Synthesis cooling water system with air coolers

Adding air coolers to cooling water system is an effective way to reduce heat load and the cost 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 fresh water, but the cost 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 target 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.