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About Cascading (Lead / Lag) -continued
Low demand: The first boiler in
sequence ignites and gradually
increases firing rate to satify the heat
demand.
Increased demand: Once the first
boiler reaches the Base Load Value
(40%) firing rate, the second boiler
ignites. After ignition, both units
modulate to half of the cascade firing
rate, then gradually increase the firing
rate together, up to the Base Load
Value .
Increased demand: Once the first two
boilers reach the Base Load Value
(40%), the third boiler ignites. After
ignition, all three units modulate to 1/3
of the cascade firing rate, then
gradually increase firing rate together,
up to the Base Load Value . This
pattern continues as demand
increases.
Approaching max demand: Once all
eight boilers reach the Base Load
Value , all units are allowed to increase
firing rate (same at all boilers) up to
maximum firing rate.
Figure 37. Lead / Lag, Demand Increase, 4 to 8 Boilers
Decreasing demand: As the demand
decreases, once the requested firing
rate reaches the Drop Load Value , the
eighth boiler turns off.
C O N F I G U R A T I O N
FT
1
2
3
30%
1
2
3
40%
1
2
3
20%
20%
1
2
3
40%
40%
1
2
3
27%
27%
27%
1
2
3
40%
40%
40%
40%
1
2
3
100%
100%
100%
100%
1
2
3
20%
20%
20%
20%
1
2
3
23%
23%
23%
23%
4
5
6
7
4
5
6
7
4
5
6
7
4
5
6
7
4
5
6
7
4
5
6
7
40%
40%
40%
4
5
6
7
100%
100%
100%
4
5
6
7
20%
20%
20%
4
5
6
7
23%
23%
23%
Page 77
8
8
8
8
8
8
40%
8
100%
8
20%
8
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