Honeywell 800 Series Manual page 12

HONEYWELL MODEL 700/800 SIGNAL PROCESSOR AND VIEWING HEAD
to sight the viewing head on the highly turbulent portion
of the flame that contains the higher frequencies. The
location of the higher frequencies can be predicted by
examining the burner with regard to where the turbulent
air enters the flame. The optimum scanner location is
parallel to the burner center line (Fig. 16). The use of a
swivel mount is encouraged to allow for line of sight.
IR Sensor Saturation
IR viewing head levels that exceed the range of the
scanner will indicate flamecounts 29 at the 700 display.
This is IR sensor saturation. Saturation may occur from
large flickering IR, or extremely high non-flickering IR
(high temp or high gain setting). This allows for IR
discrimination in low to high IR intensity applications
while preventing nuisance shutdowns. See Setup and
Adjustment Procedures for more information on proper
setup.
UV Detector
The S706, S706PF and S806 viewing head models use the
UVTron tube, with a spectral response of 185-260nm and
peak response of 210nm to ultraviolet radiation. The
output of the detector is a pulse stream of randomly
spaced pulses whose average rate is proportional to the
UV radiation present in the flame. The UV radiation is a
direct result of the combustion process as oxygen
combines with hydrocarbons in the fuel in the blue part of
the flame. The yellow part of flames, and the background
radiation from hot refractory, do not emit UV radiation.
The spectral range of the UV tube makes it ideal for
discriminating between flame and glowing refractory. As
with any UV radiation, it can be absorbed or masked by
unburned fuel, smoke, oil mist, dirt dust and other
impurities in the fuel. Care should be taken to select the
proper viewing head for the fuel used. Additionally, the
contaminants that mask UV can be diluted by providing a
strong flow of air through the sight pipe to clear a viewing
path through the attenuating material. See "Purge Air" on
page 10..
It may also be desirable to sight the detector at an area
containing fewer masking agents such as near the burner
nozzle or near the entrance of the combustion air.
Increasing the viewing area of the detector by shortening
the sight pipe or by increasing the diameter of the sight
pipe can also reduce the attenuating effects of the
masking agents.
In general, the UV viewing heads will work well on natural
gas and light oil flames. 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
previous section, "IR Detector.") The highest UV intensity
occurs near the root of the flame (Fig. 17). In addition, the
zone of higher UV intensity does not overlap the same
zones of adjacent or opposing burner so that, with proper
sighting, discrimination can be achieved.
With low NOx gas burners, the UV radiation is usually
much less in intensity and spread out. Relatively high
readings can be obtained from all over the furnace when
many burners are in service. This is particularly true when
flue gas recirculation is used. There will however, be a
relatively stronger signal near the "root" of the flame and
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the 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 a swivel mount be used when making
sighting adjustments.
Another factor that needs to be considered 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 choosing an optimum
sighting initially that will minimize signal swing due to
changing loads.
Self-Checking
The self-check circuitry guards against internal
component failure. There are several tasks that require
intelligent interaction between the viewing heads and the
signal processor. If all of these interactions do not occur
properly, the viewing head will not send pulses back to the
signal processor and the flame relay will open.
Adjustment of VH Sighting and
Gain
NOTE: Adjustment to the viewing head parameters can-
not be made unless the viewing head is con-
nected and communicating with the signal
processor.
The viewing head should be properly sighted before the
setpoints are adjusted. Adjustment can be made easier by
a 1/2 in. swivel joint, which Honeywell can supply if one is
not available (refer to "Accessories" on page 11).
While the burner is firing, vary the viewing angle while
observing the green LED on the connector at the rear of
the viewing head. Adjust the viewing angle for the
maximum pulse rate, then lock the swivel joint to preserve
this mechanical setting. If the green LED pulse rate is very
high or very low, see the two paragraphs below. The locked
mechanical setting should still be correct when Model 700
viewing heads are interchanged, because inside each
Model 700 viewing head the optical axis is aligned with
the mechanical axis within ±1/4 degree. Also, the reading
shouldn't change when a viewing head is rotated in the
mount.
For the above sighting adjustments to work properly, the
flashing rate of the green LED in the connector at the rear
of the viewing head must be reasonable. On the -PF (pipe
fitting) version, there are no LEDs. The installer must
observe the flame signal on the signal processor instead.
A count rate of 16 to 20 is recommended for proper
operation. If the displayed count is above 25, the pulses
begin to blur together, making changes in the pulse rate
difficult to observe.
If the displayed count is less than 8 or 10, it will be difficult
to maximize the count by adjusting the viewing head aim,
since the pulses occur too infrequently. In such a case the
gain should be increased. If the gain is set to a maximum
and the count rate persists below 8 or 10, the system can
still be made to work reliably as long as the count rate
drops significantly when the flame is removed. However,
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