CleaverBrooks FLX Operation, Service And Parts Manual page 75

Adjustment Procedures
Separate and independent controls affect modulated firing
and burner on-off cycling. Figure 5-3 depicts a typical setting
relationship of the operating limit control, modulating control
and the high limit control.
The burner will be "on" whenever the pressure or temperature
is less than point B and "off" whenever pressure or
temperature is greater than point A. The distance between
points A and B represents the "on-off" differential of the
operating limit control.
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 modulating
control will signal the modulating motor to be in a low fire
position. If the load demands exceed the low fire input
potential, the modulating control will increase the firing rate
proportionately as pressure or temperature falls toward point
D. The modulating motor 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. Thus it is referred to as modulated firing.
Point D represents the maximum firing rate of the burner, or
highfire. In the event pressure or temperature drops while the
burner is firing at highfire, it indicates that the load exceeds
the capacity of the boiler.
The firing graph (Figure 5-3) 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 firing rate.
! DANGER
Excessive cycling increases the potential
and severity of internal condensation. On-
Off cycling should be limited to eight (8) cy-
cles or less per hour to keep the blower mo-
tor from overheating and excessive wear on
the switch gear and pilot. Failure to follow
these instructions could result in damage
and premature failure of the equipment.
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.
Note: It is not recommended that the boiler
c o n t r o l s b e s e t s o a s to o v e r l a p t h e
750-177
CAUTION
Figure 5-1: Steam Controls
Figure 5-2: Hot Water Operating Controls
Chapter 5
5-3