Honeywell 7800 Series Manual page 4

CR7890B 7800 SERIES INTEGRATED BURNER CONTROL
Basic Requirements
The combustion flames of most carbon-based fuels emit
sufficient ultraviolet radiation to enable the CR7890B
Integrated Burner Control to prove the presence of a flame in
a combustion chamber. The CR7890 is mounted outside the
combustion chamber with its mounting flange or union
threaded to one end of a sight pipe inserted through the wall
of the combustion chamber. The ultraviolet sensor in the
control sights the flame through the pipe.
When a flame is present, the sensor in the CR7890 senses
the ultraviolet radiation emitted. The CR7890 then produces a
signal that pulls in the flame relay to allow proper system
operation.
Because it is necessary for the UV sensor to actually see the
flame, it is best to locate the CR7890 as close to the flame as
physical arrangement, temperature, and other restrictions
permit. These restrictions are described in detail in the
following paragraphs.
Determine Location
Before beginning the actual installation, determine the best
location for mounting the CR7890. Carefully consider the
factors discussed in this section before deciding on the
location.
Temperature
Install the CR7890 where the ambient temperature (outside
the case) stays within the ambient operating temperature
ratings.
To keep the CR7890 below its maximum rating, it may be
necessary to add additional insulation between the wall of the
combustion chamber and the device. A shield or screen can
be added to reflect radiated heat away from the device. If the
CR7890 continues to get too hot, additional cooling is
necessary.
Refer to the Sight Pipe Ventilation section.
Location
Humidity
install the CR7890 where the relative humidity never reaches
the saturation point. The CR7890 is designed to operate in a
maximum 85% relative humidity continuous, noncondensing,
moisture environment. Condensing moisture can cause a
safety shutdown.
Vibration
Do not install the CR7890 where it can be subjected to
vibration in excess of 1.0G continuous maximum vibration.
Clearance
Make sure there is enough room to easily mount the sight
pipe, CR7890 and all required fittings, and to remove the
CR7890 for troubleshooting and servicing.
65-0259—1
Radiation Sources (Other Than Flame)
Examples of sources (other than flame) that could actuate the
detection system:
1. Flame-simulating sources:
a. Hot refractory above 1800°F (582°C).
b. Spark:
(1) Ignition transformers.
(2) Welding arcs.
(3) Lightning.
c. Welding flames.
d. Bright incandescent or fluorescent artificial light.
e. Solar radiation.
f. Gas lasers.
g. Sun lamps.
h. Germicidal lamps.
i. Bright flashlight held close to sensing tube.
2. Gamma ray and X-ray sources:
a. Diffraction analyzers.
b. Electron microscopes.
c. Radiographic X-ray machines.
d. High voltage vacuum switches.
e. High voltage condensers.
f. High voltage coronas.
g. Radioisotopes.
Except under unusual circumstances, none of these sources,
except hot refractory and ignition sparks, would be present in
or near the combustion chamber. Verify that the CR7890 does
not respond to these sources after installation.
The CR7890 may respond to hot refractory above 1800°F
(582°C) if the refractory surface represents a significant
percentage of the CR7890 field of view. If the temperature of
the hot refractory causes the flame relay in the flame
safeguard control to pull in, re-aim the pipe so that the
CR7890 views a cooler area of the refractory surface.
Ignition spark is an intense source of ultraviolet radiation.
When installing the CR7890, make sure it does not respond to
ignition spark.
Single Burner Requirements
The CR7890 must have an unobstructed view of a steady part
of the flame it is supervising. This requires a proper sighting
angle and minimized ultraviolet radiation attenuation effects.
Supervising only one burner simplifies sighting requirements.
Sighting Angle (Fig. 2)
The first 30 percent of a flame closest to the burner nozzle
(the flame root) emits the most ultraviolet energy. Also, if the
CR7890 sights the flame at an angle instead of
perpendicularly, it views more flame depth. Therefore, the
best sighting angle is nearly parallel to the axis of the flame,
permitting the CR7890 to view a large portion of the first 30
percent of the flame closest to the burner nozzle, as illustrated
in Fig. 2.
Low angle sighting permits the CR7890 to view a greater
depth of flame, thus reducing the effects of instabilities in the
flame pattern. Also, the environment near the burner nozzle is
usually cleaner than at any other part of the combustion
chamber. This provides a clearer line of sight and can keep
the viewing window cleaner, thus reducing the maintenance
required.
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