Page 130 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

5.3 Multi-stream problem 127

Fig. 5.5(I) is based on the assumption that the hot and cold streams can match as long as the
temperature driving force is adequate. However, in reality, constraints do exist sometimes from
the considerations of safety, physical location of the two streams within the plant, etc. Therefore the
capital cost minimisation technique and ðDT min Þ optimisation is particularly applicable for systems
with fewer constraints such as in atmospheric and vacuum distillation preheat trains, FCC unit, etc. in
the petroleum refineries. Some network design software like the KBC-Super Target has capital-energy
optimisation facility.
The influence of pumping cost, a component of the operating cost, on optimum design of network
has not been considered so far. This component of cost depends on the pressure drop in the heat
exchangers as discussed in Chapter 4. The capital cost for pumps and compressors can also be included
in the cost estimate of the network. Fig. 5.5 depicts the effect of neglecting pumping cost on the
optimum value of ðDT min Þ. Typically the effect is to increase the DT min;opt value by a small amount as
can be seen in the figure.

T T
I II
q rec
q rec
H H

(A+B+C)
(A+B)
A = HE cost
Cost A B B = Utility cost
C = Pumping cost
C A+B+C = Total cost (with pumping)

I ΔT min, opt III
Δ T min
FIGURE 5.5
Trade-off between utility and exchanger costs leading to minimum cost and the influence of pumping cost on
optimumðDT min Þ.

5.3.2 Practical values of DT min
Usually ðDT min Þ is not less than 10 C in shell and tube exchangers. This requires pure counter-current

flow which is not possible even for single shell and single tube pass due to periodic cross-flow of the
shell side stream. In case of plate type heat exchangers, the value of DT min can be w5 C and the same