SCIENCE CHINA Technological Sciences, Volume 59 , Issue 10 : 1517-1523(2016) https://doi.org/10.1007/s11431-016-0160-0

## An area method for visualizing heat-transfer imperfection of a heat exchanger network in terms of temperature–heat-flow-rate diagrams

• AcceptedAug 8, 2016
• PublishedSep 12, 2016
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### Abstract

The identification of the imperfection originating from finite-temperature-difference heat transfer is an indispensable step for both the performance analysis and the better design of a heat exchanger network (HEN) with the aim of energy saving. This study develops a convenient area method for visualizing the heat-transfer imperfection of a HEN in terms of temperature–heat flow diagrams ( T-Q˙ diagrams) by combining the composite curves that have already been used in pinch analysis and the recently developed entransy analysis. It is shown that the area between the hot and cold composite curves and the hot and cold utility lines on a T -Q˙ diagram is just equal to the total entransy dissipation rate during the multi-stream heat transfer process occurred in a HEN, and this area can be used to graphically represent the total heat-transfer imperfection of the HEN. The increase in heat recovery or decrease in energy requirements with decreasing the minimum temperature difference, ΔTmin, of a HEN can then be attributed to a lower entransy dissipation rate, quantitatively represented by the decrease of the area between the composite curves and the utility lines. In addition, the differences between the T- Q˙ diagram and the pre-existing energy level–enthalpy flow diagram (Ω-H diagram) in the roles of visualizing process imperfection and designing HENs are discussed.

### Acknowledgment

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51206079, 51356001).

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• Figure 1

(Color online) The $T-Q˙$ diagram of a heat transfer process occurring in a heat exchanger with an arbitrary flow arrangement.

• Figure 2

(Color online) A two-stream heat exchanger in steady operation. (a) The enthalpy entransy of hot or cold stream at the inlet or outlet of the heat exchanger; (b) the entransy transfer rate into the cold stream is equal to the entransy transfer rate out of the hot stream minus the entransy dissipation rate during the heat transfer process between hot and cols streams.

• Figure 3

(Color online) Sketch of a HEN comprising arbitrary number of hot and cold streams. A pair of circles stands for a heat transfer process between hot and cold streams.

• Figure 4

(Color online) Proof that the area of trapezoid ④ equals to the sum of the areas of trapezoids ①, ② and ③.

• Figure 5

(Color online) Balanced composite curves for a multi-stream HEN.

• Figure 6

(Color online) Shifted composite curves for a multi-stream HEN with a smaller ΔTmin.

• Figure 7

(Color online) A typical Ω-H diagram.

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