Page 141 - Chemical engineering design

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FUNDAMENTALS OF ENERGY BALANCES
Summary
The heuristics (guide rules) for devising a network for maximum heat recovery are given
below:
1. Divide the problem at the pinch.
2. Design away from the pinch.
3. Above the pinch match streams adjacent to the pinch, meeting the restriction:

CP hot CP cold
4. Below the pinch match streams adjacent to the pinch, meeting the restriction:

CP hot ½ CP cold
5. If the stream matching criteria can not be satisfied split a stream.
6. Maximise the exchanger heat loads.
7. Supply external heating only above the pinch, and external cooling only below the
pinch.

3.17.4. Minimum number of exchangers
The network shown in Figure 3.27 was designed to give the maximum heat recovery, and
will therefore give the minimum consumption, and cost, of the hot and cold utilities.
This will not necessarily be the optimum design for the network. The optimum design
will be that which gives the lowest total annual costs: taking into account the capital
cost of the system, in addition to the utility and other operating costs. The number of
exchangers in the network, and their size, will determine the capital cost.
In Figure 3.27 it is clear that there is scope for reducing the number of exchangers.
Exchanger D can be deleted and the heat loads of the cooler and heater increased to
bring streams 2 and 3 to their target temperatures. Heat would cross the pinch and the
consumption of the utilities would be increased. Whether the revised network would be
better, more economic, would depend on the relative cost of capital and utilities. For any
network there will be an optimum design that gives the least annual cost: capital charges
plus utility and other operating costs. The estimation of capital and operating costs are
covered in Chapter 6.
To find the optimum design it will be necessary to cost a number of alternative designs,
seeking a compromise between the capital costs, determined by the number and size of
the exchangers, and the utility costs, determined by the heat recovery achieved.
For simple networks Holmann (1971) has shown that the minimum number of
exchangers is given by:
0
Z min D N 1 3.41

where Z min D minimum number of exchangers needed, including heaters and coolers
0
N D the number of streams, including the utilities