477

In cement manufacturing, formation of clinker nodules occurs at the entrance to the hottest part

of the kiln with a material temperature of around 1280°C. The clinker is preferably in the form of

10-mm to 25-mm size nodules that exit from the front end of the kiln into the cooler. It is critical

that cooling of the clinker is rapid to secure a phase composition that imparts adequate cementi-

tious properties. It is equally important that the heat exchange between clinker and air is efficient

to ensure proper cooling, and at the same time maximize the recovery of heat to secondary air,

tertiary air, and the related process requirement. The modern cooler must accomplish all of these

tasks efficiently and simultaneously.

Like other processing equipment, clinker coolers have undergone significant development over the

past years. This chapter describes the advent of clinker coolers with discussion and description of

various types of coolers presently available. The chapter also focuses on the reciprocating grate

*Technical Director, Western Region, Ash Grove Cement Co., 6720 SW Macadam Ave. #300, Portland, Oregon 97219,Tel: (503) 293-2333.

Figure 3.8.1. Grate clinker coolers.

Chapter 3.8

by Hans E. Steuch*

Clinker Coolers

cooler and the latest developments in cooler designs, while tracing the historical development of

the reciprocating grate cooler in relation to increasingly fuel-efficient kiln systems. The theoretical

mass and heat balance equations that describe the steady state and heat recuperating efficiency are

presented, followed by a more practical discussion of how to automate and optimize the operation

of the cooler. Figure 3.8.1 shows the interior of most commonly operated grate coolers in cement

manufacturing.

At the discharge end of the kiln, the clinker is red hot and contains around 1.0 million Btu per

short ton thermal energy. The clinker is also to some extent still reacting chemically toward

creation of various clinker minerals. The purpose of the clinker cooling is to recoup some of the

heat in the clinker, thereby making it cool enough to handle. We also want to stop the chemical

reactions in the clinker at the point most favorable to the cement quality.

TYPES OF CLINKER COOLERS

What governs the design and selection of a clinker cooler? Surely, today, any design project would

include some of the following requirements: low capital cost; optimum cooling rate for good

clinker quality; low clinker discharge temperature; least possible impact upon the environment;

high heat recovery; low power consumption; low wear and maintenance cost, and reliable to oper-

ate, causing minimal downtime; and easy to control so it delivers a steady flow of combustion air

at an unvarying temperature to the kiln and calciner. These criteria are of immediate interest to a

manufacturer of cement who buys a cooler for clinker. The designer of the clinker cooler looks at

these criteria and tries to optimize the design, depending upon the weight of each of these individ-

ual criteria.

Over the years, the criteria that are used to select coolers have changed. The technology of clinker

cooling has developed as well, so that many different types of clinker coolers have been applied

since the infancy of the portland cement manufacturing industry in the late l9th century. The

following sections will describe the most common clinker coolers with particular emphasis on the

reciprocating grate cooler.

Planetary Coolers

The name of the planetary cooler is

derived from the fact that it circles the

kiln like planets circle the sun. A

planetary cooler consists of a number

of cooling tubes mounted around the

circumference of the kiln shell

(Figure 3.8.2). The advantage of the planetary cooler is its simplicity: it requires no excess air to

handle, no fans or motors, and no instruments. It is self-adjusting. The power consumption is only

about 0.5 to 1 kilowatt-hours per ton of clinker added to the kiln drive and exhaust fan, making it

Innovations in Portland Cement Manufacturing478

Figure 3.8.2. Planetary cooler.

the lowest for any kind of clinker cooler. The heat losses through radiation and sensible heat in

clinker are between 0.40 and 0.45 mega-joules per kilogram of clinker for an economical dry-

process kiln even and lower for wet-process kilns. Planetary coolers have been used successfully for

kilns as big as 4000 metric tons per day, though not in North America.

These coolers were popular in the 1960s and 1970s when many dry process 4-stage preheater kiln

systems were built around the world. In North America, most of the dry process kilns were

supplied with grate coolers.

The planetary cooler does not allow withdrawal of tertiary air for a calciner. As most kiln systems

built today have calciners, the planetary cooler is becoming a relic of the past. One weakness of the

planetary coolers is that they can be costly to maintain. The cooler inlets often wear out too fast

due to the thermal, mechanical, and abrasive stress to which they are subjected. To decrease the

resulting maintenance and downtime, over the years there has been continuing improvement by

trials with inlets made of high temperature metal alloys or ceramic materials.

Rotary Coolers

Some of the earlier coolers were

almost like another kiln following

the clinker burning tube or, using

another picture, take the planetary

coolers, combine them into one

tube with its own support and

drive, and you have a rotary cooler

(Figure 3.8.3).

The modern rotary cooler is equipped with ceramic lining and lifters based upon the development

of the planetary cooler. Special seals at the kiln outlet and the cooler inlet are required. To avoid

spillage from the inlet, the cooler is inclined 2.5°

and given a speed of rotation of 3 rpm. The

power consumption for the drive is about 3.5

kWh/ton. The clinker temperature is 200°C to

250°C, but is reduced to about 150°C by water

injection in the outlet. Presently, no cooler of

this type is used in North America.

Shaft Coolers

As a curiosity, we should mention the shaft

cooler (Figure 3.8.4), which has been operating

with a 3000 metric ton per day kiln in Europe

479Clinker Coolers

Figure 3.8.3. Rotary cooler.

Figure 3.8.4. Shaft cooler.

since 1976, but apparently has not gained a foothold in the cement industry. The cooler requires

fairly even clinker size distribution. The upper part is operated as a fluid bed in order to avoid

agglomeration and to ensure even distribution. The power consumption is high, 10 to 12 kWh/ton,

because the cooling air has to be compressed to about 20 kPa. With minimum air to the cooler, the

clinker temperature is 300 °C – 350°C, but it is reduced by water injection in the lower part.

It should be added that shaft coolers of somewhat different design, such as the Niems cooler, have

been used very successfully for modern lime burning kilns. Burnt lime has a rather uniform grain

size distribution and therefore is much easier to cool in a shaft cooler than cement clinker.

Traveling Grate Coolers

It should be mentioned that travelling grate coolers have been used in the past; but, generally, they

were never developed to the same high standard of operational reliability as the reciprocating grate

cooler. Travelling grate coolers have been used mostly in connection with grate preheater kilns,

which produce a very uniform clinker size. The travelling grate cooler has the disadvantage that the

clinker is conveyed as a solid bed. To obtain effective clinker and air distribution, it is often neces-

sary to use pulsating air.

Grate Coolers

The grate cooler is by far the most common clinker cooler in North America. Where the air and

clinker move in opposite directions (also called counter current) in the planetary, rotary, and shaft

coolers, the grate cooler is based on the cooling air moving cross current to the direction of the

clinker movement. This type of cooler can produce clinker discharge temperatures around 80°C;

but it needs more air for cooling than can be used in the kiln, and the excess air has to be removed

and dedusted. The amount of air needed varies according to the clinker size distribution and to the

clinker temperature required. It is costly to cool to low temperatures. The amount generally lies

between 2.3 and 3.3 kg of air per kilogram of clinker; but in order to cope with forced conditions

and fluctuations, the cooling fan capacity is normally designed to allow the introduction of 4.5 kg

of air per kilogram of clinker. The specific load on grate coolers built since the mid-1970’s is often

35 to 45 metric ton per day per square meter grate area compared to 20 for grate coolers built in

earlier times. This is the result of the tendency to improve heat recuperation by working with a

thicker clinker bed on the grate.

The cooler consists of one or several grate sections. The sections are defined by their location or

their function, or by whether they are connected to a certain drive (for instance, ‘inlet grate,’ ‘2nd

movable grate,’ etc.). Each grate consists of a certain number of rows of plates. The plates have

been the subject of much development in the 1990s, as will be described later. The air to the grates

is supplied in various ways: through air blown into compartments under the grates or blown into

ducts (often called ‘airbeams’) connected directly to a limited number of grates.

Innovations in Portland Cement Manufacturing480

A typical cooler built between 1970 and 1990 works in the following fashion. From the kiln, the

clinker drops onto a stationary air-quenching grate. This grate may be horizontal or inclined. It

consists of one or several rows of plates. In the cooler shown in Figure 3.8.5, there are three

movable grates; the first is with an inclination of a few degrees, and the other two are horizontal.

Below the grate, the cooler is divided into a number of compartments, each provided with fans

equipped with adjustable guide vanes for automatic air flow control and minimum power

consumption. Clinker spillage through the grate is collected in hoppers and removed through

airtight flap valves to the

clinker conveyor. Since the

1990s, the underside of the

plates in the quench grate

and the first grate have

been connected directly to

cooling fans. This has

allowed better individual

adjustment of air to differ-

ent parts of the grate.

The efficient sealing between the compartments permits operation at high and different pressures

in the various compartments. With a normal clinker bed thickness of 600 mm, the pressure drop at

a constant air flow per unit area will decrease from about 5.9 kPa in the hot end to about 2.0 kPa in

the cold end. The fans are sized accordingly so that the maximum pressure decreases from 7.3 kPa

to 2.9 kPa. For trouble-free operation, it is an advantage to use more air per grate or unit area in

the hot end, up to 200 kg/min/m2, and less in the cold part, say 40 kg/min/m2.

The width of the grate is reduced in the inlet in order to spread the clinker more evenly. Together

with the high air flow and the thick layer of clinker, this helps to provide a uniform clinker bed

thickness, which in turn gives a uniform air flow over the width of the grate. This is essential not

only to avoid local overheating of the grate, but also to avoid “snowmen” – the clinker is kept

moving throughout the whole grate until the individual particles have lost their stickiness and

ability to cling together.

The clinker is pushed through the cooler by the reciprocating movement of rows of plates. Usually,

every second row of plates in a grate is movable. The other rows are stationary.

A crank arm moves the movable frame on older coolers. The rows of plates are moved by a

connecting rod which is centrally fixed to the movable frame, so that twisting is avoided. The rod

goes through the wall via an airtight seal and is driven by a direct current motor or by a hydraulic

piston. In the 1980s one supplier started to offer a pendulum suspended frame, such as shown in

481Clinker Coolers

Figure 3.8.5. Reciprocating grate cooler – side view.

Figure 3.8.6. This method of moving the frame

is claimed to be particularly effective at keeping

tight tolerances of movement to minimize wear

on side castings. The activation by a single

hydraulic cylinder with an asymmetric stroke

(slow forward, fast back), helps minimize

mixing of the clinker and, thereby, bed resistance

to airflow. The speed of frames, whatever way

they are moved, can be varied between 3 and 30

strokes/min. In normal operation, 5 strokes/min

is adequate, providing ample spare capacity.

Before the 1990s, all grate plates, both the movable and stationary, were of identical design. They

were cast with circular holes – in the front part of the cooler they were made of heat-resistant steel;

in the cold part, of cast steel. The shoes of the plates were bolted to a cross beam away from the

heat. All designs allow removal from underneath where there is easy access to the grate through the

undergrate compartments.

In the late 1980s a new type

of grate plate connected to an

airbeam was introduced. This

plate contains inclined and

curved slots rather than holes

(Figure 3.8.7). The slots are

shaped by small blades that

are easily replaced from the

top of the plate. This innova-

tion was so effective that by

the 1990s all major suppliers

were offering grate plates

with slots instead of holes for

the hot part of the cooler. The suppliers’ plate designs varied, but they all contained a pocket where

cooled clinker could rest and minimize metal wear, and they were all connected directly via an

airbeam to a fan rather than being supplied with air through an undergrate compartment. These

changes resulted in better protection of the grate from thermal and abrasive stress caused by hot

moving/sliding clinker and improved cooling of the clinker by better control of air flows.

The clinker discharges from the cooler across a grizzly to a hammer mill or hydraulic roll crusher

located in the cooler outlet. The crusher may be installed in the middle of the cooler, before the

last grate, to break up lumps and large clinker, and to ensure their efficient cooling. The thermal

stress on the crusher is obviously greater in the middle than at the end of the cooler.

Innovations in Portland Cement Manufacturing482

Seal

Bolt fornozzle

assembly

Bolt fornozzle

assembly

Stop nutLock

Seal

T-Bolt

HingeAirslot

Coolingrib

Jetnozzle

Grate beam

Through rod

Figure 3.8.7. Air beams and grates with slots.

Figure 3.8.6. Grate cooler – pendulum framefor moving grate plates.

When a cooler is operating with a thick clinker bed and evenly distributed clinker and air, and is

designed with sufficient retention time of clinker in the cold end, hot lumps do not cause severe

problems.

Grate Coolers Without Excess Air Vent Stack

The excess air from the grate cooler normally has to be dedusted and exhausted through a stack.

This is costly and may be difficult to get permission for from licensing authorities. To avoid these

problems, some plants have installed a combination of a short grate cooler and a gravity or

“G-cooler” or they have installed recirculation of the excess air.

The gravity cooler (Figure 3.8.8) is used in connection with a short grate cooler, furnished with just

the amount of air needed for combustion in the kiln and calciner. The clinker discharged from the

grate at a temperature of about 500°C is crushed and carried to the top of the gravity cooler,

through which it drops slowly at 2 - 3 cm/min, while cooled indirectly by ambient air blown

through cooling tubes. After about two hours of slow downward travel, the clinker is discharged at a

temperature of about 100°C. The power consumption for the fans of the G-cooler is around 1 to 2

kWh/ton. Control of hood pressure and the conveying of occasionally very hot clinker between

grate and gravity cooler requires special attention to make this system operate well.

Another way to avoid dedusting the excess air from a grate cooler is to cool the air in a heat

exchanger and then

recirculate it to the

grate (Figure 3.8.9).

The heat exchanger

is designed so that

ambient air is blown

on the outside of the

cooling tubes

through which the

excess air from the

cooler is drawn.

483Clinker Coolers

V = 2–3 cm/min

500°C

80°C

Figure 3.8.8. Grate cooler followed by G-cooler

1.1 kg/kg Clinker

2 kg/kg clinker250°C

100°C

2 kg/kg Clinker60°C

20 k

g/kg

Clin

ker

Figure 3.8.9. Grate cooler with recirculation of excess air.

Cross-Bar™ Cooler

In the late 1990s, a new

type of clinker cooler was

introduced. It shares the

horizontal conveying of

clinker through which a

vertically moving mass of

cooling air is blown with

the reciprocating clinker

cooler, but has several

innovative and unique

features. The most striking

is that clinker are no

longer conveyed by rows

of grate plates moving back and forth, but by wedge-shaped bars suspended above the grates,

which are all stationary (Figure 3.8.10). These bars move back and forth and have inspired the

name “Cross-Bar™ Cooler.”

Since the plates no longer move, they have been made larger. The traditional size of a cooler grate

is 30x30 cm; the cross bar cooler plates are 1x1 m. Furthermore, each plate is supplied with an

amount of air that is individually and dynamically adjusted to fit the cooling needs of the moment.

This is accomplished by a mechanical flow regulator valve located in the air supply channel affixed

underneath the grate plate. This regulator passes air from the undergrate chamber to the holes in

the plate as shown in Figure 3.8.11. This eliminates the need for airbeams between cooler fans and

grate plates, and the me-

chanical problems associ-

ated with them. Currently,

there are only a few cross-

bar coolers in cement

operation. The vendor

claims the cooler is consid-

erably more efficient at

heat recuperation than

ordinary reciprocating

grate coolers. The amount

of cooling air is reduced

from 2.8 to 1.9 kg air per

kilogram clinker, resulting

in a low power consump-

tion of 4.0 kWh/ton of

clinker cooled.

Innovations in Portland Cement Manufacturing484

Non-uniform clinker

Low bedresistance

High bedresistance

Lowervalve ∆P

Highervalve ∆P

Figure 3.8.11. Mechanical air flow regulator.

Figure 3.8.10. Cross-Bar™ cooler.

485Clinker Coolers

Table 3.8.1. Typical Operational Data for Different Cooler Used in Dry Process Kilns

Grate +Grate + recirc. Cross

Planetary Rotary Shaft Grate G-cooler vent air Pendulum bar

Air (kg/kg clinker)Fresh cooling air 1.1 1.1 1.3 2.8 1.1 1.1 1.9 1.9Excess air to vent 0 0 0 1.7 0 0 0.8 0.8

Clinker temp 1, °CAfter cooler 160 220 360 80 490 100 100 100After cooler, with water 120 160 150After secondary cooler 80

Power consumption(kWh/ton) cooler only 0.8 3.5 12 7 8.5 9.5 4 4

Thermal efficiency2, % 67 70 74 60 64 63 71 71

1Clinker from kiln: 1300°C2Thermal efficiency = (Heat in clinker from kiln – heat losses) *100

Heat in clinker from kiln

COMPARISON OF DIFFERENT COOLER TYPES

A study made in 1980 of the investment costs for grate coolers with different types of dust

collectors and grate coolers operating without excess air led to the surprising result that the total

installed equipment costs are the same within ±5%. The decisive factors for choosing between

these clinker cooling solutions are therefore their operational costs and reliability. Typical opera-

tional data for the different cooler types are tabulated in Table 3.8.1.

OPERATION OF GRATE CLINKER COOLERS

Mass and Heat Balances

In the previous chapters we have mentioned the varying amounts of secondary air and tempera-

tures found in different types of clinker cooler systems. To better understand these differences, we

might ask how much of the heat contained in the clinker dropping into the cooler has been recu-

perated to the air returned from the cooler to the kiln system?

Some of the heat entering the cooler will be lost in the cooled clinker, radiation, and possibly the

vent air. The amount recuperated is a measure of the thermal efficiency of the cooler. The more

recuperated, the more thermally-efficient it is.

To calculate the thermal efficiency, it is necessary to establish mass flows, temperatures, and heat

flows. Figure 3.8.12 shows a typical clinker cooler with its heat inputs and outputs. The thermal effi-

ciency of the cooler is defined as the relationship between the heat recuperated and total heat input

as shown in the equation in the figure. The lower the heat losses in clinker, vent air, radiation, and

convection, the higher the amount of heat recuperated in secondary air and the higher the thermal

efficiency.

Heat flow is a function of mass and temperature. The higher the mass and temperature of second-

ary air, the more heat is recuperated. The air used for combustion in the kiln and calciner, plus the

air excess creating the oxygen we measure in the kiln system, comes from the primary air supplied

through the burner(s), and the secondary and possibly tertiary air drawn from the cooler. For a

given combustion air need, to get the amount of secondary air and thus the cooler thermal effi-

ciency to increase, the quantity of primary air and/or infiltration air must be decreased. It is

important to maximize the amount of secondary air by minimizing primary and air infiltration

rates and to maximize the secondary air temperature by minimizing cooler heat losses.

Since the amount of combustion air depends on the overall fuel consumption, it becomes clear

that the type of kiln system influences the thermal cooler efficiency considerably.

A modern-type preheater kiln, consuming 3.0 million Btu per short ton clinker, should be oper-

ated at a thermal efficiency between 64% and 68%, if well adjusted. A long dry kiln, consuming

4.0 million Btu per short ton, should run between 68% and 72%; and a wet-type kiln, consuming

5.0 million Btu per short ton, between 70% and 75%. Nevertheless, many clinker coolers are oper-

ated at considerably lower thermal efficiencies. It is evident that operating a clinker cooler at peak

thermal efficiency improves overall heat consumption considerably.

Innovations in Portland Cement Manufacturing486

Secondary airH"SA

Tertiary airand/or

coal mill air,H"TA + CMA

Radiation &convection

H"R + C

Vent airH"VA

Clinker, H'Cl

Clinker, H"Cl

Clinker air, H'CA

Heat recuperating zone Final cooling zone

Definitions� = efficiencyH = enthalpy ' = input " = output

Coolerefficiency (�)

= x 100% = x 100%Heat recuperatedHeat input

Heat in secondary, tertiaryand coal mill air

Heat input

H"SA + H"TA + CMA

H'Cl + H'CA=

Figure 3.8.12. Mass and heat balance.

For any given grate cooler, one can usually measure the amount of cooling air blown into it, as well

as the amount of vent air and coal mill air exhausted. The amount of secondary air is calculated by

difference or, perhaps, from the amount of coal, backend oxygen, and primary air used. Typical

mass balances are shown in Table 3.8.2.

487Clinker Coolers

Table 3.8.3. Heat Balance for Grate Cooler

BTU/lb clinkerWet kiln Dry kiln Preheater kiln

Heat inClinker in Btu/lb clinker 534 534 534Cooling air in Btu/lb clinker 0 0 0

Recovered heat outSecondary air Btu/lb clinker 352 337 322Coal mill air Btu/lb clinker 86 67 45Total Btu/lb clinker 438 404 367

Heat lossesClinker out Btu/lb clinker 17 17 17Radiation Btu/lb clinker 22 22 22Vent air Btu/lb clinker 57 91 128Total Btu/lb clinker 96 130 167

Cooler efficiency1, % 82 75 68

1(Efficiency = recovered heat out/heat in) x 100%.

4.00

3.50

3.00

2.50

2.00

1.50

1.00

0.50

0.00Wet kiln Dry kiln Preheater kiln

kg a

ir/kg

clin

ker

(lb a

ir/lb

clin

ker)

1.59 2.24

0.65

0.99

0.50 0.34

1.96

Vent air

Coal mill air

Secondary air 1.51 1.02

Table 3.8.2. Air Mass Balance for Grate Cooler

Once the temperatures of the various material streams, their specific heat capacity, and the radia-

tion losses have been determined, one can calculate a heat balance, such as the one shown in Table

3.8.3.

When the balance is established, the cooler heat recuperation efficiency can be calculated as

follows:

(Heat entering cooler) – (Heat lost in excess air, clinker, and radiation)

(Heat entering cooler)x 100

Innovations in Portland Cement Manufacturing488

Secondary air0.99 mbtu /st 1350°F

Clinker air1.40 mbtu /st

2500°F

1.53 mbtu /st

0.48 mbtu /st0.44 mbtu /stExcess air

0.38 mbtu /st

450°F

Long dry kiln fuelconsumption 4.0 mbtu /st

st = short ton

Heat in clinker0.06 mbtu /st

Radiation loss0.04 mbtu /st

To coal mill0.06 mbtu /st

Cooler efficiency = 100 x = 69%1.53 – 0.48

1.53

Relatively high cooler efficiency due to utilizationof some vent gases for drying in coal mill.

Figure 3.8.13. Grate cooler heat balance – long dry kiln.

An example is given in Figure 3.8.13. As mentioned earlier, a modern clinker cooler should have an

efficiency of 64% or better no matter what kiln it serves. This means that it should be able to move

about two-thirds of the heat from the clinker exiting the kiln to the combustion air entering the

kiln system.

Automatic Control of Grate Coolers

Three groups of machine adjustments are usually automated to obtain: 1) constant air flow

through the clinker bed in terms of mass of air per unit area and per mass of clinker, and

2) constant, slightly negative pressure (suction) in the kiln hood. An example is shown schemati-

cally in Figure 3.8.14.

The primary objective of a clinker cooler control system is to stabilize the cooler operation and

thereby provide a more uniform flow of heated air for combustion in the kiln and possibly the

calciner. The secondary objective is to provide a controlled response during kiln upsets so that the

upsets have a minimum impact on the primary objective.

Single and cascade analog controllers, or digital equivalents of these controllers, are the most common

ones in use. Ideally, the grate cooler is controlled by one algorithm which optimizes the cooler opera-

tion during normal operation while a second control algorithm steps in during upset kiln conditions

to ensure that the cooler is not damaged by high temperatures or mechanical problems.

The pressure in the undergrate compartments and air beams (if present), the flow of air into or out

of fans, and the speed of the movable grate frame are all controlled by PID loops. Other parameters

are monitored simply to ensure they are within a desired operating range. These include fan motor

current, kiln speed, motor running status, excess air, and grate plate temperatures.

Using undergrate pressure to control grate speed is acceptable if cooler conditions remain near

ideal. To avoid problems associated with erratic first and second compartment pressure, both first

and second compartment pressures can be measured for determining a weighted average under-

grate pressure. This smoothes out the undergrate pressure and often lets the cooler run steadier.

The finer the clinker, the harder it is to blow air through the bed. With the control loop in auto-

matic mode, a decrease in clinker size will result in an increase in undergrate pressure, until the

489Clinker Coolers

SA

SA SA SA

R

R

Supervisoramplifier

Automatic air regulation

Alarm system

Interlockings, kiln, transport

P01

P11

T1

K-10

RSyn

M

M

M MM

M M M

MK-16K-15 K-13K-14 K-12 K-11

O

Figure 3.8.14. Typical automatic cooler controls.

control loop has sped up the grate, causing a lowering of the clinker bed depth. Conversely, very

large clinker will result in unusually low undergrate pressure which will decrease cooler speed and

result in excess bed depth, and may even overload the drive. The cooler control system should

include elements that detect and correct these conditions.

In the case of a two- or three-drive cooler, the second drive should be controlled by the undergrate

pressure of its first compartment. When the second drive’s first compartment is too large for its

pressure to be successfully used in connection with speed control, the second drive has to follow

the first drive. In that case, the first drive’s speed multiplied by a factor represents the second drive

speed. The second drive’s speed should always be higher than the first drive’s speed to avoid clinker

piling up between the two drives.

Occasional high grate plate temperatures in the first and second compartments can represent an

obstacle to optimizing cooler compartment airflow distribution. During upset conditions where

high grate plate temperatures occur, one may have to increase the cooler movable grate frame

speed for safety reasons. The grate plate temperature is then permitted to manipulate the under-

grate pressure setpoint. As grate plate temperature increases, it will decrease the undergrate pres-

sure setpoint which speeds up the cooler movable grate frame drive.

In one particular case, this safety interlocking resulted in no grate plate failures for two years

where, in the past, grate plate failures had been an ongoing problem.

In some cooler systems, high vent air temperatures will result in automatic opening of a tempering

damper in the vent airduct to protect downstream equipment from overheating. The vent air

volume increase caused by the opening of this damper or even just by the high vent temperature

may make the total volume exceed the capacity of the vent fan. If this is a constraint, it may be

prudent to automatically reduce undergrate compartment airflows in the latter part of the cooler

to restore kiln hood draft control when the vent air temperature (measured before the introduc-

tion of tempering air) exceeds a certain threshold valve.

In applications where vent fan capacity and high clinker discharge temperatures are a problem, the

kiln hood’s draft can be controlled as well by the last compartment fan. By doing this, it is possible

to increase the amount of cooling air and to lower the clinker discharge temperature during normal

operation. In this mode of control, the vent fan is run on fixed speed close to maximum capacity.

During upset conditions, the amount of cooling air is reduced, resulting in a higher clinker temper-

ature, which would have happened anyway. In a few coolers with limited venting capacities, this

control approach has led to considerably lower overall clinker discharge temperatures.

Finally, in order to minimize the need for control room operator involvement, other attractive

control features to strive for include automatic initialization of dampers to the full closed position

Innovations in Portland Cement Manufacturing490

on fan startup, automatic reduction in airflow on fan achievement of maximum motor current,

and automatic airflow increases programmed for kiln startup.

OPTIMIZATION OF GRATE COOLER OPERATION

A smooth cooler operation depends upon many factors. In the preceding paragraphs several of the

important design features that affect the operation, such as burner pipe location, cooler width, and

control loops, have been mentioned. In the following section, these points have been revisited,

while also dwelling on the fact that optimization of a clinker cooler can be divided into three tasks:

1) maximizing the amount of secondary air, 2) maximizing the secondary air temperature, and

3) maximizing the uniformity of the operation.

Burner Pipe Position

The first step in optimizing a cooler

operation begins in the kiln. The

burner position has a crucial influ-

ence upon the kiln and cooler

performance. Long wet and dry

kilns with a fuel consumption of

more than 4.0 million Btu per short

ton of clinker, which were common

in the past, needed high amounts of

combustion air. Low secondary air

temperatures ensured a fast clinker

cooling inside the kiln, and the

overall thermal efficiency of the

cooler was acceptable. Today’s low-

fuel-consuming kiln systems have

low combustion air requirements,

thus giving high secondary air

temperatures and slower clinker

cooling.

Figures 3.8.15 and 3.8.16 show the

difference between two burner pipe

positions. Positioning the burner tip

at the kiln nose or even into the kiln

hood (Figure 3.8.15) means that the

flame ignition takes place close to

the kiln’s discharge, thus keeping

491Clinker Coolers

� +2"W.G.

� +18"W.G.

� +16"W.G.

� 2650°F

� 7 SCFT/lb� 1900°F

� 7 SCFT/lb� 1400°F

� 2200°F� 2500°F

� 750°F

Heat consumption: �2.7 x 106 BTU/stProduction: 1500 + tpd clinkerIncl. of grate: 0-3°

Figure 3.8.15. Typical 1970’s burner pipe position.

� 0.4"W.G.

� 2650°F

� 13 SCF/lb� 1650°F

� 2200°F

�+14" W.G.�+14" W.G. �+12" W.G.

� 2500°F

Heat consumption: �2.7 x 106 BTU/stProduction: 1500 + tpd clinkerIncl. of grate: 0-3°

> + 24" W.G.

Figure 3.8.16. Recommended 1990’s burner pipe position.

the clinker hot until they drop into the cooler. This can have several undesirable consequences as

follows:

• The clinker discharged onto the clinker bed can form large clinker agglomerations, leading to

poor cooling rates because only the clinker closest to the grates are rapidly cooled.

• High temperature clinker reaches the discharge end of the cooler, resulting in elevated clinker

discharge and vent air temperatures. The overall cooler thermal efficiency decreases and fuel

consumption increases.

• High secondary air and kiln clinker discharge temperatures can result in a severe “snowman”

formation, especially if coals with high ash content are used. In addition to all these disadvan-

tages, operational and maintenance problems are likely to occur.

In contrast, positioning the burner tip approximately 1 to 2 m into the kiln (Figure 3.8.16)

improves the kiln’s own heat recuperating and cooling zone.

The pre-cooled clinker drops at a lower temperature into the cooler and the secondary air temper-

ature drops. The requirement to cool the clinker quickly is fulfilled. The clinker reaches the latter

clinker cooler zones at a lower temperature, which results in lower clinker discharge and vent

temperatures. Less required cooling air relieves the vent air system and saves considerable electrical

energy. The overall cooler thermal efficiency improves. In addition, the cooler now runs at a higher

availability and lower maintenance cost. The formation of “snowmen” is unlikely.

There may be one drawback in pushing the burner into the kiln – it might represent a problem in

regard to burner refractory life. Good results were experienced with extreme high strength low-

cement type refractories on burner pipes in very severe applications.

Maximizing the Amount of Secondary Air

Low primary air and low air infiltration rates at the kiln discharge maximize the amount of

secondary air. Low primary air rates can only be accomplished with semi-direct and indirect firing

systems that offer primary air rates as low as 6%. Low air infiltration rates at the kiln discharge can

be accomplished with good hood sealing and an effective kiln discharge seal.

Many plants now employ an effective leaf-type kiln discharge seal where overlapping sheets of high

quality steel ride on the kiln cowling. This arrangement exhibits little tendency for clinker to pry

open the seal. A puffing kiln hood does not open a gap between leaves and the air cowl. This seal

has proven itself in many applications. Repairs are easy and overall costs are low.

If the kiln system has a calciner, it is important that as much of the air as possible used for

combustion comes from the clinker cooler. Thus any potential opening to ambient air between the

calciner and the cooler should be kept as tight as possible. Such openings could be inspection

Innovations in Portland Cement Manufacturing492

doors, material discharge flaps and damper housings on tertiary air ducts, and kiln material inlet

seal and kiln riser poke holes if an in-line calciner is used.

Maximizing Secondary Air Temperature

Maximizing the secondary air temperature means getting the best heat transfer between clinker

and cooling air. The heat transfer is optimized by 1) optimization of clinker bed distribution, and

2) optimization of the cooling air distribution.

A clinker cooler basically is a heat exchanger. In contrast to most heat exchangers, both mediums –

clinker and air, come in direct contact with each other. Therefore, the effectiveness of heat

exchange largely depends upon the surface with which both mediums come into contact. In a

clinker cooler, the more uniform the clinker size distribution and the clinker granulometry, the

more effective the heat transfer.

While the clinker size, for the most part, cannot be altered, the overall heat transfer can be opti-

mized with a good, uniform clinker bed distribution.

The fact that especially large diameter type

kilns tend to discharge fine clinker on the kiln’s

load side and coarse clinker on the opposite

side can make it difficult to get good clinker

distribution. Due to the high air resistance of a

fine clinker bed, “red rivers” often are

inevitable. Studies show that “red rivers” can

cause a variation in air distribution of 1:6

between the fine and coarse clinker side and

can even cause clogging of the bed. This is why

grate plates sometimes become red hot in

places. “Red rivers” also cause an increase in

clinker discharge temperature.

Measures for improving the clinker distribu-

tion should start at the cooler inlet. Where

“snowmen” cause poor clinker distribution, the

cooler back and sidewalls can be kept clean

with the help of compressed air cannons. Some

improvements are possible by slowing down

the movement of the fine clinker bed and

diverting more fine clinker to the coarse cooler

side, thus increasing the overall clinker bed

resistance which pushes more air through the

493Clinker Coolers

ClinkerFine Coarse

Compartment #1

#2

#3

#4

#5

#6

Kiln Capacity2300 St/d Clinker

Dead GratesGrates W/O HolesWedge Grates

Figure 3.8.17. Grate arrangement to copewith “red rivers.”

fine clinker bed. This diversion can be done by using wedge-type grates with 125-mm or 200-mm

high faces. The grates are arranged in a checkerboard pattern as shown in Figure 3.8.17.

An often successful way to improve the situation is to narrow the cooler grate area on the fine

clinker side. By doing so, the clinker bed becomes narrower and often eliminates a severe segrega-

tion of fine and coarse clinker. It is recommended that the cooler inlet grate width not exceed

2.5 m for kiln capacities up to 2,500 metric tons per day of clinker.

Figure 3.8.17 shows that some air holes in corner grates are blanked off. Corner areas often have a

low clinker load which results in heavy air channeling and bypassing the clinker load. Blanked off

air holes ensure that cooling air is diverted into the clinker load.

When severe “red river” conditions exist and loss of cooler grates are experienced, “Ondufin” grates

can be applied. The grates have cooling fins on the underside which increase the cooling surface.

The grates stay cooler and last longer. In addition, if a grate is burned through, the fins prevent

large clinker spillages for a considerable time.

When “red river” conditions in a pre-1990’s style cooler are extremely severe, compartments can be

divided into two sections. Two cooling fans, one on each cooler side, assure that both grate areas,

the fine and the coarse side, receive the proper amount of air. Or, the design can be upgraded to

one with airbeams or mechanical air flow regulators for small groups of grates.

Some suppliers, borrowing from the airbeam technology, offer a grate plate design for pre-1990’s

coolers where the air has to travel through a labyrinth in the grate – first up, then down – before

exiting into the clinker bed. This provides an effective clinker seal that reduces the amount of

clinker falling through the grate plates to the undergrate compartment.

Increasing the clinker bed thickness generally improves the overall clinker distribution and heat

transfer. Good results have been experienced with clinker beds up to 1 meter deep. In addition,

lower grate speed has had a positive effect upon grate wear rates.

High undergrate pressures and airflows adversely affect the conveying action of a reciprocating

grate. High air pressures can reduce the friction between the clinker and the grate, which in turn

can speed up the movement of the clinker toward the cooler discharge. The air, which expands as it

rises in the bed, causes the clinker at the surface to be fluidized. The result might be that clinker

flows down the slope if the grate area is inclined or that the clinker can only be moved with

extremely high reciprocating speed on horizontal type coolers. To prevent clinker from flowing

forward, the single grate surface should be at least horizontal.

Experience has shown that the best results can be attained with a maximum of 4.7 to 5.5 kPa

undergrate pressures in horizontal and 3 degree inclined coolers, and 2.0 to 2.5 kPa in old

10 degree inclined coolers.

Innovations in Portland Cement Manufacturing494

AIR DISTRIBUTION VERSUS OVERALL COOLER EFFICIENCY

Optimized air distribution also improves the overall thermal cooler efficiency and prevents damage

to grates due to overheating. To achieve this goal, predefined amounts of cooling air need to be

established for every cooler compartment. Coolers with airbeams or mechanical air flow regulators

can refine the air distribution even more to sections of grate plates or to individual plates.

The optimization of airflow is especially important for the heat recuperating zone. Too high

amounts of air do not give maximum secondary air temperature. Too low amounts of air elevate

the clinker discharge temperature. Too high amounts of air also promote fluidization of the

clinker. As the finer clinker particles are likely to be entrained in the locally intensified air flow,

high amounts of dust cycles between kiln and cooler are likely. Dust particles might also be picked

up from highly fluidized areas and concentrate in others, thereby intensifying any “red rivers.”

Extremely high airflows also promote heavy air channeling, giving a poor heat exchange for a grate

cooler of 1970’s to mid-1990’s vintage. It is recommended that maximum airflow not exceed approx-

imately 140 normal cubic meters per minute per square meter of cooler grate area. Figure 3.8.18

shows a chart of optimized cooling air distribution for a typical eight-compartment reciprocating

grate cooler. The first five compartments (including quench compartment) supply secondary air and

tertiary air if applicable; compartments #5 through #8 cool the clinker to a final temperature of

approximately 100°C. Lowering the clinker discharge temperature further with more air increases the

electrical power consumption considerably. Depending upon the total amount of cooling air used,

the power consumption for the cooling fans can run between 3 and 8 kWh/ton of clinker, plus up to

4 kilowatt-hours for venting.

495Clinker Coolers

20

15

10

5

600

450

300

150

Q 1 2 3 4 5 6 7 8

1st Grate drive 2nd Grate drive 3rd Grate drive

Undergatepressure, IWR

Air pr. grate area,SCFM/sq. ft.

Compartment number

Figure 3.8.18. Air distribution in cooler.

As can be seen, the maximum specific amount of air per unit of grate area goes into the quench

compartment and compartment #1 to quench the clinker and assure low grate temperatures. The

specific airflows per unit of cooler area gradually decrease toward the cold end of the cooler.

Some older coolers still have one cooling fan for up to three compartments. The distribution of air

into each compartment is difficult since the cooling air will try to migrate into the compartment

with the lowest undergrate pressure. This is especially true when heavy loads travel down the

cooler. Employing one air fan for each compartment and making sure they are well air-sealed from

each other will result in a lower overall clinker discharge temperature and less air usage.

In order to allow deep clinker beds and defined airflows in each compartment, one needs good

undergrate compartment sealing, especially where drag chains pass through compartments. Where

drag chains are located below the cooler, the best sealing is accomplished with flap valves con-

trolled by level indicators located in the undergrate compartment. The flap valves are only oper-

ated if material inside the compartment reaches a certain level. Figure 3.8.19 shows this

arrangement.

Efforts to avoid the mixture

of low and high temperature

cooler air above the clinker

bed are important as well. If

considerable amounts of air

from the back-end compart-

ments mix with air from the

heat recuperating zone, the

secondary air temperature

drops while the vent air

temperature increases. We

can take some steps to avoid

secondary (and tertiary) air

from mixing with the vent

air. At the point in the cooler

where these two air streams

split off in different direc-

tions, an arched brick wall or

some hanging stainless steel

dampers can be installed.

From this part of the cooler,

the cooler roof should be

sloped at approximately 15°

as it approaches the cooler

Innovations in Portland Cement Manufacturing496

Electronics

Low Limit

High Limit

Dust Gates

Grate Line

Figure 3.8.19. Undergrate clinker discharge control.

throat and 5° to 10° as it approaches the vent air take off. The sloped roof changes the bullnose

from 90° to approximately 75°. The resulting lower velocity in the lower part of the cooler throat

reduces the amount of fine particles returned to the kiln.

Wherever possible, the cooler throat velocity should be held below 7 m/sec. New systems should

even be designed with velocities as low as 3.5 m/sec.

A proper and uniform distribution of the clinker upon the grate is of importance, as already

mentioned. Ideally, you would like a giant stirrer to mix the large and small clinker (that are segre-

gated as they fall into the cooler) together again, and then have them spread out in an even layer

upon the grate. Equipment that has been used for this purpose includes: 1) sloped inlet, 2) water-

cooled adjustable steel impact inlet plate, 3) reducing effective grate width (horseshoe pattern of

inlet grate plates), 4) stationary quench grates at the front of the cooler, and 5) spreader beam across

the cooler. In the 1990s another interesting method was introduced. It consists of aeration of a slop-

ing bed at the inlet end of the grate. This area is provided with a series of fixed windboxes arranged

stepwise and equipped with cast metal grate elements designed so that no particles can fall through

them (Figure 3.8.20), that is, with the airbeam and pocket grate technology mentioned earlier.

A considerable pile is built up over the grate plates, which contain pulsating air. The air expands

the pile and in particular moves and mixes the finer clinker with the coarser. At the same time

making the upper portion of the pile slide gently into the cooler while it is being spread out.

Final Words

Clinker coolers are an integral part of the kiln system. Select them carefully, keep working at opti-

mizing them, and overall plant performance is bound to improve!

497Clinker Coolers

• Static Aeration Zone• Suitable Clinker Distribution to Avoid Red River• Autogenous Wear Protection of Cooler Inlet

Figure 3.8.20. Example of fixed cooler inlet.

Innovations in Portland Cement Manufacturing498

REFERENCES

Gagnon, Denis, “Upgrading a Clinker Cooler,” Proceedings 38th IEEE/PCA Cement IndustryTechnical Conference, Los Angeles, April 1996, pages 156-170.

Herchenbach, Horst, “Cement Cooling - The Key To An Economic Kiln Operation and GoodClinker Quality,” Proceedings 21st International Cement Seminar, Rock Products, Chicago, Illinois,1985, pages 41-54.

Keefe, Brian P., and Christensen, Kim Pandrup, “The Cross-Bar Cooler: Innovative and Proven,”Proceedings 42nd IEEE-IAS/PCA Cement Industry Technical Conference, Salt lake City, Utah, May2000, pages 135-147.

Klotz, Bryan, “Design Features of the Polysius Clinker Cooler,” Proceedings 42nd IEEE-IAS/PCACement Industry Technical Conference, Salt lake City, Utah, May 2000, pages 159-170.

Labahn/Kohlhaas, Cement Engineers Handbook, 4th Edition, Bauverlag GmbH, Wiesbaden andBerlin, 796 pages, 1983.

Lecture 55, “Cooling of Clinker,” F. L. Smidth’s Cement Production Seminar, 1981.

Nobis, Rainer, “Evaluation and Optimization of Clinker Cooler Operations,” Proceedings 25thInternational Cement Seminar, Rock Products, Chicago, Illinois, 1989 pages 119-140.

von Wedel, Justus, “The IKN Pendulum Cooler,” Proceedings 42nd IEEE-IAS/PCA Cement IndustryTechnical Conference, Salt lake City, Utah, May 2000, pages 149-157.