CleaverBrooks CFC-500 Operation, Service And Parts Manual page 167

Rate Limits and Override
The Limit and Override subsystem consists of three separate
concepts:
• Safety limit functions that cause a burner control to lockout
or recycle if safety-critical limits are reached.
• Rate limit functions that limit the range of modulation due to
special or abnormal operating conditions. It is common for a
rate limit to become effective whenever conditions approach
a safety limit, to try to prevent the consequence of reaching
the safety limit.
• Rate override functions set the firing rate to a specific value
without regard to firing rate due to modulation requests or
rate limits.
Rate Limit Priorities
There are two kinds of rate limit:
— Rate reducers, those that act to limit the maximum fir-
ing rate:
• Delta-T limit - Hydronic
• Stack limit
• Slow start
• Outlet limit - Hydronic
• Forced rate (Forced rate might actually specify any
rate, but for priority purposes it is considered to be a
reducer.)
— Rate increasers, that act to increase the firing rate.
There is only one of these:
• Anticondensation - Hydronic
Anticondensation has a programmable priority vs. the other
rate limits (Hydronic only):
Anticondensation versus Delta-T
Anticondensation versus Stack limit
Anticondensation versus Slow start
Anticondensation versus Forced Rate
Anticondensation versus Outlet limit
Parameter
Delta-T inlet/outlet enable
Delta-T inlet/exch enable
Delta-T exch/outlet enable
Delta-T inlet/outlet degrees
Table 18. Delta-T Limit Parameters.
Disable, Enable Delta-T, Enable Inversion Detection, Enable Delta-T and Inversion Detection.
Disable, Enable Delta-T, Enable Inversion Detection, Enable Delta-T and Inversion Detection.
Disable, Enable Delta-T, Enable Inversion Detection, Enable Delta-T and Inversion Detection.
If either of the heat exchanger delta-Ts is enabled, the Stack Connector Type must be either
"10K single non-safety NTC" or "12K single non-safety NTC." If this condition is not met then
a lockout occurs because the exchanger input requires using the Stack sensor as two
separate sensors. Stack being S8 (J9 terminal 4) and heat exchanger being S9 (J9 terminal
6).
If this value is "disable" then all behavior associated with the Delta-T function is disabled.
If the Enable Delta-T, or Enable Delta-T and Inversion Detection options are chosen to enable
the Delta-T behavior, then the temperature gap between the temperature of "lo" and "hi" is
limited by the number of degrees given by the Delta-T degrees parameter.
If the Enable Inversion Detection or Enable Delta-T and Inversion Detection options are
chosen, the Inversion detection is active. This is implemented as a time limit on how long the
inverse of the normal temperature relationship will be tolerated.
Temperature inversion is the condition where the "lo" temperature is higher than the "hi"
temperature. If the inversion persists for longer that Delta-T inverse limit timer, then the
response given by Delta-T inverse limit response occurs.
Degrees, none
So the rate limit priority scheme uses the following steps,
where "active" means that the rate override is both enabled
and requesting its rate:
1. If Anticondensation is active and all rate reducers are
inactive, then Anticondensation determines the rate.
2. If Anticondensation is active and one or more rate reduc-
ers are also active, then the priority of Anticondensation
is compared to each active rate reducer. Of those active
rate reducers that have higher priority than Anticonden-
sation, the lowest rate requested by any of these deter-
mines the rate. However, if Anticondensation has higher
priority than any active rate reducers, then Anticonden-
sation determines the rate.
3. If Anticondensation is inactive, then the lowest rate
requested by any active rate reducer determines the fir-
ing rate.
When an "abnormal" rate limit occurs an alert is issued. The
rate limits that are abnormal are: Delta-T, Stack, Outlet, and
Anticondensation. The other two limits, Slow Start and Forced
Rate, are considered to be normal in that they always occur if
they are enabled.
Delta-T Limit (Rate Limit Only/Hydronic
only)
The Delta-T limit is designed to reduce the firing rate in case
the difference between the following is excessive:
• The Inlet and the Outlet temperature
• The Inlet and the exchanger temperature
• The exchanger and the Outlet temperature
Each will operate identically and will use either similar
parameters or shared parameters. The left name is typically at
a lower temperature than the one on the right (except when the
temperature is inverted due to a reversed flow, or some other
abnormal condition).
The "inlet temperature" is provided by S1 (J8 terminal 4), the
"exch" exchanger temperature is provided by S9 (J9 terminal
6), and the "outlet" temperature is S3S4 (J8 terminal 8, 9 and
10) dual sensor.
Comment
43
CB FALCON
750-265