Flue Gas Temperature - Bosch UT-L 1 Technical Manual

30 | Technical description
3.5.5

Flue gas temperature

The flue gas temperature is the temperature measured
inside the flue pipe, specifically at the boiler flue outlet.
It depends on the boiler load and the system
temperature ( Fig. 21 to Fig. 23). The minimum
possible flue gas temperature should generally be used
to calculate the chimney size. This will be approx. 7.5 K
below the specified flue gas temperature, relative to an
average boiler water temperature of 70 °C.
Change in the flue gas temperature
The flue gas temperature depends on the average boiler
water temperature. The flue gas temperatures in the
graphs in Fig. 21 to Fig. 23 are relative to a temperature
pair of 80/60 °C in accordance with EN 303, i.e. the
average boiler temperature is 70 °C (for conversion to
other temperature pairs  Tab. 19).
UNIMAT UT-L boiler
230
220
210
200
190
180
170
160
0 2000
Fig. 21 Flue gas temperatures for UNIMAT UT-L boilers subject to the boiler load — overview 1
η
Boiler efficiency
K
ϑ
Flue gas temperature
A
ϕ
Boiler load
K
Rated heating output
Q
K
6 720 807 794 (2013/04)
4000 6000 8000
Average boiler water
temperature
[ °C]
60
70
80
90
100
Table 19 Change in the flue gas temperature subject to
the average boiler temperature
Example
• Given
– UNIMAT UT-L boiler
– Rated output Q
– System temperatures 100/80 °C
• Check
– Change of 15 K in the flue gas temperature
( Tab. 19)
– Flue gas temperature read off ϑ
( Fig. 21)
• Result
– Flue gas temperature at full boiler load
= 198 °C + 15 K = 213 °C
10000 12000 14000
Change in the flue gas
temperature
[K]
–7.5
0
7.5
15
22.5
= 6000 kW
K
= 198 °C
A
90,7
91,1
91,5
92,0
92,4
92,9
93,3
93,7
16000 18000 20000
Q
[kW]
K
6 720 803 977-08.1ITL
UNIMAT