750-368
CBEX Elite 100-1200 HP
6-3
6.2 — Burner Operating Controls: General
FIGURE 6-1. Firing Graph
In normal operation, the burner will shut down whenever the pressure or temperature rises above setting A. At
that point the switch in the operating limit control will open. As the pressure or temperature drops back to B, the
operating limit control closes and the burner will restart. The boiler control system will signal the actuators to be
in a low-fire position. If the load demands exceed the low-fire input potential, the controls will increase the firing
rate proportionately as pressure or temperature falls toward point D. The controls will stop at any intermediate
point between C and D whenever the fuel input balances the load requirement.
As the load requirement changes, the firing rate will change accordingly. This it is referred to as modulated firing.
Point D represents the maximum firing rate of the burner, or high-fire. In the event pressure or temperature drops
while the burner is firing at high-fire, it indicates that the load exceeds the capacity of the boiler.
The Firing Graph shows that point B and point C do not coincide. Extreme load conditions could require the
points be closely matched.
When set as shown, with a time lag between B and C, the burner will be in a low-fire position upon a restart and
will fire at that rate for a short period of time before falling pressure or temperature requires an increase in the fir-
ing rate.
If points B and C overlap when restart occurs, the burner would drive to a higher firing position immediately after
the main flame was proven. It is therefore prudent to set the modulating control a few pounds or degrees below
the operating control allowing the Low Fire to “catch the load” before releasing to modulation.
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