Wednesday, August 7, 2013
THE ONLY ATTENTION THAT HEAT
EXCHANGERS SHOULD REQUIRE IS TO ENSURE THAT THE HEAT TRANSFER SURFACES REMAIN
SUBSTANTIALLY CLEAN AND THE FLOW PASSAGES GENERALLY CLEARER OBSTRUCTION.
INDICATION THAT UNDUE FOULING IS OCCURRING IS GIVEN BY A PROGRESSIVE INCREASE
IN THE TEMPERATURE DIFFERENCE BETWEEN THE TWO FLUIDS, OVER A PERIOD OF TIME,
USUALLY ACCOMPANIED BY A NOTICEABLE RISE IN PRESSURE LOSS AT A GIVEN FLOW.
FOULING ON THE SEA‑WATER SIDE IS THE
MOST USUAL CAUSE OF DETERIORATION IN PERFORMANCE. THE METHOD OF CLEANING THE
SEA‑WATER SIDE SURFACES DEPENDS UPON THE TYPE OF HEAT EXCHANGER. WITH SHELL‑ANDTUBE
HEAT EXCHANGERS, THE REMOVAL OF THE HEADER COVERS OR, IN THE CASE OF THE
SMALLER HEAT EXCHANGERS, THE HEADERS THEMSELVES, WILL PROVIDE ACCESS TO THE
TUBES. OBSTRUCTIONS, DIRT, SCALE ETC., CAN THEN BE REMOVED, USING THE TOOLS
PROVIDED BY THE HEAT EXCHANGER MANUFACTURER. FLUSHING THROUGH WITH FRESH WATER
IS RECOMMENDED BEFORE A HEAT EXCHANGER IS RETURNED TO SERVICE. IN SOME
APPLICATIONS, SUCH AS PISTON OIL COOLING, PROGRESSIVE FOULING MAY TAKE PLACE ON
THE OUTSIDE OF THE TUBES. MOST MANUFACTURERS RECOMMEND A CHEMICAL FLUSHING
PROCESS TO REMOVE THIS IN SITU, WITHOUT DISMANTLING THE HEAT EXCHANGER.
PLATE HEAT EXCHANGERS MAY BE CLEANED
BY UNCLAMPING THE STACK OF PLATES AND MECHANICALLY
CLEANING THE SURFACE OF EACH PLATE AS RECOMMENDED BY THE MANUFACTURERS. THE
PLATE SEALS MAY REQUIRE REPLACEMENT FROM TIME TO TIME AND HERE THE
MANUFACTURER’S INSTRUCTIONS SHOULD BE CLOSELY FOLLOWED.
CORROSION BY SEA‑WATER MAY
OCCASIONALLY CAUSE PERFORATION OF HEAT TRANSFER SURFACES. THIS WILL CAUSE
LEAKAGE OF ONE FLUID INTO THE OTHER BUT THIS IS NOT ALWAYS EASY TO DETECT
WHILST THE LEAKAGE IS SMALL, ALTHOUGH SUBSTANTIAL LEAKS MAY BECOME EVIDENT
THROUGH RAPID LOSS OF LUBRICATING OIL, JACKET WATER ETC.
LOCATION OF A PERFORATION IS A
STRAIGHTFORWARD MATTER IN THE CASE OF A TUBULAR HEAT EXCHANGER, WHETHER THIS IS
OF THE SHELL‑AND‑TUBE TYPE OR OF OTHER TUBULAR CONSTRUCTION HAVING DRAINED THE
HEAT EXCHANGER OF SEA‑WATER AND REMOVED THE COVERS OR HEADERS TO EXPOSE THE
TUBE ENDS, SOME FLOW OF THE LIQUID ON THE OTHER SIDE OF THE SURFACE WILL BE
APPARENT, IN THE CASE OF OIL AND WATER COOLERS, FROM ANY TUBES WHICH ARE
PERFORATED, TO TEST FOR LEAKS IN AIR COOLERS, DRAINS COOLERS ETC. EACH TUBE IN
RUM CAN BE PLUGGED AT EACH END AND PRESSURISED WITH AIR; INABILITY TO HOLD
PRESSURE INDICATES A LEAK.
TO AID THE DETECTION OF LEAKS IN A
LARGE COOLER SUCH AS A MAIN CONDENSER, IN WHICH IT IS DIFFICULT TO GET THE
TUBES DRY ENOUGH TO WITNESS ANY SEEPAGE, IT IS USUAL TO ADD A SPECIAL
FLUORESCENT DYE TO THE SHELL SIDE OF THE COOLER. WHEN AN ULTRA‑VIOLET LIGHT IS
SHONE ON TO THE TUBES AND TUBE PLATES ANY SEEPAGE IS SEEN SINCE THE DYE GLOWS
WITH IT.
