Preheater process fan is used in cement process as product separating device for
collection of fine dust particle from process air/gases as a finish product or to
restrict dust level emission from exhaust gases. Preheater Fan is consuming highest
power in pyro circuit.
There is significant loss of power in fan operation either due to overcapacity
which may occur due to either design margin or process optimization and using
dampers in air / gas ducts.
2.1 Process Operation Requirement
As per the process requirement, at the time of kiln start up, firing required induced
draft for which preheater fan to be run at low RPM. This can be achieved by
putting GRR by minimum 40% RPM, in which power loss will be higher for initial
7-8 hours. Hence it is always beneficial to go with MV/ VFD drive, losses will be
less and process control will be better.
Preheater fan is driven by AC power induction motors. The induction motors used
may be squirrel cage or slip ring type.
following starting methods of speed regulations are commonly used for induction
motors as under:
- Slip ring motor with SPRS + GRR
- Squirrel cage induction motor + MV drive
- Slip Ring Motor with SPRS + GRR
With SPRS + GRR combination, we can start fan with Initially by GRR and control
at lower speed of fan & after 60 % RPM its can bump less changeover from GRR
mode to SPRS control, above 95% speed again, changeover from SPRS to GRR to
be done due to limitation of SPRS design.
Slip Power Recovery System (SPRS) is a variable speed drive for slip ring induction
motors. It recovers and delivers the slip dependent rotor power from the motor to
the grid. In Slip power recovery system, instead of wasting the slip power in
the rotor circuit resistance, it is converted to electrical power of 50Hz AC and
pumped back to the electrical line. With this method, regulation of the speed
from 60% to 95% is achieved. The power recovery is achieved about 10% to 15%
with speed regulation.
The slip power recovery (SPR) drive is an external system connected to the rotor
circuit in place of the external resistors. The SPRS provides speed and torque
control like the resistors but can also recover the power taken off the rotor and
feed it back into the power system to avoid energy waste.
SPRS with GRR starters for existing plants process fans running with GRR starters
with slip ring motor with following advantages