BIOMASS DESIGN BEST PRACTICE
Biomass boiler installation without buffer vessel
Gas boiler
Gas boiler
Gas boiler
Gas boiler
Biomass boiler pump
Gas boiler
Biomass boiler
Back-end valve
HM
CT flow U/F flow
DHW flow U/F return
CT return DHW return
Modern biomass boilers have very different performance characteristics and requirements from either current fluid- fuelled 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 13384-13
, together with realistic
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
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
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.
24 CIBSE Journal December 2012
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.
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.
www.cibsejournal.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68