DEMM Engineering & Manufacturing
Crushers and screens in industrial plants
INTRODUCTION
A crusher is a machine used to reduce large size solids into smaller sizes; crushers are widely used in industrial and manufacturing plants to reduce the size of raw materials or other solids for processing, manufacturing, production, etc. Crushers are offered in many different models and designs; in a type of crushers, the machine holds materials between two parallel or tangent surfaces, and applies force to bring the surfaces together or to move materials toward low gap sections to crush the trapped materials. Different crushers such as jaw crushers or cone crushers work on this basis; although there are other machines working on similar principles.
Impact crushers involve the use of impact rather than pressure to crush materials. The material is usually contained within a cage
(or similar), with openings on the bottom, end, or side. Desired size of materials will be discharged and other materials would be held for further crushing. There are again many different types of impact crushers. In industrial, manufacturing and similar industries, impact crushers are more popular therefore the focus of this article would be on impact crushers particularly ring hammer crushers.
Vibrating screens have been the most important screening machines. They are used to separate feeds containing solid and crushed materials down to approximately 0.3 mm in size, and are applicable to a wide range of materials from perfectly wetted to dried. Vibrating screens are also discussed as an important parts of overall size reduction facilities.
GENERAL NOTES ON CRUSHERS
There have been many drive arrangement and configurations proposed for crushers such as VSD electric motor systems, wide ranges of mechanical variable speed systems, mechanical couplings, etc. Fluid couplings have been widely used particularly for medium size crushers.
A crusher should be capable of delivering the normal rated output even when handling materials somehow different than rated materials such as materials wetter or sticker than normal, highest moisture content than the specified ones, different sizes than rated sizes, even slightly different compositions, and so on. This is often a great challenge as a crusher should be provided to tolerate all these possible changes. No clogging or building-up of materials on the crushing elements should develop. Provisions should be kept such that the gap between different elements can be adjusted to take care of demand of output size variations or other operational changes. Uniform crushing is another important requirement which should assured by proper design and operation of a crusher.
Required power of a crusher is related to many different factors such as material properties, feeding lumpiness, discharging granularity, crusher speed, etc. It is generally difficult to compute required power of a crusher. Most likely empirical and experimental formula are used for the power sizing. In other words, most often, empirical formula or specific power consumption methods are utilised to compute power and consequently other important features such as rotational inertia, moment of flywheel, design details, etc. There have always been concerns about power calculations for crushers. Usually different calculation methods should be used to double check the rated power and other features; empirical coefficients used in such formulas are important and they should preferably be selected from upper range of offered values to provide some margins. Otherwise calculation methods come to underestimated power values. For many crushers, rated power has been sufficient on paper; but practically it has been marginal or underestimated. As the result, these crushers cannot provide rated capacities or easily get overloaded. For instance, for many crushers overload coefficient is considered between 1.3 - 1.4 whereas overload factors should often be around or above 1.8. Obviously power is underestimated with such low factors and the expected capacity and performance cannot be achieved by such an underestimated power rating and undersized driver. Also sizing of important parts such as flywheel, bearings, etc should be properly checked and verified. Monitoring and the online check of performance are important for many crushers; different monitoring sensors and elements such as vibration monitor systems, temperature detectors mounted on bearings, etc, should be provided.
RING HAMMER CRUSHERS
Ring hammer crushers break solids by impacting them with hammers that are fixed on a spinning rotor. These crushers are used traditionally for relatively soft materials such as limestone, phosphate, gypsum, shales, coals, etc. However, improvements in their designs and metallurgy have made them suitable for a wide range of materials and applications. Nowadays, ring hammer crushers are used in many applications. By comparison with other types of crushers, ring hammer crushers have many advantages such as high performance, less noise, less dust, low power ratio, etc.
Ring hammer crushers mainly make use of the impact effect to crush materials. When the materials enter the crusher, they are crashed by the high speeding ring hammer. The crushed materials gain energy from the ring hammer, rush to the crushing board at high speed, and crashed at second time. At the same time, the materials bump with each other, and are repeatedly cracked. Materials smaller than the grate bar gap are discharged, while larger ones on the grate bar are impacted, grinded, squeezed again to be crashed by the hammer, and at last the materials are extruded by the hammer from the gap to obtain products with the required size. Ring hammer crushers have usually been offered in two types: reversible and irreversible. Reversible hammer crusher is with reversible rotor, generally used for fine crushing; irreversible hammer crusher is with irreversible rotor, generally used for medium crushing.
Vibration level should be kept at minimum for both normal and abnormal/emergency situations. The crusher details and characteristics should promote this low vibration; for instance, a crusher with its drive, supporting structure and foundations should be designed adequately to take care of any unbalance forces arising out due to operation of the crusher even with maximum two broken hammers. There are many emergency cases like above-mentioned which should be considered. Another example, as the crusher hammers start wearing, imbalance of rotor takes place and vibration increase. Hence, adjustable balancing weights should be provided on both sides of the rotor to help achieving proper balance. As a very rough indication, vibration limits of a ring hammer crusher are often specified somewhere between 70 and 140 microns when all hammers are in intact condition.
Ease of maintenance is a key requirement; all hammers should be toothed hammer weighing
maximum 25 or 30 Kg. Maximum accessibility should be provided for routine inspection. On the other hand, reliability is a major consideration. The crusher rotor should usually be oversized to minimize deflection and to provide excess safety factor. The crusher housings are of suitable grade of steel with rugged design for rigidity during operation and ease of access for maintenance. The crusher frame should be provided with quick opening, large inspection doors fitted with dust tight seals. It is fitted with wear resistant thick liners from a very suitable material. The entire inside surface of crusher coming in contact with materials should be provided with abrasion resistant liners most often from suitable grade of materials such as alloy steels, etc. For ring hammers, the gap between cage bars and the hammers should be adjustable to compensate for the normal wear.
VIBRATING SCREENS
Vibrating screens usually operates at an inclined angle, traditionally varying between 0° to 25° and can go up to a maximum of 40°. Above this angle is possible and was used, nut not recommended. Flat vibrating screens or those with small angles (such as below 5°) are used in special applications; similar is true for inclined angle above 35°. Vibrating screens have become more standardised and widely adopted in material classification processes; they allow efficient cuts and fine separations. For many plants, they are important parts of size reduction and crushing systems.
Common types for screening decks are either single or double deck. Although triple decks (or even more) have been offered by some manufacturers and used in special applications. The screening performance is affected significantly by various factors such as equipment capacity, angle of inclination, details of induced vibration and others. The performance of a screen can be measured by screening efficiency and flux. Flux is defined as the amount of a desired component (undersize material) that has carried over the screening media from the feed per time per unit area. Screening efficiency is expressed as the ratio of the amount of material that actually passes through the aperture, divided by the amount in the feed that theoretically should pass. Screening efficiency is commonly used to judge and evaluate the performance of a vibrating screen.
The screening capacity is almost directly proportional to screen width. Efficiency is linked to length; this means that by increasing the length, there will be additional chances for passage, and this will usually lead to increase in efficiency. As an indication, screen length should be around two to three times the width. However, certain special situations such as restricted space may require a different ratio. Angle of inclination can be selected based on the desired performance. Increasing the slope of a screen will effectively reduce the aperture by the cosine of the angle of inclination and also the materials move across the screen faster which leads to more rapid stratification. However, the performance tends to decrease after a certain point, say above 25° or 30°, depending on application. Since the slope of the deck is too high for the feed materials and most particles will remain on the oversized stream instead of passing through the aperture, thus, it results in lower efficiency. Therefore, angles for screens are often limited to 25° or 30°; most of screens in common industrial applications have angles between 15° and 25°.
In a vibrating screen, particles are introduced to the gaps in the screens repeatedly. The vibration frequency of the screen should be high enough so that it prevents the particles from blocking the apertures and the maximum height of the particle trajectory should occur when the screen surface is at its lowest point. At low frequency, screening efficiency might be high but blinding is severe. Blinding will decrease as frequency increases but the particles will have difficulty going through the apertures. Similarly, an optimum value is available for vibration amplitude. Therefore, there is an optimum set of frequency and amplitude of vibration for each vibrating screen. Moreover, blocking and blinding are major issues in vibrating screens; therefore, all parameters and factors related to them should be dealt with great care.
The selection of the screen type and details will be based on the materials feeding to it. Therefore, each screen is specially- sized and designed equipment for each specific application. There are standard frames and models of vibrating screens, but ideally details of each vibrating screen should be tailored for each application. If the screen is not suitable for the material fed to it, there would be serious issues and problems; for instance, the materials will blind the apertures and regular maintenance will be required.
There have usually been concerns on the capacity and sizes of vibrating screens; as many screens have been undersized and they cannot achieve the claimed screening efficiencies, capacities and performances. It is often useful to ask manufacturer to provide calculations and appropriate simulations or operating references to show that the selected model, sizes and details are satisfactory for handling of the intended application and capacity.
CASE STUDY – RING HAMMER CRUSHER
This case study is for a 745-rpm ring hammer crusher to crush 700 t/h raw materials in an industrial plant. Rotor diameter and rotor working length are 1.2 m and 1.3 m, respectively. The required power of crusher is calculated by two different methods to make sure on the sufficiency of design and power rating of the electric motor driver. Using specific power consumption method, very simple linear empirical formula: P=K Q
Where:
P: Power (kW); Q: Crusher Capacity (t/h); K: Empirical Specific Power Consumption.
Based on previous experiences, the specific power consumption for ring-hammer type crusher for this service would be K= 0.35~0.50; selected factor is K= 0.48. Rated power “P” is calculated as 336 kW.
By using another empirical formula for the ring hammer crusher for this application:
P = C ×D2×L×Na×K
Where:
P: Power (kW); C: empirical coefficient, 0.1 0.15; D: rotor diameter (m); L: rotor working length (m); Na: speed (r/min); K: overload coefficient, for this case 1.8
P= 0.14×D2×L×Na
×K= 0.14×1.22×1.3×745×1.8=351 kW
The larger value, 351 kW, is considered as the brake power for sizing of other components; electric motor power is calculated as 450 kW.
CASE STUDY – VIBRATING SCREEN
The case study is for an 1800 t/ h vibrating screen for a raw material handling and crushing system for an industrial plant. This vibrating screen was of the double deck type and used for separating the raw materials of 100 mm and 35 mm. The screened materials, larger than 100 mm, are discharged to hoppers and sent back to upstream crusher (for large sizes – above 100 mm), between 35 mm and 100 mm are discharged to a downstream impact crusher (to crush materials to 35 mm), and smaller than 35 mm are discharged directly to the conveyor to be transferred to the processing unit.
The rated capacity of the vibrating screen was 1800 t/ h. The initially selected vibrating screen was a 30° angle 7 m × 2.4 m screening area; it was claimed that such a screen can handle 1800 t/ h of materials with screening efficiency of 85 percent. Questions were raised by engineers and consultants on this. Concerns were raised on claimed parameters and performances; for example, screening efficiency. It was proved that considering 30° angle, sizes and other details for this vibrating screen, screening efficiency above 75 percent could not be achieved. On this basis, the screen selection and design was modified to larger and better one; the modified vibrating screen was a 25° angle
8 m × 2.7 m screening area.