FIELD OF THE INVENTION
The present invention generally relates to a rotor position sensing device for
leakage control in Rotary Regenerative Air preheater. The present invention
particularly relates to Rotary Regenerative heat exchangers generally used as Air
preheaters and more particularly to a rotor position sensing device for Air
preheaters to measure leakage gap between a sector plate of the preheater and
radial seals of the rotor during operation of the Air preheaters.
BACKGROUND OF THE INVENTION
Air preheaters are used in Thermal power stations / industrial application to
transfer the heat from the flue gas leaving boiler / furnace to the entering
combustion air through regenerative heat transfer surface in a rotor which
revolves continuously through the air and gas streams.
A Rotary Regenerative Air preheater transfers heat from a flue gas leaving a
boiler to combustion air entering through regenerative heat transfer surface in a
rotor which revolves continuously the gas and air streams. The rotor is divided
into compartments by a number of radially extending plates referred as
diaphragms. These compartments are adapted to hold number of baskets which
contain the mass of heat absorbent material commonly formed of stacked plate-
like elements.

In a typical Rotary Regenerative heat exchanger, the hot flue gas and the
combustion air enter the rotor shell from opposite ends and pass in counter flow
directions over the heat exchange material housed within the rotor.
Consequently, the cold air inlet and the cooled gas outlet are at one end of the
heat exchanger, referred to as the cold end, and the hot gas inlet and heated air
outlet are at the opposite end of the heat exchanger, referred to as the hot end.
The air preheater is divided into a flue gas side or sector and one or more
combustion air sides or sectors by sector plates. Flexible radial seals on the rotor,
usually mounted on the top and bottom edges of the diaphragms, are in close
proximity to these sector plates and minimize leakage of air between sectors and
to separate the air and flue gas streams from each other. The radial seals are
fixed in the diaphragm plate radial to the rotor post along with seal holding bar
and is touching the sector plate. Regenerative Air preheater is rotated by the
drive system. During operation of the Air preheater, the rotor turns down axially,
due to this the gap between the sector plate and the radial seals is increased.
Through this gap, the high pressure air leaks to the low pressure gas side. In
order to measure the leakage gap between the sector plate and radial seals, it is
necessary to have measuring means for the air preheater.
The prior art leakage control system involves mechanical movement of gap
sensor and micro switches which are failing frequently due to high temperature
and high ash environment.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a sensing device for leakage
control system of Rotary Regenerative Air pre-heater, to maintain optimal gap
between sector plate of the preheater and radial seals of the rotor.
Yet another object of the invention is to propose a sensing device for leakage
control system of Rotary Regenerative Air pre-heater to maintain optimal gap
between sector plate of the preheater and radial seals of the rotor which is
immune to effects of the air pre-heater environment which is highly dusty and
having temperature in the range of 300 - 500°C.
Another object of the invention is to propose a sensing device for leakage control
system of Rotary Regenerative Air pre-heater to maintain optimal gap between
sector plate of the preheater and radial seals of the rotor.
A still another object of the invention is to propose a sensing device for leakage
control system of Rotary Regenerative Air pre-heater, to maintain optimal gap
between sector plate of the preheater and radial seals of the rotor which
continuously monitors and provides a linear indication of the rotor position which
exhibits improved reliability owing to absence of reciprocating moving parts.

A further object of the invention is to propose a sensing device for leakage
control system of Rotary Regenerative Air pre-heater, to maintain optimal gap
between sector plate of the preheater and radial seals of the rotor, which
eliminates micro switches of prior art and which are prone to fail at high
temperature and high ash environment.
A still further object of the invention is to propose a sensing device for leakage
control system of Rotor Regenerative Air pre-heater to maintain optimal gap
between sector plate of the preheater and radial seals of the rotor, which
provides data relating to the speed of rotation and gives an alarm in case of a
stall condition of the rotor.
SUMMARY OF THE INVENTION
Leakage in Rotary Regenerative Air pre-heater is dependent on the shift in the
rotor position due to rotor turn down during operation. The rotor position is
monitored based on RF admittance measurement. A static rectangular
conductive sensor plate is fixed on one side of the sector plate. An RF signal is
fed to this sensor plate continuously. One more rectangular conductive plate is
fixed on the rotor. The RF admittance at the static rectangular conductive sensor
plate is measured during rotation. The signal is varying in nature depending

upon the position of the moving plate. The signal is processed to sense the gap
between the sector plate and the rotor. This measurement can be used as an
input to a drive panel in the leakage control system to effectively control the gap
between the sector plate and radial seals.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - shows a block diagram of the RF admittance based air pre-heater
leakage gap sensing device according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The leakage gap in the Rotary Regenerative Air pre-heater is measured based on
the RF admittance between two rectangular plates, a static sensor plate [1] fixed
on the sector plate [5] and a second moving sensor plate [2] which is fixed on
the rotor [3] of the rotary regenerative Air pre-heater. When the rotor [3]
moves, the moving sensor plate [2] comes closer to the static sensor plate [1],
and moves away, once in every rotation.
The RF admittance sensor electronics [7] is connected to the static sensor plate
[1] using a steel wire [6] through ceramic beads. The RF admittance sensor
electronics [7] generates RF signal that indicates the RF admittance at the static
sensor plate [1] fixed on the sector plate [5].

The RF admittance signal is further processed in a Display Unit [8] to derive
information regarding rotor position. The Display Unit [8] monitors the RF
admittance signal continuously. The admittance values are captured as the
moving sensor plate [2] rotates along with the rotor [3]. By analyzing the pattern
of variation of the admittance, the time at which the moving sensor plate [2] has
come at close proximity of the static sensor plate [1] is identified. The amplitude
of the signal pattern captured during this period is converted into the gap
between the moving sensor plate [2] and static sensor plate [1]. This gap value
(mm) can be used as a feedback signal to the drive panel in the leakage control
system to suitably adjust the sector plate [5] to limit the leakage gap between
the sector plate [5] and the radial seals [9] and hence reduce the leakage of gas
and air between sectors. The repetition rate of the signal pattern is used to
determine the speed of rotation.

WE CLAIM
1. An RF Admittance based rotor position sensing device for leakage control
system in Rotary Regenerative Air preheaters, comprising two conductive
rectangular plates, one of which is stationary and mounted on the sector
plate and a second passive plate which is fixed on the rotor periphery
wherein an RF signal input is fed to the static sensor plate and RF
admittance is measured in terms of variation of variation of the signal
strength and wherein the pattern of variation is used to assess the
leakage gap between the sector plate and the radial seals.
2. The device as claimed in claim 1 wherein the pattern of variation of the
admittance is analyzed to arrive at the time at which the moving sensor
plate has come at close proximity of the static sensor plate.
3. The device as claimed in claim 2, wherein the leakage gap is identified
based on signal pattern captured when the moving sensor plate comes at
close proximity of the static sensor plate.
4. The device as claimed in claim 3, wherein the repetition rate of the
admittance pattern is analyzed to arrive at the speed of rotation of the air
pre-heater rotor.

5. An RF admittance based rotor position sensing device for leakage control
system in Regenerative Air pre-heater substantially as herein described
and illustrated in the accompanying drawing.

ABSTRACT

The invention relates to an RF Admittance based rotor position sensing device for
leakage control system in Rotary Regenerative Air preheaters, comprising two
conductive rectangular plates, one of which is stationary and mounted on the
sector plate and a second passive which is fixed on the rotor periphery wherein
an RF signal input is fed to the static sensor plate and RF admittance is
measured in terms of variation of the signal strength, and wherein the pattern of
variation is used to assess the leakage gap between the sector plate and the
radial seals.