750-368
CBEX Elite 100-1200 HP
4-3
4.3 — Firing Rate Controls
To comply with requirements of insurance underwriters such as Factory Mutual (FM), XL GAP, or others, addi-
tional interlock devices may be used in addition to the circuits identified in section 4.2.
For additional information see the CB780E manual, 750-234.
4.3 — Firing Rate Controls
The Cleaver-Brooks Hawk boiler control system comprises two primary subsystems: flame supervision/burner
sequencing, and modulation or firing rate control. Upon establishment of main flame, the program relay
(CB780E) relinquishes control to the Hawk modulation controls. Modulated firing is accomplished by sending
Modbus signals from the Hawk controls to the separately driven air, fuel, and FGR actuators.
4.4 — Sequence of Operation: Gas or Oil
On a combination fuel unit, the gas/oil switch must be set for the proper fuel.
The following sequence occurs with power present at the program relay input terminals and with all other operat-
ing conditions satisfied.
4.4.1 — Pre-Purge Cycle
When the Burner Switch (BS) is turned ON, and controls in the Limit and Fuel Valve Interlock circuits are closed
and no flame signal is present, the Blower Motor Start circuit is powered, energizing the blower motor starter
(BMS). The load demand light (LDL) turns on. When firing oil, the air compressor motor starter (ACMS) (if pro-
vided) is also powered.
At the same time, the air damper actuator will drive the damper to its fully open or high fire position. Opening the
damper allows a flow of purging air through the boiler prior to the ignition cycle.
The High Fire Relay (HFR) will prove that the damper actuator has driven the damper to the open position during
the pre-purge cycle.
The controls wired into the Running Interlock circuit must be closed within 10 seconds after the start sequence.
In the event any of the controls are not closed at this time, or if they subsequently open, the boiler will go into a
safety shutdown.
Components/Controls
• Low Fire Relay (LFR)
• High Fire Relay (HFR)
• Release to Modulate Relay (RMR)
• Air Actuator
• Fuel 1 Actuator
• Fuel 2 Actuator (combination burners)
• FGR Actuator
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