TABLE OF CONTENTS Basic energy targeting was described in Chapter 2. The Problem Table calculation finds the hot and cold utility requirements, pinch temperature and relationship between net heat flow and temperature for a chosen ? T min value. We will now take this further to look at more detail and a variety of special cases.
Step-by-step algorithms for calculating the Problem Table and composite curves are given in the Appendix to this chapter, Section 3.11.
We can see from the simple equation for countercurrent heat exchange ( Q = UA ? T LM) that the heat exchanger area A required is inversely proportional to both the overall heat transfer coefficient U and the temperature difference ? T on a match. So far, we have assumed that any streams with given temperatures can be matched, irrespective of their characteristics. However, some streams may be gases or viscous liquids with poor heat transfer coefficients, or may be prone to fouling heat exchanger surfaces. Heavy oils and waxes are particular culprits on both grounds. The U-value on matches involving these streams will be low, and hence the corresponding area of a match involving these streams will be alarmingly high.
Later, when we consider area targeting (in Section 3.6.3), we will see how this can be allowed for explicitly, but can we take it into account when calculating basic energy targets?
There is a simple method, which...
