Drop Stage Methods; Error Threshold; Rate Threshold - CleaverBrooks CFC Operation, Service & Parts Manual

750-322
Add-stage methods

ERROR THRESHOLD

For error threshold staging, a stage is added when the
error becomes excessive based on degrees away from
setpoint, and time.
ADD-STAGE CONDITION:
- The modulating burner(s) is at its (their) maximum
position per the rate allocation rules,
- The operating point is below the setpoint by an
amount greater than
or equal to Add-stage error threshold
When the Add-stage condition is false then AddStage-
DetectTimerN is set to zero. (If the condition is true
then AddStageDetectTimerN is not zeroed and thus
allowed to run.) If this timer reaches or exceeds LL-
Add-stage detection timeN then AddStageRequestN
is true.

RATE THRESHOLD

For rate based staging, a stage is added based on the rate
of the modulating stage.
ADD-STAGE CONDITION:
The modulating burner is at a rate that is at or above the
rate which is calculated by adding the Add-stage rate
offset to the maximum position per the rate allocation
rules.
Examples:
rate offset = 20% The add-stage condition will occur
if the modulating stage is 20% above base load for
unlimited allocations, or, if limited, when there is 20%
more rate to distribute than can be absorbed by firing
the stages at base load.
rate offset = -20% The add-stage condition will be as
described just above, but the threshold is now 20%
below the modulating stage's base load rate.
To support this, the current Rate Allocation method asks
for the current "Overflow rate" - see the Rate Allocator
section.

Drop Stage Methods

Various methods can be used to determine when a stage
should be dropped. The internal algorithms that generate
DropStageRequests are called Drop-stage methods.
One or two methods may be active at any time. If two are
active then their requests are OR'd together.
All methods work by observing various criteria such as the
Firing stages, the commanded rate, or Setpoint.
Dropping Stages Parameters:
DROP-STAGE DETECTION TIME: MM:SS
This provides time thresholds. They differ only in that:
Drop-Stage detection time is used with
DropStageDetectTimer
In the descriptions below, the relevant parameter is
referred to as LL – Drop Stage detection timeN}.
Drop-Stage method:
Disable,
Error threshold,
Rate threshold,
dError/dt and threshold,
dRate/dt and threshold
DROP-STAGE ERROR THRESHOLD: DEGREES
This provides the error threshold as defined by the
methods below.
DROP-STAGE RATE OFFSET: -100% TO +100%
This provides the rate offset threshold as defined by the
methods below.
LL boiler off options:
Options disabled,
Enable all boilers off (ABO)
Enable lead drop-stage on error (LDSE)
Enable both ABO and LDSE
This provides options for customizing the way stages are
dropped, as described below.
LL ALL BOILERS OFF THRESHOLD: TEMPERATURE OR
NONE
When the LL boiler off options specifies "Enable all boilers
off (ABO)" or "Enable both ABO and LDSE" then this
parameter provides the boiler off threshold temperature
that is used. In this case, if the temperature is the None
value then a parameter error lockout occurs.
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 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 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.
53
CB FALCON LEAD/LAG