Research on cooling systems to date has focussed on the individual components of cooling systems, not the system as a whole. Cooling water systems should be designed and operated with consideration of all the cooling system components because of the interactions between cooling water networks and the cooling tower performance. In re-circulating cooling water systems, cooling water from the cooling tower is supplied to a network of coolers that usually has a parallel configuration. However, re-use of cooling water between different cooling duties enables cooling water networks to be designed with series arrangements. This allows better cooling tower performance and increased cooling tower capacity, both in the context of new design and retrofit. A methodology has been developed for the design of cooling networks to satisfy any supply conditions for the cooling tower. A model of cooling lower performance allows interactions between the performance of the cooling tower and the design of cooling water networks to be explored systematically. In debottlenecking situations, better design of the cooling network using the new method, including increasing cooling tower blowdown, taking hot blowdown and strategic use of air coolers, can all be used to avoid investment in new cooling tower capacity and to improve the performance of the cooling tower in a systematic way.