31 750-265
If the monitored temperature is not within 12°F of the limit,
then no rate limiting occurs. The stepped rate limit shall
behave as illustrated below:
Assuming that rate limiting has not been in effect, when the
monitored temperature crosses a threshold that is 10°F away
from the limit, then the maximum allowed firing rate is reduced
by one stepsize (to 80%) and thereafter it is reduced by one
stepsize every two °F until it is reduced to the minimum
modulation rate when the 2°F threshold is crossed. Assuming
that rate limiting has been in-effect then the thresholds for
returning to a less restrictive step are shifted by 2°F to provide
hysteresis. I.e. to go from step 4 to step 5 the threshold occurs
at 2°F, but to go the other way, from step 5 to step 4, the
threshold is 4°F.
Fig. 13. Stepped modulation rate limiting.
M28038
STEPPED MODULATION RATE LIMITING
DEGREES F FROM THRESHOLD
AT THIS POINT A RESPONSE TYPICALLY OCCURS
DUE TO A SAFETY LIMIT; HOWEVER, THIS DETECTION
AND RESPONSE IS NOT PART OF THE RATE LIMIT FUNCTION.
...>12 12 10 6 4 2 0
4
5
2
3
1
0
8
LIMITED MODULATION STEP
MAXIMUM
MODULATION
RATE (NO LIMIT)
MINIMUM
MODULATION
RATE
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