TABLE OF CONTENTS We can calculate energy targets for a given ? T min, but what if our chosen value of ? T min is wrong? For a different ? T min, the streams present at the pinch may change, and we could get different pinch matches and a completely different heat exchanger network. How can we find the optimal ? T min?
We have seen that a lower value of ? T min usually gives a reduction in energy use but needs more heat exchanger area; there is a trade-off. Since energy and area are in different units, however, we need to find a basis to compare their importance, and find an optimum ? T min. The obvious basis is cost. Energy requirements will affect the operating cost, while the size of heat exchangers, heaters and coolers will affect capital cost.
In fact, ? T min affects cost in several ways. If we try to reduce our energy costs by choosing a lower ? T min:
The hot and cold utility usage falls, so energy costs fall.
The amount of heat exchange is greater, so larger heat exchangers will be needed and their capital cost is increased.
Temperature driving forces in heat exchangers are lower, again requiring larger heat exchangers and higher capital cost.
Heaters and coolers have a lower heat load, so they can be smaller and...
