Note: Descriptions are shown in the official language in which they were submitted.
AIR PULSE FAILURæ DETECTOR
The present invention relates to a solid state
pulse air flow sensor suitable or use in reverse pulse air
dust collectors.
Reverse pulse air dust collectors are used in a
broad range of air pollution control and particulate recovery
applications. In many industries, process gases laden with
dust must be cleaned before being released to the atmosphere.
One method used to clean the gases is to pass them through
filter bags. The bags trap the dust particles, and the dust
particles accumulate on the outer surface of the bay~ Once
a dust cake has been formed on the filter bags, the cake must
be periodically removed in order to preserve the eficiency
of the dust collector. A common method of removing the dust
cake is by a controlled blast of compressed air.
When the controlled blast of compressed air is blown
into the filter bag, the sudden release of energy into the
filter bag causes it to instantly expand to its maximum size
causing the dust to be thrown off the outside surface of the
filter bag. When the cleaning energy from the air pulse is
spent, the filter bag returns to its normal filtering position,
while the dust precipitation from the filter bags falls to a
collection hopper.
Should the air pulse not operate properly, serious
problems can occur. The ilter b~gs can become so laden with
dust that the process gases will no longer be able to flow.
1--
. ~
As a resultt the process gases must be discharged directly into
the atmosphere, resulting in pollution and eventually the shutdown
of the operation to allow for repair of the system. Such problems
would occur if the air pulser does not pulse, or if it is
continuously discharging air into the filter bags.
The present invention relates to a solid state device
which will detect if the air pulse has failed, or if there is an
air leak resulting in continuous air flow In addition, the
present invention is small enough in size to conveniently fit in
the compressed air manifold of a reverse pulse air dust collector.
The sensor is capable of continuous operation, is easily installed
and callibrated, and is sensitive to short air bursts in the
order of 50 milliseconds to 250 milliseconds, occuring once every
two seconds or at any other longer predetermined time interval.
secause dust collectors operate under a wide range of
process conditions, the cleaning or pulsing parameters must also
be variable. The cleanin~ air is normally under a pressure of
between 60 PSIG to 130 PSIG. The high pressure air pulse duration
is normally between 50 msec. to 250 msec.. The high pressure air
pulsing rate is normally within the range of l pulse every two
seconds to l pulse every 600 seconds. In addition, the temperature
o~ -the pulsing air can vary between -20C to ~0C, depending on
the application.
In the present invention, the sensor which will be
used to detect the presence of an air pulse will be a thermistor,
operating in the self heat mode, and having a mass such that its
resistive change, caused by the cooling effect of moving air,
--2--
will be no less than 2 to 1 in 50 msec.. A thermistor will be
disposed near the outlet of a blower so that the air pulse will
blow directly upon it. If the thermistor is placed in the path
of the cleaning air pulse, it will be cooled by the air pulse,
and accordingly, its resistance will change due to the temperature
change. The resulting change in resistance creates a voltage
change, which can be used to give an indication of the air pulse.
The supporting circuitry will use this voltage change to determine
i~ there is no air pulse, or if there is continuous air flow,
or if the timing of the air pulse is incorrect. The circuitry
incorporatlng the thermistor sensor unit must attempt to maintain
a constant level of current flowing through the sensor. Current
flowing through the thermistor must also provide enough energy
to heat the thermistor above ambient temperature, viz, the
thermistor must be in the self heat mode. As the thermistor is
internally heated, any small fluctuation in ambient temperature
will not give a false indication of the desired air source.
In aeeordance with the present invention, there is
provided an air pulse failure detector eomprising an air puffer,
a sequencing circuit to activate the air puffer at predetermined
times, sensor means responsive to the presence of an air puff,
a control eircuit which receives information from the sequeneing
eircuit and from the sensor to determine whether an air puff has
not occurred at a predetermined time, and an alarm circuit receiving
information from the control circuit and capable of causing an
alarm to activate when an air puff has not occurred at a pre-
determined time.
--3--
Thermistors have characteristic dissipa-tion constants
and time constants. If the current flowing through the thermistor
is known, the temperature oE the thermistor can be found using
the mathematical relationship between these constants. In
operation, once the thermistor has reached a stead~ state internal
temperature, and a cleaning air pulse is introduced, the
thermistor will cool. A sequencer circuit can activate a solenoid
causing the air pulse blower to operate, while at the same time,
sending a signal to a control circui-t which would cause a timer
to start. The control Gircuit would then determine if a second
signal were received as a result o~ the air puff acting upon the
thermistor. If such an air puff is presentr a second signal
will be received which can cause the timer to stop and reset.
Where the timer does not stop, the timer will time out and the
control circuit will then send a signal to an alarm circuit.
If an air puff is present, no alarm signal will be sent
out. However, if the air puff is not present, the timer will
time out and the alarm circuit will be activated~ If there is an
air leak such that the air puffer is continuous]y blowing air, the
therm.istor will not be able to heat up and prepare itself for a
sudden change of air flow. Accordingly, no signal would be sent
back from the thermistor circuit to stop and reset the timer thus
causing the timer to time out and an alarm to sound. In addition,
if -the air puffer is not activated at predetermined times,
determined by the sequencer which causes the timer -to startl the
cooling and heating up of the thermistor will not be in
synchronization with the sequencer circuit, and accordingly, the
timer will not be stopped, causing -the alarm circuit to be
activated.
Reference will now be made to the accompanying drawings.
Figure 1 shows a typical reverse pulse air dust collector
in the filtering position.
Figure 2 shows the same reverse pulse air dust collector
in the cleaning position.
Figure 3 is a block diagram of a preferred embodiment
of the present invention.
Figure 4 shows a thermistor operating in the self heat
mode.
Referring to the drawings, Figure 1 shows a typical
reverse pulse air dust collector operating in the filtering
position. Air to be filtered enters into the dust collector at 9
in the direction of the arrows. The dust laden air must pass
through filter bag 5 which is supported by cage 6. The filtered
air is passed up through venturi 3, outside secondary nozzle 2,
and exits at 10. Filter bag 5 will eventually build up a cake of
dust on its outside.
~0 Referring to Figure 2, in order to periodically clean
Eilter bag 5~ an air blast is introd~lced at 8 in manifold 1 through
pximary nozzle 4 into the interior of filter bag 5. The air blast
causes ~ilter bag 5 to expand rapidly, thereby loosening the
caked d~lst and causing it to fall in the direction indicated by
arrows 7 to a hopper. When the cleaning air blast is ended,
the filter bag 5 will return to its normal filtering position,
and after the dust precipitation has settled in the hopper, the
-5
7~
dust collector can again be operated in the filtering position.
Referring -to Figure 3, a sequencer circuit would be
used to control the timing of the air pulses. The sequencer
circuit could be adjusted to periodically send a signal to a
solenoid which would activate the air pulser. This sequencer
circuit would preferably be adjustable to send a signal from
once every two seconds to once very 600 seconds. The signal
sent would cause the air pu~fer to puff for a duration of between
50 msec. and 250 msec..
Located in the manifold 1 or primary nozzle 4 would be
a thermistor and printed circuit boards containing a control
circuit and timer circuit. The control circuit would maintain
the self heat current in the thermistor, and detect -the sudden
chan~e in resistance of the thermistor caused by an air puff~
The control circuit would also receive a signal from the
sequencer circuit causing a timer to start when the air puff
is supposed to have occurred. The signal received from the
change of resistance of the thermistor would cause the timer to
stop and reset. Where the timer stops and resets; the system
is ready for the next air puff. However, if the timer is not
stopped, it will time out and cause a si~nal to be sent to an
alarm circuit.
Referring to Figure 4, thermistor 25 is shown in the
self heat mode. Typical values are V=12 volts dc~ Rl-5.1 k ohms
R2=110 o~ls, R3=230 ohms, and thermistor 25 = 1 x 1040hms. A
typical transistor T would ~e 2 N 4123.
--6--
The timîng portion of the control circuit would be
a typical timer, and the control portion could be an AND gate
or of any other known configuration. The alarm circuit could
be a bell or light or other type of alarm.
The preferred embodiment of the present invention as
shown in the drawings is by way of example only, and is not
intended to limit the scope of the invention, which is defined in
the claims.
--7--
