CleaverBrooks CFC-500 Operation, Service And Parts Manual page 208

CB FALCON
Parameter
LL - Base load common
LL - Rate allocation method
OVERFLOW RATE AND UNDERFLOW RATE
The rate allocator knows the rate assigned to each stage, and
the requested rate, and thus can determine the difference
between these.
This difference has two forms: overflow (used by Add-stage
methods), underflow (used by Drop-stage methods).
When asked for rate overflow the threshold that is used is the
upper limit of the modulating stage per the current rate
allocation rules. Additionally this threshold may be shifted if the
Add-stage method is using a dRate/dt behavior. Rate overflow
is a positive or negative percentage offset from the threshold.
For example:
If the modulating stage is at the staging threshold position
but the LL master is not asking for more heat than this, then
the overflow rate is 0%. If it is at this location (limited) or
above this location (unlimited) and the LL master is asking
for 10% more than the threshold value, then the overflow
rate is 10%. If it is below the staging threshold position by
5%, then the overflow rate is -5%.
When asked for rate underflow the threshold that is used is the
minimum modulation rate of the last stage. Additionally this
threshold may be shifted if the Drop-stage method is using a
dRate/dt behavior.
Rate underflow is a positive or negative percentage offset from
the threshold. For example:
If the last stage is at the threshold position but the LL
master is not asking for less heat than this, then the under-
flow rate is 0%. If it is at this location and the LL master is
asking for 10% less than the threshold value, then the
underflow rate is -10%. If the last stage is 5% above the
threshold then the underflow rate is 5%.
Rate Allocation Method (
BASE LIMITED)
Allocation All stages that are Firing receive the same firing
rate. Only the LL - Base load common parameter is used for
base loading, the individual slave's base load values are
ignored.
750-265
Table 42. Rate Allocation Parameters.
0-100%
If set to zero, this parameter is disabled. For any non-zero value, it uses the individual base
load rates of each slave to be ignored by the LL master's routines and this common value to
be used instead. It is an easy way to set all base loads to the same value, without having to
set each slave.
Some rate allocation algorithms may specify the use of this parameter, and that the slave
base load settings are ignored.
Parallel common-base limited
This selects the rate allocation method. This performs three purposes:
1) it determines how the LL master allocates firing rate to each active stage,
2) the modulating stage and last stage are determined for the Add-stage and Drop-stage
methods,
3) it determines the overflow rate and underflow rate and can provide this to staging
algorithms.
PARALLEL COMMON-
Comment
As load increases:
Until all stages are Firing: No stage is requested to exceed
the common base load rate.
After all stages are Firing: There is no restriction on the
slave's commanded firing rate.
As load decreases:
As long as all available stages are Firing, there is no restriction
on the slave's commanded firing rate.
When at least one stage has been dropped: No stage is
requested to exceed the common base load rate.
Modulating stage Since all Firing stages receive the same
rate, any stage can be considered to be the modulating stage.
The one with the highest StagingOrder number is considered
to be the modulating stage.
Last stage The stage with the highest StagingOrder number is
the last stage.
Overflow and Underflow For the Parallel common-base
limited the LL - Base load common parameter provides the
overflow threshold.
For the Parallel common-base limited the minimum modulation
rate provides the underflow threshold.
Stager
The Stager is an internal program that manages the lead lag
functions. In all cases:
• The first burner turns on due to the combination of heat
demand (call for heat from a source) and setpoint demand
(operating point falls below the setpoint minus the on
hysteresis).
• The last burner (or all burners) turn off due to the loss of
burner demand which is caused by either the loss of heat
demand (no call for heat from any source) or the loss of
setpoint demand (the operating point climbs above the
setpoint plus the off hysteresis).
• In between those two extremes the Add-stage and Drop-
stage methods determine when staging occurs.
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