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