Cleaver-Brooks CBH 40 - Combustion Adjustment

Chapter 5---Adjustment and Maintenance
( 4) The following formula will provide the re-
quired gas input for a 3 minute period:
Input (BTU/HR) Heating Value
(BTU/CU. FT.) ---;- 20 = Gas input in cubic
feet for 3 minutes.
( 5) Apply any necessary pressure correction fac-
tor to this answer to obtain the desired rate.
(6) To illustrate:- using circumstances from
previous example, compute as follows:
BTU/HR---;- BTU/CU. FT.---;- 20 = 3 minute
input in cu. ft.
OR
2,929,000---;- 1000---;- 20 = 146.5 cu. ft. (base
pressure)
146.5 ---;- 1.18 (pressure correction) = 124.5
cu ft. (line pressure)
(7) If the input timed for 3 minutes does not
agree with the rating indicated by the for-
mula adjust the gas pressure regulator to in-
crease or decrease flow as required.
NOTE: The information given in this section is for
all practical purposes sufficient to set and
adjust controls for gas input. Your gas
supplier can if necessary furnish exact
correction factors that take into considera-
tion BTU content, exact base pressure,
specific gravity, temperature, etc., of the
gas used.
COMBUSTION ADJUSTMENT- GAS
Gas input adjustment is accomplished by in-
creasing or decreasing the pressure of gas down-
stream of the pressure regulator. Any required ad-
justment to the regulator should be done with bur-
ner at high fire and with the gas butterfly valve
wide open.
In the event linkage or regulator adjustments are
required proceed as follows. Initially set the
linkage by backing out the low fire stop screw so
the valve is closed. Then run screw out two com-
plete turns. Adjust the connecting rod so that ten-
sion is released.
It is not practical to list specific setting of the con-
nection points or of the angles of the arms. The
final setting shou-ld provide a coordinated move-
ment of the damper and the gas valve. If the gas
valve opens rapidly, compared to the air damper,
an overly rich fire will occur during transition be-
tween the two firing positions. This improper fuel-
air ratio can cause sooting of the fireside surfaces.
If it opens too slowly then the proportionally
greater air flow should extinguish the flame.
Points to remember are that the motor arm must
complete its full travel and that the valve arm
should travel its required arc without excessive
5-14
stretching of the over ride springs. The closer that
the swivel joint in the motor arm is to the hub the
less distance it will travel. To increase the valve
arm travel, move the connecting rod closer to the
hub or away from it if decreased travel is required.
This low fire setting should be regarded as tenta-
tive until proper gas pressure for high fire opera-
tion is established.
After operating at low fire for a sufficient period of
time to assure a warm boiler, turn the damper
positioning switch to "high''. Observe the burner as
it progresses toward high fire. In the event gas
pressure is too low or if the butterfly valve move-
ment is not properly co-ordinated with the air
damper, it is possible that increased air flow may
extinguish the flame. Immediately turn burner off.
Determine and correct condition causing flame
failure before repeating attempt.
At high fire position, the butterfly valve should be
wide open as indicated by the slot on the end of its
shaft. Set and lock the high fire stop screw until it
is just touching the valve arm.
Determine the actual gas flow from a meter
reading as outlined in another section. If cor-
rections are necessary to obtain the required input,
increase or decrease the gas pressure by adjusting
the pressure regulator. Turn its adjusting screw
clockwise
to increase pressure, or counter clock-
wise to decrease pressure.
When the high fire gas flow is established, no fur-
ther adjustment of the regulator is required.
With proper gas flow, any further adjustment
necessary to obtain a desirable flue gas analysis
must be done with the air damper. Check all ad-
justments with a combustion gas analyzer.
When the high fire air/fuel ratio is set, shut the
burner down, re-light and check low fire. It may be
necessary to re-adjust the low fire stop screw or the
linkage. To assure that low fire position of the but-
terfly valve is always the same, allow a minimum
of one turn of the stop screw for over-travel.
Any adjustment must not cause extensive stretch-
ing of the override springs. If linkage adjustments
are made, recheck to determine that high fire is not
affected.
The appearance or color of the gas flame is not an
indication of its efficiency since an efficient gas
flame will vary from transparent blue to trans-
lucent yellow.
Proper setting of the air/fuel ratios at all rates of
firing must be established by the use of a com-
bustion gas analyzer. This instrument measures
the content, by percentage, of carbon dioxide
(C02), oxygen (02), and carbon monoxide (CO) in
the flue gas.