Environmental Engineering Reference
In-Depth Information
m
C
(
T
(
−
T
)
fw
p
pr
con2
(
m
)
=
(18)
d
pr
L
av
The latent heat of vaporization
L
av
is estimated at an average temperature of [
T
ev
(1) +
T
ev
(18)]/2. The average heat transfer by each evaporator effect is obtained from:
m
Q
ev
=
ev
×
L
av
17
(19)
m
L
−
m
C
(
T
(
−
T
)
d
av
fw
p
pr
con2
=
17
The average log-mean-temperature difference (Δ
T
)
ev
is calculated as the average
temperature difference between the heating steam inside the evaporator tube bundles and the
boiling brine on the outside. Noting that the heating steam temperature in a particular effect
i
is slightly lower than the brine temperature in the preceding effect (i.e., effect
i
-1) due to:
•
the boiling point elevation (
BPE
),
•
the temperature drop across the demister inside (Δp)
demis
,
(Δ
T
)
ev
was estimated from the following relation
18
17
∑∑
=
T
(
−
T
(
18
)
−
BPE
−
(
Δ
p
)
ev
ev
demis
i
1
i
=
1
(
Δ
T
)
=
(20)
ev
17
The values of the boiling point elevation (
BPE
) and the demister pressure drop (Δp)
demis
are calculated using the correlations given in the appendix.
5.3.4. Average OHTC of Preheaters
In the 17 preheaters seawater flowing inside the tube bundles of heat exchangers is heated
up by steam condensing on the tube outside. For any preheater, the average OHTC (kcal/hr
m
2o
C) can be calculated using the equation:
Q
ph
U
=
(21)
ph
A
×
(
Δ
T
)
×
17
ph
ph
where
Q
ph
is the heat transfer rate for all preheaters , kcal/hr and
A
ph
is the heat transfer area
of each preheater, m
2
(
A
ph
= 19.5 m
2
).
Q
ph
is calculated from the measured feedwater flow
rate and the temperature difference between the outlet of preheater #1 (top preheater) and the
outlet of the condenser:
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