Heat exchanger networks temperature

The analysis of the heat exchanger network first identifies sources of heat (termed hot streams) and sinks (termed cold streams) from the material and energy balance. Consider first a very simple problem with just one hot stream (heat source) and one cold stream (heat sink). The initial temperature (termed supply temperature), final temperature (termed target temperature), and enthalpy change of both streams are given in Table 6.1. 

Example 16.1 The process stream data for a heat recovery network problem are given in Table 16.1. A problem table analysis on these data reveals that the minimum hot utility requirement for the process is 15 MW and the minimum cold utility requirement is 26 MW for a minimum allowable temperature diflFerence of 20°C. The analysis also reveals that the pinch is located at a temperature of 120°C for hot streams and 100°C for cold streams. Design a heat exchanger network for maximum energy recovery in the minimum number of units. 

While a superstructure based on the structure in Fig. 16.26 allows for many structural options, it is not comprehensive. Wood, Wilcox, and Grossmanr showed how direct contact heat transfer by mixing at unequal temperatures can be used to decrease the number of units in a heat exchanger network. Floudas, Ciric, and Grossman showed how such features can be included in a heat exchanger network superstructure. Figure 16.27 shows the structure from Fig. 16.26 with possibilities for direct contact heat transfer included. In the... 

A good initialization for heat exchanger network design is to assume that no individual exchanger should have a temperature difference smaller than AT n. Having decided that no exchanger should have a temperature difference smaller than two rules were deduced in... 

Fig. 5. Simple heat-exchange network where stream C is heated, and streams A and B are cooled (a) schematic (b) temperature—cumulative duty curve...

Determine the pinch temperatures and the minimum utility requirements for the streams set out in the table below, for a minimum temperature difference between the streams of 20°C. Devise a heat exchanger network to achieve the maximum energy recovery. 

So far, it has been assumed that the minimum temperature difference for a heat exchanger network applies globally between all streams in the network. However, there are occasions when nonglobal minimum temperature differences might be required. For example, suppose a heat... 

Table 17.7 Variation of heat exchanger network area with steam turbine exhaust temperature.

The pinch design method creates a network structure based on the assumption that no heat exchanger should have a temperature difference smaller than ATmin. Having now created a structure for the heat exchanger network, the structure can now be subjected to continuous optimization. The constraint that no exchanger should have a temperature... 

Effective temperatures. When extracting stream data to represent the heat sources and heat sinks for the heat exchanger network problem, care must be exercised so as to represent the availability of heat at its effective temperature. For example, consider the part of the process represented in Figure 19.8. The feed stream to a reactor is preheated from 20°C to 95°C before entering the reactor. The effluent from the reactor is at 120°C and enters a quench that cools the reactor effluent from 120°C to 100°C. The vapor leaving the quench is at 100°C and needs to be cooled to 40°C. The quenched liquid also leaves at 100°C but needs to be cooled to 30°C. How should the data be extracted ... 

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