Optimisation of a waste heat exchanger for ballast water treatment

Abstract

Ballast Water Treatment systems, which are type approved and commercially available, require improvements to meet stricter standards, and heat treatment could be a viable additional option. Considering the waste heat potential on a ship, a system harvesting the engine exhaust heat may be envisaged for which a heat exchanger could be vital. Design optimisation of a heater, employing the exhaust gases of an engine as utility fluid, and ballast sea water as the process fluid, was achieved using Lagrangian methods, keeping the annual cost as the objective function. Limiting the number of variables, optimal values were calculated with cost considerations for utility fluid and also pumping costs for utility and process fluids. In all, four optimum designs and three comparative designs were developed. Heat balance data from an operational tanker, specific fuel consumption values and fuel costs were considered for the design. The thermodynamic and geometric designs were worked out using computer based software for a comparison. Designs were compared on the basis of annual cost, optimum exit temperature of shell side fluid, optimum mass flow of tube side fluid and heat exchanger effectiveness. It is demonstrated that an optimal heat exchanger design can be obtained with simple optimisation procedures.