Rotary Kilns

5/20/2018 Rotary Kilns

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Rotary kilns systems have evolved considerably in form and complexity over the last 120 years, but the kilns

themselves have certain common features. This page lists these and describes their evolution.

Rotary kiln terminology

The Kiln Shell

The shell of the kiln is made of mild steel plate. Mild steel is the only viable material for the

purpose, but presents the problem that the maximum temperature of the feed inside the kiln is

over 1400C, while the gas temperatures reach 1900C. The melting point of mild steel is

around 1300C, and it starts to weaken at 480C, so considerable effort is required to protect

the shell from overheating.

Historically, the construction of rotary kiln shells has closely paralleled the construction of

boilers.

The engineers who pioneered the first rotary kilns all had a background in locomotive construction.

Shell sections were made from flat rolled plate, of thickness typically in the range 18-25 mm.

The plate was cold-rolled to the required curvature, typically into semi-circular pieces. Two of

these were then joined to make a cylinder, usually of length about equal to the diameter. The

pieces were butt-jointed together using a strap of steel plate of similar thickness attached with

rivets. The cylindrical sections were joined end-to-end in a similar manner. Short sections were

usually assembled at the factory, and final assembly was performed on site, with the kiln in

place.


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Picture: NERC: British Geological Survey Cat. No. P540333. Front end of RibblesdaleKiln 2 (constructed

1937): shell constructed mainly from staggered semi-circular sections, with all joints riveted.

Riveted construction continued until WWII. The technique of making welded joints in such

heavy plate by arc welding developed in the USA. As in the shipbuilding industry, welding was

adopted only rather slowly in the UK. Welding has the obvious advantage that a lighter

construction is possible, without the extra weight of the straps. Kiln suppliers began to use

welding after WWII, but on-site assembly continued using rivets because of the lack of the

required skill at cement plants. Typically sections of length up to about 10 m - the longest that

could be moved by road - were welded, then riveted together when in place.

Finally, from the late 1950s, all-welded kilns were installed. Although welded constructionreduced the weight of kilns, it had the distinct disadvantage that the shell, without the

reinforcement of thick straps, became somewhat less rigid, despite the adoption of thicker plate

(25-35 mm).


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Picture: Rugby Archive: Cat. No. RC-3-6-8. Back end of SouthamKiln 7 (constructed 1961): shell entirely

welded. The shell still consists of staggered half-cylindrical sections.

Except for the very earliest kilns, the shell was strengthened at the location of tyres and turning

gear by providing extra layers - "wrapper plates" - of steel, either rivetted or welded on, in order

to resist the high flexural forces encountered there.

The ends of the kiln have special features. At the back end there is often either a conical

constriction or a "closure plate" reducing the diameter, both intended to prevent the rawmix

from spilling over the back of the kiln. At the front end, clinker at 1200C or more is flowing over

the lip of the kiln, and the shell is subject to very aggressive conditions. Castings of heatresistant steel are attached to the end of the mild steel, contoured to retain the refractories of

the nose ring. Because the shell inevitably gets hot, the tendency is for the steel to "bell out" at

the end, until the brickwork will no longer stay in. Because of this, early kilns were supplied with

easily replaceable nose sections. Modern kilns keep the nose cool with elaborate systems to

duct pressurized cold air around the outside of the nose.

Shape of Kilns

Early rotary kilns were simple cylinders. However, the idea that different parts of the kiln ought

to have different diameters emerged quite early. Before 1900 in the USA, kilns were being

installed with the front (hot) half having a shell diameter 20-30% greater than the rear half. A

wider burning zone with a reduction in its diameter at the outlet end was favoured because this

was considered to produced a zone in which the material bed depth was increased, thereby

slowing material flow down, and allowing clinker to "soak" at the peak temperature. A further,

more practical reason for a wide burning zone shell was that it allowed room for thicker

refractory and for thick coating that usually forms in this zone.

The provision of expanded zones in other parts of the kiln enjoyed periods of popularity at

various times. Expanded mid ("calcining") zones were advocated in the 1920s, while various

forms of expanded rear (cold end) zones had a long and continuing popularity, these becoming

more standard as it became known that most "long" kilns are limited in their capacity by the gas

velocity at the back end. At the same time, there has always been a strong body of opinion in

favour of "straight" kilns, it being argued that the benefits of expanded sections are outweighed


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by their disadvantages - the tapered sections are mechanically weak and hard to line with

stable brickwork.

The wet process kilns at Dunstable: No 1 at the top. The kilns are 30 ft apart. These exemplify the variations -

often eccentric - in shapes of wet process kilns.

The early commercially successful rotary kilns in Britain were nearly all "straight" cylinders, the

exceptions being those at Norman(1904). Lengthening of the early kilns

at Wouldhamand Bevansresulted in kilns with enlarged burning zones, while the lengthened

kilns at Swanscombehad enlargements at both ends. Among new installations from 1909 to

1914, only 15 out of 44 were "straight", the rest having enlarged burning zones. The pattern

was repeated in the 1920s, when 46 out of 57 new installations had enlarged burning zones,

and of those, four also had enlargements at the cold end. The latter included the Leweskiln,

which was the first of what became a common FLSdesign, with elongated wide front and rear

sections, and a narrower "waist" occupying the middle third. The other back-end enlargementswere by Vickers, who during the 1930s offered short, large-diameter back-end "bulges" both on

new kilns and as retrofits to existing kilns - for example they were fitted on the FLS kilns

at Wilmingtonand Hope. These were supplied as part of the project to fit wet kilns with chains,

and led by 1938 to the development of the Vickers Desiccator, designed to act as a short, wide

heat exchanger. Many of these were fitted in order to uprate the short wet process kilns of the

1920s, and their characteristic shape remained a feature of many kilns long after their

inefficient internals had been removed.

After WWII, larger wet process kilns began to be installed, and these fell into two

camps: Vickers Armstrongsupplied kilns with enlarged burning zones only, while FLS supplied

mostly kilns with enlarged rear sections only, employing an identical design for the Long

Drykilns at Padeswood, Pitstoneand Platin.

With the abandonment of wet process, most of these embellishments have disappeared. The

larger Lepol kilns had short enlarged rear sections, although the smaller kilns were straight.

Suspension preheater kilns are invariably short, straight cylinders, with minor conical

constrictions at the inlet, and sometimes the outlet.

Mechanical Considerations

All rotary kilns essentially take the form of beams supported at a few points -

the tyres - along their length, with the added complication that they rotate. The

shell has to cope with all the forces involved, but is necessarily thin, since


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weight must be minimised. The design and maintenance of the kiln need to

keep the distortion of the structure within acceptable limits. Flexure as the kiln

rotates causes reduction in the life of the refractory lining (see below) as well

as fatiguing the shell itself.

A number of different mechanical deformations occur. Diagrams show the

nature of the distortion in an exaggerated form.

Bending of the kiln under gravity:

o Axial distortion the tendency of the kiln to sag between two

successive tyres (fig. 1)

o Transverse distortion (ovality) the tendency of the kiln to

flatten, mainly in the vicinity of the tyre (fig. 2)

Distortion due to damage:

o Blistering usually due to local over-heating (fig. 3)

o "Waisting" or "necking" usually due to the shell expanding

beyond the limit of the tyre clearance (fig. 4). This typically

happens if the shell temperature rises more than 180C

above design temperature in the vicinity of the tyre.

o Banana distortion usually due to over-heating one side of

the kiln during a crash-stop (fig. 5)

Structural defects:

o Misalignment vertical displacement of the rollers from their

correct position

o Kinks and dog-legs off-axis defects during assembly or

maintenance of the shell

Torsional distortion the twisting of the shell caused by the torque ofthe drive a very minor effect.

Thermal expansion the kiln shell expands radially and longitudinally.

Radial expansion closes up the clearance within the tyre, so reducing

ovality. Longitudinal expansion affects the location of the tyres with

respect to the rollers and of the ends of the kiln with the hood and

exhaust duct. The kiln system, of course, is designed to take up its

correct position when operating at design temperature. Since the

1930s kilns have been designed to expand 0.25-0.3%. Earlier kilns

probably expanded more than this.

The axial and transverse distortions are the main concern: distortion due to

damage and structural defects tend to amplify their effects.

The axial flexure is greatest at the tyre, and increases with the span (relative to

kiln diameter) between tyres. This is mitigated by extra layers (wrappers) of

plate under the tyres.

Ovality affects both the tyre and the shell, but is much greater for the latter

because of its thinness, and increases with the ratio of kiln diameter to shell

thickness.


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RefractoriesTo protect the shell from the high temperatures of the feed and combustion

gases, a brick lining is used. The early rotary kiln patents of Ransome, Stokes

and Hurry & Seaman simply specified firebrick (although Stokes went so far as

to require best firebrick). The 36 diameter Ransome kiln had brickwork 6

thick. In the case of the Ransome and Stokes kilns, because real clinkering

temperatures were never achieved, the quality of the bricks was a moot point.

As rotary kilns began to be used successfully in the USA, the maintenance of

the lining became a major preoccupation.

Ordinary firebrick is made from aluminosilicate clays that are relatively freefrom contaminant elements, so that when fired they are largely a compound of

silica and alumina, with the silica (at least in the cheaper grades) in

considerable excess. Naturally, siliceous bricks are attacked by the highly

basic clinker in the hottest parts of the kiln, and two strategies emerged in the

first few years of kiln operation:

maintenance of a constant thick coating of frozen clinker material on

the surface of the brick, to protect it from further attack.

employment at least in the burning zone of more expensive bricks

with increased proportions of alumina (>50%), made using bauxitic

clays.

The bricks for cement kilns have to be made in a special tapered form in order

to fit the curvature of the ki ln shell. The iron and steel industries had prompted

the production of refractories with a wide range of sophisticated chemistries,

but it took some time for rotary cement production to increase to the stage at

which these ideas were applied to cement ki ln bricks. It was not until the 1920s

that higher-alumina bricks became available in the UK, and manufacturers

wanting to try them had to import them from the USA and Germany.

Subsequently, other types of brick became available for the hotter parts of the

kiln. Clearly, to avoid chemical attack, a basic brick is required, and bricks

based on dolomite, magnesite and chromite became available.


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An odd diversion from mainstream development in the early years was the use

of "clinker" refractory. This was particularly favoured in Germany, and was

fairly consistently recommended on kilns from German suppliers. Concrete

made from graded clinker and portland cement was of course very much

cheaper than purchased brick, but its li fe was usually very short, and its use

mostly died out in the 1930s. It was last used at Masonsin the late 1940s. It

continued in use in the burning zones of the Anhydrite Processkilns.

With a wide variety of brick types to choose from, complex zoned bricking

arrangements developed. Cheap siliceous brick was used in the coolest

zones, grading up to higher alumina in the hotter parts, and the hottest parts

were provided with basic brick, the type selected depending on the nature and

thickness of coating produced by the local raw material. The selection of more

sophisticated brick types was always a compromise between the enhanced

brick-life expected and the greatly increased price per brick. From the 1980s,

the use of chromite-containing bricks was phased out due to environmental

regulations. By these strategies, the burning zone brickwork of wet process

kilns in the post WWII period could be expected to last for a full years

operation. Bricks in the cooler zones would usually last for many years or

decades. A typical long-term-mean refractory consumption of a wet process

kiln would be in the range 1-2 kg per tonne of clinker made, so the refractory

was still a significant running cost.

With the advent of more efficient dry processes, and particularly precalciners,

the relative cost of refractories has been reduced, mainly because of the larger

output that can be obtained from a given sized kiln tube. Parallel with this is a

reduction in the relative amount of heat wastage shell losses radiatedfrom the surface of the kiln.

The desirable characteristics of refractories which must be achieved by careful

selection are:

refractoriness i.e. the ability of the brick to retain its physical

properties at the operating temperature

volume stability i.e. no excessive expansion

chemical resistance i.e. resistance to the attacking species in the

feed and kiln atmosphere in the zone in question

abrasion resistance

low thermal conductivity

coatability in general a porosity or surface texture allowing the

clinker liquid to glue coating to the surface

Factors tending to reduce refractory life include:

intermittent kiln operation stops and starts cause thermal shock to

the bricks and the coating

variable clinker chemistry good bonding of coating requires that the

chemical and thermal environment should remain constant

poor running in of linings. Many types of brick undergo chemical

changes during warming up, and this process must be well regulated


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overheating due to excess fuel or periods of thin feed

distortion and/or flexure (ovality) of the kiln shell

badly directed or impinging flame

Tyres and RollersThe purpose of tyres (often called riding rings) and rollers is to support the kiln and allow it to rotate with minimal

friction. Rotary kilns are among the largest i tems of permanently moving industrial machinery, the largest

examples weighing in their fully-loaded form several thousand tonnes. Despite the challenges of their size and

their high temperature, the best examples of rotary kiln rotate on their rollers almost frictionlessly, the power

supplied by the drive being almost entirely in order to oppose the eccentric load of the contents of the kiln. On

cutting the power to a kiln, the kiln will roll back and unless a brake is applied, will continue to swing l ike a

pendulum for ten or fifteen minutes before coming to a standstill. This finely-tuned mechanical condition requires

sophisticated design of the kilns supports.

A standardised design evolved during the first three decades of the twentieth century, allowing the great

escalation in size of kilns that then followed.


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Tyre mounting

The tyre itself is usually a single steel casting, machined to accurately circular dimensions and with a mirror-

smooth texture on all surfaces. Early tyres were occasionally produced as half-sections that could be easily

assembled and replaced, but this was very soon abandoned because of the resulting rapid and erratic wear at

the joints.

In the standard design, the tyre was mounted loosely on the kiln shell. Inevitably, the tyre is cooler than the kiln

shell, and so a small gap allows differential expansion to take place. The gap is usually designed to be about

0.2% of shell diameter at normal operating temperature. The kiln tube bears down upon the inside of the tyre

through smooth-surfaced chairs which also have lugs bracketing the tyre, preventing it from slipping along the

kiln axially. The spacing of the chairs also reduces the amount of heat conduction from the kiln shell to the tyre.

The tyre needs to remain relatively cool because so large a casting would be unlikely to survive a large radial

temperature differential during heating up of the ki ln. Another effect of the gap is that tyres would gradually

precess around the kiln, with one complete turn in every 500 turns of the kiln. Measuring the rate of precessionwas a rough-and-ready way of assessing the width of the expansion gap while the kiln was in operation. Small

changes due to wear could be adjusted by adding shims.

The expansion gap leads to distortion as discussed above, with the shell sagging within the loose fit of the tyre,

which causes the refractories to flex and break. If the kiln becomes sufficiently over-hot to close up the expansion

gap, then permanent damage to the shell occurs. The early kilns had the chairs attached directly to the shell, and

damage to the shell and refractories in this area soon led to the provision of one or two extra layers of shell plate

- "wrapper plate" - in the tyre area. From the 1920s, all kilns had triple-thickness shell under the tyre chairs, and

this made ovality problems manageable until kilns over 5 m in diameter started to be constructed.. The largest

diameter British kilns at Northfleet(where the burning zone internal diameter was 6.096 m) suffered major

difficulties with short refractory life. Since scale-up of modern precalcinerkilns requires the use of large

diameters, splined tyres have been developed since the 1990s (although these are occasionally encountered in

more primitive form before that). These allow the tyre to interlock with the shell (while maintaining an air gap) in

such a way that the kiln is suspended from the 3 oclock and 9 oclock positions rather than have the weight of

the kiln entirely concentrated at the 6 oclock position as in the traditional design. This has the effect of reducing

the magnitude of ovality distortion by 75% or more, although the resulting design is much more complex and

therefore expensive. This expense is easily offset by savings in refractory costs, and all recent kiln installations

have used this new design.

Rollers

The basic design of rollers has changed little over the years. The rollers are mounted on a massive cast iron or

steel base plate which provides the inward horizontal forces on the rollers and distributes the weight of the kiln

over the pier. The spacing between the rollers has to be small enough to prevent large horizontal forces, but

large enough to keep the kiln laterally stable. Rollers are designed to subtend 60at the tyre centre, and this

seems always to have been the case. Minor adjustment is allowed so that the kiln can be kept aligned (i.e. to

keep the centres of the tyres co-linear) as small changes take place, such as wear of the tyre or settlement of the

pier. The roller outer face is made wider than that of the tyre, mainly to allow for contraction of the kiln during

shut-down. This poses a problem: if the tyre remains in one position relative to the roller, wear or plastic

deformation causes a depression to form on the roller face. It is therefore normal practice to deliberately make

the kiln float (i.e. regularly move uphill and downhill across the rollers) so that wear is evened out. Because the

kiln slopes (typically 1.5 to 3.5) it has a natural tendency to slip downhill as it turns. From the earliest times, thistendency was compensated by cutting the rollers skewing their axes by a very small angle so that an uphill


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screw action is imparted to the tyre. This action relies upon the friction between the tyre and roller surfaces, and

operators could therefore make the kiln move up or down by adjusting the amount of friction. As a further

precaution to prevent the kiln from falling off its rollers, thrust rollers bearing upon the side of the tyre are used.

These are usually located on the roller beds nearest the drive, where movement most needs to be restricted.

Relying upon friction, cutting of rollers necessarily increased the rate of wear, and after being standard practice

for many years, it was abandoned from the 1950s onward in favour of the use of mechanical thrusters to float the

kiln. These usually take the form of hydraulic rams attached to the thrust rollers, which are automatically

controlled to impart a saw-tooth axial oscillation to the position of the kiln, with an amplitude of a few centimetres.

Number of tyres

As mentioned above, a rotary kiln is essentially a rotating beam, so its tendency to sag between the supports

means that the distance between the supports must be limited, and longer kilns must therefore have more tyres.

The earliest kilns had only two supports, so that there was no need to confront the problem of kiln algnment. Infact, during the early 1890s, the prospect of these problems was a disincentive to building kilns over 30 ft long.

But the new patent kilns of the late 1890s were extended to 60 ft with three tyres. The question of whether it was

feasible to progress to longer kilns was settled with Edison's kilns of 1905. These 150 ft kilns bizarrely had one

tyre on each of their 10 ft cast iron sections - a total of 15 tyres. These kilns were an evolutionary dead end, but

at least demonstrated that there need be no limit to the length of kilns.

Three kilns had eight tyres: West Thurrock kiln 6, Westbury kiln 2and Masons kiln 5. Note: planetary cooler

outrigger tyres not included.

Since the amount of sag between supports depends on the ratio of the span to the diameter, the actual number

of tyres employed depends upon the kiln's length/diameter ratio, but also upon the load that could be

accommodated by the tyre/roller systems of the time. The ratio of between-tyre span to kiln diameter settled

down to a mean value of about 6. The total mass of wet processkilns increased in proportion with their output,

and kilns were designed with as many as eight tyres. The emergence of dry process kilns brought about a return

to the use of short kilns with three tyres. The following charts show the evolution of these factors with time, as

running mean of five.


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The most recent precalcinerkilns, because most of the processing is done in the preheater, can have very low

length/diameter ratios (


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Turning Gear

Until the advent of frictional drives (see below), kilns only ever had one turning gear and this

supplies all the torque to turn the kiln, so in the case of a long kiln, it is usually positioned

somewhere near the middle (strictly speaking, the centre of mass) to minimise the amount of

torsional distortion produced in the shell. Preferably a relatively cool section of the kiln is

chosen. The gear is placed near to a tyre so that it is accurately aligned with the kiln axis, with

minimal wobble. It is normal for the nearby tyre to be fixed in position with thrust rollers, so that

as the kiln expands on warming up, the turning gear position remains fairly constant, while the

nose and tail of the kiln expand outward. The pier of the nearby tyre is usually extended to

include the pinion mounting bed, the gearbox and the motor, although on early kilns it was

common to mount the motor on the kiln house floor, and connect it to the gearbox with a flat

belt. In the case of shorter dry process kilns with preheaters, it has been normal practice to

locate the turning gear next to the rear tyre, at the coolest part of the kiln.

Early turning gears were attached directly to the shell. Differential expansion in such

circumstances causes the gear to break and the kiln shell to "neck", and this practice was soon

abandoned in favour of some sort of flexible mounting. Various mountings have been used, but

by far the most common is the tangential mounting, which emerged in the first decade of the

twentieth century. Flexible plates are rivetted (and later welded) to the kiln shell tangentially,

the other end being fixed to the gear ring through a flexible coupling. This allows expansion of

the shell to take place unfettered. It also results in minimal heat transfer to the gear ring, so that

the latter remains cool enough for conventional lubricants to be used. Tangent plates must

operate in tension, and so they differ from longitudinal mountings in that the kiln may not be runin reverse.


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There is an urban legend that, when the first kiln at Westburywas commissioned, the chains were hung in the

wrong direction, screwing the feed uphill instead of downhill. Rather than delay the Grand Opening, the polarities

of the motors were reversed, and the kiln was run backwards until the dignitaries had departed.

Power Train

Until the 1960s, kilns had a single pinion engaging with the gear wheel. These were almost

invariably located on the rising side of the kiln. This places it underneath the feed bed, which is

marginally cooler than the other side which is in contact with hot combustion gases. It also

places the turning effect closest to the source of the eccentric load. The rotation of the kiln lifts

the feed up the side of the kiln, and the energy required to maintain its centre of gravity above

the lowest (6 o'clock) point is the main component (80-90%) of the energy consumed. In the

case of kilns containing curtain chains, these also produce an eccentric load.

Rotary kilns have always had variable speed drives. From the earliest times there was always,

at least, an option of "full speed" and "half speed". This allows the operator to vary the rate atwhich the feed advances down the kiln, and in particular, allows the kiln to be warmed back up

again if for some reason the burning zone has become too cold to sinter the clinker. On early

drives, speed change was brought about by use of "fast and loose" pulleys of various sizes.

However, with DC motors it was also possible to vary the speed of the motor itself, and this

ability was one of the main reasons for the early adoption of electric power. However, variable

speed was not viable for more powerful motors, so this placed a limit on the size of kiln that

could be turned with a single motor, the maximum being around 250 kW.

An alternative strategy for larger kilns is to have two pinions acting on the gear wheel, one on

each side of the kiln. This was problematic for earlier technology, because of the problems of

having two motors competing to supply torque at varying speed, but from the 1960s, advancesin motor control allowed dual drives to be installed on larger kilns. Because it is related to

eccentric load, kiln rotating power is more or less proportional to speed. The need for higher

rates of rotation began to emerge with suspension preheaterkilns in the 1960s, and much

higher speeds of 4 rpm or more are required for short precalcinerkilns. Modern drives, in line

with the large, high speed kilns being installed, can be much larger (>500 kW from each motor

in a dual drive), with speed varied over a wide range by means of solid state controls.

However, the largest British drives appear to have been the pair of 580 kW motors on each of

the six kilns at Northfleet


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Picture: NERC: British Geological Survey Cat. No. P539361. Drive of PlymstockKiln 1 (constructed 1961). As

is normal with short, dry process kilns, the drive is close to the rear - the back end seal is visible top right. The

girth gear is placed close to a tyre for stability, and axial movement is restricted by a horizontal thrust roller,

visible under the kiln. The drive pinion is behind the left hand roller. Behind that is the gearbox, and the 67 kW

electric motor is in the rectangular enclosure on the left edge. The roller bearings, gearbox and motor all have

heat shields to protect them from radiant heat from the kiln shell. From the position of the girth gear tangent

plates, it can be seen that the kiln turns clockwise when viewed from this direction, and the drive pinion, as is

normal, is on the "rising" side of the kiln, under the feed.


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Picture: Rugby Archive: Cat. No. RC-10. Helical girth gear on RochesterKiln 6 during construction (1978). The

drive was on the enlarged (5.3 m) back end section of the kiln, making this one of the larger girth gears. Because

of the high power requirement (580 kW?), this is a dual drive. The rising-side pinion is shown: the other is off-

frame to the left, and the other gear-box is just visible through the hole in the girth gear.

Auxilliary Drives

An additional feature of kilns from the 1950s onward was the provision of an "auxilliary drive"

which is engaged in the event of failure of the main drive. Once a kiln has been raised to

operating temperature, it must be kept turning, at least intermittently, because the upper part

cools faster than the lower part which contains the hot feed bed. If this situation continues,

differential contraction will cause the kiln to bend. A further problem is that, at the hot end of the

kiln the feed is partly liquid, and will "freeze" into a solid block unless turned over by kiln

rotation. Frozen feed, on finally turning the kiln, will pull out the underlying refractory lining. The

earliest "barring gear" on smaller kilns consisted simply of a highly geared-down capstan that

could be turned by hand. More modern systems commonly consist of a small diesel engine that

can be started up and engaged with the gear-box, even if there has been a complete power

failure, turning the kiln at about 0.2 rpm.

Friction Drives

With the re-emergence of two-tyre kilns on precalcinersystems, some kilns have been supplied

without girth gears, the torque being supplied through the rollers. This relies upon the friction

between roller and tyre, and the critical requirement is that the friction should be sufficient to

start a heavily-loaded kiln from the stalled condition. On older kilns, this was never a viable

proposition, but the large-diameter two-tyre kilns have a sufficiently large roller loading that

tangential friction is greatly in excess of the likely requirements. Drive through the tyres serves

further to simplify the design of two-tyre kilns. Torque is applied to the roller shaft(s) by either

an electric motor and gearbox or by a hydraulic drive.


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The Kiln Hood

The purpose of the kiln hood is

to provide an insulating front closure for the kiln to provide a secure entry-point to the kiln for the firing pipe

to provide a relatively safe place for the operator to view the formation of clinkerin the

hottest part of the kiln

to duct the hot secondary air from the cooler into the kiln with minimal leakage and

wastage of heat.

The last of these would today be considered to be the most important requirement, but in the

early days the importance of secondary air was not always appreciated, and some (including

Lathbury and Spackman) maintained that all combustion air should enter through the firing

pipe. For this reason, early kiln hoods were usually very short, and communicated with a small,

restrictive cooler throat.

Picture: from article in The Engineer: NormanKiln A1 hood viewed westwards in 1904.

Typical Fellner & Zieglerhood design. Note the firing pipe entering below and to the right

of the kiln center-line towards the clinker bed - the kiln turns anti-clockwise. Early short

kilns had difficulty concentrating the heat into the burning zone. The clinker fell into

a rotary coolerbelow the firing floor. The cooler air was used in coal drying, and little

entered the kiln directly.

The hood receives direct radiant heat from the white-hot clinker and refractories and the flame,

and so is refractory lined for protection from this. It also needs to be strongly constructed to

cope with pressure variations that may occur. On the other hand, there is a need to gain

access to the kiln for maintenance of the refractories. This was particularly the case during the

experimental first decade of the twentieth century, when the service life of refractories was

often very short. For this reason, from the first patents onward, the kiln hood was mounted on

wheels or rollers so that it could be rolled back from the kiln nose. The need to do this later

became a reason for restricting the size and cross-section of kiln hoods.


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Hood of WilmingtonKiln 4 from the south in 1921. Typical FLSdesign of the time with a

concentric cooler below the kiln.

A significant step-change in hood design came with the Fuller grate cooler patent, which put

heavy emphasis on the speed of cooling of the clinker and the aerodynamics of the secondary

air. This led to the installation of deeper hoods, and the escalation of both hood depth and kiln

diameter led to the abandonment of moveable hoods in the early 1960s. Modern large hoods

have doors in the front large enough for small vehicles to enter the kiln.


19/19

Picture: NERC: British Geological Survey Cat. No. P539367. Hood of PlymstockKiln 1 (constructed 1961): a

larger, fixed hood. The kiln rotates anticlockwise, so the feed bed is on the right hand side of the kiln. The

operator is viewing the feed diagonally from the left, under the flame, in order to get a long perspective view.

Note the oil-fed burner. Below is a Fuller grate cooler.

For maximum thermal efficiency, modern kilns use a relatively small amount of cool primary air

through the firing pipe, and coolers produce secondary air at high temperature, so hoods are

carefully aerodynamically designed to ensure that the secondary air envelopes the flame in a

manner that optimises combustion.

The use of the hood as a view-point for the operator was essential in early practice. The peak

temperature of the feed in the front of the kiln is critically important, since a fall in temperature

causes the free-lime content to rise rapidly, and clinkering (i.e. sintering) may cease altogether,

causing fine feed to rush forward into the cooler. On the other hand, too high a temperature is

liable to cause loss of coating and damage to the kiln. Although various kinds of pyrometer for

temperature measurement have been available throughout the history of rotary kilns, it can besafely said that they were not an effective means of control until the end of the twentieth

century. The sole control of temperature was the expert eye of the operator. Criteria were the

colour and brightness of the clinker, the height to which the clinker climbs the kiln wall (which is

related to the amount of liquid formed) and the position where clinkering starts. The control

panels were therefore located on the firing floor and the operator interspersed scrutiny of the

instruments with frequent visits to the ki ln hoods to monitor progress. Cement plants stood or

fell by the round-the-clock expertise of their kiln operators.

From the late 1960s, instrumentation started to improve, and the operators reliance on visual

information was aided by TV cameras mounted on the hood inspection ports. This led to the

possibility of centralised control rooms remote from the kiln. More modern cameras are

provided with infra-red sensitivity and the image is processed to show colour-coded

temperature. Modern firing floors are usually deserted unless something has gone wrong.

Kiln Seals

The seals connect the ends of the kiln to the kiln hood and the kiln exhaust

duct. They prevent leakage of cold air into the system at these points. Gases

are moved through the kiln by the suction provided by a fan in the exhaust or

in the preheater, so the efficiency of the fan relies upon minimal inleak at the

back end seal. Furthermore, even if the fan is capable of handling a large

amount of inleaking air, the dust control equipment and the preheater (if

present) will be inefficient if they have to handle an excessive amount of gas.

The front-end seal ensures that there is sufficient suction to draw the

secondary air from the cooler into the kiln.

Both these seals have to deal with high temperatures, and so must be either of

a simple heat-proof design, or must be kept cool by means of external fans.

Both must also be capable of remaining air-tight as the kiln expands and

contracts, and must cope with rotation of a kiln that may be slightly distorted.

Dylan Moore 2013: last edit 09/02/14.

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Rotary Kilns