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Mill Heat Balance and Water Spray

Almost all the heat in a cement mill comes from the motor, not the clinker. This works out how hot the mill and the cement will actually run, how many litres of water an hour you must spray to hold them where you want them, and whether your gypsum is getting the temperature it needs — too cold and it never gives up its water, too hot and it turns to plaster and your cement sets in the silo. Press YOUR MILL on the board to type your numbers in.

Why this matters now

Clinker reduction, high electricity prices and tighter cement specifications put the grinding circuit under pressure. The fastest gains usually come from finding the real constraint before changing equipment or operation.

What this tool helps decide

Separate grinding heat, feed moisture, air and shell losses so temperature control protects gypsum, cement quality and production rate.

What subscription adds

Build a connected grinding audit: save every mill separately, reload previous measurements, compare changes over time and issue an editable report for the plant team.

Connected to the industry conversation around clinker-factor reduction, grinding energy, blended cement and quality control reported in World Cement, CemNet and Global Cement.See process-tool plans

How hot it runs, and what holds it there

What you want

Degrees C. Most mills are held between 95 and 110 so the gypsum gives up its water without turning to plaster.
Litres per hour, all sprays added together. Put 0 if you spray none, and the board will tell you how hot it runs without it.

The weather outside

Degrees C.
Per cent — the humidity from the weather report.

What goes into the mill

Tonnes per hour, the finished product.
Tonnes per hour.
Tonnes per hour. Everything else is additive.
Per cent.
Per cent.

Material coming back round

If not, the tool works it out from three sieve samples, the same way the Tromp curve tool does.
Per cent retained on the same sieve, from three places. Take all three at the same moment. The product must be the finest and the return the coarsest.

Air through the mill

Almost nobody knows it in kilogrammes an hour. If you do not, give the speed you want the air to move at and the tool works the flow out, the same way the mill ventilation tool does.
Metres per second. Cement mills run 1.0 to 1.5. Below 1.0 the fine material stays in the mill and coats the balls; above 1.5 you drag coarse material out unground.
Metres.
Per cent.
Per cent of that gas that leaks in rather than being drawn in on purpose.

The motor and the shell

Kilowatts at the shaft.
Per cent. About 90 for a ball mill, 75 for a vertical mill. The rest leaves as noise and vibration.
Metres.
Metres.
Kilocalories per square metre per hour. 750 is a normal warm shell.

Temperatures you measured

Degrees C.
Degrees C.
Degrees C. Measure both once and the difference stays much the same.
Degrees C.
Degrees C. Optional. If you type it, the board compares it against what the heat balance says it should be — the gap is heat nobody has accounted for.

What counts as too hot

Degrees C.
Degrees C.
Degrees C.
WHAT YOUR MILL WILL DO
WHERE THE HEAT COMES FROM AND GOES
LIMITS AND CROSS-CHECKS
Mill outlet gas will be
—
Cement will leave at
—
Water to hold your target
—
Water you spray now
—
Gypsum verdict
—
New heat entering
—
Of that, from the grinding
—
New heat per kg of cement
—
Water evaporated in total
—
Brought in by the air alone
—
Rate where cement hits the limit
—
Extra air instead of water
—
Lost through the shell
—
Without any water it runs
—
Measured minus calculated
—

Every stream, in and out

What to do about it

How the numbers are worked out

The balance is your own finish-mill-balance.js, term for term. Heat goes in with the clinker, the gypsum, the additive, the separator returns, the air, all the moisture they carry, the water you spray, and the motor. Heat goes out with the sweep gas, the cement, the water it takes to reach 100 °C, the far larger amount it takes to evaporate, the vapour carrying on heating, and the shell. Nothing is fitted. Every constant is either an exact conversion or a published property: cp air 0.24, cp cement 0.19, cp gypsum 0.22, cp water 1.009, cp vapour 0.45, latent heat 539.1 kcal/kg, 859.845 kcal/kWh.

The motor and the grinding media are not two heat sources — they are one. Electricity goes into the motor, the motor turns the shell, the shell lifts the balls, and the balls fall and rub against the material and each other. That is where the energy becomes heat. The balls make no energy of their own; they carry the motor's energy and convert it. So it is counted once, where it enters, and the board calls it the grinding. Counting the motor and then the media as well would be counting the same kilocalories twice.

New heat, and heat that is only going round in a circle. The separator returns come back into the mill hot and leave again with the discharge at almost exactly the same temperature. Counted as an input they look like about 40 % of the heat, which makes the grinding look far smaller than it really is — but nothing was added by that loop. The board therefore reports the grinding share against the new heat: what the motor puts in, what the clinker brings in above ambient, and the moisture. On the reference mill that moves the grinding from a misleading 43 % to a truthful 72 %. The full gross balance, returns included, is still in the table so you can audit it.

Why this one answers instead of checking. Your file asks for the mill temperature and tells you whether the balance closes. But the balance is a straight line in temperature and a straight line in spray water, so it can be solved rather than tested. Type the temperature you want and it gives the litres. Type the litres and it gives the temperature. The two are exact opposites of each other and are tested that way: solve for the water at a target, put that water back in, and the temperature has to land back on the target.

The air is much wetter than your file assumed. It fixed the moisture in the inlet air at 2.5 g per kg of dry air. Real air at 25 °C and 60 % holds 11.9 g/kg, and at 35 °C and 80 % it holds 28.9 g/kg. On your own sweep rate that is the difference between 149 and 1,727 kg of water an hour arriving for free — over a million kilocalories an hour of cooling you were not counting. It is now worked out from the air temperature and the humidity, using the same ASHRAE saturation correlation that is already in your psychrometrics.js.

Ambient temperature is now an input too, where your file fixed it at 25 °C for the inlet air, the gypsum, the additive and the false air. Set it back to 25 and every one of those terms returns to exactly the value your file gives.

The temperature limits. Gypsum gives up its crystal water between about 95 and 120 °C. Below that it stays as it is and does nothing for your setting time. Above about 120 °C it goes past hemihydrate to soluble anhydrite, which takes water back up again in the silo and gives you false set — cement that stiffens in the first minutes of mixing. Cement to the silo is held to 110 °C. All three limits are on the panel and you can change them.

What this cannot tell you. It assumes the mill is at steady state and that your shell loss rate is right. The shell figure is the softest number in the whole balance — it is an assumption, not a measurement, and on a big mill it is worth a few degrees. If your measured temperature and the calculated one disagree, that is the first place to look.