SETUP & OPERATION
HeatNet Control V3 3.x
Page 22
Example Systems:
Figure 8
Non-Mixed Boiler System
System
MMBTU
Effective
Turndown
MOD
MAX
KN 5:1
10.0
25:1
70%
KN20, KN20,
KN20, KN20, KN20
5.0
25:1
70%
KN10, KN10,
KN10, KN10, KN10
3.0
25:1
70%
KN6, KN6, KN6,
KN6, KN6
With the traditional non-Mixed boiler system, the effective
turndown increases by the turndown ratio for every boiler
added. The min fire rate is equal to the minimum BTUs that
can be delivered to the system.
Number of boilers * Turndown Ratio = Effective System
Turndown: 5 * 5:1 = 25:1.
Figure 9
Mixed Boiler System
System
MMBTU
Effective
Turndown
MOD
MAX
Priority 1
5:1
Priority 2
5:1
4.2
35:1
60%
KN6, KN6
KN10, KN10,
KN10
3.8
42:1
70%
KN4, KN4
KN10, KN10,
KN10
2.2
55:1
81%
KN2, KN2
KN6, KN6,
KN6
3.6
90:1
72%
KN2, KN2,
KN2
KN10, KN10,
KN10
With the mixed boiler system, a lower minimum fire
rate/BTU can be delivered to the system by using small
boilers with larger boilers. This works in much the same
way as base loading.
Figure 10
KN Boiler Btu Chart (MBH)
KN2
KN4
KN6
KN10 KN20 KN30
Max Input
200M 400M 600M
1MM
2MM
3MM
Min Input
5:1
40M
80M
120M 200M 400M 600M
Mod Max
80%
160M 320M 480M 800M 1.6MM 2.4MM
Mod Max
70%
140M 280M 420M 700M 1.4MM 2.1MM
Mod Max
60%
120M 240M 360M 600M 1.2MM 1.8MM
Mod Max
50%
100M 200M 300M 500M
1MM 1.5MM
When selecting the Priority
1
boiler(s) for a high effective
system turndown, the BTU Min Input is selected first. Next,
the MOD-MAX value of this Priority 1
boiler needs to be
greater than
: Mod MAX % =
(Priority 1‘s Min Input + Priority 2‘s Min Input)
Max Input of the Priority 1 boiler
The reason for this is keep the continuity of BTUs linear
without a BTU bump (discontinuity) when boilers are added
or shed. This is illustrated in the
Boiler System Response 2
graph.
If redundancy is not required, the min inputs of the
Priority 1 boilers may be summed to lower the Mod Max %
value so smaller Priority 1 boilers can be used. The sum of
the min inputs would then need to be divided by the sum of
the Max Input of the Priority 1 boilers. The effect of this
would create a higher turndown. See:
EXCEPTION NOTES:
Mod MAX % =
(((Priority 1 Min) * (#Priority 1’s)) + Priority 2 Min)
Max Input of Priority 1 boiler * (#Priority 1’s)
Example:
(2) KN6s, (2) KN20s
Redundancy:
(120 + 400)/600 = 87%
No Redundancy: (120 * 2) + 400)/(600*2) =54%
EXCEPTION NOTES:
1.
Mixing more than two different size/type boilers
becomes more complex than the scope of this manual.
2.
If using more than one Priority 1 boiler and the
calculated value is <
Priority 1Min * 2
Priority 1 Max Input
Use this result PLUS note 3 value as the
ModMax%.
3.
Always add a few % (3-5%) to the calculated MOD
MAX % value to allow a guard band (tolerance).
4.
If boilers are of different sizes, try to use larger
Priority 2 boilers.
If the calculated Mod MAX % value is greater
than 99%, the combination cannot be used
since short cycling will occur.
Once the Priority 1 and Priority 2 boilers are selected, they
can be multiplied in each Priority set to achieve the desired