Environmental Engineering Reference
In-Depth Information
outside of the tubes. Heat is therefore transmitted from the hot water inside the tubes to
seawater on the outside. The overall heat transfer coefficient for the heater can be expressed
as
Q
U
=
h
(13)
h
A
(Δ
T
)
h
h
where
Q
h
is the rate of heat transfer (kcal/h),
A
h
is the heat transfer area (
A
h
= 24.9 m
2
) and
(Δ
T
)
h
is the log-mean-temperature difference obtained from the equation:
{
T
−
T
(
)}
−
{
T
−
T
(
)}
hw1
ev
hw
2
pr
(
Δ
T
)
=
(14)
h
{
T
−
T
(
)}
ln
hw1
ev
{
T
−
T
(
)}
hw
2
pr
The rate of heat transfer can be estimated from
Q
=
m
C
(
T
−
T
)
(15)
h
hw
p
hw
1
hw
2
The specific heat
C
p
is calculated at the average temperature (
T
hw1
+
T
hw2
)/2 .
5.3.3. Average OHTC of Other Evaporator Effects
In the 2
nd
-18
th
effects the mechanism of heat transfer is different from that in the first
effect. In these effects, boiling takes place on the outside of the horizontal tubes while
condensation occurs on the tube inside. The average overall heat transfer coefficient for the
17 evaporators was estimated from the equation
Q
ev
Δ
U
=
(16)
ev
A
(
T
)
ev
ev
where
Q
ev
is the average heat transfer rate for each evaporator effect, (Δ
T
)
ev
is the average
log-mean-temperature difference for each evaporator and
A
ev
is the heat transfer area of each
evaporator (
A
ev
= 63.1 m
2
). The heat transfer rate
Q
ev
is obtained from the fact that the
distillate production consists of two components: production by the 17 (2
nd
-18
th
) effects and
production by the 17 (1
st
- 17
th
) preheaters:
m
=
(
m
)
+
(
m
)
(17)
d
d
ev
d
pr
where (
m
)
ev
is the distillate production by the 17 evaporator effects and (
m
)
pr
is the
m
)
pr
is calculated from a heat balance equation over all the
production by the 17 preheaters. (
preheaters:
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