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.