Industries and materials
A wide variety of materials are handled in powdered and granular form, and a large number of different industries have processes which involve their transfer and storage. Some of the industries in which bulk materials are conveyed include agriculture, mining, chemical, pharmaceuticals, paint manufacture, and metal refining and processing.
In agriculture very large tonnages of harvested materials such as grain and rice are handled, as well as processed materials such as animal feed pellets. Fertilizers represent a large allied industry with a wide variety of materials. A vast range of food products from flour to sugar and tea to coffee are conveyed pneumatically in numerous manufacturing processes. Confectionery is a particular industry in which many of these materials are handled.
In the oil industry fine powders such as barytes, cement and bentonite are used for drilling purposes. In mining and quarrying, lump coal and crushed ores, and minerals are conveyed. Pulverized coal and ash are both handled in very large quantities in thermal power plants. In the chemical industries materials include soda ash, polyethylene, PVC and polypropylene in a wide variety of forms from fine powders to pellets. Sand is used in foundries and glass manufacture, and cement and alumina are other materials that are conveyed pneumatically in large tonnages in a number of different industries.
Mode of conveying
Much confusion exists over how materials are conveyed through a pipeline and to the terminology given to the mode of flow. First it must be recognized that materials can either be conveyed in batches through a pipeline, or they can be conveyed on a continu- ous basis, 24 h a day if necessary. In batch conveying the material may be conveyed as a single plug if the batch size is relatively small.
For continuous conveying, and batch conveying if the batch size is large, two modes of conveying are recognized. If the material is conveyed in suspension in the air through the pipeline it is referred to as dilute phase conveying. If the material is con- veyed at low velocity in a non-suspension mode, through all or part of the pipeline, it is referred to as dense phase conveying.
Dilute phase
Almost any material can be conveyed in dilute phase, suspension flow through a pipe- line, regardless of the particle size, shape or density. It is often referred to as suspension flow because the particles are held in suspension in the air as they are blown or sucked through the pipeline. A relatively high velocity is required and so power requirements can also be high but there is virtually no limit to the range of materials that can be conveyed.
There will be contact between the conveyed material and the pipeline, and particu- larly the bends, and so due consideration must be given to the conveying of both friable and abrasive materials. With very small particles there will be few impacts but with large particles gravitational force plays a part and they will tend to ‘skip’ along horizon- tal pipelines.
Many materials are naturally capable of being conveyed in dense phase flow at low velocity. These materials can also be conveyed in dilute phase if required. If a high velocity is used to convey any material such that it is conveyed in suspension in the air, then it is conveyed in dilute phase.
Dense phase
In dense phase conveying two modes of flow are recognized. One is moving bed flow, in which the material is conveyed in dunes on the bottom of the pipeline, or as a pulsatile moving bed, when viewed through a sight glass in a horizontal pipeline. The other mode is slug or plug type flow, in which the material is conveyed as the full bore plugs sep- arated by air gaps. Dense phase conveying is often referred to as non-suspension flow.
Moving bed flow is only possible in a conventional conveying system if the mater- ial to be conveyed has good air retention characteristics. This type of flow is typically limited to very fine powdered materials having a mean particle size in the range of approximately 40–70 /-Lm, depending upon particle size distribution and particle shape.
Plug type flow is only possible in a conventional conveying system if the material has good permeability. This type of flow is typically limited to materials that are essentially mono-sized, since these allow the air to pass readily through the interstices between the particles. Pelletized materials and seeds are ideal materials for this type of flow.
Conveying air velocity
For dilute phase conveying a relatively high conveying air velocity must be main- tained. This is typically in the region of 12 m/s for a fine powder, to 16 m/s for a fine granular material, and beyond for larger particles and higher density materials. For dense phase conveying, air velocities can be down to 3 m/s, and lower in certain cir- cumstances. This applies to both moving bed and plug type dense phase flows.
These values of air velocity are all conveying line inlet air velocity values. Air is compressible and so as the material is conveyed along the length of a pipeline the pres- sure will decrease and the volumetric flow rate will increase.
For air the situation can be modelled by the basic thermodynamic equation:
|
p1V˙ =
T1
p2V˙ T2
|
.
(1.1)
where p is the air pressure (kN/m2·abs), V, the air flow rate (m3/s), T, the air tempera-
ture (K) and subscripts 1 and 2 relate to different points along the pipeline.
If the temperature can be considered to be constant along the length of the pipeline this reduces to:
|
p1V˙ =
p2V˙
(1.2)
|
Thus if the pressure is one bar gauge at the material feed point in a positive pressure conveying system, with discharge to atmospheric pressure, there will be a doubling of the air flow rate, and hence velocity in a single bore pipeline. If the conveying line inlet air velocity was 20 m/s at the start of the pipeline it would be approximately 40 m/s at the outlet. The velocity, therefore, in any single bore pipeline will always be a minimum at the material feed point.
It should be emphasized that absolute values of both pressure and temperature must always be used in these equations. These velocity values are also superficial values, in that the presence of the particles is not taken into account in evaluating the velocity, even for dense phase conveying. This is universally accepted. Most data for these val- ues, such as that for minimum conveying air velocity are generally determined experi- mentally or from operating experience. It is just too inconvenient to take the presence of the particles into account.
Particle velocity
In dilute phase conveying, with particles in suspension in the air, the mechanism of con- veying is one of drag force. The velocity of the particles, therefore, will be lower than that of the conveying air. It is a difficult and complex process to measure particle velocity, and apart from research purposes, particle velocity is rarely measured. Once again it is generally only the velocity of the air that is ever referred to in pneumatic conveying.
In a horizontal pipeline the velocity of the particles will typically be about 80% of that of the air. This is usually expressed in terms of a slip ratio, defined in terms of the
velocity of the particles divided by the velocity of the air transporting the particles, and in this case it would be 0.8. The value depends upon the particle size, shape and density, and so the value can vary over an extremely wide range. In vertically upward flow in a pipeline a typical value of the slip ratio will be about 0.7.
These values relate to steady flow conditions in pipelines remote from the point at which the material is fed into the pipeline, bends in the pipeline and other possible flow disturbances. At the point at which the material is fed into the pipeline, the mater- ial will essentially have zero velocity. The material will then be accelerated by the conveying air to its slip velocity value. This process will require a pipeline length of several metres and this distance is referred to as the acceleration length. The actual distance will depend once again on particle size, shape and density.
There is a pressure drop associated with acceleration of the particles in the air stream and it has to be taken into account by some means. It is not only at the material feed point that there is an acceleration pressure drop. It is likely to occur at all bends in the pipeline. In traversing a bend the particles will generally make impact with the bend wall and so be retarded. The slip velocity at exit from a bend will be lower than that at inlet and so the particles will have to be re-accelerated back to their steady-state value. This additional element of the pressure drop is usually incorporated in the overall loss associated with a bend.
Solids loading ratio
Solids loading ratio, or phase density, is a useful parameter in helping to visualize the flow. It is the ratio of the mass flow rate of the material conveyed divided by the mass flow rate of the air used to convey the material. It is expressed in a dimensionless form:
4> =
m˙ p
3.6 m˙ a
(1.3)
where 4> is the solids loading ratio (dimensionless), m˙ p, the mass flow rate of material (tonne/h) and m˙ a, the mass flow rate of air (kg/s).
Since the mass flow rate of the conveyed material, or particles, is usually expressed in tonne/h and the mass flow rate of the air is generally derived by calculation in kg/s, the constant of 3.6 in Equation (1.3) is required to make the term dimensionless. A par- ticularly useful feature of this parameter is that its value remains essentially constant along the length of a pipeline, unlike conveying air velocity and volumetric flow rate, which are constantly changing.
For dilute phase conveying, maximum values of solids loading ratio that can be achieved are typically of the order of about 15. This value can be a little higher if the conveying distance is short, if the conveying line pressure drop is high, or if a low value of conveying air velocity can be employed. If the air pressure is low or if the pipeline is very long, then the value of solids loading ratio will be very much lower.
For moving bed flows, solids loading ratios need to be a minimum of about 20 before conveying at a velocity lower than that required for dilute phase can be achieved. Solids loading ratios, however, of well over 100 are quite common. For much of
the data presented in this Design Guide on materials such as cement and fine fly ash, solids loading ratios in excess of 100 are reported, whether for horizontal or vertical flow.
In conveying barytes vertically up the author has achieved a solids loading ratio of about 800 with a short pipeline. Conveying at very low velocity is necessary in order to achieve very high values of solids loading ratio in moving bed flow. This is because air flow rate is directly proportional to air velocity and air flow rate is on the bottom line of Equation (1.3).
For plug type flow the use of solids loading ratio is not as appropriate, for the num- bers do not have the same significance. Since the materials have to be very permeable, air permeates readily through the plugs. Maximum values of solids loading ratio, therefore, are only of the order of about 30, even with high values of conveying line pressure drop. If a material is conveyed at a solids loading ratio of 10, for example, it could be conveyed in dilute phase or dense phase. It would only be with the value of the conveying line inlet air velocity that the mode of flow could be determined.