Mill outlet temperature
The mill outlet temperature is decisive for drying the material to be ground. The mill outlet temperature is
determined by the inherent raw material moisture content and the set value is adjusted accordingly during
commissioning.
The plant equipment, such as dedusting systems, fans etc. following the mill have to be designed to withstand
the mill outlet temperature.
Too high mill outlet temperature is not dangerous only for bearing and seals in the grinding rollers but also for
the plant equipment following.
Too low mill outlet temperature results in inadequate drying, has a negative influence on the grinding process
and runs a risk of a temperature fall below the water dew point. The mill outlet temperature must be held
constant.
Limit values
MIN Not enough heat is available and the grinding process can no longer be
guaranteed. If the heat source to the mill is already at maximum, the material
feed to the mill must be reduced.
MAX 1 Warning
MAX 2 Hot gas supply –OFF and the stopping or switch over sequence is initiated.
Measurement and control loop
The mill outlet temperature is measured in the duct at the mill outlet.
The following regulating units could be used to control the temperature.
1) A burner of an air heater
2) A damper in the cold air duct
3) A damper in the hot gas duct (Influence the heat flow)
4) A speed controlled booster fan
5) A control valve with solenoid valve and nozzle arrangement at the cooling tower.
6) A combination of the above mentioned arrangements is possible.
The control system is only operative while the mill is running. Manual control is also possible.
Certain process situations may arise whereby the regulation unit is not in a position to retrieve the set
point. For this reason limit monitors are installed for alarm purposes and to take the necessary steps in
the process control logic to ensure the plant equipment is protected accordingly.
Mill inlet pressure (for vacuum pressure driven plants)
The criterion that a medium flows between two points in a system is that a differential pressure exists. The
static pressure at the mill inlet as opposed to atmospheric pressure is a point for developing a pressure
difference to a previous system.
The mill inlet pressure depends on the air volume rate of flow in the plant, developed by the mill fan. For this
reason the mill inlet vacuum pressure is responsible for drawing process gases into the system. For grinding plants which operate in compound with clinker producing plants, the mill inlet region is the interface between grinding plant and kiln.
The difference in pressure between the mill inlet and the kiln outlet ensures that the kiln waste gases are drawn in for the dry grinding process.
In plants where the kiln waste gas volume flow is too low that the grinding plant has to be run with
recirculation gases, the mill inlet pressure is decisive for guaranteeing the flow of recirculated gases.
Depending on operating conditions this recirculating gas may be supplement with fresh air.
In central grinding plants the mill inlet pressure influences the supply of gas from either of the following:
1) hot gas generator
2) fresh air supply
3) Recirculation gas supply
4) combination of the above (depending on the plant design)
In all of above mentioned systems the mill inlet pressure is a vacuum pressure.
The mill inlet pressure is maintained constant.
The regulation unit for the mill inlet pressure could be
control damper in the recirculation duct
control damper in the exhaust stack
fresh air damper in the mill inlet duct
damper or van controller from a fan parallel to the grinding plant
Measurement and control loop
The mill inlet pressure signal is transformed into a standard 4 – 20 mA signal. Should this signal be required to be electrically damped, this may take place during commissioning. The pressure control loop only operates during mill operation. The controller (PID) may be switched to manual at any time.
Certain situations may arise wherby the regulation unit is not in a position to retrieve the set point. For this
reason limit monitors are installed for alarm purposes. It is not normal to take any action in the process control logic.
Limit values
MIN 1 warning MAX 1 warning
MIN 2 alarm MAX 2 Alarm
Mill air volume flow
The volume rate of flow of gas through a mill per unit time influences the grinding, classifying and dedusting process in a grinding plant and for this reason must be held constant.
The volume rate of flow per unit time depends on the design and capacity of grinding plant
In general:
During the normal grinding operation, the volume rate of flow must be held constant according to the design criteria independent of pressure and temperature fluctuations in the system.
Measurement and control loop
The volume rate of flow in a grinding plant is determind by a venturi tube or an orifice plate situated after the pre-selection or dedusting system accordingly. Certain plants, because of their general arrangement, prehibit such measurement. An alternative to this would be summing two separate measurements made in the exhaust stack and recirculation duct.
The differential pressure measurement taken from the venturi or the orifice plate is transformed into the standard 4-20 mA signal proportional to the flow and is required for the control loop. This signal must be damped electronically and will be set accordingly during the commissioning.
The following regulation units could be used to control the flow
1) A damper at the fan inlet
2) A vane controller at the fan inlet
3) A variable speed fan drive motor
4) A combination of damper or vane control with the variable speed fan drive motor
For 1) and 2) a three point PID controller is required
For 3) a continuous PID controller is required
For 4) a three point stepping and a continuous controller or a multifunction controller is required
The mill air volume flow control loop is only active during grinding operation Manual control is also possible.
Certain process situations may arise whereby the regulation unit is not in a position to retrieve the set point. For this reason limit monitors are installed for alarm purposes and to take the necessary steps in the process control logic to ensure the plant equipment is protected accordingly.
Limit values
MIN 1 at app. 75 % of rated flow : warning
MIN 2 at app. 70 % of rated flow : switch off or switch over the grinding plant
Loesche documentation
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