VRM Kiln Cooler: Complete Technical Guide

The VRM, the kiln and the cooler are the three heavy machines of the modern cement line: the vertical roller mill that grinds the raw meal and the cement with the rolling pressure of the hydraulic cylinders, the rotary kiln that sinters the clinker at 1,450°C, and the grate cooler that quenches the hot clinker and recovers the heat into the kiln air. The Excel workbook of this article is a process calculation tool that covers the three machines together: the vertical roller mill calculations (the grinding pressure, the table speed, the dam ring, the power and the production), the kiln calculations (the loading, the retention time, the drive power, the thermal load) and the cooler calculations (the grate loading, the air flows, the recuperation and the clinker discharge temperature), the working numbers that the process engineer of the production line uses for the daily performance checks and the design reviews.

This article walks the reader through the complete calculation set of the three machines: the VRM process and its key formulas with the worked numbers, the kiln mechanical and thermal calculations, the cooler performance calculations, and the interlinked picture of the three machines as one system. The Complete Cement Technical Package (931 files: the handbooks, the courses, the Excel tools and the presentations, $249.99 one-time, instant download through the secure PayPal payment) delivers the VRM-Kiln-Cooler workbook together with the complete library of the cement process calculation tools.

1. The Three Machines as One System

The VRM, the kiln and the cooler of the article title are not three separate calculations but the three links of the same production chain: the raw mill prepares the meal, the kiln converts the meal into the clinker, and the cooler recovers the heat of the clinker back into the kiln air. The workbook of the package calculates the three links with the shared production basis, and the numbers of the three sheets connect:

• The VRM: the vertical roller mill of the raw grinding: the material bed crushed between the rotating table and the hydraulically loaded rollers: the mill produces the kiln feed at the required fineness and the moisture content: the raw mill capacity must match the kiln demand with the availability margin (the typical design of the 105–115 percent of the kiln feed demand);
• The kiln: the rotary kiln of the burning: the raw meal heated through the preheater, calcined and sintered: the kiln defines the production rate of the line, and the kiln loading, the retention time and the thermal loads are the fixed numbers of the process;
• The cooler: the grate cooler under the kiln: the clinker quenched from the 1,350–1,450°C to the 100–150°C, the secondary and the tertiary air recovered to the kiln and the calciner, and the excess air exhausted: the cooler performance affects both the clinker quality (the quench rate) and the heat consumption (the recuperation);
• The system balance: the three machines share the flow sheet of the line: the VRM feed of the 333 t/h at the 1.60 factor feeds the 208 t/h clinker kiln of the 5,000 t/d line, and the cooler handles the 208 t/h of the clinker at the 1,350°C: the workbook’s production basis runs through the three sheets, and the engineer who changes the kiln rate recalculates the mill and the cooler requirements together.

The system view of the workbook is the view of the production engineer: the three machines calculated on the one basis and read as one line, the way the plant actually runs.

2. The Vertical Roller Mill: The Process Basics

The vertical roller mill grinds the material between the rotating grinding table and the rolling rollers: the material bed is squeezed, the fines are carried out by the air stream to the classifier, and the coarse particles return to the bed. The VRM is the standard mill of the modern raw grinding and increasingly of the cement grinding because of its energy efficiency (the 17–25 kWh/t of the raw grinding against the 20–30 of the ball mill) and its integrated drying capacity:

• The grinding principle: the bed of the material on the table is crushed by the rollers under the hydraulic pressure: the specific grinding force of the industry practice at the 400–800 kN per square meter of the roller (the 40–80 bar of the hydraulic pressure depending on the mill type and the material): the force is the master variable of the mill: too low, the bed slips and the mill vibrates; too high, the power rises and the vibration and the wear increase;
• The table and the rollers: the table speed of the 25–45 rpm for the typical mill diameters (the peripheral speed of the table at the 5–9 m/s), the roller diameter of the 2–3 meters and the roller width: the geometry of the mill defines the material throughput per revolution;
• The airflow: the hot gas of the kiln or the hot gas generator carries the ground material to the classifier: the gas velocity in the mill at the 60–100 m/s at the nozzle ring, the gas flow of the 2.0–2.8 Nm3 per kg of the feed: the drying and the transport gas of the mill;
• The classifier: the dynamic classifier at the mill top separates the product: the fineness of the raw meal at the 12–16 percent residue on the 90 micron and the 1.5–2.5 percent on the 200 micron, the cement at the 1–4 percent on the 45 micron depending on the grade: the classifier speed is the fineness control;
• The moisture handling: the drying capacity of the VRM handles the raw materials with the 5–20 percent moisture with the kiln exit gas at the 280–380°C: the mill and the kiln are thermally coupled: the gas temperature at the mill inlet at the 250–350°C and the mill exit at the 90–120°C.

The VRM process combines the grinding, the drying and the classifying in the one machine, and the calculation sheet of the workbook quantifies each function: the forces, the speeds, the flows and the fineness that define the mill operation.

3. The VRM Calculations: The Worked Numbers

Follow the VRM calculation sheet of the workbook through the worked example of the raw mill for the 5,000 t/d line. The kiln feed demand: the clinker of the 208.3 t/h at the factor of the 1.60 gives the raw meal of the 333.3 t/h, and the mill design with the 110 percent margin at the 366 t/h.

• The specific power: the raw grinding at the 19 kWh/t (the VRM against the 22–26 of the ball mill): the mill power demand of the 366 t/h × 19 = 6,950 kW, the installed motor at the 7,500 kW with the drive losses: the power line of the mill in the cost sheet;
• The table diameter: the mill of the 366 t/h at the specific throughput of the 75–90 t/h per square meter of the table area requires the table area of the 4.2–4.9 m2: the table diameter of about the 2.4–2.5 meters (the area = πD2/4 = π × 2.45^2 / 4 = 4.71 m2): the geometry basis of the mill;
• The table speed: the peripheral speed of the 6.5 m/s gives the table speed of the 6.5 × 60 / (π × 2.45) = 50.7 rpm: the speed setting of the table drive;
• The grinding force: the specific force of the 600 kN/m2 over the roller projection area: with the roller width of the 0.85 m and the diameter of the 2.0 m, the roller projection area = the width × the diameter = 1.7 m2, and the force per roller = 600 × 1.7 = 1,020 kN: the hydraulic cylinders at the 40–60 bar deliver the force through the rocker arms;
• The gas flow: the mill gas at the 2.4 Nm3 per kg of the feed: the 366 t/h × 2.4 = 878,000 Nm3/h of the drying and the transport gas: the mill fan at the 900,000 Nm3/h at the system resistance: the fan power of the 2,200–2,600 kW;
• The fineness: the classifier speed set for the raw meal residue of the 14 percent on the 90 micron: the fineness of the kiln feed that the preheater and the kiln chemistry require.

The worked numbers of the VRM sheet give the complete design picture of the mill: the power, the geometry, the force, the flow and the fineness, and the engineer verifies the operating mill against the calculated values: the deviations flag the wear, the bed problems or the instrument errors.

4. The Kiln Calculations: The Loading and the Retention

The kiln sheet of the workbook computes the mechanical and the process parameters of the rotary kiln, the numbers that define the burning capacity of the line:

• The kiln loading: the degree of the material fill of the kiln cross-section: the loading = (the clinker production × the retention factor) / (the kiln volume × the bulk density): the practical values of the 5–10 percent of the cross-section, with the kiln volume of the example (the diameter of the 4.4 m and the length of the 60 m: V = π/4 × 4.4^2 × 60 = 912 m3): the kiln loading of the 208.3 t/h / 912 m3 = 0.228 t/h per m3, the healthy range of the modern kilns;
• The retention time: the time of the material in the kiln: the retention time = (1.77 × L × √θ) / (D × n × s), with the L the length (60 m), the θ the angle of the repose of the material (the 35–50 degrees), the D the diameter (4.4 m), the n the rotation speed (the 3.2 rpm) and the s the slope (the 3.5 percent): the retention of the example = (1.77 × 60 × √40) / (4.4 × 3.2 × 3.5) = (1.77 × 60 × 6.32) / 49.28 = 672 / 49.28 = 13.6 minutes: the 12–20 minutes of the practice: the time of the clinker formation in the kiln;
• The degree of the filling: the percent of the kiln volume occupied by the material: the 5–10 percent: the loading and the retention together define the material depth and the heat exchange of the bed: the operator adjusts the speed and the feed to hold the filling in the designed range;
• The drive power: the kiln drive power: P = 1.5 × D^2.7 × L × n × f / η, with the f the filling factor: for the example: 1.5 × 4.4^2.7 × 60 × 3.2 / 0.94: the drive power of the 500–650 kW range: the twin pinion drives of the modern kilns with the variable speed;
• The specific thermal load: the heat release per unit of the kiln cross-section: the burning zone load of the 4–7 million kcal/h per m2 of the kiln cross-section: the flame intensity that the refractory and the coating tolerate: the design limit of the kiln.

The kiln sheet of the workbook gives the engineer the mechanical numbers of the burning: the loading, the retention, the filling and the drive power that the daily operation watches, and the same numbers serve the design reviews of the capacity upgrades.

5. The Kiln Thermal Calculations: The Flame and the Heat

The thermal side of the kiln sheet completes the picture of the burning: the fuel, the flame and the heat terms that the kiln engineer computes with the workbook:

• The fuel flow: the clinker production of the 208.3 t/h at the specific heat consumption of the 760 kcal/kg gives the heat demand of the 208,300 × 760 = 158.3 million kcal/h: at the coal of the 5,260 kcal/kg the fuel flow is the 158,300,000 / 5,260 = 30,100 kg/h = 30.1 t/h of the coal: the split of the 40–60 percent to the calciner and the rest to the main burner;
• The flame and the burning zone: the burning zone temperature of the 1,400–1,500°C measured by the pyrometer, the flame temperature of the 1,800–2,000°C, the flame length of the 3–6 kiln diameters: the flame shape controlled by the burner momentum: the flame calculations of the package’s burner workbook feed this sheet;
• The secondary air: the hot cooler air entering the kiln at the 800–1,100°C: the secondary air flow of the 0.9–1.1 Nm3 per kg of the clinker: the heat recovered from the cooler: the link between the kiln and the cooler sheets of the workbook;
• The kiln exit gas: the gas leaving the kiln to the preheater at the 900–1,100°C: the gas flow of the 1.35–1.45 Nm3 per kg of the clinker: the heat source of the raw mill drying and the preheater exchange;
• The clinker temperature: the clinker leaving the kiln at the 1,350–1,450°C into the cooler: the enthalpy of the clinker at the 320–350 kcal/kg, of which the cooler recovers the 65–75 percent back to the process: the cooler sheet computes the recovery in detail.

The thermal sheet of the kiln connects the burning to the fuel and the heat flows, and the numbers of the sheet feed the heat balance of the package: the same fuel, the same gas and the same clinker terms that the balance splits into its losses.

6. The Cooler Calculations: The Quenching and the Air

The cooler sheet of the workbook computes the performance of the grate cooler, the machine that receives the 1,400°C clinker and returns it to the process as the 100–150°C product with the recovered heat:

• The grate loading: the clinker per square meter of the grate per day: the standard of the modern grate coolers at the 40–60 t/d per m2 of the grate area (the high-capacity coolers to the 70–90): for the 5,000 t/d line the grate area of the 5,000 / 50 = 100 m2: the loading is the capacity basis of the cooler design;
• The cooling air: the total cooling air of the 2.0–2.6 Nm3 per kg of the clinker: the example at the 2.2 Nm3/kg: the 208,300 kg/h × 2.2 = 458,000 Nm3/h of the cooling air through the grate: the split of the air: the secondary air to the kiln (the 25–35 percent), the tertiary air to the calciner (the 20–30 percent) and the excess air to the cooler exhaust (the 40–50 percent): the air distribution is the core control of the cooler;
• The recuperation: the heat recovered from the clinker into the secondary and the tertiary air: the recuperation efficiency of the 65–75 percent for the modern coolers: the clinker heat of the 340 kcal/kg at the 1,400°C with the recovery of the 70 percent returns the 238 kcal/kg to the process, and the remaining 102 kcal/kg leave with the clinker and the cooler exhaust;
• The clinker discharge temperature: the clinker leaving the cooler at the 100–150°C (the ambient + 70–130): the cooling rate decides the cement quality: the quick quench locks the alite in the glassy structure and improves the grindability and the strength: the discharge temperature of the example at the 120°C corresponds to the clinker sensible loss of the 0.19 kcal/kg°C × 95°C = 18 kcal/kg;
• The cooler fans: the multiple grate zones with the fans under the grate: the fan pressures of the 500–1,000 mbar, the power of the 1,500–2,500 kW for the 5,000 t/d line: the fan control by the under-grate pressures and the clinker bed height: the power line of the cooler in the cost sheet.

The cooler sheet of the workbook computes the loading, the air and the recuperation of the cooler, and the numbers connect to the kiln sheet through the secondary air and to the heat balance through the clinker loss term: the cooler performance is the third pillar of the line’s efficiency.

7. The Worked Numbers: The Complete Line Picture

Assemble the numbers of the three sheets into the complete picture of the 5,000 t/d line, the way the workbook presents the system in its summary:

The summary table of the workbook gives the production engineer the complete view of the line: the mill prepares the feed at the 366 t/h, the kiln burns the clinker at the 208 t/h with the 13.6 minutes of the retention and the 30.1 t/h of the fuel, and the cooler handles the 208 t/h of the clinker with the 458,000 Nm3/h of the air at the 70 percent recuperation: the three machines, the one line, the one set of the numbers.

8. The Interaction Between the Machines: The Operational Coupling

The calculations of the three sheets are not independent: the machines of the line are coupled operationally, and the workbook’s system view captures the couplings that the plant engineer manages daily:

• The mill and the kiln gas coupling: the raw mill dries with the kiln exit gas: the kiln exit gas temperature and the mill moisture demand set the mill gas flow: when the raw materials are wet, the mill pulls the maximum gas and the kiln operation is constrained by the drying demand: the two machines share the gas balance of the line;
• The kiln and the cooler air coupling: the secondary and the tertiary air temperatures seen by the flame and the calciner depend on the cooler performance: the over-aerated cooler dilutes the secondary air with the cold air and cools the flame: the cooler operation directly changes the kiln fuel consumption and the burning zone condition;
• The cooler and the clinker quality: the quench rate of the cooler decides the clinker structure: the slow-cooled clinker with the large alite crystals grinds harder and develops the strength differently: the cement grinding of the VRM and the ball mill reads the clinker quality of the cooler: the three machines of the quality chain;
• The production rate coupling: the line rate is limited by the weakest of the three machines: the kiln at the 5,000 t/d, the mill at the 110 percent and the cooler at the 100 percent: the capacity upgrade of the line starts with the bottleneck analysis of the three sheets;
• The heat balance coupling: the three machines exchange the heat: the kiln gas heats the mill, the clinker heats the cooler air, the cooler air heats the flame: the heat balance of the package integrates the three exchanges, and the specific heat consumption of the line is the result of the three machines working together.

The coupling chapter is the process wisdom of the workbook: the engineer who calculates the three machines separately sees the parts, and the engineer who calculates them together sees the line: the VRM, the kiln and the cooler as one optimized system.

9. The Optimization Practice with the Workbook

The calculations of the workbook support the daily optimization of the three machines, and the practice of the well-run plants uses the numbers as the control framework:

• The VRM optimization: the working points of the mill tuned against the calculated values: the grinding pressure, the table speed and the classifier speed adjusted to the power and the fineness targets: the vibration and the bed instability corrected before the power loss: the mill log of the workbook tracks the daily parameters against the calculated set points;
• The kiln optimization: the kiln loading and the retention watched against the calculated ranges: the feed and the speed adjusted to hold the filling, the burner settings tuned to the flame, the secondary air temperature watched against the cooler: the kiln stability of the well-run plant is the kiln calculated;
• The cooler optimization: the air distribution tuned to the maximum recuperation: the grate zones balanced, the clinker bed height held, the discharge temperature minimized: the cooler optimization of the practice saves the 10–30 kcal/kg of the clinker and improves the quality: the measured improvement appears in the heat balance of the package;
• The power optimization: the combined power of the line (the mill at the 6,950 kW, the kiln drive, the cooler fans at the 2,000 kW, the process fans) minimized against the production: the specific power of the line at the 75–90 kWh/t of the cement with the VRM: the cost sheet of the package quantifies the power savings;
• The monitoring cycle: the daily logs, the weekly trend reviews and the monthly balances: the workbook of the package gives the plant the calculation base of the cycle, and the deviations from the calculated values trigger the investigations before the production losses accumulate.

The optimization practice closes the loop of the workbook: the calculations of the three machines are not the one-time design numbers but the standing framework of the daily operation, and the plants with the discipline run the three machines to the numbers of the file.

10. The Fan and the Draft Calculations

The three machines of the workbook run on the fans and the draft of the line, and the fan calculations complete the process picture of the VRM, the kiln and the cooler:

• The process fan duties: the preheater fan (the ID fan) pulling the gases through the preheater and the kiln at the flow of the 1.35–1.55 Nm3 per kg of the clinker and the draft of the 600–1,000 mbar, the mill fan of the VRM at the 2.0–2.8 Nm3 per kg of the mill feed, the cooler fans under the grate zones and the cooler exhaust fan: the fan sheet of the workbook computes the flow, the pressure and the power of each duty;
• The fan law calculations: the fan performance follows the affinity laws: the flow proportional to the speed, the pressure to the speed squared, and the power to the speed cubed: the sheet’s fan rows use the laws for the speed changes: the 10 percent speed reduction cuts the power by the 27 percent: the variable speed drives of the modern fans exploit the cubic law for the energy savings;
• The fan power example: the preheater fan of the 5,000 t/d line: the flow of the 850,000 m3/h at the process conditions, the total pressure of the 750 mbar and the efficiency of the 78 percent: the power = the flow × the pressure / (the efficiency × 3,600 × 1,000) = 850,000 × 75,000 / (0.78 × 3,600,000) = 63,750,000,000 / 2,808,000 = 22,700 kW: the fan power of the line: the power line of the cost sheet that the fan optimization reduces;
• The draft profile: the pressure profile along the system (the cooler under-grate pressures, the kiln inlet draft, the preheater stage drafts, the fan suction) is the diagnostic of the blockages and the false air: the draft measurements of the audit campaign read against the calculated profile: the deviations flag the cyclone blockages and the seal leaks;
• The fan optimization: the damper positions versus the variable speed drives, the impeller conditions and the duct resistances: the fan optimization of the practice saves the 3–8 percent of the line’s power: the measured before and after fan currents document the savings in the workbook’s log rows.

The fan and the draft calculations complete the system view of the workbook: the VRM grinds with its mill fan, the kiln pulls with its ID fan, and the cooler blows with its grate fans, and the three fan systems are the power and the draft backbone of the line: the numbers of the sheet hold the energy and the diagnostics of the whole system together.

11. Frequently Asked Questions

Why is the vertical roller mill more efficient than the ball mill?

The VRM grinds the material in the compressed bed between the table and the rollers rather than by the impact of the tumbling balls, and the bed grinding uses the energy more efficiently: the raw grinding at the 17–25 kWh/t against the 20–30 of the ball mill, and the cement grinding at the 22–32 against the 30–40. The VRM also combines the grinding, the drying and the classifying in the one machine, eliminating the separate dryer and the classifier circuits.

What is the typical retention time of the clinker in the kiln?

The retention time of the material in the rotary kiln is the 12–20 minutes for the modern dry process kilns, computed from the kiln geometry, the slope, the rotation speed and the material angle of repose: the worked example of the article (the 4.4 m × 60 m kiln at the 3.2 rpm and the 3.5 percent slope) gives the 13.6 minutes, the healthy range of the practice.

What does the cooler recuperation efficiency mean?

The recuperation efficiency is the share of the clinker’s heat that the cooler returns to the process in the secondary and the tertiary air: the 65–75 percent of the modern grate coolers. The recovered heat raises the flame and the calciner temperatures and reduces the fuel consumption, and the remaining heat leaves with the clinker and the cooler exhaust: the efficiency of the cooler is one of the largest levers of the line’s heat consumption.

How is the grate loading of the cooler calculated?

The grate loading is the clinker production per square meter of the grate area per day: the 5,000 t/d of the clinker divided by the grate area gives the loading in the t/d per m2. The modern coolers are designed for the 40–60 t/d per m2, and the loading check of the existing cooler tells whether the capacity upgrade of the kiln requires the cooler modification.

How are the three machines of the workbook coupled?

The raw mill uses the kiln exit gas for the drying, the cooler returns the heat to the kiln through the secondary and the tertiary air, and the clinker quality of the cooler decides the cement grinding: the three machines share the gas, the heat and the quality of the line, and the workbook calculates them on the shared production basis so the engineer sees the system rather than the parts.

Is the VRM-Kiln-Cooler workbook included in the package?

Yes: the Complete Cement Technical Package (931 files) includes the original VRM-Kiln-Cooler calculation workbook with the calculation sheets of the three machines, together with the VRM training materials, the kiln process references, the cooler courses and the complete library of the cement engineer: the purchase below delivers the file and the package.

12. Conclusion

The VRM, the kiln and the cooler are the three heavy machines of the modern cement line, and the workbook of the package calculates them on the shared production basis: the vertical roller mill with the grinding force, the table speed, the gas flow and the power (the 366 t/h at the 6,950 kW of the example), the kiln with the loading, the retention time and the fuel (the 208.3 t/h at the 13.6 minutes and the 30.1 t/h of the coal), and the cooler with the grate loading, the air flows and the recuperation (the 70 percent recovery and the 120°C discharge). The three sets of the numbers are the three pillars of the line’s capacity, the quality and the efficiency, and the engineer reads them together.

The workbook, the worked examples of the article and the complete library of the process calculation tools and the training materials are part of the Complete Cement Technical Package: the 931 files, the one-time payment of $249.99, the instant download and the lifetime access. The purchase button below delivers the VRM-Kiln-Cooler workbook and the whole package of the cement engineer in one download.

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