Long-term operation optimization of circulating cooling water systems under fouling conditions

Abstract

Fouling caused by excess metal ions in hard water can negatively impact the performance of the circulating cooling water system (CCWS) by depositing ions on the heat exchanger's surface. Currently, the operation optimization of CCWS often prioritizes short-term flow velocity optimization for minimizing power consumption, without considering fouling. However, low flow velocity promotes fouling. Therefore, it's crucial to balance fouling and energy/water conservation for optimal CCWS long-term operation. This study proposes a mixed-integer nonlinear programming (MINLP) model to achieve this goal. The model considers fouling in the pipeline, dynamic concentration cycle, and variable frequency drive to optimize the synergy between heat transfer, pressure drop, and fouling. By optimizing the concentration cycle of the CCWS, water conservation and fouling control can be achieved. The model can obtain the optimal operating parameters for different operation intervals, including the number of pumps, frequency, and valve local resistance coefficient. Sensitivity experiments on cycle and environmental temperature reveal that as the cycle increases, the marginal benefits of energy/water conservation decrease. In periods with minimal impact on fouling rate, energy/water conservation can be achieved by increasing the cycle while maintaining a low fouling rate. Overall, the proposed model has significant energy/water saving effects and can comprehensively optimize the CCWS through its incorporation of fouling and cycle optimization.

1. Introduction

Circulating cooling water systems (CCWS) are widely applied in industrial production as water treatment systems for cooling, heating and conveying. They are used to release industrial waste heat to the environment. Optimizing circulating water systems improves water utilization and reduces operating costs. Several studies have optimized individual components when designing circulating water systems, including pumps [1,2], cooling towers [3], heat transfer networks [4], or through air coolers [5], variable frequency drives (VFD) [6,7] for system optimization. The main optimization methods are the pinch method [8] and the mathematical programming method [9], etc.

Despite numerous experimental and simulation studies, predicting and preventing fouling is still an unsolved problem in engineering, resulting in huge economic losses and environmental damage [10]. More than 90% heat exchangers in various industries are experiencing fouling [11]. Circulating water contains a large number of mineral ions, such as calcium, magnesium, etc., evaporation in the cooling tower makes the ions constantly concentrated. When the circulating water in the exchanger is heated, the solubility of minerals is reduced, previously dissolved in the fluid calcium carbonate began to precipitate, and deposited on the surface of the pipeline. Fouling of circulating water system includes (i) particle fouling, (ii) crystalline fouling, (iii) corrosion fouling, (iv) biofouling, etc [12]. Fouling increases the thermal resistance between the fluid and the heated surface, decreasing the heat transfer performance. The inner diameter of the heat exchanger tube decreases continuously with the accumulation of fouling. The velocity of the heat exchanger tubes will increase, results in a rise of pressure drop under the same flow rate.