Optimal Selection of Steam Mains in Total Site Utility Systems

Abstract

This paper examines the optimal location (pressure) and the number of steam levels required to meet the external heating and cooling demands of individual site processes. The model developed makes use of Mixed Integer Linear Programming (MILP) techniques, implemented in a Visual Basic/Excel environment and linked to existing simulation software in order to extract the appropriate data for the total site. The model makes use of various methods of calculating shaftwork produced from the expansion of steam from simple single stage turbines operating between the steam levels involved. The shaftwork targeting methods include the TH model (Raissi, 1994), simple isentropic expansion, Willans’ line methods, and methods developed recently by Ghannadzadeh et al (2012) . Different models of shaftwork calculation are required depending on the nature of the data available. Results from various case studies are validated by comparison with simulation, and show that the optimal location of a fixed number of steam levels can significantly change depending on the method of shaftwork calculation used. Similarly the number of steam levels has an influence on the overall site heat recovery through the steam mains and the steam needed to be supplied by the boiler. This new approach to the selection of the appropriate and optimal pressure of the steam mains across total sites can also be applied to existing total sites in order to improve operational performance. The procedure can also be applied in the total site context to examine improvements in waste heat utilisation and consequently in distributed energy systems.