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As the load increases:
•
Until all units are firing, no unit is requested to exceed
the base load value.
•
Additional units are added once the Base Load Value
has been reached and the Min On Time timer has
expired.
•
As long as all boilers are firing, the base load value can
be exceeded, as long as all units maintain the same
firing rate.
Low demand: The first boiler in sequence ignites and
Low demand: The first boiler in sequence ignites and
gradually increases firing rate to satisfy the heat demand.
gradually increases firing rate to satify the heat demand.
Increased demand: Once the first boiler reaches the Base
Increased demand: Once the first boiler reaches the Base
Load Value (50%) firing rate, the second boiler ignites. After
Load Value ( 50%) firing rate, the second boiler ignites. After
ignition, both units modulate to half of the cascade firing rate,
ignition, both units modulate to half of the cascade firing rate,
then gradually increase the firing rate together, up to the Base
then gradually increase the firing rate together, up to the
Load Value .
Base Load Value.
Increased demand: Once the first two boilers reach the Base
Increased demand: Once the first two boilers reach the
Load Value (50%) firing rate, the third boiler ignites. After
Base Load Value (50%) firing rate, the third boiler ignites.
ignition, the three units modulate at 1/3 of the cascade firing
After ignition, the three units modulate at 1/3 of the cascade
rate, then gradually increase firing rate together, up to the Base
Load Value.
firing rate, then gradually increase firing rate together, up to
the Base Load Value.
Approaching max demand: Once all three boilers reach the
Approaching max demand: Once all three boilers reach
Base Load Value (50%), all units are allowed to increase firing
the Base Load Value (50%), all units are allowed to increase
rate (same at all boilers) up to maximum firing rate.
firing rate (same at all boilers) up to maximum firing rate.
Decreasing demand: As the demand decreases, once the
Decreasing demand: As the demand decreases, once the
requested firing rate reaches the Drop Load Value , the third
requested firing rate reaches the Drop Load Value, the third
boiler turns off.
boiler turns off.
Continued decreasing demand: As the demand decreases,
Continued Decreasing demand: As the demand
decreases, once the requested firing rate reaches the Drop
once the requested firing rate reaches the Drop Load Value , the
second boiler turns off.
Load Value, the second boiler turns off.
Demand Satisfied: When the heat demand is satisfied or
Demand Satisfied: When the heat demand is satisfied or the
the temperature reaches set point + off hysteresis, the final
temperature reaches set point + off hysteresis, the final boiler
will turn off.
boiler will turn off.
Figure 36.
Lead / Lag, 3 Boilers
LAARS Heating Systems
As the load decreases:
•
As long as all units are firing – the base load value can
be exceeded, as long as all units maintain the same
firing rate.
•
As the firing rate decreases below the Drop Load
Value, the last unit to ignite turns off first (last on/first
off), following this pattern until the heat demand is
satisfied and all units are off.
Units in Lead / Lag mode maintain local boiler limiting
features (firing rate limiting based on outlet or flue
temperature) when in Lead / Lag mode operations.
1
2
30%
1
2
50%
1
2
50%
50%
1
2
50%
50%
1
2
20%
20%
1
2
20%
20%
1
2
20%
Lead/Lag - 3 Boilers
3
3
1
2
25%
25%
3
1
2
33%
33%
3
1
2
100%
100%
50%
3
1
2
20%
20%
20%
3
1
2
20%
3
1
2
3
3
33%
3
100%
3
3
3
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