination of the volumetric flow rate of air available and the conveying limit for the materials. The shape and slope of the curves representing the conveying limits for the materials are both additionally dictated by the compressibility of the conveying air. With the limit for the pulverized fuel ash being a conveying line inlet air velocity of about 600 ft/min, testing will be possible with air supply pressures up to 100 psig if required. For the silica sand, however, with a minimum conveying air velocity of about 2600 ft/min, conveying is limited to a maximum air supply pressure of about 35 psig within the limit of free air flow rate of 200 ft3/min. 2.3
Pipeline Bore
It would be recommended that the minimum diameter of conveying pipeline that should be considered should be 2 inch. Anything less than this would not be given credibility by the industry. It is unlikely, however, that a pipeline bore greater than
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Conveying Test Facilities
655
about 4 inch would be necessary for the vast majority of test work. In recent years, however, many companies have installed test facilities with 6 inch bore pipelines and so the data presented here has been extended to that diameter of pipeline for reference. Since pipeline bore has a very significant influence on the specification of many of the components that comprise the conveying system, a range of pipeline diameters are considered from 2 to 6 inch, so that cost implications can be taken into account in the decision making process. 2.4
Free Air Flow Rate
In a positive pressure conveying system the velocity of the conveying air at the end of a pipeline, in which the material is discharged at atmospheric pressure, is approximately at free air conditions. The recommended value of this velocity has been set at about 9000 ft/min and so the values of free air flow rate for the range of pipeline bores to be considered will be as follows: Pipeline Bore
- inch
Free Air Flow Rate
- ftVmin
3
2
2'/2
3
200
300
450
4
6
800
1800
SYSTEM COMPONENTS
Some major pieces of equipment are required for a pneumatic conveying test facility, and the size, and hence the cost of these items, is very dependent upon the pipeline bore selected. 3.1
Compressor Specification
Since the air supply pressure has been recommended, and the free air flow rates have been evaluated, the compressor specifications for the range of pipelines bores considered are as follows: Pipeline Bore
- inch
2
Air Supply Pressure
-psig
100
Free Air Flow Rate
- cfrn
Approximate Power Required
-hp
3
4
6
100
100
100
100
200
300
450
800
1800
50
80
120
210
470
2'/2
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656
Chapter 23
It must be emphasized that these power requirements are approximate values and are for guidance only. It would be recommended that the compressor should supply oil free air. Since the conveyed material is to be re-circulated it would also be recommended that the compressed air should be cooled for conveying purposes. If it is proposed that hygroscopic materials, such as alumina and soda ash are to be conveyed, consideration will need to be given to the provision of an air drier, although this could be a later addition. 3.1.1 Compressor Type Positive displacement blowers are not worth considering here because of the pressure limitation on these machines. For any additional test facilities, however, such a compressor would be ideal, particularly for dilute phase conveying with either a low pressure rotary valve or a low pressure blow tank. A screw or reciprocating compressor would be recommended for the duty. 3.1.2 Air Receiver In the first instance an air receiver is not a necessity. With future development, however, it would be useful to have an air receiver located between the compressor and the conveying facility, particularly if further compressors and test facilities are added. 3.2
Pipeline Feeding Device
In order to utilize high pressure conveying air, and to test materials capable of dense phase conveying, as well as dilute phase test work, a blow tank would be recommended for feeding materials into the pipeline. If it is envisaged that much work will be undertaken with abrasive materials, such as fly ash, cement and alumina, a blow tank would be ideal. For test work a continuously operating pneumatic conveying system is not necessary. Test work can conveniently be carried out on the basis of conveying a batch of material. A single blow tank fed from a hopper above, therefore, will be adequate and it will not be necessary to incorporate a lock hopper in the facility. Consideration, however, must be given to batch size and material flow rate to ensure that a reasonable period of steady state conveying can be achieved during the conveying of the single batch. The choice now is between top and bottom discharge types of blow tank. The best for dense phase conveying is top discharge, but can be unsuitable for granular materials, for as they tend to be permeable it is often difficult to get them to discharge. Bottom discharge can be used to convey most materials. The ideal solution would be to have one of each. A typical solution to the problem is to have a common tank to which alternative bottom sections can be attached, one for top and another for bottom discharge. If only one blow tank is to be employed a bottom discharge blow tank would be recommended.
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Conveying Test Facilities
657
3.2.1 Blow Tank Size The batch size of material to be conveyed has to be large enough to ensure a reasonable period of steady state conveying during the conveying cycle. It would be suggested that the blow tank capacity be sized on the basis of a minimum of two minutes of conveying for the highest material flow rate to be expected. This will be in the shortest pipeline to be tested. Blow tank sizes for the range of pipeline bores being considered here are approximately as follows:
Pipeline Bore
- inch
2
2'/2
3
4
6
Maximum Material - ton/h Flow Rate
30
45
70
120
270
1
11/2
21/2
4
9
75
120
200
450
Batch Size
- ton
Blow Tank Volume - ft3
3.3
50
Supply/Reception Hopper
For test work it is necessary to re-circulate the conveyed material, and so it is most convenient to discharge the material from the end of the conveying pipeline back into the supply hopper. Thus the supply hopper that feeds material into the blow tank doubles as the reception hopper. Normally the entire batch of material in the supply hopper will be discharged into the blow tank to be returned to the supply/reception hopper. Since the material at the end of the pipeline will be in a highly aerated state, the size of the supply/reception hopper typically needs to be about 20% greater than that of the blow tank, as follows:
Pipeline Bore
- inch
Supply/Reception Hopper Volume -
ft3
2
2'/2
3
4
6
60
90
150
250
550
A conical or pyramid type section will be required on the bottom of the supply hopper, depending on whether a square or circular design is adopted. In either case as steep a wall slope as possible would be recommended in order to minimize flow problems in the filling process for the blow tank. If head room does not allow, consideration must be given to the use of discharge aids, such as those based on air, vibration, etc.
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658
Chapter 23
3.3.1 Support Structure The supply hopper will need to be mounted above the blow tank and a support structure will be required for this purpose. It would be useful to incorporate a platform for access to the filtration plant in this structure, as well as the provision of access from the ground. 3.4
Filtration Unit
The supply/reception hopper will need to be fitted with a filtration unit, probably mounted on top of the reception hopper. A standard reverse air jet type of bag filter would be recommended for the duty. The size will be dictated by the free air flow rate to a large extent:
Pipeline Bore
- inch
Free Air Flow Rate
- rf/min
2
2!/2
3
4
6
200
300
450
800
1800
The filter should be sized on the basis of handling cement or very fine fly ash, at these air flow rates. 3.5
Plant Layout
A typical layout of blow tank, supply/reception hopper and filtration unit is shown in Figure 23.2. With the filter mounted on top of the hopper, the conveyed material will remain within the conveying system. In this arrangement the filter unit does add to the overall height of the conveying plant. If this is too high, the air could be ducted from the hopper to a filter unit positioned alongside, possibly on the ground. This arrangement, however, will mean that much of the fine dust from the material will not be returned automatically to the bulk of the material. This, however, may be an advantage and so the alternatives must be considered. 3.5.1 Material Re-circulation With a need to re-circulate the material, for the convenience of carrying out many tests, once the material is loaded into the conveying system, a decision will need to be made on whether to keep the fine material within the system or to extract this material. If the material being conveyed is friable, to the extent that a change in particle size distribution might result in a gradual change in the conveying characteristics for the material, as illustrated with soda ash in Figures 11.18 and 19, it would always be recommended that fresh material should be used for every test. A vent line between the blow tank and the hopper should also be provided. This will need to be opened when loading the blow tank with material. It can also be used to de-pressurize the blow tank at any time, should this be necessary.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
659
Conveying Test Facilities
Filter
Return to Hopper
Figure 23.2
Sketch of typical conveying plant test facility.
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660
3.6
Chapter 23
Conveying Pipeline
A convenient routing for the pipeline needs to be established, preferably with a single loop, incorporating four bends, having a conveying distance of about 300 ft. The provision for additional loops also needs to be considered so that the conveying distance can be extended, possibly in units of 300 ft. The ratio of 4 bends in 300 ft of pipeline should provide a typical pipeline balance. A larger bore line could be added in future so that the performance of stepped pipelines can be investigated. The pipeline should be reasonably accessible so that changes in bend types and routing can be conveniently made. The possibility of having one or two sight glasses in the pipeline, for flow visualization purposes, should be considered. This would be of particular value when demonstrating the operation of the test facility to clients, and is always of value as a research facility since much can be learnt from observation of the flow. 3.6.1 Orientation For convenience it would be suggested that the pipeline loops be located entirely in the horizontal plane. No attempt need be made at this early stage to incorporate any vertical lift into the pipeline, other than that necessary to accommodate changes in elevation between the blow tank discharge and entry to the reception hopper. 4
SERVICE FACILITIES
A number of service facilities will be required for the test facility, mainly centered on the handling and storage of the materials to be conveyed. The size of some of these units will depend upon the batch size to be handled, and hence the pipeline bore selected. 4.1
Material Loading
A convenient means of loading a batch of material into the supply hopper will be required. A small low pressure blow tank would be ideal for this purpose, which need not be large, since the load could be charged in small batches. Alternatively a mechanical or aero-mechanical conveyor could be used for the purpose. The filtration unit on the hopper should be sufficient for the purpose. If a dedicated line is employed for material loading an isolating valve will be required. 4.2
Material Off-Loading
When test work with a particular batch of material is completed, the batch of material will need to be off-loaded from the test rig. The conveying system itself can conveniently be used for this purpose, possibly via a short section of the convey-
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Conveying Test Facilities
661
ing pipeline, into an off-loading hopper. The off-loading hopper will need to be of a similar size to that of the reception hopper and to be fitted with a filtration unit. A reduced quantity of conveying air could probably be used for this purpose so that the filtration unit would not have to be as large as that mounted on the reception hopper. 4.3
Storage Hoppers
If materials are to be stored for possible re-use a number of such storage hoppers will be required for the purpose. It would be an advantage to have these hoppers elevated so that the contents could be discharged back into the supply hopper, by means of the loading facility, when required. Alternatively the material could be loaded back into sacks for subsequent disposal after use. A valve would also be needed at the outlet for these purposes. If provision needs to be made for the storage of a number of materials, the storage hoppers could be inter-linked. By this means they could be loaded from a common pipeline via diverter valves, and they could all be vented through a single filter unit. 5
INSTRUMENTATION
A number of measuring instruments will be required in order to take measurements of pressures, temperatures and flow rates. 5.1
Air Pressure
A minimum of two pressure measurements need to be recorded. These are of the pressure in the blow tank and of the air pressure at inlet to the conveying pipeline. These can be Bourdon type pressure gauges, with values recorded manually with respect to time during each test. Alternatively pressure transducers can be employed that give a digital display. If on-line computer analysis is to be employed, suitable pressure transducers should be used. The monitoring of pressure along the length of the pipeline would not be recommended as an initial instrumentation requirement, but it is suggested that it should be given high priority for future development, particularly if research work is to be undertaken. In common with most plants that involve the flow of fluids, the measurement of pressure in pneumatic conveying systems is equally important to the efficient operation of any such plant. Gas-solids flows are not as amenable to mathematical analysis, as single-phase flows, and as a result the monitoring of pressure is a common requirement. Technical difficulties in measuring pressure in pneumatic conveying system pipelines, however, tend to be much greater when compared with similar problems in single-phase flow [1]. Even the interpretation of the pressure readings obtained from gas-solid flow systems requires specialized analytical techniques, as discussed in relation to the
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Chapter 23
662
measurement of pressure drop across bends with Figure 8.14. Since theoretical design methods are severely limited by the complex behavior of gas-solid flows, the design of pneumatic conveying systems relies heavily on experimental methods. This applies to both dilute and dense phase modes of conveying. 5.1.1 Pressure Tappings The reliable measurement of pressure along pneumatic conveying system pipelines requires pressure tappings, and any connecting lines to a pressure measurement device, to remain unblocked by the conveyed material. Pressure tappings invariably block at the start of a conveying cycle, or as a result of pressure pulsations that may occur during conveying. An increase in pressure will cause some of the fines in the material being conveyed to surge into the connecting lines, where the material may be deposited, and a gradual build-up is likely to result in a blockage. The shortening of connecting lines will help to reduce the problem of material ingress. Another solution is to pass these lines vertically upward wherever possible, so that particulate material will drain out of the lines, but this is not always possible. In most cases filters are inserted near the tapping point. A typical example is illustrated in Figure 23.3 [1]. Filter pads will become covered and impregnated with conveyed material, and so it is usually necessary to provide a reverse flow of high pressure air in order to purge all such pads clean periodically. It is also common practice to have more than one pressure tapping at each location along a pipeline. Three typical arrangements are illustrated in Figure 23.4 [2J.
Filter Pad
Figure 23.3
Typical pressure tapping point on a pneumatic conveying system pipeline.
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663
Conveying Test Facilities
(a)
(c)
Figure 23.4 Typical arrangements of static pressure tappings in pneumatic conveying system pipelines, (a) All four inter-connected, (b) three inter-connected, and (e) separate. The normal procedure is to link all three or four pressure tappings together, as shown in Figures 23.4a and b. The advantage of this arrangement is that if one of the tappings becomes blocked, a valid pressure reading will still be obtained. Only for very specific research purposes would the individual tappings each be provided with a dedicated pressure measuring device, as shown in Figure 23.4c. 5.7.2 Bend Pressure Drop Measurement The difficulties of pressure measurement in pneumatic conveying system pipelines are highlighted most effectively with the problem of measuring the pressure drop across a bend in a pipeline, as illustrated with Figure 8 . 1 1 . It is not just a matter of recording the pressure at inlet to and outlet from the bend and subtracting the two readings. It is necessary to record the pressure at regular intervals along the sections of pipeline both before and after the bend. Part of the problem lies in the complexity of the flow in the region of a bend. The conveyed particles approaching a bend, if fully accelerated, will have a velocity that is about 80% of that of the conveying air. This velocity, of course, depends upon the particle size, shape and density, and the pipeline orientation. At outlet from a bend the velocity of the particles will be reduced and so they will have to be re-accelerated back to their terminal velocity in the straight length of pipeline following the bend. 5.7.3 Particle Deflection Influences Reliable pressure measurement at any given point requires the flow to be both steady and wholly axial. If this is not the case a dynamic element of pressure will exist, in addition to the static element, and inconsistent or false readings may result. The dynamic element may add to or subtract from the static value depending
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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Chapter 23
upon the geometry of the flow. This is a situation that can occur at outlet from a bend in a pipeline. In a long radius bend centrifugal force will tend to take the particles to the outer wall. In a short radius bend the particles may bounce through the bend. Following the bend the particles will gradually establish a steady flow regime some distance downstream. In a horizontal pipeline, large or heavy particles will have a tendency to 'skip' through the pipeline when conveyed in dilute phase. This is because the gravitational force on the particles is relatively high compared with the drag force. Poorly welded pipe joints and misaligned flanges can cause particles to stream and deflect from the discontinuity in flow. This streaming of particles can be particularly pronounced in worn bends. Mason and Smith [3] carried out tests with a Perspex bend in order that the change in flow pattern and wear over a period of time could be visually observed. Alumina particles were conveyed and the flow was from vertical to horizontal. The results of one of their tests was shown earlier in Figure 20.25. Pronounced streaming of particles was observed from a number of wear sites that had formed, including the straight section of pipeline following the bend. Mason and Smith |2], monitoring pressures around 90° bends, and using the array of pressure tappings illustrated in Figure 23 Ac, recorded pressures at outlet from a bend. Their work has shown that the upper tapping can record a pressure that is greater than that at entry to the bend, from which it might be deduced that the pressure drop around the bend is negative. The flowing suspension impacts on the wall surface at an angle of about 20° and the dynamic pressure contribution gives an apparent gain in 'static' pressure. A deflecting flow away from the surface can induce a suction effect, however, leading to an apparent excessive pressure loss. Such turbulence in pneumatic conveying system pipelines is unavoidable, particularly after a change in direction, but its effects can be identified if pressure measurements are taken at regular intervals along a pipeline. In straight pipeline without any fittings a reasonably regular pressure gradient should exist and so if an isolated reading gives an inconsistent value it can generally be disregarded. It could also indicate that the pressure tappings at this point are blocked. Inconsistencies in pressure readings should not be dismissed, however, without examining all possible causes for, as mentioned earlier, gassolid flows are very complex and measurement of pressure drop requires great care. 5.1.4 Straight Pipeline Pressure Gradient Although the energy loss due to bends in a pneumatic conveying system pipeline can be very significant, particularly if there are a large number of bends, the pressure drop in the straight pipeline generally dominates in most pipelines. The method of determining the pressure drop, or pressure gradient, in straight pipeline is much as shown in Figure 8.14.
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Conveying Test Facilities
665
Because of all the problems enumerated above it is generally recommended that a long section of pipeline should be instrumented with at least six sets of pressure tappings. By this means reasonable data can be obtained even if one or two of the tapping points block. From Figure 8.14 it will be seen that the first of the series of pressure tappings should be located well down-stream of a bend in the pipeline, or any other fitting that is likely to cause an initial disturbance to the flow in the straight section of pipeline. With pressure tappings along a straight length of pipeline, data in the form of pressure gradients can be obtained, in isolation from the total pipeline. In Chapter 8 pressure gradient data was presented for a number of materials for flow in pipelines both vertically up and vertically down. The pipeline used to generate this data was shown in Figure 8.2 and the routing included long sections of vertical pipeline specifically for this purpose. 5.2
Conveying Air Temperature
The temperature of the air at inlet to the pipeline also needs to be recorded for reference. On a conveying plant the material could well be at a high temperature and so the influence that this might have on the temperature of the suspension has to be established. With a test facility it is unlikely that tests would be conducted with material at an elevated temperature. 5.3
Air Flow Rate and Control
The air flow rate needs to be set and controlled at a reasonably precise given value for each test undertaken. The most convenient way of doing this with high pressure air is to use a set of convergent-divergent nozzles. Two sets will be required for a blow tank, one for the conveying air and another for the blow tank air supply. The modeling and use of such nozzles was considered in detail in Chapter 6 at section 3.1. A 2:1 progression in volumetric flow rate capacity is suggested for the nozzles, starting at about 4 cfm, so as to give a very wide and uniform range of flow rates over which the air flow rate can be varied. It is suggested that for a 2 inch bore pipeline two sets of 6 nozzles would be required. Two sets of 7 nozzles would be needed for the 21/2 and 3 inch bore lines and two sets of 8 nozzles for the 4 inch bore pipeline. 5.4
Conveyed Material Flow Rate
The most convenient method of measuring the mass flow rate of the conveyed material is to use load cells. These can be used on the blow tank to measure loss in weight, or on the receiving hopper to record gain in weight. The mounting of the receiver on three load cells is generally the best configuration. The read out of the three load cells is usually summed and then the values can either be displayed on an instrument for manual recording with respect to time; be recorded on a chart for
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Chapter 23
666
subsequent analysis; or be fed into a data logger or computer for possible on-line analysis, depending upon the level of sophistication required. The rating of the load cells will depend upon the size of batch to be conveyed and the weight of the reception hopper, both of which will depend upon the choice of pipeline bore. For the 2 and 2'/2 inch bore pipelines it is suggested that three 1 ton load cells would be required, and for the 3 and 4 inch bore pipelines three 2 ton load cells would be needed. 5.4.1 Load Cells The most commonly used device for the measurement of material flow rate is the load cell. A typical arrangement for a positive pressure pneumatic conveying system is illustrated in Figure 23.5 [4], The situation with regard to a vacuum conveying system would be essentially the same. Whether the load cells are used in conjunction with the supply hopper or the reception hopper is mostly a matter of convenience. If the supply hopper is chosen a loss in mass will be recorded and with the reception hopper there will be a gain in mass. In terms of steady state readings there should be little or no difference between the two. Although Figure 23.5 is shown with both supply and reception hoppers mounted on load cells, only in special cases would it be necessary to mount both hoppers on load cells. Such cases would include non-steady state conveying and the need to monitor material deposition in the conveying pipeline. Filtration Unit
Figure 23.5 Sketch of typical positive pressure conveying system with hoppers mounted on load cells.
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667
Conveying Test Facilities
5.4.1.1 Flexibility For load cells to provide repeatable and reliable recordings it is essential that the hopper should be allowed to 'float' as feely as possible on the load cells. No anchoring or restraining of the hopper should be employed that will apply any component of vertical force. Connecting pipelines often present a problem in this respect but this can be overcome quite reasonably by means of a flexible connection in the pipeline, close to the hopper, with the pipe/hose connection furthest from the hopper being supported. The pipeline feeding device and air supply/exhaust lines may also prove difficult to accommodate, and for these reasons load cells are generally used on reception hoppers for positive pressure conveying systems and on supply hoppers for vacuum conveying systems. Provided that they do not interfere with the vertical component of force, any filtration plant, feeders and offloading facilities associated with the hopper can be taken into account with the tare weight of the hopper itself. This weight, together with the maximum expected load of material in the hopper, will be used in determining the size of load cells to be employed for the duty. 5.4.2
Analysis of Data
The output from the load cells is either fed into a data logger or computer, or is recorded on a chart. Either way the signal is integrated with respect to time to give the material flow rate. A typical trace, with respect to time, for the conveying of a 500 Ib batch of material is given in Figure 23.6.
25 u 3 cti M
20
500
Air Pressure
400 300
15
200 10
o
100 1
— I"
t:\
L where p = material bulk density T = time k" = de-aeration constant A/? = pressure drop across bed and L = bed height
-
lb/ft 3 min ft/min lbf/in 2 ft
Integration of this expression between suitable experimentally derived limits will yield the de-aeration constant. High values of this constant indicate a high settling rate and, therefore, poor air retention capability. A further method of monitoring rapid transients is to use an electronic differential pressure transducer. If this is connected across the pressure tappings on the column of material on the permeameter, it will provide a suitable trace of the pressure decay following the shut off of the air, for evaluation of the constant. The value of the de-aeration constant obtained will give some indication of the capability of a material for dense phase pneumatic conveying, without the need for air addition along the length of the pipeline. It will also give an indication of the effect that aeration might have on the material, for aiding its discharge from hoppers. 7. 6. 2 Vibrated De-aeration Constant If the bed of material in the de-aerated condition is vibrated, the height will fall in a similar manner to that described above, in which the fluidized bed height falls when the air supply is cut off. A comparison of the two de-aeration plots of bed height versus time is illustrated in Figure 23. 1 7 [7]. It is possible, therefore, that this vibration test could generally be of more value than the permeameter method. For materials that exhibit poor air retention characteristics, and hence de -aerate rapidly, the rate of change can be slow enough to observe visually. On the other hand, for some very air retentive powders, the settling time can run into hours and even days, and vibration can speed up the process considerably.
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683
Conveying Test Facilities
Fluidized Condition
£P 'S
I
Compacted Condition
ffl
Settlement under the Influence of Gravity
Settlement under the Influence of Vibration
Time - T Figure 23.17
Comparison of de-aeration curves.
It is also very much easier to apply to cohesive and other materials that are difficult to aerate. Vibration is applied in the vertical plane, but only a narrow band of frequencies have a settling effect on materials. If the frequency is too low it has little effect, and if it is too high dilation will occur instead of compaction. Also, the higher the frequency, the lower the penetration of vibration. 7.6.2.1 Analysis An idealized graph showing the change in bed height with respect to time was shown above in Figure 23.17. This compares settlement under the influence of gravity and vibration. It can be seen that the relationship in each case is similar and, therefore, it is not unreasonable to apply the analysis proposed by Sutton and Richmond for the settlement of powders under the influence of gravity to the settlement of powders under the influence of vibration. The application of the analysis of Sutton and Richmond to this case yields: JP
^~
~
K k"
(6)
v I
where k"v = vibrated de-aeration constant
and Ap = py-, - p
- ft/min - lb/ft 3
This expression can be put into a form where it can be integrated and the following boundary conditions applied:
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684
L = L,
at r = 0,
T = oo,
L = L,x
The result is:
In
= k"
(7)
L where £/
= initial bed height
and LOO = final bed height
- ft - ft
This equation can be written in the form of a straight line graph, the slope of which is the vibrated de-aeration constant. Thus
H = k" v r
(8)
= L
(9)
where
H
In
A detailed test procedure is given in Reference 7. These tests are relatively easy to undertake and take little time to carry out. A small sample of the material is all that is required and the equipment needed to carry out the tests manually is relatively simple and inexpensive. 7.7
Permeameter Design
A permeameter, is an invaluable device both for determining the minimum fluidizing velocity of a bulk particulate material, and for observing the fluidization behavior of different materials. It can also be used to measure the permeability of powdered and granular materials, as well as the air retention characteristics of such materials. It also provides an easy means of determining the resistance of porous membrane materials.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
685
Conveying Test Facilities
Air Mover
Membrane
Plenum Chamber Air Flow Measurement and Control
Figure 23.18
Typical layout of permeameter and components.
The authors are not aware of any company that manufactures and markets permeameters. Most companies and research organizations that find that they have a need for a permeameter, generally make their own. In order to provide some general advice and guidance on the design and construction of a permeameter, notes are appended here. For reference purposes a sketch of a permeameter is given in Figure 23.18. This shows the associated components in relation to the permeameter. The main items that are required are considered in the notes that follow. A range of sizes are also considered. 7. 7 /
Material Column
The heart of the device is the vertical column, or permeameter, in which the bulk particulate material is fluidized. The behavior of the material in the permeameter requires to be observed, and in particular the height of the free surface. For this reason the column needs to be made of a clear material such as glass or Perspex. Perspex is the material most commonly used. The column is open to the atmosphere and so the pressure within the device is very low. A sketch is given in Figure 23.19. 7.7.1.1 Dimensions The column can be square, circular, or of any other section, but it is usually circular and of constant diameter. The primary dimension of the device is the inside diameter, d, of the cylinder used to contain the particulate material. Diameters of 2, 4 and 6 in will be considered.
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686
Cylinder Material Bed
2ft
in Pressure Tappings
- 8 in
Valve
\ Membrane Figure 23.19
Plenum Chamber
Sketch of material column.
Two inch is typically the smallest diameter used, and is probably the most common, as only small quantities of material are required for testing purposes. With larger diameters, however, the wall effects are minimized and membrane influences on fluidization are easier to detect. Diameters not less than 4 in are generally recommended whenever possible. Larger diameters also help to increase the accuracy of air flow measurement, and hence the determination of superficial gas velocity. This is particularly a problem with very fine powders since fluidizing velocities can be very low. Regardless of diameter, the column of material under test needs to be about one foot high. The height of the Perspex cylinder needs to be about double this at two feet. The cylinder should be much higher than the bed of material in order to allow for expansion of the bed when fluidized, and possible violent agitation when fluidized at high velocity. Also, some materials will rise en masse above the membrane when fluidized, and a reasonable column height will allow time to switch off the air, or stir the material, before it discharges itself over the top. 7.7.1.2 Pressure Tappings It is suggested that pressure tappings should be provided on the cylinder about 8 in apart, with the lower one about 2 in above the base. Depending upon the type of pressure measuring device employed it may be necessary to add a gauze to filter dust from the device. It may also be necessary to cap the tappings, if the pressure measuring device is removed, such as when fluidizing in order to observe flow behavior.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
687
Conveying Test Facilities
Material Column
Pressure Tappings
Membrane
Clamps
Hinges
-9t
Th Gasket or Seal
Air Supply
Dia
Depth
T Gap
D Figure 23.20
Sketch of plenum chamber.
7.7.2 Plenum Chamber A plenum chamber is required for supporting the membrane and material column, and distributing the air to the bed of material. The plenum chamber can be square or circular and needs to be vertical. A sketch of a typical plenum chamber is given in Figure 23.20. 7.7.2.1 Hinged Unit The plenum chamber can be hinged or entirely welded. In Figure 23.20 it is shown hinged. A hinged unit is very convenient in allowing the contents of the material column to be emptied simply by inverting the cylinder. A disadvantage is that the hinged top must be provided with an air-tight seal, since the air flow rate is measured upstream of the plenum chamber and needs to be determined accurately. Although an entirely welded unit eliminates air leakage, some means has to be found for removing the material from the permeameter. A vacuum cleaner is typically used for this purpose. 7.7.2.2 Pressure Tapping A pressure tapping on the plenum chamber will allow the pressure drop across different membrane types and materials to be measured directly. This, of course, is carried out with no material in the permeameter. If the pressure measuring device is removed, the pressure tapping must be capped to prevent air leakage. 7.7.2.3 Dimensions Approximate sizes for the plenum chamber, for the range of permeameter diameters considered, are as follows:
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Permeameter Chamber Width or Diameter Diameter D d in in 8 10 12
2 4
6
Gap
Depth
in
in
0-2 0-4 0-6
3 4 5
Air Supply Pipe Diameter in 3
/4
1% 2
A short stub of pipe, of the same diameter as the permeameter, needs to be fixed beneath the membrane, as shown in Figures 23.19 and 20. The spacing above the base (gap) will help provide a uniform flow of air across the membrane for fluidizing the material in the permeameter. The other dimensions are also in proportion to the permeameter diameter, and hence air flow rate. 7.7.3 Membrane A range of membrane materials and types may need to be tested and so ease of changing and testing needs to be incorporated into the design. A suggestion for a fixing arrangement is given in Figure 23.21. Screwing to the top surface of the plenum chamber with studs is probably the most convenient, and will accommodate a wide range of membrane thicknesses. Washers or gaskets will have to be provided on each side to prevent air leakage.
Material Column
Sealing Washers / Gaskets
Membrane Top Surface of Plenum Chamber
Y Figure 23.21
Sketch of fixing arrangement for membrane.
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Conveying Test Facilities
689
It is unlikely that any membrane material under test will need support in the permeameter against either the air pressure beneath or the weight of material above. If it should be necessary, however, support can be provided on either side by means of wire mesh. 7.7.4 A ir Supply A small air mover is required to supply the air for fluidizing. Measurement and control of the air flow are essential requirements. 7.7.4.1 Rating The rating of the air mover is in terms of delivery pressure and volumetric flow rate. 7.7.4.1.1 Pressure The pressure required for the air mover is mainly that necessary to fluidize the bed of material. A small allowance will have to be made for the porous membrane and the resistance of the air flow measuring device, together with all the connecting pipe-work. The model for the fluidized bed was presented earlier in Equation 23.4:
Ap = £^±144 gc taking p g L and gc
= = = =
lbf/in2
60 Ib/ft as a typical value (eg alumina) 32-2 ft/s2 } ft 32-2 ft Ib/lbf s2
gives Ap
= 0-4 lbf/in 2
To enable the permeameter to be used with materials having a much higher bulk density, such as barite and metal powders, it would be advisable that the air mover have a pressure capability somewhat higher, at about 1-2 lbf/in (33 in wg). This will allow tests to be undertaken with materials having more than double the density of alumina, and also accommodate the pipe-work and flow measuring device losses. Since the bed height remains constant, with increase in permeameter diameter, the pressure required will also remain constant with the diameter of the permeameter. 7.7.4.1.2 Flow Rate The volumetric flow rate of air to be delivered, V , is given by air velocity times flow area, which in this case is:
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
690
Chapter 23
,2
V
=
U mf
x
— 576
ft3/min
- - - - - -
where U m f = minimum fluidizing velocity (see Figure 23.15) and d = diameter of permeameter
(10)
- ft/min - in
From Figure 18.16 it will be seen that U m r can vary over an exceptionally wide range, depending mainly upon the mean particle size of the bulk solid. For 20 micron sized particles having a particle density of 60 lb/ft3, for example, it is about 0-04 ft/min, and for 500 micron sized particles having a particle density of 300 lb/ft3 it is about 60 ft/min. It is necessary for fluidization tests to be undertaken with air velocities much higher than the value of the minimum fluidization value and so it is recommended that a permeameter should be designed to provide a maximum fluidizing air velocity of 100 ft/min. For the range of permeameter diameters being considered, the air flow rates required are as follows:
Permeameter Diameter - d
Air Flow Rate
in
ft3/min
2 4 6
2 8 19
7.7'.4.2 Air Mover From this pressure drop and flow rate rating it will be seen that a small fan or blower would be suitable. A power rating well below 1 hp would be required. 7.7.4.3 Measurement As discussed in relation to Figure 18.16, a very wide range of fluidizing velocities have to be catered for. With large particulate materials 100 ft/min will be required and the maximum air flow rate available will have to be used. With fine powders, however, the maximum fluidizing velocity required may be well below 1 ft/min. In this case less than 1% of the air flow rate will be required as a maximum, and it will be necessary to accurately measure air flow rates to 1% accuracy below this value. It will be appreciated from this data, just why sealing of the membrane and plenum chamber are so important. A very small air leak can represent a very large error in the value of the fluidizing velocity for a fine powdered material.
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For a permeameter to be capable of testing a wide range of powdered and granular materials, therefore, the measurement of air flow rate is critical, and the measuring device is a major feature of the permeameter. There is a need for the air flow rate to be reduced by a factor of at least 10,000:1. This is not likely to be achieved with a single flow meter, but will require staging and isolating valves. A three stage device, based on rotameters, is shown in the sketch in Figure 23.18. With a 10:1 turn down ratio on each rotameter, or flow measuring device, stage one could cater for fluidizing velocities from 0 to 1 ft/min, the second from 0 to 10 ft/min and the third from 0 to 100 ft/min. By this means reasonable control and accuracy could be obtained in the testing of any material. Rotameters are ideal for this purpose as they provide a direct visual display, do not take up too much space, and can be easily plumbed into the system. In terms of the three sizes of permeameter being considered, approximate volumetric flow rates required, in ftVmin, for a three stage measuring device are as follows:
Permeameter Diameter
Fluidizing Air Velocity Range
d
ft/min
in
0 - 1
0 - 10
0 - 100
2 4 6
0-02 0-08 0-19
0-2 0-9 1-9
2-0 8-4 19-0
The diameter of the air supply piping into the plenum chamber, for the different permeameter diameters, was given earlier in the table in section 7.7.2.3. These same diameters can apply to the pipe-work throughout the entire air supply and flow measuring system. 7.7.4.4 Control It is unlikely that either a fan or a blower would be capable of achieving such a wide turn down ratio. To overcome this problem it is suggested that a tee piece with valves should be fitted between the air mover and the flow measuring device, as shown on Figure 23.18, so that air not required can be discharged to atmosphere. It is only loss of air downstream of the flow measuring device that must be prevented. The valve on the air supply line, at entry to the plenum chamber, is not used for flow control. It is either fully open or fully closed. It does, however, need to be capable of rapid closure. This facility is required when the permeameter is used to measure the de-aeration constant for a bulk material, and in any emergency situation, such as the bed of material rising en masse in the permeameter.
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692
Fluidizing Column
Air Flow Measuring Device
Membrane Discharge to Atmosphere Plenum Chamber
Figure 23.22 7.7.5
Possible working layout for permeameter.
Layout
It should be possible to mount the entire device on a small table. The air mover can be placed on a shelf below, the flow measuring device can be mounted on a board behind, and the permeameter itself at the edge of the table, if it has a hinged top. A sketch of such a layout is given in Figure 23.22. REFERENCES D. Mills. Measuring pressure on pneumatic-conveying systems. Chemical Engineering. Vol 108, No 10, pp 84-89. Sept 2001. J.S. Mason and B.V. Smith. Pressure drop and flow behavior for the pneumatic transport of fine particles around 90° bends. Proc Pneumotransport 2. BHRA Conf Paper A2, 16 pp. Guildford, England. Sept 1973. J.S. Mason and B.V. Smith. The erosion of bends by pneumatically conveyed suspensions of abrasive particles. Powder Technol, Vol 6, pp 323-335. 1972. 4. D. Mills. Material flow rates in pneumatic conveying. Chemical Engineering. Vol 109. No 4. pp 74-78. April 2002. D. Mills. Optimizing pneumatic conveying. Chemical Engineering. Vol 107. No 13. pp 74-80. December 2000. 6. J.S. Mason and D. Mills. 20 years of pneumatic conveying with the Powder and Bulk Solids Conference. Proc 20th Powder and Bulk Solids Conf. pp 3-40. Chicago. May 1995.
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693
M.G. Jones and D. Mills. Product classification for pneumatic conveying. Powder Handling and Processing. Vol 2. No 2. pp 117-122. June 1990. 8. R. Siegel. Effect of distributor plate-to-bed resistance ratio on the onset of fluidized bed channeling. AlChE Jnl. Vol 22. No 3. pp 590-592. 1976. 9. A.M. Sutton and R.A. Richmond. Improving the storage conditions of fine powders by aeration. Trans Inst Chem Engrs. Vol 51. 1973. 10. A.M. Sulton and R.A. Richmond. How to improve powder storage and discharge in hoppers by aeration. Process Engng. Sept 1973.
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