Chapter 6 — Adjustment Procedures
6-6 Part No. 750-184
• Eliminates frequent burner on-off cycling.
Separate and independent controls affect modulated firing and
burner on-off cycling. Figure 6-6 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.
Figure 6-6 Firing Graph
Modulated Firing
Range
Firing Rate
100%
0%
Minimum Input
(Low Fire)
Burner Off
Increasing
Falling Temp.
or Pressure
“ON - OFF”
Differential
Rising Temp.
or Pressure
Increasing
Increasing
(Burner ON)
(Burner OFF)
(Burner OFF)
Boiler Temp.or Press
High Limit Control
Safety Shutdown
Operating Limit Control
Response
Modulation Control Response
D
C
B
A
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