Heat Balances Of Kilns And Coolers And: Complete Guide & Dow

The heat balance of the kiln system and the cooler is the most informative single calculation a cement plant can perform: it accounts for every megajoule that enters the burning process and names every megajoule that leaves: the result is the thermal efficiency of the kiln, the losses through the shell, the exhaust gas, the cooler waste air, the clinker carry-over heat and the radiation surfaces: it converts the vague feeling of “the kiln is not economical” into the numbered statement of where the energy goes: the energy audit is the unchallenged starting discipline of every fuel-saving project in cement.

This file of the Complete Cement Technical Package (931 files, $249.99 one-time via PayPal with the instant download; library of cementequipment.org) is the working reference of the heat balance: the method of the mass-and-heat accounting, the calculation tables, the worked examples of the plant (the preheater kiln with the precalciner, the grate cooler with the tertiary air) and the related topics of the kiln air balance, the cooler balance, the shell losses, the refractory and the energy efficiency: this article leads the reader through the file in the order that the audit itself proceeds.

The reading plan: the terms and the framework of the balance first, the mass balance that feeds the heat balance second, the walk of the kiln’s heat inputs and outputs third, the cooler balance and the related calculations fourth, and the operational conversion of the results last: by the end, the reader opens the Excel sheets of the package, inserts his own numbers and sees the profile of his own kiln.

1. The Vocabulary of the Heat Balance: The Terms That the Audit Speaks

The balance reports are written in a fixed vocabulary, and the file pins the definitions before the first number:

• Heat balance: the statement of the conservation of energy: the heat input equals the sum of all heat outputs, within the measurement noise of the audit window;
• Lower heating value (LHV): the enthalpy available from the fuel when the water vapor of the combustion is not condensed: the basis of the cement industry, by convention;
• Higher heating value (HHV): includes the condensation heat of the water vapor: the value that appears in the fuel certificates and that must be converted to the LHV;
• Specific heat consumption: the thermal energy consumed per kg of clinker, in kJ/kg clinker or kcal/kg clinker: the headline number of the whole balance;
• Burning zone input: the fuel energy actually released in the kiln flame, against the energy of the material;
• Preheating: the recovery of the hot gas in the cyclone tower: the heart of the dry process energy efficiency;
• Cooling air / recovery: the hot air from the cooler returned to the kiln and the precalciner (secondary and tertiary air): the heat recuperated into the process;
• Waste heat: the flue gas and the cooler waste air that leave the system unrecovered: the target of the modern waste-heat recovery plants;
• Shell losses: the heat radiated and convected from the refractory and the kiln shell surfaces: the greatest non-recoverable item;
• Formation heat: the endothermic energy required to form the clinker minerals from the raw materials: the theoretical minimum that the system must at least overcome;

The vocabulary above walks every page of the balance report; the same words appear in the quotations of the suppliers, the guarantees and the emission norms, and the engineer who controls them reads the entire technical correspondence of the plant without translation.

2. The Framework of the Balance: The System Boundaries

The heat balance is defined by where the engineer draws its border: the choice of the boundary decides which flows enter and which leave:

• The system “kiln + preheater”: the boundary through the kiln feed pipe, the fuel entry ports, the cooler inlet and the kiln exit gas: the common boundary of the specific heat report;
• The system “kiln plus preheater plus cooler”: the full thermal system of the plant: the boundary crosses the raw materials at the feed top and the clinker at the cooler discharge: the complete thermal account of the process;
• The fictitious balance surfaces: the audit often draws the surfaces through the measurement points of the fixed plant instruments, so the “inside” is the volume between the sensors: the result is the “system balance” of the particular audit run;
• The cooler as its own balance: the cooler balance is computed so: air in (ambient + auxiliary), clinker in (hot) vs air out (secondary, tertiary, waste) and clinker out: the tool of the cooler audits;

The selection of the boundary the audit is never negotiable: the guarantee test of a kiln and the daily energy audit use the same boundary, or the numbers cannot be compared: the file states with every worked example exactly which flows are in and which are out, and the reader repeats the drawing before he repeats the arithmetic.

3. The Inputs Sheet of the Kiln Balance: Every Kilojoule That Enters

The input side of the kiln balance is short but crucial: the discipline of the heat feed includes all the entries:

• The fuel heat: the fuel flow × the lower heating value of the fuel: the coal, the petcoke, the gas, the waste-derived fuels: the dominant line of the balance: ±1-2 % of its own accuracy dominates the total;
• The sensible heat of the fuel: usually small, but for the hot petcoke or the gas at the burner it is added: the fuel temperature times the specific heat times the flow;
• The sensible heat of the raw material feed: the temperature of the raw meal entering the preheater: usually the ambient; if the raw meal comes from the hot storage, the value adds;
• The moisture / water in the feed: if the raw meal enters wet (wet process) the latent heat of its moisture is an input item that must be accounted:
• Net radiation to the flame and the burner air: the hot air of the combustion: the enthalpy of the secondary/tertiary air at the measured temperature: this is actually a circulation within the boundary, accounted through the cooler;

In the precalciner setting, the total fuel is split: the secondary fuel at the precalciner (often 50-65 % of the total, the “decarbonation” fuel) vs the main burner fuel: the balance reports both: the ratio is a design and operating variable of the system, with its effects on the gas temperature, the NOx and the clinker quality, captured in the file tables.

4. The Outputs Sheet: Where All the Heat Leaves the Kiln

The outputs of the kiln balance define the efficiency and the target of every optimization project: the classification of the outputs is standard:

• Theoretical heat requirement: the enthalpy needed to the formation of the clinker from the dry feed: 1700-1800 kJ/kg of the clinker in the modern dry process (the endothermic reaction of the clinker formation): the “useful” component of the whole heat leaves in the clinker chemical composition;
• Heat in the clinker at the cooler discharge: the clinker leaves the cooler at 90-150 °C in the good plants: the remaining thermal content of the product: 100-150 kJ/kg;
• Heat in the exit gas: the preheater gas leaves at 280-380 °C in the dry 5-stage systems: 500-600 kJ/kg at the kiln exit gas: the biggest single recoverable stream: the raw mill and the WHR power plants feed from it;
• Heat in the dust: the kiln dust exits with the gas: its heat and its material: the dust loops return it or the bypass releases it: the accounting of the dust in the exit flow;
• Cooler waste air heat: the excess air after the recuperation leaves at 200-350 °C: 250-400 kJ/kg depending on the temperature, the recovery and the cooler design;
• Radiation and convection from the shell: the heat leaves through the kiln shell, the preheater steel, the ducts, the cooler casing: 50-150 kJ/kg of the typical insulation; the “shell losses” of the audit, measured by the surface temperature and the thermal imaging;
• The total heat: the enthalpy of the process, the summation of the above closed the balance; the missing fraction is the measurement error, the short benchmark of the audit quality (<5 % absolute of the total in the good audits);

Each line of the outputs is a line of the file’s table, with the representative values of the kilns of the world for the 5-stage, 6-stage and the modern kilns: the specific heat of each output stream and the method of each measurement: the table below reproduces the classical figures of a well-driven 5-stage kiln with a grate cooler:

Output stream	kJ per kg clinker	Share of total	Recoverability
Clinker formation (required)	~1750	55 – 60 %	obligatory
Exit gas from preheater	500 – 600	17 – 20 %	raw mill, WHR plant
Cooler waste air	200 – 300	7 – 10 %	coal mill, WHR plant
Radiation + convection	120 – 180	4 – 6 %	reduced by insulation
Clinker heat at discharge	70 – 120	2 – 4 %	return or WHR
Dust, bypass, others	up to 100	1 – 3 %	various

The table is the portrait of one good kiln: the engineer compares his own numbers against these and instantly names the chapters where his plant diverges: the exit gas too hot, the waste air too cold, the radiation high: the numbers of the balance become the prioritized list of the energy projects.

5. The Physics of the Theoretical Heat: The Clinker Formation from the Raw Meal

The heart of the “useful” line of the balance is the theoretical heat of the clinker formation: the fixed arithmetic that the process cannot break, and that determines the floor of the specific heat:

• The endothermic reactions: the dehydration of the clay minerals (100 – 200 °C), the decarbonation of the limestone (750-950 °C, ~1743 kJ per kg printed CaCO₃), the early clinker phases at the solid state; all consume heat;
• The exothermic reactions: the formation of the clinker melts and the belite/alite crystallization (that release), the total exotherm may recover part of the heat consumed in the clay and the limestone stages;
• The resulting net: the well-established net enthalpy of the clinker formation lies near 1700-1800 kJ/kg of clinker (some authors 1750, the variations by the raw material elemental): the “theoretical” specific heat that no plant can cross below by the first law of the process;
• The inverse side: the rest of the heat the plant puts in 2900-3200 kJ/kg ends somewhere that the list of the outputs has named: the difference between the theoretical minimum and the actual is the inefficiency of the process: the modern kilns run 1.55-1.7× the theoretical;

The file gives the computation methods of the theoretical heat from the raw meal composition (the “dry process heat” from the Bogue constituents, the Kumba’s method of the national laboratories) so the audit does not need a reference number by heart: the calculation sheet of the package yields the theoretical heat of the site’s own meal, and the operator watches the gap: the only lever of the plant within the limit.

6. The Mass Balance First: The Kiln System and the Cooler

The heat balance is only as true as its companion mass balance; the audit starts with the mass flow of every stream:

• The raw material flow: the kiln feed (dry matter + moisture + the dust): the basis of the balance, kg/h;
• The fuel flow: the kg/h of the coal/petcoke, the m³/h of the gas at the burner: the measured flow and the heating value of the fuel of the moment;
• The air streams: the primary air, the secondary air (through the clinker bed), the tertiary air (to the calciner), the cooler airs, the leak air: the round-the-plant measured volumes converted to the mass by the temperature and the pressure;
• The product: the clinker out of the cooler, the finished cement vs dust waste: the material balance of the kiln (the dust, the discards) closes within the measurement;
• The water vapor: the moisture of the fuel, the raw and the quench water: part of the stream: its latent heat enters the heat output sheet;

Then the heat balance becomes a summing: the mass × the specific heat × the temperature for the sensible items; the flow × the LHV for the fuel; the heat of the dry air: the file walks the entire example of a 5000 tpd plant, from the flow sheet to the final numbers in a downloadable sheet, and the tables of the stream data are provided in the reference: the balance is a discipline, not a formula: and the discipline begins with the flow measurements that the companion file on Gas Flow Measurement documents.

7. The Cooler Balance and the Recovery: The Secondary and Tertiary Air

The grate cooler is both a heat exchanger and a regenerator of the process air, and its balance is a separate chapter of the audit:

• The inputs of the cooler: the hot clinker from the kiln (temperature 1250-1400 °C, the heat of 1100-1200 kJ/kg), the ambient cooling air (with its humidity), the auxiliary airs (the vents, the trimming);
• The outputs: the secondary air (to the kiln flame, 850-1100 °C), the tertiary air (to the calciner, 800-1000 °C), the cooler exhausts (the waste air, 250-350 °C depending on the cooler stages), the clinker at the cooler discharge (100 + 40 °C excess), the radiation of the cooler casing;
• Recovery efficiency: the ratio of the heat returned to the process (secondary + tertiary) to the total air heat: the modern reciprocating and the cross-bar coolers reach 62-72 %, the older grate coolers 50-60 %;
• The parameters: the clinker bed height, the grate speed, the under-grate compartment pressures and the recovery: the “specific cooling air” of the cooler (1.5-2.5 Nm³/kg cl) and the exit clinker temperature correlate;

Cooler type	Heat recovery share	Specific cooling air, Nm³/kg cl	Clinker discharge temp	Specific power
Rotary (old)	45 – 55 %	2.5 – 3.5	200+ °C	6 – 9 kWh/t
Reciprocating grate	60 – 68 %	1.8 – 2.4	120 – 160 °C	8 – 12 kWh/t
Cross-bar (modern)	65 – 72 %	1.6 – 2.0	80 – 120 °C + excess	10 – 13 kWh/t

The cooler audit of the file measures the temperatures and the airs of the compartments, computes the secondary and tertiary recovery, and compares with the design numbers: the modern cooler audits also combine with the kiné of the nozzle clogging, the air distribution, the gap sealing — the mechanics that rob the heat balance of its recovery stated in the mechanical chapters of the package.

8. The Bypass, the Dust and the Unorthodox Heat Flows: The Barely-seen

Every real kiln carries the flows that the textbook balances silently ignore, and the honest audit accounts or blesses them:

• The kiln gas bypass: (3-9 % of the gas, in the kilns of the alkalis) takes a fraction of the hot gas with its heat and its dust: the balance subtracts the bypass flow from the main gas and accounts its heat content (a large extraction: 60-80 kJ/kg cl kept out of the system):
• The dust to the filter: the preheater dust (2-15 % of the feed mass) carried to the baghouse: its heat (the gas exit t°),its return to the main system or the discarding: the dust loop of the kiln (the recirculation in the cyclones) changes the effective feed:
• The coal moisture and the quench: the water of the fuel, the injection of the quench at the cooler/kiln blowers: the latent and the sensible effects on the cooling;
• Scale accumulation on the transfer: the changes the heat transfer, the surface temperature and the radiation of the heat exchanger: the audit compares the running with the clean design; the preheater cleaning is reflected by the balance at the next audit:

The treatment of the minor flows by a rigorous file: at first the audit gets the whole of the picture: then the sensitivity of the balance to the bypass amounts (the closure percent) is displayed so the director reads which numbers decide the interpretation: the discipline of the balance closes the gap with the short explanations in the footer of the sheet.

9. The Energy Audit of the Kiln: From the Measured Data to the Savings Plan

The balance leaves the desk and walks the plant when it becomes the template of the energy audit of the kiln:

• Data collection day: the measured flows 24 h: the fuel, the feed, the power, the temperatures, the hourly data registration; the measured LHV on the lab sample;
• The calculation plate: the runs the balance with the Excel sheet of the file: the specific heat (kJ/kg), the outputs sheet, the closure check;
• The comparison against the benchmark: the same capacity, the same process at 3100-3300 kJ/kg vs the current: the gap named numerically per stream;
• The priority list: the items in the order of the recoverable amount and the cost: exit gas recovery (raw mill or WHP), cooler waste (WHP, coal mill), shell insulation, clinker transport heat, the dust, the stable operation (the campaigns): the file provides the evaluation form of each;
• The savings accounting: kg coal per kg cl × the tons/year × the coal price: the savings in MWh and the currency: the management accepts the project list on this table alone;

The table of the energy audit below is representative of what the majority of plants measure; every user looks for its own numbers in the file: the specific heat of the kiln (net, the producer-side) vs the number of stages and the efficiency of the cooler:

System	Typical specific heat kJ/kg	Exit gas °C	Cooler type
Wet process kiln	5600 – 6800	200 – 300 (after wet)	Rotary / grate
Dry Lepol grate kiln	3800 – 4600	250 – 400	Reciprocating
Dry 4-stage	3200 – 3600	360 – 420	Reciprocating
Dry 5-stage	3000 – 3300	320 – 360	Cross-bar
Dry 6-stage	2900 – 3150	280 – 320	Cross-bar

The same table behind the file: the whole world of the specific heat with the benchmark: the plant started from 3500 on its way to 3100 has the chapters to read and the projects to run: the audit converts the engineering into the P&L of the plant.

10. The Operation of the Heat Balance: The Daily and the Monthly Routine

The professional heat monitoring is a calendar discipline, not a one-off visit:

• The daily seven: the fuel flow × the LHV, the feed, the temperatures (kiln gas, exit gas, cooler): the daily specific heat with the daily clinker: the trend line of the month:
• The weekly cooler audit: the air of the compartments and the clinker temperature at the gates: the clinker 1:1 (the recovery trend, the wear of the grates):
• The monthly complete: the full balance from the weekly data plus the laboratory: the monthly report of the specific heat and the waste streams by the month:
• The annual external audit: an independent full mass/heat balance with the flue gas analysis and the isokinetic: the official number of the year:
• Reporting format: the temperature, the reports to the management read: the specific heat, the recovery, the losses: the traffic light table and the line: the carbon footprint (CO2/t):

The daily discipline is what separates the plants that “lose the trend” from the plants that “see the drift early”: the drift of the specific heat by 30 kJ/kg means the refinery, the moisture of the raw, the burner tip, the refractory: early seeing of the profile the file keeps: the monthly shape is the narrative of the operation, and the sheet makes the narrative one glance.

11. The Related Topics: The Auxiliary Heat Exchangers of the System

“The related topics” in the title of this reference: the heat balances of the companion equipment that completes the picture:

• The heat balance of the raw mill / dryer: the exhaust gas heat to dry the raw: the gas 380 → 90 °C, the water evaporated, the recirculation: the drying balance closes the raw mill audit;
• The heat balance of the coal mill: the hot gas dries the coal: the balance sets the coal mill drying requirements and the safety temperature limits (the ignition/whe point of the coal dust):
• The heat balance of the kiln shell cooler: the heat exchange across the refractory, from the charge to the shell: the thermal profile of the shell and the hot spots (alignment, coating dynamics);
• The waste heat recovery unit: the waste heat boiler after the kiln: its balance (and the power produced) removes the “losses” from the waste streams: a new chapter of the modern plant portfolio;

The related balances follow the identical discipline of the main balance: the inputs, the outputs, the closure, the errors: the file devotes a section to each and draws every sheet in Excel so the engineer copies the row structure and replaces his own numbers: the audit of the “whole plant heat economy” is the sum of the chapters: the leader of the optimization turns each chapter into an action item: the balance is the mastermind of the energy plan.

12. The Instrumentation the Balance Depends On: The Measurement Quality

Between the kiln and the balance sheet sits the measuring instrument, and the quality of the balance collapses if the measurement chain is weak: the file lists the required instruments and the acceptance quality per stream:

• The fuel metering: the coal weigh feeders (accuracy ±1 % long term), the gas meters (the turbine or the orifice), the waste fuel mass: the core of the input, highest priority of the calibration:
• The temperature measurements: the gas temperatures of the tower, the shell surfaces (thermal camera pyrometer), the material (the clinker: the infrared after the cooler): the poor contact measures yield 20-50 kJ/kg of data error each;
• The flow measurements: the air of the cooler and the preheater (the venturi, the annubar), the pitot traverse audits once: the fan testing is the source of the air data: their ±10 % produces the ±2-3 % of the balance items;
• The lab data: the LHV and the ultimate analysis of the fuel, the moisture of the raw feed, the chemical of the meal (the theoretical): the lab quality is part of the measuring chain:

The rule of the uncertainty book: the balance is the near-1% of the measuring chain; a chain of independent ±% instruments gives the closure the ±5% power; the modern audit budget communicates the uncertainty of each stream: the file provides the audit check-list the plant runs before launching any balance: the calibration, the spans, the filters the zero adjustments — the prerequisites of the honest numbers.

13. Frequently Asked Questions

What closure percentage should a good kiln heat balance achieve?

With the careful measurements, the closure of the balance (input minus outputs, relative to the input) should be within ±5-10 %; a good audit runs 3-5 %; a figure above 10 % indicates a missing or erroneous stream — a fuel flow error, a missing bypass flow, an unaccounted quench: the check of the audit quality is the small closure it closes with.

What is the largest recoverable waste in a modern 5-stage kiln?

The exit gas of the preheater carries 500-600 kJ/kg of clinker — the largest recoverable stream, over 17 % of the input: its uses: the raw mill drying (the classic), the coal mill, and the wasteful heat recovery power plant: the waste cooler air is the second candidate: the order of size: exit gas > cooler waste air > shell losses.

How much does one lost kilogram of kiln bypass cost?

In the specific heat: each percentage of the treated gas bypassed carries out some 30-60 kJ/kg of the clinker (with the dust the range grows): the 3 % bypass costs the modern kiln the 90-180 kJ/kg, i.e. ~3-6 % of the heat: the decision is made on the alkali and the SO3 control — the balance quantifies the price tag of that control:

Should the heat balance use LHV or HHV?

The cement industry uses the lower heating value (LHV) throughout: the exhaust gas leaves the system above the condensation point so the water vapor leaves without releasing its full condensation heat: the reporting is always identified LHV @; — the HHV must be converted to LHV with the classic formula before any benchmark: the file includes the conversion for the coal, petcoke, the gas and the waste fuels:

How does the cooler recovery affect the specific heat number?

Directly and strongly: improving the cooler recovery by 5 % points dampens the specific heat by about 30-50 kJ/kg cl: the secondary/tertiary temperatures rise, the flame burns the fuel more efficiently, the clinker leaves colder: the cooler balance is the single largest mechanical lever of the kiln’s specific heat and its maintenance is the audit’s first recommendation.

14. Conclusion

Heat balances of the kilns and the coolers: the report card of the thermal efficiency of the cement plant: the inputs and the outputs, the theoretical minimum and the recoverable streams, the exit gas, the cooler waste, the shell: the audit converts the fuel bill into the line items of the projects, and the balance disciplines make the plant keep its place in the benchmark of the industry: the kiln is the largest energy consumer, and its balance is the map of its improvement.

The Complete Cement Technical Package carries this Heat Balance reference with the tables, the worked 5000 t/d example, the Excel calculation sheets of the kiln and the cooler, and the audit check-lists: 931 files, $249.99 one-time via PayPal, the instant download: for the engineer the file is the difference between the mystery of the fuel bill and the science of the savings: the balance calculated, the process improved, the career the package-wide: get the package, run the numbers, save the plant.

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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.

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