CleaverBrooks CFC-500 Operation, Service And Parts Manual page 210

CB FALCON
Parameter
LL - Drop-stage error threshold
LL - Drop-stage rate offset
Drop-stage Methods
Error threshold For error threshold staging, a stage is
dropped when the error becomes excessive based on degrees
away from setpoint and time.
Drop-stage condition:
• The modulating burner(s) is at its (their) minimum position
per the rate allocation rules,
• The operating point is above the setpoint by an amount
greater than or equal to LL - Drop-stage error threshold
When the Drop-stage condition is false then
DropStageDetectTimerN is set to zero. (If the condition is true
then
DropStageDetectTimerN is not zeroed and thus allowed to
run.) If this timer reaches or exceeds
LL - Drop-stage detection timeN then
DropStageRequestN is true.
Rate threshold For rate based staging, a stage is dropped
based on the rate of the last stage.
Drop-stage condition
The modulating burner(s) is at a rate that is at or below the
minimum modulation rate plus a rate offset.
Examples:
rate offset = 20% The Drop-stage condition will occur when
the last stage is less than a threshold that is the minimum
modulation rate plus another 20%.
Parameter
LL - Lead selection method
LL - Lag selection method
LL - Lead rotation time
LL - Force lead rotation time
Sequencer Add Boiler Selection
The sequencer selects the next boiler to be added according to
a sorted order. This description assumes this is implemented
by assigning an ordering number and that the lowest numbers
are the first to be added.
• Any Available slaves that have a mode of Use First will have
the lowest ordering numbers. If two or more Use First
boilers exist, they are numbered according to their assigned
LL - Slave sequence order or Modbus address if this value
is zero, as described above.
750-265
Table 45. Dropping Stages Parameters. (Continued)
degrees
This provides the error threshold as defined by the methods below.
-100% to +100%
This provides the rate offset threshold as defined by the methods below.
Table 46. Sequencer Parameters.
Rotate in sequence order, Measured run time
This determines the selection method for lead selection and sequencing, as described below.
Sequence order, Measured run time
This determines the selection method for lag selection and sequencing, as described below.
hh:mm or None
This determines the lead rotation time as defined below.
hh:mm or None
If this parameter is a non-zero time, then it is used to force the rotation of the lead boiler if it
stays on longer than the time specified.
Comment
rate offset = 0% The Drop-stage condition will occur when
the last stage is at the minimum modulation rate.
rate offset = -20% The Drop-stage condition will occur if the
last stage is at minimum modulation and there is 20% less
rate to distribute than can be absorbed; that is, the rate allo-
cator would like the minimum modulation rate to be lower
than it is.
To support this, the current Rate Allocation method asks for the
current "Underflow rate" - see "Rate Allocation Methods" on
page 84.
Sequencer
The sequencer determines which Falcon is next whenever an
Add-stage event occurs. It maintains the following variables:
LeadBoilerSeqNum - sequence number of the current lead
boiler in the Slave Status table.
Lead BoilerRunTime - the cumulative time that the current
lead boiler has been running
In all cases, if a boiler sequence number is needed and LL -
Slave sequence order is 0, then the boiler's modbus address is
used as its sequence number.
In all cases, if two boilers being compared have the same
effective sequence number, then the one that is selected is
undefined (either may prevail).
Comment
• Next are slaves that have the mode of Equalize Runtime.
When the add boiler routine gets to this group it first invokes
the Voluntary Lead Rotation routine (to make sure this is
done, but only once) and then selects an Available boiler, if
any, ordered according to:
• The first is the lead boiler per the
LeadBoilerSeqNum parameter.
• The rest are the other slaves ordered according to
the LL –Lag selection method} parameter:
• If this parameter is "Rotate in sequence order", then they
are ordered according to their LL – Slave sequence order or
Modbus address if this value is zero, as described above.
86