Cooling water systems are the most common method for the rejection of waste heat in chemical processes. Conventional re-circulating cooling water systems have a network of coolers in a parallel configuration, demanding both large cooling water circulation and cooling towers. Although reuse of cooling water reduces the amount of water that is necessary in the system and, hence, increases the cooling tower performance and its capacity, it may significantly increase the pressure drop due to series-parallel arrangements. This paper develops a methodology for the pressure drop calculation based on the Graph Theory. From the adjacency matrix of coolers network, it uses the topological sorting and critical path algorithms in order to evaluate the pressure drop in a network with coolers in series-parallel arrangements. Applying this methodology to coolers network with cooling water reuse, it is possible to analyze the impact of increased pressure drop when compared to a network with no reuse.