CleaverBrooks CFC-1000, Operation, Service And Parts Manual

Page: 207 / 314

CB FALCON
83 750-265
RATE ADJUSTMENT
When the LL - 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 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 maxi-
mums 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”
bit: the changing of this bit 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
anticondensation 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.
• An add-stage is in-progress and all firing burners are at their
limits (max modulation rate or base load) and then the new
stage becomes available.
This also applies when the system is first starting up, that is, all
firing burners are at their limits (zero) because non are firing,
and thus when the add-stage is finished the system transitions
from no modulation at all, to modulating the first stage.
LEAD LAG BURNER DEMAND
Lead Lag burner demand will be present when Frost protection
burner demand is true, as described the section on Frost
protection. For the CH, and DHW demand sources, Lead Lag
burner demand will be true when one of these is true and also
setpoint demand from the hysteresis block is true.
Rate Allocation
The rate allocator first generates the LL - Slave Command.
Except for the Firing state, the value ultimately depends only
upon the SlaveState. The values are:
•
Available
• AddStage
• SuspendStage depending on whether any other slave stage
is firing, no matter what SlaveState it is in.
• Firing
• OnLeave - same as SuspendStage This ensures that when
a slave returns and is already firing, it will remain firing until
the master decides what to do about that, or if it is not firing
it will remain off.
• Disabled - same as Available
• Recovering - same as Available
The rate allocator next fills in the modulation rate for all Firing
boilers.
The rate allocator also provides functions to return
identification of the modulating stage and the last stage, for
use by the Add-stage and Drop-stage methods.