(83b) Operational Optimisation of Industrial Cooling Water Systems

Operational optimisation of industrial cooling water systems

Fei Song, Nan Zhang, Robin Smith

Centre for Process Integration, School of Chemical Engineering and Analytical Science,

The University of Manchester, UK M13 9PL

Email address: fei.song-2@postgrad.manchester.ac.uk

Cooling water systems are used to remove wasting heat from processes, in which cooling towers are used to cool hot water from cooler networks by heat and mass transfer between bulk water and air to produce cold water for cooler network, and water pumps are employed to provide the pressure for water transportation. Cooling water systems consume a large amount of water and energy in chemical plants. The investigation on cooling water systems is important in water and energy conservation of the process industry. The key components, namely cooling towers, coolers network and pumps, interact with each other in cooling water systems. Therefore, minimum water and energy consumption in a cooling water system should be determined by considering these components simultaneously. 

Research [1,2,3] on operational optimisation of cooling water systems to date has considered the components in cooling water systems simultaneously when determined operating conditions. However, there are some limitations in the previous work. A simple and less accurate cooling tower model was employed in cooling water system modelling. In addition, the previous work has focused on simple cooling water systems with coolers in parallel configuration and a single cooling tower, instead of considering the combination of series and parallel cooler configurations and multiple cooling towers.

In this work, a mathematic model of overall cooling water systems including cooling towers, cooler networks and pumps is developed, in which a simplified cooling tower model is regressed out based on a rigorous cooling tower model[4,5] for forced draft counter flow wet cooling towers. The cooling tower model is validated to provide reliable results of evaporation rate and water outlet temperature of cooling towers. Cooler networks are modelled for the combination of parallel and series configurations. As the model of cooling water systems is formulated as an NLP problem, CONOPT solver is then applied for optimisation of relevant operating conditions of cooling towers and water coolers. Three case studies are presented to illustrate the application of the models developed in this work for both simple and complex cooling water systems: a single cooling tower and a cooler network in parallel arrangement, a single cooling tower and a cooler network in series and parallel arrangement, and multiple cooling towers and a cooler network in series and parallel arrangement. After optimisation, circulating water flowrate in the three cases is reduced by 6.4%, 7%, 8% respectively, make-up water is reduced by about 2%, and the total operating cost is decreased by 6.4%, 2.3% and 3.6% respectively. The results show that higher inlet water temperature of cooling towers with lower circulating water flowrate tends to reduce the operating cost.

References:

[1] M.M. Castro, T.W. Song, J.M. Pinto, Minimization of operational cost in cooling water systems, Trans IChemE 78(Part A) (2000) 192–201

[2] G.F. Cortinovis, M.T. Ribeiro, J.L. Paiva, T.W. Song, J.M. Pinto,  Integrated analysis of cooling water systems: Modeling and experimental validation. Appl Therm Eng 2009.

[3]G.F. Cortinovis, J.L. Paiva, T.W. Song, J.M. Pinto, A systemic approach for optimal cooling tower operation, Energy Conversion and Management 50 (2009) 2200–2209

[4] M. Poppe, H. Rogener, Berechnung von Ruckkuhlwerken, VDI-Warmeatlas (1991), Mi 1–Mi 15

[5] J.C. Kloppers, D.G. Kroger, A critical investigation into the heat and mass transfer analysis of counterflow wet-cooling towers, International Journal of Heat and Mass Transfer 48 (2005) 765–777