HeatNet Smith Series, Руководство пользователя

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SETUP & OPERATION HeatNet Control REV 1.38-1
Page 13
the condensing boilers first. This is also true when
using the OA T setting to start the priority 1 boiler set
when the OA T is above the start setpoint. To use
temperatures as start and stop conditions, the system
design temperatures must be known.
Selecting Mixed Boilers
There are a few factors to consider when choosing which
type of boilers to use in a mixed system. These factors
need to be considered when boilers are added or shed.
When BTUs are introduced into the system by adding
boilers, the amount of introduced BTUs should be
smooth (linear). If these factors are not considered,
discontinuity in BTUs may occur when boilers are added
and as a result, short cycling will occur.
1.
Turndown
: This is the ratio of minimum fire rate
to maximum fire rate: Example: a 20% minimum
modulation = 5:1 turndown (100%mod / 20% mod).
A (1) million BTU boiler = 200,000 BTUs
minimum input.
2.
MOD MAX CLAMP
: This value determines the
maximum modulation % at which the boilers will
fire to until all available boilers are firing.
3.
Total System BTUs
.
4. Desired Effective Turndown . This is the lowest
firing rate of the system relative to the maximum
firing rate of the system. The larger the value, the
lower the BTUs that can be delivered to a light load.
5.
Piping
.
Mixed System:
High System Turndown
Condensing/Non-Condensing
The following examples are of mixed boiler systems with
high effective system turndown, fault tolerance, and
condensing/non-condensing applications. When boiler types
are the same, the system turndown is limited to the boiler’s
min input and fault tolerance is always present. When the
system has mixed boiler types, consideration needs to be
taken on what types can be mixed properly to achieve a high
system turndown and provide some fault tolerance.
Fault tolerance allows for one boiler in the Priority 1 system
to fail and any boiler(s) in the Priority 2 system to fail and
still provide near linear (continuity) BTU response when
adding boilers. This is illustrated in the following examples
using the Boiler System Response graphs.
The Smith Mixed Boiler System (examples) is
advantageous in providing low BTU input for light loads
and high BTUs for heavy loads. The effective system
turndown minimizes short cycling when light loads are
present by assigning smaller boilers to Priority 1, running
them first, and then stopping them last.
In order to achieve the high effective
turndown, smaller boilers are required
(plumbing considerations need to be
considered here due to differing flow/volume
characteristics through the large and small
boilers).
Example Systems:
Figure 4 Non-Mixed Boiler System
System
MMBTU
Effective
Turndown
MOD
MAX
MB/MW 4:1
3.5 9:1 70%
28A-W-5, 28A-W-5,
28A-W-5
13.8 12:1 70%
28A-W-11, 28A-W-11,
28A-W-11, 28A-W-11
23.1 15:1 70%
28A-W-18, 28A-W-18,
28A-W-18, 28A-W-18,
28A-W-18
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 * 3:1 = 15:1.
With the mixed boiler system, a lower minimum fire
rate/BTU can be delivered to the system by using small
boilers with larger boilers. Condensing and Non-
Condensing of different sizes can also be mixed as
illustrated in
Figure 5 - Mixed Boiler System chart.
Figure 5 Mixed Boiler System
System
MMBTU
Effective
Turndown
MOD
MAX%
Priority 1
5:1
Priority 2
3:1
3.5 29:1
*54
86%
KN6, KN6 28A-W-5, 28A-W-5
6.46 32:1
*47
62%
KN20, KN20 28A-W-9, 28A-W-9
11.12 28:1
*57
87%
KN20, KN20 28A-W-14, 28A-W-14
13.25 33:1
*67
99%
KN20, KN20 28A-W-18, 28A-W-18
* The MOD MAX% column has (2) numbers for the MOD
MAX %. The first # does not allow for redundancy while
the second # does. Redundancy provides for continuity in
BTUs when a new priority boiler is added and one is offline.
If one of the priority boilers goes offline and redundancy is
not provided for, short cycling will occur due to
discontinuity in BTUs. The second # allows for redundancy
so one of the priority boilers can be taken offline with no
discontinuity. This will be explained more in the following
examples.
When selecting the
Priority 1
boiler(s) for a high effective
system turndown, the BTU Min Input is selected first. (See: