Honeywell 7800 Series Manual page 5

DETECTOR IN GOOD
SIGHTING POSITION
(LOW ANGLE SIGHTING)
BURNER
NOZZLE
Fig. 2. CR7890 sighting angle.
NOTE: When possible, it is desirable to tilt the CR7890 and
sight pipe downward to prevent the buildup of soot in
the pipe or on the viewing window.
In most installations, the CR7890 needs to respond to the pilot
flame alone, then the pilot and main burner flame together,
and finally the main burner flame alone. The CR7890 must
meet all sighting requirements that apply:
• Pilot flame alone—the smallest pilot flame that can be
detected must be capable of reliably igniting the main
burner.
• Pilot and main burner flame together—the CR7890 must
sight the junction of both flames.
• Main burner flame alone—the CR7890 must sight the most
stable part of the flame for all firing rates.
Screening Effects
Smoke, oil mist, dirt and dust are masking agents that
attenuate the ultraviolet radiation that the flame emits. If they
absorb too much radiation, the amount of ultraviolet radiation
reaching the CR7890 is reduced. The flame signal can then
become too low to hold in the flame relay, resulting in burner
shutdown.
Diluting the contaminants can eliminate the problem. A strong
flow of air through the sight pipe clears a viewing path through
the attenuating material. Refer to the Sight Pipe Ventilation
section.
It is also desirable to sight the CR7890 in 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 CR7890 by shortening the sight pipe or by
increasing the diameter of the sight pipe also reduces the
attenuating effects of masking agents.
Install the Sight Pipe (Fig. 4)
After you have determined the location and sighting angle,
select the sight pipe. A black iron pipe with a diameter of at
least 1-1/2 in. (38 mm) is recommended. Do not use stainless
steel or galvanized pipe because they reflect ultraviolet
radiation internally and complicate aiming the pipe.
FLAME DEPTH–
ANGLE VIEW
UNBURNED FUEL
FLAME DEPTH–
PERPENDICULAR VIEW
DETECTOR IN POOR
SIGHTING POSITION
M1956
CR7890B 7800 SERIES INTEGRATED BURNER CONTROL
Sight pipes with diameters 2 to 3 in. (51 to 76 mm) produce
better results for horizontal rotary burners, which require wide
viewing angles. A wide viewing angle can also be obtained by
using a short sight pipe.
BLACK IRON SIGHT
PIPE (1-1/2 TO 3 in.
[38 TO 76 mm] DIA.)
REFRACTORY
FLARED
HOLE
1 IF VENTILATION OF THE SIGHT PIPE IS REQUIRED, ADD PIPE
TEE, PERFORATED NIPPLE, OR OTHER SUITABLE DEVICE
FOR VENTILATION.
Fig. 3. Typical mounting of CR7890.
Prepare Hole in Combustion Chamber Wall
Cut or drill a hole of the proper diameter for the sight pipe in
the wall of the combustion chamber at the selected
location.Flare the hole to leave room for small adjustments of
the sighting angle. The taper of the hole should be about 1 in.
for every 3 in. (25 mm for every 76 mm) of wall thickness.
Mount Sight Pipe
Thread one end of the pipe to fit the mounting flange, union or
required coupling. Cut the pipe to the desired length (as short
as practical) and at an angle so it fits flush with the wall
of the combustion chamber. Tack weld the pipe to the wall in a
trial position. Do not weld the sight pipe permanently in place
until after completing the Adjustments and Checkout.
NOTE: If you use 118367A Swivel Mount and you are
positive about the location and sighting angle, you
can permanently weld the pipe.
Install Fittings
In some cases, the sight pipe does not directly fit theCR7890
mounting flange or union. Also, it may be desirable or
necessary to ventilate the sight pipe. You may also want to
use a swivel mount or an antivibration mount. Each of these
cases can require additional fittings.
Reducer
For sight pipes of larger diameter than the mounting flange
connector or union, install a reducer as illustrated in Fig. 3.
The reducer requires a close nipple with 3/4 in. NPT external
threads.
5
REDUCER
1
CLOSE NIPPLE
TEMPORARY
TACK WELD
COMBUSTION
CHAMBER WALL
M6814GR
65-0259—1