Evaluation of sodium hypochlorite for fouling control in plate heat exchangers for seawater application

Plate heat exchangers are often preferred in Ocean Thermal Energy Conversion (OTEC) plants because of the large area they provide for heat transfer leading to higher efficiencies and also because of the compact size of such heat exchangers. Tight control over heat exchanger cleanliness is required as the gap between successive plates is only 3.7 mm and hence fouling can easily impede flow. Based on preliminary evaluation liquid sodium hypochlorite was selected as a possible biocide for fouling control in OTEC operation based on its effectiveness, easy availability, low cost and ease in mid sea replenishment of stock. Subsequently, experiments were conducted in near-shore waters, at residuals of 0.2, 0.5, 0.8, 1.0, 1.2 and 1.5mgL-1 in simulated flow chambers (Pederson device) and slime control was achieved at a continuous residual level of 1.2mgL-1. A log reduction 0.8 times in bacterial density between control and chlorinated plates was observed at this regime. Later on experiments were conducted at an OTEC site (65 km offshore off Tuticorin, on the east coast of India) by changing the regime to an intermittent mode, with a view to reducing the amount of chlorine carried on the OTEC barge. These studies showed that an intermittent chlorination regime of 1.2mgL-1 residuals, 2 h on/2 h off, was effective for slime control. Since the control of fouling is critical for heat transfer efficiency, the dose and frequency evolved in coastal and oceanic waters using flow chambers was validated using a model plate heat exchanger at the oceanic site onboard a research vessel. In these experiments, a significant log reduction (0.6 times) was observed in bacterial density between control and chlorinated plates in an intermittent mode of 1.2mgL-1 once 2h-1 for 2 h. In addition, experiments were conducted by varying the frequency of chlorination from 0.5 to 2 h at a dose of 1.2mgL-1 to gain a better understanding of biofilm development and the cleanliness efficiencies achievable for plate heat exchangers used for seawater applications.