Modes of Pneumatic Conveying
Dr. Peter Wypych
Director, Key Centre for Bulk Solids Handling & Particulate Technologies
Faculty of Engineering, University of Wollongong
Wollongong N.S.W. 2522 Australia
This article summarizes the main modes of pneumatic conveying that have been developed over the past few decades to take advantage of the different behavioral properties of bulk materials and also to meet the increasingly demanding requirements of industry in the areas of system reliability, product quality control and energy efficiency.
Dilute-Phase - (Suspension Flow)
“Traditional” pneumatic conveying is based on the simple concept of providing sufficient air to entrain, suspend and transport particles along the entire length of pipeline. Several terms are employed to describe this mode of flow, such as dilute-phase, lean-phase or suspension-flow. This is simplest and most common mode of flow used in industry. The following different modes of dense-phase conveying, also referred to as non-suspension flow, have been developed to improve product quality and system reliability and also expand the future potential of pneumatic conveying.
Fluidized Dense-Phase
Fluidized dense-phase is considered often as the most reliable and efficient method of conveying certain powders or fine granular bulk solids over distances ranging from only a few feet up to 1 or 2 miles. It takes advantage of the fluidization and air retention properties of the bulk material [1]. The suitability of a given material to this mode of conveying and the best type of feeder for these systems (e.g. blow tank, rotary valve), including optimal operating conditions and the method of air injection [2], usually are determined by test work. Some materials that are being conveyed successfully in fluidized dense-phase include cement, fly ash, pulverized coal, soap powder, zircon sand, crushed bauxite, electrolytic manganese dioxide, lead dust, limestone and flour.
Low-Velocity Slug-Flow
This mode of dense-phase pneumatic conveying [3] has been developed to allow friable and/or granular products to be conveyed with extremely low levels of product degradation or particle damage (e.g. sugar, wheat, barley, skim milk powder, poly pellets, peanuts, milled grain, semolina, muesli, powdered and granulated coffee, sand grinding media). Some typical low-velocity slug-flow results obtained on sugar, poly pellets, wheat and duralina have been presented previously [4,5]. Due to the extremely high levels of concentration that occur during transportation, the subsequent operating conditions depend quite strongly on the nature and physical properties of the material being conveyed. It is important for this reason that large-scale pneumatic conveying tests are carried out prior to the design or selection of suitable equipment. The main features of this technology that allow friable or easily damaged products to be transported in this manner are listed below.
· The average material transport velocity can be controlled and maintained easily between 50 and 800 fpm (depending on degradation/throughput requirements). Even products, such as granulated sugar, have been conveyed successfully without even scratching the crystal surface.
· Due to material characteristics (e.g. permeability) and the relatively low velocities that are used, the conveying cycle can be stopped and restarted at any time.
· Due to the high volumetric concentration of product inside the pipeline, reasonable conveying rates still can be obtained despite the relatively low velocities that are used for transport.
· There is very little inter-particle movement in the full-bore moving slugs and hence, segregation effects are avoided (even around bends). This aspect was confirmed in two recent cases studies, where milled/mixed grains (with particles of different size and density) and muesli (with rolled oats, sugar, coconut, sultanas, etc) were conveyed successfully in low-velocity slug-flow.
Low-Velocity Plug-Flow
At first glance, this mode of flow [3] appears similar to slug-flow. However, the main differences are that low-velocity plug-flow does not “produce” a stationary layer of material and also is suited to more cohesive or sticky powders, such as full-cream milk powder, drinking chocolate and cocoa powder. Usually, a plug-forming method or device is employed at the feeder to ensure stable plugs are generated along the pipeline. The advantages and features of this mode of dense-phase are similar to those listed previously for low-velocity slug-flow.
Bypass Conveying
Internal Bypass
External Bypass
A relatively unique range of “gritty” bulk materials (e.g. alumina, poly powder, fine sand, coarse fly ash) are “troublesome” in any of the above modes of dense-phase and can cause severe plugging, pipe vibrations and/or pressure surges in “conventional” pipeline systems. Although these materials usually display good fluidization behavior, they deaerate quite quickly (especially compared with the powders suited to fluidized dense-phase) and generate high friction forces when allowed to build up inside the pipeline. Hence, it is usually necessary to employ dilute-phase for such materials and also purge the pipeline prior to any shutdown operation.
However, by employing specially designed bypass technology, it is still possible to convey such materials in dense-phase. Various types of bypass technology are available [6], such as multi-point injection, external bypass and internal bypass. The main concepts involved with this technology are controlling the length of material “build-up” along the pipeline and preventing the conveying air from being “forced” through this material.
Single-Slug Conveying
This dense-phase mode involves the transportation of a limited batch of material per conveying cycle. A detailed description of this method of transport together with typical performance results has been presented by Wypych and Arnold [7]. It can be used to transport granular materials (e.g. crushed coal, sand, grains, diamond ore aggregate, petroleum coke, food products, bone char) over relatively short distances (e.g. up to 600 ft). Note: the materials suited to low-velocity slug-flow also can be conveyed successfully in single-slug mode, but this would result in inefficient conveying performance. Single-slug conveying is no longer considered as the “brute-force” technology of pneumatic conveying. In fact, the following advantages now are realized.
· Average conveying velocities are relatively low (e.g. 600 to 1200 fpm). In dilute-phase, the same material may need to be transported at 4000 to 8000 fpm, depending on the size and density of the particles.
· Consequently, system erosion (e.g. pipe, bends) is minimized.
· For a large range of materials, the only alternative is dilute-phase producing high rates of erosion and product degradation. That is, the other modes of dense-phase are not possible.
Extrusion Flow
Occasionally, it may be beneficial to maintain the total conveying pipeline full of material and produce an extrusion mode of flow. Usually, specially designed blow tank feeders are employed for this purpose. Some successful applications of this technology, include the extrusion flow of:
· meat lumps for canned dog food, where the product basically is conveyed in the form of a long sausage along the pipeline;
· chopped fish chunks and gravy, as well as whole fish pieces and gravy, for canned cat food;
· artificially formed meat lumps and gravy for canned pet food.
Other possible applications include the transportation of softened grains, vegetables, etc for food processing and canning operations. It is important to emphasize that:
· the dilute-phase option would cause excessive damage to such products;
· these types of material are not suited to most of the other dense-phase options;
· the single-slug mode of conveying could be used for such materials, but would be relatively inefficient in terms of conveying capacity and maintaining a constant product velocity along the pipeline (especially if the length and/or diameter of pipeline is significant);
· mechanical pumps can be used for these applications but may cause excessive damage to the particles – a properly designed blow tank feeder is preferred.
Air-Assisted Gravity Conveying
Air-assisted gravity pneumatic conveying is often not considered as a mode of pneumatic conveying [8]. However, it is actually one of the most efficient modes of dense-phase due to its relatively high solids loadings, low conveying velocities and low specific air power requirements. For example, a Roots-type blower or even a centrifugal fan is only required, as opposed to a compressor that quite often is selected for the other modes of dense-phase. Conveying capacities as high as 1100 tons per hour of alumina over 1000 ft have been achieved with this mode of dense-phase.
Conclusions
Dilute-phase systems are quite common in industry but in many instances can produce a wide range of unique problems, such as wear, product damage, segregation and relatively high power consumption. Different modes of dense-phase (non-suspension flow) have been developed mainly to eliminate/minimize these problems and hence, expand the future potential of pneumatic conveying. Due to the product being the dominant phase in dense-phase conveying, test work usually is necessary to confirm dense-phase suitability, accurate operating conditions and product quality.
References
1. N.J. Mainwaring and A.R. Reed, Bulk Solids Handling, Vol. 7, No. 3, 1987, pp. 415-425.
2. O.C. Kennedy, P.W. Wypych and P.C. Arnold, The effect of blow tank air injection on pneumatic conveying performance, Pneumatech 3, Jersey, Channel Islands, U.K., 1987.
3. P.W. Wypych and G. Hauser, Design considerations for low-velocity conveying systems & pipelines. Pneumatech 4, Glasgow, Scotland, 1990.
4. P.W. Wypych, P.C. Arnold and W.R. Armitage, Developing new methods for the pneumatic transport of bulk solids through pipelines, Chemeca, Sydney, 1988.
5. R. Pan, B. Mi and P.W. Wypych, Pneumatic conveying characteristics of fine & granular bulk solids, KONA Powder and Particle, No. 12, 1994, pp. 77-85.
6. J. Klintworth and R.D. Marcus, A review of low-velocity pneumatic conveying systems, Bulk Solids Handling, Vol. 5, No. 4, 1985, pp. 747-753.
7. P.W. Wypych and P.C. Arnold, Plug-phase pneumatic transportation of bulk solids and the importance of blow tank air injection, Powder Handling and Processing, Vol. 1, No. 3, 1989, pp. 271-275.
8. S.J. Ashenden, A.N. Pittman and M.S.A. Bradley, An economic assessment of air assisted gravity conveying as an alternative to pneumatic conveying. 5th International Conference on Bulk Materials Storage, Handling and Transportation, Newcastle, Australia, 1995.
