CTRLMax Manual

55
CHAPTER 8
CHAPTER 8 - CASCADE
8.1. Operating Principles of an CTRL-
Max-Controlled Cascade (PRESTIGE and
INSTINCT only)
PRESTIGE and INSTINCT boilers can be setup in a cas-
cade conguration (max. 6 boilers) that is controlled by
the CTRLMax Boiler Control System.
The boilers can operate together without the need for
an external cascade controller. One unit will be selected
as the Master and will be wired to accept all the low
voltage control signals and all the system pumps. The
Master appliance organizes the distribution of the heat
demands over the appliances in the cascade
The other boilers will be designated as Slaves and will
only respond to calls from the Master Boiler and will not
respond to internal sensors. The internal supply sensor
will still limit the local.
The CH heat demands follow the same logic as the heat
demands in a single appliance: the target setpoint at
rst start-up is calculated, following a temperature rise
of 7.2°F/min. The start-up sequence calculates the de-
sired capacity according to the target setpoint.
The rst appliance starts and as soon as it reaches twice
the minimum capacity of the appliance, the second
appliance will be started when the stage delay has ex-
pired. Both appliances are then running at minimum
capacity.
The capacity increase continues following the rise of
the calculated target setpoint. The third appliance will
be started as soon as the two appliances have reached
3 times the minimum capacity. The cascading strategy
is to have more boilers running at lower speeds than
less boilers running at higher speeds
The control parameters to determine the control speed
of the cascade is derived from the individual appliance
capacity parameter.
As soon as the System temperature comes in reach of
the target system temperature the capacity increase will
stop and the normal temperature control takes over.
Be aware that in a cascade the temperature
control is done on the system temperature and
NOT on the local temperature. That means that
the local temperature can be easily 9 to 12°F
higher than the system temperature depending
on the position of the system sensor.
NOTICE
The Cascade control works as a power control on the
local appliances. The system temperature is the param-
eter to regulate and the local temperatures are of no
avail, they are only used to limit the local power in case
the max. temperature is reached.
Additional characteristics :
1. The Cascade function allows up to six identical boil-
ers to operate together in a single heating system.
2. Parallel Modulation res as many boilers as possible
to maximize system eciency.
3. The Auto rotation function rotates the lead boiler
every time a call for heat is received when a Ther-
mostat option is chosen in CH Demand or every
24 hours when a Constant option is chosen in CH
Demand.
8.2. Cascade Algorithm
8.2.1 Boiler Enabling Algorithm
The Master boiler uses the following algorithm to deter-
mine when the next boiler can be enabled:
Once the currently red boilers reach the calculated r-
ing rate, the next boiler can be enabled without aect-
ing the overall cascade ring rate.
For example, the calculation for a Cascade System con-
sisting of two PRESTIGE Solo 399s would be:
Once the rst PRESTIGE Solo 399 ring rate reaches 50%,
the second PRESTIGE Solo 399 can be enabled. Both will
then re at the minimum 25% ring rate so that the over-
all output from the Cascade System remains the same.
8.2.2 Boiler Disabling Algorithm
Once the ring rate of all currently red boilers decreases
to the Minimum Firing Rate, a boiler can be disabled. The
boilers which continue to re will increase their ring rate if
required to replace the output of the disabled boiler.
Stable cascade operation requires that all boilers in a
Cascade System be the same size. Mixing boiler sizes in
a Cascade System could lead to temperature uctua-
tions and erratic cascade operation.
Number of Boilers Firing +1
Number of Boilers Firing
x Min. Firing Rate = Individual Boiler Firing Rate
1+1
1
x 25% = 50%
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