Biofouling and Cleaning Tests of OTEC External Flow Heat Exchanger Tubes.

摘要

Heat transfer measurements before and after ultrasonic cleaning were done on sections of 3-in.-O.D., Alclad aluminum tubing (alloy 3004 clad with 7072) that had been biofouled by ocean surface water at a NOAA data bouy 170 nautical miles west of Tampa, Florida, in the Gulf of Mexico. The tubes were sealed inside and exposed to seawater flow at 2.5 ft/sec on the outside. They were arranged in a water box in the geometry of the folded-tube, Ocean Thermal Energy Conversion (OTEC) heat exchangers being developed by the Applied Physics Laboratory (APL). The biofouling exposure periods varied from 3-1/2 to 13 weeks. The heat transfer measurements on some of the fouled tubes were at sea, but some had to be done (due to program limitations) 3 to 6 weeks after their removal from the data buoy, and some of those were exposed to air due to seal failures of the plastic shipping bags. Thus, the thickness and character of the fouling on the tubes varied considerably, but no barnacles were seen on any of them. For the specimens not exposed to air no appreciable corrosion or scale was seen; when scale and/or corrosion were present, much longer ultrasonic cleaning times (using a tank with bottom-mounted tranducers generating 0.7 W/cm exp 2 ) were required. The test results indicated that a building of fouling resistance coefficient R/sub f/ of about 0.00005 Btu exp -1 hr-ft exp 2 - exp 0 F per week occurred. After four weeks, and R/sub f/ approx. = 0.0002, a 25- to 30-sec exposure to ultrasonic radiation at an intensity of 0.7 W/cm exp 2 reduced R/sub f/ to the visually judged cleaned condition, where only a slight discoloration of the surface remained. (ERA citation 09:016158)