Page 456 - Advanced thermodynamics for engineers

CHAPTER
19
PINCH TECHNOLOGY

In recent years, a new technology for minimising the energy requirements of process plant has been
developed: this has been named pinch technology or process integration by its major proponent,
Linnhoff (Linnhoff and Senior (1983); Linnhoff and Turner (1981); Smith (1995)). Process plant, such
as oil refineries or major chemical manufacturing plant, requires that heating and cooling of the feed
stock take place as the processes occur. Obviously it would be beneficial to use the energy from a
stream which requires cooling to heat another which requires heating; in this way the energy which has
to be supplied from a high temperature source (or utility) is reduced, and the energy which has to be
rejected to a low temperature sink (or utility) is also minimised. Both of these external transfers incur a
cost in running the plant. Pinch technology is an approach which provides a mechanism for automating
the design process, and minimising the external heat transfers.
Pinch situations also occur in power generation plant; for example, in a combined cycle gas turbine
(CCGT) plant (see Fig. 19.1) energy has to be transferred from the gas turbine exhaust to the working
fluid in the steam turbine. A T-s diagram of a CCGT plant is shown in Fig. 19.2, where the heat transfer
region is shown: the pinch is the closest approach in temperature between the two lines. It is defined as
the minimum temperature difference between the two streams for effective heat transfer, and is due to
the difference in the properties of the working fluids during the heat transfer process (viz. the exhaust
gas from the gas turbine cools down as a single phase but the water changes phase when it is heated) –
this limits the amount of energy that can be taken from the hot fluid. The heat transfer processes are
shown on a Temperature – Enthalpy (T–H) transfer diagram in Fig. 19.3, where the pinch is obvious.
Perhaps the easiest way of gaining an understanding of pinch techniques is to consider some simple
examples.

19.1 HEAT TRANSFER NETWORK WITHOUT A PINCH PROBLEM
This example has a total of seven streams, three hot and four cold, and it is required to use the heating
and cooling potential of the streams to minimise the heat transfer from high-temperature utilities, and
the heat transfer to low-temperature utilities. The parameters for the streams involved in the processes
are given in Table 19.1. The supply temperature, T S , is the initial temperature of the stream, and the
target temperature, T T , is the target final temperature that must be achieved by heat transfer. The heat
flow capacity, mC, is the product of the mass flow and the specific heat of the particular stream, and the
heat load is the amount of energy that is transferred to or from the streams.
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