Information Technology Reference
In-Depth Information
√
R
2
+
1ln[
(
1
−
S)/(
1
−
RS)
]
(R
−
1
)
ln
2
−
S(R
+
1
−
√
R
2
+
1
)
2
F
=
(9.5)
+
√
R
2
+
1
)
−
S(R
+
1
where
T
hot
in
−
T
hot
out
R
=
(9.6)
T
cold
−
T
cold
out
in
T
cold
−
T
cold
out
in
S
=
(9.7)
T
hot
in
−
T
cold
in
When used in simulation mode, the state of the exchanger feeds must be specified.
Depending on the complexity of the model chosen, the heat transfer area,
A
, is either
specified or calculated from the heat exchanger physical layout. Heat transfer coeffi-
cients are calculated from correlations such as the Hewitt (1992) correlation prepared
by Gnielinski, which involves the Nusselt
N
NU
=
hD/k
, Reynolds
N
RE
=
DG/
μ
,and
Prandtl
N
PR
=
c
p
μ
/k
numbers, and the Darcy friction factor, given by
1
2
/
3
(9.8)
h
i
D
i
k
1000
)N
PR
1
+
12
.
7
√
F
D
/
8
(N
2
/
3
(f
D
/
8
)(N
RE
−
D
L
N
NU
=
=
+
−
1
)
PR
1
.
64
)
−
2
f
D
=
(
1
.
82 log
10
N
RE
−
(9.9)
Here
h
i
is the inside pipe heat transfer coefficient,
D
i
the inside pipe diameter,
k
the thermal conductivity,
G
the mass flow rate,
μ
the viscosity,
c
p
the heat capacity,
and
f
D
the Darcy friction factor. Complete documentation of all correlations used in
Aspen Plus can be found in Help: Heatx Reference and Model Reference. Depending
on which model is chosen,
U
is either specified or calculated iteratively during the
convergence process. The four heat-exchange-related models can be found in the model
library under the tab Heat Exchangers.
9.1 HEATER BLOCK
An example of the primary input form of the Heater block, which shows the possible
specifications, is shown in Figure 9.1. The Heater block offers a variety of ways to
specify the output stream state, all of which result in calculation of the energy required
to heat (or cool) a stream. Alternatively, one may specify the energy added to or
removed from a heater, which is used by the block to establish the state of the output
stream.
An important capability is the use of two heaters to model a heat exchanger
bypassing the use of equations (9.3) and (9.4), as shown in Figure 9.2. Note the use
of a heat stream to connect the two heaters. The heat stream should be aligned in the
correct direction, which depends on which heater will receive the heat, either positive
or negative. Care must be taken with the sign of the heat transferred (heat added is
positive). In this example the outlet temperature of heater H2 is specified and the heat,
stream 5, flows to heater H1. This example may be found at Chapter Nine
Examples/Heaters.
