Figure 9 Plot of DTmin against total annual
cost for the crude pre-heat train
Results as presented in Figure 9 show the plot of total cost index
against minimum temperature approach for Crude Pre-Heat Train of CDU I
Unit, KRPC. Capital cost decreases with increasing minimum temperature
difference while operating cost increases with the minimum delta T.
Along with variation in the minimum approach temperature, the
corresponding total annualized cost of the HEN was calculated in order
to find a ΔTmin that yields the minimum total annual
cost. The plot shows that optimum minimum temperature approach desired
is 2.95 K which is similar to the one obtained in one of the examples
solved by Yee and Grossmann . This value of minimum temperature approach
was determined by parametric optimization. Here, the least total annual
cost of $4.88 million/yr was attained at ΔTmin of 2.95
K, with a total of seventeen heat exchanger units to achieve maximum
energy recovery. The DTmin that gives the minimum total
annual cost was found to be 2.95K and the corresponding total annualized
cost was $4.88 million/yr.
The minimum temperature approach (ΔTmin) was found to
affect the pinch location, minimum number of heat exchangers, and the
heat exchanger area. In the plot of pinch temperature against minimum
Delta T, the pinch point of 519.96 K occurred at minimum temperature
difference of 38 oC. The dependence of number of heat
exchangers on the Delta minimum is shown in the minimum number of heat
exchangers vs. DTmin plot. The plot revealed that the
minimum number of heat exchangers of 21 was obtained at minimum
temperature approach of 38 oC. Also, the area target
plot showed that there is an exponential relationship between the total
heat exchange area and the minimum temperature difference.