Note: Descriptions are shown in the official language in which they were submitted.
- ,
10404S7
This invention relates to gas sampling devices
and more particularly to gas sampling devices incorporating
means for the analysis of at least one gas.
In the analysis of gaseous mixtures for net oxygen
content it is known to aspirate or by-pass a sample of gas
for analysis by a solid-state electrochemical sensor in
a known temperature environment generated by an electric
furnace. The disadvantages associated with small diameter
aspirators (sample line blockage and aspirator failures)
can be overcome by connecting a large diameter gas by-pass
means to the source to be monitored. However, such by-pass
means have the disadvantages that (1) the sensor is arranged
in the gas flow and is therefore subject to particle
deposition, damage and wear, and (2) a large furnace is
required to heat the gas in the by-pass resulting in
distortion of the by-pass pipe leading to gas sealing
problems. Furthermore, since the furnace must surround the
pipe, it is difficult to remove for maintenance.
The present invention therefore provides a gas sampling
device comprising a sampling tube (as hereinbefore defined),
said tube being open at one end and closed at the other end,
means for sealingly supporting a gas sensor within said tube,
sample heating furnace means surrounding said tube, said tube
being adapted for external connection at said open end to a gas
bypass, aspirator or other flowing gas source such that said tube
is open to said gas source at said open end and the longitudinal
axis of the tube is perpendicular or up ~ 30 from the perpen-
dicular to the direction of flow of said flowing gas source to be
sampled, whereby gas flows from said source into and along said
tube and out of said open end of said tube back into said source.
-- 2
~04Q457
The term "tube~ is intended to embrace a hollow
body having a passage other than cylindrical in configuration
although a cylindrical tube is preferred for practical
reasons.
Preferably the tube is closed at one end by a sealing
nut which sealingly supports said gas sensor while a
connecting flange is secured at the other end of the tube.
-'~ Alternatively, the end of the tube is closed by a plate
and the gas sensor passes through the wall of the tube so
as to be transverse to the sample flow.
The furnace means preferably comprises a heat
resistant tube which is a sliding fit with respect to said
sampling tube, said heat resistant tube acting as a former
for an electrical winding which acts as the heat source
of'the furnace. Preferably, a thermocouple receiving tube
is arranged between the winding and the heat resistant
tube. The furnace means is preferably surrounded by an
i insulation filled casing while the whole assembly is
'i protected by a loose cowling member having sufficient air
space to promote adequate cooling of the device.
In use, the gas sampling device is connected to
the source, e.g. a by-pass pipe, with the axis of the tube
transverse, preferably vertical and perpendicular, to the
direction of flow of gas. However, offsets of up to 30
can be tolerated. The gas flow creates eddies at the base
:,
.. . .
1040457
of the tube and the eddies cause the transfer of part of
the gas flow into the tube and out again so that a
continuously changing sample is presented to the sensor.
Since the sensor is not in the direct gas,stream it does
not suffer the problems of direct impingement of solid
particles. Furthermore, since the sample flow within the
tube is slower than the gas flow in the source, the heating
requirements of the furnace are far less onerous. Similarly,
the detachable nature of the preferred furnace makes
maintenance easy while the "stand-off" orientation of the
tube allows easy access for maintenance and covering for
protection from the working environment.
Preferred forms of the invention will now be
described with reference to the accompanying drawings in
15 which: ',
; Figure 1 is a cross-sectional elevation of the
sampling device attached to a by-pass or aspirator pipe, and
Figure 2 is a similar cross-sectional elevation in a
more schematic form of another sampling device having an
alternative positioning of the sensor.
In the embodiment of Figure 1 the analyser comprises
a tube 1 about 38 mm in diameter and 310 mm long attached
as shown to a by-pass or aspirator pipe P., A gas sensor 2
is supported in a gas-tight manner by a gland nut 3 to which
a heat sink 4 may be attached if required for dissipating
heat from the nut 3.
-
104~)457
The sensor 2 is a known electrochemical cell
comprising a closed end oxygen ion conducting zirconia
tube coated on both surfaces with platinum electrodes.
Since the outside surface is exposed to the sample gas
and the inside surface exposed to the ambient air, the
oxygen partial pressure of the sample gas can be calculated
from the voltage generated by the cell making due allowance
for the temperature of the cell measured by a thermocouple
arranged in the cell.
~- 10 A gas sample heating furnace 5 comprising an
alumina tube 6 around which is wound the furnace wiring
element 7 is slidingly fitted over the tube 1. The element
7 also surrounds a small alumina tube for receiving a
thermocouple (not shown) for monitoring the furnace
, 15 temperature. The furnace 6 is enclosed by a casing 8
- filled with insulation I and which forms a~ assembly with
the furnace 6. The assembly is held in place on the tube 1
by a collar 9 receiving a grub screw. Thus, by removing
the heat sink 4 and the collar 9, the furnace/casing
assembly may ~e readily slid off the tube 1 for servicing
or replacement.
A cowling 10 surrounds the furnace assembly to
protect the device against its working environment. The
cowling lO has a loose fitting cap to facilitate adequate
cooling of the furnace assembly.
'`
~ - 5 -
16114~457
In use the tube 1 is attached such as by the
fittings shown on the by-pass or aspirator pipe and part
of the gas flowing therethrough is induced into the tube 1
by means of the eddies in the gas flow at the inside
S surface of the pipe, as previously described. If desired,
a sample scoop 11 comprising a rectangular gas flow
disruptor may be arranged in the pipe near the opening to-
the tube 1 to ensure that an adequate sample flows into the
tube 1. In the tests conducted to date, it has been found
that the device works well without the sample scoop.
The above described embodiment has been used with
success on a 400 t/day limestone burning kiln. The kiln
is heated with coke ovens gas and fuel oil and the analyser
;~ is mounted as illustrated in Figure 1 near one end of a
40 mm internal diameter pipe which penetrates about 2.5 m
into the feed or back end of the kiln at the opposite end
to the burners. The sample including lime dust is
extracted using an air aspirator attached to the analyser end
of the pipe and samples combustion products at the rate of
about 100 litres/minute from a position inside the kiln,
well removed from the open ring seal at the end of the kiln
to avoid sample contamination by ingressed air. The sample
is not filtered. The analyser responds to 90% of change
in eight minutes. The analyser requires minimal maintenance -
every three months and the main sample pipe requires cleaning
- 6 -
; 1~4V457
out every four weeks. The analyser does not have to be
protected during this operation and no sample scoop is
fitted.
In the alternative shown schematically in Fig. 2,
S which alternative has been successfully used to monitor
oxygen content in a pulverised fuel boiler, the sensor 2 -
is arranged transversely to the flow of sample gas in
the tube 1, the end of tube 1 being closed by a plate 12 as
shown. The sensor 2 is housed in a tube 13 which passes
through tube 1 with the top half of the portion passing
`', through tube 1 cut away so that the lower portion 14 acts as
a shield. In all other respects the sampling device is the
same as in the previous embodiment.
In use,the main sample ~ipe P is connected between the waste
lS heat recovery system and the inlet to the precipitator.
This 40 mm pipe has a large pressure drop along its length
. .
ensuring high volume sample flow. The sample does not need
conditioning and all the fly ash passes through the pipe with
~ the sample. The analyser is mounted at right angles to this
- 20 pipe and requires minimum maintenance every twelve months.
The response time of the analyser to 98% of change is about
- three minutes. A sample scoop 11 is fitted.
If desired, the plate 12 may be replaced by a plug
; having a fitting for the extraction of part of the sample
from the tube 1 or for the introduction of a calibration
~ , ,,
~,:
-
~(94~4S7
sample. Alternatively the fitting may be used to allow
a small degree of aspiration of the sample in the tube 1
to speed the analyser response where this is justified.
The two forms of the invention described above
have been thoroughly tested and the following table shows
the response times obtained for different flow rates.
In each case the response time is to 98% of the change
and the sample gas temperature was about 25C. The flow
rate in the main sample pipe is measured at the base of
the analyser.
TABLE
Response Flow Rate
(Minutes) Fig. l Analyser Fig. 2 Analyser
(litres/minute) (litres/m
4 450 500
6 250 375
8 lS0 325
100 250
, .
The response time to 90% of change is about half that
to 98% at all flow rates. The fitting of a sample scoop
reduces the response time of the Figure 2 analyser to about
half that without the disruptor and the Figure 1 analyser
to about two thirds that without the disruptor. This
means that the response time of a Figure 1 analyser to 90%
of change with a flow disruptor in a sample gas flow at the
base of the analyser of 450 litres per minute is about
one minute.
1~4~457
~ It will be appreciated that the above response
times are relatively slow compared to other analysers.
However, the analyser is able to handle sample that other
analysers cannot handle. Furthermore, the response times
are generally compatible with high energy combustion
systems. Blockages occur always (in the practical
experience to date) in the main sample pipe which is easily
~ cleaned out without removal of the analyser.
,,. :
. .
:;
