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4.1.2
Maximum airflow through heat exchanger
enclosure
Special attention must be given to ensure
that the temperature rise of the air,
passing through the enclosure around the
heat exchanger tubes, is high enough to
avoid condensation forming within the
tubes
. This condensation forming could
occur by super cooling of the products of
combustion. Such condensation can be the
source of severe corrosion and damage to
tubes or part of the tubes.
Figure 6 indicates the probability of
condensation forming within the tubes
reference :
-
the temperature rise of the air through
the enclosure around the tubes (see
table 2- fig. 4).
-
the inlet air temperature passing over the
heat exchanger
In function of the expected minimum inlet air
temperature,
the
minimum
required
temperature rise (to avoid condensation)
must be derived from the chart shown in
figure 6.
Example :
•
expected minimum inlet temperature = -
5°C
•
minimum required temperature rise shown
on the chart = +13°K
Table 3 indicates the heat output (kW) of the
different models.
The maximum allowable airflow
[
through the
enclosure (duct channel) around the tubes
]
can
be calculated as follows :
0.3423
minimum
T
1000
(kW)
put
out
heat
(m³/h)
max
V
×
×
=
Δ
Example :
<
Tmin = 13K (with min. inlet temp. = -5°C)
model 8075 09 at 100% heat output : 75,00kW
results into Vmax
≈
16854m³/h
Remark
:
For air heaters with the option 2 stage gas valve
(50% minimum heat input) or the option
modulating gas valve (30% minimum heat input)
attention must be given to determine the
maximum allowable airflow in function of the
appropriate heat input.
Example :
<
Tmin = 13K (with min. inlet temp. = -5°C)
model 8075 09 at 30% modulation (see table 3
‘output’) : 21.20kW
results into :
h
/
³
m
4764
0.3423
13
1000
(kW)
21.200
max
V
=
×
×
=
Figure 6 : Dew-point occurrence chart (condensation zone)
-20
░
C
0
░
C
20
░
C
40K
20K
0K
T
T
IN
LE
T
AI
R
TE
MP
ER
AT
UR
E
30K
CONDENSATION ZONE
AT
T
HE
H
EA
T
EX
CH
AN
GE
R
10K
TEMPERATURE RISE OVER THE HEAT EXCHANGER
20°C
0°C
-20°C
