Falcon Lead Lag Appendix E
E-54
Demand and Rate
On/Off Hysteresis Includes hysteresis shifting at turn-on, turn-off
LL OFF HYSTERESIS: DEGREES OR NONE
LL ON HYSTERESIS: DEGREES OR NONE
The LL hysteresis values apply to all setpoint sources. The behavior of the hysteresis function is identical to the
behavior of the stand-alone CH hysteresis function, except:
• where stand-alone CH hysteresis uses the on/off status of a single burner, the LL hysteresis uses the on/off
status of all slave burners: this status is true if any slave burner is on, and false only if all are off.
• where stand-alone CH hysteresis uses time of turn-on and turn-off of a single burner, the LL hysteresis uses the
turn-on of the first slave burners and the turn-off of the last slave burner.
LEAD LAG PID
The behavior of the Lead Lag PID function is identical to the behavior of the stand-alone CH PID function. The
same gain scalars and algorithms are used.
Additionally:
RATE ADJUSTMENT
When the Slave dropout/return compensation parameter specifies a rate adjustment and a rate compensation
event occurs (a slave leaves while firing, or a slave returns) then rate adjustment will alter the integrator value so
that the commanded rate compensates for the added or lost capacity.
INTEGRATOR COMPENSATION
A stand-alone CB Falcon includes a feature to smooth the response when a rate override has occurred (such as
delta-T rate limit) causing the PID output to be ignored.
Whenever an override has occurred then, at the moment the override ends, the integrator is loaded with a value
that causes the PID output to match the current rate, whenever this is possible within the integrator’s limits. The
Lead Lag PID will implement similar behavior: The rate allocator will provide a trigger that causes the integrator's
value to be recomputed and this trigger will activate whenever a rate allocation limit is released; that is, this event
will occur any time the system transitions from the condition in which it is not free to increase the total modulation
rate, to the condition where this rate may increase.
Implementation:
The examples below are ways in which this may occur, but in implementation what is necessary, first of all, is to use
a rate allocator that assigns rate to each slave and can detect when all of the assigned rate is absorbed, or if there
is excess requested rate that the firing stages could not absorb.
Then:
1. Whenever the system is rate limited, that is, when A) all firing stages are commanded to their respective max-
imums and also B) the PID is asking for more heat than that, note that this has occurred by setting a flag and
also record total rate that the system absorbed (the total of the commanded maximums, not the PID's
requested rate which might include excess).
2. Whenever the rate allocator completes an execution pass and detects that both conditions of step 1 are no
longer true (demand has decreased) then it clears the flag.
3. Whenever the rate allocator completes an execution pass and detects both conditions of step 1 are true, and
it also detects that the total rate potentially absorbed by the system (the commands have not yet been sent)
has increased from the value that was saved when the flag was set, then it re-computes the integrator value
based on the old commanded maximum, clears the flag, and actually allocates the old rate that was saved
when the flag was set.
Examples include:
• The rate allocator has encountered a limit such as base load (for a "limited" rate allocation scheme) and this limit
is released.
• All stages are at their maximum (base load, or max modulation) and one or more stages are rate-limited (such as
due to slow-start or stepped modulation limiting due to high stack temperature, etc.) and the rate limited stage
recovers, changing from rate-limited to free to modulate.
(This is indicated by the Slave Status "slave is modulating": the changing from false to true is not, itself, a trigger, but
while it is true the rate allocator can assign to the slave only the firing rate that it is reporting; thus the release of this
might allow more rate to be absorbed by the system. It also might not do this, if for example the slave was in anti-
condensation and thus the rate limit was maximum modulation rate.)
• All firing stages are at their maximum (base load, or max modulation) and a stage which was OnLeave returns in
the firing state and is available for modulation.
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