Design and evolution of rotary cement kilns

19/04/2019 Cement Kilns: Design features of rotary kilns
https://www.cementkilns.co.uk/kiln_design.html 1/15
Picture: ©NERC: British Geological Survey Cat. No. P540333. Front end of Ribblesdale Kiln 2
(constructed 1937): shell constructed mainly from staggered semi-circular sections, with all
joints riveted.
Please note that all the pages of this website (but particularly this one) describe the history of
cement manufacture, and provide background information for historians, geographers and
indutrial archaeologists. They must not be used as a guide to cement industry practice.
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 1400°C, while the
gas temperatures reach
1900°C. The melting
point of mild steel is
around 1300°C, and it
starts to weaken at
480°C, 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.

Picture: ©Rugby Archive: Cat. No. RC-3-6-8. Back end of Southam Kiln 7 (constructed 1961):
shell entirely welded. The shell still consists of staggered half-cylindrical sections.
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.
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 construction reduced 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).
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 riveted 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 1200°C or more is flowing over the lip of the kiln, and the shell is subject
to very aggressive conditions. Castings of heat resistant 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

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.
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 by their disadvantages - the tapered sections are mechanically weak
and hard to line with stable brickwork.
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 Wouldham and Bevans resulted in kilns with enlarged burning zones, while the lengthened kilns
at Swanscombe had 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 Lewes kiln, which was the first of what became a
common FLS design, with elongated wide front and rear sections, and a narrower "waist" occupying the
middle third. The other back-end enlargements were 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
Armstrong supplied kilns with enlarged burning zones only, while FLS supplied mostly kilns with enlarged
rear sections only, employing an identical design for the Long Dry kilns at Padeswood, Pitstone and 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 (riding rings) - 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 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:
Axial distortion – the tendency of the kiln to sag between
two successive tyres (fig. 1)
Transverse distortion (ovality) – the tendency of the kiln to
flatten, mainly in the vicinity of the tyre (fig. 2)
Distortion due to damage:
Blistering – usually due to local over-heating (fig. 3)
"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 180°C
above design temperature in the vicinity of the tyre.
Banana distortion – usually due to over-heating one side of
the kiln during a crash-stop (fig. 5)
Structural defects:
Misalignment – vertical displacement of the rollers from
their correct position
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 of the 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.

Refractories
To 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 3’6” 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 free from 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 kiln
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 kiln 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.
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 life was usually very short, and its use mostly died out in the 1930s. It was last
used at Masons in the late 1940s. It continued in use in the burning zones of the Anhydrite Process kilns.
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 year’s 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” – radiated
from 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
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 Rollers
The 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 items 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 like a pendulum for ten or fifteen minutes before coming to a standstill. This finely-
tuned mechanical condition requires sophisticated design of the kiln’s 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.
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 kiln. 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
precession was 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 precalciner kilns 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 o’clock” and “9 o’clock” positions rather than have the weight of the kiln entirely concentrated at
the “6 o’clock” 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 60° at 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, this tendency was compensated by “cutting”
the rollers – skewing their axes by a very small angle so that an uphill 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 alignment. In fact, 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 2 and 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 process kilns 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.

The most recent precalciner kilns, because most of the processing is done in the preheater, can have very
low length/diameter ratios (<14) and this re-introduces the possibility of mechanically simplified two-tyre
kilns. Splined tyres are combined with self-aligning roller assemblies and through-the-roller drives (see

below), allowing the system to continue to function even if the kiln is bent. This has been adopted for the
kilns at Rugby, Ballyconnell, Kinnegad, Tunstead and Platin.
If a kiln has length 14D and has two tyres located at 25% and 75% of the length, then the span between the tyres is 7D, which is
about the practicable limit. The Kinnegad kiln has a girth-gear drive.
The Kiln Drive
Ever since the first Ransome kiln (until recently) rotary kilns were turned by means of a single girth gear
(known as the turning gear) surrounding the kiln. The early kilns turned very slowly, the girth gear meshing
with a worm gear. Subsequently full-speed rotation of kilns in the range 0.5-1.5 rpm became standard, and
the gear meshed with a pinion running at 10-20 rpm. The pinion shaft was driven by a gearbox. Some kiln
gearboxes derived their power from layshafts driven by a common electric motor or a steam engine, but
this was comparatively uncommon in Britain, and most kilns had their own dedicated electric motor. In
many older plants, rotary kiln drives represented the first use of electricity for anything other than lighting.
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

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.
Picture: ©Rugby Archive: Cat. No. RC-10. Helical girth gear on Rochester Kiln 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.
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
riveted (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 run in
reverse.
There is an urban legend that, when
the first kiln at Westbury was
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 at which 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, advances in 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
preheater kilns 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
Auxiliary Drives
An additional feature of kilns from the 1950s onward was the provision of an "auxiliary 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 precalciner systems, 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.

Picture: from article in The Engineer: Norman Kiln A1 hood viewed westwards in 1904.
TypicalFellner & Ziegler hood design. Note the firing pipe entering below and to the right of the
kiln centre-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.
Hood of Wilmington Kiln 4 from the south in 1921. Typical FLS design of the time with a
concentric cooler below the kiln.
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 clinker in 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: ©NERC: British Geological Survey Cat. No. P539367. Hood of Plymstock Kiln 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
Fullergrate cooler.
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.
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.
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 unburned 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 be safely said that they were not an effective means
of control until the end of the twentieth century: all methods are optical, and instrumental methods are
defeated by the difficulty of discriminating between radiation from the various sources - clinker, flame,
brickwork and swirling dust. The sole control of temperature was therefore the expert eye of the operator.
Visible criteria were the colour and brightness of the clinker, the height to which the clinker climbs the

rotating kiln wall (which is related to the stickiness caused by the amount of liquid formed) and the
positions where clinkering starts and finishes. The control panels were therefore located on the firing floor
and the operator interspersed scrutiny of the instruments with frequent visits to the kiln 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 operator’s 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 2012: commenced 04/02/2012:
last edit 28/02/2019.
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