HEAT EXCHANGER NETWORK...........

 HEAT EXCHANGER NETWORK 

 THE HEN is a heat exchanger network synthesis program that was developed by Professor F.C. Knopf at Louisiana State University. It integrates the networks of heat exchangers, boilers, condensers and furnaces for best energy utilization. It uses thermodynamic pinch analysis as the basis for designing a network. 

THEN uses pinch analysis for the optimum design of heat exchanger network. It employs three concepts: the composite curves, the grid diagram of process stream and the pinch point; and these are applied to minimize the energy use in the process. This program takes the necessary streams information from the user and decides the best arrangement of heat exchangers, heaters and coolers so that the amount of utilities needed such as cooling water and steam is minimized. It plots the Grand Composite Curve (GCC), which shows the enthalpy of the process streams as a function of temperature. It also shows the heat exchanger network design in a graphical form as the network grid diagram for visualization of the network configuration.

Equipment Selection for Heat Exchange in the Heat

Exchanger Network.....

 The shell and tube heat exchanger is the most common of various types of 
unfired heat transfer equipment used in industry. Although it is not especially 
compact, it is robust and its shape makes it well suited to pressure operation. 
It is also versatile and it can be designed to almost any application. 
A shell and tube heat exchanger consists of a shell, invariably cylindrical 
containing a nest of tubes plain or finned, which run parallel to the 
longitudinal axis of the shell, and are attached to perforated flat plates, 
baffles at each end. The tubes pass through a number of baffles, along their
length which serve to support them and to direct the fluid flow in the shell.
The assembly of tubes and baffles is a tube bundle held together by a system 
of tie rods and spacer tubes. The fluid which flows inside the tubes is directed
by means of special ducts, known as stationary and near heads or channels (12). 
 
One Fluid stream flows through the inside of several tubes in parallel on 
the tube side of heat exchanger, while the other fluid flows over the outside of 
the tubes on the shell side of the heat exchanger. Baffles are used on the shell 
side to make the fluid flow back and fourth across the tubes at the desired
velocity (13)
. 
 The amount of heat exchanged depends on the flow rates, temperature 
difference, and thermal properties of the fluids, as well as the design of heat 
exchangers, in particular the heat exchange surface area. 
 In co-current operation the hot and cold streams pass through the 
exchanger in the same direction, and in counter current operation the streams 
flow in opposite directions. The direction of flow has a significant effect on 

the exchanger.

Work and Heat Exchange Networks

While each of the five formulations have their specific advantages, the question that arises from an application perspective is which of these formulations should be used? That is the question we were trying to answer in a paper published last year. The application we considered was Work and Heat Exchange Networks.

Work and Heat Exchange Networks relates to how to design compression and expansion processes. Distinct parts of an industrial process typically operate at different pressures that require process streams to be compressed or expanded. Compression is typically done with electrically driven turbo compressors while turbo expanders drive electric generators to produce electricity.

Thus the goal of Work Exchange Networks is to design processes that consume the least electricity for compression while generating the maximum electricity from expansion

HOW DOES THIS RELATE TO HEAT INTIGRATION ?

When a stream is compressed, not only does its pressure increase, there is also an associated increase in temperature, while for an expansion process it is the other way around where both pressure and temperature of a process are decreased. Thus it is possible to provide cooling requirement to a process by cooling produced by the expansion process.

This is indeed a standard in refrigeration processes used in the industry and in your refrigerator at home. Hence it is not possible to fix temperature for heat integration when dealing with processes with compressors and expanders that require optimisation. Thus simultaneous process and heat integration is required here.

Our work showed that for small sized problems, the smooth approximation reformulation worked best. The reformulations of the Duran-Grossmann model can only deal with small to moderate scale problems. Since there is a large number of binary variables in the model, it is quite computationally expensive for large-scale problems with the four reformulations evaluated in this study.

Heat Exchanger networking methods:

1)Heuristics method 

2)Temperature interval method 

3)Pinch technology 

Heuristics method

 The general techniques that have been developed previously for solving 
HEN problem included the heuristics approach based on the use of rules of 
thumb. The selection rules which favor the use of a given piece of equipment 
in certain phases of system synthesis evolve from experience and are thought 
to be part of the empirical skill of successful process designers. These rules 
may be wrong on occasion and will lead to non minimum cost systems, but 
the experienced designer requires only that the rules lead to efficient designs
frequently enough to warrant their use. Heuristics rules are useful empirically 


Temperature Interval Method

 The temperature –interval method was developed by Linnhoff and Flower 
(10) following the pioneering work of Hohmann. Any network will solve the 
problem may be thought of as an array of sub networks. Each of these sub 
networks include all streams (or part of streams), which fall within a defined
temperature interval. The temperatures T1, T2, T3… Tn+1 are deduced from the
problem data in the following way: Each stream supply and target 
temperatures are listed after the temperatures of the hot streams have been 
reduced by the minimum temperature difference ∆Tmin. The highest 
temperature in the list is called T1, the second highest T2, and so on.
Generally, the following expression holds:- 
N = 2Ζ −1 ….. (2.1) 
Where N represents the number of sub networks can obtain for the system and 
Z: The number of streams. 
Each sub network represents a separate synthesis task. However, since all
streams in a sub network run through the same temperature interval, the 
synthesis task is very easy (10). 
As will be seen, a systematic procedure unfolds for determining the 
minimum utility requirements over all possible HENs, given just the heating 
and cooling requirements for the process streams and the minimum approach 
temperature in the heat exchangers, ∆Tmin

 Pinch technology

 The term "Pinch Technology" was introduced by Linnhoff and 
Verdeveld (21) to represent a new set of thermodynamically based methods 
that guarantee minimum energy levels in design of heat exchanger networks. 
Over the last two decades it has emerged as an unconventional development
in process design and energy conservation .The term pinch analysis is often 
used to represent the application of the tools and algorithms of pinch 
technology for studying industrial process.
Pinch technology presents a simple methodology for systematically 
analyzing chemical processes and the surrounding utility systems with the 
help of the first and second laws of thermodynamics. The first law of 
thermodynamics provides the energy equation for calculating the enthalpy 
changes (∆H) in the streams passing through a heat exchanger .The second
law determines the direction of heat flow .That is heat energy may only flow 

in the direction of hot to cold.