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Unformatted text preview: Abstract: In most of chemical plants at some stages heat must be added to a system or extracted from it. In this experiment heat is transferred to water flowing inside a vertical tube from steam condensing on the outside of the tube. Condensation occurs when saturated vapor contacts with a surface with lower temperature. There are two types of condensation known as filmwise condensation and dropwise condensation. Heat transfer coefficient (h) can be obtained for filmwise condensation from an equation which is simplified by using a lot of assumptions. So that there a large difference between the heat transfer coefficient calculated experimentally ( exp h ) and theoretical heat transfer coefficient th h which calculated from equation (16). Also, in this experiment the effect of air on the heat transfer was checked by measuring the difference between the water inlet and outlet temperatures before and after adding air and it was about 8.9 C and 2.6 C respectively for the dropwise condensation. Introduction: Condensation takes place when a saturated vapor comes in contact with a surface whose temperature is lower than the saturation temperature ( sat sur T T < ), corresponding to the thermodynamics vapor pressure. The liquid or condensate thus formed on the cold surface will somewhat subcooled by contact with the cooled surface, and more vapor will condense on the exposed surface and upon the previously formed condensate. The role of condensation using steam for power production, refrigeration and distillation, and to convey heat has a long history and its use in these filed likely to continue into foreseeable future. In all applications, the steam must be condensed as it transfers heat to a cooling medium. This could be the cold water in the condensers of a generating station, the hot water in a heating calorifier a sugar refinery, etc. Theoretical Background: During condensation, very high heat fluxes are possible and provided the heat that can be quickly transferred from the condensing surface into cooling medium, steam using heat modes, known as filmwise and dropwise. For the same temperature difference between the steam and the surface, dropwise condensation is several times more effective than filmwise. I) Filmwise Condensation Most materials used in the construction of heat exchangers are wettable and during condensation a film of condensate spreads over the surface. More vapor condenses onto the outside of this film, increasing its thickness and causing it to flow downward and drip from the lowest points. The heat given up by the vapor during condensation is conducted through the film to the metal beneath and from this, to the coolant. The liquid is a comparatively poor conductor of heat, and although the film may be relatively thin, its thermal resistance is appreciable. It is this resistance which accounts for large difference between the effectiveness of filmwise and dropwise condensation....
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This note was uploaded on 09/27/2011 for the course CHEMICAL E CHE 309 taught by Professor M.elgaily during the Fall '09 term at King Fahd University of Petroleum & Minerals.
- Fall '09