Heat-exchanger networks

In an industrial process there will be many hot and cold streams and there will be an optimum arrangement of the streams for energy recovery by heat exchange. The problem 

Consider that there are M hot streams, Shi(i = 1, 2, 3. M) to be cooled and N cold streams Scjij = 1, 2, 3. A) to be heated each stream having an inlet temperature tf, or an outlet temperature to, and a stream heat capacity Wi. There may also be Suk(k = 1, 2, 3. L) auxiliary steam heated or water-cooled exchangers. 

Porton and Donaldson (1974) suggest a simple procedure that involves the repeated matching of the hottest stream (highest tf) against the cold stream with the highest required outlet temperature (highest fo)- 

A general survey of computer and manual methods for optimising exchanger networks is given by Nishida et al. (1977) see also Siirola (1974). 

The design of heat exchanger networks is covered in more detail is Section 3.17. 

In the 1980s, there was a great deal of research into design methods for heat exchanger networks see Gundersen and Naess (1988). One of the most widely applied methods that emerged was a set of techniques termed pinch technology, developed by Bodo Linnhoff and his collaborators at ICI, Union Carbide, and the University of Manchester. The term derives from the fact that in a plot of the system temperatures 

In this section the fundamental principles of the pinch technology method for energy integration will be outlined and illustrated with reference to a simple problem. The method and its applications are described fully in a guide published by the Institution of Chemical Engineers, IChemE (1994) see also Douglas (1988) and Smith (2005). 

The development and application of the method can be illustrated by considering the problem of recovering heat between four process streams two hot streams that require cooling and two cold streams that must be heated. The process data for the streams is set out in Table 3.2. Each stream starts from a source temperature, Tj, and is to be heated or cooled to a target temperature, Tj. The heat capacity flow rate of each stream is shown as CP. Eor streams where the specific heat capacity can be taken as constant, and there is no phase change, CP will be given by 

The heat load shown in the table is the total heat required to heat or cool the stream from the source to the target temperature. 

There is clearly scope for energy integration between these four streams. Two require heating and two require cooling and the stream temperatures are such that heat can be transferred from the hot to the cold streams. The task is to find the best arrangement of heat exchangers to achieve the target temperatures. 

---

Of course, some processes do not require a reactor, e.g., some oil refinery processes. Here, the design starts with the sepauration system and moves outward to the heat exchanger network and utilities. However, the basic hierarchy prevails. 

Having found the best nonintegrated sequence, most designers would then heat integrate. In other words, the total problem is not solved simultaneously but in two steps. Moving outward from the center of the onion (see Fig. 1.6), the separation layer is addressed first, followed by the heat exchanger network layer. 

Heat Exchanger Network and Utilities Energy Targets... 

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. 

After maximizing heat recovery in the heat exchanger network, those heating duties and cooling duties not serviced by heat recovery must be provided by external utilities. The outer-most layer of the onion model is now being addressed, but still dealing with targets. 

The energy cost of the process can be set without having to design the heat exchanger network and utility system. These energy targets cam be calculated directly from the material and energy balance. Thus... 

Cerda, J., Westerberg, A. W., Mason, D., and Linnhoff, B., Minimum Utility Usage in Heat Exchanger Network Synthesis—A Transportation Problem, Chem. Eng. ScL, 38 373 1983. 

In addition to being able to predict the energy costs of the heat exchanger network and utilities directly from the material and energy balance, it would be useful to be able to calculate the capital cost, if this is possible. The principal components that contribute to the capital cost of the heat exchanger network are... 

Let us take each of these components in turn and explore whether they can be accounted for from the material and energy balance without having to perform heat exchanger network design. 

To understand the minimum number of matches or units in a heat exchanger network, some basic results oigraph theory can be used. A graph is any collection of points in which some pairs of points are... 

FIgur 7.4 If film transfer coefficients difier significantly, then nonvertical h t transfer is necessary to achieve the minimum area. (Reprinted from Linnhoff and Ahmad, Cost Optimum Heat Exchanger Networks I. Minimum Energy and Capital Using Simple Models for Capital Cost," Computers Chem. Engg., 7 729, 1990 with permission from Elsevier Science, Ltd.)... 

Calculate the capital cost target for the mixed specification heat exchanger network from Eq. (7.21) using the cost law coefficients for the reference specification. 

Different utility options such as furnaces, gas turbines, and different steam levels can be assessed more easily and with greater confidence knowing the capital cost implications for the heat exchanger network. 

The design of the heat exchanger network is greatly simplified if the design is initialized with an optimized value for... 

---