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FUNDAMENTALS OF ENERGY BALANCES
pressure is expanded over the throttle value and fed to the condenser, to provide cooling
to condense the vapour from the column. The vapour from the condenser is compressed
and returned to the base of the column. In an alternative arrangement, the process vapour
is taken from the top of the column, compressed and fed to the reboiler to provide heating.
The “efficiency” of a heat pump is measured by the coefficient of performance, COP:
energy delivered at higher temperature
COP D
energy input compressor
The COP will depend principally on the working temperatures.
The economics of the application of heat pumps in the process industries is discussed
by Holland and Devotta (1986). Details of the application of heat pumps in a wide range
of industries are given by Moser and Schnitzer (1985).

3.17. PROCESS INTEGRATION AND PINCH TECHNOLOGY

Process integration can lead to a substantial reduction in the energy requirements of a
process. In recent years much work has been done on developing methods for investigating
energy integration and the efficient design of heat exchanger networks; see Gundersen
and Naess (1988). One of the most successful and generally useful techniques is that
developed by Bodo Linnhoff and other workers: pinch technology. The term derives from
the fact that in a plot of the system temperatures versus the heat transferred, a pinch
usually occurs between the hot stream and cold stream curves, see Figure 3.22. It has
been shown that the pinch represents a distinct thermodynamic break in the system and
that, for minimum energy requirements, heat should not be transferred across the pinch,
Linnhoff and Townsend (1982).
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).

3.17.1. Pinch technology
The development and application of the method can be illustrated by considering the
problem of integrating the utilisation of energy between 4 process streams. Two hot
streams which require cooling, and two cold streams that have to be heated. The process
data for the streams is set out in Table 3.3. Each stream starts from a source temperature
T s , and is to be heated or cooled to a target temperature T t . The heat capacity of each
stream is shown as CP. For streams where the specific heat capacity can be taken as
constant, and there is no phase change, CP will be given by:

CP D mCp

where m D mass flow-rate, kg/s
Cp D average specific heat capacity between T s and T t kJ kg 1Ž C 1