RBI FUTERA III Series Instruction Manual download pdf (Page 12)

Version 3.37 – Control Board Manual used for Futera III, Futera Fusion and Futera XLF

ers

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XLF

Figure 9 Boiler System Response 1

(2) MB/MW 500s, (3) MB/MW 1250s

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 1,000,000 2,000,000 3,000,000 4,000,000 5,000,000

System Load, Btu/Hr

Blr 1+2+3 (2250 MBTU)

Blr 1+2+3+4 (3500 MBTU)

Blr 1+2+3+4+5 (4750 MBTU)

When running non condensing boilers at low input

rates, the risk of condensing should be considered.

 e Boiler System Response 1 chart illustrates how each boiler (in

the example) is brought on and  res to 60%, drops to a lower  re

rate and then adds the next boiler (vertical dashed lines). Once all

boilers are  ring, the modulation is released allowing all boilers to

 re to 100%.

Now, if (1) MB/MW 500 (one of the MB/MW 500s was brought

o ine) were used with (3) MB/MW 1250s and the Mod-Max is

set to 60%, the MB/MW 500 would  re to 300 MBTUs and wait

for the MB/MW 1250 (Boiler System Response 2 graph). Now,

the minimum input rate would be 312M (MB/MW 1250) + the

125M (MB/MW 500) (already running, but dropped to low  re

when the MB/MW 1250  red), the total being 437M. With a 60%

MOD-MAX clamp, there would be 137 MBTUS more than needed

and added to the system when the MB/MW 1250  red.

 e PID algorithm would then compensate for the discontinuity

(bump) in BTUs and the MB/MW 1250 could shut o (short

cycle).

 is discontinuity is observed in the graph below, (Boiler System

Response 2) where the jump from the MB/MW 500 @60% to the

 ring of the MB/MW 1250 is apparent.

Figure 10 Boiler System Response 2

(1) MB/MW 500, (3) MB/MW 1250, 60% Mod-Max

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0.10

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0.30

0.40

0.50

0.60

0.70

0.80

0.90

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0 1,000,000 2,000,000 3,000,000 4,000,000 5,000,000

System Load, Btu/Hr

Blr 1+2 (1750 MBTU)

Blr 1+2+3 (4250 MBTU)

Blr 1+2+3 (3000 MBTU)

To correct this would require the MB/MW 500 to set the MOD-

MAX to roughly 90% (Boiler System Response 3: not as e cient

as it could be when using CB/CW Fusion boilers) in order to have

a linear BTU transfer when the MB/MW 1250 is added ( red).

Figure 11 Boiler System Response 3

(1) MB/MW 500, (3) MB/MW 1250, 60% Mod-Max

0.00

0.10

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0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 1,000,000 2,000,000 3,000,000 4,000,000 5,000,000

System Load, Btu/Hr

Blr 1+2 (1750 MBTU)

Blr 1+2+3 (4250 MBTU)

Blr 1+2+3 (3000 MBTU)

An MB/MW 500 running with a MB/MW 1250 may not be an

optimal choice unless (2) MB/MW 500s are used in the Priority1

set or (3) MB/MW 500s and one is allowed to be taken o ine.

A system employing this redundancy where (1) is allowed to be

taken o ine is listed in the MIXED BOILER SYSTEM chart.

 is system uses (3) MB/MW 500s and (3) MB/MW 1500s. Two

of the MB/MW 500s are treated as one when adding the min inputs

of the Priority1 set.

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RBI FUTERA III Series Instruction Manual Page 12

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