p. 47/69
low temperature of the ambient air (for example -10
o
C or even lower) the
water vapor in the flue gases could condense and light smoke rake will be
visible above the exit orifice of the chimney.
EXPLANATION: the flow rate of the air, drawn in for the combustion
process strongly influences the operating conditions of he boiler – its
thermal capacity as well as its efficiency – in case the air flow rate is below
the optimal one, then the fuel will be partially oxidized, i.e. low combustion
efficiency, at high air flow rate the combustion process will be cooled down
and again the efficiency will be decreased.
5.3.8.
Estimating the thermal capacity of the boiler
The estimation of the thermal capacity of the boiler in practice could be
done by measuring the fuel flow rate, i.e. the consumption rate and to estimate
the heating value of the fuel, as well as the efficiency of the boiler. In the
following section is described in details:
o
Measure the average fuel flow for defined (or assumed) period of time
(for example one hour) – m
wood_logs
= 10.7 kg/h. Calculate the instant fuel
flow rate, by dividing the averaged mass of the fuel by 3600 (1 hour =
3600 seconds) and get the result m’
wood_logs
=
0.002972 kg/s;
o
Estimate the heating value of the fuel – H
wood_logs
= 13.9 MJ/kg = 13900
kJ/kg; In case that the moisture content is not known, then it has to be
estimated (or assumed)– see figure 2.1 , table 2.1;
o
Estimate (or assume) the efficiency of the boiler at nominal operating
conditions –
η
boiler
=85%=0.85 (or the efficiency at the estimated thermal
capacity of the boiler);
o
Calculate
the
thermal
efficiency
of
the
boiler
-
12
.
35
002972
.
0
*
13900
*
85
.
0
'
*
*
log
_
log
_
=
=
=
s
wood
s
wood
boiler
boiler
m
H
P
η
kW;
o
In case that the thermal capacity of the boiler should be estimated at
different than the nominal operating conditions as well as other fuel is
utilized (the type or the moisture content could be different) the same
approach should be utilized;
EXPLANATION:
•
At any change of the thermal capacity of the boiler (for example at
change of the fuel type and especially its moisture content)it is
recommended to make appropriate adjustment of the operating
parameters of the boiler – the position of the of the primary air
control flap as well as the position of the flap, which controls the
total air flow rate drawn in the boiler;
