BIOMASS DESIGN BEST PRACTICE
30 minutes to two hours after the call for heat ceases. For efficient operation, this heat should not be lost up the flue, but stored in a buffer vessel or thermal store. A full controllability review is recommended at an early stage in the design – controls cannot just be added on at the end of the design process. Variable flow is essential for some systems to work effectively and, together with temperature control from the thermal store, can maximise thermal store stratification and hence its effectiveness. A major cause of underusing biomass
Very few people working now have ever sized chimneys
What happens if that water supply is from a cold-water boost set? Water in a break tank can run out. The emergency cooling water supply must be testable for flow, and monitored for supply pressure by the BMS.
Effective control of biomass boilers Biomass boilers are far more complex to control effectively. Biomass systems must be designed to take account of the complete system operation, recognising that biomass boilers produce heat for
Biomass boiler with thermal store and condensing boiler AAV tp
Drench valve
P HM 1 T
Safety heat
Drench valve supply
Biomass boiler
SV tf T2 MV2 tf P CS1 T T
Load mixing valve
T3 T CS3 tp
boilers is that the auxiliary fossil fuel boilers are not effectively controlled and inhibited from operation when heat is available from the biomass boiler and thermal store. Once enabled, the fossil fuel boilers normally respond more quickly to demand and take over the lead role. The most effective control strategies frequently use heat-load control, rather than simple temperature-based control. Heat-load control has been available for many years but is not often used due to the additional cost of flow/heat meters. Fortunately, costs have significantly decreased in recent years. Any flow meter – including heat meters – must be correctly installed. Normally, a minimum of 10 diameters upstream and five diameters downstream of straight pipe are required, although some types of meter need more. Meters with analogue outputs are normally preferable for control functions.
exchanger T
Biomass boiler
DC CS1 pumpset
Back-end Valve
T1 System
pressurisation & expansion
MV3 DC CS2 SV AAV Burner
Condensing boiler with modulating burner
DC T4
Fossil fuel
boiler pumpset MV1 tr tp P T
4-port thermal Store
T T T5 T HM 2 Tr tp T5 Split headers
26
CIBSE Journal December 2012
www.cibsejournal.com
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