It monitors the degrading condition of the machine. Trending degradation permits planning for repairs. Trending degradation permits scheduling repairs.
Limits the extent of damage of machine save money in costly repairs as well as in loss of opportunity to produce. Presence of contaminants, if any, such as: Dust, Dirt, Metal wear particles, Soluble and insoluble acidic bodies in lubricating oils.
Purpose To measure surface temperature of shell, tyre, support roller, girth gear tooth flanks. Purpose To measure surface temperature of: shell, tyre, support roller, girth gear tooth flanks. Purpose Data collection, machinery vibration analysis and monitoring e. Should do early detection of bearing defects or gear tooth wear, electric motor monitoring and field machinery balancing. Damage to men, machine or building. Support with drawings and photographs Poor Material ii.
Long operating life iii. Wrong operations iv. Negligence in maintenance. Roller shafts must be on the same slope as me kiln though they do deflect due to the kiln vertical load by as much as l-5mm. All shafts on a given pier must be parallel to avoid generation of unnecessary thrust bearing loads. Shafts should be slightly skewed relative to the kiln shell's theoretical axis at each pier to generate some thrust on the kiln tyre that pushes it, and consequently the kiln, uphill, On a properly skewed set of bearings, rollers ride downhill against their thrust bearings.
Slight changes in skew can relieve the rollers' downhill thrust. Although all kiln thrust rolls are designed to support the entire kiln's downward load, only certain types, such as those with hydraulic actuators, are designed to operate this way continuously.
When all rollers are properly skewed, and under stable process conditions, each support roller imparts a small uphill thrust to the riding rings and, thus, to the kiln shell so that the kiln will contact its thrust rolls only inter- mittently during each revolution. Evidence of excessive support roller thrusting can be detected by temperature comparisons of the thrust bearings and from bearing wear rates. Unfortunately, many kilns do not have bearing metal thermocou- ples so that temperature measurement must be indirect.
The two most common methods are to monitor the temperature of the bearing hous- ing where the thrust bearing 'button' is mounted, or to measure the temperature of the roller shaft thrust shoulder using an infrared pyrom- eter directed through the bearing housing hand-hole. This is often referred to as 'checking for fish-scales' since the roller surface will feel rough in one direction and smooth in the other.
It should be remembered that roller and tyre surfaces can be very hot during kiln operation. Kiln seals are required at the inlet and discharge ends to exclude false air. The feed end seal must protect against cm WG differential pres- sure compared to 1cm or less for the discharge seal. However, false air at the feed end displaces potential combustion air on draft limited kilns which reduces production capac- ity and efficiency.
Also, introducing cold air in the middle of any volatile cycle, tends to cause serious build-up which inter fere with both mater- ial and gas flow.
There are numerous designs of kiln seal and most work reasonably well if they are properly maintained. Even within this type there are many variations in design. At the discharge end, the most common cause of poor scaling is overheating of the sheet metal plates.
Many kiln designers have found that it is important to protect the inter- nal surfaces of the seal plates from exposure to radiant heat. This is usually achieved by appropriate provision for radiant heat shielding and cooling air flow. Apart from overheating, the most common cause of seal failure is mate- rial loss resulting from stationary and rotating component contact.
Since wear between these components is unavoidable, regular inspec- tion and opportune maintenance is essential. It is also important that the rotating component run out be held within the seal's capability. Kiln shell design has historically been based on consideration of the kiln shell as a beam of cylindrical cross section. Effective designs are characterised by tyre locations that balance the load uphill and down- hill on mid-kiln tyres and yield shell overhangs between one and two , kiln diameters from feed and discharge end tyres.
The shell thickness is selected to maintain calculated material stress levels well within the steel's capabilities and manufacturers utilise historically proven stress limits that accommodate variation from design assumptions.
There is ample evidence that for acceptable fatigue life, no step change should exceed 20mm while the thicker plate should be scarfed to provide at least a taper down to th e thickness of the thinner plate. Infrared imaging systems are used increasingly for this purpose and also to provide information on refractory condition, coat- ing thickness, and tyre creep.
Creep should never be zero and may typically be up to about 2cm per revolution. Continued operation at or above this temperature will generally result in perma- nent shell deformation or crack initiation. Generally there are two thicknesses of shell at each tyre; the thicker plate directly under the tyre is known as the tyre course and the thin- ner plates uphill and downhill are known as flanking plate.
The plate between tyre sections is even thinner than the flanking plate. The most common location for shell cracking is at the transition between the flanking plate and the thin shell plate that spans between piers.
Failures generally occur at the toe of the weld joint on the thin plate side. It is often acceptable simply to mark the extent of these cracks while con- tinuing to operate until an opportune shutdown. Drilling a ' crack stop- per' hole at the end of a crack is a common practice but it will generally not be effective unless it is at least 25mm in diameter. Magnetic particle or ultrasonic inspection should be used to determine that the entire crack has been removed. Welding should be performed with appropriate filler metals laid down in straight beads.
Each bead should overlap the one below it by about half the bead width to provide heat for relief of weld shrinkage stress- es in the underlying bead. The lay- ers of weld bead should be built up until the toe of each weld bead lies on the line connecting the lips of the groove; ie the weld should form a lapered surface from the thicker plate to the thinner.
At the first opportunity, the repair should be accessed from the inside and the full length again burned or gouged to remove the root pass and reweld as described for the initial repair. Apart from overheating, the most common cause of shell cracking is probably fatigue due to excessive alternating stress generated from kiln rotation with high tyre pad clearance.
Though tyre creep should be logged daily for each tyre, creep is not the best indication of tyre pad clearance. This should be measured with a device commonly known as the Obourg Pen Tester Chapman; Recommended Procedures for Mechanical Analysis of Rotary Kilns; Fuller Company , Figure 10 , which yields a trace of the relative motion between tyre and shell dur- ing several revolutions.
More importantly, regular ovality measure- ments should be taken at each tyre to verify the actual shell deflection. On tyres adjacent to the kiln gear, it is common to find normal or even low oval- ity while tyre pad clearance is high. When tyre pad clearance on these tires reaches a level which is associated with excessive ovality on the other tyres, corrective action should be instigated, Although the gear is able to hold the kiln shell with minimal ovality, it is not designed to handle the stress caused by this situation.
Tyre creep varies with shell temperature and the continuous measurement offered by some shell scanners is valuable. Some creep is essential at all times and it should not normally exceed about 1cm per revolution; correction is effected by shimming of tyre pads. Tyre thrusting or excessive thrust loading on a tyre is indicated by hard contact between a kiln tire and its retaining mec hanism.
The most fre- quent cause of excessive tyre thrust loading is a slope difference between support rollers and the kiln axis through the tyre. Other, less common, causes are conical wear on tyre or roller and excessive tyre pad clearance. Kiln drives generally utilise girth gears and pinions designed to give over 20 years of continuous service if lubrication and alignment are maintained.
Lubrication should be in accordance with the gear suppli- er's recommendations for viscosity at operating temperature. An infrared pyrometer is used to measure the gear and pinion tooth flank temperatures and a t least three measurements across the gear face should be recorded monthly. The tip to root clearance between gear and pinion should be measured routinely and after any refractory failure which may have resulted in shell damage. Any changes in support roller position should be considered for their effect on gear alignment and it is, therefore, normal to move tyre axes on all piers except the drive pier.
However, excessive kiln misalignment will greatly increase the base load power required. Motor designs provide for short term loading of up to about 2. Precalciner technology and the desire to minimise retention time of material between calcination and sintering have resulted in a trend to increased kiln rotation speed. As older kilns are upgraded it is common for drive speeds to be increased and this is usually accom- plished in one of three ways: gear reducer ratio changes weakening of d.
If a motor is run close to its current limit, load changes resulting from operating conditions may cause uncontrolled changes in kiln speed which will seriously exacerbate the problems of kiln control. Thus, a marginal drive motor should be replaced before attempting to increase kiln speed.
Kiln alignment. Allowable bearing pressures are determined by the bearing materials used, either brass or babbitt, so that the length of the bearing journal must be selected to keep these pressures within design limits.
It is essential to inspect the shaft surface routinely during shut downs andre- machine before excessive circumphcrential scoring may cause pene- tration of the oil film, temperature elevation and bearing failure. Kiln designs make generous allowances for the weight of the kiln charge, refractories, and accretions. To operate reliably the support roller journal surface and bearing clearances must be adequate as must the viscosity, supply, and cleanliness of the bearing lubricant.
Variations in coating pattern resulting in non-uniform shell temperature distribu- tions can cause temporary changes in the shell's theoretical axis of rota- tion. These temporary, process induced, bends in the shell axis combined with other permanent shell deformations can cause excessive bearing pressures and subsequent bearing failures. The lowest risk operating state then, is the one that allows the kiln to accommodate as much transient shell misalignments as possible.
To achieve this state, the kiln shell's theoretical axis must be measured while the kiln is operating. This is actually done somewhat indirectly by measuring the position of the riding rings, assuming the rings and shell are perfectly round, and calculating the location of the kiln axis at each tyre.
Then imaginary straight lines are drawn between each tyre axis to determine if the tyres are high, low, left, or right of a theoretical straight line representing the kiln shell's axis. The vertical locations of the tyre axes relative to the the- oretical straight line kiln axis may vary significantly from pier to pier among kilns. Generally, on a three support kiln, the middle support rollers bear the highest load and often must be set low relative to the theoretical kiln axis in order to avoid excessive bearing pressures.
The most effective means to make the final alignment adjustments is to utilise an ovality gauge to measure the shell deflections at each tyre. The support rollers should then be adjusted to give equal deflection between left and right rollers on a given pier as well as between differ- ent piers. This is not, however, accomplished by obtaining the same ovality percentage at each pier because ovality is significantly influ- enced by the tyre pad clearance. Bearing temperatures also are indica- tors of the support roller loading and, consequently, should be monitored closely.
Kiln support rollers are designed to bear the weight of the kiln as well as some of the downhill thrust acting along the axis of the inclined kiln. The support rollers on each pier should have their axes aligned parallel to the theoretical axis of the kiln between each pier and slightly cut to impart an upward thrust to the tyre on each pier.
This will result in the support rollers lightly touching against their thrust bearings. The sum of all support roller thrusting should keep the kiln from contacting its thrust rollers continuously when internal coating is normal and uni- formly distributed. Note that shell expansion at operating temperature is approximately 20cm and it should be confirmed that the tyres are cen- tred on the rollers when hot.
Corrosion of the kiln shell is not normally a serious problem unless high levels of sulphur or chloride are present. Refratechnik Symposium, , pg The best system is a stand-by diesel generator of ca 1MW which starts automatically upon failure of the main power supply.
Certain manual procedures are then essen- tial such as withdrawing the burner pipe from the kiln hood an d open- ing doors at the top of the preheater if there are no automatic vents. The more reliable the main power supply and the more infrequent the use of the emergency system, the more important is maintenance of the stand-by equipment and rehearsal of the procedure.
Serious distortion of the kiln shell will result if it is not turned within minutes of a crash stop. If: is, however, liable to be, at least intermittently, at considerably higher temperature as dis- charged from the cooler. In combination with cement storage, there should be adequate clinker capacity to maintain cement shipments during kiln maintenance shut- downs and; if the market is cyclical, to bridge low and high shipping periods.
While total clinker capacity should be equivalent to at least 14 days of kiln production, there must also be separate storage for differ- ent types of clinker if produced and for high free lime clinker. Clinker storage serves also to blend the clinker and a silo should not be filled and discharged to milling at the same time unless the silo has multiple discharge points to avoid short-circuiting. It is inadvisable habitually to run clinker silos down tothe steel cone as abrasion will eventually cause structural failure.
High free lime clinker must be blended into mill feed with circumspec- tion to ensure that the cement is not expansive. A maximum composite free lime for the cement should be established and used as a control parameter. The desirability of large capacity storage buildings has resulted in a range of structures including sheds, silos, conical buildings, and domes of various shapes either clad steel structures or concrete shells sprayed on inflatable forms.
Certain clinker silo designs leave substantial quantities of dead material without the possi- bility for access; such silos, upon construction, are better pro-filled with limestone and run down to refusal to avoid a perpetual inventory of high-value clinker. Outside stockpiling of clinker is not usually worthwhile if subject to rainfall; it is also prohibited in some locations. Partial hydration of clinker before grinding seriously reduces strength and increases setting time.
Recovery from the stockpile should not contribute more then 1. Although the Blame fineness may need to be raised, the fraction should be maintained at the normal level. If clinker must be put outside, it is advisable first to screen out fines -4 to minimize fugitive dust and hydration.
The process and equipment circuits are sim- ilar to those employed for dry raw milling. In recent years there have been numerous plant capacity increase pro- jects as well as the construction of new lines- Increased milling capaci- ty has often been achieved by adding pregrinding, principally roll presses, to existing ball mills. New lines have increasingly incorporated vertical roller mills due to improved reliability, lower specific power consumption, and their ability to grind blast furnace slag both sepa- rately and in blended cements.
Ball mills however are still the most common and will be discussed here roller mills were covered in Section 3. Though some single-compartment and open-circuit mills remain, two-compartment ball mills in closed circuit with separators predominate. The diaphragm separating the mill compartments allows the first to be charged with large media appropriate to raw clinker while the second contains small balls which more efficiently achieve fine grinding.
Finish grinding involves the largest unit consumption of power in cement manufacture and should be optimised. It must, however, be recognised that clinker grindability is largely governed by clinker chem- istry and burning conditions so that kiln and finish mill should be con- sidered together.
It is impor- tant to avoid variable or hard burning mixes as the harder burning and longer retention time involved in controlling free lime result in large alite and, worse, large belite crystals which cause poor grindability. The final major factor is the rate of reaction in the kiln. After calcination is complete, transition to melt formation should be as rapid as possible to minimise growth of belite and CaO crystals.
This transition is delayed by a long, lazy flame which may be due to poor mixing, coarse coal, or insufficient burner tip momentum Weihrauch; Influences of Burning Process on Clinker Grindability; Polysius Corporation Technical Seminar, October Higher clinker SO3, gives harder grinding and hiher free lime gives easier grinding increasing FL by 0.
Any reduction in power which can be obtained by varying mix design and burning practice must be considered in the context of overall manufacturing cost and cement quality. The first compartment is primarily to break feed clinker nodules which may be up to 30mm; lifting liners and balls from 50mm up to 90mm are employed to effect impact.
While coarse grinding benefits from a range of ball sizes, greater efficiency of fine grinding, which involves mainly attrition, may be achieved with single sized small mm balls. If a range of ball sizes is used in the second compartment, classifying liners are employed to retain the larger balls close to the diaphragm and the small balls at the discharge.
If a single ball size is used, simple wave liners are appropriate. Note that the discharge screen slots must be at least 3mm wider than the diaphragm slots and 5mm less than the smallest ball. Charges should be dumped and sorted, preferably once each year,to maintain the optimum size profile and to remove tramp metal. The quantity of balls, the type and condition of the shell liners, and the mill speed determine the power draw of the mill. Air flow serves to remove water evaporated from wet feed materials, from dehydration of gypsum, and from water injected to the mil!
Air sweep also assists with transportation of material within and out of the mill and with the direct removal of heat due to hot clinker and to mill power dissipation.
Normal air velocity is 0. Static pressure across the mill is a good measure of air sweep. Water sprays are used to control mill temperature but it is essential that the water is evaporated and does not give rise to cement hydration or to build-up on liners or screens.
It is easier to add water to the first compartment but, where the maximum cooling is required, the best effect is achieved by spraying at the diaphragm co- currently into the second compartment. Specific power consumption usually decreases with lower charge loading but so does production rate. Since other circuit power is essentially fixed, the design charge loading and rated mill power should normally be maintained.
A useful test of mill condition is to shut the mill down on load after steady state has been maintained for hours. The fan should be stopped immediately to avoid sweeping fines out of the mill. The mill needs to be positioned so that the doors can be opened, preferably using an inching drive. In the second compartment, the bails should be covered to a depth of mm.
At each interval, l-2kg samples should be taken from just below the surface along a line perpendicular to the mill axis. Ball charge gradation is also determined during the crash stop by col- lecting totals of approximately balls from the first compartment and from the second. Equal numbers are taken at each of the locations used for screen samples and random sampling maybe assured by spray painting the balls in a line across the width of the mill.
Ball diameters are measured using calipers and each ball is either weighed or its weight calculated using a specific weight of 7.
Diaphragm and discharge screen should be inspected for wear, holes, and plugging. The energy efficiency of ball mills is very low, particularly fur coarse grinding. In recent years, various circuits have been introduced incor- porating hammer mills, roll-presses, and roller mills for pregrinding ahead of the ball mill.
Many early roll presses suffered from roll surface and bearing failures but, progressively, operating pressures have been reduced, roll sizes increased, and metallurgy improved to achieve satisfactory performance. The most common circuits are now pregrinding with slab recirculation and semi-finish grinding "s". In the former Figure 5. Greater capacity increase can be achieved if fines from roll press product are removed using a V-separator with subse- quent selection by a high-efficiency separator between final product and finish grinding in a ball mill Figure 5.
The roller is in free rotation but is hydraulically pressed against the shell. The mill may operate either in closed circuit with a separator or be used for pregrinding in a ball mill circuit. The overall performance of a milling circuit is best summarised by its specific power consumption. Abnormally high power consumption may be due to mill inefficiency, but is as likely to be caused by over-burned clinker. High mill temperatures also exacerbate materi- al agglomeration and coating of balls and liners, significantly increasing specific power consumption.
The optimum addition rate should be determined which balances enhanced grinding against power savings to minimise cost. Bulletin Author : United States. Minerals Yearbook Author : United States. Contains statistical data on materials and minerals and includes information on economic and technical trends and development. Includes chapters on approximately 90 commodities and over countries. Popular Books. The Becoming by Nora Roberts. Interparticle convection.
The axial mixing component is attributed to the mechanism that results in an overall convection, and causes the bulk of the mate- rial to move from the inlet of the cylindrical drum to the outlet with an average velocity equal to the plug flow velocity. The radial mixing component, however, involves mechanisms at the smaller scale that cause local constraints on individual particles and result in velocity components both in the axial direction and the transverse direction.
Both axial and transverse mixing coefficients tend to increase with an increase in kiln rotational speed. For low rates of rotation, one expects a spread in the residence time distribution due to the influence of the velocity profile Wes et al. The effect of the drum size, particle rheology, and drum internal features are therefore major design con- siderations.
The effect of the drum rotational speed on the transverse flow pattern is illustrated later. For now we present the features of the rotary reactor as a contactor by providing a quantitative description of the dispersion mechanisms, the resultant effects of which are critical to bed heat transfer during material processing. Other pertinent features include the internals, such as constriction dams and lifters, that impact the residence time.
The empirical relationship developed by the US Geological Survey in the early s relating the residence time and the kiln geometry has become a design mainstay even to this date Perry, The shape of the free surface the interface between the bed and freeboard is dependent upon the operational requirements, that is, the feed rate, the drum rotational rate, and the material properties. As a result, the sizing of the rotary kiln depends on the application, typically, the feed rate capacity and related transport properties such as temperature, gas flow rates, and bed material velocities that ultimately will deter- mine the residence time.
For example, in dry processing applications, cylinder length-to-diameter ratios on the order of 5—12 are typical depending on whether the heat exchange is contact or non-contact. The movement of a charge in a rotating cylinder can be resolved into two components mentioned earlier, that is, movement in the axial direction, which determines residence time, and movement in the transverse plane, which influences most of the primary bed processes such as material mixing, heat transfer, and reaction rate physical or chemical , as well as the axial progress of the charge.
Although this linkage between particle motion in the transverse plane and particle velocity in the axial direction was established several decades ago, the literature generally deals with these two types of bed motion as independent phenomena until recent advances in the characterization and application of granular flow theories could be applied to powder processing in such devices Boateng, This is an extreme condition in which all the bed material rotates with the drum wall.
Cascading, which also occurs at relatively high rates of rotation, is a condition in which the height of the leading edge shear wedge of the powder rises above the bed surface and particles cascade or shower down on the free sur- face Figure 2. Slumping 1. Rolling 0. Cascading 0. Cataracting 0. Froude numbers Fr are given for each of the different modes.
Henein, For example, starting at the other extreme, that is, at very low rates of rotation and moving progressively to higher rates, the bed will typically move from slipping, in which the bulk of the bed material, en masse, slips against the wall; to slumping, whereby a segment of the bulk material at the shear wedge becomes unstable, yields and empties down the incline; to rolling, which involves a steady discharge onto the bed sur- face.
In the slumping mode, the dynamic angle of repose varies in a cyclical manner while in the rolling mode the angle of repose remains constant. The ranges of Froude numbers for the various modes are shown in Figure 2. In the rolling mode Figure 2. The particular mode chosen for an operation is dependent upon the intent of the application.
A survey of various rotary drum type operations Rutgers, has indicated that most operations are in the 0. The geometric features of a typical rolling bed are depicted in Figure 2. Hence the bed cross section occu- pied by material can be defined by this angle.
The chord length, Lc , the longest distance traveled by particles on the free surface path of steep- est decent , can also be defined in terms of this angle. The fraction of the cross sectional area occupied by material is the kiln loading. This is usually defined as the volume percent occupied by material in the Figure 2.
Nonetheless, no clear definition has ever been given for the range of operation encompassed by the two cases of kiln loadings. As shown in Figure 2. The active region is usually thinner than the passive region because particles there are not restricted and they move faster. This is due to higher particle velocity there, although there is more to this phenomenon than simple mass conservation.
Most of the mixing in the drum cross section and also dissociation reactions occur in the active region. The deeper the active layer, the better the mixing. In order to increase the active layer depth, it is essential to increase kiln speed.
Tracer studies have shown that particles move forward along the kiln only through the active region Ferron and Singh, Therefore, for every kiln rev- olution, the bed material makes several excursions possibly 3 or 4 in the cross section, thereby resulting in an axial advance. Increasing the kiln speed will result in an increase in the number of excursions and ultimately in increased mixing.
Kiln speed increase, however, will decrease the residence time of the material, since the bed will move faster axially. It is therefore important for the kiln operator to know, based on the nature of the application, what the critical residence time should be in order to achieve the desired product quality.
Process control by kiln speed is therefore critical if one can provide adequate mixing and maintain sufficient residence time to process the material. Studies involving rotary kiln bed behavior have resulted in many ways of estimating how the bed will behave at any given opera- tional condition. One such tool is a bed behavior diagram Henein, which presents a typical behavior for a sand bed for a 41 cm 1.
Given the angle of repose, kiln geometry, and speed, users of such diagrams can predict what bed behavior to expect within the kiln cross section. However, since these delineation curves were generated from room temperature exper- iments, their industrial use has been limited. Despite this shortcoming, the bed behavior diagram can be helpful for the drying zone and, for the most part, the preheating zone of long kilns. In the calcination zone, however, any softening of the material that increases stickiness or agglomeration of the material will result in an increase in the angle of repose.
Because of the importance of mixing on product uniformity, we will elaborate further on rolling bed behavior. Mapping of bed behavior regimes for dif- ferent operation conditions.
Some of the observed flow and transport phenomena in rotary kilns that have provided insights into particulate flow behavior, which lead to accurately stating the mathematical problem or modeling the rotary kiln transport phenomena, are described.
Dependent upon the bed depth and the operational conditions, the flow behavior constrained in the transverse plane can be purely stochastic, purely deterministic, or a hybrid of both; hence mixing can either be modeled by a random walk for very shallow beds Fan and Too, or by a well-defined bulk velocity profile estimated by shear flows similar to boundary layer problems Boateng, Early workers used tracer particles to observe and characterize mixing Zablotny, ; Ferron and Singh, ; and others.
Lately, such works have been extended to the use of nonintrusive techniques, such as nuclear magnetic resonance NMR; Nakagawa et al. Boateng used experiments on the continuous flow of granular material in the transverse plane of a rotating drum to elucidate the rheological behavior of granular material in rotary kilns. Additionally, for the first time such work provided data for mathematical modeling of granular flow in the boundaries of a rotary kiln.
Using granu- lar materials that varied widely in their physical properties, specifi- cally, polyethylene pellets, long grain rice, and limestone, the mean depth and surface velocities were measured using optical fiber probes Boateng, ; Boateng and Barr, From these, granular flow behavior including the velocity fluctuations and the linear concentra- tion of particles could be computed. The rotary drum used consisted of a steel cylinder of 1 m diameter and 1 m length.
It had a glass end piece with a center opening providing access for flow measurements. In light of the shape and thickness of the active layer, experiments confined to the observation of the general behavior of material flow in the cross section could be carried out. Because shear stresses generated in the induced flow involve collisional elasticity, particles with a relatively low angle of repose, for example, polyethylene pellets, experience an easy transition from the potential energy position in the plug flow region to kinetic energy in the active layer.
Conversely, low coefficient of restitution materials, such as rice and limestone, have a relatively high friction angle and the energy dissipation is lower than that for polyethylene pellets. As a result, high potential energy is built up during the transition and is accompanied by material buildup prior to release into the active layer. Flow instabilities result, which manifest themselves into formation of multiple dynamic angles of repose with unsteady velocity distribution at the exposed bed surface.
The active layer depth and bed flow properties depend on the coef- ficient of restitution of the material. The flow properties of interest include granular temperature, which is a measure of kinetic energy in random motion of particles, and dilation. Granular temperature was found to be high at regions of low concentration with high mean veloc- ity. However, its thickness depends on the physical properties of the material and the operational parameters such as rotational rate.
After transferring from the plug flow region into the active layer, particles accelerate rapidly up to around mid-chord of the surface plane before decelerating with streaming, kinetic, and gravity effects playing various roles in momentum transfer.
Increasing the degree of fill pro- vides a longer chord length for material to travel and, for larger kilns, the velocity at the exposed bed surface can become fully developed by mid-chord.
For deep beds, surface velocities can reach as high as 4. Similar observations have been observed using nonintrusive flow measurement techniques Parker et al. For each excursion within the cross section, a particle on the free surface travel- ing on the chord length makes an axial move either backward or for- ward.
This is due to the random nature of flow within the active layer. The forward advance is based on the cylinder slope in the axial direc- tion as well as the apparent forward angle resulting from the transverse flow pattern. Although these derivations lack the rigor of a true granular flow, they have com- bined key empirical relationships to provide design formulas still in use today. Nicholson assembled some pertinent formulas that have been used to characterize axial bulk movement in kilns that are purely based on geometrical considerations.
The major ones are from the early works by Seaman who calculated the material flow properties in the axial direction based purely on geometry of the equilibrium position Figure 2. Multiplying the axial transport distance per cascade by the number of cascades per revolution and the kiln rotational speed yields the average axial transport velocity, uax , for a particle at position r anywhere in the radial plane.
From the geometry Figure 2. Several early and recent investigators have derived variations of the throughput, axial velocity, and residence time expressions Hogg et al. Recognizing that this derivation might be conve- nient for estimating these parameters for all kilns. This is due to the random nature of solids motion alluded to earlier. For each cascade or excursion, a particle can move forward in line with the forward geometric projection as discussed, or backward due to several factors including changing material dynamic angle of repose.
Evidence of this lies in the situation where large agglomerated particles notoriously known in the industry as logs which form in high-temperature kilns have been observed to travel back and forth without being discharged. Also in the cross sectional plane, particles in the plug flow region can emerge in the active layer in a sequence following the birth-death phenomenon, that is, entirely by random walk Ferron and Singh, Hence residence time is a result of axial dispersion, which in turn depends on transverse dispersion and is truly a distribution func- tion.
It is not surprising, therefore, that residence time has been the subject of many tracer experiments. Parker et al. They have observed in their experiments a one-to-one relationship between the radial entry of a tracer particle and its exit within a bed of industrially processed materials. These relationships between residence time and bed depth can be plotted and used in tandem for kiln design.
Combined with process data, these can be manipulated to achieve optimum kiln size and operating conditions. A plot of the dimensionless residence time and flow rate for a 3. References A. Multiphase Flow, 24, —, References 31 A. Boateng and P. Fluid Mech. Brimacombe and A. B, 9B, —, Fan and J. Ferron and D. Gorog, J. Brimacombe, and T. B, 12B, 55—70, Guruz and N. Hogg, K. Shoji, and L. Particle Dynamics in Rotary Cylinders. Nakagawa, S. Altobelli, A. Caprihan, E. Fukushima, and E.
Fluids, 16, 54—60, Thesis, University of Queensland, Parker, A. Dijkstra, T. Martin, and J. Perry and D. Chemical Engineering Handbook.
McGraw-Hill, New York, Tscheng and A. Wes, A. Drinkenburg, and S. The goal is to describe the characteristics of confined jets that determine burner aerodynamic mixing and, in turn, combustion efficiency, and flame shape and its character.
Fluid flow through the kiln freeboard comes from several sources, including combustion air, combustion products, and the air infiltrated into the vessel. In direct-fired kilns, especially those with pulverized fuel p. Burner pipe nozzles range from 25—61 cm 10—24 in.
Hence with a chamber much larger than the burner pipe, the fuel emerges as a jet. The freeboard flow phenomenon near the com- bustion zone therefore exhibits the properties of jets, how they are entrained, and how they mix with the surrounding fluid.
Therefore, the gross pattern of flow in the region near the burner is determined by the geometry or the physical boundaries surrounding the burner, typically involving a jet confined in a cylindrical vessel and by the manner in which the fuel is discharged.
The latter may be introduced through the inlet surrounding the primary air or from discharge cool- ers that recuperate some of the energy in the discharge product and return it into the kiln to improve combustion and fuel efficiencies. In the region further away from the combustion zone, the flow field is made up of combustion products and any other gases that are released as a result of the bed reactions. The combustion products must be induced into air pollution control devices to be cleaned up before dis- charge into the atmosphere.
Atmospheric discharge is accomplished by an induced draft I. Because the I. The higher the pressure i. Owing to a slight vac- uum, air infiltration into the kiln is almost always evident, originating from the joints between the cylinder and around the fire hood.
Infil- tration air also comes from several sources including kiln attachments, intended or unintended open access points, such as cleavages around burners, take-off pipes, holes, and so on, with the quantity dependent upon the efficiency of the seals around these curvatures.
Kiln seals are therefore a big design and operational challenge, the most prominent being the point between the stationary fire hood and the rotating cylinder. It can be rightfully said that the kiln is like a big conduit and the I.
The flow area kiln freeboard depends on the kiln loading or the degree of fill, thereby determin- ing the turbulent intensity of the freeboard flow in the region further away from the combustion zone.
Near the feed end the high turbulent kinetic energy can result in increases in dust generation and discharge through the exhaust. In the near field, however, the interaction of the primary air jet and secondary air in a confined environment introduces intense mixing involving recirculation eddies that return combustion products into the flame region, a phenomenon that underlies mix- ing, the mainstay of turbulent diffusion flames found in rotary kilns.
Such flow properties exhibited by confined jets underlie the freeboard aerodynamic phenomena and will be described further in more detail. However, it is more complex in the near field involving entrained jets. Most theoretical considerations in fluid dynamics are based on the concept of perfect fluids thereby requiring that the fluid is frictionless and incompressible.
The no-slip condition in an incom- pressible fluid means that two contacting layers acting on each other exert only normal or pressure forces but not tangential forces or shear stresses.
However, this assumption falls short in real fluids since they offer internal resistance to a change in shape. This results in the con- cept of viscosity whereby the existence of intermolecular interactions causes the fluid to adhere to the solid walls containing them.
Because shear stresses are small for fluids of practical importance such as that encountered in rotary kilns their coefficients of viscosity are small and agree with perfect fluids.
For more on the outline of fluid motion with friction the reader is referred to Schlichting The rotary kiln is among the largest type of moving machines made and is subjected to extreme temperatures power failures atmospheric conditions varying loads and other operating conditions which affect its wear and alignment Chat Online Kiln Mechanics PDF Free Download edocsite Alignment of Kiln Alignment of Kiln View More Details.
Rotary kilns run 24 hours a day and are typically stopped only for a few days once or twice a year for essential maintenance One of the main maintenance works on rotary kilns is tyre and roller surface machining and grinding works which can be done while the kiln.
Rotary kilns have numerous industrial applications including cement production Frequent operational problems such as low thermal efficiency refractory failure and poor product quality have prompted extensive efforts to improve and optimize their design.
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