Page 135 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

132 Chapter 5 Heat exchanger network analysis

5.4.2 The pinch principle
The pinch design method splits the problem into two independent designs e a hot section design and a
cold section design. Hot section comprises of all streams or parts of
streams hotter than the hot pinch temperature. This involves only
process exchange and utility heating. All streams or parts of streams
Significance of the Pinch
below the cold pinch temperature belong to the cold section where
only process exchange and utility cooling may be involved. Thus no
heat transfer occurs across the network pinch temperature. This is valid provided no heat exchanger
has a temperature difference lower than ðDT min Þ and ensures minimum achievable requirements of
both utilities.
To summarise, the necessary and sufficient conditions to achieve minimum utility consumption
targets for a process are
• No heat transfer across the pinch temperature
• No use of cold utility above the pinch
• No use of hot utility below the pinch
The logic behind this can be explained as follows
Any heat transferred (say x kW) from above to below the pinch must have been supplied from hot
utility in addition to the minimum requirement. Enthalpy balance below the pinch shows that this heat
transfer also increases the cold utility above the minimum required by the same amount. Any heat
transfer across the pinch therefore doubles the total utility load in the network. This is somewhat
similar to a situation in a distillation column where any additional heat (beyond the minimum boil up
requirement) added to the reboiler will lead to additional condenser cooling load.
In real life complex network designs, exchangers and utility heaters and coolers will almost
inevitably be placed in positions which violate the pinch. This results in using more than the minimum
amount of both hot and cold utility. For retrofit applications, equivalent arguments apply and the design
procedure ‘corrects’ the exchangers which are ‘at fault’, i.e. violate the pinch principle and prevent a
minimum utility design. Often such retrofit in crude preheat exchanger trains in petroleum refineries
lead to increase in crude preheat temperature at the crude heater (furnace) inlet and results in saving in
fuel (hot utility).

5.4.3 Design strategy
Design for the hot and the cold sections start from the pinch around which the restrictions on the stream
matches are maximum. The two problems are then solved independently.
Key to the pinch design method lies in identifying appropriate
stream matches for exchangers operating at the pinch. The three
Feasibility Criteria at the Pinch feasibility criteria for matching the streams are listed in
Table 5.5. These not only identify appropriate stream matches
but also reveal the need for stream splitting.