The Department of Defense (DoD) operates vessels that use heat exchangers (HXs) to cool ships’ operating fluids and gases (i.e., water, compressed air, lubricants, etc.). Many of these HXs use the ocean water as the cooling medium where heat transfer occurs from the service fluid to the cooling water and the cooling water, now warmed, is returned into the ocean. Fouling of these systems reduces the efficiency of the HX, increasing ships’ fuel use and the generation of greenhouse gases. If the foul is microbiological in nature, increased corrosion can also lead to significant discharges of heavy metal ions.
The objective of this demonstration was to validate the safety, effectiveness, ease of implementation, and cost savings of the elemental iodine (I2) bubble infusion technology as a means to significantly reduce the fouling rate within a functioning HX in a marine environment. The project team also intended to show that I2 bubble infusion technology when used onboard a ship as part of a cleaning protocol can significantly reduce the overall cleaning cost as well as the generation of hazardous wastes. These demonstrations were performed on a full scale HX onboard a retired Navy warship that is now operated as a Navy test ship. It was shown that the I2 bubble infusion technology offers environmental, safety, and occupational health benefits as compared to the existing acid cleaning method.
Three approaches currently used to address bio-fouling are: no action, chemical/mechanical cleaning, or electro-chlorination. To restore a HX to full performance, it must be cleaned through either mechanical or chemical cleaning or a combination of both. Expensive and time consuming, this process produces a liquid containing high levels of dissolved metals and is normally a hazardous waste regulated by the Resource Conservation and Recovery Act (RCRA). After cleaning, either no action is taken or the system may be maintained through electro-chlorination. In either case, once water enters the HX, the system begins to foul immediately with fouling primarily indicated by both an increase in the temperature drop across the HX as well as an increase in HX inlet pressure.
Halogens (iodine, bromine, and chlorine) have long been used in water purification. Electro-chlorination, currently used by the Navy, is the electrolytic production of sodium hypochlorite from seawater. Unfortunately, recently enacted environmental regulations are challenging the use of chlorination. The I2 infusion protocols demonstrated in this project strip I2 from iodinated resin beads using compressed air to form a perfusion of micro and macro bubbles within a fluid for remote disinfection. This patented technology was developed by I2 Air Fluid Innovation, Inc. The iodinated bubbles interact with the cell walls of microorganisms (bacteria, larvae, etc.) within the fluid or on surfaces providing elemental iodine transfer. Iodine vapor offers a number of benefits including rapid disinfection and iodination of some foulants. The easily integrated infusion device is a safe, cost-effective system requiring little maintenance and energy.
The I2 infusion process is at the core of two methodologies developed by I2 Air Fluid Innovation used to retard bio-fouling: the I2 Cleaning Protocol (I2CP) and I2 Maintenance Protocol (I2MP). I2CP uses bubbles in conjunction with mild acid or alkaline cleaners to remove existing foul within a HX. It mechanically disrupts foul, forces cleaner through bio-films, and re-distributes cleaner to improve foul solubility. Where minerals are the foul and acid is the cleaner of choice, the solution retains low pH through vapor acidification. This allows for the use of milder acid cleaners, and due to bubble perfusion, the need for less volume of cleaner.
This project was completed in three phases: laboratory testing, field testing, and a shipboard demonstration. The overall goal was the rehabilitation of an already fouled exchanger and a reduction in foul progression under normal operating conditions.
In the first phase of the project in the laboratory, the team verified that the non-metallic and metallic materials commonly used within shipboard HXs were compatible with the chemicals used during the I2 protocols. The project team also verified that the iodinated bubbles did not increase the erosion rate on HX materials.
In the second phase, the team performed field testing at the National Energy Laboratory of Hawaii Authority’s (NELHA) facility in Kona, Hawaii. At this facility, Makai Engineering, a subcontractor to I2 Air Fluid Innovation, designed, installed, and monitored a device to determine foul retardation and metal erosion rates for five common HX metals using warm Pacific Ocean seawater. Testing was performed both in unlit conditions, emulating the HX interior, and sunlight conditions, fostering the growth of algae. Testing showed that the I2 infusion process was not inhibitory to algae growth. Although initial qualitative indications showed a reduction in foul formation, numerous performance problems by Makai Engineering resulted in the project team not achieving the intended goals as specified in the Demonstration Plan.
In phase three, onboard the Self Defense Test Ship (SDTS), two identical Low Pressure Air Compressor (LPAC) HXs (Numbers 1 and 2) were used for the demonstration. Both received the I2CP treatment to form a performance baseline. Both HXs were cleaned on the same day, requiring only 3.5 hours each without the need for disassembly. Waste was collected and measured for volume and sampled for metal content. The time required and the effluent collected met the performance objectives for the I2CP.
LPAC No. 1 was designated to receive the infusion protocol, I2MP. The demonstration was performed over a period exceeding 9 months, with resin cartridges changed approximately monthly. No equipment maintenance was required during the demonstration period. Measurements of the inlet and outlet temperatures and inlet pressure readings were recorded on calibrated ship gages. Water samples were periodically obtained to measure metallurgical elution and sublimation of iodine.
Although the project team had asked that each exchanger be used 50% of the time, in actuality, LPAC No. 1 was in use approximately 85% of the time. As expressed by the crew, typically this exchanger would have been cleaned every 3 to 6 months. At the end of the demonstration, the temperature and pressure parameters were still within the acceptable range. Water sampling indicated low metal and iodine levels within the effluent. LPAC No. 1 metal ion elution did not vary greatly whether the system was infusing or not.
At the end of the demonstration period, the LPAC units were disassembled and viewed for foul progression. The units were re-assembled and had an I2CP performed using an acidic cleaner. The effluent from each was analyzed for Total Suspended Solids (TSS), Total Organic Carbon, and Total Dissolved Solids (TDS) as well as Ti, Cu, Ni, Zn, and Pb content before and after the cleaning. The inspection showed that the tubes were relatively clear of solidified foul. The cleaning solutions for both HXs indicated less metal elution than with normally used Navy cleaning procedures.
Unfortunately, this project did not result in a definitive clear indication of success. The fact that the LPAC No. 1 exchanger was used 85% of the time meant the control HX saw very little use. Ideally, the demonstration would have been continued until such time that the ship needed to perform a HX cleaning. Because the ship normally cleans the exchangers every 3 to 6 months, the project team was able to show that use of the I2 bubble infusion technology did achieve the most important goal of extending the period between cleanings by 50%.
Two follow-on Navy demonstrations of this technology have been initiated. The technology is undergoing study at the Undersea Naval Warfare Center in Newport, Rhode Island, as a hull foul retardant in conjunction with air bubble curtains. In addition, the protocol is under study at Pearl Harbor Naval Shipyard to prevent bio-fouling in a submarine salt water heat exchange support system that is used both pier side and in dry-dock. For this application, the I2 technology would replace an existing electro-chlorination system. Although not approved at this time, the project team’s technology integration plan includes demonstrating the technology on a Navy combat ship to determine how long the I2 technology can extend the period between HX cleanings.