Clinker Cooling: Complete Technical Guide

Clinker cooling is the process step that follows the burning: the hot clinker leaves the kiln at 1,350 to 1,450 degrees and must be cooled quickly and controllably to the temperature of the storage: the cooling is not a mere transport problem: it decides the quality of the cement, the recovery of the process heat and the availability of the kiln line: this article is the complete technical guide to the clinker cooling: the physics, the equipment, the parameters, the heat recovery and the control.

The Complete Cement Technical Package (931 files including the courses, the books, the Excel tools and the presentations: $249.99 one-time: instant download via the PayPal payment) includes the Clinker Cooling module of the cement process technology course: the module covers the cooling process in the depth that the industry practice demands: the purpose of the cooling, the physics of the quench, the cooler types, the heat recovery, the operation and the maintenance: this article follows the module chapter by chapter in the honest technical voice of the course.

Why the cooling deserves a full chapter: the modern grate cooler is the recovery machine of the process: it preheats the combustion air of the kiln and the calciner, saving hundreds of kilocalories per kilogram, and it quenches the clinker in a way that fixes the glassy phase and the strength: a badly operated cooler costs the plant heat, quality and hours: the chapter of the module, like this article, gives the complete understanding of this deceptively simple-looking equipment.

1. The Purpose of the Clinker Cooling: Why We Cool the Clinker

Before the equipment, the purpose: the clinker cooling serves four distinct masters, and the module opens with them:

• The heat recovery: the cooling air absorbs the heat of the clinker and returns it to the process: the secondary air of the kiln at 900 to 1,200 degrees and the tertiary air of the calciner at 850 to 1,000 degrees: the recovered heat displaces the fuel: the recovery efficiency of the modern grate cooler is 60 to 75 percent of the clinker heat;
• The clinker quality: the rapid cooling of the clinker below 1,200 degrees fixes the alite crystal structure and prevents the decomposition to the secondary phases: the slow cooling produces the dull, dusty clinker with the poor grindability and the lower strength: the cooling rate is a quality control parameter of the burning line;
• The material handling: the clinker must be cooled before the storage and the grinding: the conveying equipment, the silo linings and the cement mill can only handle the clinker below the material-safe temperature: the target temperature of the cooled clinker is 70 to 130 degrees for the modern coolers;
• The process stability: the stable clinker temperature and the stable cooling air flow stabilize the kiln draft and the combustion: the cooler is the last link of the pyro system, and its instability travels directly to the flame and the burning zone;

The cooling is therefore not an end-of-line detail: it is the interface between the burning and the grinding, between the heat input and the heat recovery, between the clinker quality and the cement quality: the module states the four purposes early and returns to them in every subsequent chapter.

2. The Physics of the Cooling: The Heat Transfer in the Clinker Bed

The cooling process is the heat transfer between the hot clinker particles and the air that passes through them: the module teaches the physics before the machines:

• The heat balance: the clinker enters with the sensible heat of 1,350 to 1,450 degrees, roughly 350 to 450 kilocalories per kilogram: the air removes this heat: the outlet temperature of the clinker and the temperature of the heated air are the two sides of the balance: the module works the numbers of a 5,000 t/d cooler;
• The air-to-clinker ratio: the cooling air flow is typically 2.2 to 2.6 kilograms of air per kilogram of clinker: of this the combustion air takes about 0.8 to 1.0 kilograms: the remainder leaves the cooler as the exhaust: the ratio is the central operating variable: more air cools faster but wastes more heat through the exhaust;
• The counter-flow and the cross-flow: the clinker moves horizontally on the grate while the air blows upward through the bed: the heat exchange is a cross-flow system: the hottest clinker meets the hottest air at the kiln end: the module explains why the cross-flow gives the highest recovery;
• The bed depth and the particle size: the cooling depends on the contact surface: the deep bed of 600 to 1,000 millimetres with the coarse clinker offers the pressure and the heat transfer of the bed: the module teaches the relation between the bed height, the air velocity and the cooling result;

The physics of the module is practical: the reader who understands the heat balance of the cooling air can read the cooler instruments and know whether the recovery is healthy: the temperature of the secondary air, the exhaust temperature and the cooled clinker temperature tell the whole story of the cooler in three readings.

3. The Cooler Types: From the Rotary Cooler to the Grate Cooler

The industry has cooled its clinker in several generations of machines, and the module presents them in the honest historical and technical order:

• The rotary cooler: the earliest mechanical cooler: a rotating drum with the internal lifters where the clinker tumbles in the counter-flow air: its recovery was limited and it disappeared from the large lines: the module covers it for the completeness and the small plants of the older stock;
• The planetary cooler: the satellite tubes attached around the kiln shell: the clinker passes through the tubes and the air enters the kiln directly: the planetary cooler needs no fan and no drive, but its size limits the kiln diameter: it survives on some medium lines: the module explains its layout and its limits;
• The grate cooler (generation one): the horizontal reciprocating grate where the clinker bed is pushed by the moving grate plates while the air blows upward: the first generation (the first and the second stages) reached a recovery of 55 to 65 percent: the module draws the construction of the grate plates and the drive;
• The modern grate coolers: the third generation designs with the aeration of the bed through the fixed grate and the variable air distribution: the air beams, the hydraulic drive, the recuperation zones with the high-pressure air: the recovery reaches 70 to 75 percent and the specific air flow falls below 2.0: the module compares the generations with the measured performance;

The module is honest about the trade: the rotary and the planetary are simple and robust but their recovery is low: the grate cooler is complex and expensive but it is the machine that feeds the hot air of the modern calciner: the choice of the cooler type belongs to the design of the line, and the module gives the comparison table of the types.

4. The Heat Recovery of the Cooler: The Secondary and the Tertiary Air

The recovery is the economic heart of the cooler: the module teaches the air flows and their temperatures in detail:

• The secondary air: the air entering the kiln hood from the first section of the cooler: the temperature of the secondary air is 900 to 1,200 degrees for the well-operated coolers: the higher the secondary air temperature, the more the heat returned to the flame and the less the fuel: the hood pyrometer is the recovery indicator of the kiln;
• The tertiary air: the air ducted from the mid-cooler to the calciner at 850 to 1,000 degrees: the tertiary air allows the calciner to burn its 55 to 65 percent of the fuel with the preheated air, which is the essence of the precalciner efficiency;
• The heat recovery rate: the recovered heat of the modern grate cooler is about 300 to 350 kilocalories per kilogram of clinker, which is 60 to 75 percent of the incoming clinker heat: the remainder leaves with the exhaust air at 250 to 350 degrees and the cooled clinker at 70 to 130;
• The losses of the cooler: the radiation and the convection from the cooler casing, the false air ingress, and the heat of the bypassed air: the module teaches the measurement of the losses and the improvement of the recovery: the insulation of the casing, the sealing of the grate and the control of the excess air;

The heat recovery of the cooler is the second largest economy of the pyro system after the preheater: the module gives the recovery calculation and the influence of the operating parameters, so the engineer can quantify the value of the cooler discipline in the fuel bill of the plant.

5. The Effect of the Cooling on the Clinker Quality

Beyond the heat, the cooling rate changes the clinker itself: the module teaches the metallurgy of the clinker cooling:

• The rapid cooling and the glass: the quick cooling below 1,250 degrees freezes the liquid phase into the glass and prevents the crystallization of the secondary phases: the glassy phase is more reactive in the grinding and the hydration: the rapidly cooled clinker grinds easier and delivers the higher early strength;
• The slow cooling and the degradation: the slow cooling in the hot bed converts the alite to the belite and the free lime, the so-called the “dusting” of the clinker: the clinker loses strength, its grinding consumes more energy and its storage deteriorates: the module: the alite decomposition mechanism in the 1,250-1,100 range;
• The magnesia and the periclase: the slow cooling allows the periclase crystals to grow large and the cement to show the expansion problems: the rapid cooling keeps the periclase fine: the module connects the cooling to the soundness of the cement;
• The grindability: the cooling state changes the hardness of the clinker: the well-quenched clinker shows the better grindability and the lower mill energy: the module presents the grindability index of the clinker and the influence of the cooling practice on the cement mill consumption;

The module connects the cooling rate to the cement quality in the laboratory language: the engineer who controls the cooling controls the strength potential of the product: the cooler is a quality instrument, not a handling machine, and this is the message that the chapter repeats at every opportunity.

6. The Operation of the Grate Cooler: The Parameters and the Control

The grate cooler is operated with a small set of parameters, and the module teaches them with the normal values of the industry:

• The grate speed and the bed depth: the grate speed sets the residence time of the clinker on the grate: the target bed depth of the modern coolers is 600 to 1,000 millimetres at the feed end: the deeper the bed, the better the heat recovery and the more stable the secondary air: the grate speed adjusts automatically to the kiln output;
• The cooling air distribution: the air flow is distributed along the grate in zones: the high-pressure air at the feed end where the clinker is hottest, the lower pressure towards the discharge: the air flow per zone is trimmed to hold the bed temperature profile: the module: the zone control of the air and the hydraulic of the drive;
• The under-grate pressure: the pressure below the grate, typically 5,000 to 9,000 Pa, is the indicator of the bed resistance: the rising pressure with the constant air flow means the bed is compacting: the falling pressure means the bed is breaking or the clinker is coarse: the module teaches the reading of the pressure and the response;
• The clinker temperature at the discharge: the target is 70 to 130 degrees: the temperature is controlled by the total air flow and the bed depth: the hot discharge clinker means the cooling deficit and the damage of the downstream equipment: the module: the control loop of the final temperature and the alarms;

The cooler operation is the daily balance between the recovery and the cooling result: the module gives the operating window of the grate cooler and the logic of the automatic control: the operator who masters the cooler parameters keeps the kiln stable, the heat recovered and the clinker cold: the three goals of the cooler in the three readings.

7. The Cooler and the Kiln: The Two-Way Coupling

The cooler and the kiln are a coupled system, and the module teaches the coupling because most cooler problems surface in the kiln:

• The draft coupling: the combustion air of the kiln is drawn through the cooler bed: the bed resistance and the under-grate pressure influence the kiln draft and the flame: a compacted bed can starve the flame of air, a broken bed can over-draw the cold air into the hood;
• The clinker discharge and the kiln rings: the clinker size distribution at the kiln outlet is set by the burning zone: the large clinker balls and the snowmen block the cooler feed and disturb the bed: the module teaches the protection of the cooler: the clinker breaker, the hydraulic hammer and the air cannons;
• The kiln upset response: when the kiln discharges a hot or a large clinker, the cooler must absorb the surge: the module: the emergency mode of the cooler, the extra air and the grate protection: the interaction of the cooler alarms with the kiln control;
• The tertiary air extraction: the calciner air demand pulls the air from the cooler: the competition between the kiln air and the calciner air is settled in the cooler: the module teaches the air split control and its effect on the temperatures of both flames;

The coupling chapter closes the loop of the pyro system: the kiln, the cooler and the preheater are one gas circuit and one heat circuit: the module gives the engineer the systems view that the operators of the modern lines use daily: the cooler is the keystone of that view.

8. The Maintenance of the Cooler: The Wear Parts and the Life

The grate cooler is a heavy-wear machine, and the module covers the maintenance as a chapter of its own:

• The grate plates: the cast grate plates carry the hot clinker and the cooling air: they wear, distort and crack: the typical life of the grate plates is 1 to 3 years of operation: the module teaches the inspection, the replacement planning and the materials of the plates;
• The drive and the hydraulic: the reciprocating drive of the grate, the hydraulic cylinders and the oil systems: the module: the maintenance of the drive, the adjustment of the stroke and the lubrication: the drive failure stops the cooler and the kiln;
• The refractory of the hood and the inlet: the feed end of the cooler operates at the clinker temperature of the kiln outlet: the refractories and the castables of the hood and the inlet section protect the steel: the module teaches the lining design and the repair of the inlet;
• The instrumentation: the thermocouples of the clinker, the pressure transmitters of the zones and the pyrometers of the secondary air: the module: the calibration and the maintenance of the cooler instruments, because the cooler cannot be controlled without them;

The maintenance chapter is built on the reliability thinking: the cooler is the availability bottleneck of the pyro section, and its shutdowns are the kiln shutdowns: the module teaches the planned maintenance of the cooler, the wear-based inspection and the spares policy: the cooler reliability is the line availability.

9. The Frequently Asked Questions

Why must the clinker be cooled quickly?

The rapid cooling below 1,250 degrees freezes the liquid phase into the reactive glass and prevents the decomposition of the alite: the quickly cooled clinker has the higher strength potential, the better grindability and the sounder cement: the slow cooling produces the dull clinker with the degraded phases and the higher grinding energy.

What temperature does the clinker leave a modern grate cooler?

Typically 70 to 130 degrees Celsius at the discharge: the target depends on the cooler design and the clinker handling downstream: the temperature above the target signals the cooling deficit, the bed problems or the excess kiln output, and the discharge temperature is one of the control variables of the cooler operation.

How much of the clinker heat is recovered?

In the modern grate cooler, 60 to 75 percent of the sensible heat of the clinker is recovered into the secondary and the tertiary air: that is about 300 to 350 kilocalories per kilogram of clinker: the rest leaves with the cooler exhaust air and the cooled clinker: the recovery rate is the prime indicator of the cooler performance.

What is the specific air flow of the grate cooler?

The total cooling air is typically 2.2 to 2.6 kilograms of air per kilogram of clinker, and the modern coolers with the recuperation optimize below 2.0: of this only about 0.8 to 1.0 kilogram is the combustion air of the kiln and the calciner: the excess air leaves the cooler as the exhaust, and its temperature is the loss of the cooler.

Why is the under-grate pressure monitored?

Because it is the live indicator of the bed: the bed resistance depends on the bed depth, the clinker size and the compaction: the pressure rises when the bed thickens or the clinker fines accumulate, and falls when the bed breaks: the pressure is the feedback of the grate speed control and the alarm of the bed problems.

Can the clinker cooler influence the clinker quality of the cement?

Yes: the cooling rate fixes the phase composition: the rapid cooling preserves the alite and forms the glass, the slow cooling allows the degradation and the periclase growth: the cooled clinker also grinds differently: the cooling practice of the plant is a quality parameter that the cement laboratory and the pyro department must coordinate.

10. Conclusion

Clinker cooling is the silent step of the pyro process with the loud consequences: the heat recovery of 60 to 75 percent feeds the flames of the kiln and the calciner, the rapid quench fixes the quality of the clinker and the cement, and the discipline of the grate operation keeps the whole line stable: the module of the course teaches the physics, the machines, the operation and the maintenance of the cooling in the depth of the industry practice: this article has followed the module section by section, and the reader now holds the complete map of the clinker cooling.

The Complete Cement Technical Package (931 files including the courses, the books, the Excel tools and the presentations: $249.99 one-time: instant download, lifetime access) includes the Clinker Cooling module and the companion chapters of the pyro course: the preheater, the kiln and the coolers: the complete sequence of the burning line under one roof: the engineer who works through the course reads the pyro section of any plant with the trained eye: the purchase of the package is the purchase of the full course, and the PayPal button below opens the door.

12. The Final Cooling and the Storage: The Closed Loop

The cooling train of the clinker ends far after the grate: the closed loop of the cooling and the storage completes the journey of the clinker:

• The last cooling: the clinker leaves the cooler at 70-100°C with the cross-flow of the air; the later dry sections of the eccentric cooler bring the temperature below the 60°C for the comfortable storage: the cooling continues on the belt;
• The storage rules: the clinker silo and the hall receive the hot material with the caution of the moisture: the hot clinker absorbs the humidity and the strength of the cement suffers: the storage time of the clinker is measured days to the weeks, the caking and the moisture watched;
• The dust collection: every cooler step withdraws the air through the filters: the fines of the clinker recovered and returned to the mill feed: the environmental compliance of the loop: the dust of the cooler is the raw material of the finish mill;
• The heat recovery accounting: the quantified share of the clinker heat recovered in the secondary and the tertiary air and the hot water: the plant charges the kiln with the recovered 30-50% of the clinker heat: the balance sheet of the energy, closed;

The cooling loop is the quiet half of the finishing: the plant that cools the clinker well grinds it better, because the mill receives the stable, dry, warm material it was designed for, and the kiln receives the preheated air: the two directions of the recovery: the design course of the department closes the loop of the clinker heat.

The cooling of the clinker is judged by three readings of the plant: the clinker temperature at the exit, the secondary air temperature at the hood, and the volume of the vented air: the three readings are the health of the cooler, and the weekly trend of the three sits in the file’s tables: the drifting clinker temperature announces the heat recovery falling and the power rising; the plant corrects the feed, the bed, or the vent before the finish mill suffers: the trending of the cooler: the numbers that keep the burning line honest.

13. The Cooling Numbers in the File Table

The cooling demonstration of the file is worth repeating: the full cycle of the modern grate cooler burns the numbers of the balance:

• The clinker enters the cooler at 1350-1400°C with the heat of about 1000-1100 kcal per kg of the clinker (the total of the clinker heat);
• The recovery: the secondary air to the kiln recovers the 10-25% of the heat, the tertiary air for the calciner 25-35%, the vented air the remaining: the cooler efficiency, the balance;
• The exit: the clinker leaves at 75-100°C (modern coolers at 70-80), the conveying the remainder: the heat balance of the cooler closes at the file’s table;
• The power: the cooler fans the 15-25 kWh per ton of the clinker, the higher for the old grates, the lower for the modern grates: the cooling energy bill of the burning;

The quantities of the table give the numbers of the discussions: the plant that wants the energy to win replaces the old grate with the modern and saves the 30-50 kcal per kg in the heat and the 5-10 kWh in the fans: the payback measured in the months: the file opens with the numbers the engineers argue about.

The Start-up and the Shutdown of the Cooler

The transient of the cooler matters as much as its steady state: the start-up of the kiln line brings the grate to life first, the feed of the clinker spreading across the cold steel: the operator walks the bed by the hand, watching the distribution from the side doors, and the fans run at the low settings until the hot clinker arrives:

On the start-up the rule is heat first: the grate speeds up with the bed, the fan ramps with the temperature of the clinker, and the first clinker rushes through the crusher while the undergrate temperatures are still cold: the dangerous phase is the first hour, when the clinker may be too hot for the belt and the equipment adjusts in the minutes: the file gives the start-up procedure with the checklist of the interlocks: the belts on, the dampers at the set, the drive on the minimum: the crew who follows the checklist starts the cooler clean: the shutdown the mirror: the housekeeping of the grate, the dust and the clinker removed from the chamber, the doors sealed for the next start: the weekly inspection of the chamber is the mechanical eye of the grate: the steam, the red spots, the cracks of the welds: the lines of the maintenance book.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.