conveying of solid in pfpp chem engg.pptx

PFPP Conveyor Of Solid methods

1. Conveying of Solid Dr. Saurabh Meshram

2. Introduction Within plant and long distance. Transportation from mining site to the primary crusher; primary crushers to intermediate crushers and further to secondary crushers and grinders; oversized materials from a screen back to a crushing or grinding unit for further sizing; sized materials into and out of a storage vessel; sized materials to a classifier, thickener, clarifier, electrical separator, jig, spiral concentrator, flotation cell, etc. Transportation of solids are more difficult to transport than fluids. Types of transportation depends on capacity, material characteristics, horizontal or vertical transport.

3. Types of Conveying 1- Mechanical Conveyor i ) Belt Conveyor ii) Chain Conveyor Apron, Bucket, Scraper iii) Screw Conveyor 2- Hydraulic Conveyor 3- Pneumatic Conveyor

4. BELT CONVEYOR 1901 by Sandvik application in a wide variety of industries. most versatile among all the conveying equipment. They can be used both for short-as well as long-distance transport. can be operated horizontally or on an incline.

5. Consists of a continuous belt passing around two large pulleys at the two ends, one of which is a drive pulley and the other is a tail pulley. Solids are loaded on the upper surface of the belt near the tail pulley through a feed hopper and are carried to the other end of the belt and are discharged over the drive pulley. The loaded belt is supported during its carrying run by closely spaced rollers, known as idlers, while during the returning run the belt is supported by widely spaced idlers. The idlers are so spaced to prevent the sagging of the belt during its operation.

6. The conveyor belts can be operated under flat or troughed conditions, which are created by the arrangement of idlers. Flat belts are generally used to transport boxes, solid units, and solid particles with a high angle of repose. The capacity of flat belts is low for solid particles with a low angle of repose. The capacity is higher for troughed belts. The angle of inclination of the belt is less than the angle of repose of solid particles to be transported and is usually between 10 and 20°. The belts are made up of canvas or rubber and are generally reinforced with steel wire to impart strength. Neoprene, vulcanised rubber, and other special types are available for handling hot and moist materials. Installation cost is high.

7. The capacity of a belt conveyor depends upon i ) width and speed of the belt; ii) friction between the surface of belt and the solids; iii) angle of repose of solid particles; iv) angle of inclination of the belt; v) stickiness of solid particles; vi) degree of troughing; and vii) shape, size, and specific gravity of the solids.

8. Capacity The maximum capacity of belt conveyors is given by where, Q b = maximum capacity of the belt conveyor, kg/s A b = cross-sectional area of load on the conveyor belt. m 2 V= linear speed of the belt, m/s ρ b = bulk density of the solid materials, kg/m 3 Here, where, Ka= a constant whose value depends on the flowability of the material and the angle of inclination of side rollers. Ci = correction factor for inclination. b1 = width of the belt carrying the load, m = 0.9 b-0.05 Where b = width of belt, m  

9. Example 8.1 A belt conveyor with an inclination of 15° to the horizontal is to be used for the transportation of iron ore from the mine to the washing plant. The iron ore particles are of size 10 to 30 mm and of bulk density 2600 kg/m3. Production at the mine is 1600 tonne /h. The materials with around 10% moisture may be taken as medium flowable. For a belt speed of 1.6 m/s, calculate the width of the belt to be used for the above purpose. Data: Ka = 0.067; Ci = 0.95 [Table 10.2 and 10.3 - Narayanan, 2003] Solution: Capacity of the belt conveyor is Given that, V= 1.6 m/s, ρ b = 2600 kg/m 3 , K a = 0.067, and C i = 0.95. From b 1 = 1.295 m Thus, using b 1 = 0.9 b-0.05, we have b = 1.494 m, say 1.5 m, is the required width of the belt  

10. Charts To determine the Belt speed By relating the load, density, lump size and speed To determine the power By relating the belt width, speed, length and power

11. SCREW CONVEYOR Screw conveyors are one of the oldest type of conveying equipment, operated horizontally or at a slight incline (up to 20°) and are extensively used for transporting finely divided solids; sticky materials; and semisolid materials including food waste, municipal solid wastes, and boiler ash. These are generally used for transporting over short distances, which may be about 40 m in the horizontal and 30 m in the vertical direction. A screw conveyor essentially consists of a U-shaped trough inside which a screw or spiral flight mounted on a shaft is placed parallel to the trough bottom . The shaft is supported on a bearing at each end and is generally driven at feed/one end. The solid particles to be conveyed are fed to the trough through a feed hopper and as the shaft along with the screw rotates, the solid particles are pushed forward towards the discharge end along the front face of the spiral.

12. Inside the trough, the materials travel at such a level that the upward lifting force is just balanced by the downward gravity force. This is due to the friction between the particles and the spiral surface. The spiral surfaces are available in continuous, bladed, or cut forms. Continuous spirals are used for transporting dry, granular, and free-flowing materials, while discontinuous spirals are used for wet, muddy, and thick materials. The bladed or cut types of spirals are chosen when mixing during transportation is desired. The screw conveyors with various pitch designs are also available- with a pitch equal to its diameter (standard pitch), long pitch, short pitch, and double pitch.

13. The clearance between the screw flight and the inside surface of the trough is nearly equal to the average size of particles in the feed. The maximum particle size in the feed that can be transported depends upon the diameter of the flight. Standard screw conveyors are 3 to 20 inches in diameter and 8 to 12 ft long. Screw-conveyor capacities are generally limited to around 4.72 m 3 /min (10,000 ft 3 /h).

14. Advantages simple design and ease of maintenance; little headroom is required; slurry or sticky materials can be transported; with the increase of pitch spacing, the capacity can be increased without increasing the rotational speed of the screw; and apart from conveying and mixing, these can be used for heating, cooling, or drying of solids.

15. Disadvantages high wear of screw and trough materials; size reduction of feed materials; higher power consumption; capacity decrease with the increase of angle of inclination; and due to the stresses developed in the shaft, the conveyor length is restricted.

16. Capacity The capacity of screw conveyors depends on ( i ) screw diameter; (ii) screw pitch; and (iii) speed of rotation. The capacity is given by where, Q s = capacity of screw conveyor, kg/s C i = correction factor for inclination which varies from 1.0 for horizontal conveyor to 0.6 for inclination of 20 o . Other values are 0.9, 0.8, and 0.7 for inclinations of 5, 10, and 15 degrees, respectively.  D s = screw diameter excluding shaft diameter, m P = screw pitch, m N = speed of the shaft, rpm ρ s = density of solid materials, kg/m 3 C f = Filling coefficient values depending primarily on the t ype of materials (values range from 0.125 to 0.4 depending on heavy/light, abrasive/non-abrasive)  

17. Example 15 tonne /h of boiler ash is to be transported to the ash pond by a horizontal screw conveyor. With the following operational and material data, specify a suitable screw arrangement for the service. Data: bulk density of the material = 1400 kg/m 3 Filling coefficient = 0.125 Lead of the screw (screw pitch)/Diameter of the screw =0.8 Speed of the screw shaft = 30 rpm. Solution Here, C; = 1.0, since it is to be transported horizontally. Given that, ρ s = 1400 kg/m 3 , P/D s = 0.8, C f = 0.125, 4.167 = 54.98 D s 3 D s = 0.423 m. Thus, the screw pitch = 0.8 D s =0.3384 m  

18. BUCKET ELAVATOR Bucket elevators are used only for vertical transport of bulk solids. Can handle fine powder as well as sticky material. A bucket elevator essentially consists of a number of buckets attached to a continuous double-strand chain or belt which passes over two sprockets or pulleys located at different elevations inside a casing, as shown in Fig. Solid materials are directly fed into the buckets partly and are scooped up from the boot partly, which are carried up vertically and are discharged into a hopper as the buckets turn over the upper sprocket. The emptied buckets faced downward travel vertically downward and again scoop up materials as they pass the lower sprocket.

19. Bucket elevators are mainly of three types: centrifugal-discharge type - buckets are spaced; mainly used to handle free-flowing materials or small-lump materials continuous type - buckets are very closely spaced; used for large-lump materials. positive-discharge type buckets are spaced and the return belts or chains are snubbed back beneath the upper sprocket to invert them for positive discharge. used to lift sticky materials or cohesive solids and are slow speed equipments .

20. Centrifugal discharge is the most common discharge option. It has great travelling speed of 1.2 and 1.4 m/s. It’s loading is carried by dredging the material at the bottom of the elevator. The separation distance between the buckets is 2 to 3 times the bucket height. For gravity or continuous discharge, it’s travelling speed is lower which are 0.5 and 1.0 m/s. The positive discharge is similar to the gravity elevator safe that buckets are fitted at the edge with two cords. Bucket speed is low are appropriate for light, aired and sticky materials.

21. Scraper Conveyor Used for light load and short runs. Can handle large pieces and large inclination Operation cost is high.

22. Apron Conveyor Used for heavy loads and short runs. Wooden bars or steel bars are attached between two chains.

23. BUCKET CONVEYOR Used generally in handling coal in power houses. Can handle variety of material for large inclined transport.

24. PNEUMATIC CONVEYOR Pneumatic conveying involves the transport of particulate materials by air or other gases. It is generally suitable for the transport of particles in the size range 20 μ m to 50 μ m. Finer particles cause problems arising from their tendency to adhere together and to the walls of the pipe and ancillary equipment. Large particles may need excessively high velocities in order to maintain them in suspension or to lift them from the bottom of the pipe in horizontal systems. Pneumatic conveying lines may be horizontal, vertical or inclined. It is important to keep changes in direction of flow as gradual as possible use velocities of flow sufficiently high to keep the particles moving, but not so high as to cause serious erosion.

25. Due to the compressibility of the gas, the diameter of pipe is to be changed to maintain the same linear velocity as at the beginning of pipe. In practice, "stepped" pipelines are commonly used with progressively larger pipes used towards the downstream end. Horizontal pipelines up to 500 m long are in common use and a few 2000 m lines now exist; vertical lifts usually do not exceed about 50 m. The pneumatic transport of particulate solids is broadly classified into two flow regimes: dilute (or lean) phase flow; and dense phase flow. Dense phase flow exists when, for horizontal flow, the gas velocity is insufficient to support all particles in suspension, and, for vertical flow, where reverse flow of solids occurs.

26. Dilute Phase flow Dense Phase flow High gas velocites (>20 m/s) Low gas velocities (<1-5 m/s) Low solids concentrations (<1% by volume) High solids concentrations (>30% by volume) Capable of operation under negative pressure High pressure drop per unit length of pipe Solid particles behave as individual Much interaction between the particles Fully suspended in gas Particles are not fully suspended High attrition Less attrition High wear Less wear

27. * Used for conveying solid with the help of compress air or by vacuum.

28. Flow Patterns In a horizontal pipeline the distribution of the solids over the cross-section becomes progressively less uniform as the velocity is reduced. The following flow patterns which are commonly encountered in sequence at decreasing gas velocities have been observed in pipelines of small diameter. Uniform suspended flow The particles are evenly distributed over the cross-section over the whole length of pipe. 2. Non-uniform suspended flow The flow is similar to that described above but there is a tendency for particles to flow preferentially in the lower portion of the pipe. If there is an appreciable size distribution, the larger particles are found predominantly at the bottom. 3. Slug flow As the particles enter the conveying line, they tend to settle out before they are fully accelerated. They form dunes which are then swept bodily downstream giving an uneven longitudinal distribution of particles along the pipeline.

29. 4. Dune flow The particles settle out as in slug flow but the dunes remain stationary with particles being conveyed above the dunes and also being swept from one dune to the next. 5. Moving bed Particles settle out near the feed point and form a continuous bed on the bottom of the pipe. The bed develops gradually throughout the length of the pipe and moves slowly forward. There is a velocity gradient in the vertical direction in the bed, and conveying continues in suspended form above the bed. 6. Stationary bed The behaviour is similar to that of a moving bed, except that there is virtually no movement of the bed particles. The bed can build up until it occupies about three-quarters of the cross-section. Further reduction in velocity quickly gives rise to a complete blockage. 7. Plug flow Following slug flow, the particles, instead of forming stationary dunes, gradually build up over the cross-section until they eventually cause a blockage. This type of flow is less common than dime flow.

30. Hydraulic Conveying In hydraulic conveying the densities of the solids and fluid are of the same order of magnitude, with the solids usually having a somewhat higher density than the liquid. Practical flow velocities are commonly in the range of 1 to 5 m/s. In pneumatic transport, the solids may have a density two to three orders of magnitude greater than the gas and velocities will be considerably greater—up to 20-30 m/s. In a mixture with a volume fraction of solids, of say 0.05, the mass ratio of solids to fluid will be about 0.15 to 0.20 for hydraulic conveying compared with about 50 for pneumatic conveying.