IRIS S550, Applications Manual, Page: 8 / 19

The S550 and S552 respond to flicker frequencies
16 Hz and above. They ignore the lower
frequencies so it is important to sight the viewing
head on the highly turbulent portion of the flame
that contains these higher frequencies. One can
predict where these higher frequencies will be by
examining the burner with regard to where the
flame envelope begins and where the turbulent air
enters this flame. This spot can be chosen by
examining the drawings of the burner but it is
better to do this empiri cally by using a swivel
mount on the viewing head.
UV DETECTOR
The S550 viewing head with its dichroic mirror
and UV tube have a combined spectral response of
190-215 nm. The UV tube is inher ently a pulsing
device, that is, the power supply has been
designed so that the tube emits pulses when UV
radiation is present. Its output is pulse stream of
randomly spaced pulses whose average rate is
propor tional to the UV radiation present.
The very narrow spectral response of the S550
viewhead makes it ideal for discrimi nating
between flame and glowing refractory and other
burners. How ever, because the tube responds to
only the very short wavelengths, problems can
occur because this radiation is easily absorbed and
masked, for example with pulverized coal, by
unburned fuel or other impurities in the fuel. Sour
gas (H
2
S) will readily absorb these short 200nm
wavelengths as well as other waste fuels resulting
in a dropout of the flame signal. Even standard
glass will absorb this wavelength, hence, the
necessity of using a quartz lens with this viewing
head.
In general, the UV viewing head will work well on
natural gas flames. It will respond to oil flames but
with a lower signal level. The sighting for both oil
and gas flames should be parallel to the axis of the
burner and aimed at the root of the flame, as with
the IR detector. (See pre vious section
IR
DETECTOR.
) Photo graphs of gas and oil flames
using visual and UV-sensitive film show clearly
that the UV zone is much smaller than the visible
zone, and the highest UV intensity occurs near the
flame root. In addition, the zone of higher UV
intensity does not overlap the same zones of
adjacent or opposing burners so that, with proper
sighting, discrimi nation is predictable.
As a general rule, the sighting of the S550 when
using the UV detector, will be more likely a “line-
of- sight” than the IR flicker detector which will
read flame signals bouncing off the side of the
sight pipe and other “reflectors”. Sometimes the
reading with the IR detectors can be greater with a
reflected signal than the direct line of sight. The
UV detector on the other hand, will have to be
aimed at the part of flame that has the highest
concen tration of UV, which usually is at the root
of the flame (at the throat of the burner) and in a
very narrow region. The angle of view of the S550
UV is somewhat wider than the S550 IR but this
does not offset the fact that the UV radiation
present is from a very small region.
This small region emitting UV is not the case
how ever, for low NO
X
gas burners. The UV
radiation is usually much less in intensity and
spread out with this type of burner, such that one
gets relatively high readings from all over the
furnace when many burners are on. This is
particu larly true when flue gas recir cu lation is
used. There will be, how ever, a relatively stronger
signal near the “root” of the flame and this more
intense spot should be located during the aiming
or sighting process. This “root” or intense spot
may be further out than with the standard gas
burner so it is imperative that one use a swivel
mount when making these sighting adjust ments.
Also, another factor one must con sid er when
aiming the viewing head is the load condition of
the boiler. The flames from a burner can be
radically different at different loads. This is one of
the reasons for trying to pick a sighting initially
that will minimize this changing flame at different
loads. If the flame moves further out at higher
loads then the parallel to the burner axis sighting
will be the best. Using a large diameter sight pipe
with a swivel mount is recom mended for all
instal la tions on large multi burner boilers.
ORIFICING
There are many appli ca tions where the burner
flames can be so bright they will saturate the S550
sensors. With these condi tions orificing the
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IRIS MODEL S550 APPLICATION MANUAL