AERCO BMS 168 - Manual (Page 38)

4.4.4 START LEVEL and STOP
LEVEL Keys
These keys allow you to set the energy
levels (firing rate percentages) at which
additional boilers are staged-in or staged-
out of the system during Sequential or
Combination mode operation. While the
start level can be set at any percentage up
to 100%, it is best to keep it below 60%.
This allows the system to operate at
optimum efficiency under low loads. The
BMS is preset with 45% start and 18% stop
levels, which generally provide the best
performance.
When changing the start or stop levels, it is
important to remember that load-balancing
affects bumpless transfer. A sufficiently
wide gap between the start and stop levels
must be maintained to prevent cycling
between units. For example, in a two-boiler
system with a start level of 60% and a stop
level of 30%, when the first boiler reaches
60%, the BMS will distribute the load
between the two boilers, so that each will
have a firing rate of 30%. However, since
the stop level is 30%, the BMS will
automatically shut down one boiler and
allow the firing rate of the second boiler to
increase to 60%. The BMS will then repeat
this undesirable cycling. To avoid this, the
start level could be set to 65%, leaving the
stop level at 30%. When the BMS
distributes the load after the first boiler
reaches a 65% firing rate, the individual
load would be 32.5%, sufficiently above the
stop level. Conversely, the stop level could
be changed to 25%, leaving the start level
at 60%. In this case, when the BMS
distributes the load, there will be 5% margin
between this load (30%) and the stop level
(25%).
• Pressing the START LEVEL key
displays firing rate percentage at which
the boilers will start.
• Pressing the STOP LEVEL key displays
firing rate percentage at which the boilers
will stop.
4.4.5 BLR OP MODE
This key is used to select the Boiler
Operating Mode. It allows you to make
choices concerning the effective and
efficient way to fire the boilers based on the
building load. The mode choices include:
Parallel, Sequential, or Combination Mode.
Pressing the BLR OP MODE key displays
BOILER OP MODE followed by the
currently selected mode (PARALLEL,
SEQUENTIAL or COMBINATION). AERCO
presets the mode to SEQUENTIAL. If
necessary, use the ▲ and ▼ arrow keys to
toggle between the 3 available modes. The
following paragraphs describe the
advantages of each mode selection.
4.4.5.1 Sequential Mode
The Sequential Mode provides a greater
turn-down ratio than Parallel Mode. This is
because the turn-down ratio in Sequential
Mode is equal to the number of boilers
multiplied by 14 for KC Series boilers and
by 15 for Benchmark boilers. In Parallel
Mode, the turn-down ratio is fixed at 14:1 for
KC Series boilers and 15:1 for Benchmark
boilers, and does not consider the number
of boilers in the plant.
In Sequential Mode, each boiler is started
one at a time based on the load and
start/stop levels programmed in the BMS.
The BMS will start a single boiler when
there is a load demand. Once the first boiler
reaches the start level, a second boiler will
be started and the load will be distributed
evenly between the two boilers.
For instance, if a start level of 50% is
chosen, when the first boiler reaches 50% a
second boiler will start (after a 30 second
delay), and the BMS will distribute the load
25% for each boiler. If the firing rate of both
boilers reach the start level, a third boiler is
started by the BMS (after a 30 second
delay), and the load will be distributed
across all three boilers. This sequence will
continue base on load demand and the
number of boilers connected to the BMS.
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