Drying Technology: Complete Technical Guide

Drying technology in the cement context is the removal of the water from the raw materials, the fuels and the products before and during the processing: the limestone from the quarry carries 3-8% of the moisture: the clay 15-30%: the slag 10-20%: the coal 8-15%: the raw mill, the vertical mill, the flash dryer and the rotary dryer exist to dry and grind: the drying technology is the discipline of the heat, the air and the water: the engineer who masters it controls the mill throughput, the fuel consumption and the filter operations: this file is the complete manual of the drying in the cement plant.

The Complete Cement Technical Package (931 files including the books, the courses, the Excel tools and the presentations: $249.99 one-time: instant download via the PayPal payment) includes this drying chapter with the psychrometric tables, the heat and mass balance workbooks, the dryer selection guide and the plant case sheets: the package is the engineer’s library of the whole process: this article walks the drying file page by page.

This article follows the file structure: the theory of the moisture, the psychrometrics, the balance, the dryer types, the mill-dryer systems, the operation, the troubleshooting and the examples: the reader can open the document and follow the same order with the tables and the sheets.

1. The Moisture of the Materials: The Enemy of the Mill

Every solid of the cement plant carries water: the water exists in the free and the bound forms:

• The surface moisture (the free): the water film on the particles: removable by the evaporation: the quarry rain, the washing, the settling: the moisture of the limestone 3-8%, the clay up to 30% in the wet seasons;
• The bound water (the combined, the hydrate): the chemically combined water in the clay minerals, the gypsum, the limestone (some): not removable by the simple drying: the calcination releases it at the temperature;
• The effect on the milling: the moisture above ~5% in the raw mill: the coating on the balls, the pack in the diaphragm, the reduced grinding: the throughput collapses: the mill vents hot gas to dry the feed: the moisture is the first enemy of the mill;
• The effect on the process: the wet feed jams the bins and the feeders: the wet coal: the flame instability: the wet slag: the transport problems: the drying is the precondition of the smooth production;

THE MEASUREMENT of the moisture: the oven-drying method (the 105°C to the constant mass), the infrared balances, the microwave, the online NIR: the moisture is the process variable that the plant must know at the hour level: the file: the sampling and the test methods: the quick test of the field: the plant that measures the moisture feeds its mill with the data: the file: the moisture map of the materials and the seasons.

2. The Psychrometrics: The Language of the Humid Air

The drying happens in the air: the psychrometric chart is the language of the air and the water vapor:

• The absolute humidity: the grams of the water vapor per kilogram of the dry air: the capacity of the air to hold the vapor: the air enters with x1 and leaves with x2: the difference (x2 – x1) is the water taken per kg of the air;
• The relative humidity: the percent of the saturation: the RH of 100%: the dew point: the air cannot absorb more: the drying stops: the wet-bulb temperature: the chart lines;
• The temperature terms: the dry-bulb, the wet-bulb, the dew point, the adiabatic saturation: the practical: the drying uses the sensible heat of the hot gas: the gas cools as the water evaporates: the outlet temperature must stay above the dew point of the mixture: the condensation in the mill and the filter is forbidden;
• The dew point protection: the outlet gas of the raw mill: the 90-100°C typical: the dew point of the mixture with the moisture: the filter above the dew: the insulation: the condensation: the corrosion, the blinding: the file: the dew point calculation sheet;

THE PSYCHROMETRIC CHART of the file: the plotting of the drying processes: the heating (the constant moisture), the evaporation (the constant enthalpy), the mixing of the hot and the cold air: the engineer reads the chart the way the designer reads the P&ID: the air quantities and the outlet conditions fall out of the plot: the Excel psychrometric: the input the T, the RH: the output the absolute humidity, the enthalpy, the dew point: the tool of the daily calculations.

3. The Heat and Mass Balance of the Dryer

Every dryer is a balance of the heat and the mass:

• The mass balance: the wet feed = the dry product + the water evaporated: the water removed = the feed × (m1 – m2) / (100 – m2) in the percent terms: the kg/h of the water: the target of the drying:
• The heat balance: the heat in = the heat of the gas in, the heat of the feed, the heat of the reaction: the heat out = the gas out, the product out, the water vapor out, the losses: the difference is the heat of the evaporation + the heating:
• The specific heat consumption: the kJ per kg of the water removed: the ideal ~2,600 kJ/kg (the latent heat of the water), the practical 3,000-5,000: the efficiency of the dryer: the comparison of the plants:
• The hot gas source: the kiln exhaust (the waste heat: the free), the hot gas generator (the burner: the fuel), the cooler exhaust: the economics: the waste heat first: the file: the selection of the source:

THE BALANCE WORKBOOK of the file: the input the feed rate, the moistures, the gas temperature and the flow: the output the water evaporated, the outlet temperature, the gas requirement, the dew point: the engineer runs the sheet for every case: the summer and the winter, the wet and the dry feeds: the balance is the backbone of the drying design: the numbers decide the fan, the filter and the dryer size.

4. The Drying Inside the Grinding System: The Mill as the Dryer

The most common drying in the cement plant happens inside the mill:

• The raw mill with the hot gas: the kiln gas (the 300-400°C at the mill inlet) enters the ball mill: the grinding and the drying in one machine: the gas sweeps the mill, carries the fines, dries the feed: the most efficient drying-grinding combination of the classic plants;
• The vertical roller mill (VRM): the modern raw grinding: the hot gas in the mill body: the drying of the high moisture (up to 20-25% for the special designs) with the high gas flow: the VRM is the champion of the drying-grinding: the file: the VRM drying limits;
• The parameters: the gas temperature at the inlet (the 150-400°C), the gas velocity in the mill (the 1.5-2.5 m/s in the ball mill), the outlet temperature (the 90-120°C), the dew point margin, the gas quantity per ton of the material: the file: the tables of the standard values:
• The product moisture: the target of the raw meal below 1%: the balance between the drying and the grinding: the wetter feed needs the hotter gas or the lower throughput: the drying capacity is often the throughput limiter of the mill: the file: the “drying-limited vs grinding-limited” analysis:

THE MILL-DRYER OPERATION: the gas flow and the temperature control: the inlet damper, the cold air, the hot gas from the kiln, the preheater exhaust: the control loops (the temperature at the mill outlet, the pressure): the sequence of the startup (the mill warms first, the feed after): the file: the operating windows of the raw mill with the seasonal feed: the case: the winter feed at 6% vs the summer at 3%: the same mill, the different settings: the operator’s guide of the file.

5. The Dedicated Dryers: The Rotary, the Flash and the Fluid Bed

When the drying must happen separately from the grinding, the plant uses the dedicated dryers:

• The rotary dryer: the inclined rotating drum with the flights: the material showers through the hot gas: the classic dryer of the slag, the clay, the coal: the 60-80% moisture materials: the retention 20-40 minutes: the robust, the simple, the high capacity: the large footprint and the heat consumption;
• The flash dryer (the pneumatic): the suspension of the fine material in the hot gas stream: the drying in seconds: the ideal for the fine cohesive powders (the clay, the chalk, the kiln dust): the compact, the efficient: the gas velocity the transport: the cyclone and the filter downstream;
• The fluidized bed dryer: the air through the distributor plate: the fluidization: the gentle drying of the granules: the temperature control: the specialty applications: the limestone fillers, the cement additives:
• The selection: the particle size, the moisture, the stickiness, the temperature sensitivity, the capacity: the file: the selection matrix: the clay (the sticky): the flash or the rotary with the chain curtain: the slag (the granular): the rotary: the fuel (the safety): the special:

THE DRYER DESIGN: the volume, the retention, the gas flow, the drum diameter: the file: the volume per the water removal (the 10-30 m³ per t/h of the water), the gas velocity, the flight design: the worked example of the file: the slag dryer: the 30 t/h of the water: the rotary: the diameter 3.2 m, the length 22 m, the gas 50,000 m³/h: the numbers: the tender: the drawing: the complete design of the dryer from the balance to the motor.

6. The Drying of the Coal and the Fuels: The Safety Dimension

The fuel drying adds the explosion safety to the drying technology:

• The coal mill drying: the hot gas (the kiln exhaust or the generator) dries the coal to the 1-2% moisture: the mill grinds the coal: the product feeds the kiln flame: the drying and the grinding of the coal in the same mill:
• The explosive atmosphere: the coal dust in the air: the lower explosion limit: the ignition sources: the drying temperature is the critical: the mill inlet gas below the 300-400°C limits (the design): the O2 control: the inertization with the CO2/N2: the CO monitoring: the explosion venting and the suppression:
• The safety interlocks: the high CO trips the mill, the high temperature trips, the inerting system: the file: the safety chapter of the coal drying: the standard of the design (the codes): the fire fighting: the deluge:
• The alternative fuels: the RDF, the SRF, the plastics, the tyres: the drying of the waste fuels: the metals and the chlorides: the drying technology of the alternative fuels: the feed systems: the file: the cases of the co-processing:

THE SAFETY FIRST of the fuel drying: the file: the “ten rules of the coal mill” (the temperature, the CO, the inerting, the housekeeping, the spark detection, the fire fighting): the plant that respects the rules runs the coal drying safely for the decades: the plant that relaxes pays with the explosions: the file: the honest chapter of the risk and the mitigation: the same discipline applies to the drying of the alternative fuels with the metals and the chemical energy.

7. The Coolers and the Condensation: The End of the Drying Line

The drying line does not end at the dryer: the downstream equipment:

• The dust collection: the bag filters after the dryers and the mills: the gas with the dust and the moisture: the filter must run above the dew point: the pre-coating of the bags (the start: the wet conditions), the filter insulation, the compartment isolation:
• The condensation: the water vapor condenses on the cold surfaces: the hopper of the filter, the duct walls: the corrosion (the acid: the SO3), the material build-up: the prevention: the insulation, the gas temperature, the condensate drains: the file: the condensate management:
• The heat recovery: the waste heat of the drying gas (the preheating of the combustion air, the water heating, the power generation at the large plants): the “waste heat recovery” of the cement plant: the ORC systems: the economics of the modern plants:
• The recirculation: the part of the drying gas recirculated (the energy saving: the higher humidity): the balance of the recirculation: the file: the recirculation ratio: the systems:

THE LINE DESIGN: the dryer, the duct, the cyclone, the filter, the fan: the pressure balance along the line: the velocities (the dust transport 15-25 m/s), the drops, the fan selection: the file: the “drying line” chapter: the complete flow: the drawing: the control: the operator’s view of the whole: from the hopper of the wet feed to the stack of the filter: the line is one system: the file treats it as such.

8. The Operation of the Drying: The Control of the Process

The daily operation of the drying systems: the control variables and the practices:

• The controls: the outlet temperature of the dryer/mill (the master), the inlet gas temperature (the damper/hot gas), the feed rate, the gas flow (the fan): the loops: the setpoints: the interplay with the grinding:
• The feed variations: the moisture of the quarry changes with the rain: the operator reads the moisture report, adjusts the gas: the “anticipatory” control: the file: the decision table: the moisture vs the settings:
• The winter vs the summer: the ambient temperature and the humidity change the drying: the summer at the high RH: the wet-bulb limit: the winter at the cold: the condensation risk: the seasonal operating guide of the file:
• The start and the stop: the sequence: the warm-up, the feed-in, the normal, the feed-out, the purge: the “no feed no heat” rule: the prevention of the fires and the blockages: the checklists of the file: the shift routines:

THE OPERATOR’S MANUAL of the drying: the normal conditions, the alarms, the responses: the case of the raw mill: the outlet temperature falls (the wetter feed): the operator raises the gas temperature, watches the dew point, checks the filter: the sequence of the responses in the file: the practice: the operator runs the sheet and the loop: the control of the drying is the daily craft of the plant: the file: the skills of the shift.

9. The Troubleshooting of the Drying: The Symptom and the Cause

The drying systems fail with the recognizable symptoms: the file maps them:

• The low outlet temperature: the wet feed, the low gas temperature, the low gas flow, the overfeed: the check the moisture, the dampers, the fan: the steps of the file:
• The high outlet temperature: the dry feed, the underfeed, the high gas: the risk of the hot gas to the filter: the response:
• The filter blinding: the condensation, the pre-coat failure, the high humidity: the delta-P of the filter: the cleaning: the insulation: the file: the blinding checklist:
• The blockages: the sticky feed in the chutes and the dryers: the chain curtains, the air lances, the cleaning doors: the prevention: the feed control:
• The product moisture high: the insufficient drying: the balance check: the throughput reduction or the gas increase: the file: the moisture audit of the product:
• The material carry-over: the too high gas velocity, the cyclone undersized, the feed fines: the loss of the product to the filter: the velocity check:

THE DIAGNOSTIC TABLES of the file: the symptom, the possible causes, the check procedures, the fixes: the plant records its own cases: the file grows with the experience of the plant: the troubleshooting is the practical half of the drying technology: the engineer who knows the failures prevents them: the file: the case histories of the drying at the end of the chapter.

10. The Energy Economics of the Drying

The drying consumes the heat and the power: the economics of the drying matter:

• The specific consumption: the 3,000-5,000 kJ per kg of the water: the dryer type, the gas temperature, the insulation: the comparison of the plants: the audit: the target of the improvement:
• The waste heat first: the kiln gas drying is the free heat: the dedicated generator the cost: the plant designs the drying to use the waste heat wherever the distance and the temperature allow: the file: the heat mapping of the plant:
• The power of the gas handling: the fan power of the drying gas: the kWh per ton of the material: the velocity optimization: the pressure drops: the VFD fans: the power economics: the file: the fan power workbook:
• The improvement projects: the insulation, the recirculation, the dryer control optimization, the pre-drying of the wet materials, the moisture measurement: the list of the improvements with the paybacks: the file: the investment ranking:

THE ECONOMICS: the drying is a cost center of the plant: the fuel and the power of the drying appear in the specific energy of the cement: the plant that optimizes the drying improves its competitive position: the file: the energy audit of the drying: the calculation of the payback: the case: the mill with the wet feed: the drying capacity limits the throughput: the small investment in the gas control returns the t/h: the economics of the drying in the hands of the engineer: the file.

11. The Dryer Design Data: The Tables of the File

The reference tables of the file: the numbers the designer uses every day:

• The moisture data: the typical moistures of the materials: the limestone 2-8%, the clay 12-30%, the sand 3-6%, the slag 10-25%, the coal 8-15%, the alternative fuels 10-30%: the seasonal variations: the data table:
• The gas data: the kiln exhaust: the temperature 250-450°C, the dust load, the flow per ton of the clinker: the cooler exhaust 200-350°C: the hot gas generator: the fuel and the flow: the table of the sources:
• The dryer data: the specific volumes, the velocities, the retention times, the heat consumptions: the rotary vs the flash vs the VRM: the comparison table: the selection:
• The psychrometric data: the saturation values, the enthalpy at the standard temperatures: the chart in the appendix: the interpolation: the quick table:

THE TABLES: the designer opens the file and finds the numbers: the quick estimate without the heavy calculation: the check of the vendor’s data: the tender evaluation: the tables are the reference of the whole drying chapter: the engineer’s day: the file: the tables in the appendix: the same data in the Excel for the calculations: the complete data package of the drying.

12. The Moisture Measurement and the Process Data: The Instruments

The drying control begins with the knowledge of the moisture: the instrument chapter of the file:

• The sampling and the oven method: the classic reference: the sample dried at 105°C to the constant mass: the hours: the off-line: the daily calibration of the online instruments: the file: the procedure card of the laboratory:
• The online moisture analyzers: the NIR (the near-infrared) on the belt: the microwave resonance: the measurement of the process stream: the water content of the feed to the mill: the input of the automatic control: the file: the technologies, the accuracies, the positioning:
• The balance with the mill parameters: the indirect indicators: the mill outlet temperature (the drying status), the differential pressure, the product moisture: the operators trained to read the hints: the file: the diagnostic table:
• The data organization: the moisture records by the material and the season: the quarry report, the mill report: the moisture curve of the year: the planning of the drying operation: the file: the data sheets and the logs:

THE MEASUREMENT: the first-class practice: the plants that know their moisture run the drying efficiently; the plants that guess their moisture run the drying blind: the file invites the engineer to install the measurement and to log the trend: the same data supports the process control, the purchasing (the moisture is paid in the tonnage) and the energy planning: the measurement is the smallest investment with the largest return in the drying discipline: the instrument chapter of the file: the complete: the drying: the measures: the numbers: the process: the cement: the professional: the guide.

13. The Drying in the Raw and the Finish Milling: The Grinding-Drying Balance

The integrated drying is the everyday reality of the mills: the grinding-drying balance chapter of the file:

• The raw mill: the hot gas dries and the mill grinds: the inlet temperature, the gas flow, the outlet temperature: the two functions compete for the gas: the balance: the file: the raw mill drying chart:
• The vertical mill: the high velocity: the internal classifier: the drying within the mill body: the hot gas to the full moisture capability: the file: the VRM drying limits table:
• The ball mill and the drying:” the ball mill with the hot gas for the moderate moisture: the gas velocity: the retention: the file: the ball mill drying section:
• The finish cement: the moisture in the clinker and the additives: the small: the vent temperature: the gypsum dehydration: the water injection: the file: the finish mill moisture case:
• The optimization: the moisture measurement ahead, the gas distribution, the waste heat mapping: the drying and the grinding: the one supervisory: the file: the integrated:

THE BALANCE: the drying and the grinding compete for the same gas and the same heat: the plant: the optimum: the combo: the file: the combined duty: the raw mill: the drying shown in the Excel-case: the file: the chapter: the complete: the drying: the grinding: the cement: the modern plant: the instrumentation: the file: the package: the drying: the professional: the complete: the numbers.

14. Frequently Asked Questions on the Drying Technology

Why does my raw mill slow down when the material is wet?

The moisture coats the grinding media and blocks the diaphragm: the grinding efficiency falls: the mill also spends the heat to dry the feed: the throughput drops until the drying and the grinding balance: the fix: the hotter gas, the better ventilation, the moisture control of the feed, and the knowledge that the wet season changes the capacity: the file: the drying-limited analysis of your mill.

What is the difference between the drying and the calcination?

The drying removes the free water by the evaporation below 100-150°C: the calcination releases the chemically bound CO2 and water at the high temperature (the limestone at 850-900°C, the clay dehydroxylation): the drying is the process of the raw preparation: the calcination belongs to the kiln: the two processes must not be confused in the balance: the file: the boundary of the two.

The gas of my mill is condensing in the filter: what do I do?

The outlet temperature is below the dew point of the mixture: raise the gas temperature at the mill inlet, reduce the water input (the feed moisture, the water injection), insulate the ducts and the filter hopper, and check the cold air in-leakage: the file: the dew point calculation and the filter protection checklist: the condensation is the silent killer of the bags: the fix is the temperature discipline.

Is the vertical mill better than the ball mill for the wet raw materials?

Yes, generally: the VRM handles the high moisture (up to 20-25% with the special design) with the large gas flow and the internal drying: the ball mill dries up to about 8-10% with the same gas: the wet materials and the modern plants choose the VRM for the raw grinding: the ball mill with the external dryer or the pre-dried feed remains the classic solution: the file: the comparison of the two systems with the numbers.

The drying consumes the fuel: can I dry without the generator?

Use the waste heat first: the kiln and the cooler exhaust gases dry the raw materials without the additional fuel: the design locates the mills near the heat sources: the remaining duties (the slag, the coal, the additives) may need the hot gas generators: the economics: the free heat vs the fuel: the file: the heat mapping of the plant: the answer: minimize the generator, never eliminate it where the source is far.

15. Conclusion: The Water Leaves, the Production Flows

The drying technology: the discipline of the water in the cement plant: the moisture, the air, the heat and the balance: the mill-dryer systems, the dedicated dryers, the safety of the fuel drying, the operation and the troubleshooting: the water leaves the material, the production flows: the engineer with the drying file controls the throughput of the mill in every season: the knowledge of the drying is the knowledge of the whole plant: the file of the package: the complete manual of the moisture.

The Complete Cement Technical Package (931 files: $249.99 one-time: instant download via the PayPal payment) includes the drying chapter with the psychrometric chart, the balance workbooks, the dryer design data and the case sheets: the complete library of the cement process: the kiln, the mills, the fans, the bins: the one-time price: the instant access: the engineer’s library: the drying file: the tool of the daily work: the cement knowledge: the PayPal button: the download: the library of the professional.

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.