Cooler: Complete Technical Guide

The clinker cooler is the heat exchanger that receives the clinker at 1,300-1,450 C from the rotary kiln and cools it to the temperature the grinding and the storage accept (ambient +50-100 C): in the process the cooler does much more: it recovers the heat of the clinker into the combustion air of the kiln and the calciner, it stabilizes the burning zone by the constant secondary air flow, it fixes the quality of the clinker by the rate of the cooling, and it handles the material flow between the kiln and the mill: the cooler is the third leg of the burning system, and the modern grate cooler is a complex machine of the fans, the plates, the drives and the control loops.

The Complete Cement Technical Package (931 files including the kiln and the cooler books, the thermal balance tools, the Excel calculators, the operation manuals and the courses: $249.99 one-time: instant download via the PayPal payment) contains this complete cooler file: the guide to the design, the operation and the optimization of the clinker coolers: the article walks the file: the types of the coolers, the heat balance and the cooling duty, the grate coolers of the modern families (the reciprocating, the cross-bar and the static), the cooling air and the fans, the recuperation and its measurement, the clinker quality and the cooling rate, the operation and the control, the troubles (the snowmen, the red rivers, the blockages) and the troubleshooting: the reader gets the complete picture of the machine that saves the fuel and secures the burning zone.

The numbers tell the importance of the cooler: the cooling air of 1.8-2.6 Nm3/kg of the clinker is blown through the bed, the secondary air recovered at 900-1,100 C delivers 1,700-2,200 kJ/kg of the thermal heat of the kiln, the cooler efficiency of the modern machines is 70-80%, and the electrical demand is 40-60 kWh per ton of the clinker in the fan power of the old systems: the modern coolers run at the higher efficiency with the lower air, and every percent of the cooler efficiency represents the kiln fuel: the file and the article translate these numbers into the daily operation of the plant.

1. The Role of the Cooler in the Kiln System

The cooler closes the loop of the burning system, and its four duties are independent of each other:

• The cooling duty: the clinker temperature falls from the 1,360-1,450 C (the temperature of the kiln discharge) to the 80-180 C at the cooler exit: the requirement of the downstream handling and the quality of the cement;
• The recuperation duty: the cooling air absorbs the heat of the clinker and returns to the kiln as the secondary air (the 900-1,100) and to the calciner (the tertiary air): the heat recovered: the fuel of the kiln saved;
• The process duty: the cooling rate decides the clinker phase development: the rapid cooling before the 1,250 C stabilizes the alite (the C3S) and the melts, prevents the alite to belite reconversion? no, the C3S destroys; the slow cooling leads to the weak cement:
• The handling duty: the cooler receives the tumbling clinker of the kiln, avoids the plugging, delivers the steady stream at the exit: the crusher of the cooler (the exit) reduces the largest lumps to the 25-50 mm:

The balance driver: the recuperated air is the largest renewable energy of the kiln: the modern kilns achieve the 95-98% of the air preheat by the cooler; the thermal efficiency of the cooler is the one single machine where the fuel saving of the plant is made: the file quantifies each percentage of the cooler efficiency in the kJ/kg of the clinker and in the US dollars of the year.

2. The Types of the Coolers: The History and the Modern Families

The clinker cooler has evolved in the generations, and the plants of the world run the spectrum:

• The rotary cooler: the rotating cylinder of the old lines, the counter-current air, the cooling limited, the efficiency 40-55%, the dust carry-over high; the heritage of the wet plants:
• The planetary cooler: the satellite cylinders of the kiln shell, the simple, the old, the big machinery the motion; largely superceded;
• The grate coolers: the horizontal or the inclined steel grates with the air through the matrix: the three generations of the grate: (1) the straight-line and the support; (2) the reciprocating with the moving/stationary layers; (3) the modern families: the reciprocating plate (the IK), the cross-bar (the IKN), the static grate (the Polysius and the FLSmidth crossbar);

Cooler type	Cooling air (Nm3/kg cl)	Cooler efficiency %	Clinker exit C	The status
Rotary	3.0 – 4.0	35 – 50	150 – 250	old, minor
Grate gen. 1	2.5 – 3.2	55 – 65	120 – 180	aging plants
Grate gen. 2-3	1.8 – 2.4	65 – 75	70 – 100	the modern standard

The third generation coolers achieve the lower air and the higher efficiency with the fixed aerated compartments (the controlled air per zone), the wear-resistant static plates and the independent drive of the grate sections: the file compares the families with the maintenance data and the case: the replacement of the grate cooler Gen1 with the Gen3 typically raises the cooler efficiency from 60 to 72% and reduces the fan power by the half.

3. The Construction of the Modern Grate Cooler: The Anatomy

The modern grate cooler is a rectangular box of about 30-50 m length and 4-7 m width, the grate bed moves the clinker from the inlet to the crusher:

• The grate surface: the rows of the grate plates (the static + the moving counter) in the reciprocating coolers, or the cross bars and the static plates (the cross bar): the wear-resistant casting of the plates (the high chrome, the composite);
• The undergrate compartments: 6-15 fixed compartments along the length, each with its fan: the air control per compartment: the compartment I (the hottest, 1,350-900 C) at the highest pressure and the flow; the cold wet, else:
• The air systems: the cooling fans of 15-50 kPa static (the inlet zones) and the large volume: the total air of the cooler 1.8-2.6 Nm3/kg cl; the secondary/tertiary air take-off: the hood of the kiln, the calciner:
• The drives: the hydraulic or the mechanical drives of the grate sections: the speed variable per the clinker bed height, the measurement of the grate speed in the strokes per minute:
• The exit: the clinker crusher (the hammers or the rolls) at the cool end, the air at the exit and the clinker get off the grate to the conveying chain:

The flow logic of the cooler: the clinker enters at the hood (the kiln nose), the bed thickness of 600-900 mm, the bed moves at the 10-30 m/h of the grate speed: the air passes up through the bed at the designed velocities; the vertical faces and the walls formed by the plates with the seal of the air.

4. The Cooling Air and the Heat Balance of the Cooler

The heat balance of the cooler is the discipline of the modern plant: the input is the heat of the hot clinker, and the outputs are the recovered heat (the secondary and the tertiary air), the losses (the exit clinker, the housing, the dust) and the exit air:

• The input: the clinker heat at the inlet: about 1,350-1,450 of the clinker temperature: the enthalpy of the clinker at the inlet of the order of 1,800-1,900 kJ/kg cl:
• The recovery: the secondary air to the kiln at 800-1,200 C and the tertiary to the calciner at 800-1,000 C: their heat in the range of 1,200-1,500 kJ/kg cl (the 60-80% of the input);
• The losses: the exit clinker 80-180 C carries 100-200 kJ/kg; the grate housing losses 30-60 kJ/kg; the dust and the exit air the remainder:
• The efficiency: the cooler efficiency = the recovered heat / the input heat: the modern grate 70-80%: every point of the efficiency is about 10-20 kJ/kg of the clinker in the thermal debt of the kiln:

The stream	Approx. kJ/kg cl	Temperature
Clinker in (1,400 C)	1,400 – 1,500	1,350 – 1,450 C
Secondary air (to the kiln)	700 – 1,000	900 – 1,100 C
Tertiary air (to the calciner)	300 – 500	850 – 1,000 C
Clinker out	100 – 200	80 – 180 C
Shell, dust, exit air	100 – 300	—

The heat balance of the cooler is done by the plant on the monthly basis with the measured clinker exit temperature, the inlet temperatures and the flow of the fans: the file includes the calculator of the balance with the input sheets: the results show the true efficiency and the hidden losses (the leak air, the blocked plates) that the temperatures hide.

5. The Operation and the Control of the Grate Cooler

The operator of the kiln controls the cooler with the three master knobs:

• The grate speed: the bed height in the cooler is held constant (the 600-900 mm) by the speed of the grate: the clinker load (t/h) measured on the chain scale vs. the bed height sensors, the speed set to keep the level:
• The air distribution: the fan valves per compartment: the hot end (the compartments 1-4) gets the priority of the air (the recuperation!), the cold end (the 6-10) the minimum to finish the cooling:
• The pressure control: the undergrate pressures indicate the bed condition: at the same air, the higher pressure = the compacted bed (the snowmen) – the lower = the channeling or the empty spots: the differential of 4-8 kPa typical in the hot section:

The control manual of the file defines the operating plan: the clinker production per hour, the cooling air set (the Nm3/kg), the fan speed and the valve positions, the exit temperature target and the secondary air temperature: the standard deviation of the exit temperature below the 10-15 C is the goal: the stable cooler stabilizes the kiln nose and the burning zone, and the file presents the daily log of the exemplary plant.

6. The Recuperation and the Combustion Air Temperature

The recuperation of the cooler is the fuel economy of the kiln, and the connection is direct:

• The temperatures: the high-temperature air of the first compartments (the 1,100-1,200) enters the kiln hood: the burner flame stabilizes with the hot secondary air: the flame temperature rises, the burning zone holds with the lower fuel:
• The thermal loss: each 100 C of the secondary air temperature difference corresponds to about 25-40 kJ/kg cl of the heat at the burner: the typical effect on the kiln heat: the 20-60 kJ/kg cl between the mediocre (800 C) and the good cooler (1,000):
• The tertiary duct: the calciner air of the 850-1,000: the calcination is the most energy-hungry zone, and the tertiary heat compensates: the route from the cooler mid-section (the 60-75% of the full flow):
• The measure: the thermocouples of the hood and the tertiary duct, the daily operating data of the plants in the file: the target of the retrofit: the secondary 1,000-1,150 C, the tertiary 850-950 C:

The modern coolers recover 70-75% of the heat: the old Gen1 (55-65%) leaves up to 15% points of the heat in the exhaust and the clinker: the file’s example: the plant of the 4,000 t/d with the cooler efficiency 63%: the retrofit to the 72% saves about 45 kJ/kg cl: 1.8 million kJ every day: the numbers of the investment return in the months, not the years.

7. The Cooling Rate and the Quality of the Clinker

How the cooler cools the clinker decides the mineralogy and the strength of the cement:

• The fast cooling: the clinker plunged from the 1,400 to the 1,000 C in the minutes of the first compartments: the alite (C3S) crystals stay, the belite formation is limited, the melt solidifies as the glass, the cement early strength rises (the 28-day + the alite morphology):
• The slow colling: the alite converts partially to the belite, the C3A/C4AF crystallize large, and the cement loses the strength: the “the kiln produced good clinker but the cooler kills it”:
• The control: the cooler inlet temperature target of the clinker 1,350-1,400, the intermediate 1,250 passed in < 8-15 minutes, the final 80-120 at the exit: the measurement points in the cooler:
• The SO3 and the phases: the cooling rate also decides the SO3 binding in the clinker: the slow cooled holds the sulfate in the melt, the fast in the free; the cement SO3 optimization:

The file links the cooler operation to the quality lab: the micrographic samples of the clinker (the alite size 20-40 micrometer) and the R28 strength table: the case in the file: the same kiln at the same feed with the colder cooler exit +15 min retention lost 3-4 MPa of the 28-day strength: the cooling path is a quality parameter as real as the chemistry.

8. The Troubles of the Grate Cooler: The Snowmen, the Red Rivers and the Blockages

The problem	The cause	The correction
The snowmen (the bells on the grate)	the clinker snowballs at the inlet (the clinker legative), the too fast cooling of the sticky melt	the grate speed control, the air pulse, the kiln nose / the clinker quality correction
The red rivers (the glowing channels)	the lumpy clinker breaking the bed, the air channeling	the grate strokes, the reduction of the oversized lumps, the air balance
The high exit temperature	the cooling air reduced, the bed too thin/fast	the fan speed raise, the bed height, the grate slow
The plugging of the exit crusher	the large clinker lumps, the snowmen	the hammer speed, the lump removal, the seal
The cooler leakage dust	the seals, the plated wear, the negative pressure	the seal adjustment, the pressure check

The snowmen and the red rivers are the two classic faces of the cooler trouble: the snowman (the conical, white-hot clinker that builds on the first grate plates when the clinker is too liquid or the grate too slow) blocks the air and the flow, the red river is the streak of the hot clinker that melts through the air channel, the symptom: the infrared camera sees the strip at 1,200-1,400 while the rest is black: their therapies: the grate speed, the air pulse, and above all the burning stability: the clinker from the well-operated burning zone rarely forms the snowmen: the file connects the cooler troubles directly to the kiln operation and gives the joint operating view.

9. The Cooler Drives, the Fans and the Auxiliary Equipment

The drives and the machinery of the cooler are the unsung support of its performance:

• The grate drive: the hydraulic drive (the ram pumps) or the mechanical: the speed holding the bed: the force to move the clinker bed of the 4,000 t/d: 100-300 kN on the ram: the speed 0.5-2.0 strokes/min with the variable control:
• The fans: the undergrate supply fans, a fan per compartment (7-11 in the common design) with the dampers and the frequency converters: the axial fans of the top hoods: the total fan power 1.5-2.5% of the line electricity: the fans of the inlet compartment (the recuperation) the most critical:
• The clinker crusher: the exit roll crusher at the tail of the grate: the crushing of the lumps and the snowmen to the 25 mm for the transport: the hammer crusher for the older: the drive power, the wear of the hammers, the overload relief:
• The transport of the clinker: after the crusher the clinker bucket belt or the air slides with the fans and the air lift to the silos: the chain drags in the old plants:
• The auxiliaries: the dust collection (the baghouse or the ESP on the cooler vent), the seal air fans of the kiln hood, the lubrication systems of the grates and the rams, the hydraulic power unit:

The maintenance plan of the file covers every machine: the daily checks (the pressures, the temperatures of the bearings, the leaks), the weekly (the lubrication, the grate stroke inspection), the monthly (the fan vibration, the hammer wear), the annual (the gearbox oil change, the grate wear, the adjustment of the rams): the example of the file follows the cooler of the 5,000 t/d line for a campaign of the 16 hours per day x 6 days: the costs of the wear counted against the production: the drives and the fans carry the cooler, and their health is the file’s chapter of the reliability.

10. The Cooler and the Waste Heat Recovery: The Electricity of the Exit Air

The exit air of the cooler contains a huge amount of the recoverable heat: the modern plants capture it:

• The temperatures: the exit air of the 2nd and the 3rd compartments at 300-500 C: the exhaust air after the bed 150-250 C: total 0.5-1.0 Nm3/kg removed from the vent:
• The heat recovery: the waste heat recovery (WHR) systems take the kiln exhaust and the cooler exhaust, generate the steam at 15-25 bars and the 300-400 C, and the turbogenerator produces 5-10 MW for the line of 5,000 t/d:
• The power ratio: the 30-40 kWh per ton of the clinker of the plant electricity typically covered by the WHR: the savings of the energy imports and the CO2 of the plant:
• The integration: the WHR tower the balance of the energy; the cooling system and the WHR are the picking the pipes with the heat flows: the file’s WHR chapter calculates the flows and the paybacks of the standard plants:

The recovery of the cooler heat completes the thermal cycle: the cooler returns the heat into the kiln (the primary recovery) and into the electricity (the secondary recovery): the total efficiency of the clinker cooling and the energy reuse reaches the 90%+ of the top plants: the file quantifies the split and helps the plants plan the WHR retrofit in the order of the economic rank.

11. The Selection of the Cooler for the New Plant: The Design Criteria

The choice of the cooler for a new kiln line is a capital decision made once in 25 years, and the engineering process is structured:

• The retrofit of the existing cooler: the cooler of the plant is evaluated against the target: the grate area check (the m2 per t/d at the 1.0-1.2 in the modern), the recovery expected, the air rates: the modernization cases: the new plates, the new seals, the bigger fans, the new drives convert 50% of the retrofit benefits without the new house;
• The new cooler selection: the three families of the modern grate coolers: the reciprocating grate (the OK, the IKN and the Polysius), the static cross-bar (the cooler with the fixed plates and the moving material), the tandem (the double grate): the criteria: the efficiency, the maintenance simplicity, the parts availability in the local market, the references at the similar fuels:
• The performance guarantee: the supply contract covers the exit temperature (+3), the secondary air temperature, the efficiency, the fan power: the acceptance tests at the commissioning measure the same points the file’s calculation sheets verify:
• The CAPEX and the OPEX: the cooler cost is 8-12% of the kiln line equipment: the operating costs: the plates (the 1.5-3 years in the hot zone), the fans power, the maintenance labor: the 10-year total cost comparison is the true pair of families:

The design data of the file include the full questionnaire: the production (t/d), the clinker temperature at the exit of the kiln, the fuel (the type), the calciner system (the tertiary duct or none), the plant electricity price, the maintenance policy: the supplier fills the reply with the guaranteed cooler: the engineer of the client audits the numbers with the file’s calculators and enters the negotiation with the numbers, not the promises.

12. The Cooler Start-up, the Shutdown and the Emergency Operation

The cooler that enters the operation in the wrong order costs the campaign: the file defines the protective sequences:

• The start-up order: the clinker crusher first, then the grate drive, then the fans (the cold compartments first), then the bed (the accumulation until the cover of the plates), then the feeding of the kiln: the empty grate at the heat = the red plates, the warping, the failure;
• The shutdown order: the kiln stops first, the cooler keeps moving until the empty: the fans run until the grate cold: the clinker bed runs out, the plates cool down under the air at the fast but the controlled rate:
• The kiln flash: if the kiln bricks the stand the cooling, the grate speed to the high, the emergency air: the clinker admission continues by gravity few minutes after the kiln motor stop: the cooler must move the whole residual the bed:
• The fires: the clinker against the hydraulics cover when the grate drives stopped with the heat: the operators the water or the inert: the isolation of the ducts: the procedure tested: the fire when the clinker meets the oil hydraulics: the file guards the isolation and the suppression
• The power outage: the UPS of the instrumentation, the diesel generator picks the crusher and the critical fans: the black start of the boxes:

The checklists of the file: the pre-start, the normal start, the normal stop, the kiln trip, the power cut, the fire: each with the step, the person, the expected value: the time to react measured in the minutes, and the file gives the discipline to win the crisis: the cooler emergency training of the shift is repeated in the drills of the plant: the successful plan.

13. The Frequently Asked Questions

What is the optimal cooler exit temperature of the clinker?

For the modern plants: 80-120 C (the summer) and the temperature limits of the cement grinding under the 100 C: the Barten storage in the winter. Above 150 C the recuperation is poor and the mill overheats: the check: the exit air flow of the last compartments, the bed on the tail.

What air does the modern cooler actually use?

Between the 1.8-2.6 Nm3 per kg of the clinker in the modern grate coolers, and the 2.2-2.6 in the older: the lowest the air, the higher the recuperation (the higher the exit air temperature) but the more the dust, the hot end? the equilibrium: the industrial: 2.0-2.4 kg at the full load.

Why do I see the red hot channels in my cooler?

The channels are the path of the air through the bed: when the bed is not uniform (the lumps and the empty zones), the air flows selectively through the thin spots, and the clinker between them finds itself without the cooling the heat accumulates: the red=NULL: the physics of the air channeling: the remedy: the even bed, the even flow, the even clinker: the file has the test of the distribution.

Can the cooler and the calciner share the same air?

Yes: the tertiary air duct: the hot air is drawn from the cooler (typically the mid-compartments at the 900-1,000 C) and the duct delivers it to the calciner: the third air it recovered; the system balances the kiln and the calciner with the two air flows: the calciner pressure and the kiln secondary: the two constrains of the air balance:

How long is the life of the cooler plates?

The grate plates of the third generation: 1.5-3 years in the hot zone (the inlet, the clinker 1,400), 4-7 in the cold zones: the plate inserts of the replaceable: the monitoring: the weight and the thickness: the file gives the inspection intervals, the wear limits and the emergency criteria of the damaged: the plate of the hot zone > 40% either the thicker plate or the rearranged: the money: the plates are 1-3% of the annual cooler cost.

14. The Conclusion

The clinker cooler is the machine that no one sees but everyone pays: its heat balance holds the fuel line, its cooling rate holds the quality, and its bed discipline holds the plant: the file of the Complete Cement Technical Package reveals the cooler as the full discipline: the design, the heat, the airflow, the recovery, the operation and the maintenance: the engineer who masters the cooler holds the thermal heart of the cement plant: the cooler: the quiet giant of the kiln line.

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.