Environmental Engineering Reference
In-Depth Information
To calculate the HEN estimated surface area from the composite curves, utility streams
must be included with the process streams in the composite curves to obtain the balanced
composite curves. The resulting balanced composite curves should have no residual demand
for utilities.
Then the balanced composite curves are divided into vertical enthalpy intervals to
calculate the total minimum area targets assuming constant overall heat transfer coefficient
and pure vertical counter current heat transfer using the famous formual in heat transfer
Q=U*A*T_
L.M.T.D
[4]. Nowadays due to widespread use of computer programs for HEN
automated design such area targeting calculation is not very beneficial any more in industry.
Now we will start the design of the HEN using the well known pinch design method. A
good initialization of this design is to assume that no individual heat exchanger will have a
temperature difference smaller than ∆Tmin calculated from the targeting phase and there must
be no heat transfer across the pinch by process to process heat transfer or/and inappropriate
use of utilities.
These rules are important for the HEN design to achieve the energy target, given that no
individual exchanger should have a temperature difference smaller than ∆Tmin. To comply
with these two guidelines the design problem needs to be divided at the pinch and using the
grid diagram as shown in figure 20.
HEN
D
ESIGN
M
ETHOD
Four Streams Problem Example
The graph below shows the stream data of a very simple HEN problem drawn on a grid
diagram [1,2,3], where the pinch temperature is shown on both the hot and the cold sides
using a minimum approach temperature,
∆
T_min=10 ºC
The Grid Diagram for the Step -By-Step HEN Design
Pinch
CP
(kW/ ºC)
310 ºC
QH_min = 2870 kW
520 ºC
330 ºC
H1=10
380 ºC
300 ºC
H2=5
550 ºC
300 ºC
C1=20
380 ºC
320 ºC
C2=2
QC_min = 50 kW
300 ºC
Figure 20. Grid Diagram for HEN Design
