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iomass design best practice
Biomass boiler installation without buffer vessel
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Gas
boiler
Gas
boiler
Gas
boiler
Gas
boiler
Gas
boiler
Biomass
boiler
Biomass
boiler
pump
Back-end
valve
Training will be needed for
architects, civil engineers,
cost consultants, town
planners, project managers,
construction managers,
commissioning engineers
and technicians, facilities
managers, maintenance
fitters and site staff
A properly designed low-loss header helps avoid
interaction between water flowing in boilers –
normally at a constant temperature and flow rate,
and the variable temperature and flows in heating
and hot water circuits. This hydraulic isolation
helps the biomass boiler work efficiently and within
manufacturer’s limits. Low loss header and its
associated circuits should be designed as follows:
Rule 1. The flow along the header must always be
in a forward direction. This requires the total of
flows from the primary circuits to be greater than
the total of flows of the secondary circuits at all
times.
Rule 2. The flow velocity along the header should
not exceed 0.15m/s at full load. A rule of thumb
to achieve this is to ensure that the diameter of
the header is at least three times that of the largest
pipe attached to the header.
Rule 3. The header should be mounted vertically.
250 per month. It has been estimated by
Colin Ashford, lead author on the CIBSE
Application Manual that, to meet the UK
government’s targets for decarbonising
buildings by 2020, 400 fully trained
professional engineers and 4,000 design
technicians will be required.
Sizing chimneys
In ‘slumber mode’, the boiler fire bed
remains alight with minimal combustion
air. Under these conditions, flue gases
have been measured to be within the
lower and upper explosive limits. The
fire bed contains the ignition source, and
explosions have been reported.
Planners will need support to make
rational decisions, but very few people
working now have ever sized chimneys.
Common header (low-loss header) design rules
HM
At low flow velocity any sludge in the hydraulic
system will accumulate in the header. The header
must be mounted vertically in order to trap sludge
at the bottom and be able to drain it. Any air in the
system will rise to the top of the header, where it
can be removed with an automatic air valve.
Rule 4. The header should operate at neutral
pressure. To achieve this, the suction (inlet) side
of all pumps in the system should be connected
directly to the header. This is consistent with
good practice design, where boiler pumps should
pressurise boilers to avoid kettling, and secondary
load pumps pressurise load circuits.
Rule 5. System pressurisation should be directly
onto the header. The pressurisation connection
should be above the level at which sludge could
collect and below the lowest primary circuit
connection. This ensures that every pump is
pressurised on its inlet to avoid cavitation.
Modern biomass boilers have very
different performance characteristics and
requirements from either current fluidfuelled
boilers or former coal boilers. Most
of the relevant skills base has now retired.
To stop flue gases causing a nuisance,
planning authorities must be informed by
flue height calculations, such as specified
in BS EN 13384-1 3 , together with realistic
topographical appraisal. This chimney
sizing calculation method was available in
2003. In addition, dispersion modelling
will be advisable if the flue has nearby tall
buildings – that is, a distance of less than
seven times the building’s height.
There is some confusion about the
applicable guidance and regulations on
flue heights. HMIP Technical Guidance
Note (Dispersion) D1: Guidelines on
discharge stack heights for polluting emissions
is available from the Environment Agency.
That note assumes efflux velocities greater
than 10 m/s, which is associated with older
boilers with high flue gas temperatures.
Biomass boiler flues have efflux velocities
in the order of 3 m/s and occasionally up to
5 m/s. D1 calculations are not applicable.
Also, HETAS guidance applies to biomass
and solid fuel domestic heating appliances
and associated services.
Safety on loss of electrical supply
Few designers realise the need to maintain
flue draught on power failure. When
power fails on a biomass installation, it
does not stop combustion or generation
of flue gases, so installations may need
an uninterruptible power supply (UPS)
configured as a safety critical system.
This has to maintain the biomass boiler’s
control system and the main building’s
building management system (BMS), run
pumps to take residual heat out of the
boiler, as well as the flue fan if installed,
until the installation has safely cooled. This
may need to run for 90 to 120 minutes.
Biomass boilers have a far higher
thermal inertia than oil or gas boilers.
Designers need to ask: ‘What happens if
the power fails?’ Water flow through the
boiler will stop, but the heat in the fire-bed
and firebricks must be removed if the
boiler is to avoid damage from overheating.
Flues with long horizontal runs will often
have fans – but these will stop. Then,
minimal heat will escape via the flue. Some
biomass boilers have a separate cooling
water facility to minimise overheating and
damage to the fire grate and fire-bricks.
The heated water will run to waste.
CIBSE Journal December 2012 www.cibsejournal.com
CT flow
U/F flow
DHW flow
U/F return
DHW return
CT return