Worked example — a 100 t/h finish mill running at 32 kWh/t draws about 3,200 kW electrical input: the operator’s balance measure: the product leaves with about 600 kW, the shell hands 450 kW to the room, the air carries about 1,450 kW (60,000 Nm³/h heated by 80 K at cp 1.0 kJ/kgK), and the water spray accounts for the remaining 600-700 kW: the numbers demonstrate that neither the air alone nor the water alone can hold the total, the two channels live together: the air handles the permanent duty, the water injects on the summer peaks and the overload hours.

5. The Ventilation as the Carrier of the Fines: Sweep and Backfeed

The ventilating air is not only a cooling medium: it is a transport medium that influences the classifier balance of the circuit:

• The share carried: in a closed circuit the mill ventilation often carries 10-25% of the finished product directly to the separator.
• The carry velocity: the gas velocity inside the mill organises an “elutriation” effect: the air leaving the second chamber carries the enriched fine fraction; the coarser particles are returned by the longitudinal transport of the mill;
• The cut size: the effective cut of the sweep at the outlet region sits at the coarse end of the cement fraction, the air cannot lift the oversized pieces and the separator must do the fine classification: the two sorting stages cooperate;
• The balance of the mill: when the sweep weakens, the fines accumulate inside the mill, the internal circulation rises, the mill power climbs and the product coarsens; when the sweep strengthens, the fines course ahead of the separator returns, the circuit becomes “air-bound”: the vegetation equals the classification capacity;

The operator reads the balance in the classic trio: mill outlet temperature, mill Δp, separator fineness: a growing mill out with a constant feed and a rising temperature typically means the air is difficult to pull the fines out: the ventilation becomes a diagnostic instrument of a mill that is becoming filled — “surcharged” — before the motor current even tells the same story.

6. Water Injection for Cement Cooling: The Second Channel

When the climatic duty exceeds the air capacity, the standard industry solution is the water injection inside the mill:

• Water is sprayed with atomising nozzles into the second chamber (and in many plants into the cooling of the separator feed) at a pressure of 2-6 bar through fine droplets;
• The evaporation absorbs about 2.45 MJ per kilogram of water, so each 0.5% of water on the feed typically depresses the mill outlet temperature by roughly 10-20 °C;
• The practical limit of the injection is about 1.5-2.5% of the feed rate: above it the moisture enters the product, the separator and the filters of the mill; the cement moisture must stay below the handling and packing limits;
• The nozzles demand filtered cold water below 30 °C; the scaling water kills the atomisation; each dead nozzle shows as a mysteriously rising temperature band on the shell;
• The same water improves the grinding aid effect for many cements and reduces the coating tendencies of the hot mill, but the dosage is always governed by temperature, never by taste: the temperature controller arms the spray above 105-110 °C and stops it below;

The loading philosophy: the air covers the base load 365 days of the year, and the water spray covers the peak hours of the summer and the high-load guild: the plants that hold both channels tuned get the stable gypsum, the stable strength and the stable filter all at once; a mill without the spray on a hot climate is a mill that is always at the edge of its temperature limits.

7. The Gypsum Limit: Why 115 °C is the Ceiling

The ventilated temperature is not a matter of comfort: the chemical constitution of the cement is at stake:

• Portland cement receives its calcium sulfate as gypsum (CaSO4·2H2O), the retarder of the C3A reaction; the dihydrate must survive the grinding;
• Above about 120-125 °C the dihydrate distills its crystal water and turns into the hemihydrate and the soluble anhydrite: the grinding environment becomes a “dehydration zone” and the false set of the concrete appears;
• The control band 95-115 °C exists to keep the gypsum intact while maximising the evaporation of the residual moisture; the plants grinding very hot clinker (late sandstone, extreme summer) may deliberately let 120 °C on short periods and correct by feed;
• The flowing water injection that appears on very hot days must be watched in the same logic: the vaporised water cools the gas but the outlet temperature can be raised only at the price of the cement quality: the temperature sensor of the outlet must be representative (in the spout, in the gas1), not at the shell of the chamber;

The ceiling is enforced by the control logic: the outlet temperature alarm at 115 °C and the permissive on the water spray at 105 °C: the operator who sees the trend of 119 °C immediately looks at the spray and the air, not at the pump pressure: the gypsum chemistry is the discipline of the temperature, and the ventilation is the tool that holds it.

8. The Instruments and the Control Loops of the Ventilation

The plant instrumentation issues the operator the daily steering data, and the table shows the typical ranges and the meaning of the alarms:

The control philosophy is a cascade: primary loop the volume controller on the fan; secondary the temperature of the outlet feeding the ROI set point of the fan and the spray; tertiary the filter status: the plants with good cascades report mill temperatures rectilinear during the shifts, and their product quality follows.

9. The Practical Troubleshooting of the Mill Air

The failure modes of the ventilation decide the typical plant calls: the operator’s quick cross table:

The method: always read the pressure and temperature trend over the last 30 minutes before opening anything: the trial inspection opens the airflow plan: if the ΔP and the temperature are normal, the sensor is the first suspect—a wet sensor, a clotted pitot port: the instrumentation discipline saves time: thereafter the biggest favourites: leaks (false air forced), the unloaded fans and the nozzles of the spray: the man who respects the numbers of the mill back comes home with the diagnosis in the hand.

10. The False Air, the Leaks and the Audit of the Flow

The false air is the silent thief of the ventilation balance: the plant that ventilates with 80,000 Nm³/h on the calendar actually moves less than the design through the grinding because 10-20% enters with the leaks:

• The leak points: the mill seals and the spout manholes, the duct flanges, the filter doors, the expansion joints on the outlet side;
• The effect: the false air dilutes the gas, cools the contact, raises the fan volume and the filter load, and degrades the moisture: the mill gets nowhere… the measured export flow is lower;
• The measure: the audit measures the flow at the fan and the flow at the mill inlet (the difference is the false air), repeated quarterly on the schedule;
• The remedy: the optimization of the mill seal packings, the screen of the spouts, the pressure testing of ducts on the shutdown, and the elimination of the manhole leaks once a year.

The discipline of the false air pays double: the mill keeps its temperature, and the fan consumes less energy: the audible audit (the hiss of air on the leaks) is the primitive instrument that every expert still uses; the flow audit is the repeatable documentary one: the file contains the leak survey sheets for the plants.

11. The Water Cooling of the Vent and of the Product: the Packing Side

The cement Factory faces the same temperature on two more fronts in order: the finish silos and the packing plant:

• The cement that enters the silo at 110-120 °C demists the silos, increases the lumping of the walls and degrades the flow of the aerated silo floors;
• The plant’s final barrier: the cement cooler with a fluidised bed, a plate cooler or a tube cooler downstream of the mill outlet for the hot cement (the so-called “finish cooler”); the packaged products recommend a temperature that enters the packer at approx. 70-90 °C;
• In principle the same logic applies to the clinker cooler of the clinker yard, but in the finish side the cooling happens properly in the mill room: the ventilation and the spray do their work first, the cooler of the silo only tuning the final 20-30 °C;

Where the miller ends, the packer begins: the total chain “mill-outlet temperature → silo inlet → packer” is written into the quality assurance plans, and the ventilation controls the first link of this chain: the silo and packing sections reuse the same heat balance logic with less sophisticated equipment: the same numbers, the same verdict.

12. The Start-up and Shutdown Sequences: The Meaning for the Filter and the Seasonal practice

The life of a mill ventilation is measured in the wet start-ups: the humidity and the dust of the start become the deposits of the filter:

• The cold start: the filter is preheated (the clean gas at 80-100 °C for about 15-20 minutes) before the mill feed starts, so the humid dust does not condense on the bags;
• The zero-feed period: the mill runs empty for the first few minutes with ventilation only: the thermal mass of the mill shell drives the temperature up faster than the dust
• The winter protocol: the inlet air of cold plants is preheated/admixed with hot recirculated gas to keep the dew point safely under the metal temperature, and the mill is rarely completely stopped in frost when the moisture risk is high;
• The shutdown: the ventilation continues 10-20 minutes after the feed stops, sweeping the residual dust; the mill is cooled with the spray to 70-80 °C; the stop of an overheated, humid mill is the recipe for the coating and the lumping;
• The weekend: the plants that stop at the weekend keep the fans running at the reduced speed during the frost, or cover the inlet with dry blankets, so the internal condensation never starts;

The operation of the mill stands or falls on the understanding that the ventilation is never “turned on”: it is a closed circuit whose state must be plausible in advance: every start opens the door of the dust, and the mill that ignores its own dew point pays with the filter life and the quality of the first product after every restart.

13. The Frequently Asked Questions

Why is the mill called “ventilated” when the air flows with gravitational direction?

Centrifugal ventilation is one of the fundamental air-flow concepts of the industry: the term “ventilation” covers the entire controlled exchange of the gas atmosphere of the mill: the same air that cools, the same stream that sweeps, the same control of the pressure and the moisture: German and American texts use “ventilation”, the computer of the plant the same word: in daily practice “mill ventilation” is synonymous with “mill air flow”

How does the ventilation influence the strength of the cement?

Through the gypsum state and the fineness: an overheated mill dehydrates the dihydrate (the false set), a flooded mill produces a coarse uncontrolled product: the correct ventilation holds the temperature, limits the water loss, and keeps the separator feed constant: the strength at 2 and 28 days is the Y effect of the guarantee of the stable mill.

Can the vertical roller mill use the same numbers?

No: the VRM dries with the mill gas (higher temperature, 90-110 °C at the outlet) and the material transport is pneumatic through the throat and the classifier: the VRM has no water injection, the airflow IS the transport: the design values of this file apply to the tube ball mills; the VRM numbers are the separate chapter of the package.

Why does the filter go “wet” in the wet house condensate?

Because the dew point of the saturated air (35-60 °C) comes above the metal temperature of the filter (cold surfaces in winter): the hoppers of the incorrectly insulated filter collect liquid water: the cure is the heated, insulated ductwork and the correct water spray control: the rooms of the bag house are the humid-sensitive province of the air discipline.

Is the recycled air from the filter good for the mill?

Yes, in the flue value: recirculating the cleaned air at 60-80 °C into the inlet reduces the fresh air, the vapor make-up, the heating load and the total filter capacity: the percentage is balanced by the ground: too much retour raises the moisture request of the gas and the outlet temperature; the duty of the expertise is the balance, and the file draws the control logic.

What is the quick sign that the mill is “air bound”?

The pressure differential of the mill rises above the historical value at the same feed, the outlet temperature crawls up despite the water spray, and the separator coarsens with the rising mill “power” empty: the air column is closed; immediately reduce the feed 10%, open the damper and cool: the sign and remedy of the air-bound mill are covered in the troubleshooting chapter.

14. Conclusion

The mill ventilation is the first and the last instrument of the mill atmosphere: the air sweeps the fines, carries the vapour, transports the heat: the temperature band 95-115 °C protects the gypsum chemistry and the filter: the flow is the conveyor of the most important product: stable: the operators of the cement mill measure, control and audit their air as their most honest partner.

With the Complete Cement Technical Package (931 files, $249.99, instant download via the PayPal) the reader receives the full ventilation chapter with the complete tables, the heat balance calculator and the checklists of the plants: the air of the mill — the mastered dimension: the complete library of the cement plant engineer in the fee-less: the guide is inside the package, and this article is its preview.

The Ventilation Optimization and the Cement Temperature Law

The optimization of the mill ventilation is the constant target of the operation: the three dials the operator holds:

• The mill exit velocity profile: the velocity must pull the fines from the mill at the designed rate: the too-slow velocity holds the fines and overgrind occurs, the too-fast velocity carries the coarse particles to the separator: the classic curve of the velocity vs the fineness efficiency of the separator; the velocity of the in-height of the mill is a fixed movie of the airflow;
• The false air: the air leakages through the seals, the door frames and the joints dilute the grinding gases and add the oxygen for the reactions: the infr-red survey of the mill surfaces amends the diff: the leak clamping economics: each 1% of the false air costs the ventilation capacity in the separator;
• The temperature law at the mill: the cement exits the mill at a temperature governed by the grinding heat and the ventilation heat uptake: the vent air carries the kW out (the airflow x the specific heat x the delta), and the cement and the vent approach the same order: the exit temperature is the direct indicator of the ventilation success;
• The target windows: the standard practices stop the cement temperature below 110-115&degC (the gypsum threshold) and the mill temperature at the inlet shell 70-90&degC, the air-cooling flows and the water spray optional the order: with the window control overlay, the operation keeps the quality and the flows;

The ventilation is the author of the cement temperature: the operator who understands the velocity, the leak, the temperature links never fails the cooling of the mill: the chapter of the file closes the cooling subject with the temperature law as the reference page of the operation.

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.