UTS Faculty of Engineering

46316 Materials Handling Assignment 1 – 2016

Thiago Luiz Lara Oliveira - 12224390

1. A rotary screw conveyor is to be designed to discharge Willow wood chips from the below ground bin illustrated, at a suitable rate of X tones per hour (to be estimated by students), into the furnace. The conveyor is to be a Y m long inclined screw conveyor which elevates the material 5 m. It requires a suitable pitch screw and may be selected from one of the standard diameters given in the tables. The student may choose to use a shaft-less screw. However, the student must justify his/her choice of screw giving the full reasoning supporting their choice. The equipment is in a covered area.

Preliminaries:

Read the reference article “Energy from Willow” and make your estimates of X and Y based on data in the paper.

From CEMC (Screw Conveyor Manual v2.2) Table B:

[pic]

The estimative of tons per hour follow the example from the school consumption of 300 tons per year. Considering that there are 150 working days per year, 8 hours per year, there is a consumption of 0.25 tons or 552 lbs. per hour.

Due to the capacity of the conveyor decreases dramatically with the increase of angle, the chosen angle is 20 degrees. Thus Y or L=15 meters or 50 feet (approximately).

According to Table B, the density W= 20lbs. per cubic feet. In volume, there is a flow of (Q) 27.6 cubic feet per hour.

The chosen conveyor model is the short-pitch screw. Due to the short length it was also possible to use the shaft-less or even standard model. However, a short-pitch screw is used in incline and the pitch is less than diameter of the screw. In this case study it is not required any another special characteristic for the screw.

Determine:

i) The required screw diameter (mm),

According to Table D - 30%B, the screw diameter must be determined by the capacity at maximum RPM, through load and lump size. Required radial clearance considered is 1.25” (class ratio 2 and maximum lump size over 1/2”).

Thus, the screw diameter is 9” or 228.6 mm.

ii) The required screw speed (rpm), Considering a short pitch, standard flight and none special screw mixing paddle.
|[pic] |[pic] |
|N = 27.6/5.7 = 4.85 RPM |N = 27.6*1.5*1*1/5.7= 7.4 RPM |

iii) The power required at the drive shaft (kW),
|[pic] |[pic] |[pic] |
|= 50*7.4*31*55/1000000 |= 41.4*50*0.6*1*1/1000000 |= 41.4*20*16/1980000 |
|= 0.63085 |= 0.00492 |= 0.0067 |
|[pic] |= (0.63085+0.00492+0.0067)*2.3/(0.85) = 1.74 |

iv) The maximum length this conveyor could be without exceeding the torsional ratings of standard pipe, shaft and coupling bolts listed in – Torsional Ratings of Screw Conveyor.
A 2.5 HP motor driving a conveyor at 20 rpm will produce:

|[pic] |=63025*2.5/20=7878.125 inch. lbs. of torque |

From Table Q, line 5:
Pipe TQ=13832 L = 124 feet
Shaft TQ=8798 L = 79 feet
Cou. B. TQ=7888 L = 70 feet
Note: The length was determined after several manipulations of Torque and HP total formulas, with an aim to isolate L as unique variable. v) Discuss and compare advantage and disadvantages between (i) Conventional straight shaft screw conveyor and (ii) a flexible, shaft-less screw conveyor
The straight shaft screw is the most common used conveyor, employed to convey a wide variety of products. It can be used with variations, such as cut flight screw or with paddles. The shaft-less screw is recommended for conveying sticky, gummy or viscous substances (or where the material tends to stick to the flighting at the pipe) and also employed with stringy products that would wrap on screw pipe.
While a conventional shafted screw conveyor has a centre pipe to support the screw flights (and the pipe is supported on each end by bearings), a shaft-less conveyor is designed to be supported by a liner that conforms to the radius of the trough. Thus, the shaft-less screw conveyor is quite similar to a conventional shafted screw conveyor in design and construction. Typically, there is a half-inch gap between the outside diameter of the shafted screw and the inside diameter of the trough and therefore, the screw never touches the bottom of the trough. The weight of the shaft-less spiral is distributed over the full length of the spiral on the liner. The drive is connected to one end of the spiral by means of a flanged and bolted drive shaft and spiral end plate, while the tail end of the spiral is not connected and is allowed to rotate freely.
The shaft-less spiral allows higher filling rates and lower rpm’s resulting in less wear and consequently less maintenance and down time. They can be made in horizontal, inclined and vertical conveyors with no intermediate or end bearings, low rpm’s and low power usage is a space-saving, extremely reliable conveying option. Further, maintenance is significantly reduced when compared to traditional conveying methods. The elimination of a central shaft allows a much higher fill rate resulting in lower rpm’s; more efficient conveying and consequently less wear and power usage. However, there is a length limitation on shaft-less conveyors. The maximum length is 25 meters and when used as inclined, there are more limitations due to deflection of the spiral.
In contrast, the conventional shaft screw addresses a cost-effective when compared to other conveying devices and also is an efficiently distributes bulk materials to various locations using multiple inlet and discharge points. It also permits a wide variety of designs and is more resistant in terms of mechanical demands. Shaft screw conveyors can be manufactured in lengths of up to 50 to 75m long, the centre shaft support bearing reduces the deflection of the spiral.

vi) What special problems (list at least three) should be anticipated with this system?
The deflection on the screw;
Avoid the screw flight tips to bent;
The possible insufficient lubrication system;
Alignment of hangers.

vii) How should the special problems of (v) be combated?
The deflection on the screw - Use of center shaft if necessary.
Avoid the screw flight tips to bent – Use a hardener material. If the tips are damaged is possible to straighten, or replace the screw.
The possible insufficient lubrication system – Increase the lubrication or change the lubricant.
Alignment of hangers – Precise assembly, use of correct tools. If there is already a problem, then replace the bearing and realign the hanger so that the bearing’s centerline aligns with the rough’s centerline

2. What secondary equipment will be needed? For example: How should the machines be fed? What equipment will be suitable at the entry of each machine? How will the material enter the furnace? Any special equipment? Carefully read the paper and perform any extra research that you consider to be necessary to assess the requirements for extra equipment directly associated with the screw conveyor. Marks will be awarded if it is evident from your report that you have carefully studied the requirements of devices and parameters in supplying and controlling the fuel feed to the boiler see appropriate manufacturers catalogues.

A control system would control the wood chip supply; the screw conveyor would drop willow chip to boiler house directly. Use of a fire damper on inlet in order to avoid backfire.

3. What alternative equipment may be used to perform the function of the screw conveyor?

A pneumatic system also would be suitable in this case study.

4. Tabulate and compare all relevant parameters for two methods of lifting/feeding this material into the furnace?

|Screw Conveyor |Pneumatic Conveyor |
|High efficiency. |Addresses complex designs. |
|When using a shaft-less screws is possible to save money, space and |High control of the process parameters. |
|simple layout. |Independence from weather and terrain conditions. |
|Effective cost of the solution. |Possible computerized control of integrated transportation. |
|Can have multiple inlet and discharge points. |Continuous flow basis. |
|More resistant to internal and external corrosion. | |

5. Submit your answers as a summary report, comparing the two systems. Illustrate with sketches. Discuss all factors in your comparison of the two methods above: Costs (capital and running); Size; Convenience; Environment; Access; Maintenance. Attach references and relevant websites with your submission.

Comparison the cost of a pneumatic conveyor and a screw conveyor, the screw conveyor requires less investment and is more suitable for short distances. However, a modification of the process (if the company wants to fed two boilers) would require an entire new system of screw conveyor. In contrast, a pneumatic system only needs a simple diverter valve and pipe for the new boiler would be installed, saving the expense of adding a great deal of new mechanical equipment. Pneumatic conveyors are far easier to work with in situations where the scope of the system could increase. In the end, pneumatic conveyor can provide a substantial savings on the cost of expansion, while a screw conveyor requires less capital to begin.

Screw systems require a large amount of floor space. Pneumatic conveyors, on the other hand, are far easier to route. Because the convey line consists of simple pipe, they usually require minimum floor space and the line can flow in both horizontal and vertical directions. Frequently, the pneumatic convey line can be attached to existing structures, columns, or rafters, thereby eliminating the need for special foundations and support frames. Most pneumatic systems actually free up floor space if replacing a mechanical system, allowing for a high level of design flexibility in material routing. Convey lines can be easily adapted to existing equipment, and can be built to avoid or go around obstructions. Due to the straight route required by screw conveyors, any equipment in its path must be relocated during construction, adding expense.

Another point for screw conveyors are well suited for heavy, granular materials that may be moist, doughy, and packable. It is more practical for materials with widely varied particle distribution; materials with both small and large particles combined.S Screw conveyors can mix, cut and handle with wider variety of materials than pneumatic conveyor. Materials commonly transferred by mechanical screw convey systems include whole grains, crushed rock, gravel, wet sand, and large food particles, among others. Ideal materials for pneumatic convey systems are fine, fluidizable, dry powders. These particular characteristics allow the material to be aerated (fluidized) and successfully pumped through a convey line. Some types of granular and pelletized materials are also good candidates for pneumatic conveying.

The system to clean the screw conveyor can be problematic. However, purging of material from a rotary valve or pressure tank and convey line are ordinary features of pneumatic conveying systems. Furthermore screw conveyors have an abundance of moving parts and all of these parts need to be maintained. If proper maintenance is not completed, failure could result. The individual maintenance and inspection of all the working components of screw conveyors drive up the cost of operation. Pneumatic conveyors break down into only a few key working parts: an air compressor or vacuum pump, an in-feed device, piping or tubing for transference, and a separating device. Each part of this system is repairable at the component level with the equipment remaining in position, so the entire system will not need to be removed or replaced. Pneumatic systems rely on the airflow created by a fan or blower rather than the multiple parts that make up mechanical systems. The overall result of having so many fewer moving parts is less susceptibility to mechanical failure.

Another consequence that comes from working with an abundance of moving parts is a need for greater safety awareness. Gears, bearings, belts, and other parts all contribute to the chance that a worker could suffer injury from getting too close to the moving components. The motors, drives, chains, belts, and other moving parts that are essential for screw conveyor systems also create a noisier environment. Pneumatic systems essentially eliminate this risk since all components are sealed to contain pressure or vacuum and thus have few exposed moving parts. The fewer working parts in pneumatic systems create a quieter operation, ultimately making for a more hospitable and safer working environment. By placing the air source in an enclosed environment, noise is virtually eliminated and the compressed air is easily piped to the point of entry into the pneumatic system. However, the pneumatic conveyor suffers more with corrosion and pipelines exposure. Due to the pressures inside, there are critical parts of the line which concentrate tensions and are more exposed to corrosion process.

References

All notes from Materials Handling subject.

A.J. Burnett, The Use of Laboratory Erosion Tests for the Prediction of Wear in Conveyor Bends, Ph.D. Thesis, The Wolfson Centre for Bulk Solids Handling Technology, University of Greenwich, London, UK, 1996.

A.W. Roberts, The influence of granular vortex motion on the volumetric performance of enclosed screw conveyors, Powder Technol. 104 (1999) 56–67.

Conveyor Eng. & Mfg. Co., 2012. Screw conveyor components & Design, Digital Version 2.20.

G. Towler, R. Sinnott, Specification and design of solids-handling equipment, Chem. Eng. Des. (Second Edition) (2013) 937–1046 (Chapter 18).

M. Bortolamasi, J. Fottner, Design and sizing of screw feeders, in: Proc. Partec 2001, Int. Congress for Particle Technology, Nuremberg, Germany, Paper 69, 2001, pp. 27–29.

Y. Yu, P.C. Arnold, Theoretical modelling of torque requirements for single screw feeders, Powder Technol. 93 (1997) 151–162.

P.A. Cundall, O.D.L. Strack, A discrete numerical model for granular assemblies, Géotechnique 29 (1979) 47–65.

Y. Shimizu, P.A. Cundall, Three-dimensional DEM simulation of bulk handling screw conveyors, J. Eng. Mech. 127 (9) (2001) 864–872.

P.W. Cleary, DEM modelling of particulate flow in a screw feeder, Prog. Comput. Fluid Dyn. 7 (2/3/4) (2007) 128–138.

T. Deng, et al., Effect of Particle concentrations on erosion rate of mild steel bends in a pneumatic conveyor, Wear 258 (2005) 480–487.