(679g) Optimal Design of Aging Systems: A-Frame Coolers Design Under Fouling

Everyday operation causes fatigue to materials and dirt deposits build on the surface of equipment. As a result, these units suffer losses in performance that affect the overall yield of the unit and of the facility. Beyond the lost in performance, failures may occur if the units operate for longer periods. Eventually, some of the units or sections must be replaced or repaired. It is estimated that $20,000 -$30,000 of losses per hour can occur in refineries due to failures due to lack of proper maintenance (Tan and Kramer, 1997). Industry schedules maintenance stops to evaluate, mitigate, clean and repair sections and entire units. There is a large number of methods in the literature to evaluate maintenance strategies (Dekker, 1996). However, the most common optimization is minimum cost subject to average uptime or downtime (Wang, 2002) and mitigation strategies are developed based on the performance decay or aging process (Beaurepaire et al., 2012). To the best of our knowledge, simultaneous design and operation of the units considering performance decay is not addressed, although for instance the design of structures considers aging (Dekker, 1996).

Concentrated solar power (CSP) plants are typically located in regions with high solar incidence but limited water availability (Martín and Martín, 2013). The large cooling needs, as any other thermal facility, represent the strongest link between water consumption and power production (DOE, 2018). A frames are a special type of air coolers that allows decoupling the water –energy link in power plants. However, these systems consume up to 10% of the power generated in the facility to power the fans. The design of such units has typically been carried out based on rules (Krogen, 2004). Furthermore, A-frame units are located in open air and fouling, in particular, particle fouling is an important issue for their operation and performance. Fouling does not only affect the heat transfer coefficient, but deposit also block the cross sectional area, generating an additional pressure drop across the system (Pu et al., 2009; Sarfraz and Bach, 2016).

In this work we present a general methodology for the simultaneous optimal design and operation of units or processes whose yield is affected by aging or performance decays. We propose a parametric programming design procedure to determine the design and the cleaning/maintenance schedule. We use it for the design of A-frames under fouling conditions.

A detail equation based model for the units developed including mass and energy balances, design equations for the geometry and pipes layout, , the fan power curves, heat transfer coefficients, etc. and the aging process characterized. A two-stage procedure is proposed. First, the unit is optimally designed for the worst case scenario, just before maintenance. In a second stage, a multiperiod problem is solved for the optimal the operation of the unit over time including cleaning costs. The methodology is applied to A-frame dry cooling systems under fouling conditions, where fouling affects the pressure drop and the global heat transfer coefficient. Due to the sigmoidal deposition profile and assuming cleaning costs, the optimal cycle time is 8 yr. This design allows reducing the energy required to around 4% of the energy produced by the concentrated solar power plant. It is a promising result that can be affected by plant layout and ground availability.

References

Dekker, R. 1996 Application of maintenance optimization models: a review and analysis. Rel. Eng. Sys. Saf. 51 229-240
Beaurepaire, P., Valdebenito, M.A., Schueller, G.., Jensen, H.A. 2012. Reliability –based optimization of maintenance scheduling of mechanical components under fatigue. Compt. Methods. Appl. Mech Eng. 1(221-222), 24-40

Kröger, D.G., 2004. Air-cooled heat exchangers and cooling towers: thermal-flow performance evaluation and design. Volume II. Pennwell: Oklahoma, Tulsa.

Tan J.S.,Kramer M.A., 1997. A General Framework For Preventive Maintenance Optimization In Chemical Process Operations. Computers and Chemical Engineering, 21(12), 1451-1469.

Pu, H., Ding, G.-l., Ma, X.-K., Hu, H.-T., & Gao, Y.F., 2009. Effects of biofouling on air-side heat transfer and pressure drop for finned tube heat exchangers. International Journal of Refrigeration, 32(5), 1032-1040.

Sarfraz, O., Bach, C., 2016. A Literature Review On Heat Exchanger Air Side Fouling In Heating, Ventilation And Air conditioning (HVAC) Applications International Refrigeration and Air Conditioning Conference. Paper 1663.