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Category: Heat Transfer Technology
Question: Are fin tubes necessary for steam heating a liquid?
Keywords: finned,tubes
Answer: Fins are placed (or machined) on tubes in order to makeup for the rather small ("controlling") film coefficient that is on the finned side of the tubes. In a system of steam-liquid, fins are not required nor justified. To explain the practical use of finned tubes, consider a fin-fan cooler. This is an air-cooled nest of horizontal, finned tubes that normally are used to condense or cool inside the tubes. The air on the fin-side has a terrible film coefficient as compared to the liquid inside the tubes. The air film coefficient is, therefore, the "controlling coefficient" or the one that sets the max amount of heat transfer regardless of how high the tube-side coefficient is. To make up for this air film handicap, additional area on the air side is introduced as fins. Most gases are terrible heat transfer applications. Witness the rather low (10 to 30 Btu/hr-ft2-0F) overall U for gas coolers. Liquids almost always give better heat transfer coefficients. You really should never use fin tubes unless you need them for an engineering reason. The excessive area can only give further problems like surface fouling, hotspots for heat-sensitive compounds and sometimes can cause the phenomena of "cycling": a reboiler or evaporator can be so excessive in area that the heat flow rate is high enough to cause rapid vaporization on the outer tube surfaces and results in a blanket of vaporized liquid shrouding the steam tubes. Remember how terrible gas film coefficients are? What results then is an abrupt halt in the heat transfer due to the vapor blanketing the tubes. This lasts until the vapor disperses in the liquid and the heatup cycle starts again.... It's common for reboilers to be over designed by inexperienced engineers who think they are being conservative and safe in giving more area to the steam bundle. In most cases a plug results or you'll have to remove tubes from the bundle to reduce the heat transfer area.


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