mill ventilation measurement

8/17/2019 Mill Ventilation Measurement

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MILL VENTILATION MEASUREMENT

- The ventilation of a ball mill is extremely important and this for several reasons: removal of the very

fine particles, dedusting of the internal equipment, keeping an acceptable temperature, avoiding

coating on balls and/or linings, better drying of the material for the raw

mills.

- Then, it is important to know the quantity of air or hot gas inside the mill.

- Four calculators are available to determine the required ventilation:

For cement mills:

1) Mi ll Venti lation and Mi ll Fan Sizing:

http://www.thecementgrindingoffice.com/millvent.html

2) Heat balance of the cement mill (Calculation of the water injection required)

http://www.thecementgrindingoffice.com/termbalwater.html

3) Heat balance of the cement mill (Calculation of the cement temperature at mill outlet)

http://www.thecementgrindingoffice.com/balancetemp.html

For raw mills:

4) Calculation of drying capacities for raw mill

http://www.thecementgrindingoffice.com/drying.html

- Determine the measurement point(s) in the circuit: The next step to ensure that the ventilation is the right one is the checking on site. The flow

measurement points depend of the configuration of the grinding circuit. Generally, the best points are

in a place where there are no elbows or other connections. Finally, where the flow of air is the least

disturbed.

In fixing the measuring point, the following recommendations should be observed:

1) Constant shape and size of the pipeline

2) Length of the duct segment before the measuring point higher than 5D (D = duct diameter)

3) Length of the duct segment after the measuring point higher than 4D

See the figure below:



Usually, the measurments are taken in the vertical duct at mill outlet and in the duct after the fan.

Other points of the circuit can also be measured in case of necessity like the separator ventilation.

Concerning the raw mill, we have to consider the problem of the high dust loaded pipelines and the

measurements points must be well-chosen.

The duct is often circular, but also may be square or rectangular.

- Location of measuring points in the plane:

Hydraulically smooth ducts don't show uniform velocity over the whole cross-section. For this reason,

the measuring section (plane) is divided into circular rings of e qual area or according the

Log. Tchebycheff method.

See both figures below:



Or use the table before which gives the number of points in relation with the duct diameter:

The result will be the average of all measurements.

- Pitot L tube:

The measuring device used is the Pitot L tube.



A Pitot tube is a small tube with its open end aligned into the flow so as to sense the full impact

pressure of the flowing fluid. The tube is also provided with static pressure holes.

The Pitot L tube gives both total and static pressures.

Nowadays, there are connected with a digital manometer.

The lecture can be the pressure and the velocity of the air. One can often choose the unity (English

or Metric).

For further details, you must refer to the ISO 10780 standard (Stationary source emissions --

Measurement of velocity and volume flowrate of gas streams in ducts)

Here below, a representation of the Pitot tube operation:

- Equations used to know the air quantity (Bernoulli):



where: ρdry air is the dry air density in kg/m3, Find P:

P is the air pressure in Pa in function of the al ti tude, Alti tude 0 m

R is a specific gas constant for dry air = 287,05 J/(kg.K) and P (air pressure) 101325 Pa

T is the air temperature in °K.

Find ρdry air:

Temperature 0 °C

Air density 1,2923 kg/m3

where: V is the air velocity in m/sec

p is the pressure given by the Pitot tube (dynamic pressure) in Pa and

ρdry air is the dry air density in kg/m3.

Find V:

P (Pitot tube)0

Pa

Velocity 0,00 m/sec

where: Q is the air quantity (volumetric flow) m3/h, 10

S is the section (area) of the duct in m2 and 15

V is the air velocity in m3/sec. 20

25

Find Q: 30

Duct diameter 0 mm 35

Air flow (Q) 0 m3/h 40

45

- Air quantity inside the ball mill: 50

False air % (at mill outlet)

Mill ventilation 0 m3/h

- In function of the results of the air flow measurements, the staff has to take the following actions:

1) Comparison with the theoretical need of the ball mill

2) Calibration of measuring devices in the control room

3) Calibration of the damper fan opening

4) Adjustment of the fan damper

5) Check up of the mill outlet if necessary (to decrease the false air)

6) Increase the fan capacity if necessary

End of the presentation

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mill ventilation measurement

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