Chapter 16 - Minimum number of units (MNU) and minimum total surface area (MTA) targets

Abstract

This chapter focuses on minimum number of units (mnu) and minimum total surface area (mta) targets. Targeting approaches for minimum number of units (MNU) and minimum total surface area (MTA) have to account for the type of heat exchangers applied. Most often they are limited to shell-and-tube apparatus with pure counter-current flow, which are known as matches or 1-1 units (1 shell–1 tube pass). Such methods are able to determine the minimum number of matches (MNM target) and the minimum total surface for matches (MTA-m target). In order to calculate the MNU and MTA targets, one should know the total number of hot streams, which include hot process streams and heating utilities, and the total number of cold streams, which include cold process streams and cooling utilities, that are considered for exchanging heat in a heat exchanger network. The necessary data for process streams are the same as those required for calculating the MUC target. Additionally, to calculate minimum area target, one has to know individual heat transfer coefficients for all streams or, alternatively, overall heat transfer coefficients for all pairs of hot and cold streams. The first method for reaching the MTA-m target was developed by Townsend and Linnhoff in 1984, which is commonly referred to as the “Bath formula.” The key idea in this method is to apply the composite curves to simulate overall counter-current heat exchange in a HEN.