Page 133 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

130 Chapter 5 Heat exchanger network analysis

temperature interval T 3 to T 5 , the composite curves do not overlap and hence heat transfer is not feasible
in this temperature range. Relative slopes of the two curves in the specified interval govern the amount of

heat that can be recovered. This is overcome by shifting the hot composite curve lower by ðDT min = 2Þ
than specified and the cold composite higher by ðDT min =2Þ. This makes the shifted composite curves
touch at the pinch as shown in Fig. 5.6B. Heat balance between the shifted composite curves within a
shifted temperature interval is now possible since the hot streams are actually hotter than the cold
streams in each shifted temperature interval ðDT min Þ. The vertical shifting eliminates the problem of
ensuring heat transfer feasibility within the temperature intervals but does not alter the horizontal overlap
between the curves, i.e. the extent of heat recovery and utility requirement remains unaltered.

(A)

Composite
curves
T 5
T→ T 4

T 3
ΔT min
T 2

T 1
H→
(B)

Shifted
composite
curves
T→

H→

FIGURE 5.6
(A) Composite curves with infeasible heat transfer in the interval T 3 to T 5. (B) Shifted composite curves with
feasible heat transfer in the temperature range.