Technical data

14
In practice, heating systems rarely operate at full load conditions and so
configuring the system to optimise the part load performance is an effective
way of maximising the energy saving of the heating plant.
Purewell VariHeat Boiler Sequencing Cascade Controller
The boiler sequencing cascade controller available with Purewell VariHeat boilers
can be configured to sequence boilers in traditional cascade mode or in unison mode.
Cascade Control
Steps a boiler module on at its lowest rate and then modulates it to its
maximum rate, before switching on the next boiler module at its lowest rate,
to match the system load. Maintains the lowest number of boiler modules in
operation for a given heat load.
Unison Control
Steps each boiler module on in turn at its lowest rate, then modulates all
boiler modules simultaneously to higher rates to match the system load.
This method of sequencing can offer higher operating efficiencies, taking
advantage of the higher part load efficiencies available at low firing rates.
Using BMS Signals to Initiate Cascade
Temperature control 0 -10 Volt DC analog input.
The cascade controller can be configured to accept a BMS analog input to
initiate heat generation. Where analog inputs are used, the cascade controller
provides heat generation according to the magnitude of the input signal only.
NOTE: When using the BMS to initiate cascade control via a 0-10 volt analogue
signal, then the internal time clock and remote enable circuit functions are
disabled. Input signals to the RVA47 cascade control must be temperature
configured. The input signal is translated to a temperature set-point for the
flow temperature, and translation is according to a linear graph from 5°C to
an upper limit set by parameters during commissioning. 10 Volts corresponds
with the upper limit with a maximum 90°C setting.
Optimising Life Time Performance
In addition to providing system security,
accurate load matching is one of the major
advantages of using multiple boilers.
When first introduced, modular boilers
operated in a basic mode, where modules
were switched on or off to meet the
system demand for heat. The requirement
for turndown was met by increasing
or decreasing the number of modules
(boilers) in an installation.
As boiler designs advanced, two stage
modular boilers became more popular,
and high/low firing boilers changed the
parameters for turndown.
Today, most high efficiency boilers have fully
modulating capability and this once again
changes the methodology for achieving close
control in a multiple boiler installation.
When selecting modulating high efficiency
boilers, consideration should be given to the
life time costs associated with service and
maintenance of the boilers. Opt for too many
small output (kW) boilers and the servicing
costs will be higher, whereas too few large
output (kW) boilers will result in less back
up and system security.
It is important therefore to select the
optimum size boiler output and number
of modules to achieve a balanced life time
cost and ensure system security.
The Hamworthy range of Purewell VariHeat
boilers has been designed to satisfy a wide
range of commercial systems. Available as
condensing and non-condensing boilers,
with fully modulating burners, multiple
boiler applications enable accurate load
matching using the optimum number
of boiler modules.
Each module (boiler) can modulate
to part load conditions, where efficiency
performance will be the greatest.
Setting the boiler sequence controller
to fire each boiler at low fire before
modulating up to high fire will not only
provide the best efficiency performance,
but also enable the boilers to share the
system load and even out wear
and tear on the plant.
Multiple Boilers
With Modulating Burners
When there is no analog input, the boilers will not fire unless the frost protection
function of the boiler is activated. It is recommended that the BMS has a frost
protection setting higher than 5°C water temperature, to prevent the boiler
from firing without circulation. Please refer to the section on pump control, page 15.
Whilst operating with an analog input signal, the cascade controller will
provide lead boiler rotation programmable for number of hours run.