Optimum
Outlet Temperature: This is the
temperature of an exchanger at its highest efficiency of operation. It can be found as follows:
Example
5.10: What is the water outlet temperature that could
reasonably be obtained, and the heat that could reasonably be recovered from a
co-current exchanger, counter-current exchanger, and 1-2 heat exchanger? Use the following information.
Inlet
temperature of hot fluid, T1 = 350 °F,
Inlet
temperature of cold fluid, t1 = 70 °F,
Specific heat
of hot fluid, C = 1 Btu/(lb·°F),
Specific heat
of cold fluid, c = 1 Btu/(lb·°F),
Mass flow rate
of hot fluid, W = 10000 lb/hr,
Mass flow rate
of cold fluid, w = 20000 lb/hr
Solution:
For the given situation, R, can be
found as
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(1)
Co-current arrangement: A practical approach at the outlet
of the exchanger cannot be less than 5 degrees or Δt = T2
- t2 = 5 °F.
$
t2:
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$
Outlet temperature of hot fluid, T2 = t2
+ Δt = 166.7 °F.
$
$
Heat transfer rate, Q1:
|
|
(2)
Counter-current arrangement: In this arrangement practical approach cannot be
less than 10 degrees or T2 - t1 = 10 °F.
$
Outlet temperature of hot fluid, T2 = 70 +
10 = 80 °F.
$
Outlet temperature of cold fluid, t2:
|
|
$
Heat transfer rate, Q2 (5.31 or 5.32) =
2.7´106
Btu/hr
(3)
1-2 heat exchanger, given R = 2, Use Figure 5.21 to obtain P
$
P = 0.375 (Fig 5.21)
$
Outlet temperature of cold fluid, t2:
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$
Outlet temperature of hot fluid, T2:
|
|
$
Heat transfer rate, Q3 = (5.31 or 5.32) =
2.1´106
Btu/hr
The
value of Q is the highest for a counter-current arrangement, Q1.