Innovative Environmentally Safe Heat Exchanger Cleaning System

Currently Navy vessels use seawater heat exchangers to provide the cooling capacity needed to meet the ship’s heat load.  Unfortunately, seawater contains many organisms that attach to the walls of the heat exchanger and produce biofilm.  Accumulation of biofilm will reduce the heat exchanger efficiency and can severely inhibit seawater flow.  The current Navy practice to remove biofouling from heat exchangers includes in situ chlorination through electrochlorination, which produces hypochlorous acid (HOCl) and hypobromous acid (HOBr).  Unfortunately, these compounds decompose into less effective forms and the extent of this process is increased as seawater temperatures rise. In addition, chlorination is not effective for removing biofilm from fouled heat exchangers, and therefore a more effective method for biofilm control is needed.  Reaction Systems’ approach to control biofouling in seawater heat exchanges is to inject an alternative biocide into the seawater flow.  The alternative compound is more effective than hypochlorous acid and should have lower emission levels of total residual oxidants (TRO).  In the Navy SBIR Phase I and Phase II projects, Reaction Systems demonstrated the effectiveness of this approach in small scale tests with coupons and in larger scale tests using shell and tube heat exchangers.  In the recently completed Phase II Option project, Reaction Systems conducted a six-month field test using raw seawater from the Pacific Ocean.  The test results showed that the new method prevented biofilm from accumulating in the seawater heat exchangers and it also removed biofilm that had collected previously in an untreated heat exchanger.  Perhaps most importantly, the field test results showed that with a small modification the process can remove biofilm in less than 24 hours.  Currently, fouled heat exchanges must be disassembled and cleaned manually. Therefore, the ability to clean these units in a short period by injecting a biocide into the seawater flow represents a significant savings in labor.  The overall objective of the work proposed here is to design and build a prototype biofilm control system that will be installed and used in a Navy maintenance facility to clean fouled heat exchangers on active Navy ships.  The initial tasks in the project include a field test in a warm water site where the work will be directed at optimizing the biocide levels for preventing biofilm and removing the material in a rapid cleaning cycle.  In all tests, analyses will be performed to monitor TRO emissions produced over a range of injection levels.  Reaction Systems will also work with Navy personnel to identify a specific Navy application for a prototype system that will be installed and operated at the facility to demonstrate the effectiveness of this process.