Abstract
Adding air coolers to cooling water system is an effective way to reduce heat load and the biaya of cooling water system. It is also an effective method to prevent fouling and save water in the region where water is scarce. There is a trade-off between air cooler system and cooling water system. When heat load of air cooler is high, cooling tower consumes less segar water, but the biaya of air cooler can be high. Conventionally, the two systems are optimized separately. This paper presents an optimization model for synthesizing cooling water system with air coolers. The water coolers, air coolers, pumping scheme and cooling tower are simultaneously optimized. Each hot stream can be cooled down by air cooler to certain degree and then cooled down by water cooler to obyek temperature. Or it can be cooled down by the air cooler or water cooler exclusively. The model is formulated as mixed-integer nonlinear programming (MINLP) problem. The objective is to formulate the cooling water system with the minimizing total annual cost. The case is optimized under two cities with different prices of water and electricity. Results show that optimization model yields 29.4% and 13.1% TAC reduction. Results also indicate that it is particularly necessary to add air coolers to cooling water system in region where water is scarce and electricity price is low.
Introduction
Water cooling and air cooling are among the two most common methods that are employed to reject industrial waste heat to environment. Because water has suitable thermal properties and non-harmful chemical composition, cooling water system by far, has been widely used and thoroughly studied. The pioneer work conducted by Kim and Smith (2001) introduced mathematic model that emphasizes interaction between cooling tower and cooler network. Cooling water is reused in mixed parallel/series cooler arrangement, and cooling tower has better performance owning to low cooling water flow rate and high water return temperature. To reduce complexity and improve flexibility of cooler network, Feng et al. (2005) proposed a cooling water network with an intermediate main that is easy to control and operate. Introducing intermediate mains allows water to be reused which in turn leads to increase of tower efficiency. Research conducted by Castro et al. (2000) also focuses on reducing the operational biaya of cooling water system by minimizing water flowrate. Because water cooling consumes segar water and produces waste water, the system has certain impact on local environment. Panjeshahi et al. (2009) optimized cooling water system that involved environmental considerations as well as energy conservation. However, all the models previously mentioned mainly focus on minimizing water flowrate. In order to achieve the heat exchanging service, more contact areas are required. Later, Ponce-Ortega et al. (2007) proposed an MINLP model that considered the capital biaya of coolers and cooling water biaya simultaneously. In their work, stage-wise cooler network was proposed. The objective was minimizing the total biaya and this model was more economical than other model previously mentioned. Few years later, they studied the detailed design of cooling tower, the optimization of cooling tower was based on MINLP model (Serna-González et al., 2010) and rigorous poppe model (Rubio-Castro et al., 2011). For optimizing cooling tower, Singh and Das (2017) optimized performance parameters and energy consumption of cooling tower simultaneously.